Age-related decline in osteoblastogenesis and 1a-hydroxylase/CYP27B1 in human mesenchymal stem cells: stimulation by parathyroid hormone

With aging, there is a decline in bone mass and in osteoblast differentiation of human mesenchymal stem cells (hMSCs) in vitro. Osteoblastogenesis can be stimulated with 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and, in some hMSCs, by the precursor 25-hydroxyvitamin D3 (25OHD3). CYP27B1/1a-hydroxylase activates 25OHD3 and, to a variable degree, hMSCs express CYP27B1. In this study, we tested the hypotheses (i) that age affects responsiveness to 25OHD3 and expression/activity of CYP27B1 in hMSCs and (ii) that parathyroid hormone (PTH) upregulates CYP27B1 in hMSCs, as it does in renal cells. There were age-related declines in osteoblastogenesis (n = 8, P = 0.0286) and in CYP27B1 gene expression (n = 27, r = -0.498; P = 0.008) in hMSCs. Unlike hMSCs from young subjects (=50 years), hMSCs from older subjects (=55 years) were resistant to 25OHD3 stimulation of osteoblastogenesis. PTH1-34 (100 nm) provided hMSCs with responsiveness to 25OHD3 (P = 0.0313, Wilcoxon matched pairs test) and with two episodes of increased 1,25(OH)2D3 synthesis, of cAMP response element binding protein (CREB) activation, and of CYP27B1 upregulation. Both increases in CYP27B1 expression by PTH were obliterated by CREB-siRNA or KG-501 (which specifically inhibits the downstream binding of activated CREB). Only the second period of CREB signaling was diminished by AG1024, an inhibitor of insulin-like growth factor-I receptor kinase. Thus, PTH stimulated hMSCs from elders with responsiveness to 25OHD3 by upregulating expression/activity of CYP27B1 and did so through CREB and IGF-I pathways.