Background: In steroid-induced osteonecrosis, hypertrophy and hyperplasia of marrow fat cells and lipid deposition of osteocytes can be found in the femoral head. However, the precise reason is not clear yet. The aim of this study was to observe the effect of dexamethasone (Dex) on differentiation of marrow stromal cells (MSCs), and to investigate the pathobiological mechanism of steroid-induced osteonecrosis.
Methods: MSCs in cultures were treated with increasing concentrations of Dex (0, 10(-9), 10(-8), 10(-7), and 10(-6) mol/L) continuously for 21 days. The cells, which were exposed to 0 mol/L (control) or 10(-7) mol/L Dex for 4 - 21 days, were then cultured for 21 days without Dex. MSCs were stained with Sudan III. Number of adipocytes was counted under a light microscope. The activity of alkaline phosphatase (ALP) of MSCs treated with 0, 10(-8), 10(-7), and 10(-6) mol/L Dex for 12 days, and that treated with 0 mol/L and 10(-7) mol/L Dex for 8, 10, or 12 days were determined. The levels of triglycerides, osteocalcin and cell proliferation of MSCs treated with 0 mol/L and 10(-7) mol/L Dex were detected. The mRNA expression levels of adipose-specific 422 (aP2) gene and osteogenic gene type I collagen in MSCs treated with 0 mol/L and 10(-7) mol/L Dex for 6 days were analyzed by whole-cell dot-blot hybridization. Statistical analysis was performed using Student's t test and analysis of variance. P values less than 0.05 were considered significant statistically.
Results: The number of adipocytes in cultures increased with the duration of MSCs' exposure to Dex and the concentration of Dex. The level of ALP activity in the MSCs decreased with concentration of Dex. In the control group, it was 8.69 times of that in the 10(-7) mol/L Dex group on day 12 (t = 20.51, P < 0.001). The level of triglycerides in 10(-7) mol/L Dex group was 3.40 times of that in the control (t = 11.00, P < 0.001). The levels of cell proliferation and osteocalcin in the control were 1.54 and 2.42 times of that in the 10(-7) mol/L Dex group respectively. As compared to the control, the mRNA expression of adipose-specific 422 (aP2) gene in 10(-7) mol/L Dex group was significantly increased (t = 36.48, P < 0.001), and that of osteogenic gene type I collagen was decreased (t = 42.07, P < 0.001).
Conclusions: Dex can directly induce the differentiation of MSCs into a large number of adipocytes and inhibit their osteogenic differentiation, which provide a novel explanation for the pathologic changes of steroid-induced osteonecrosis.