Pathophysiologic unity of the development of osteoporosis and vascular atherosclerosis

In the structure of mortality of the population of developed countries, the leading place is occupied by diseases of the circulatory system. Cardiovascular diseases (arterial hypertension, ischemic heart disease, myocardial infarction), which are based on atherosclerosis, are rightly called the epidemic of the 21st century.

According to WHO, over 17 million people die from cardiovascular diseases every year in the world, and by 2015 the number of fatalities will increase to 20 million. Along with this, one of the leading causes of functional insufficiency and loss of working capacity in the adult population is osteoporosis (OP) - the most well-known and common disease of the skeletal system in the world with age-associated prevalence. Osteoporosis is a multifactorial polygenic skeletal disease, which is the most common form of metabolic osteopathy. The disease is characterized by loss of bone mass, disruption of their microarchitecture (destruction of trabeculae), decreased strength and is accompanied by a high risk of fractures.

It is fractures, the most severe of which are fractures of the femoral neck and the radius in the lower third of the forearm, that determine the medical and medical-social significance of the disease, including increased mortality and significant economic losses associated with them. The peculiarity of osteoporosis is that this disease mainly affects elderly and old people. A significant increase in osteoporosis incidence, observed since the second half of the 20th century, naturally reflects the demographic changes that occur in the population and are manifested by the aging of the population in all industrialized countries of the world. Numerous epidemiological studies conducted recently in the world and Europe indicate a positive correlation between cardiovascular diseases and pathologies of the skeletal system. At the same time, many authors associate osteoporosis with the progression of atherosclerosis, including calcification of the vessel walls. In women with osteoporotic fractures, an increase in the incidence of aortic and coronary artery calcification was observed, the severity of which correlated with a decrease in bone mineral density (BMD).

The studies by S. O. Song et al. revealed a relationship between a decrease in the BMD of the spine and proximal femur and an increase in the calcium content in the coronary arteries according to electron beam computed tomography. M. Naves et al. found that in women with postmenopausal osteoporosis, a decrease in BMD by one standard deviation from peak bone mass is associated with an increased risk of overall mortality by 43% and premature death from cardiovascular pathology. Other studies also found that patients with a decrease in BMD are more likely to have an increase in blood lipid concentrations, develop more severe coronary atherosclerosis, and significantly increase the risk of stroke and myocardial infarction. The presented data suggest that an increase in the incidence of osteoporosis, ectopic calcification, and atherosclerosis in the same patients has a common pathogenetic basis. The concept that cardiovascular disease and osteoporosis are linked through markers that simultaneously affect vascular and bone cells has been supported by extensive experimental studies.

A candidate for the role of such a marker is the recently identified protein osteoprotegerin (OPG), which belongs to the family of tumor necrosis factor receptors and is part of the RANKL-RANK-OPG cytokine system.

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Bone remodeling and the role of the rankl-rank-opg system

Osteoporosis is a disease based on the processes of bone remodeling disorders with increased bone resorption and decreased bone synthesis. Both processes of bone tissue formation are closely interconnected and are the result of cellular interaction of osteoblasts (OB) and osteoclasts (OC), originating from precursors of different cell lines: osteoblasts - from mesenchymal stem cells, osteoclasts - from macrophage-monocytic cells of the bone marrow. Osteoblasts are mononuclear cells involved in the process of bone formation and mineralization of bone matrix cells. Osteoblasts play a fundamental role in modulating bone remodeling and regulating the metabolic activity of other bone tissue cells. They secrete a number of biologically active substances, through which they influence the maturation process of the osteoclast precursor cell, transforming it into a large multinucleated cell capable of participating in resorption, i.e., the absorption of bone tissue, acting only on mineralized bone, without changing the actual matrix of bone tissue.

Maturation and differentiation of osteoblasts are carried out under the influence of various specific factors affecting the transcription process, the most important of which is the protein Cbfal (core-binding factor oil; also known as runt related transcription factor 2; RUNX2). In mice with Cbfal/RUNX2 deficiency, a significant slowdown in the bone formation process is observed, and maturation of OB cells is not observed. In contrast, administration of recombinant Cbfal to animals causes expression of genes inherent in osteoblasts in non-osteogenic cells. The significant role played by Cbfal/RUNX2 in the differentiation and maturation of osteoblasts is also manifested in the ability of the protein to regulate the function of many genes involved in the synthesis of bone tissue proteins: collagen type 1, osteopontin (OPN), osteocalcin and sialoprotein. The growth and functional capacity of the OB are also influenced by paracrine and/or autocrine factors regulating the activity of intranuclear transcription processes, synthesis of OPN and osteocalcin. These include a number of cell growth factors, cytokine modulators, and hormonal biologically active substances. The assumption that the activation and regulation of bone tissue remodeling are a consequence of the interaction of osteoblasts and osteoclasts has been confirmed in numerous research studies. Significant progress in understanding the processes of bone remodeling was achieved with the discovery of the cytokine RANKL-RANK-OPG system, which plays a key role in the formation, differentiation and activity of osteoclasts. The discovery of this system became the cornerstone for understanding the pathogenesis of osteoporosis, osteoclastogenesis and regulation of bone resorption, as well as other processes involved in local bone remodeling. Regulation of osteoclastogenesis is carried out mainly by two cytokines: receptor activator of nuclear factor kappa-B ligand (RANKL) and OPG against the background of the permissive action of macrophage colony-stimulating factor (M-CSF).

RANKL is a glycoprotein produced by osteoblastic cells, activated T lymphocytes, which belongs to the tumor necrosis factor (TNF) ligand superfamily and is the main stimulus for osteoclast maturation. The molecular basis of intercellular interactions involving the RANKL-RANK-OPG system can be represented as follows: RANKL expressed on the surface of osteoblasts binds to the RANK receptor located on the membranes of OC precursor cells and induces the process of osteoclast differentiation and activation. Simultaneously, bone marrow and OB stem cells release M-CSF. This polypeptide growth factor, interacting with its high-affinity transmembrane receptor (c-fms), activates intracellular tyrosine kinase, stimulating proliferation and differentiation of the osteoclast precursor cell. The proliferative activity of M-CSF significantly increases when OB is exposed to parathyroid hormone, vitamin D3, interleukin 1 (IL-1), TNF and, conversely, decreases under the influence of estrogens and OPG. Estrogens, interacting with intracellular OB receptors, increase the proliferative and functional activity of the cell, simultaneously decreasing the function of osteoclasts, stimulating the production of OPG by osteoblasts. OPG is a soluble receptor for RANKL, synthesized and released by osteoblastic cells, as well as stromal cells, vascular endothelial cells and B lymphocytes. OPG acts as an endogenous decoy receptor for RANKL, blocking its interaction with its own receptor (RANK), and thus inhibits the formation of mature multinucleated osteoclast cells, disrupting the process of osteoclastogenesis, reducing the activity of bone tissue resorption. Synthesized and released by OB cells, RANKL is a specific factor necessary for the development and functioning of OC. RANKL interacts with its tropic receptor RANK on the membrane of the OC precursor cell (a common precursor for osteoclasts and monocytes/macrophages), leading to intracellular cascade genomic transformations. RANK affects nuclear factor kappa-B (NF-kB) through the receptor-associated protein TRAF6, which activates and translocates NF-kB from the cytoplasm to the cell nucleus.

Accumulation of activated NF-kB increases the expression of NFATcl protein, which is a specific trigger that starts the process of transcription of intracellular genes that form the osteoclastogenesis process. The differentiated osteoclast takes a certain position on the bone surface and develops a specialized cytoskeleton that allows it to create an isolated resorption cavity, a microenvironment between osteoclasts and bone. The OC membrane facing the cavity formed by the cell forms many folds, acquires a corrugated appearance, which significantly increases the resorption surface. The microenvironment of the created resorption cavity is acidified by electrogenic pumping of protons into it. The intracellular pH of the OC is maintained with the participation of carbonic anhydrase II through the exchange of HCO3/Cl ions through the antiresorptive membrane of the cell. Ionized chlorine penetrates into the resorption microcavity through the anion channels of the corrugated resorption membrane, resulting in the pH in the cavity reaching 4.2-4.5. The acidic environment creates conditions for the mobilization of the bone mineral phase and forms optimal conditions for the degradation of the organic matrix of bone tissue with the participation of cathepsin K, an enzyme synthesized and released into the resorption cavity by the "acid vesicles" of OK. Increased RANKL expression directly leads to the activation of bone resorption and a decrease in skeletal BMD. The introduction of recombinant RANKL led to the development of hypercalcemia by the end of the first day, and by the end of the third - to a significant loss of bone mass and a decrease in BMD. The balance between RANKL and OPG actually determines the amount of resorbed bone and the degree of change in BMD. Animal experiments have shown that increased expression of OPG in mice leads to increased bone mass, osteopetrosis, and is characterized by a decrease in the number and activity of osteoclasts. In contrast, when the OPG gene is turned off, a decrease in BMD, a significant increase in the number of mature, multinucleated osteoclasts, a decrease in bone density, and the occurrence of spontaneous vertebral fractures are observed.

Subcutaneous administration of recombinant OPG to mice at a dose of 4 mg/kg/day for a week restored BMD indices. In the model of adjuvant arthritis in rats, administration of OPG (2.5 and 10 mg/kg/day) for 9 days at the initial stage of the pathological process blocked the RANKL function and prevented the loss of bone and cartilage tissue mass. The experiments indicate that the function of OPG mainly consists in decreasing or significantly "switching off" the effects caused by RANKL. At present, it has become obvious that maintaining the relationship between RANKL and OPG is an important condition for maintaining the balance between bone resorption and formation. The conjugation of these two processes, the relative concentrations of RANKL and OPG in bone tissue determine the main determinants of bone mass and strength. Since the discovery of the RANKL-RAMK-OPG system as the final pathway for the formation and differentiation of osteoclasts, many researchers have confirmed the leading role of this cellular and molecular mechanism in the pathogenesis of osteoporosis.

The role of the rankl-rank-opg cytokine system in the process of vascular calcification

The assumption about the presence of a common pathogenetic basis for osteoporosis and atherosclerosis, a certain similarity between the mechanisms of osteoporosis development and vascular calcification is confirmed by many experimental and clinical observations. It has been demonstrated that bone and vascular tissues have many identical properties both at the cellular and molecular levels. Bone tissue and bone marrow contain endothelial cells, preosteoblasts and osteoclasts - derivatives of monocytes, while all of them are also normal components of the cellular populations of the vascular wall. Both bone tissue and the wall of arterial vessels under the conditions of the atherosclerotic process contain OPN, osteocalcin, morphogenetic bone protein, matrix Gla-protein, collagen type I, and matrix vesicles. In the pathogenesis of atherosclerosis and OP, monocytes are involved with differentiation into macrophages with foamy cytoplasm within the vascular wall and into osteoclasts in bone tissue. In the vascular wall there are cellular elements that differentiate into osteoblasts in accordance with the stages of formation of bone OB, producing the mineral component of bone.

Of fundamental importance is the fact that the RANKL-RANK-OPG cytokine system, initiating osteoblastogenesis and osteoclastogenesis in bone tissue, induces, among other things, differentiation of osteoblasts and OC, as well as the process of vessel wall mineralization. Among the components of this system, directly indicating the existence of a relationship between osteoporosis and atherosclerosis, OPG attracts the greatest attention of researchers. It is known that OPG is expressed not only by bone tissue cells, but also by cardiovascular cells: myocardiocytes, smooth muscle cells of arteries and veins, and vascular endothelial cells. OPG is a modulator of vascular calcification, which was confirmed in the experimental work of S. Moropu et al., performed on intact mice and animals with a disruption/absence of the gene providing OPG expression. It was found that mice with impaired ability to synthesize OPG (OPG-/-), unlike the control group of animals, show activation of the process of arterial calcification in combination with the development of osteoporosis and multiple bone fractures. On the contrary, the introduction of the gene synthesizing it to animals with insufficient expression of OPG contributed to the suppression of both the process of bone resorption and vascular calcification.

Inflammation plays a key role in all stages of atherosclerosis development, accompanied by a significant increase in the concentration of inflammation markers in the blood plasma - cytokines (interleukin-1, a-TNF), which, in turn, induce bone resorption. According to the inflammatory nature of atherosclerosis development, the expression and release of OPG into the bloodstream and surrounding tissues by endothelial cells and vascular smooth muscle cells are carried out under the influence of the above-mentioned proinflammatory factors. Unlike stromal cells, endothelial cells and vascular smooth muscle tissue do not respond to changes in the content of vitamin D3 or parathyroid hormone (PTH) in the blood plasma by increasing the synthesis and release of OPG. OPG prevents vitamin D3-induced ectopic calcification in vessels, simultaneously increasing the content of OPN, the main non-collagenous matrix protein of bones, which acts as an inhibitor of vascular mineralization and as a trigger for the synthesis and release of OPG by endothelial and smooth muscle cells. OPN, inhibiting the process of hydroxyapatite matrix formation (in vitro) and vascular calcification (in vivo), is synthesized and released in sufficiently high concentrations by smooth muscle cells of the media of the vascular wall and by macrophages of the intima. OPN synthesis occurs in areas with predominant mineralization of the vascular wall and is regulated by proinflammatory and osteogenic factors. Together with avb3 integrin, synthesized by endothelial cells at the sites of atherogenesis, OPN causes the NF-kB-dependent effect of OPG on maintaining the integrity of endothelial cells. Thus, the increased plasma and vascular OPG concentrations observed in cardiovascular diseases may be a consequence of endothelial cell activity both under the influence of inflammatory markers and as a result of the OPN/avb3-HHTerpnHOBoro mechanism.

Activation of NF-kB in macrophages of the arterial wall and in TC is also one of the important mechanisms linking osteoporosis and atherosclerosis. Increased NF-kB activity occurs as a result of the action of cytokines released by activated T cells in the vascular intima, which contributes to an increase in the activity of serine/threonine kinase (Akt, protein kinase B), an important factor for the function of, first of all, vascular endothelial cells.

It has been established that as a result of increased protein kinase B activity, stimulation of eNOS and increased synthesis of nitric oxide (NO), involved in the mechanism of maintaining the integrity of endothelial cells, are observed. Similar to OPG, the synthesis and release of RANKL by endothelial cells is carried out under the influence of inflammatory cytokines, but not as a result of the action of vitamin D3 or PTH, which are capable of increasing the concentration of RANKL in OB or stromal cells.

The increase in the RANKL concentration in arterial and venous vessels is also achieved as a result of the inhibitory effect of the transforming growth factor (TGF-Pj) on the process of OPG expression, the content of which is significantly reduced under the influence of this factor. It has a multidirectional effect on the RANKL content in bone and vessels: in bone tissue, TGF-Pj promotes the expression of OPG OB and, as a result, OPG, binding RANKL, reduces its concentration and osteoclastogenesis activity. In the walls of blood vessels, TGF-Pj increases the RANKL/OPG ratio and, as a consequence, the RANKL content, interacting with its RANK receptor on the surface of endothelial cell membranes with the participation of intracellular signaling systems, stimulates vascular cell osteogenesis, activates the process of calcification, proliferation and cell migration, and matrix remodeling. The result of the new concept based on the current understanding of the cellular and molecular mechanism of bone remodeling in osteoporosis and the atherosclerosis process, and the elucidation of the leading role of the cytokine RANKL-RANK-OPG system in the implementation of these diseases, was the synthesis of a new generation drug - denosumab. Denosumab (Prolia; Amgen Incorporation) is a specific human monoclonal antibody with a high degree of tropism for RANKL, blocking the function of this protein. Numerous laboratory and clinical studies have established that denosumab, demonstrating a high ability to reduce RANKL activity, significantly slows down and weakens the degree of bone resorption. Currently, denosumab is used as a first-line drug, along with bisphosphonates, in patients with systemic osteoporosis to prevent bone fractures. At the same time, S. Helas et al. established the inhibitory effect of denosumab on the ability of RANKL to implement the process of vascular calcification. Thus, the obtained data open up new possibilities for slowing the progression of osteoporosis and vascular atherosclerosis, preventing the development of cardiovascular complications in osteoporosis, and preserving the health and life of patients.

S. Sagalovsky, Richter. Pathophysiological unity of development of osteoporosis and atherosclerosis of vessels // International Medical Journal - No. 4 - 2012