Objective To assess the coordinated regulation and the molecular mechanisms of 17β-estradiol and 1,25-dihydroxyvitamin D3 [ 1,25-( OH ) 2 D3 ] on the proliferation and differentiation of osteoblastic MC3T3-E1 cells.Methods MC3T3-E1 cells were cultured in phenol-red free α-MEM medium supplemented with 10％ FBS,MTT assay was performed to determine the effects of 17β-estradiol and 1,25-( OH )2 D3 on MC3T3-E1 cells proliferation.After cells were treated with different agents,cell cycle related genes [ cyclin E,proliferation cell nuclear antigen ( PCNA ),and cyclin-dependent kinase inhibitor 2b ( Cdkn2b ) ] and markers of osteoblastic differentiation [ type Ⅰ collagen ( COL Ⅰ ),alkaline phosphatase ( ALP),osteopontin ( OPN ) ] were detected with SYBR green-based quantitative PCR.ALP activity was detected with BCIP/NBT method.Results 17β-estradiol could promote proliferation of MC3T3-E1 cells,which was accordant to its ability to increase cyclin E and PCNA and to inhibit Cdkn2b mRNA expression in MC3T3-E1 cells.However,1,25-( OH)2D3 had no effect on the proliferation of MC3T3-E1 cells and also did not enhance the proliferation of MC3T3-E1 cells stimulated by 17β-estradiol.On the other hand,17β-estradiol promoted the gene expression of differentiation markers Col Ⅰ,ALP,and OPN,and 1,25-(OH) 2 D3 synergistically increased the expression of these genes with 17 β-estradiol.Conclusion As two of the most important hormones which regulate bone metabolism,estrogen and vitamin D may coordinately promote osteoblast differentiation,but may not regulate osteoblasts proliferation synergistically.