Introduction

Osteoporosis is a complex and degenerative bone metabolism disease, which is characterized by the reduced bone mass and deteriorated microarchitecture of bone tissue [1]. Osteoporosis-induced fracture has been a major health hazard, especially the fracture in hip and vertebrae [2]. Actually, Si et al. [3] predicted that the number of osteoporosis-related fracture in China would double annually by 2035 and could increase to 5.99 million in 2050. Meanwhile, in the United States, the national expenses on bone fracture care will increase from $17 billion in 2005 to $474 billion by 2025 for the enhancing rate of osteoporosis [4].

It is conventionally considered that menopause and old age are the major determinant factors for bone loss due to the reduction in gonadal function [5, 6]. Additionally, epidemiological data showed that the common factors including excessive glucocorticoids use [7], diabetes [8], kidney diseases [9], weightlessness [10] and smoking [11] were proved to lead to osteoporosis. As fully demonstrated by animal and cell experiments as well as clinical evidence, the central research parameters for osteoporosis largely involved the reduction in bone quality, changes in serum and urine biochemical markers, a decrease in bone formation, and an increase in bone resorption. Moreover, from the cell and molecular level, the mechanism of bone loss is generally attributed to the altered osteoclast differentiation and bone resorption over osteoblasts bone formation [12]. The research indexes and relevant parameters of osteoporosis were described in detail in Table 1.

Table 1 Osteoporosis indexes and relevant parameters in vivo and in vitro studies
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Recently, hormone replacement therapy (HRT) drugs and bisphosphates are the major agents to treat osteoporosis in clinical practice due to their inhibition to bone resorption. However, several lines of researches have demonstrated a series of potential adverse effects in the process of treatment with HRT. For example, the long-term treatment with estrogen will pose increasing risk for the morbidity of the secondary diseases including breast cancer, endometrial carcinoma, heart attack, blood clots and stroke [13, 14]. Similarly, it has been reported that bisphosphates partly would result in the complication of osteonecrosis in jaw [15, 16]. Thus, to reduce these adverse effects, a new insight into complementary and alternative medicine is becoming an indispensable exploration in the treatment of osteoporosis.

Traditional Chinese medicine (TCM) are being gradually recognized and approved in dealing with osteoporosis as one of the remarkable complementary and alternative therapy, because of their own less adverse reactions as well as significant curative effects [17]. TCM theory pointed out that the growth and development of bones are closely related to a normal function of the kidney-yin, an essential substance for nourishing the bones [18]. Alternatively, in case the kidney-yin is insufficient, there will be increasing risk of some TCM syndromes like “Guwei”, “Guku” and “Gukong”, which are similar to the disease of osteoporosis in western medicine [19, 20]. Thus, the traditional Chinese herbs categorized into tonifying the kidney-yin played crucial roles in the clinical treatment and preclinical researches of osteoporosis. A large number of researches have been carried out on this topic. However, only scattered rather than systematic reports are available in previous literature with regard to anti-osteoporotic effects of tonifying kidney-yin herbs and formulas. Therefore, the purpose of this study is to comprehensively review the previous researches on the effective herbs and formulas including Fructus Ligustri Lucidi, Eclipta prostrata, Fructus Schisandrae Chinensis, Fructus Corni, Rhizoma Polygonati, Radix Polygoni Multiflori, Radix Angelicae Sinensis, Erzhi Wan, Siwu Tang, Liuwei Dihuang Wan and Zuogui Wan, as well as their potential targets.

Fructus Ligustri Lucidi

Fructus Ligustri Lucidi (FLL) is mostly used to nourish kidney-yin for maintaining healthy energy and treating age-related disease, which was initially recorded in Shennong Bencaojing, the oldest Chinese materia medica monograph in China (Anonymous, CA. 200 BC). Many studies have confirmed that FLL had various pharmacological effects including anticancer [21], immunoloregulation [22], antioxidant [23], and anti-osteoporosis [24].

Bone density is determined by the peak value reached during young adulthood. Furthermore, reaching the higher peak bone mass (PBM) may prevent and reduce the risk of osteoporosis in the later years [24]. In vivo studies, with the administration of FLL extract for 4 months till PBM time point, the FLL-treated rats exhibited higher bone mineral density (BMD) on femur than the control ones. The μCT testing indicated that FLL improved their bone microarchitecture with higher trabecular bone volume fraction and thickness. Additionally, FLL-treated group had higher dry weight value, calcium retention and calcium absorption. The serum parameters including 25(OH)D3, 1,25(OH)2D3, and osteocalcin (OCN) were remarkably elevated, while the level of C-terminal telopeptide of type I collagen (CTX-I) was decreased by FLL when compared to the control group [25, 26]. And, the bone biomechanical parameters in ultimate load and ultimate deformation were also increased by FLL treatment [25]. These effects could be elucidated that, on the one hand, the ratio of receptor activator of NF-κB ligand (RANKL) to osteoprotegerin (OPG) in tibia was decreased and the genes expression of 1α-hydroxylase (1-OHase), transient receptor potential vanilloid 6 (TRPV6), calcium transporter calbindin-D9k (CaBP-9 k) and vitamin D receptor (VDR) in kidney and duodenum were enhanced, consequently the treatment with FLL regulating the absorption of calcium [25, 26]. On the other hand, FLL inhibited the high bone turnover rate of biochemical markers including the increased urinary deoxypyridinoline (DPD) and serum OCN in ovariectomized (OVX) rats, which are the special bone resorption and formation markers respectively [27]. Moreover, a combination of FLL with high calcium managed to synergistically increase calcium level in serum and bone, despite the report from Zhang et al. that the decreased bone calcium level in OVX mice was difficult to be rectified by single high calcium administration. This significant effect on calcium balance was mainly attributed to the up-regulation of CaBP-9 k and down-regulation of calcium-sensing receptor (CaSR) in kidney [28].

In vitro studies, FLL administration realized the anti-osteoporotic effects by significantly stimulating the differentiation of osteoblasts. FLL enhanced Alkaline phosphatase (ALP) activity in mesenchymal stem cells (MSCs) and significantly attenuated the mineralization time for MSCs. These actions may be related to the increased genes expression of β-Catenin, BMP2, Cyclin D1, MT1-MMP, OPG, and TBX3 by FLL administration, which were the differentiation regulators of osteoblasts [29]. It has already showed that when the osteoblast MC3T3-E1 cells were treated with FLL for 7 days, the mRNA levels of ALP, bone sialoprotein (BSP) and OCN were elevated, and the ratio of RANKL to OPG was decreased, resulting in the positive effect of FLL on the cells differentiation [25].

In addition, the anti-osteoporotic effects of FLL were determined by means of studying the interaction between FLL and other positive anti-osteoporosis herbs, such as Herba Epimedii (HE) and Radix Puerariae (RP) [30, 31]. For example, Liu et al. has demonstrated that the osteoporosis induced by retinoic acid was alleviated by a combination therapy of FLL with HE, because they enhanced the levels of bone mineral content, biomechanical parameters in the model rats, and the bone formation markers, as well [32]. But, when FLL and RP were applied into treating the OVX rats together, their respective positive effects including inhibiting the bone loss and regulating mineral metabolism wore off [33]. Although more researches about the herb-herb interaction should be further done, after all, the above effects have proved that FLL will be a potential candidate for the prevention and treatment of osteoporosis.

Eclipta Prostrata

Eclipta prostrata (EP) is originated from dried aerial parts of Eclipta prostrata L. in compositae family. It possesses many pharmacological effects, including anti-breast tumor [34], hypolipidemic capacity [35] and promotion of hair growth [36]. Recently, researchers attached too much importance to its anti-osteoporotic effect.

The EP extract exhibited positive effects on inhibiting osteoporosis in similar ways of FLL. When the OVX female rats were administrated with EHE for 3 months, they had remarkably higher BMD than the control rats, which was equivalent to the positive drugs administration group. The impaired histomorphology of bone was also ameliorated by EP extract [37]. These vivo findings that EP extract reversed the increased interleukin-6 (IL-6) and decreased calcitonin (CT) as well as RANKL in tibiae were partly responsible for its protective effects on the bones of OVX rats [37]. Furthermore, Deng et al. found that Echinocystic acid (EA), one of bioactive components in EP, significantly improved the damages in the bones induced by estrogen deficiency, such as lower BMD, impaired trabecular architecture and biomechanical properties, and the excessive expression of pro-inflammatory cytokines such as IL-1β and TNF-α. Consequently, they concluded that EA may be an important substance for EP to play roles in inhibiting bone loss [38].

Meanwhile, in the vitro study, it has been found that wedelolactone (extracted from EP) may be another important bioactive compound to treat osteoporosis. Liu et al. [39] found that wedelolactone significantly inhibited the proliferation and differentiation of pre-osteoclastic RAW264.7 cells, while it was capable of stimulating the proliferation and differentiation of BMSCs. They inferred that wedelolactone still showed an excellent anti-osteoporotic effect at low concentration in spite of the fact that the high dose of wedelolactone leaded to some cytotoxic effects on BMSCs [39]. These findings indicated that wedelolactone may be another potential bioactive compound to work in EP for anti-osteoporosis.

Fructus Schisandrae Chinensis

Fructus Schisandrae Chinensis (FSC) is supposedly categorized as the tonic due to its powerful and effective action on nourishing kidney-yin [40]. Currently, the FSC extract showed anti-inflammatory effects to treat asthma and sepsis [40, 41]. FSC also exerted oxidant inhibition to treat alcoholic liver injury by preventing hepatocyte apoptosis and fatty degeneration [42]. But more importantly, the FSC was attracting a lot of interests from the researches on the osteoporosis.

In vivo studies, the treatment of FSC not only attenuated the pathomorphological changes in femur head and condyles of OVX mice, such as growth plate hypertrophy and bone marrow pores, but also reversed some key parameters due to the lack of estrogen including the involvement in bone loss as well as the deficiency of uterus. The data proved that FSC indeed enhanced the levels of BMD and serum OCN, and, moreover, the level of serum estradiol and the expression of ER-α and ER-β in uterus were significantly increased by FSC. Additionally, FSC obviously downregulated the relevant oncogene proteins in uterus including c-fos and c-Jun [43]. Schisantherin A (SA) is one kind of lignans extracted from FSC. He et al. found that SA inhibited osteoclast formation and Ti particle-induced osteolysis by decreasing osteolysis area, osteoclasts activity and osteoclasts number [44].

In vitro studies, He et al. further explored whether the SA worked at the cellular level. It turned out that SA suppressed the differentiation and osteoclastogenesis of BMMs and RAW 264.7 cells. This group also clarified the result that SA destroyed osteoclast-induced bone resorption may be fully understood by its roles in RANKL signaling pathways involving the down-regulation of the phosphorylation of IκB, JNK and ERK1/2, as well as the genes expression of NFATc1 and c-Fos [44]. Parallel study has demonstrated that FSC stimulated the proliferation of osteoblasts UMR 106 cells and increased the activity of ALP, indicating the positive effects of FSC on osteoblasts [45]. These data proved that FSC is a promising herb for the treatment of bone loss induced by osteoclast.

Fructus Corni

Fructus Corni (FC) is usually used in the Orient as a source of food resulting from redundant nutrition greatly enriched in sarcocarp. In China, however, it is often known in medical literature that the core medicinal uses of FC involve treating the deficiency of kidney-yin. In recent decades, the uses of FC have been expanded into treating diabetes [46], neurotoxicity [47] and platelet aggregation [48]. But, from the nourishing kidney-yin viewpoint, FC has recently been researched as an ideal medicinal component for anti-osteoporosis.

In vivo studies, it is believed that the spinal cord-injured (SCI) surgery was considered as another promising osteoporotic model. As long as 6 weeks after SCI surgery, the rats encountered the deterioration of biomechanical parameters, such as maximal load, energy absorption and structural stiffness, which is similar to those of OVX model, the main osteoporotic model. But, these abnormal biomechanics were effectively improved by the administration of FC with a dose-dependent manner. Meanwhile, SCI surgery brought about the lower BMD and BMC and a certain amount of morphological damage including inner diameter, internal and external areas in tibia diaphysis, all of which were also partly reserved by FC [49]. Several lines of studies further found that FC served as a rich source of cornel total glycoside, which is well-known for its ability to significantly elevate the levels of serum E2 and BMD and improve the damaged bone trabecula, especially that of rat model of OVX [50, 51].

In vitro studies, Sweroside extracted form FC significantly increased the proliferation of human MG-63 cells and osteoblasts, and enhanced the activity of ALP and osteocalcin, as well. In addition to its proliferation effects, the involvement of anti-apoptosis effect on the osteoblast was found in the treatment of sweroside [52]. The data manifested that developing FC as well as its extracts could potentially provide certain benefits for osteoporosis that have been implied by laboratory animal and cell studies.

Rhizoma Polygonati

Rhizoma Polygonati (RP) belongs to the genus polygonatum family of plants. Huge amount of RP are consumed as food-medicine dual plants in the East Asia countries, despite the fact that it is widely distributed from Asia to Europe. In China, RP is the ideal tonic for the ones who need to strengthen body’s kidney-yin. Current studies showed that RP was usually used to prevent and treat metabolic disorders such as obesity [53], diabetes [54] and cardiovascular disease [55]. Additionally, some progress has been made in its anti-osteoporotic effect and its pharmacological mechanism.

In vivo studies, the administration of RP at a dose of 0.4 g/kg/day for 35 days significantly enhanced the level of BMD, and decreased the occurrence of malalignment in the tibia trabeculae of the OVX rats, which is similar to that of 17-ethinylestradiol, the excellent conventional agent for deficiency of estrogen. These effects have been proven to be related to the up-regulation of basic fibroblast growth factor (bFGF) and BMP and the suppression on excessive expression of bone Gla protein (BGP) and TRAP in tibiae [56]. This animal experiment indeed indicated the anti-osteoporotic activity of RP.

In vitro studies, Zong et al. [57] reported that RP increased the viability of BMSCs with the greatest stimulation at a dose of 0.5 g/ml. They then clarified this relevant mechanism by detecting the effects of RP on the levels of ALP, OCN, PINP and BMP-2 in BMSCs as well as the protein expression of BSP (bone sialoprotein) and SPARC (secreted protein, acidic and rich in cysteine). The results demonstrated that RP showed powerful and effective effects on these parameters, indicating a potential effect from RP on promoting the proliferation and differentiation of osteoblasts [57]. The vivo and vitro studies have confirmed the RP might be a potential alternative medicine for treatment of osteoporosis.

Radix Polygoni Multiflori

Radix Polygoni Multiflori (RPM) is originated from the dried root tuber of Polygonum multiflorum Thunb. (Fam. Polygonaceae). In China, RPM is used in the prevention and treatment of deficiency of the kidney-yin, which has been documented in detail by Bencao Gangmu (Shizhen Li, AD 1578), one of most authoritative Chinese pharmacopeias. Previously, the modern medicinal uses of RPM have been found that include the promotion of hair growth [58], increasing longevity [59], the prevention and treatment of cerebrovascular diseases [60], and anti-osteoporotic ability.

In vivo studies, it was reported that OVX rats experienced 15 days of RPM administration showed an increase in the bone ALP. Subsequently, the level of ALP in serum was elevated by RPM when treating the OVX rats for 30 days, and the TRAP level in RPM group exhibited a decreased trend. Meanwhile, the impaired bone morphological features were significantly improved by the intervention of RPM [61], suggesting the anti-osteoporotic effect of RPM on animal experiments.

In vitro studies, exposure to H2O2 remarkably suppressed the differentiation of MC3T3-E1 osteoblast cells by reducing the genes expression of ALP, collagen I (COL-I) and osteocalcin (OCN), resulting in the increase of dead cells. However, 2,3,5,40-tetrahydroxystilbene-2-O-b-D-glucoside (TSG), one of useful ingredients in RPM, prolonged the survival of cells to maintain their function, which was proven to work by significantly inhibiting the increased levels of RANKL, IL-6, reactive oxygen species (ROS) and lipid peroxide (MDA) that were induced by H2O2 [62]. And the suppressed expressions of osteogenic differentiation genes (ALP, COL-I and OCN) were significantly enhanced by TSG. These evidences suggested that the role of RPM in treating osteoporosis is mainly associated with its antioxidant activities.

Radix Angelicae Sinensis

Radix Angelicae Sinensis (RAS) is the dried root of Angelica sinensis (Oliv.) Diels. RAS is a well-known herb with the efficacy of nourishing the liver-blood. However, according to the principle of traditional Chinese medicine, the homogeny of the liver-blood and kidney-essence or kidney-yin expanded the clinical uses of RAS into treating the deficiency of the kidney-yin, even if RAS has been alleged to be useful for the treatment of deficiency of live-blood [63]. In vitro and in vivo studies have shown that RAS and its extracts exhibited antioxidant activity [64], neuroprotective role in ischemic injury [65], and the improvement of memory impairment in Alzheimer [66]. Additionally, there are a number of reports concerning the uses of RAS in the treatment of osteoporosis in recent years.

In vivo studies, 0.3 g/kg of the RAS extract daily for 4 weeks increased the BMD of the femur in OVX rats, which is similar to that of 17β-estradiol, another excellent conventional agent for deficiency of estrogen. Moreover, RAS extract also significantly reserved OVX-induced excessive expressions of ALP, CTx, and OCN in serum, but it has been verified that the RAS extract did not bring about uterus impairment even if adding the dose of RAS extract to 1 g/kg or 2 g/kg. Results of this study suggested that RAS extract inhibited OVX-induced bone loss without adverse toxicity [67]. In other studies, more successful treatment of osteoporosis induced by estrogen deficiency was further found after administration of the ultrafine RAS when compared with the coarse RAS. Administration of RAS for 8 weeks was effective in up-regulating the level of serum estradiol and reducing the ratio of bone-alkaline phosphatase (BALP) to total-alkaline phosphatase in bone (TALP), especially the ultrafine RAS, indicating the improvement of bone formation. More importantly, the ultrafine RAS reversed the low BMD and the trabecular and cortical bone loss that were induced by estrogen deficiency, and consequently ultrafine RAS attenuated the biomechanics impairment. These positive activities of ultrafine RAS were deemed to be associated with its more active ingredients such as decursin and decursinol than the coarse RAS [68]. In another study, Kim et al. [69] proved the above inference by exploring the effects of decursin on lipopolysaccharide (LPS)-induced bone loss. Results demonstrated that the treatment of decursin effectively inhibited the bone loss induced by LPS by enhancing the BMD, trabecular thickness (Tb.Th) and trabecular number (Tb.N).

In vitro studies, decursin isolated from the RAS inhibited NFATc1 that directly and positively related to RANKL signaling pathway [69]. Since the RANKL signaling pathway are known to have pronounced effects on the differentiation of osteoclasts, the inhibitory activities to RANKL may explain some of the reported effects of RAS extract on osteoclastogenesis. Furthermore, the decursin showed significantly suppressed effects on pre-osteoclasts. This mechanism of action appeared to be via down-regulation of the genes of DC-STAMP and β3 integrin, leading to less fusion and migration of pre-osteoclasts. In another study, Kong et al. and Ahn et al. [70, 71] pointed out that the administration of RAS extract and Angelica tenuissima water extract (ATWE) inhibited the formation of filamentous actin (F-actin) ring and the differentiation of osteoclasts, as determined by reducing the phosphorylation of p38, ERK, JNK, p65, and I-κB, as well as the protein expression of c-Fos, c-Jun and NFATc1. These results indicated that RAS and its extracts will be potential candidate for anti-resorptive agent in osteoporosis via RANKL pathway.

Erzhi Wan

The traditional Chinese formula Er Zhi Wan (EZW) was famous for its tonifying kidney-yin effect that was firstly recorded in Fushou Jingfang (AD 1530). EZW was consisted of FLL and EP at a ratio of 1:1. Besides the pharmacological applications in liver injury [72], menopausal syndrome [73] and breast cancer [74], EZW also showed some potent inhibition effects on bone loss.

In vivo studies, the treatment with EZW for consecutive 26 weeks significantly increased the BMD level of OVX rats by inhibiting the bone loss on femur, tibia and the 4th lumbar vertebra, and ameliorated the impaired trabecular parameters including BV/TV, connectivity density (Conn.D), trabecula number (Tb.N), trabecular thickness (Tb.Th), trabecula separation (Tb.Sp) and structure model index (SMI) in femur, with the results that the OVX-induced osteoporosis was effectively dealt with by EZW. In addition, the bone biomechanical parameters including maximum load, energy, maximum stress and elastic modulus were enhanced by EZW administration, but there was rarely side effects such as the endometrial hyperplasia [75]. Similarly, Sun et al. found that EZW was capable of inhibiting the degradation of trabecular microarchitecture and bone loss in mandibles induced by estrogen deficiency, because of its regulation effects on the important bone metabolism markers for osteoporosis, such as serum E2, bone-specific alkaline phosphatase (BALP) and tartrate-resistant acid phosphatase 5b (TRAP5b). Furthermore, these actions of EZW in osteoporosis induced by estrogen deficiency may be related to the up-regulation of wingless-related MMTV integration site 3a (Wnt3a), lipoprotein receptor-related protein 5 (LRP5), β-catenin expression, and the down-regulation of dickkopf homolog 1 (DKK1) expression [76].

In vitro studies, although EZW-containing serum showed few effect on the proliferation of primary cultural osteoblasts and osteoblast-like UMR106 cells, the proliferation and differentiation of osteoclast precursors RAW264.7 induced by M-CSF and RANKL were positively inhibited by EZW-containing serum administration [77]. These positive effects indicated that EZW was beneficial to inhibit bone resorption and treat osteoporosis.

Siwu Tang

Siwu Tang (SWT) is a classical formula that is widely used for the treatment of gynecological diseases in Chinese medicine. SWT consists of four herbs, Radix Angelicae Sinensis, Radix Rehmanniae Preparata, Radix Paeoniae Alba and Rhizoma Ligustici Chuanxiong (usually in mixture ratio 1:1:1:1). Many evidences have shown that SWT has special clinical significance in the relief of women’s diseases such as emmeniopathy, climacteric syndrome and dysmenorrhea [78, 79]. The latest studies reported that SWT exhibited promising efforts in the treatment of cancers including breast and primary colorectal cancers [80, 81]. Furthermore, due to the valuable efficacy for nourishing the yin and blood, SWT has been verified as a potential treatment method for osteoporosis.

In vivo studies, administration of SWT extract to OVX rats increased the levels of ALP, BMP-2 and OPN in serum that were related to the bone formation, and conversely inhibited the level of C-terminal telopeptides of type I collagen, a biomarker of bone resorption. These results of treatment of SWT extract ultimately brought about a higher BMD in OVX rats, leading to significant inhibition for the bone loss [82].

In vitro studies, Wu et al. [82] found that SWT promoted the differentiation of osteoblast by increasing the differentiation-related genes including BMP-2, ALP, and osteopontin (OPN). Meanwhile, administration of SWT inhibited the protein expression of phosphatidylinositol 3-kinase (PI3K), Akt and NF-κB in osteoblasts, resulting in the stimulation of bone mineralization nodules. There was no evidence to support that SWT affected the osteoblasts viability with examined MTT examination, but the positive action of SWT for osteoblast differentiation suggested a potential therapeutic effect on osteoporosis.

Liuwei Dihuang Wan

Liuwei Dihuang Wan (LWDHW) was initially formulated in Song Dynasty, and has been widely used till now as basic tonic prescription for the deficiency of kidney-yin [83]. It is made up of six crude herbs, including Radix Rehmanniae Preparata, Fructus Corni, Rhizoma Dioscoreae, Cortex Moutan Radicis, Rhizoma Alismatis and Poria. Modern pharmacological studies showed that LWDHW was effective to improve spatial memory and neurogenesis [84]. In other studies, the novel medicinal purposes of LWDHW were found that included the prevention and treatment of diabetes [85], breast cancer [83] and osteoporosis.

In vivo studies, administration of LWDHW (0.4 g/kg body weight) daily for 12 weeks elevated the decreased BMD in right femurs induced by OVX, leading to the attenuation of bone loss. Meanwhile, the estrogen deficiency caused the morphologic impairments of femur including trabecular malalignment, empty bone lacunae and fractures, and they were significantly improved after the treatment of LWDHW. Moreover, the result of biomechanical test in L2 vertebra showed that the administration of LWDHW to OVX rats reversed the suppression of maximum loading as well as elastic modulus. Results of this study indicated that LWDHW is effective to the prevention and treatment of osteoporosis [86].

In vitro studies, LWDHW-containing serum studies demonstrated that administration of LWDHW to osteoblasts significantly increased the cells viability, proliferation and ALP activity. LWDHW also stimulated the formation of mineralization nodules when administered to the osteoblasts. LWDHW enhanced the OVX-mediated downregulation of Lrp-5, β-catenin, Runt-related transcription factor-2 (Runx2) and Osx genes, as well. These results suggested that the up-regulation of the canonical Wnt/β-catenin signaling pathway induced by LWDHW administration produced a stimulation on bone formation that facilitating the treatment of osteoporosis [86]. Further study found that administration of morroniside and loganin isolated from LWDHW stimulated the differentiation of MC3T3-E1 cells in the whole process, as determined by the increased production of ALP and osteocalcin [87]. Meanwhile, the morroniside and loganin inhibited the apoptosis when administered to the osteoblasts. The mechanism of action appeared to be via the inhibition of caspase-3, capase-9 and RANKL genes as well as the upregulation of bcl-2 gene [87]. These studies may explain the mechanism of anti-osteoporotic effect in LWDHW.

Zuogui Wan

Zuogui Wan (ZGW) was firstly conceived and developed by Jingyue Zhang during Ming Dynasty in Jingyue Quanshu that has been listed as an authorized Chinese medicine book. There are 8 ingredients in ZGW: Radix Rehmanniae Preparata, Rhizoma Dioscoreae, Fructus Corni, Fructus Lycii, Semen Cuscutae, Colla Corni Cervi, Carapax et Plastrum Testudinis, and Radix Achyranthis Bidentatae, with mixed weight ratio of 8 : 4 : 4 : 4 : 4 : 4 : 4 : 3 [88]. Recent studies has proven that ZGW exhibited neuroprotective effects on autoimmune encephalomyelitis [89] and hypoglycemic effects on gestational diabetes mellitus [88]. However, the clinical applications of ZGW were extensive due to the excellent effects on nourishing the kidney-essence, which was consistent with the traditional Chinese medicine theory in Huangdi Neijing that the kidney is responsible for the birth, growth and reproduction in body, including bone growth of course. So, ZGW is indispensable to researchers interested in the prevention and treatment of osteoporosis with traditional Chinese formula. Actually, a number of studies are trying to clarify how ZGW managed to inhibit bone loss and benefit the osteoporosis.

In vivo studies, it has been reported that ZGW was capable of improving OVX-induced rat model of osteoporosis by up-regulating the levels of BMD and serum calcitonim (CT). It was believed that the inhibition on high bone turnover rate of BGP and TRAP in serum may partly explain the mechanism of action for ZGW [90, 91]. Another supplementary study found that ZGW with their ability to inhibit the bone loss was largely responsible for the downregulation of TGF-β1/Smad 4 gene [92]. Since the glucocorticoid can exert a rapid and powerful damage in rat bone histomorphology, glucocorticoid-induced rodents are developed as a stable and repeatable osteoporotic model. It has been reported that as long as 8 weeks for intraperitoneal injection of glucocorticoid, the rat tibia tissue showed the reduction of trabecula volume percent (TBV%) and trabecular forming surface percent (TFS%), as well as the enhancement of trabecula absorption surface percent (TRS%), indicating a worse bone histomorphology. But, these impairments caused by glucocorticoid were ameliorated by administration of ZGW [18]. However, the mechanism of action may be further understood by other studies that ZGW upregulated glucocorticoid-induced decrease in serum bone r-carboxyaluatamic-acid-containing proteins (BGP), insulin-like growth factors (IGF-I) and E2, and inhibited the increased serum parathyroidhormon (PTH), as well [93, 94].

In vitro studies, administration of ZGW mainly regulated the relevant protein of osteoblasts and MSCs in canonical Wnt/β-catenin signaling pathway including Wnt1, LRP-5 and β-catenin, resulting in promoting the differentiation, development and proliferation of osteoblasts that facilitated to maintain the normal bone metabolism [18]. These studies may help to understand the mechanism of anti-osteoporotic effect in ZGW.

Conclusion

In summary, with the increasing number and accelerated aging in the world population, osteoporosis has become an urgent healthy and social problem. However, the side effects of current agents to treat osteoporosis promote the development of new drugs. The tonifying kidney-yin herbs and formulas have been identified as the effective anti-osteoporosis agents with few adverse effects. Thus, the present study comprehensively reviewed the effects of tonifying kidney-yin herbs and formulas on improving bone density, bone microstructure, biomechanics and other parameters as well as their potential mechanisms. Their relevant applications and mechanism were described in detail in Tables 2 and 3. But the mechanisms of the imbalance between bone formation and resorption in osteoporosis are really complicated. In addition to the canonical Wnt pathway and the RANKL/RANK pathway (Fig.1), some papers have reported that there were several other signaling pathways liking BMP/Smad signaling pathways [95], TGF-β signaling pathways [96], PTH pathways [97] and Notch signaling pathways [98] being involved in the metabolism of bone. And some tonifying kidney-yin herbs and formulas exhibit duplicate effects via different pathways on the treatment of osteoporosis. Therefore more studies examining the intricate process of anti-osteoporotic effects with particular pathway in kidney-yin herbs and formulas are required. However, this current study will contribute to understand the functions of tonifying kidney-yin herbs and formulas in inhibiting osteoporosis, and also provide direct evidence for promoting the use of tonifying kidney-yin herbs and formulas in the treatment of osteoporosis.

Table 2 Anti-osteoporotic effects of tonifying kidney-yin herbs and formulas in vivo studies
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Table 3 Anti-osteoporotic effects of tonifying kidney-yin herbs and formulas in vitro studies
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Fig. 1
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Some tonifying kidney-yin herbs and formulas regulate bone metabolism via Wnt pathway and RANKL/RANK pathway

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