Could hydroxyapatite toothpaste cause hypercalcaemia?

BONE METABOLISM

Arno Helmberg

This script is a learning and overview aid for my lecture in the module "Musculoskeletal System" at the Medical University of Innsbruck. I would like to encourage all students to develop a good basis in medical English and therefore make the script available in an English version. The pdf version is suitable for printing.

Version 1.7 © Arno Helmberg 2009-2020

1. BONE BUILDING AND REMOVAL

Living bone is constantly being remodeled. Our bones are always close to a balance between building and breaking down. In childhood and adolescence, the build-up until the maximum bone mass between twenty and thirty is reached, followed by breakdown. The constant renovation has two reasons. On the one hand, it enables constant adaptation to changing loads. An example is the amazing ease with which orthodontists move teeth in the jaw with constant light pressure. On the other hand, constant remodeling is necessary to repair small cracks caused by constantly occurring microtraumas. In a typical microscopic bone structure (basic multicellular unit) bone tissue is removed in about 3 weeks by specialized cells, so-called osteoclasts. This resorption lacuna is then filled with new bone tissue in about 3 months by osteoblasts. Bone tissue comes in two forms: as substantia compacta and substantia cancellous. Bone saves weight as much as possible. Only the outer layer, the compacta, is solid; with the long bones in the form of a tube. Inside is the cancellous bone, a three-dimensional pillar framework made of trabeculae, which is constantly being rebuilt and adjusted to the changing loads, e.g. B. in the vertebral bodies or at the ends of the long bones.

The basic unit of compact bone tissue is the osteon or Havers system. It consists of a central vascular canal with massive lamellae of mineralized fibers arranged concentrically around it. The fibers of the basic bone substance are arranged in successive lamellae alternating right-hand and left-hand spirals, which gives additional stability. Occasional bone cells, osteocytes, sit between the lamellae.

Only two cell types are actually responsible for bone metabolism: osteoblasts and osteoclasts. Osteocytes are nothing more than osteoblasts that have walled themselves in. Osteocytes remain connected to one another by long cell processes and thus form a network through which they can use gap junctions Ions can pass on. Osteocytes can receive mechanical loads and in this way transmit corresponding signals to the bone construction crews.

Osteoblasts differentiate from bone marrow stromal cells. They produce the organic basic bone substance, which in its not yet mineralized form is known as an osteoid. Below are three functionally important proteins from a much larger number of molecules:

  1. Type I collagen makes up the majority of this protein matrix. It consists of two α1 chains and one α2 chain, which are post-translationally hydroxylated on lysines and prolines and form a procollagen triple helix in the endoplasmic reticulum of the osteoblast. These procollagen monomers are secreted. The N- and C-terminal procollagen peptides are split off extracellularly, whereupon many triple helix monomers spontaneously assemble to form long fibrils that are covalently cross-linked via the hydroxylated lysines. Vitamin C is required as a coenzyme for the hydroxylation of lysines and prolines; A lack of vitamin C leads to scurvy due to the instability of the poorly cross-linked collagen.
  2. Osteocalcin is a small protein that is carboxylated on glutamic acid residues with the help of vitamin K. Glutamic acid already contains a COOGroup as the end of the side chain; the carboxylation on the γ-carbon atom now adds a second one. The two negative charges of the neighboring COO-Groups are an ideal attachment point for the doubly positively charged Ca2+-Ion. Osteocalcin binds hydroxyapatite approx5(PO4)3(OH), however, is not a prerequisite for its formation: Osteocalcin-null mice even show increased bone mineralization. The hydroxyapatite crystallization, which ensures that the poorly soluble calcium phosphate precipitates in the bone and not in other parts of the body, must therefore be based on other osteoblast products. However, the fracture strength of the bone in these mice is greatly reduced; Apparently, osteocalcin limits the propagation of cracks by stretching and dissipating mechanical energy. Osteocalcin therefore probably acts as a shock absorber and cushioning between organic and inorganic matrix components. Vitamin K is also required to provide coagulation factors II, VII, IX, X with approx2+- equip loyalty points; A deficiency in vitamin K therefore leads to coagulation disorders first. A second vitamin is important for osteocalcin: its transcription is stimulated by the activated vitamin D receptor. In a second function, non-carboxylated osteocalcin acts as a metabolic hormone that increases the effect of insulin. Via a G-protein-coupled receptor, it promotes the proliferation of β cells in pancreatic islets and increases their insulin secretion. It also promotes the release of adiponectin from fat cells, which increases their insulin sensitivity. The bone metabolism thus has an influence on the energy metabolism.
  3. Osteonectin is another osteoid component that binds both hydroxyapatite and collagen and thus also contributes to the connection of organic and inorganic components of the matrix.

In addition, osteoblasts take approx2+ and phosphate and secrete them in a targeted manner, so that local supersaturation occurs and the newly formed basic substance is mineralized through precipitation. The alkaline phosphatase located on the outside of the osteoblast cell membrane, which provides phosphate ions possibly by splitting off phosphorylated organic molecules and by splitting pyrophosphate, plays a role in this process that has not yet been adequately understood.

One could compare the statics of bones with that of reinforced concrete. Hydroxyapatite is pressure-resistant, while the built-in collagen fibers absorb the tensile forces that occur. This picture helps to understand why bone loss and bone formation are an inseparable functional unit. Due to the mechanical stress, microcracks constantly appear in the bone. There the collagen ("steel") fibers are torn. A repair can only be carried out at this point if there is a greater resorption mood so that new, intact fibers with sufficient anchoring on both sides of the original crack can be installed ("embedded in concrete"). Pure mineralization ("plastering") of the crack would not restore the original load-bearing capacity. Similarly, there are rare genetic diseases in which a defect in the bone breakdown mechanism results in very dense, but nevertheless fragile (easily breaking) bones, as numerous inadequately repaired ("plastered") microcracks (osteopetrosis or marble bone disease) are pervaded by it.

When osteoblasts have completely walled themselves in, they change their expression pattern and become Osteocytes. Osteocytes secrete sclerostin, which also prevents osteoblasts in the immediate vicinity from further forming bone substance by blocking their LRP5 / 6 receptor and thus the Wnt signaling pathway. Sclerostin thus promotes "sclerosis", the solidification of the bone. Factors that promote bone remodeling, such as mechanical stress, parathyroid hormone and prostaglandin E, inhibit the production of sclerostin.

Pharmacological cross bracing:Romosozumab is a monoclonal antibody that binds and inactivates sclerostin. In the development phase it showed good efficacy against osteoporotic fractures, but there were more adverse cardiovascular events compared to controls. Romosozumab was approved subject to conditions in 2019.

Osteoclasts are large, multinucleated cells that arise in the bone marrow from hematopoietic progenitor cells. The developmental sequence is the same that leads to the formation of macrophages and neutrophils. A number of cytokines can trigger the differentiation of these progenitor cells into osteoclasts. The simplest signal pattern consists of M-CSF (macrophage colony stimulating factor) and RANKL (explained in the Parathyroid Hormone section), both of which are produced by osteoblasts. But also cytokines, which are mainly formed by macrophages in the course of inflammation, strengthen osteoclast differentiation: IL-1, IL-6, TNFα and prostaglandin E. Bone degradation takes place like lysosomal degradation through acidification and activation of hydrolases, only extracellularly . Osteoclasts close off a certain area of ​​mineralized bone surface like a suction cup and acidify it with a proton pump. They transport HCO for intracellular acid-base balance3to their back. The acidification leads to the dissolution of the hydroxyapatite crystals and thus dissolves Ca2+ from the bone. A concerted increase in osteoclast activity therefore increases extracellular Ca2+-Concentration. Proteases with a pH optimum in the acidic range, such as cathepsin K, degrade the melted protein matrix.

Long bones cannot grow in length in the actual bone, but rather in the cartilage of the epiphyseal plate. Three zones of chondrocytes of different differentiation can be observed. In all three zones, the chondrocytes secrete proteins and proteoglycans that make up the extracellular cartilage matrix, such as collagen and aggrecan. The reserve zone with chondrocytes, which act as precursor cells, is located near the epiphysis. Next comes the proliferation zone, in which the spatial orientation of the cell division creates long columns of chondrocytes parallel to the longitudinal axis of the bone. These cells produce the type II collagen that is typical of hyaline cartilage. The hypertrophic zone follows the metaphysis, in which the chondrocytes differentiate terminally, increase in volume and collagen type X and VEGF (vascular endothelial growth factor) secrete. The hypertrophic chondrocytes die at the border zone. The secreted VEGF leads to the sprouting of vessels. The existing cartilage is initially mineralized (chondral ossification), but soon it is secondarily converted into lamellar bone by migrating osteoclasts and osteoblasts. In this way, cartilage growth in the epiphyseal plate leads to the longitudinal growth of the bone. This process is regulated in a complex manner. Genome-wide association studies (GWAS) have shown around 200 gene loci that influence human height. The proliferation of the chondrocytes is caused by a large number of paracrine factors, e.g. B. a gradient of BMPs (bone morphogenetic proteins) or C-type natriuretic factor, as well as many endocrine factors such as growth hormone, IGF-1, sex hormones and leptin. The endocrine factors ensure that rapid growth only occurs when sufficient nutrition is available.

A second mechanism of bone formation, desmal ossification, is the direct conversion of connective tissue into bone. This is how large parts of the skull are created; The healing of broken bones also takes place in this way.

2. REGULATION OF BONE ALTERATION

2.1 Calcium and phosphate balance

We have already dealt with the calcium and phosphate balance in connection with the kidneys. Now let's look at it in terms of how it interacts with our bones.

2.1.1 Soluble approx2+, Hydroxyapatite and calcitonin

Calcium (approx2+) is found in huge amounts in the body as it is a major component of our bones. On the other hand, we fine-tune the relatively low extracellular Ca2+-Concentration for controlling extremely delicate functions, not to mention the importance of the even lower intracellular concentrations. The reason that both are possible at the same time is because of the low solubility product of Ca.2+ and phosphate (PO43-): if one of the two ions is in solution and the second is added, the majority precipitates as calcium phosphate.

In the bone we find the two ions together with a hydroxyl ion (OH-) in the form of the very hard mineral hydroxylapatite Ca5(PO4)3(OH), which forms hexagonal crystals. Hydroxyapatite accounts for up to 70% of bone weight; Tooth enamel consists almost exclusively of the extraordinarily hard mineral that gives the tooth surface its mechanical resistance. This has one disadvantage: As mentioned, hydroxyapatite is sensitive to acids. Tooth enamel is attacked by acid in the mouth through the same mechanism that osteoclasts use to break down hydroxyapatite. If you bite into an orange or if bacteria in the dental plaque convert sugar into lactic acid, a proton H will tear+ the OH- out to a water molecule H2O to form, and the remainder breaks down into 5 Ca2+ and 3 PO43- Ions: the hydroxyapatite dissolves. Ultimately, this process creates tooth decay. The molecule is much less soluble in an acidic medium if the OH- Ion by a fluoride ion F- is replaced: fluorapatite approx5(PO4)3F. Fluoroapatite forms spontaneously when there are enough fluoride ions, which can be achieved by adding fluoride to toothpaste, salt or, in some countries, drinking water.

The concentration of approx2+ in the blood plasma is regulated in the narrow band from 2.2 to 2.7 mM. This measured approx2+ represents the sum of three forms: protein, mainly albumin, bound (approx. 45%), complexed with small organic anions (approx. 10%) and free ionized Ca2+ (approx. 45%). The total approx2+ is therefore dependent on the plasma protein concentration. The relevant, regulated quantity is the free Ca2+.

The regulation of the Ca2+- The household is mainly managed by two hormones: parathyroid hormone and calcitriol (1,25-dihydroxy vitamin D). Parathyroid hormone is responsible for the short-term regulation of the concentration of free Ca.2+ at the expense of bone storage. Vitamin D strategically maintains total Ca2+- The body's memory. A third hormone, Fibroblast Growth Factor 23 (FGF23), regulates the excretion of phosphate via the kidneys and thus influences the Ca2+-Household.

A fourth approx2+-regulating hormone, Calcitonin, is of little importance in humans. It is secreted in the thyroid by the parafollicular C cells and briefly lowers the Ca.2+-Mirror, but the system swings back quickly to a neutral position. Neither the failure of the calcitonin-producing C cells (e.g. after thyroid surgery) nor prolonged calcitonin therapy interferes with the correct regulation of the Ca.2+Household. Calcitonin is likely an evolutionary holdover. Animals whose living conditions vary greatly in the approx2+- Bring recording with it, such as B. the salmon alternating between fresh water and the very calcium-rich sea, are heavily dependent on the action of calcitonin.

Pharmacological cross bracing:Salmon calcitonin is used to treat patients, although today it is produced genetically or synthetically. Why not the humane version? Salmon calcitonin is about ten times as effective, so only a tenth as many molecules need to be injected. Although it differs from the human peptide in 14 of the 32 positions, immunological complications are surprisingly rare.Due to its small size, calcitonin can also be used as a nasal spray. Calcitonin is used in acute hypercalcaemia to reduce the Ca2+-Lower levels quickly. In addition, it is used in diseases with high bone resorption to intermittently slow down osteoclast activity, e.g. B. in osteoporosis, Mb. Paget and bone metastases, which sometimes also has a pain-relieving effect.

2.1.2 Parathyroid hormone

Parathyroid hormone (PTH) is named after its production site, the four tiny epithelial bodies of the parathyroid glands (Glandulae parathyroideae). The concentration of free ionized Ca increases2+, the membrane-bound calcium-sensitive receptor (calcium-sensing receptor, CaSR) activates the main cells of the epithelial bodies; these then reduce the secretion of parathyroid hormone. A high concentration of 1,25-dihydroxy vitamin D also lowers the secretion of parathyroid hormone. So the message of the increased calcitriol is: "Stop nibbling our bones, I'll be able to do more approx2+ from outside! "PTH consists of 84 amino acids and has a very short half-life of about four minutes. PTH increases the Ca.2+-Concentration in two ways: by release from the bone and by affecting the kidney.

The net effect in the bone is an increase in bone breakdown by osteoclasts. This effect is achieved in an indirect way; Osteoclasts do not express a PTH receptor. PTH is recognized by osteoblasts, which by releasing IL-1, IL-6 and other cytokines stimulate the osteoclasts to become more active. In addition, the osteoblasts produce signaling molecules that stimulate the formation of new osteoclasts: M-CSF (Macrophage Colony-Stimulating Factor) and RANKL.

RANK ligand (RANKL) is a molecule from the TNF superfamily. It appears as a trimer; partly as a transmembrane protein on the cell surface of the osteoblasts, partly "cut off" as a soluble signaling molecule. In the bone marrow, M-CSF and RANKL meet hematopoietic progenitor cells of the macrophage and neutrophilic granulocyte lineage. These express the transmembrane receptor RANK (receptor activator of NFκB), a transmembrane protein of the TNF receptor superfamily. If precursor cell RANK is trimerized by osteoblast RANKL, the precursor cells first differentiate into mononuclear osteoclasts and then fuse to form polynuclear osteoclasts. Osteoblasts produce a second molecule, Osteoprotegerin (OPG)which looks like a soluble receptor for RANKL. It's called one decoy receptor (Deception receptor), which neutralizes its own RANKL and thus makes it ineffective. The extent to which osteoblasts induce the formation of osteoclasts therefore depends on the ratio of RANKL to OPG that they produce in various physiological situations. PTH induces the formation of RANKL and inhibits the expression of OPG, thus accelerating the formation of new osteoclasts.

There would be little point if parathyroid hormone only increased the concentration of approx2+ increase: due to the low solubility product with phosphate, it would precipitate again immediately. PTH therefore simultaneously lowers the phosphate level by inhibiting phosphate reabsorption in the kidneys in the proximal and distal tubules. This is done by removing the Na phosphate cotransporter from the luminal membrane into the underlying vesicles. At the same time, PTH increases the approx2+-Resorption in the distal tubule, so that even the small amounts that are normally excreted are retained in the body. The third PTH function in the kidney stimulates the last hydroxylation that is necessary to activate vitamin D: the hydroxylation of the C-atom 1. This replenishes the Ca2+Pools initiated.

Pharmacological cross bracing:Cinacalcet (Mimpara®, Sensipar®) is a small molecule that binds "outside" to the calcium-sensitive receptor, making it more allosterically sensitive to free Ca2+ power (i.e. less PTH release with a low Ca2+-Concentration causes). Its main use is the treatment of secondary hyperparathyroidism in patients with chronic kidney failure on dialysis. Damaged kidneys excrete too little phosphate and activate too little vitamin D. This results in a high phosphate and a low Ca in the plasma2+-Concentration, an imbalance that is carried via PTH on the back of the bone substance.

2.1.3 Vitamin D

Vitamin D is actually a hormone that is made in the skin itself. Sunlight is necessary for this, as vitamin D is only created from 7-dehydrocholesterol by breaking the second ring of the cholesterol skeleton if there is sufficient UV B exposure. This light-dependent process is the likely cause of Caucasians having pale complexions. Until they emigrated from Africa about 60,000 years ago, all modern humans were probably dark-skinned. The further north people settled, the less their sun exposure became and those with paler skin had a selection advantage, as they could still produce enough vitamin D.

(The selection advantage of fair-skinned individuals in the northern regions probably resulted not only from bone stability. UV B-generated vitamin D also plays a role in defense against infection. Decreased vitamin D levels correlate with susceptibility to infections of the respiratory tract during the winter months, albeit The mechanisms are not yet fully understood. It is likely that vitamin D is important for the function of macrophages, which can even activate vitamin D themselves by expressing the enzyme 1α ‑ hydroxylase, which is otherwise only expressed in the kidneys. Synthesize upon vitamin D stimulation Macrophages strengthen the antibacterial peptide cathelicidin.Based on empirical experience, patients in the tuberculosis clinics built in the mountains in the 19th and early 20th centuries were exposed to the sun every day during the cool season, which meant stronger UV radiation at high altitude.

The ability of macrophages to activate vitamin D is of particular importance in sarcoidosis, which is often associated with hypercalcemia caused by this mechanism. In the macrophages, the expression of 1α-hydroxylase does not take place under the control of PTH. Hypercalcemia has occasionally been described for other granulomatous diseases such as tuberculosis or leprosy.)

Ethnic groups that mainly nourished themselves from the sea, such as the Inuit, supplied enough vitamin D with their food and were therefore able to remain more pigmented. The fat-soluble vitamin D3 can also be obtained from animal food (particularly abundant, for example from fatty fish such as cod - liver oil! -, mackerel, salmon). The lack of sunlight was linked to rickets only in the late 19th century.

The alternative to UV production or the inclusion of the precursor cholecalciferol (vitamin D3) is the absorption of a very similar molecule, ergocalciferol (vitamin D2) from vegetables, but this is usually too small in terms of quantity to cover the need.

D3 and D2 are converted into active calcitriol in two hydroxylation steps: in the liver, the 25-hydoxylation takes place at the end of the side chain, in the kidney, the 1-hydroxylation on the carbon six-membered ring. This decisive, last step takes place in the proximal tubular cell and is precisely regulated: PTH promotes hydroxylation, while the end product calcitriol as well as FGF23 and / or phosphate inhibit hydroxylation. 1,25-dihydroxy-cholecalciferol (calcitriol) distributes itself in the body like a hormone and binds to the vitamin D receptor, which belongs to the superfamily of nuclear receptors. As a ligand-dependent transcription factor, it induces, among other things, genes that are responsible for maintaining Ca2+-Memories are essential.

The most important target organ in this regard is the duodenum. Several proteins are induced here, which in interaction with the Ca2+- Promote intake from food. While the approx2+-Concentration in the intestinal lumen and in the blood is in the mM range, it is significantly lower inside the cell; too much free approx2+ in the cytosol would be dangerous. So there is an approx2+Channel induced by the Ca2+ Passively flows into the cell, inside the cell the affine Ca2+-binding protein calbindin to neutralize passing calcium and on the blood side an ATP-driven Ca2+-H+-Antiporter as well as a Na+-driven approx2+-N / A+-Antiporters, both of which pump calcium into the blood against a steep concentration gradient. Calcitriol also promotes the absorption of phosphate in the small intestine.

Like PTH, calcitriol promotes the reabsorption of Ca in the kidneys2+ in the distal tubule, but the effect is much weaker. In contrast to PTH, calcitriol also promotes the reabsorption of phosphate: both ions are necessary to refill the bone stores.

The sum of these vitamin D effects leads to the solubility product being exceeded, so that Ca2+ and phosphate precipitates in the osteoid ideally suited for this; In addition, vitamin D-induced transcription of the osteocalcin gene promotes breaking strength. Overall, this indirect effect outweighs the direct, receptor-mediated effect on osteoblasts and osteoclast precursors, which increases the Ca.2+-Mobilization would promote.

 

2.1.4. FGF23

FGF23 is released by osteocytes and osteoblasts, stimulated on the one hand by phosphate uptake from food and on the other hand by 1,25 ‑ dihydroxy vitamin D. It increases renal phosphate excretion by increasing the number of Na-Pi cotransporters in the apical membrane of the proximal tubule reduced. In this function it acts similarly to PTH. In contrast to PTH, it counteracts this by inhibiting the 1 ‑ hydroxylation of vitamin D. This lowering of active vitamin D leads to a lower Ca.2+- Intestinal absorption.

CKD-MBD (chronic kidney disease- mineral and bone disorder): For many old people, one problem arises from the fact that we absorb all of the available phosphate through the intestines, but excrete the excess that is not required via the kidneys under the control of FGF23. As the glomerular filtration rate declines with age, FGF23 rises ever higher in order to excrete an ever higher percentage of the filtered phosphate. Calcitriol is kept low by FGF23, the Ca2+-Concentration in the blood can only be maintained by increasing the parathyroid hormone. This secondary hyperparathyroidism permanently damages the bones.

Phosphate diabetes (X-linked hypophosphatemia): The X-linked PHEX gene (Phosphate-regulating neutral endopeptidase, X-linked) encodes a peptidase that inhibits FGF23. A defect in this peptidase leads to an overactivity of FGF23 and thus to phosphate losses via the kidneys and to a rickets-like clinical picture in the affected children that cannot be adequately treated with vitamin D. Instead, an antibody against FGF23, burosumab, helps.

2.2 Growth hormone and IGF-1

Growth hormone (GH, growth hormone) is essential for bone growth. It is produced by somatotrophs (cells) in the anterior pituitary under the control of the hypothalamus (stimulated by GH-releasing hormones - GHRH -, slowed down by somatostatin), released in a pulsatile manner, only during sleep and during physical exertion (so there is no point in determining growth hormone during the day in a child at rest). Growth hormone triggers some rapid effects that are opposite to those of insulin: lipolysis in adipose tissue, gluconeogenesis in the liver and an inhibition of glucose uptake in the muscles. However, the growth-promoting effect on cartilage and bones is indirect. The liver is stimulated by GH, Insulin-like growth factor-I (IGF-I) to secrete into the plasma. In addition, chondrocytes and osteoblasts, like many other cell types, produce GH-dependent IGF-I, which has a paracrine effect on these and neighboring cells.

IGF-1 is closely related to insulin, with a little less than 50% identical amino acids. Like insulin, it binds to a heterotetrameric receptor made up of two extracellular α and two transmembrane β chains with a tyrosine kinase domain. There are "mixed-chain" insulin / IGF-1 receptors that can be activated by either hormone. IGF-1 is bound by IGF-binding proteins in the organic matrix, protected from proteolysis and concentrated. Along with other growth factors such as TGFβ and PDGF (transforming growth factor βand platelet-derived growth factor) a part is walled in by mineralization in the bone, so that a growth factor reservoir is created, which only becomes active again when the bone is broken down. (This is probably one reason why metastatic tumor cells often find fertile soil in the bone.) IGF-1 has a paracrine effect on the cells, e.g. B. it stimulates chondrocytes in the epiphyseal plates as well as osteoblasts to increased cell division. IGF-1 is directly dependent on the pulsatile fluctuating GH, but shows a much more constant effect through these buffering mechanisms. A deficiency in growth hormone, like a deficiency in IGF-1, leads to dwarfism, while an overproduction of growth hormone in childhood leads to gigantism.

Pharmacological cross bracing: Growth hormone was the second genetically engineered drug after insulin and was approved for Genentech in 1985. Before that, growth hormone was purified from cadaveric pituitary glands. Indications are growth hormone deficiency on the one hand, and symptomatic use in Turner syndrome and kidney failure in children to increase body size on the other. In some countries, pediatricians are coming under increasing pressure from parents to increase the size of healthy children by giving them growth hormone, as height is seen as a social advantage. IGF-1 is used for children with growth hormone receptor defects that otherwise lead to so-called Laron dwarfism.

[Growth hormone and IGF-1 have more functions than just stimulating growth. Bovine growth hormone is used in some countries, e.g. B. the USA, used to increase the milk yield of cows. This is only possible with optimally nourished animals, so unfortunately it is no help for those countries that need it most urgently. After intensive discussions, an agreement was reached in the EU not to allow this application.]

2.3 Thyroid hormone

Growth hormone and IGF-I are necessary but not sufficient for bone growth and maintenance. Thyroid hormone and, depending on gender, estrogens or androgens are also necessary. Like IGF-I, thyroid hormones and sex hormones are indirectly under the control of the CNS. The precise molecular mechanisms by which these hormones act on bones cannot be adequately described. Virtually all cells in the body express receptors for thyroid hormone, and many tissues express estrogen and androgen receptors. The three types of receptors are related. All three are members of the superfamily of "nuclear receptors" (nuclear receptors), as well as the vitamin D receptor and the glucocorticoid receptor. All receptors in this family are ligand-activated transcription factors that gradually regulate a large number of genes. In the presence of the corresponding hormone, many genes are increasingly expressed and even more genes are slowed down in their activity. Unfortunately, it is still insufficiently understood which of these genes are relevant for bone growth and maintenance.

While other receptors of the nuclear receptor superfamily only switch from the cytoplasm to the nucleus after ligand binding, the thyroid hormone receptors (α and β) sit on the DNA from the outset. As long as the ligand is absent, they often inhibit the transcription of the corresponding gene. Binding of triiodothyronine (T.3), and to a lesser extent thyroxine, turns the receptors into active transcription stimulators. Chondrocytes, bone marrow stromal cells, osteoblasts and osteoclast precursors express T.3Receptors. It is unclear whether T3 also has effects in mature osteoclasts.

Deficiency of thyroid hormone in childhood leads to growth retardation. But too much is also unhealthy: hyperthyroidism leads to secondary osteoporosis.

2.4 Estrogens, Progesterone and Androgens

The importance of sex hormones for bone metabolism has also been made clear by clinical observations. A deficiency of these hormones in various forms of hypogonadism regularly leads to osteoporosis. Estrogen or androgen excess in childhood initially leads to an acceleration of growth (as is normally the case during puberty), but then to a premature epiphyseal closure and thus ultimately to a reduced body size. Postmenopausal osteoporosis is initiated by a drop in the concentration of estrogen.

Both sexes express both estrogen and androgen receptors. While there is only one androgen receptor, there are two variants of the DNA-binding estrogen receptor, ERα and ERβ. ERα is mainly expressed in the ovary, uterus, and breast, ERβ is also expressed in many other tissues, but both types are expressed in bone cells. In addition to these classic receptors, which move between the cell nucleus and cytoplasm, there is also an estrogen-binding G-protein-coupled protein in the membrane of the endoplasmic reticulum, which is so far not known to function in the bone. Numerous different mechanisms have been proposed and supported by data for the anabolic effects of estrogens on bones. On the breakdown side, estrogens inhibit the number and activity of osteoclasts. This effect runs partly through the RANK system. Activated estrogen receptors do not intervene directly on the promoter in the transcription of RANKL and RANK genes, but estrogens regulate this system indirectly via many different points of attack. Estrogens increase z. B. the production of OPG by osteoblasts. They inhibit the production of M-CSF, IL-1, IL-6 and TNFα. The result is less osteoclast formation. These and other mechanisms also reduce the activity and lifespan of osteoclasts. In sum, estrogens slow down bone loss. There is a lot of evidence that they also actively promote bone formation, but there is still no agreement on the corresponding mechanisms.

Progesterone, on the other hand, promotes the expression of RANKL and the formation of osteoclasts. The highest concentrations of progesterone are reached during pregnancy. This promotes the release of calcium from the maternal bones to facilitate mineralization of the fetus's bones. In addition, the progesterone-driven RANKL expression promotes the proliferation of cells of the mammary epithelium so that new glandular tissue is available for milk production.

Similar to TSH, a direct effect on osteoclasts has also been described for the follicle-stimulating hormone (FSH), which is superordinate to estradiol, which is said to be opposite to that of TSH, i.e. activates the osteoclast effect. Before menopause, this effect is more than offset by the anabolic effects of the estrogens; after menopause, however, it could be responsible for the accelerated breakdown phase.

In men, the drop in androgens occurs at a later age, but it also results in osteoporosis. Probably the bone mass-promoting mechanisms of the androgens largely overlap with those of the estrogens. There is, however, a second possibility: Androgens are partially converted into estrogens by the enzyme aromatase in adipose tissue, even in men. It is therefore discussed that the bone-protective effect could also be caused by estrogens in men.

2.5 Cortisol and other glucocorticoids

Glucocorticoids intervene both on the build-up and on the break-down side of bone metabolism. Glucocorticoids inhibit the function of the osteoblasts, e.g. b. by inhibiting the transcription of collagen and osteocalcin genes (this also happens in other tissues: in the skin the inhibition of collagen formation is sometimes directly visible in the form of striae). The survival time of osteoblasts is also shortened. On the degradation side, glucocorticoids induce RANKL in osteoblasts and at the same time reduce the expression of OPG. Combined, these two glucocorticoid effects increase the number and activity of osteoclasts. Both the effects on the build-up side and on the break-down side thus promote the development of osteoporosis.

2.6 Mechanical load

Exercise in the form of physical activity is essential for building bone mass. The bone trabeculae are constantly being remodeled according to the requirements of the mechanical load. Inactivity, e.g. B. bedridden, quickly leads to a loss of bone mass. The reduction of stress in astronauts through the elimination of gravity leads to the same result. The osteocytes sitting between the lamellae of the osteons can perceive mechanical stress on the bone. They then reduce their release of sclerostin and change other growth and differentiation factors for osteoblasts. Unfortunately, the molecular mechanisms of this regulatory system have not yet been adequately clarified. One mechanism discussed is based on the fact that stress on the bone leads to a flow of fluid through the porous matrix of the bone. This deforms the osteocytes (like laundry in the wind), which could lead to the opening of mechanosensitive ion channels. Such ion flows could then go through gap junctions are forwarded from osteocyte to osteocyte to a point where groups of bones (basic multicellular units) can be formed.

2.7 Nutritional situation: leptin

Leptin was drawn to the attention of a strain of mice, which according to the phenotype "obesity" (obesity) was inbred. Analysis of whether / whetherMice showed that the gene for an extracellular signaling molecule was defective, which is why it is called "leptin" (Greek leptus= thin) called. Homozygous mice were not only fat but also sterile. Interestingly, they had increased bone mass, while hypogonadism is otherwise regularly associated with osteoporosis. The molecular mechanisms of the action of leptin have been shown in the mouse for experimental reasons. As far as individual aspects could be checked in humans, these findings probably also apply to humans.

The signal protein leptin is mainly secreted by adipose tissue cells (an "adipokine"). Its long-term plasma level is proportional to the individual's fat stores. Around this level there is an oscillation dependent on the daily meals, usually with a low for breakfast and a high in the late evening. In addition, changes in the nutritional situation lead to temporary deviations in the leptin level. Leptin reacts to hunger phases over a few days with a decrease, to festive calorie intake with an increase. Leptin crosses the blood-brain barrier and influences the autonomic nervous system via centers in the hypothalamus. Decreased leptin levels lead to an increased feeling of hunger, increased leptin to satiety. For a while there was hope that leptin egg would be the answer to the widespread obesity problem. Unfortunately, very overweight people show a central leptin resistance (similar to the insulin resistance in Diabetes mellitus Type 2), d. that is, they do not respond to high plasma leptin concentrations with decreased appetite.

The influence of leptin on bone metabolism is apparently also directed via the autonomic nervous system. Impulses from the hypothalamus are sent via sympathetic neurons directly into the bone, where osteoblasts are influenced by noradrenaline via adrenergic β-receptors. Impulses coming in via this signal path have effects in the osteoblasts that are influenced by the circadian molecular clock in the cells. Depending on the phase of this clock, these impulses lead to an acceleration of osteoblast cell division and function, and to a delay. The osteoclast function is also influenced via this adrenergic pathway. It has been known for many years that markers of bone metabolism such as osteocalcin in the blood follow a circadian rhythm. It would be B. plausible that remodeling processes in the bones are easier to accomplish during sleep.

Leptin is therefore a signal that feeds the current nutritional situation into the central nervous system. This reacts to this by, in addition to eating behavior and reproductive behavior, e.g. B. also adapts the bone metabolism. With regard to bone metabolism, the calculated output is also embedded in a meaningful day-night rhythm. Genetically determined complete leptin deficiency leads to the whether / whether-Mouse anyway to increased bone mass.

Pharmacological cross bracing: If leptin inhibits new bone formation via a β-adrenergic mechanism, β-blockers should have a positive effect on osteoporosis. According to retrospective studies, this does appear to be the case. More conclusive prospective studies have to be carried out first; a first small prospective study indicated a positive effect of β-blockers.

3. BONE ALTERATION DISORDERS

3.1 Osteoporosis

By far the most common form of osteoporosis affects people in their second half of life. It is known as primary or idiopathic osteoporosis. Although the mechanisms are likely to be the same in women and men, symptoms appear earlier in women because their estrogens drop earlier than androgens in men. Therefore, one speaks of postmenopausal osteoporosis in women.

Symptoms of osteoporosis are essentially broken bones. These often affect the femoral neck or the vertebral bodies (impression fractures). Bone fractures naturally occur with peak loads such as falls, but with comparable loads, the more frequently the lower the bone mass. If the mechanical stability of a bone is so weakened that it breaks in the event of a minor trauma, we speak of a pathological fracture. We have the largest bone mass between the ages of 20 and 30 years. From then on, the net effect of the many factors affecting bone metabolism is slightly negative. In women, this net breakdown accelerates with the decrease in estrogen after menopause. In the first 5-10 years immediately after menopause, an accelerated loss of bone mass with increased osteoclast activity is observed (high turnover bone loss), later a slow further loss, which is more due to a small deficit in osteoblast activity compared to normalized bone resorption (low turnover bone loss).

In primary osteoporosis, several factors contribute to the negative net effect:

1.    Decrease in estrogens and androgens

2.    Decreased physical activity

3.    More common relative malnutrition in terms of vitamin D and calcium

4.    Less exposure to light reduces the endogenous production of vitamin D.

5.    Kidney damage caused by causes such as diabetes, arteriosclerosis or analgesic abuse reduce the 1-hydroxylation of vitamin D secondarily (also via FGF23)

When bone resorption slightly predominates, serum Ca increases2+-Mirror. Parathyroid hormone decreases, so that approx2+ is less reabsorbed in the kidney. The parathyroid hormone-dependent 1 ‑ hydroxylation of vitamin D to calcitriol is also reduced, so that less Ca2+ is absorbed through the intestine. The approx2+-Balance follows the bone mass balance (anything else would also make little sense: where should the excess Ca2+ because deposit?) and is negative overall.

Interestingly, being overweight has a protective effect against osteoporosis to a certain extent. It has not yet been sufficiently clarified whether this is due to the increased mechanical stress or to an increased residual estrogen formation from androgens through the aromatase of the adipose tissue.

Diagnosis

The most relevant property of bone would be its resistance to fracture. Of course, this cannot be tested directly. The determinable substitute values ​​are, on the one hand, the bone density and, on the other hand, molecules in the serum that arise during bone formation or degradation.

The Bone density is mostly done by the DXA method (dual energy X-ray absorptiometry) certainly. This is based on the fact that lower-energy ("soft") X-rays are absorbed to a greater extent in tissue of lower density than higher-energy ("hard") ones, while the opposite is true in denser tissue. In other words, from two X-ray images recorded with different energies, one can draw conclusions about the density of the irradiated tissue, and with the help of a few assumptions, after a lot of arithmetic, also about the bone density. The result is given as the multiple of the standard deviation from the mean of the 30-year-olds of the corresponding sex, the so-called T-value. A T-value less than -2.5 (i.e. a bone density that is more than two and a half standard deviations below the average bone density of a 30-year-old) means osteoporosis by definition. Another, technically more complex and expensive method for measuring bone density is what is known as quantitative computed tomography (QCT), which provides more information, e.g. B. Compacta and cancellous bone can be analyzed separately.

Bone augmentation can be followed by measuring serum osteocalcin as this is only produced by osteoblasts. Useful values ​​also result from fragments that arise during the extracellular collagen assembly, since most of the collagen type I synthesis takes place in the bone. Specifically, the procollagen I C-terminal propeptide (PICP) and the procollagen I N-terminal propeptide (PINP) can be determined. Another marker is bone-specific alkaline phosphatase (ostasis).

Bone loss osteoclasts also break down cross-linked collagen. The resulting C-terminal, cross-linked ("x") fragment of the collagen I triple helix (CTx-I, also known as Crosslaps can be measured in plasma and is used as a measure of bone loss. When these collagen fragments continue to break down, the chemical structure of the actual cross-linking between hydroxylysines, pyridinoline, remains. Pyridinoline (Pyr) and the bone-specific deoxypyridinoline (D-Pyr) can be measured in urine (this value is often simply referred to as Crosslinks designated).

Because of the circadian rhythm of bone metabolism, it is important to always take blood samples for check-ups at the same time of day.

Therapeutic options:

Estrogen replacement?

Hormone replacement therapy has been popular for some time to help alleviate the uncomfortable side effects of menopause. Initially, little thought was given to possible long-term side effects. When the necessary, elaborate randomized double-blind studies were finally carried out (Women's Health Initiative study in the USA, Million Women Study in the UK), the result was sobering.While there was a reduction in the number of femoral neck fractures, this was more than offset by the increased incidence of breast cancer, endometrial cancer, heart attacks, strokes and pulmonary embolisms. With regard to heart attacks in particular, the opposite was assumed, since women before menopause suffer less heart attacks than men of the same age. General postmenopausal hormone replacement therapy was thus off the table; more recent studies suggest that it can be used for a limited period in the first few years immediately after menopause.

A possible alternative is Raloxifene Raloxifene is a SERM (selective estrogen receptor modulator) like tamoxifen. These lipophilic ligands bind to the estrogen receptor and lead to a conformational change that leads to a somewhat different spatial structure than if estradiol were bound. As a result, these molecules have estrogen-like effects in some tissues but antagonistic effects in other tissues. This depends on the individual equipment of the target tissue cells with other molecules that influence transcription (coactivators and corepressors), which bind the complex modified in this way better or worse than the original (estradiol) complex. Raloxifene has an estrogen-like effect in the bones and can be used to brake osteoporosis. Raloxifene has an estrogen-antagonistic effect in the mammary gland and even reduces the risk of breast cancer. With regard to endometrial cancer and vascular diseases, raloxifene is largely neutral according to data so far, with the exception of a slight increase in venous thrombosis.

Bisphosphonates

Phosphonate is similar to phosphate, but differs in that the central phosphorus is surrounded by three oxygen atoms instead of four as in phosphate. In bisphosphonates, two such groups are attached to one carbon atom, with the phosphorus atom being bonded directly to the carbon atom (not via an oxygen atom, as is the case with organic phosphate). While phosphates can easily be split off enzymatically by phosphatases, this is not the case for the P ‑ C ‑ P bonds; the bisphosphonates are therefore very stable in the body. Once they have reached the bone, they have a half-life on the order of years. In many ways they behave like phosphate: they form with Ca2+ insoluble complexes and are therefore difficult to absorb from the intestine (they are therefore often administered parenterally). They preferentially attach to hydroxyapatite. From there they are taken up by "nibbling" osteoclasts and prevent them from working through several mechanisms; many osteoclasts go into apoptosis after a while. Bisphosphonates such as alendronate (e.g. Fosamax®) slow down bone loss and restore a certain balance in osteoporosis. There is also evidence that bisphosphonates inhibit the development of bone metastases. There is also evidence that bisphosphonates counteract the "hibernation" of micrometastases in the bone marrow (see section on metastasis). Problematic side effects arise when the bisphosphonate toxicity spreads to other bone cell types, especially when this leads to the rare but dreaded osteonecrosis of the jaw.

Denosumab

As a natural RANKL-blocking protein, osteoprotegerin was a logical candidate for the treatment of osteoporosis and was also introduced into early clinical trials by the biotechnology company Amgen. However, there were several questionable aspects. OPG not only binds RANKL, but also other members of the TNF superfamily. OPG is also expressed by endothelial cells and natural OPG levels correlate with coronary artery disease.

As an approach with a likely higher level of security, Amgen developed a monoclonal antibody, denosumab (Prolia®), with an OPG-like function. The advantages are a higher RANKL specificity and a lower risk of triggering the formation of neutralizing antibodies against the body's own OPG. The human monoclonal IgG2 antibody (IgG2 activates complement significantly less than IgG1 or IgG3) is injected subcutaneously every six months and is effective against postmenopausal osteoporosis and the osteolytic effect of breast cancer metastases. Denosumab has been approved since May 2010 for the treatment of women with postmenopausal osteoporosis with an increased risk of fractures and for men with prostate cancer who are at increased risk of fracture due to androgen suppression therapy. Since RANKL knockout –Mice also have immunological problems (these occur at an early stage of development), possible side effects with long-term use must be kept in mind.

Parathyroid hormone analogs (teriparatide, abaloparatide)

While PTH has a physiological effect on bone degradation and this effect is even intensified in the case of hyperparathyroidism, a therapeutically administered PTH analogue can surprisingly have a bone building effect. The difference is obviously in the temporal pattern: the intermittent, short-term peaks of the once-daily administered PTH analog activate osteoblasts more than osteoclasts and thus have the opposite effect of chronically elevated PTH. For this reason, parathyroid hormone analogs are also used therapeutically against osteoporosis for a limited time, especially if fractures have already occurred as a result of postmenopausal osteoporosis.

Vitamin D and Ca2+

To counteract the various reasons for vitamin D deficiency, it makes sense to give vitamin D orally. Since that can only work if enough approx2+ is present in the intestine, approx2+ substituted for security.

Movement and light

Physical activity significantly slows the progression of osteoporosis and has numerous other beneficial metabolic and psychological effects. Resistance / impact training of high intensity, i.e. strong mechanical stress on the bone, works best. Unfortunately for most older people this is not very realistic. Any form of activity is better than inactivity. In connection with this, the light exposure increases the own vitamin D production.

Secondary osteoporosis

In addition to its primary or idiopathic occurrence, osteoporosis can also be caused by a number of underlying diseases, such as: B.

  • Hypercortisolism
  • Hyperparathyroidism
  • Hyperthyroidism
  • anorexia
  • diffuse neoplasms such as B. multiple myeloma, which is often diagnosed only after a pathological fracture.

Pharmacological cross bracing: Long-term therapy with glucocorticoids increases the risk of fractures considerably. To a lesser extent, the risk of fracture is also increased by SGLT2 inhibitors, which diabetes patients take for a long time: Since with the reabsorption of glucose, the reabsorption of Na+ is slowed down in the proximal tubule of the kidney, there is a higher tubule Na+-Concentration available for other fecal transport processes. In this way, the phosphate reabsorption via the Na-Pi cotransporter can be increased. Secondly, this results in: Lowering the approx2+-Concentration, increase in parathyroid hormone release, increase in FGF23 and thereby decrease in calcitriol.

3.2 Rickets and osteomalacia

A lack of vitamin D leads to rickets in children and osteomalacia in adults. In both cases the sufficiently formed organic matrix is ​​not mineralized enough. The different symptoms come from the fact that the child's bones are still growing, are too soft due to a vitamin D deficiency and deform under load. In children, therefore, one finds skull, chest and leg deformations (square skull, Harrison furrow along the diaphragmatic attachment and bow legs) as well as wide and distended epiphyseal plates and rib attachments (rachitic rosary). Logically, there are also tooth enamel defects and functional disorders due to the reduced serum calcium (restlessness, sweating, muscle weakness, frog belly, constipation, tetany). Adult osteomalacia leads to bone pain and insidious pathological fractures. Rickets and osteomalacia can be caused by vitamins D and Ca2+-Giving well treated or avoided. Breast-fed infants should also receive regular vitamin D drops.

3.3 Bone degradation in the context of inflammatory diseases

In the context of chronic joint inflammation, e.g. B. Rheumatoid arthritis, activated macrophages secrete IL-1, IL-6, TNFα and prostaglandin E. On the one hand, this leads to the induction of proteases in surrounding cells such as synoviocytes, which lead to the breakdown of cartilage and bone matrix. On the other hand, these inflammatory cytokines also induce the differentiation of osteoclasts in the adjacent bone. This contributes to the fact that several types of cells in the inflamed tissue, e.g. B. Synoviocytes and T cells, express RANKL. All in all, these mechanisms can lead to massive destruction of the bone near the joint.

The same mechanism is responsible for the appearance of painful tooth necks and eventual tooth loss due to periodontal disease (formerly "periodontal disease"). Bacteria in plaque on the gumline lead to chronic inflammation of the gums. Inflammatory cytokines induce osteoclasts, which gradually break down the thin bone around teeth.

3.4 Paget's disease (Osteodystrophia deformans or Osteitis deformans)