Vitamin D Alleviates Osteoarthritis Progression by Targeting Cartilage and Subchondral Bone via Myd88-TAK1-ERK Axis Suppression
Abstract
Osteoarthritis, commonly abbreviated as OA, represents a pervasive and debilitating chronic joint disease characterized by the progressive and often irreversible degeneration of articular cartilage, leading to significant structural damage within the joint. Despite the profound impact of OA on global public health and quality of life, current therapeutic interventions primarily focus on symptom management, such as pain relief, and fall short of comprehensively addressing the multifaceted and complex pathological mechanisms that drive the disease’s progression. A growing body of scientific evidence has increasingly pointed towards metabolic dysfunction within the subchondral bone—the layer of bone directly beneath the cartilage—as a potential initiating factor or a key contributor to the onset and progression of OA. This suggests a more intricate interplay between bone and cartilage health than previously understood. Concurrently, vitamin D, a vital micronutrient, is exceptionally well-established for its crucial role in the regulation of bone metabolism and mineral homeostasis, particularly its therapeutic efficacy in conditions like osteoporosis. However, despite numerous observational studies hinting at potential protective effects, the precise therapeutic utility of vitamin D in the context of osteoarthritis has, until now, remained largely ambiguous and underexplored. Our meticulously designed integrated study, encompassing both in vivo animal models and in vitro cellular experiments, provides compelling evidence that vitamin D exerts a dual beneficial action in OA: it not only demonstrates robust chondroprotective capabilities, safeguarding cartilage from degradation, but also exhibits significant osteogenic actions, promoting healthy bone formation and remodeling.
Methods
To systematically investigate the therapeutic potential of vitamin D, our in vivo experimental phase utilized a cohort of Sprague-Dawley rats, randomly assigned into three distinct groups, with eight rats in each group: a sham operation group, serving as a control for surgical procedures; an osteoarthritis model group, in which OA was experimentally induced; and an OA group receiving vitamin D treatment. The OA model was reliably established through the intra-articular injection of Monosodium iodoacetate (MIA), a well-recognized method for inducing cartilage degeneration and OA-like symptoms. Following MIA induction, oral cholecalciferol, a specific form of vitamin D, was administered at a carefully determined daily dosage of 2.34 micrograms per kilogram of body weight. This treatment regimen was sustained for a period of six weeks, allowing for comprehensive evaluation of its effects on disease progression. The therapeutic efficacy of vitamin D in this animal model was thoroughly assessed through a series of detailed in vivo experiments, which included macroscopic and microscopic evaluations of joint tissues and biochemical analyses. Complementing the in vivo studies, an in vitro experimental phase was conducted utilizing human chondrocyte C28 cells, a well-established cell line used to model cartilage biology. To simulate the inflammatory conditions characteristic of osteoarthritis, these C28 cells were first pretreated with tumor necrosis factor-alpha (TNFα) at a concentration of 1 nanogram per milliliter, which is known to induce an inflammatory injury response in chondrocytes. Subsequently, the pretreated cells were exposed to 1,25(OH)2 D3, the biologically active form of vitamin D, at a concentration of 10-2 micromolar for a duration of 72 hours. This in vitro setup allowed for a focused assessment of vitamin D’s direct effects on chondrogenesis, the process of cartilage formation, and provided a crucial platform to delve deeper into its underlying molecular mechanisms of action within chondrocytes.
Results
In the established osteoarthritis rat model, the administration of vitamin D yielded remarkably significant and positive outcomes, fundamentally altering the course of the disease. Specifically, vitamin D treatment demonstrated a potent ability to suppress the progressive degradation of femoral cartilage, a hallmark of OA pathology. This chondroprotective effect was quantitatively evidenced by a substantial 567.76% increase in the preserved cartilage area compared to the untreated OA group, indicating a dramatic preservation of joint tissue. Concurrently, the Osteoarthritis Cartilage Histopathology (OACH) score, a comprehensive metric used to assess the severity of cartilage damage, showed a significant reduction of 39.13%, further affirming the protective effect on cartilage integrity. Beyond its direct impact on cartilage, vitamin D also exerted beneficial effects on the subchondral bone, manifesting as enhanced bone mass. This osteogenic action was quantitatively demonstrated by a 61.81% higher bone volume fraction (BV/TV), indicating a greater density and health of the bone beneath the cartilage. At the intricate molecular level, vitamin D treatment was found to critically downregulate the expression of cartilage matrix metalloproteinase 13 (MMP13), a key catabolic enzyme known to play a destructive role in cartilage matrix breakdown. This downregulation resulted in a substantial reduction of MMP13 expression by 74.72% when compared to the untreated OA group. Furthermore, our investigations revealed that vitamin D actively inhibited inflammatory signaling pathways, particularly targeting the MyD88-TAK1-ERK axis within chondrocytes, which is a crucial cascade involved in mediating inflammatory responses. This anti-inflammatory action was further supported by a measurable decrease in serum interleukin-6 (IL-6) levels, a prominent pro-inflammatory cytokine often elevated in OA. Mechanistic validation of these critical molecular findings was robustly demonstrated by a significant reduction in the protein expression of several key components of the inflammatory pathway: MyD88 expression was reduced by 31.22%, phospho-ERK1/2 by 66.11%, AP-1 by 61.43%, and NFκB by 34.36%, all compared to the untreated OA group. These reductions collectively pinpoint the specific molecular targets through which vitamin D exerts its anti-inflammatory and cartilage-protective effects. Moreover, in the in vitro experiments utilizing C28 chondrocyte cells, vitamin D successfully rescued the loss of cell viability that had been induced by ethanol, suggesting a direct protective effect on chondrocyte health. Concurrently, it significantly upregulated the expression of various cartilage anabolic markers, which are indicators of cartilage matrix synthesis and repair, further highlighting its role in promoting chondrogenesis and tissue regeneration.
Conclusion
These comprehensive and integrated findings, derived from both robust in vivo animal models and detailed in vitro cellular investigations, collectively establish vitamin D as a truly multimodal therapeutic agent for osteoarthritis. Its therapeutic utility is underpinned by its dual capacity to simultaneously target and counteract cartilage catabolism, the destructive processes that break down cartilage tissue, while concurrently promoting beneficial subchondral bone remodeling, fostering a healthier bone environment essential for joint integrity. Critically, VX-11e these beneficial actions are mechanistically linked to vitamin D’s ability to modulate specific intracellular signaling cascades, notably the MyD88-TAK1-ERK axis. By influencing this pivotal pathway, vitamin D effectively dampens inflammatory responses and shifts the cellular balance towards cartilage preservation and bone regeneration. Therefore, our study provides strong evidence that vitamin D could offer a novel and comprehensive therapeutic strategy for OA, addressing its complex pathology at both cartilage and bone levels.
Keywords: bone, cartilage, osteoarthritis, rat, vitamin D.