The regulation of intestinal epithelial cell proliferation by 1,25-dihydroxyvitamin D(3) and transforming growth factor beta.

Abstract

The effects of the active metabolite of vitamin D$\sb3$, 1,25 (OH)$\sb2$D$\sb3$, and of transforming growth factor $\beta$1, TGF$\beta$1, on the proliferation and differentiation of an epithelial cell line derived from the rat small intestine (IEC-6) were investigated. IEC-6 cells contain the vitamin D receptor, which is upregulated by serum. TGF$\beta$1 has no effect on the activities of two markers of differentiation, alkaline phosphatase and sucrase. However 1,25 (OH)$\sb2$D$\sb3$ elevates alkaline phosphatase activity 8.9-fold within 72 hours. Both 1,25 (OH)$\sb2$D$\sb3$ and TGF$\beta$1 inhibit DNA synthesis in a concentration-dependent manner. Within 24 hours, 0.8 ng/ml TGF$\beta$1 and 500 nM 1,25 (OH)$\sb2$D$\sb3$ reduce $\sp3$H-thymidine incorporation by 55% and 60% respectively. A transient time course of DNA synthesis results from TGF$\beta$1 treatment while 1,25 (OH)$\sb2$D$\sb3$ treatment produces a sustained effect. The effect of 1,25 (OH)$\sb2$D$\sb3$ on DNA synthesis is partially blocked with neutralizing antibodies to TGF$\beta$ which suggest that active TGF$\beta$ is produced in response to 1,25 (OH)$\sb2$D$\sb3$. TGF$\beta$1 induces an accumulation of cells in the G$\sb0$G$\sb1$ phase of the cell cycle while 1,25 (OH)$\sb2$D$\sb3$ does not. However both 1,25 (OH)$\sb2$D$\sb3$ and TGF$\beta$1 inhibit the phosphorylation of the product of the retinoblastoma tumor suppressor gene. In conclusion, we have shown that 1,25 (OH)$\sb2$D$\sb3$ is a potent inhibitor of intestinal epithelial cell proliferation and have obtained indirect evidence to support a role for TGF$\beta$ in this effect. We speculate that modulation of IEC-6 cell proliferation may involve TGF$\beta$, whereas effects of 1,25 (OH)$\sb2$D$\sb3$ on differentiated markers, such as alkaline phosphatase, are mediated independently of TGF$\beta$.