Background: Vitamin D is an important steroid that can regulate bone metabolism including osteoclast (OC) differentiation. Transient receptor potential cation channel subfamily V member 5 (TRPV5), is a calcium channel protein involved in OC differentiation. However, the impact of vitamin D on TRPV5 expression during OC differentiation is not clear. Objectives: To determine if 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) regulates the expression of TRPV5 during OC differentiation. Methods: Bone marrow mononuclear macrophage (BMMs) were induced to differentiate into OC with or without treatment with 10 nM 1,25(OH)2D3. The expression levels of vitamin D receptor (VDR) and TRPV5 were examined. The expression of several OC markers, including tartrate resistant acid phosphatase (TRAP), carbonic anhydrase II (Ca II), cathepsin K (CTSK), and vacuolar-type H + -ATPase (V-ATPase) were also detected. Results: We found that the VDR was expressed in murine bone marrow-derived macrophages at the early stage of OC differentiation. TRPV5 expression was increased during OC differentiation, which was down-regulated by 1,25(OH)2D3 after a prolonged exposure. The 1,25(OH)2D3 and TRPV5 inhibitors inhibited OC differentiation. Conclusions: 1,25(OH)2D3 can inhibit TRPV5 expression as well as TRPV5 inhibitors during OC differentiation. This suggests that 1,25(OH)2D3 may suppress OC differentiation by inhibiting TRPV5 expression.

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Bones play an important role in maintaining the level of calcium in blood. They provide support for soft tissues and hematopoiesis and undergo continuous renewal throughout life. In addition, vitamin D is involved in regulating bone and calcium homeostasis. Galectin-3 (Gal-3) is a β-galactoside-binding protein that can regulate bone cell differentiation and function. Here, we aimed to study the regulatory effects of Gal-3 on vitamin-D-regulated osteoclastogenesis and bone resorption in chicken. Gal-3 expression in bone marrow stromal cells (BMSCs) from 18-day-old chicken embryos was inhibited or overexpressed. BMSCs were then co-cultured with bone marrow monocytes/macrophages (BMMs) with or without addition of 1α,25(OH)2D3. The results showed that 1α,25(OH)2D3 upregulated the expression of Gal-3 mRNA and receptor activator of nuclear-factor κB ligand (RANKL) expression in BMSCs and promoted osteoclastogenesis, as shown by the upregulated expression of osteoclast (OC) markers (CtsK, CAII, MMP-9, and TRAP) and increased bone resorption, a method for measuring the bone resorption area in vitro. Knockdown of Gal-3 by small-interfering RNA (siRNA) in BMSCs downregulated the expression of RANKL mRNA and attenuated the effects of 1α,25(OH)2D3 on osteoclastogenesis and bone resorption. Conversely, overexpression of Gal-3 in BMSCs enhanced the effects of osteoclastogenesis and bone resorption by increasing the expression of RANKL mRNA. These results demonstrated that Gal-3 mediates the differentiation and bone resorption of osteoclasts regulated by 1α,25(OH)2D3.