PTH upregulates IGF-I [12] which stimulates bone formation in hMSCs [8]. PTH upregulates CYP27B1 expression and stimulates 1α,25(OH)2D3 production [12], and at higher concentration, 1α,25(OH)2D3 downregulates CYP27B1 [8]. In addition, IGF-I upregulates CYP27B1 expression and stimulates 1α,25(OH)2D3 biosynthesis [8]. 1α,25(OH)2D3 induces IGF-I and bone formation in hMSCs [8].