Introduction

Osteoporosis is the most common type of bone disorder, resulting from a disruption of balance between bone formation and resorption. It is characterized by degeneration of bone microstructure, reduction of bone mass, and leads to a considerably higher increased risk of fractures [1]. The estimated number of individuals with osteoporosis and osteopenia, the precursor to osteoporosis, continues to increase, especially with the aging of the population [2]. Osteoporosis poses a serious public health problem, particularly threatening postmenopausal women and senior citizens. The total number of bone fractures due to osteoporosis reported in the USA exceeded 2 million in 2005, costing nearly $17 billion, with indirect and direct costs expected to increase substantially annually between 2005 and 2025. Currently, bone mineral density (BMD) scans which utilize dual-energy X-ray absorption (DEXA) and provide two-dimensional information for an area of the bone, are the most commonly used method for the diagnosis of osteoporosis. Quantitative computed tomography (QCT) has the advantage of detecting the volumetric, rather than areal, BMD, allowing measurements of bone geometric parameters and regional bone strength, leading to improved sensitivity and accuracy of BMD measurements. Finite element analysis (FEA) utilizes high-resolution images and three dimensional models to analyze the changes in bone strength under different conditions, and to predict the load and location of potential fractures [3]. Other than BMD, several characteristics of the bone, including degree of mineralization, hydroxyapatite crystal size, collagen structure, heterogeneity of bone microstructure, connectivity of trabeculae, and microdamage, are of clinical significance.

Bone remodeling occurs continually and is mediated through the coupled cycle of bone resorption and bone formation. Bone loss and the resultant osteopenia and osteoporosis occur due to an imbalance in this process. At the cellular level, osteoclasts promote bone resorption by secreting acids and enzymes that disassemble and digest bone mineral and proteins, while osteoblasts promote bone formation by creating a protein matrix consisting primarily of collagen that is soon calcified, resulting in mineralized bone. Strategies to prevent and treat osteoporosis include general preventive measures (such as diet and exercise) and pharmacologic interventions [4]. Drugs commonly used in clinical settings are divided into two categories: (1) bone resorption inhibitors, such as hormone (estrogen) therapy, estrogen agonists, calcitonin, bisphosphonates, and isoflavones, which exert their effect by interfering osteoclast resorption activity; and (2) bone formation promoters, such as fluoride and parathyroid hormone, which enhance bone mass and strength through stimulating osteoblast activity. However, clinical use of most anti-resorption drugs is limited due to the adverse side effects of suppressing bone formation in the long term [5]. For example, although short-term (3 years or less) bisphosphonate use appears to be well-tolerated in children and adolescents, adverse side effects after long-term use of bisphosphonates have been reported, including the upper gastrointestinal bleeding, acute phase response, hypocalcaemia and secondary hyperparathyroidism, musculoskeletal pain, osteonecrosis of the jaw, and ocular events [6]. The most commonly reported adverse reaction of Calcitonin (Miacalcin, Fortical, Nasal calcitonin) is nasal irritation [7]. Subcutaneous injections of teriparatide (Forteo) and abaloparatide (Tymlos), the parathyroid hormone medications currently FDA-approved for osteoporosis treatment, are highly effective in building the bone and reducing fractures, although teriparatide is very costly (approximately $850 per month) and cannot be used for more than 2 years due to the tumorigenic potential observed in rats [8]. Similarly, abaloparatide, like teriparatide, is subject to a total treatment duration limit of 24 months, due to the potential risk for osteosarcoma incidence, especially in susceptible individuals [9, 10].

More researchers are turning to traditional Chinese medicines to search for preventive or alternative therapies to treat osteoporosis. Increasingly, natural products, particularly flavonoids, are being explored for their therapeutic potentials in reducing bone loss and maintaining bone density. Icariin, 2-(4′-methoxylphenyl)-3-rhamnosido-5-hydroxyl-7-glucosido-8-(3′-methyl-2-butylenyl)-4-chromanone, is the most abundant flavonoid constituent in Herba Epimedii, and has been shown to be effective for bone regeneration and repair [11]. H. Epimedii is a centuries-old traditional Chinese medical plant used in the treatment of fractures, joint disease, and gonadal dysfunctions [12]. Naturally isolated icariin is fast becoming an attractive alternative in the prevention and treatment of osteoporosis. Commercially purified icariin used in most research studies is commonly extracted using high-performance liquid chromatography (HPLC). Evidence is steadily accumulating that icariin may play a dual role in bone health by stimulating bone formation while simultaneously inhibiting bone resorption [13]. Importantly, icariin can be steadily and locally released, using biomaterials, making it an attractive osteoinductive candidate for bone tissue engineering [14,15,16]. Angiogenesis also plays an important role in bone regeneration, and it has been reported that icariin could increase angiogenesis via stimulating endothelial cell migration, proliferation, and tubule genesis in vivo [17]. In this review, we update and summarize the current model of the bone remodeling cycle, the role of icariin in bone formation and bone resorption, its pharmacokinetics and pharmacological effects, and compare its efficacy to other flavonoids.

Icariin: activation of bone remodeling

Bone remodeling is comprised of two basic physiological processes—osteoblastogenesis and osteoclastogenesis [18, 19]—which comprise a highly coordinated, dynamic, and continual physiological cycle of bone formation and bone resorption [20]. The process of bone remodeling is respectively executed by two distinct lineage cells—osteoblasts and osteoclasts. Osteoblasts are derived from bone marrow-derived mesenchymal stem cells (BMSCs) and mature into specialized, terminally differentiated osteocytes [21]. The group of osteoblasts and the bone matrix form the osteon, the basic structural unit of mature bones. Osteoclasts, originating from monocyte/macrophage hematopoietic progenitors in the bone marrow, are multinucleated cells which secrete acids and collagenolytic enzymes to resorb the bone [22]. Osteoclast differentiation and activity are dependent on the receptor activator of nuclear factor kappa-B (NF-kB) ligand (RANKL), secreted by osteoblasts and BMSCs [23] (Fig. 1).

Fig. 1
figure 1

Icariin stimulates bone formation by promoting osteoblastogenesis and the bioactivity of the osteoblasts. Icariin, a prenylated flavonol glycoside isolated from Epimedium herb, increases the number of BMSC-derived osteoblasts and augments the maturation of pre-osteoblasts, meanwhile inhibits the adipocytic transdifferentiation of primary osteoblasts. Icariin-mediated osteogenesis is the result event of multiple signaling transduction pathways, including up-regulation of BMP, NO, MAPK, Wnt pathways, and down-regulation of ERK and JNK pathways. Icariin also can exert the osteogenic effect by inhibiting the ROS generation

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Normal bone remodeling requires balance between bone formation and bone resorption, but this balance is disrupted under certain circumstances, such as estrogen deficiency following menopause or in some autoimmune diseases [24, 25]. Estrogen plays a role in bone remodeling, through ERs (estrogen receptors) in the osteoblasts and osteoclasts, with ERα mediating the predominant effect [26]. Estrogen can enhance the proliferation and maturation of primary osteoblasts, inhibit their apoptosis, and enhance their bone-inducing activity [27], while it also suppresses osteoclastogenesis. Inhibition of the ER signaling pathway resulted in bone loss in a mice model [28]. Estrogen is also associated with osteoimmunology, and its deficiency leads to augmentation of activated T cell activity, osteoclastogenesis, and increased bone resorption [29]. Icariin and its derivatives can restore the balance of bone remodeling through an estrogen mimetic effect which regulates osteogenic progenitor cell fate commitment, proliferation, maturation, and matrix mineralization [13].

Icariin stimulates the osteogenic differentiation of BMSCs into osteoblasts

BMSCs are multipotent stem cells located within the bone marrow stroma and have the potential of giving rise to several cell linages, including osteoblasts, chondrocytes, adipocytes, cardiomyocytes, and endothelial cells [30]. Thus, BMSCs provide a primary source of osteoblasts. Icariin treatment of pre-osteoblastic MC3T3-E1 cells and mouse primary osteoblasts in vitro promotes the expression of osteoblast marker genes, Runx2 (runt-related transcription factor 2), and Id-1 (inhibitor of DNA-binding 1) [31]. Icariin also enhances the self-renewal ability, and augments the osteogenic differentiation of BMSCs in vitro and in vivo in female Sprague-Dawley rats (6 months old) [32, 33]. However, it should be noted that the skeleton of 6-month-old rats is still growing and has not reached its peak bone mass; therefore, 9-month-old ovariectomized female rats are recommended as a more accurate model for simulating postmenopausal osteoporosis [34].

Multiple signaling pathways are involved the osteogenic differentiation, including BMP (bone morphogenetic protein), NO (nitric oxide), MAPK (mitogen-activated protein kinase), and the canonical Wnt/β-catenin pathways [31, 35,36,37,38]. BMP-4 is induced under icariin treatment and subsequently initiates the BMP signaling pathway, as established in multiple cell models, including human MSCs and mouse-derived pre-osteoblastic MC3T3-E1 and C3H10T1/2 MSCs, and wild-type C57BL/6N mouse primary osteoblasts [31]. The BMP-2/Smad4 signal transduction pathway is reported to be activated by icariin in both human osteoblastic hFob1.19 and murine osteoblastic MC3T3-E1 cell lines [39, 40]. Treatment with NO pathway inhibitors diminishes the osteogenic effect of icariin in rat primary bone marrow stromal cells in vitro [35]. In MC3T3-E1 cells, icariin induces ERK (extracellular signal-regulated kinase) and JNK (c-Jun N terminal kinase) activation, but p38 kinase activity is not affected [36]. Blocking estrogen-mediated signaling attenuates icariin’s bone-inducing effects [36]. Icariin can also induce β-catenin mRNA expression, β-catenin subcellular redistribution, and enhances the GSK-3β (glycogen synthase kinase-3) phosphorylation levels in rat BMSCs [37]. The mechanism of activation of these signaling pathways perhaps arises from the estrogen mimetic properties of icariin [41, 42] and induced production of estrogen by icariin [43].

It appears that icariin also promotes human BMSCs differentiation by epigenetic regulation. ROS (reactive oxygen species) are one of the factors controlling human BMSCs differentiation, and excessive ROS promotes adipogenic differentiation, and is clinically associated with corticosteroid-induced osteonecrosis [44]. Intracellular ROS levels, MMP (mitochondrial membrane potential), P-gp (P-glycoprotein) activity, methylation of ABCB1 (ATP-binding cassette subfamily B member 1), and other important indices of oxidative stress are significantly up-regulated in BMSCs of patients with steroid-associated osteonecrosis [45]. However, treatment with icariin alleviates oxidative stress and reduces CpG island hypermethylation of ABCB1 in BMSCs isolated from steroid-associated osteonecrosis of femoral head (ONFH) patients [45]. One study suggested that icariin can also protect DNA from excessive oxidative stress in an AAPH (2,2′-azobis(2-amidinopropane) dihydrochloride)-induced oxidative damage of DNA model [46], although apparently, the 10−5–10−4 M concentration adopted in the study is cytotoxic, and the extent to which icariin protects DNA from oxidative damage is debatable in cell and animal models. The osteogenic differentiation of human BMSCs induced by icariin is dose-dependent [32, 47], and the optimal concentration in vitro ranges from 10−9 to 10−5 M, with concentrations above 10−5 M resulting in cytotoxicity [32].

Icariin suppresses adipocytic transdifferentiation of primary osteoblasts

BMSCs are multipotent and have the potential of giving rise to both adipocytes and osteoblasts [48]. Icariin increases the number of mature osteoblasts simultaneously through an alternative pathway, by suppressing the adipocytic transdifferentiation of primary osteoblasts [49, 50]. Adipogenesis-related genes, peroxisome proliferator-activated receptor γ (Pparg), and CCAAT/enhancer-binding protein β (Cebpb) genes are down-regulated when the primary osteoblasts are treated with icariin, and inhibition of adipogenic transdifferentiation leads to further osteoblastic differentiation [49].

Icariin stimulates the bone formation activity of osteoblasts

Icariin also can facilitate the maturation of primary osteoblasts and bone remodeling activity of osteoblasts. Icariin treatment induces expression of terminal differentiation markers, ALP (alkaline phosphatase) and Col I (collagen type I), and mineralization of osteoblasts [36, 51,52,53]. In addition, icariin attenuates cell apoptosis and preserves cell viability in rat calvarial osteoblasts exposed to hypoxic conditions (2% oxygen) by counteracting the effects of oxidative stress [53]. In the glucocorticoid-induced osteoporosis Sprague-Dawley rat model, the bone mass increase observed with icariin (125 mg/kg, daily for 12 weeks, i.g.) treatment was comparable to that of alendronate (0.3 mg/kg daily for 12 weeks, i.g.), significantly increasing ALP, a bone formation marker, and reducing CTX (carboxy-terminal collagen cross-links), a bone resorption marker. Furthermore, icariin displays a robust anti-apoptotic effect and a concentration of 10−7 M was shown to completely annihilate dexamethasone-induced apoptosis in osteocytes [51].

Icariin regulates bone homeostasis mainly by activating the ER and ERK signaling pathways, simultaneously inducing the mRNA expression of OPG (osteoprotegerin) and activating the Wnt signaling pathway. Icariin has been shown to activate ER by phosphorylation at Ser 118 and Ser 167 and prevent glucocorticoid-induced apoptosis in osteocytes by activating ERK signaling via ER. OPG, a decoy receptor of RANKL, also known as OCIF (osteoclastogenesis inhibitory factor) or TNFRSF11B (tumor necrosis factor receptor superfamily member 11B), belongs to the TNF superfamily and plays an essential role in restraining bone loss by counteracting the effects of RANKL [54, 55]. The anabolic response of trabecular bone was diminished in OPG knockout C57BL/6J mice when they were orally administered icariin (300 mg/kg) every day for 8 weeks [56]. Another report shows that icariin, 5 mg/kg per day by local injection over the calvaria surface of OPG-deficiency mice, induces bone formation and reverses the osteopenia phenotype [57]. In vitro experiments on BMSCs were conducted using the bone marrow of the femur and bilateral tibia of C57/BL6 mice, and showed that treatment with 50 μM icariin for 2 days induces the activation of Wnt/β-catenin and BMPs/Smads/Runx2 signaling, and augments the expression of signaling pathway associated genes, including BMP2, BMP4, RUNX2, OC, Wnt1, Wnt3a, AXIN2, DKK1, TCF1, and LEF1 [57]. Although the dosages adopted by the two research groups are incomparable due to different administration protocols, it is reasonable to speculate that icariin functions in a dosage-dependent manner, mediated by both OPG and ERs.

Icariin: inhibition of bone resorption

Icariin inhibits osteoclastogenesis by suppressing osteoclastic differentiation

Inhibition of osteoclastogenesis partially contributes to the anti-osteoporosis efficacy of icariin and its derivatives. Tartrate-resistant acid phosphatase (TRAP) is commonly utilized as an osteoclast differentiation marker. Icariin treatment of osteoclast precursor cells (isolated from 8-month-old female imprinting control region (ICR) mice) leads to a significant decrease of TRAP-positive multinuclear cells in a dose-dependent manner [58], and icariin directly quells the RANKL-induced differentiation of hemopoietic cells from which osteoclasts are formed [59]. Icariin and its derivatives increase the anti-osteoblastogenic mRNA expression of ALP, OC (osteocalcin), COL-1 (typeIcollagen), and OPG, suppressing that of RANKL in primary osteoblasts, resulting in an indirect suppression of osteoclastic differentiation [60]. Icariin also prevents mRNA expression of inflammatory cytokines and inflammation-induced progenitor osteoclast differentiation [58]. Besides regulating osteoclastic differentiation, icariin induces a pause in the cell cycle of precursor osteoclast cells, leading to apoptosis. Icariin treatment of RAW 264.7 (a mouse-derived cell line that has the potential of differentiation into osteoclast upon RANKL induction) leads to G2/M cell cycle arrest, inhibition of proliferation, and apoptosis, in an ER-dependent manner [61] (Fig. 2).

Fig. 2
figure 2

Icariin inhibits bone resorption by suppressing osteoclastogenesis and the bioactivity of the osteoclasts. Icariin acts upon monocyte/macrophage-derived cells (pro-osteoclasts) and inhibits their differentiation by regulation of RANKL, OPG, ALP, and Col-1. Bone resorptive activity of osteoclasts could be inhibited by icariin. Meanwhile, another way of reducing bone loss could be through the alleviation of inflammatory signaling pathways by icariin

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Icariin inhibits the osteoclast activity

Icariin (10−8 M) has an inhibitory effect on osteoclast activity by suppressing inflammatory signaling pathways, such as p38, ERK, NF-κB, and JNK in primary osteoclasts isolated from 8-month-old female ICR mice [58]. A notable decrease of osteoclastic resorption area was observed after the osteoclasts isolated from Sprague-Dawley fetal neonatal rats were treated with icariin and its derivatives at concentrations of 10−5–10−8 nM [60], indicating that icariin undermines osteoclastic activity by restraining motility and bone resorption. In vitro experiments in 1~2-day-old Japanese white rabbits show that icariin can also suppress the activities of osteoclasts by eliciting the decline of superoxide anion (·O2−) generation, decreasing size and number of actin rings, and intracellular calcium concentration [62].

Icariin: another way of reducing bone loss by regulating immune response?

Bone remodeling and the immune system are closely linked. Normal bone remodeling is disrupted in autoimmune diseases such as arthritis, and activated T cells can directly lead to the production of OPGL, a ligand of OPG, and subsequent bone loss [63]. Inflammatory signaling pathways also induce primary osteoclast maturation and bone resorption activity [64]. Clinically, bone loss is closely associated with estrogen deficiency in ovariectomized mice, as well as in postmenopausal women. Icariin can increase biosynthesis of estrogen and activate the ER-mediated signaling pathways [36, 43]. Since estrogen and ERs are also involved in modulating immune responsiveness [65], it is reasonable to speculate that icariin could regulate the immune response, and exert its effect on bone formation via an ER-dependent way.

Immune responses activated through the LPS (lipopolysaccharide) pathway also result in osteoclastogenesis and bone loss. Icariin treatment was shown to prevent immune-related bone loss, reduce RANKL expression levels, and enhance OPG expression levels in a New Zealand rabbit model of antigen-induced arthritis [66]. Icariin can inhibit the proliferation of CD4+ T cells stimulated with mitogens, or specific antigen ovalbumin, and can suppress Th1 and Th17 cell differentiation, and inhibit cytokine production in mice [67, 68]. These studies indicate that icariin can potentially prevent inflammatory bone loss by inhibiting the immune response.

Clinical trials and extended applications of icariin

Novel therapeutic methods based on icariin are being developed

The use of icariin in the prevention and treatment of osteoporosis has been studied [69,70,71,72]. A 24-month randomized double-blind placebo-controlled clinical trial in healthy, late postmenopausal women, was conducted to explore the efficacy of icariin in preventing bone loss. Results show that, compared to the placebo group (n = 50), the intervention group (n = 50, a daily dose of 60 mg icariin, 15 mg daidzein, and 3 mg genistein) had a significantly reduced bone loss. Treatment with icariin maintained BMD at 12 months (femoral neck 1.1%, p = 0.285; lumbar spine 1.0%, p = 0.158) and 24 months (femoral neck 1.6%, p = 0.148; lumbar spine 1.3%, p = 0.091) [69]. A long-term (up to 12~24 months) administration of icariin products resulted in an improved BMD in the lumbar spine and femoral neck in a time-dependent manner. However, the effect of icariin in maintaining BMD was not impressive, as the improvement in BMD was less than half of that shown to occur in studies of similar duration, investigating estrogen replacement or treatment with bisphosphonates, and small compared to the effects of PTH (parathyroid hormone) treatment. However, there is no incidence of breast cancer and cardiovascular events in the postmenopausal women after icariin treatment for 2 years [69]. In contrast, long-term compliance with ERT (estrogen replacement therapy) was poor, due to its malignant effect in reproductive organs [70, 71] and potential risks for cardiovascular diseases [73]. A recent longitudinal follow-up study validates these controversial side effects and shows that ERT resulted in an increase in cancellous bone volume, together with an increase in endometrial thickness [74]. These findings position icariin as an attractive alternative therapy due to its low side effects. In another 24-month randomized double-blind and controlled clinical trial in osteoporosis patients (consisting of 36 males and 324 females, aged 50~70, treatment group n = 360, control group n = 120), Epimedium total flavone capsule showed higher efficacy in terms of symptom relief, measured by scores for back pain and leg pains (90.83 to 75.00%), and ratio of BMD enhancement (47.38 to 34.23%), compared to Gusongbao capsule (approved to prevent and treat osteoporosis by the State Food and Drug Administration, China, 2010) [72]. Adverse events reported in this study included rash, constipation, diarrhea, cardiopalmus, tinnitus, and gastrointestinal dysfunction, with an incidence rate of 6.67% compared to the control group of 5.00% [72]. Though icariin has been tested in animal models of bone loss [33, 51, 75,76,77], to date, no studies have been reported on icariin tested in a non-rodent model. FDA guidelines (1994) recommend potential therapeutic agents to be tested in at least two animal species, including one rodent species and a second, non-rodent model. In addition, further clinical trials on a larger scale and novel delivery systems are needed to explore the efficacy of icariin and its derivatives on bone formation and regeneration in humans, as well as any possible occurrences of side effects with long-term usage.

Thus, novel therapeutic methods utilizing icariin are being developed for bone regeneration and treatment of osteoporosis. An enzymatic hydrolysis method using β-glucosidase has been developed to produce icariside II, the main effective component in vivo after administration of icariin [78]. Further research focuses on the development of drug delivery systems for icariin. Multiple materials have been adopted as scaffolds to deliver icariin, including chitosan/nano-sized hydroxyapatite, porous PHBV (poly 3-hydroxybutyrate- co-3-hydroxyvalerate), gelatin/hyaluronic acid composite microspheres, and small intestine submucosa [14, 79,80,81]. The evidences from these studies are limited, and further exploration for the novel delivery systems of icariin should be addressed in the future.

Icariin improves the bioactivity and biocompatibility of bone implant materials

Periprosthetic osteolysis (PIO), which can lead to implant instability and failure, is a major orthopedic problem after total joint arthroplasty (TJA) [82, 83]. Icariin effectively induces bone formation and inhibits bone resorption in a murine macrophage cell line (RAW264.7) induced by titanium (Ti) particles [84]. In addition, icariin exhibits bone-protecting effect, increases bone mass, and decreases bone loss in titanium-particle-induced osteolytic sites in a C57BL/6 mouse calvarial model [85]. Icariin was shown to improve the biocompatibility of Ti substrates in another pilot study. Ti nanotubes were loaded with icariin and sealed with a chitosan/gelatin multilayer coating. The fabricated Ti nanotube adjusted the icariin release profile, modulated the biocompatibility of Ti substrates, and elicited bone formation efficacy in primary osteoblasts isolated from 3-day-old Sprague-Dawley (SD) rats [86]. These studies show that icariin has the potential of being developed as a complementary or substitutive therapy for revision surgery. Research also shows that an expanded application of icariin could enhance the bioactivity and biocompatibility of porous b-TCP (b-tricalcium phosphate) ceramic disks. Porous b-TCP ceramics have been widely utilized as bone substitution material in clinic, and a 3-month investigation showed that icariin loaded onto b-TCP ceramics induced new bone formation after intramuscular implantation of Wistar Albino rats [87].

Pharmacological effects and pharmacokinetic properties of icariin

The metabolites of icariin in human are icaritin and desmethylicaritin [88]; it is the metabolites themselves that bind to the ER and exert the bone-protective effect [13]. Sprague-Dawley rats (ovariectomized at 6 weeks) were administered with a standard extract of traditional Chinese medicinal plant Epimedium (300 mg/kg body weight), and icariin, icariside I, icariside II, icaritin, and desmethylicaritin were detected in their sera [89]. An additional metabolite, demethylicaritin, was detected in rats, showing that the metabolism of icariin is species-specific [89], and that a comprehensive metabolite profile needs to be further established in human. The pharmacokinetic profile showed that icariin and icariside II reached t max 0.5–1 h, icariside I, icaritin, and desmethylicaritin peaked at t max 8 h, and micromolar levels of icaritin (clinically irrelevant high dosage) were detectable 72 h after administration in rats [89]. After administration of an aqueous decoction of H. Epimediumii, icaritin but not demethylicaritin was detectable from 1 h, reaching a peak at 8 h (1.51 ± 1.6 nM) in sera of human volunteers [88]. These data are preliminary, and the clinical pharmacological effects and pharmacokinetic properties of icariin and its derivatives have not yet been fully elucidated, warranting further research.

The intestinal absorption mechanisms of icariin-related flavonoids were examined in the human intestinal Caco-2 cell model and the perfused rat intestinal model. Results show that flavonoids were absorbed by intrinsic permeation and transporter-mediated efflux [90]. The report also shows that heating processes help preserve the bioactivity of flavonoids [90].

Comparison of bone-protective efficacy between icariin and other flavonoids

Certain flavonoid compounds including epimedin and genistein have the potential to promote osteogenesis, and these bioactive constituents exert bone-protective effect in a similar mechanism to icariin. Epimedin is also isolated from H. Epimedii and promotes osteoblast proliferation and maturation in an ER-dependent manner [91, 92]. Genistein is a flavonoid rich in soy and exhibits a similar bone-protecting effect in an ER-dependent manner [93]. One report suggested that icariin is more potent than genistein in terms of osteogenic potential [94].

Conclusions and prospects

Osteoporosis is a bone disease where bone loss generates porous and weak bone structure, caused by the disruption of the balance of bone formation and resorption. Icariin is an osteoinductive flavonoid compound with a mimetic property of estrogen that induces bone formation and inhibits bone resorption. At the in vitro level, icariin stimulates the osteogenic differentiation of BMSCs and suppresses the adipogenic transdifferentiation of primary osteoblasts; at the same time, icariin inhibits bone loss by suppressing osteoclastogenic differentiation, immune response, and bone resorption activity. Concurrently, the efficacy of induction of osteogenesis and inhibition of bone resorption has been studied at the in vivo level. However, the limitations of animal models in evaluating the therapeutic efficacy and pharmacological properties of icariin should be taken into consideration. For example, due to the small size of rodents, their skeleton consists of a proportionally smaller portion of cancellous bone mass and a larger portion of cortical bone mass, compared to their human counterparts [95]. Meanwhile, species differences at the cellular and biochemical levels may also negatively influence the usefulness of animal models of osteoporosis. There are also characterized differences between human and mouse physiology regarding the actions of estrogens and estrogen analogs [96]. Animal studies use very high dosage of icariin, which make the results clinically irrelevant; therefore, information generated from skeletal studies of rodents should be approached with extreme caution. The anti-osteoporotic activity of icariin and its derivatives needs further verification using other mammalian models, primates, or human clinical data. Meanwhile, the continued investigation of the pharmacological effects and pharmacokinetic properties of icariin and its derivatives in humans, contributes to the development of icariin-related therapies.

Icariin is a potentially useful treatment for bone regeneration, in view of its osteogenic bioactivity. Pharmacological evidence based on animal models has been accumulating about the mechanisms by which icariin regulates osteoblastogenesis and osteoclastogenesis, but the efficacy of prevention and treatment of osteoporosis is lacking of strong evidence in clinical trials. Furthermore, icariin provides a plausible and intriguing prospect for treating autoimmune-induced bone loss, and further research on the molecular mechanisms of icariin regulating immunity may help expand its application in treating bone diseases.