Progesterone and promegestone stimulate human bone cell proliferation and insulin-like growth factor-2 production

Abstract

Recent clinical studies suggest that progesterone may be involved in the regulation of bone turnover and could promote bone formation. This study was undertaken to evaluate whether progesterone and promegestone (a 19 nor-PG derivative) may have a direct effect on human bone cells and, if so, whether growth factor production could be involved in promoting this effect. The osteosarcoma cell line TE85 and untransformed normal human osteoblastic cells derived from iliac crest were used as in vitro model systems. Progesterone and promegestone were found to significantly increase [3H]thymidine incorporation in TE8 5 cells in a dose-dependent manner at concentrations ranging from 10−12to 10−8 mol/l after four days of cultivation (p<0.01, ANOVA). Consistent with this response in the TE85 cells, progesterone and promegestone increased cell number in human osteoblastic cells after six days of treatment (p<0.05. ANOVA). To determine whether this effect on cell proliferation was mediated by the insulin-like growth factor (IGF) regulatory system, the levels of IGF-1, IGF-2 and IGF binding protein (IGFBP) were measured in the conditioned media of both TE85 and human osteoblast cells. While no significant changes in IGF-1 levels were found in the conditioned media of progesterone and promegestone treated cultures, progesterone and promegestone at the concentration of 5 nmol/l significantly increased IGF-2 levels 2.4 and 1.5-fold respectively, at 48 h in the conditioned medium of TE8 5 cells as compared to control. Similarly, a 4.1 and 1.9-fold increase in IGF-2 levels was found upon treatment with progesterone and promegestone in human osteoblastic cells. Consistent with the increased secretion of IGF-2 into the conditioned medium, IGF-2 mRNA levels were found to be increased in TE85 cells. A 4.9 kb transcript was increased 2.7 and 3.7-fold respectively after 6 h of exposure to 5 nmol/l of progesterone and promegestone as compared to control. Western ligand blot analysis of conditioned medium collected from TE85 and human osteoblast cell cultures treated with progesterone and promegestone revealed no changes in the levels of IGFBP-3 and IGFBP-4 after 48 h of treatment. Consistent with these results, the IGFBP-4 mRNA level was unaffected. These data suggest that both progesterone and promegestone stimulate human bone cell proliferation and that the mechanism may in part involve increased IGF-2 secretion. Because IGF-2 has been proposed to play a potential role in the coupling of bone formation to bone resorption, it follows that progesterone deficiency may be involved in the uncoupling that occurs in postmenopause. In any case, the findings that progesterone and promegestone have direct effects on bone formation could have physiological implications.