Vitamin D and bone health: from physiological function to disease association

Abstract

Vitamin D (VD) is a pleiotropic secosteroid hormone with well-established roles in calcium homeostasis, bone metabolism, and emerging functions in immune regulation, inflammation, and chronic disease modulation. In this paper, we provide a comprehensive summary of the current research on the significance of VD for bone health, with emphasis on its mechanism of action and its clinical significance in bone health. This review starts with an overview of VD metabolism, with emphasis on the enzyme transformation of vitamin D3 (VD3) and vitamin D2 (VD2) into the active 1,25-dihydroxyvitamin D (1α,25(OH)₂D) and their genomic and non-genomic signaling pathways through the Vitamin D receptor (VDR). Then, we discuss how VDR polymorphisms affect disease susceptibility and the dual role of VD in promoting innate immunity as well as inhibiting over-adaptive immunity. Our main focus is placed on VD’s involvement in bone destruction diseases, including osteoarthritis (OA), osteoporosis (OP), rheumatoid arthritis (RA), and bone tuberculosis. For OA, there is conflicting evidence on whether VD supplementation reduces cartilage degradation or pain. In OP, vitamin D deficiency aggravates bone loss, but the effectiveness of supplementation is dependent on baseline and calcium supplementation. For RA, the immunomodulatory effects of VD may decrease the activity of the disease, whereas in tuberculosis, VD increases the clearance of macrophage-mediated mycobacterial clearance, although the clinical study data are still inconclusive. This review underscores VD as a critical mediator of bone-immune crosstalk while calling for rigorous translational research to clarify its therapeutic potential across diverse diseases.

Introduction

Vitamin D (VD) is a secosteroid hormone that plays a crucial role in calcium absorption and bone mineralization. There are two primary forms of VD: VD3, which is important for animals and synthesized in the skin, and VD2, which is produced by plants [1]. The primary source of VD is from exposure of the skin to ultraviolet B (UVB) radiation. Additionally, VD can be obtained from limited dietary sources, such as egg yolks [2]. In addition to its crucial role in calcium absorption and bone mineralization, VD is essential for the proper functioning of the immune system [3]. Research indicates that VD significantly impacts protection against bacterial and viral invasions [2]. Laboratory studies and epidemiological data have demonstrated a correlation between vitamin D deficiency and various conditions, including skeletal diseases, inflammatory bowel diseases, respiratory tract infections, and autoimmune disorders. As research on VD progresses, an increasing number of its functions have been identified, which is now recognized as an important signaling molecule involved in numerous significant physiological processes. Whether it is VD3 or VD2, both forms associate with the VD-binding protein (VDBP), which facilitates their transport to the liver [4]. Importantly, various cytochrome P450 (CYP) enzymes found in the Liver and kidney have been recognized for their involvement in the conversion of VD into 1α,25(OH)₂D [5, 6]. The initial hydroxylation of VD occurs mainly in the liver, although this process is not exclusive to it [7]. VD undergoes hydroxylation at the C25 position, resulting in the formation of 25-hydroxy VD [25(OH)D], the primary circulating metabolite [8]. This mechanism is characterized as non-regulated and substrate-dependent [7]. Subsequently, 25(OH)D is transported to the kidney via VDBP, where it dissociates from VDBP before being taken up by renal proximal tubule cells [4]. The second hydroxylation occurs at the carbon in the C1 position, resulting in the formation of 1α,25(OH)₂D, which is the biologically active form of VD [9].

The biological activity of 1α,25(OH)₂D as signaling molecules and metabolic regulators is mediated through its binding to the vitamin D receptor (VDR), which is a member of the nuclear receptor superfamily and functions as a transcription factor [10]. The VDR is found in a variety of cell types and is expressed in over 38 tissues, including keratinocytes in the skin, T lymphocytes, bone marrow macrophages, monocytes, intestines, bones and kidneys [11]. VDR contains two critical domains: a DNA-binding domain (DBD) and a ligand-binding domain (LBD) [12]. The LBD is capable of forming a Ligand-binding pocket that effectively binds 1α,25(OH)₂D, which subsequently induces a conformational change in the VDR [10]. Research has demonstrated that the downstream network of genes regulated by the 1α,25(OH)₂D-VDR complex is linked to the cancer, immune system and metabolic processes [13]. The primary biological function of VDR signaling is to regulate intestinal calcium absorption through 1α,25(OH)₂D, thereby maintaining calcium homeostasis, which is crucial for preventing rickets. Recent studies have identified the VD/VDR pathway as crucial for maintaining intestinal homeostasis and regulating the interactions between microbes and their hosts [14]. Mice without VDR gene exhibited sever inflammatory bowel diseases (IBDs) and alternations of the microbiota [14]. Furthermore, 1α,25(OH)₂D-VDR complex regulates the expression of at least 11 genes that are essential for skeletal and mineral homeostasis, with implications for promoting healthy aging [13].

The function of the VDR in combination with co-activators relies on the binding of 1α,25(OH)₂D and the involvement of the retinoid receptor (RXR) [13]. Upon binding with 1α,25(OH)₂D, the VDR undergoes a conformational change that facilitates its interaction with RXR [10, 13]. The VDR-RXR heterodimer complex allows for the recognition of VD response elements (VDREs) in the DNA sequences of genes regulated by VD [10, 13]. Ultimately, the activated VDR-RXR-DNA complex regulates the transcription of protein-coding genes by recruiting various co-activator and co-repressor complexes, including PU.1, CEBPα, GABPα, ETS1, as well as chromatin modifiers and remodelers to target at genes including osteocalcin, CAMP, CYP24A1 and Atg16l1 [10].

VD is an essential hormone that facilitates calcium absorption and bone mineralization, which are positively correlated with bone mineral density (BMD). The metabolite of VD3, 1α,25(OH)₂D, enhances the mobilization of bone minerals by promoting intestinal calcium absorption [15]. The microenvironment of osteoblasts plays a pivotal role in determining the final outcomes of 1α,25(OH)₂D action [16]. The effects of 1α,25(OH)₂D on osteogenic cells are diverse and can lead to either bone resorption or formation, a process that may be affected by the developmental stage of osteoblastic maturation [17, 18]. However, 1α,25(OH)₂D also supports osteoclastogenesis by binding to the VDR in osteoblasts, thereby enhancing the expression and release of nuclear factor-κB ligand (RANKL) from osteoblasts and this interaction increases the receptor activator for RANKL to osteoprotegerin (OPG) ratio, influencing bone resorption [4, 11, 16]. Additionally, other factors such as parathyroid hormone (PTH) and interleukin (IL) −6 also act on osteoblasts to induce RANKL expression, promoting osteoclast differentiation [11]. In conclusion, VD is widely involved in the maintenance and regulation of bone homeostasis.

The component and functions of VD

VD is derived from two main sources: the endogenous pathway and the exogenous pathway. The synthesis of VD is influenced by several factors, with geographical location being a key determinant of its synthesis efficiency. Furthermore, lifestyle, environmental, and physiological factors also impact VD production [3]. In the endogenous pathway, when exposed to UVB radiation, the covalent bond between carbon-9 and carbon-10 of 7-dehydrocholesterol (7-DHC) can be cleaved, resulting in the formation of the unstable 9, 10-seco-sterol, termed pre-VD3, which subsequently undergoes isomerization to generate VD3 (cholecalciferol) [1]. However, excessive UVB exposure leads to the photodegradation of VD3 into inactive metabolites, lumisterol and tachysterol [19]. The synthesis of 1α,25(OH)₂D involves two hydroxylation reactions. The initial hydroxylation occurs at the 25-carbon position, facilitated by microsomal and mitochondrial CYP enzymes in the liver, specifically CYP2R1, CYP27A1, CYP3A4, and CYP2D5. The first oxidation product is 25(OH)D. The subsequent hydroxylation takes place at the 1-carbon position, catalyzed by CYP27B1 in the proximal tubular cells of the kidney; however, this reaction may also occur in extrarenal tissues such as bone, placenta and intestine [20]. The catabolism of 25(OH)D can also be catalyzed by CYP24A1 to form 24,25(OH)₂D in kidney [21]. With a higher affinity of CYP24A1 than 25(OH)D, 1α,25(OH)₂D can also be transformed to 24,25(OH)₂D which is a key Metabolite of 1α,25(OH)₂D [22]. This Metabolic regulation by 24,25(OH)₂D is crucial for modulating the biological and clinical actions of 1α,25(OH)₂D, ensuring the maintenance of VD homeostasis within the body [23]. The degradation of 1α,25(OH)₂D involves a series of metabolic steps that convert VD into water-soluble metabolites. Calcitroic acid, which is the terminal product of this catabolic pathway, is ultimately excreted via the biliary route [20]. Various factors can influence Metabolism and catabolism of 1α,25(OH)₂D, including the concentration of 1α,25(OH)₂D, calcium levels, PTH levels, serum or extracellular fluid phosphorus levels, fibroblast growth factor 23 (FGF23), klotho, etc [24]. FGF23, a phosphaturic hormone, plays a crucial role in regulating calcium and phosphate metabolism [25]. There are three known triggers in the osteocyte to regulate FGF23, namely phosphate, leptin, and 1α,25(OH)₂D. 1α,25(OH)₂D stimulates transcription factors (GATA3, CREB, cEts1, and STAT1) to jointly activate the nearest promoter, inducing the FGF23 gene, which in turn represses PTH to inhibit CYP27B1 activation to feedback-repress VD bioactivation [26]. (The synthesis and metabolism diagram of VD is shown in Fig. 1)

Fig. 1.

Synthesis and Metabolism of VD. (1) Sources of VD: UVB Exposure, dietary Source, and drugs. (2) Conversion to 25(OH)D: Once ingested or synthesized, VD is converted into 25(OH)D in the liver. This conversion is facilitated by the enzymes CYP2R1 and CYP27A1. (3) Further Conversion to 1α,25(OH)₂D: The 25(OH)D is then transported to the kidneys where it is further converted into the active form of VD, 1α,25(OH)₂D, by the enzyme CYP27B1. (4) Metabolism and Excretion: The 25(OH)D can also be metabolized through two different pathways leading to its inactivation. C23 lactone pathway results in the formation of 26,23-lacton. C24 oxidation pathway involves the enzyme CYP24A1 and results in the formation of calcitroic acid

The genomic activity of 1α,25(OH)₂D initiates from the combination with VDR. On the one hand, according to the free hormone hypothesis, free VD and its derivatives can enter cells directly via cell Membrane despite that it only accounts for a very small proportion; on the other hand, the complex of 1α,25(OH)₂D bound to megalin/cubulin complex (LRP2-CUBN complex) can be internalized via receptor-mediated endocytosis [27]. VDR, a DNA-binding transcription factor, forms an active signal transduction complex through the heterodimerization of 1α,25(OH)₂D-bound VDR and unliganded RXR [28]. After binding to 1α,25(OH)₂D, the heterodimerization is able to connect with VDREs in the DNA sequence of VD-regulated genes and is translocated to the nucleus [13]. The VDR comprises two core functional domains: the highly conserved N-terminal DBD and the more variable C-terminal LBD [29]. VDREs typically exhibit either a direct repeat of two hexanucleotide half-sites separated by a three-nucleotide spacer (DR3) or an everted repeat of two half-sites separated by a six-nucleotide spacer (ER6), with DR3 motifs being predominant [30]. In DR3 positive VDREs, the VDR preferentially binds to the 3’ half-site, while the RXR associates with the 5’ half-site [29, 30]. Genes containing VDREs can be categorized into major biological networks: (a) bone homeostasis, (b) mineral metabolism, (c) detoxification pathways, (d) cell life cycle (including proliferation, differentiation, migration, and apoptosis), (e) immune regulation, and (f) metabolic processes (involving amino acids, lipids and carbohydrates) [31]. Upon the binding of the VDR-RXR heterodimer to the VDREs, alterations in gene expression are facilitated by the liganded receptor’s capacity to recruit transcriptional coactivators. The p160 family, steroid receptor activator 1, 2, and 3 (SRC-1, SRC-2, and SRC-3), as main co-activators, can bind to the AF2 domain of liganded VDR. Proteins recruited by p160 coactivators such as CBP/p300 serve as secondary coactivators [24]. Actually, the transcriptional activity of VDR is affected by numerous transcription factors. Studies have found C/EBPβ, Runx2, SWI/SNF, the methyltransferases, CARM1, Ras-activated Ets transcription factor and G9a can regulate VDR-mediated transcription collectively [32]. Some VDR-interacting proteins function as co-repressors, such as NCOR1, COPS2 and MED1 [10]. Instead of co-activators and co-repressors proteins, chromatin-modifying enzymes, such as histone acetyltransferases, histone deacetylases, lysine demethylases like KDM6B and KDM1A or chromatin remodeling proteins, such as bromodomain-containing (BRD) 7 and 9, also involve in the composition of VDR-interacting proteins [10]. Taken together, VD exerts epigenetic effects by modulating transcription factors binding and histone modification levels, as well as influencing chromatin accessibility and the three-dimensional organization of chromatin [33].

However, the pleiotropic activities of VD are unable to be fully explained by VDR-mediated transcriptional regulation, which demonstrate the existence of rapid and non-genomic activity of VD. G-coupled membrane receptor (GPCR) was found to involve in VD signaling and after 1α,25(OH)₂D binds to these membrane receptors, the secondary messengers including cAMP and calcium are rapidly mobilized [34, 35]. Protein disulfide-isomerase A3 (PDIA3), one of these receptors, is localized within caveolae, where it forms complexes with phospholipase A2-activating protein (PLAA) and caveolin-1 (Cav-1). This association activates a downstream signaling cascade mediated by Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), phospholipase A2 (PLA2), phospholipase C (PLC), and protein kinase C (PKC), ultimately leading to the activation of the extracellular regulated protein kinases1/2 (ERK1/2) pathway within the mitogen-activated protein kinase (MAPK) family. Researchers have found disruption of the PDIA3 gene leads to the inhibition of rapid calcium transport and diminishes PKC signaling, consequently impairing the swift non-genomic responses to 1α,25(OH)₂D. However, the exact Mechanism of interactions between 1α,25(OH)₂D and PDIA3 are still not fully understood. Furthermore, VD and its derivatives are found to directly bind to ion channels or relevant proteins so that ions flux can be quickly influenced, such as calcium influx [34].

VD plays a pivotal role in regulating the innate immune response, including the upregulation of cathelicidin, activation of cytokine production, promotion of autophagy, and other related mechanisms [36]. VD orchestrates the production of antimicrobial peptides, including cathelicidin, cationic peptides, and defensins, which act as endogenous antibiotics capable of directly damaging microbes, particularly within the mucosal immune system [37]. TLRs, which are integral to the innate immune response, activate macrophages by enhancing the expression of VDR and CYP27B1 thereby stimulating the production of antimicrobial peptide [38]. Research has shown that VD not only induces the production of antibacterial peptides but also enhances the autophagic capabilities of macrophages, thereby bolstering antibacterial defense mechanisms [39]. As a key regulatory factor of calcium metabolism, VD inhibits the expression of mTOR and Bcl-2 by increasing the level of free cytoplasmic calcium, offsetting their inhibitory effects on autophagy [40]. VD is posited to exert immunosuppressive effects by modulating various components of the immune response: (1) By binding to its receptor, VD mediates downregulation of major histocompatibility complex (MHC)-II and co-stimulatory molecules on dendritic cells (DCs), including CD40, CD80, and CD86, thereby inhibiting T cell activation [41], thereby reducing the production of pro-inflammatory cytokines such as IL-2, IL-5, IL-17, and tumor necrosis factor (TNF) -α, and consequently diminishing cytokine-driven immune responses [42, 43]; (2) It can inhibit the differentiation of naïve T cells into Th17 cells and suppress the secretion of cytokines IL-17 and IL-21, which are derived from Th17 cells, while simultaneously promoting the development of Treg cells to reestablish the body’s Th17/Treg cell balance [43].

The VDR gene has various polymorphisms, such as TaqI, ApaI, BsmI, FokI, and Cdx2, which are associated with a spectrum of diseases. Specifically, the FokI SNP is linked to an increased risk of coronary artery disease (CAD) [44]. Similarly, all genetic models associated with the ApaI SNP, except the recessive model, have been found to significantly increase the risk of CAD in comprehensive analyses [45]. In addition, the ApaI and BsmI polymorphisms are predictive of mild cognitive impairment (MCI), while the TaqI polymorphism is associated with an increased risk of Alzheimer’s disease (AD) [46]. Furthermore, aggregated data have revealed a substantial correlation between the ApaI polymorphism and the risk of developing Behcet’s Disease (BD) across all studied populations [47]. A significant correlation has also been identified between the BsmI bb genotype and the incidence of Type 1 Diabetes (T1D), with the BsmI variant notably associated with a heightened risk in Asian populations [48]. The FokI SNP’s F allele and genotypes FF and Ff are linked to increased genetic susceptibility to Rheumatoid Arthritis (RA) and Systemic Lupus Erythematosus (SLE), while the BsmI bb genotype is associated with higher genetic risk for Multiple Sclerosis (MS) and RA, and its B allele, BB, and Bb genotypes are linked to SLE predisposition [48]. Additionally, the TaqI SNP’s TT genotype is linked to an increased genetic risk for RA and may confer protection against MS [48]. The FokI C allele has been associated with an increased genetic predisposition for both Hashimoto’s thyroiditis (HT) and Graves’ disease (GD) [47]. The FokI and ApaI variants within the VDR gene have been correlated with the risk of ovarian cancer, and the BsmI and FokI variants notably linked to breast cancer [49]. The BsmI and Cdx2 gene polymorphisms have been associated with a reduced risk of lung cancer [50].

Vitamin D deficiency

Vitamin D deficiency is a significant global concern, particularly among pediatric and geriatric populations. This deficiency is influenced by various factors, including gender, age, latitude, weather conditions, cultural and social aspects [51, 52]. Infants are at the highest risk of developing vitamin D deficiency, particularly those who are exclusively breastfed in developing countries [52]. Interestingly, childhood VD levels have a significant impact on peak bone mass in males, but not in females. This difference may be attributed to the estrogenic effects in females [53]. Furthermore, vitamin D deficiency is prevalent in chronic childhood conditions, which further deteriorate skeletal health and contribute to existing morbidity [52]. In both adolescent males and females, there are moderate associations between VD levels measured in prepuberty, adolescence, and early adulthood [53].

Maternal VD insufficiency during pregnancy is a global public health concern [54]. Numerous Mechanisms have been discovered in relation to this issue. The fetal 25(OH)D levels are entirely dependent on the supply from the maternal kidney, with the necessary amount absorbed from the maternal intestinal Ca²⁺. The placenta plays a role in regulating these levels by activating or deactivating 25(OH)D [54, 55]. It has been confirmed that there is a positive correlation between adequate childhood VD levels and improved bone mineralization. The ideal 25(OH)D threshold for childhood bone health is 75 nmol/L [56]. The markers of bone absorption are relatively low during the first trimester of pregnancy, but they increase up to twice the normal levels during the second half of pregnancy [57]. By mid-pregnancy, the extra mineral in the mother’s body is resorbed, resulting in a positive calcium balance for the fetus [58]. However, the mother herself may be in a neutral to slightly negative calcium balance. It has been confirmed that PTH is the main stimulator of CYP27B1, with Parathyroid Hormone-Related Protein (PTHrP) serving as the secondary stimulator. The binding of PTHrP to the PTH/PTHrP receptor is less tight and of shorter duration compared to PTH [59]. However, during the first trimester, PTH is inhibited while the level of PTHrP gradually increases, reaching up to three times its initial level [57]. Additionally, it has been observed that maternal 25(OH)D is positively associated with bone mineral content and density in boys, but not in girls [54]. Although neonates of mothers with adequate VD status have a higher rate of ossification centers compared to neonates of mothers with low VD status, it remains uncertain whether VD levels in pregnant women can influence their children’s bone mass or not [54]. Additionally, if mothers do not have sufficient VD, their infants with the FF or Ff polymorphism tend to have lower mean birth weight compared to infants with the ff polymorphism [60]. When maternal VD levels are insufficient, the PTH levels increase in order to regulate bone resorption and maintain proper maternal serum calcium levels, which can lead to a transient neonatal hypoparathyroidism and hypocalcemia [54]. In cases where the mother has low VD levels, the offspring may develop a larger bone area in order to adapt to mechanical strain, resulting in a condition known as craniotabes [61]. However, children born to mothers with sufficient VD levels who receive high-dose supplementation during pregnancy have been found to have a 60% reduced occurrence rate of fractures [56]. Therefore, it is important to screen the VD levels of pregnant women who are at high risk of vitamin D deficiency. High-risk groups include women who are obese, have dark skin, do not cover themselves well, are receiving corticoid treatment, have hypertension, pre-gestational diabetes mellitus, or autoimmune diseases [54].

Multiple digestive system diseases can lead to vitamin D deficiency. CKD is a prevalent disease worldwide, and it can result in a high risk of vitamin D deficiency. This deficiency is associated with various adverse outcomes, such as bone disease, cardiovascular disease, increased mortality and secondary hyperparathyroidism (SHPT) [62]. However, the safety and efficacy of high-dose cholecalciferol supplementation remain uncertain. Although it increases serum calcium in most CKD patients and reduces PTH and SHPT, it has no adverse effect on FGF23 levels [25]. Digestive disorders often result in deficiency of VD due to malabsorption [63]. In such situations, the presence of steatorrhea, duodenal exclusion or pancreas dysfunction further exacerbates the issue of vitamin D deficiency [63, 64]. Furthermore, pancreatic diseases, such as acute and chronic pancreatitis, also result in deficiency of VD because of the deficiency of pancreas, malabsorption and inflammation [65, 66]. The major mechanisms of vitamin D deficiency in liver diseases include impaired hepatic hydroxylation, decreased intestinal absorption, malnutrition and inadequate sunlight exposure [67–69]. The key points related to vitamin D deficiency and diseases are presented in Table 1.

Table 1.

Vitamin D deficiency and diseases

Risk population and risk diseases​​	​​Key characteristics​​	Bone health consequence	​​Clinical recommendations​
Infants & children	The highest risk of developing vitamin D deficiency (exclusive breastfeeding in developing countries)	Peak bone mass decrease (males) Decreased bone mineralization	Maintain 25(OH)D ≧ 75 nmol/L
Pregnant women	Maternal obesity Autoimmune diseases	Neutral to slightly negative calcium balance	VD supplementation Screen the VD levels of pregnant women
Chronic kidney disease (CKD)	Increase serum calcium High risk of vitamin D deficiency	Bone diseases SHPT	High-dose cholecalciferol supplementation (the safety and efficacy uncertain)
Malabsorption syndromes	Malabsorption Result in deficiency of VD	Osteomalacia	VD supplementation

Vitamin D in the pathogenesis of bone-destructive diseases

Osteoarthritis (OA)

OA, one of the most common degenerative joint disorders, has affected more than 500 million population worldwide, especially the elderly [70]. Risk factors involved in OA include growth of age, adiposity, increased biomechanical loading of joints, genetic factors, Oxidative stress, mechanical stress and joint injury [71, 72]. OA mainly affects knees, hips, hands and spine, causing cartilage deterioration, synovial inflammation, subchondral bone destruction, ligament laxity, osteophyte formation [73]. The extracellular matrix of chondrocytes is mainly composed of collagen and aggrecan. Matrix metalloproteinases can lead to the degradation of extracellular matrix, which triggers the onset of early osteoarthritis [74]. Several cytokines including IL-1β, TNF, IL-6, IL-21 and IL-8 are demonstrated to change in OA [74]. Inflammatory cytokines secreted by macrophages can promote the expression of matrix decomposition proteins, and then aggravate the development of osteoarthritis [75].

Evidences suggest that VD plays an important role in the progression of OA [76, 77]. Vitamin D deficiency is associated with the early stages of OA [78]. Patients with knee early OA demonstrate more serious pain intensity, disability, anxiety and depressive symptoms, and social performance in the presence of vitamin D deficiency [79]. A case-control study demonstrated Vitamin D deficiency was strongly associated with increased matrix metalloproteinases (MMPs) activity and oxidative stress in OA patients [80]. However, low serum VD concentration is not linked with incremental incidence of knee and hip OA [76, 81]. Vitamin D deficiency expedites the progression of age-related knee OA (KOA) [82]. A cross-sectional analysis of 524 participants shows VD correlates with reduced pain in male KOA patients [83]. Besides, men may benefit more from VD supplementation in terms of mortality reduction than that of women especially those postmenopausal women in OA patients [84].

Sufficient 1,25(OH)₂D instead of 25(OH)D levels are more effective for the prevention of vitamin D deficiency-induced KOA. Pacharee Manoy et al. found regularly supplement of 1α,25(OH)₂D for six months is beneficial to improve the life quality of OA patients by decreasing oxidative protein damage and relieve pain [85]. The study found that 1α,25(OH)₂D combined with VDR increased the ubiquitination level of NLRP3 (NOD Like receptor pyrin domain 3 inflamasome) in macrophages, and then reduced the secretion of IL-1β and IL-18 to prevent the loss of extracellular matrix, cartilage degeneration and osteophyte formation [86]. 1α,25(OH)₂D is beneficial to prevent the progression of KOA through suppressing the degradation of extracellular matrix, senescence of chondrocytes and senescence-associated secretory phenotype (SASP) via Sirt1 mediation [82]. Reverse Transcription-Polymerase Chain Reaction (RT-PCR) results demonstrated VD treatment can reduce mRNA levels of MMPs, nuclear factor kappa-B (NF-κB), TNF-α, and IL-6 in OA mice while increasing IL-10 levels [87]. Research found VD induced chondrocyte autophagy to ameliorate OA inflammation through AMPK/mTOR signaling pathway mediated by VDR-NF-κB interaction, which might be a novel way for OA treatment [87, 88]. However, the optimal level of autophagy activity in patients with KOA remains unknown [89]. The gut microbiome among KOA patients with or without Vitamin D deficiency is different [90], VD modulates gut microbiota composition and metabolites via the “gut-immune-bone axis”, enhances intestinal barrier function, suppresses pro-inflammatory pathways, regulates immune balance and bone metabolism, ultimately alleviating OA inflammation and cartilage degradation [91]. Z. Qu et al. found no casual effect of serum 25(OH)D levels on OA, but the decrease of serum PTH level was concomitant with VD administration, suggesting the treatment effect of VD on OA might come from the change of PTH [92]. In addition, the study found that VD increased the expression of VDR and its downstream mediated signaling pathways by promoting the secretion of TGF-β1 in chondrocytes [93].

Patients with sufficient VD is less likely to develop OA, and VD sufficiency and supplementation reduce articular cartilage degeneration radiographically. Zhiqiang Wang et al. found among patients who did not have knee surgery, there is a significant improvement in Western Ontario and McMaster Universi ties Osteoarthritis Index (WOMAC) function in VD supplementation group compared to the placebo group [94]. VD supplementation can reduce pain and improve physical function in OA patients with low 25(OH)D status (< 50 nmol/L) [77, 95]. Although supplement of VD more than 2,000 International Units (IU) VD is beneficial in improving the WOMAC pain and function in patients with OA, it fails to prevent cartilage loss [96]. A mendelian randomization study found there was a lack of causal association between VD and OA, suggesting VD supplementation may be unlikely to treatment hip or KOA [97]. A post hoc analysis manifested that maintaining VD sufficiency is conducive to decrease cartilage loss and improve effusion-synovitis and physical function in people with symptomatic KOA though the effect is small [98]. Therefore, there is insufficient evidence to support the use of VD supplementation in preventing the progression of KOA [99, 100].

It is reported expression of VDR in the articular cartilage of OA patients, but not in that of healthy volunteers [76, 95]. Polymorphisms of VDR are associated with the development and formation of osteophytes in OA. However, the association between VDR gene poly-morphisms and OA are inconsistent in different population and researches [101]. A meta-analysis suggests VDR BsmI and TaqI polymorphisms have relevance to OA susceptibility in the spine, but the association between VDR ApaI polymorphism and OA susceptibility is not significant [102].

The effect of VD in OA treatment is controversial. In the past, the majority of studies have shown that VD supplementation has no effect on pain and function improvement in patients with KOA. However, with further in-depth researches coming out, VD were demonstrated to have a strong association with OA. Although supplementation of VD is instrumental to relieve pain in patients of OA to some extent, the evidence is not enough on the effect of VD supplementation on OA prevention [95, 103]. However, these inconsistencies may stem from multiple factors, including study design, population characteristics, or VD assessment methods [104]. There is a lack of specific guidelines on the use of 1α,25(OH)₂D in the treatment and prevention of OA, but VD is still a potential choice [105]. (Fig. 2. Right section)

Fig. 2.

VD and bone destruction diseases. It displayed the roles of VD in various physiological processes related to bone health and immune function. (Left Section) It illustrates how VD acts on dendritic cells to promote a tolerant immune response, which can help in reducing inflammation in conditions Like RA. It also shows the impact of Vitamin D deficiency on osteomalacia leading to OP. 1α,25(OH)₂D interacts with the VDR to influence bone metabolism, including bone formation by Osteoblasts and bone destruction by Osteoclasts. (Middle Section) This part explains the role of VD in modulating the immune system. It affects the differentiation of T helper cells (Th17 and Treg cells), which are crucial in adaptive immunity. (Right Section) It shows how VD and its receptor (VDR) enhance the clearance of Mycobacteria in tuberculosis through macrophage activity. It also highlights the effects of vitamin D deficiency on chondrocyte health, leading to oxidative stress and chondrocyte aging, which can contribute to OA. The interaction between VD and the AMPK/mTOR pathway is also depicted, suggesting a role in autophagy within chondrocytes

Osteoporosis (OP)

OP is a common systematical skeletal disorder among the elderly. With the aging of population, OP has risen to a major public health problem in the past decade. As a metabolic bone disease, OP is characterized by reduced bone mass, micro-structural deterioration and bone fragility, which contributes to the low bone mineral density and increased risk of fractures [106]. Osteoporotic fractures are most commonly found in the hip, wrist or spine. And vertebral compression fractures (VCFs) are the typical clinical presentation in OP. Many factors are associated with OP including the use of glucocorticoids, smoking, low serum concentration of calcium and VD, increasing age, female sex, gene, etc [107]. Compared with men, women are more predisposing to develop OP, especially those postmenopausal women [108]. Although the exact mechanism of OP is still unknown and it remains a chronic incurable disease, the symptoms can be alleviated via aggressive treatment [109]. And it is demonstrated in many studies that VD may play a role in the treatment and prevention of OP.

Vitamin D deficiency is associated with OP, which is significant to the calcium homeostasis and bone mineralization in our bodies. Vitamin D deficiency can lead to secondary hyperparathyroidism, leading to bone turnover, osteomalacia, bone loss and osteoporotic fractures [110, 111]. With the increase of age, aging is significantly related to the decrease of TRPV6, calcium binding protein D9k and intestinal VDR expression, as well as the decrease of VD cycle activity due to the reduction of renal 1-α hydroxylation, which will lead to osteoporosis [112]. 1α,25(OH)₂D inhibits bone marrow mesenchymal stem cells (BM-MSCs) senescence, stimulates osteoblastic bone formation and alleviates osteoclastic bone resorption through up-regulated Ezh2-H3K27me3 and repressed p16, demonstrating 1α,25(OH)₂D may be effective to treat and prevent OP induced by aging [113]. In high-risk patients, there is evidence that 25(OH)D ≥ 30 ng/ml should be maintained to treat osteoporosis. Higher calcium intake (> 800 mg per day) plus VD intake (> 600 IU) was helpful to inhibit OP in relatively healthy postmenopausal women, though did not considerably decrease the risk of fracture [114]. However, VD could improve the tail effect of alendronate on BMD in postmenopausal women who received OP treatment followed by discontinuation [115]. Compared with VD alone, the combined use of VD and calcium confers a moderately greater anti-fracture benefit and increases the total BMD in high-risk patients [110, 114]. Furthermore, a phase 2 randomized controlled trial (RCT) demonstrates high-dose weekly VD supplementation significantly reduces hip and femoral neck BMD loss in prostate cancer patients undergoing androgen deprivation therapy, especially among those with lower baseline serum 25(OH)D levels [116].

However, research results about the effect of VD supplementation on OP and fractures are inconsistent. VD supplementation in healthy middle-aged and elderly people may be ineffective in reducing the fracture risk [117]. Pang Yao et al. thought there is a lack of association between standard doses of VD supplementation alone and reduction of fracture risk. Because both intermittent and daily dosing with standard doses of VD alone failed to reduce the risk of fractures [118]. The anti-fracture effect of VD was almost only observed in fractures of femur and non-vertebral bones [111]. A meta-analysis found there was no significant effect of VD on bone mineral density in either the spine or total hip [119]. Besides, the benefits only appear in patients with vitamin D deficiency (< 30 ng/mL), not population with sufficient VD [120]. A randomized controlled trials demonstrated that VD3 supplementation (2000 IU/day) in healthy midlife and elderly adults who did not have vitamin D deficiency and OP fails to reduce the risk of total fractures [121]. VD supplementation may increase hip fracture risk in older healthy women [122]. A response-adaptive and randomized clinical trial found high-dose VD supplementation is ineffective in the prevention of falls [123]. Excessive VD intake can even lead to lower femoral neck BMD and bone strength [114]. Moreover, high doses of VD supplementation (1000 to 4000 IU/day) might contribute to the higher incidence of fractures in first time falls compared to 200 IU per day [124]. (Fig. 2. Left section)

Rheumatoid arthritis (RA)

RA is a chronic, immune, and systemic inflammatory disease that can lead to OP, disability, and even mortality [125–127]. It affects approximately 1% of the global population [128, 129]. Additionally, it is one of the most common rheumatic diseases in children and adolescents [130]. Various factors impact on RA, including age, gender, region, and season [131]. It is characterized by systemic inflammation, autoantibody production, bone destruction, low serum concentrations of 25(OH)D, and synovial inflammation [132, 133]. Particularly, synovial fibroblasts (SFS) are highly involved in the development of RA as they produce amounts of cytokines, chemokines, and matrix-degrading enzymes. TNF-α stimulates the production of MMPs in SFS, which in turn degrade cartilage components, such as collagen type II and proteoglycan aggrecan [86]. Th1 cells are involved in the acute phase of rheumatoid arthritis. Th17 cells mediate synovial inflammation by inducing macrophages, synovial fibroblasts and chondrocytes to produce pro-inflammatory cytokines and chemokines, reducing the synthesis of collagen and proteoglycan in chondrocytes, increasing osteoclast differentiation, and ultimately leading to cartilage degradation and bone erosion [134].

In recent years, the association between vitamin D deficiency and RA has gained significant attention [128, 135]. The abnormal metabolism of VD has been shown to be linked with the pathogenesis of RA [136]. Various studies have proven that Vitamin D deficiency is associated with increased mortality, disease activity, disability, and radiographic progression of RA by aberrant Disease Activity Score 28 (DAS28), Simplified Disease Index (SDAI), Clinical Disease Activity Index (CDAI), Health Assessment Questionnaire (HAQ), and C-reactive protein (CRP) [125, 126, 130, 131, 135]. Genomic studies have further demonstrated that specific polymorphisms in the gene encoding VDR and VDBP are associated with susceptibility to RA [137]. A study has reported an association between RA and the VDR FokI variant, leading to an increased susceptibility to the disease in carriers of the f(T) allele and the RA-related osteopenia in carriers of the BsmI/ApaI Ba (AC) haplotype [138]. The methylation levels of the VDR and CYP27B1 genes exhibit a significant association with the risk of RA and could potentially serve as supplementary biomarkers for diagnosing this condition [136]. Additionally, VD has been found to play a potential role in nociceptive thresholds and allodynia scores of RA, suggesting that it could be used as a prophylactic treatment to reduce the future risk of RA [139].

The fundamental mechanisms by which VD affects RA have been widely reported [140–142]. VD exerts its effects on immune regulation, anti-inflammatory functions and cell-cell interactions, including antifolate resistance and osteoclastogenesis regulation, through the NF-κB signaling pathway targeting at TNF, CASP3 and TP53 [139, 143]. The 1α,25(OH)₂D has been shown to have the ability to decrease the presentation of antigens to T cells by promoting an immature and tolerogenic dendritic cell (DC) phenotype [144]. 1α,25(OH)₂D can inhibit the expression of toll-like receptors on monocytes, leading to low production of proinflammatory cytokines and high production of anti-inflammatory cytokines [137]. In RA, reduced synovial VDBP compromises rheumatoid arthritis synovial fibroblast (RASF) viability and promotes their apoptosis, whereas 1,25(OH)₂D₃ up-regulates VDBP expression in RASF and this restored VDBP reciprocally amplifies the 1,25(OH)₂D₃-mediated suppression of viability and induction of apoptosis in RASF [145]. On the other hand, 1α,25(OH)₂D regulates the balance of Th cells from Th1/Th17 to Th2/Treg, inhibits the production of B cells and autoantibodies, reduces pro-inflammatory cytokines, and inhibits the proliferation of synovial cells and the secretion of MMPs [130]. Moreover, the 1α,25(OH)₂D can inhibit the activation of synovial fibroblasts (SF) via the pro-inflammatory loop between CCR6 + Th cells and SF and the pathogenicity of Th17 cells [143]. (Fig. 2. Left section)

Bone tuberculosis

Tuberculosis is a chronic infectious disease caused by Mycobacterium tuberculosis and has put a huge burden on global public health. According to the World Health Organization global tuberculosis report in 2024, among the 30 countries with high tuberculosis burden, China ranked third in the estimated number of tuberculosis cases. Osteoarticular tuberculosis is one of the most common extrapulmonary tuberculosis, with approximately 95% secondary to pulmonary tuberculosis. The proportion of drug-resistant tuberculosis is also rising year by year [146]. Consequently, novel tuberculosis treatments are urgently needed. More and more evidences suggest that vitamin D deficiency and tuberculosis infection are highly associated that low serum VD levels increase the risk of tuberculosis infection [147, 148].

Research has found Vitamin D deficiency is associated with the setup and development of spinal tuberculosis. The serum concentration of 25(OH)D in spinal tuberculosis group was lower compared with healthy control group. People with low serum level of VD (< 25 nmol/L) were more likely to develop necrotic lesions than patients with high serum VD concentration (≥ 25 nmol/L) [149]. However, in subjects with active tuberculosis, levels of 1α,25(OH)₂D tended to increase [150]. Studies have found that adequate VD can reduce the risk of contracting tuberculosis after exposure, limit the progression of latent tuberculosis, and, as an adjunct to antimicrobial therapy, shorten the duration of treatment and improve treatment effectiveness [151].

VD, specifically its active Metabolite 1,25(OH)₂D, plays a crucial role in the innate immune defense against tuberculosis. VD inhibits the replication of Mycobacterium tuberculosis in vitro and has shown a promising role in the treatment of tuberculosis due to its link to oxidative balance [150]. In addition, VD has immunostimulatory and immunosuppressive effects linked with the body’s anti-mycobacterial response [151]. There might be two possible approaches: the first is that VD mediates innate immune protection against tuberculosis through direct VDR-dependent nuclear regulation of cathelicidin expression upon exposure to infection; the second is the restriction of the growth of Mycobacterium tuberculosis within the cell by enhancing macrophage activity [147]. VD may promote bone formation and reduce bone resorption via its regulatory effect on tuberculosis-related immunity [149].

Due to ultraviolet radiation, PTH, VDR gene polymorphism and other factors, the specific effect and dosage of VD in the treatment of tuberculosis are still uncertain. Clinical evidence regarding VD supplementation for tuberculosis prevention or treatment remains mixed. While some studies, like a prospective case-control study in India, found a significantly higher prevalence of tuberculosis among VD-deficient individuals compared to sufficient one [152], but among VD-deficient Mongolian children, the supplemental VD group did not show a significantly reduced risk of primary tuberculosis infection compared with the placebo group [153]. Major trials of VD in patients with active tuberculosis have failed to show a major benefit overall [151]. But some research has found that the concentration of 25(OH)D detected in patients was positively correlated with tuberculosis cure rates [148]. Given the low incidence of side effects even at high doses and the low cost, it is recommended to assess VD levels in patients with active tuberculosis and those at high risk and get adequate VD supplementation. Further research and clinical trials are needed to evaluate the effectiveness of VD supplements in preventing active tuberculosis [150]. (Fig. 2. Right section)

Discussion

Previous studies mainly focused on the classical roles of VD in calcium metabolism and bone mineralization. Now, it has been discovered that VD not only regulates bone homeostasis but also functions as a signaling molecule to modulate the immune, inflammatory and microbiota responses of the body. The genomic and non-genomic signaling pathways mediated by the VDR underpin its diverse biological effects, spanning osteoblast-osteoclast crosstalk, innate and adaptive immunity, and chronic disease modulation. While the skeletal benefits of VD in preventing rickets and osteomalacia are well-established, its therapeutic potential in complex bone destruction diseases such as OA, OP, RA, and tuberculosis, remains contentious. For instance, in OA, conflicting clinical evidence highlights the dual nature of VD, where supplementation may mitigate cartilage degradation and pain in select cohorts but fails to universally alter disease progression. Similarly, in OP, vitamin D deficiency exacerbates bone loss, yet the efficacy of supplementation is highly dependent on baseline levels, concomitant calcium intake, and genetic polymorphisms in VDR and CYP enzymes. The immunomodulatory effects of VD further complicate its role in autoimmune-driven conditions like RA, where it suppresses pathogenic Th17 responses while promoting regulatory T-cell activity, yet fails to consistently correlate with reduced radiographic damage in clinical studies.

A critical gap in current understanding Lies in defining context-specific thresholds for VD sufficiency, particularly for non-skeletal outcomes. While 25(OH)D levels > 50 nmol/L are widely accepted for bone health, optimal thresholds for immune regulation or Metabolic modulation remain undefined. Furthermore, the interplay between VDR polymorphisms and disease susceptibility underscores the need for personalized supplementation strategies, as genetic variants influence receptor function, Ligand affinity, and downstream gene expression. Due to improper use of VD, reports of VD poisoning have increased significantly since 2010. The toxicity of VD is mainly mediated by hypercalcemia. The symptoms can range from mild (such as thirst and polyuria) to severe (such as seizures, coma and death), and are more common in children and the elderly [154]. In another retrospective study, it was found that 25(OH)D was inversely J-shaped correlated with mortality, and had potential adverse effects when the physiological dose was exceeded [155]. Intermittent high-dose administration of VD or the concentration of 25(OH)D in blood greater than 112 nmol/L will not only lead to hypercalcemia and hypercalciuria, but also reduce BMD and increase the risk of falls [156]. Therefore, the supplement of VD should be appropriate and have an upper limit. Emerging evidence also suggests that extraskeletal benefits of VD, such as antimicrobial peptide induction in tuberculosis or β-cell preservation in diabetes, are modulated by gut microbiota and environmental factors, necessitating a systems-level approach to unravel its pleiotropic mechanisms [157, 158]. Future studies ought to emphasize large-scale longitudinal clinical trials to elucidate current ambiguities, while incorporating multi-omics datasets for comprehensive mapping of VD’s interactome. Furthermore, investigators should pursue synergistic therapeutic interventions targeting complementary molecular pathways, with the ultimate goal of optimizing clinical outcomes in skeletal disorders.

Emerging evidence highlights the complex disease-modifying roles of VD across bone destruction disorders, though therapeutic translation remains nuanced. In OA, VD’s putative anti-inflammatory and chondroprotective mechanisms including suppression of matrix metalloproteinases and enhancement of autophagy via AMPK/mTOR signaling contrast with inconsistent clinical outcomes. While observational studies correlate Vitamin D deficiency with accelerated cartilage degradation and pain severity, randomized trials demonstrate marginal structural benefits despite modest symptomatic improvements in WOMAC scores. This discrepancy may reflect threshold-dependent effects, as higher serum 1,25(OH)₂D shows stronger cartilage preservation, suggesting tissue-specific activation kinetics and VDR expression variability in diseased joints. For OP, calcium-homeostatic function of VD dominates therapeutic rationale, and meta-analyses reveal calcium co-supplementation as a critical determinant of fracture risk reduction. Paradoxically, excessive VD dosing associates with paradoxical bone loss, likely through FGF23-mediated phosphate wasting and suppressed osteoblastogenesis. The aging osteoblast niche further complicates responses, where VD-VDR signaling enhances Ezh2-mediated epigenetic repression of senescence markers but requires baseline 25(OH)D > 30 ng/mL for efficacy. In autoimmune-driven bone erosion like RA, VD exhibits dual immunomodulation: suppressing Th17 polarization via IL-6/STAT3 inhibition while upregulating Treg differentiation. However, VDR polymorphisms exhibit ethnic-specific modulation of​ disease predisposition among populations, mandating implementation of stratified pharmacogenomic approaches. Tuberculosis-associated osteolysis presents unique VD dynamics, where calcitriol enhances macrophage mycobactericidal activity through cathelicidin induction yet fails to reduce spinal tuberculosis progression in VD-replete populations, which is a paradox potentially explained by pathogen-driven CYP27B1 overexpression in granulomas. These disease-specific Mechanisms underscore the need for precision approaches addressing baseline VD status, genetic modifiers, and comorbidity interactions. Future trials must prioritize biomarker-guided dosing such as 1,25(OH)₂D in OA and FGF23/PTH ratios in OP to optimize skeletal outcomes while mitigating off-target immunologic effects.

Compared with previous systematic reviews on VD and bone-related diseases, this review systematically elucidates the molecular-cellular-disease cascade mechanisms of VD in multifactorial bone-destructive disorders such as OA, OP, RA and tuberculosis, covering immune regulation, autophagy-mediated osteocyte homeostasis maintenance, classical VDR-RXR nuclear transcriptional regulation, and non-genomic membrane receptor-mediated rapid calcium signaling responses. It further integrates epigenetic regulatory mechanisms with VDR gene polymorphisms, addressing the shortcomings of existing reviews in the integration of cross-disease mechanisms and the depth of epigenetic regulatory mechanisms [159–162]. The concept of the “bone-immune-metabolic axis” is proposed, emphasizing that VD not only acts as a bone metabolism regulator but also serves as a core mediator connecting the immune microenvironment and metabolic pathways, providing a new theoretical paradigm for understanding the pleiotropy of VD in complex bone diseases. However, this study has certain limitations. For instance, the strength of clinical translational evidence is insufficient. Although mechanistic explorations are thorough, direct data supporting key endpoints such as fracture risk and bone mineral density improvement rate remain limited, with inadequate validation from large-scale prospective cohorts or RCTs. Additionally, human validation is incomplete, as some conclusions rely on animal experiments or in vitro models, necessitating further clinical verification. Moreover, the analysis of heterogeneity in special populations is incomplete. While VDR gene polymorphisms are mentioned, investigations into VD metabolic heterogeneity caused by comorbid chronic diseases or racial differences are insufficient, and precision supplementation strategies have not yet been established. The evolving understanding of VD’s pleiotropic roles demands a paradigm shift from universal supplementation to precision medicine. To unlock its therapeutic potential, future research must integrate multi-omics approaches to delineate tissue-specific VDR interactomes, validate context-dependent optimal thresholds, and stratify populations by genetic/epigenetic profiles. Large-scale randomized trials should prioritize adaptive designs to address comorbidities, pharmacogenomic variability, and microbiota interactions. Concurrently, exploring synergistic regimens combining VD with immunomodulators or osteoanabolic agents may overcome current limitations, ultimately advancing personalized strategies for bone-immune axis disorders.

Author contributions

ZHZ and JZD designed and directed the review. YL and WW contributed to collecting the resources and writing the manuscript. YL, WW and YSY contributed to designing and drawing the figures and tables. All authors critically read and approved the final manuscript.

Funding

This work was supported by a grant from the National Natural Science Foundation of China (81972082) and a grant from the Chongqing Postdoctoral Program for Innovative Talents (CQYC202105037).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Competing interests

The authors declare no competing interests.

Compliance with ethics requirements

This article does not involve any studies with human or animal subjects.