The role of Pdia3 in vitamin D signaling in osteoblasts
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1a,25-Dihydroxyvitamin D3 (1a,25(OH)2D3) is a major functional metabolic form of vitamin D. 1a,25(OH)2D3 has drawn increasing attention due to its functions in addition to maintaining calcium phosphate homeostasis. It directly regulates mineralization by osteoblasts, matrix production and remodeling by chondrocytes, and contraction of cardiomyocytes. 1a,25(OH)2D3 and its analogues have shown beneficial effects in treating multiple sclerosis, diabetes and various types of cancer. In order to maximize the pharmaceutical potential of 1a,25(OH)2D3, a better understanding its cell signaling pathway is necessary. 1a,25(OH)2D3 regulates osteoblasts through both classical nuclear vitamin D receptor (nVDR) mediated genomic effects and plasma membrane receptor-mediated rapid responses. The identity of the plasma membrane receptor for 1a,25(OH)2D3 is controversial. Protein disulfide isomerase associated 3 (Pdia3) has been hypothesized as one of the putative plasma membrane receptors for 1a,25(OH)2D3. The overall goal of this thesis was to understand the general role and the molecular mechanism of Pdia3 in 1a,25(OH)2D3-initiated rapid responses, and to determine the role of Pdia3 and its dependent signaling in osteoblast biology. The results show that Pdia3 is required for membrane-mediated responses of 1a,25(OH)2D3. Moreover, both Pdia3 and nVDR are critical components of the plasma membrane receptor complex for 1a,25(OH)2D3. Finally, Pdia3 and signaling via Pdia3 regulate osteoblast differentiation and mineralization. Taken together, this study demonstrates the role of Pdia3 in rapid responses to 1a,25(OH)2D3 and osteoblast biology, reveals the unexpected complexity of the 1a,25(OH)2D3 plasma receptor complex and opens the new target, Pdia3, for pharmaceutical application and tissue engineering.