Study and analysis on the mechanisms of action of cytokines on osteoclasts in os

Jin-Mei Zhang, Yan Yin, Xin-Qi Liu, Li-Juan Du, Li-Qin Wang, Lei Tan

Weifang Hospital of Traditional Chinese Medicine, Shandong, China.

Abstract Osteoporosis is caused by unbalanced bone reconstruction processes which leads to areduction in bone mineral density and changes in bone microstructure, resulting in increased fracture risk.Osteoclasts are the only bone resorption cells in the body of human being, and are closely related to the pathogenesis of osteoporosis.In recent years, there have been more in-depth studies on osteoclasts and bone resorption.The pathogenesis of osteoporosis has always been an important research topic for many medical specialists.Cytokines, as important humoral factors that regulate bone metabolism, play an important role in osteoporosis, especially in the formation, differentiation,and apoptosis of osteoclasts.On the basis of referring to the related studies at home and abroad, this paper analyzes the action mechanism of cytokines such as interleukin-1, interleukin-6, nuclear factor KB ligand receptor activator (RANKL) and tumor necrosis factor (TNF) family members on osteoclasts, and clarifies the relationship between cytokines and osteoporosis, which provides a new angle for the prophylaxis and therapy of osteoporosis.

Keywords: Cytokinesm, Osteoporosis, Osteoclasts, Action mechanism

Introduction

Under the action of multiple pathogenic factors, human bone metabolism is abnormal, resulting in the decrease of bone mineral density and the destruction of bone microstructure, which is called osteoporosis.On this basis, the increase of bone brittleness is easy to induce fracture.There are about 8.9 million patients with osteoporotic fractures worldwide every year.Normal bone tissue of the body is in a dynamic equilibrium of bone absorption, caused by osteoclast (OCS), and bone formation,caused by osteoblast (OBs).Bone formation and maintenance are processes of continual shaping and repair by osteoblasts and osteoclasts.The imbalance between the two can lead to loss of bone mass [1,2].Osteoclasts, derived from hematopoietic stem cells,are cells related to monocytes and macrophages,which interact with osteocytes and osteoblasts to perform normal bone turnover[3,4].Under pathological conditions, if the activity of osteoclasts is too strong or is weakened, it will lead to loss of bone mass and osteoporosis.

At present, many studies have reported the mechanisms underlying the development and progression of osteoporosis.Systemic body fluid factors are significant controllers of bone metabolism,and the role of cytokines as local regulators cannot be ignored.Cytokines are not only critical regulators of bone resorption and bone formation, but also have important effects on bone metabolism.The dysfunction of cytokine regulation may lead to the imbalance of bone formation-absorption coupling and result in osteoporosis.Cytokines are divided into interleukins (IL), interferons, the tumor necrosis factor (TNF) superfamily colony stimulating factors,chemotactic factors, and growth factors, among others[5].The cytokines closely related to osteoporosis are interleukin-1, interleukin-6, nuclear factor KB ligand receptor activator (RANKL) and tumor necrosis factor(TNF), which are important factors in regulating bone resorption under pathological conditions

1.Interleukins

Many studies have shown that bone cytokine such as Interleukin-1 (IL-1) and Interleukin-1 (IL-6) can inhibit osteoblast formation and its activity, thus inhibiting bone formation and accelerating bone loss

1.1 Interleukin-1(IL-1)

IL-1 is an important immune system regulator that was discovered in 1972, and is mainly produced by mononuclear macrophages.In addition, it can also be produced by various other cells and has a wide range of biological effects.The role of IL-1 in promoting osteoclast formation and stimulating bone resorption.

IL-1 affects OC generation, differentiation, and activation, and thus participates in the regulatory mechanisms of bone destruction, including [6-9]: 1)Promoting OC generation and activation it induces OBS/SC to synthesize prostaglandin E2 (PGE2), and PGE2 acts on OBS/SC, increasing RANKL expression and promoting OC generation.In addition, it remodels the OC cytoskeleton and activates Ocs by acting on intracellular c-Src and the tumor necrosis factor receptor- related factor 6 (TRAF6) signaling system.Furthermore, it inhibits OC apoptosis and prolongs OC life.2) Promoting OC osteolysis: It stimulates OCS to secrete enzymes that destroy bone matrix proteins,such as matrix metalloproteinases 2 and 9 (MMP2/9),thus enhancing the osteolysis function of OCS.In addition, in the synovium, IL-1 can induce fibroblast proliferation, stimulate synovial cells to synthesize IL-6 and GM-CSF, and cause fibroblasts to produce more collagenase, aggravating bone destruction.

1.2 Interleukin-6 (IL-6)

IL-6 is a cytokine with multiple functions produced by a variety of cells, which usually plays a significant part in acute phase reaction，immune response,and hematopoietic regulation.IL-6 is a potential bone resorption factor.In the presence of both IL-6 and soluble interleukin-6 receptors (SIL-6R), the expression of RANKL increases and significantly stimulates the occurrence of osteoclasts [10].The regulatory mechanisms of IL-6 in bone destruction include [11,12]: 1) IL-6 can induce the formation of IL-I and TNF-α, and play a role in bone destruction via these two cytokines.2) OBs produce IL-6 stimulated by IL-1 and TNF-α, and IL-6 plays an critical role in regulating bone resorption.3)IL-6 can stimulate bone marrow granulocyte and macrophage colony formation,and continue to stimulate them further until they form osteoclast precursor cells: IL-6 then stimulates Oc activity, which promotes bone resorption, and thus leads to bone destruction.4) IL-6 can enhance the expression of vascular endothelial adhesion molecules and affect the adhesion of osteoclast progenitor cells[13].

2.Tumor necrosis factor (TNF)

2.1 RANKL

At present, it is believed that the RANKL-RANKOPG axis composed of RANKL of the TNF family,its receptor (RANK), and osteoprotegerin(OPG), is involved in regulating the equilibrium between bone loss and bone formation [14].RANKL is a type-Ⅱtransmembrane protein, which is currently the only factor found to induce osteoclast differentiation,development, and functioning.RANKL not only promotes the differentiation of osteoclasts by binding to RANK, but also activates mature osteoclasts in a dosedependent way to improve the bone resorption ability[15].OPG is a secretory glycoprotein with the function of inhibiting bone absorption.During the culture of mesenchymal stem cells, RANKL can be added to form osteoclasts with perfect function, while OPG can completely inhibit the function of osteoclasts.Further studies have found that OPG inhibits bone resorption by competing with RANKL to bind to RANK.The balance among RANKL, RANK and OPG regulates the differentiation and activation of osteoclasts and affects bone metabolism.RANKL-RANK-OPG axis is directly involved in the proliferation and apoptosis of osteoclasts [16].

2.2 TNF-α

TNF-α, a multifunctional bioactive factor secreted by monocytes and macrophages, is a powerful bone absorption inducer both in vivo and in vitro.which can directly excite the growth and differentiation of osteoclast precursor cells, induce osteoclast formation,and enhance osteoclast activity.TNF-α can also directly or indirectly act on osteocalcin precursors to induce the differentiation into matur osteocalcin and enhance bone absorption capacity, thus reducing bone formation [17] and inducing osteoporosis.Studies have shown that TNF-α can also induce apoptosis in bone cells together with IL-6, aggravating the progression of glucocorticoid-induced osteoporosis.Dischereit et al [18] researched the effect of inflammatory factors on fracture and bone metabolism , and found that the levels of TNF-α in the plasma of the elderly was significantly higher than that in young people and TNF-α had a significant negative correlation with bone mineral content.Zhu et al [19] studied the role of TNF-α and IL-6 gene knockout in preventing bone loss and regulating bone metabolism in mice, which showed that TNF-α or IL-6 gene knockout could significantly up-regulate the mRNA expression of bone formation--resistant genes ( Runx2 and Collal), and down-regulate the expression of bone resorption -resistant mRNA tartrate-resistant acid phosphatase, MM9, and cathepsin K in vivo and in vitro.In addition, TNF-α has more potential than IL-6 in inhibiting bone formation and enhancing osteoclast formation mediated by Tnf-related activation-induced genes.

TNF- α induces the expression of Dickkopf-associated protein I (DKK-1).DKK-1 inhibits bone formation by inhibiting Wnt signal transduction.Under the induction of TNF-α, the expression of DKK-1 increased, the signal transduction of Wnt was inhibited, and the decrease of OPG production led to the increase of RANKL/OPG ratio and promoted bone resorption leading to bone loss.In the presence of RANKL,TNF-α increases the differentiation of osteoclasts in a dose-dependent manner.The minimum dose of TNF-α or RANKL alone make no difference on the differentiation of osteoclasts.While the same minimum doses of TNF-α and RANKL used in combination significantl reases the differentiation degree of osteocl.

Discussion

Based on the viewpoints of the above medical specialists.the authors believe that the expression of IL-I and IL-6 play a key role in bone resorption processes, and both participate in the regulatory mechanisms of bone destruction.The above research is based on the conclusion drawn from animal experiments and still needs a large number of clinical studies for verification.Anti-il-6 antibody tocilizumab(TCZ), as a recombinant humanized monoclonal antibody of IL-6 receptor antagonist, can inhibit osteoclast activation.Through analysis of the metacarpophalangeal joint bones of 20 RA patients receiving TCZ, it was found that TCZ plays a critical role in local bone remodeling [20]; however, further evidence of clinical application and research are needed to elucidate the action mechanisms of TCZ.In addition to participating in the regulation of bone resorption, RANKL and TNF-An also plays a pivotal role in the formation, differentiation and apoptosis of osteoclasts.The currently availabled denosumab is an anti-RANKL monoclonal antibody that interferes with the proliferation and differentiation of osteoclasts to prevent bone resorption.It has been proven in Phase Ill clinical trials that it can reduce centrum and hip fractures, and it can be used to treat postmenopausal women with osteoporosis [21].Research on the applicationis still needed for further observation In clinical application, it is found that tumor necrosis factor-α antagonist infliximab and Adamumab can alleviate large area bone loss; infliximab can prevent bone loss in the backbone and hip joints of RA sufferers but has a weak inhibitory effect on local bone loss[22].Wiibrandts et al [23] found that after oneyear of treatment with adalimumab, the BMD of 50 patients with active RA was not significantly increased but the average progressive bone loss was inhibited.

In the past few decades, the pathological mechanisms of osteoporosis have been identifiedat the molecular,cellular, and tissue level.The treatment of osteoporosis has gone through many years of research and development, and has achieved good results.With the rapid development of science and technology,there have been more in-depth studies on osteoclasts and bone resorption in recent years, and a variety of targeted anti-bone resorption drugs have been developed, making it possible to treat osteoporosis faster and more effectively.Therefore, through an indepth study of the relationship between various related cytokines and osteoporosis, we can further understand the pathogenesis of osteoporosis and provide a new direction for prevention and treatment.