Ｒeview For Treatment Effect And Signaling Pathway Regulation Of Kidney-tonifying Traditional Chinese Medicine On Osteoporosis

［Abstract］ The treatment effect and signaling pathway regulation effects of kidney-tonifying traditional Chinese medicine on osteoporosis have been widely studied,but there is no systematic summary currently． This review comprehensively collected and analyzed the traditional Chinese medicines on the treatment and signaling pathway regulation of osteoporosis in recent ten years,such as Epimedii Folium,Drynariae Ｒhizoma,Cnidii Fructus,Eucommiae Cortex,Psoraleae Fructus and Dipsaci Ｒadix． Based on the existing findings, the following conclusions were obtained: ① kidney-tonifying traditional Chinese medicine treated osteoporosis mainly through BMPSmads,Wnt /β-catenin,MAPK,PI3K/AKT signaling pathway to promote osteoblast bone formation and through OPG/ＲANKL/ ＲANK,estrogen,CTSK signaling pathway to inhibit osteoclasts of bone resorption． Epimedii Folium,Drynariae Ｒhizoma,Cnidii Fructus and Psoraleae Fructus up-regulated the expression of key proteins and genes of BMP-Smads and Wnt /β-catenin signaling pathways to promote bone formation． Epimedii Folium,Drynariae Ｒhizoma,Cnidii Fructus,Eucommiae Cortex,Psoraleae Fructus and DipsaciＲadix inhibited the bone resorption by mediating the OPG/ＲANKL/ＲANK signaling pathway． ② Kidney-tonifying traditional Chinese medicine prevented and treated osteoporosis through a variety of ways: icariin in Epimedii Folium,naringin in Drynariae Ｒhizoma,osthole in Cnidii Fructus and psoralen in Psoraleae Fructus can regulate BMP-Smads,Wnt /β-catenin signaling pathway to promote bone formation,but also activate OPG/ＲANKL/ＲANK,CTSK and other signaling pathways to inhibit bone resorption． ③The crosstalk of the signaling pathways and the animal experiments of the traditional Chinese medicine on the prevention and treatment of osteoporosis as well as their multi-target mechanism and comprehensive regulation need further clarification

Keywords:  kidney-tonifying traditional Chinese medicine; osteoporosis; signaling pathways; bone metabolism; osteoblast; osteoclast

NEW HERB CISTANCHE FOR KIDNEY HEALTH 

Osteoporosis is a systemic metabolic bone disease characterized by decreased bone mass, deterioration of bone microstructure, increased bone brittleness, and susceptibility to pathological fractures [1]. In traditional Chinese medicine theory, osteoporosis belongs to the category of "bone atrophy" and "bone paralysis". As early as in the Suwen·Wei Lun Pian, it was recorded that "the kidney governs the body's bone marrow". The kidney stores essence, and essence produces marrow. The marrow is located in the bone cavity to nourish the bones. When the kidney essence is deficient, the bones wither and the marrow decreases. Therefore, kidney deficiency is considered to be the main cause of osteoporosis. The strength of the bones is determined by the rise and fall of kidney essence. Therefore, kidney-tonifying Chinese medicine is widely used to prevent and treat osteoporosis. Modern molecular biological studies have shown that kidney-tonifying Chinese medicine can regulate the proliferation and differentiation of osteoblasts and osteoclasts through multiple signaling pathways, so that the activities of the two reach a dynamic balance, thereby playing a role in preventing and treating osteoporosis [2]. This article reviews the regulatory effects of classic kidney-tonifying Chinese medicines such as Epimedium, Drynaria, Psoralea corylifolia, Cnidium monnieri, Eucommia ulmoides, and Dipsacus asper on signaling pathways such as BMP-Smads, Wnt/β-catenin, MAPK, PI3K/AKT, OPG/ＲANKL/ＲANK, estrogen, and CTSK in the prevention and treatment of osteoporosis, providing a reference for explaining the scientific connotation of "kidney governs bones" and further research and utilization of kidney-tonifying Chinese medicines.

1 Related signaling pathways of osteoporosis
Modern studies have shown that the related signaling pathways of osteoporosis mainly include two categories: promoting bone formation and inhibiting bone resorption. Among them, BMP-Smads, Wnt/β-catenin, MAPK, PI3K/AKT, estrogen and other signaling pathways mainly promote bone formation by acting on osteoblasts; OPG/ＲANKL/ＲANK, estrogen, and CTSK signaling pathways mainly inhibit bone resorption by acting on osteoclasts. Moreover, each signaling pathway does not exist independently, but interacts with each other.

1.1 Signaling pathways related to bone formation

1.1.1 BMP-Smads signaling pathway

Bone morphogenetic proteins (BMP) are multifunctional growth factors belonging to the TGF-β family [3]. They are mainly present in the bone matrix. When BMP increases, it promotes bone formation and increases bone mass, thereby preventing and treating osteoporosis. BMP mainly exerts its osteogenic effect by activating Smads protein signal transduction and regulating osteogenic gene transcription. Smads have multiple subtypes, among which Smad1, Smad5, and Smad8 are the main effector molecules of BMP. Inhibition of the BMP Smads pathway will inhibit the expression of Osterix and Runx2, genes related to osteogenic differentiation. Studies have shown that when the BMP inhibitor Noggin is overexpressed or the BMP receptor BMPＲ1 is missing in mouse osteoblasts, the proliferation, differentiation and mineralization of osteoblasts are inhibited, and mice may even develop osteoporosis［4-5］. From the above, it can be seen that the BMP-Smads signaling pathway may regulate the entire life cycle of osteoblasts, including the differentiation of bone marrow mesenchymal stem cells (BMSCs) into osteoblasts and the proliferation and mineralization of osteoblasts.

1. 1. 2 Wnt/β-catenin signaling pathway
The Wnt signaling pathway is currently a hot topic in the research on the pathogenesis of bone metabolism-related diseases, among which the Wnt/β-catenin signaling pathway plays an important role in promoting bone formation. The Wnt/β-catenin signaling pathway can promote the differentiation of BMSCs into osteoblasts, inhibit their differentiation into chondrocytes or adipocytes, and inhibit osteoblast apoptosis, thereby promoting bone formation［6］. When β-catenin was conditionally knocked out in osteoclasts, the number of osteoclasts and the degree of bone resorption increased significantly, resulting in a decrease in bone mass; while conditionally activating the expression of β-catenin resulted in an increase in bone mass [7], indicating that the Wnt/β-catenin signaling pathway can promote the differentiation and mineralization of osteoblasts and inhibit the differentiation and bone resorption of osteoclasts.

1. 1. 3 MAPK signaling pathway

The MAPK signaling pathway is an important pathway for regulating the growth and differentiation of osteoblasts. Studies have shown that it mainly includes the ERK pathway, the p38 pathway, and the JNK pathway. The ERK signaling pathway can upregulate the expression of Osterix mRNA in osteoblasts, inhibit bone resorption, and increase bone density [8-9]. Inhibition of the ERK signaling pathway will downregulate the expression of osteoprotegerin (OPG) and the ratio of OPG/RANKL [10]. The p38 signaling pathway is indispensable for the differentiation of osteoblasts and can promote the mineralization of osteoblasts and the expression of alkaline phosphatase (ALP) and BMP-2, thereby promoting bone formation [11]. After JNK protein inactivation, the expression of osteocalcin (OCN) and osteopontin (OPN) decreased significantly in the late stage of osteoblast differentiation, indicating that the JNK signaling pathway mainly acts on the late stage of osteoblast differentiation［12］.

1. 1. 4 PI3K/AKT signaling pathway

When AKT and its target genes are activated, they can promote the differentiation and mineralization of osteoblasts. In AKT1 and AKT2 gene knockout mice, obvious symptoms of bone loss appeared; when the endogenous inhibitor of PI3K, PTEN, was deficient, the AKT signaling pathway was significantly enhanced and bone mass increased［13］, suggesting that osteoblasts may promote bone formation through the PI3K/AKT signaling pathway.

1.2 Signaling pathways related to inhibiting bone resorption

1.2.1 OPG/ＲANKL/ＲANK signaling pathway

The OPG/ＲANKL/ＲANK signaling pathway plays an important role in the differentiation and maturation of osteoclasts. OPG is a member of the tumor necrosis factor receptor superfamily and has the function of inhibiting osteoclast differentiation and bone resorption. OPG can competitively bind to the receptor activator of NF-κB ligand (ＲANKL), inhibit the interaction between ＲANKL and (receptor activator of NF-κB, ＲANK), thereby inhibiting the proliferation and differentiation of osteoclasts and promoting their apoptosis. Increased OPG levels or increased OPG/ＲANKL ratio can reduce the activity of osteoclasts and inhibit the differentiation and maturation of osteoclasts［14］.

1.2.2 Estrogen signaling pathway

Estrogen can inhibit the differentiation of osteoclasts by inhibiting the activity of RANKL [15]. Nakamura et al. [16] selectively knocked out the estrogen receptor α (ERα) of osteoclasts and found that it could prolong the survival time of osteoclasts, reduce their apoptosis, and reduce trabecular bone mass. This indicates that after estrogen binds to the receptor, it can affect the cell cycle and induce osteoclast apoptosis, inhibit the activity and differentiation of osteoclasts, and ultimately inhibit osteoclast bone resorption and prevent osteoporosis [17].

1.2.3 CTSK signaling pathway

CTSK is a proteolytic enzyme mainly present in osteoclasts. It promotes bone resorption by promoting the degradation of bone collagen fibers. It is mainly involved in the degradation of type I collagen (Collagen I), OPN, osteonectin, etc. [18]. Therefore, some scholars believe that inhibiting the degradation of bone tissue by CTSK can inhibit bone resorption, thereby achieving the purpose of improving osteoporosis.