Oleanolic Acid Modulates 25-Hydroxyvitamin D3 1-alpha-hydroxylase in Osteoblasts and Human Mesenchymal Stem Cells

Abstract Objectives: 25-Hydroxyvitamin D3 1-alpha-hydroxylase (CYP27B1) catalyzes the hydroxylation of 25-hydroxyvitamin D3 (25(OH)D3) to 1alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3), the bioactive form of vitamin D3. Our previous studies suggested that oleanolic acid (OA), a pentacyclic triterpenoid presents in many food and herbs, can improve circulating 1,25(OH)2D3 in ovariectomized (OVX) mice and increase CYP27B1 expression in human renal proximal tubular cells (HKC-8). However, the role of OA in regulating CYP27B1 in bone is far from clear. The present study is designed to study the effects of OA on CYP27B1 expressions and 1,25(OH)2D3 production in bone cells. Methods: The mRNA and protein expressions of CYP27B1, as well as bone metabolism markers were determined in osteoblast-like UMR-106 cells in response to treatments of parathyroid hormone (PTH) (10–7 M) or OA (10–9 - 10–5 M) for 24 hours. By using excessive 25(OH)D3 (10–6 M) as substrate, cellular production of 1,25(OH)2D3 was measured to determine CYP27B1 activity. To mimic the physiological condition, human mesenchymal stem cells (hMSCs) were pre-treated with 25(OH)D3 (10–7 M) for 12 hours, followed by OA treatment (10–9 - 10–6 M) for another 24 hours, the osteogenic effects of OA on alkaline phosphatase (ALP) activity and CYP27B1 expression were evaluated. Results: PTH (10–7 M, p<0.001) and OA (10–9 M, p<0.05) significantly upregulated mRNA and protein expressions of CYP27B1 in UMR-106 cells. 4-hour treatments of PTH (10–7 M) and OA (10–9 M) also stimulated the 1,25(OH)2D3 production by 46.02 % (p<0.001) and 17.60 % (p<0.01), respectively. Moreover, the mRNA expressions of ALP and osteocalcin (OCN) involved in osteoblast differentiation, were upregulated in response to PTH and OA in UMR-106 cells (p<0.05). The protein expression of CYP27B1 was upregulated by treatment with OA in hMSCs supplemented with 25(OH)D3 (10–8 M, p<0.05 vs. supplement alone). Furthermore, OA (10–8 M) potentiated the effects of 25(OH)D3 on osteogenesis in hMSCs by enhancing ALP activity by 47.77 % (p<0.01). Conclusions: Our results indicated that the bone anabolic effects of OA are associated with its actions to improve local bioactivation of vitamin D3 in osteoblasts and hMSCs, suggesting the involvement of paracrine or autocrine activities of 1,25(OH)2D3 in mediating the actions of OA in bone. Funding Sources: This work is supported by research studentship of Wen-Xuan Yu, The Hong Kong Polytechnic University.