Activation of retinoic acid receptor gamma induces proliferation of immortalized hippocampal progenitor cells.

In the present study, we report evidence that activation of RARgamma promotes cell proliferation in immortalized hippocampal progenitor cell line HiB5. We found that treatment of HiB5 cells with all-trans- (all-trans-RA) or 9-cis-retinoic acid (9-cis-RA) significantly increased the number of dead floating cells as well as viable cells in serum-free defined medium (N2). Flow cytometric analysis of DNA contents revealed that the proportion of apoptotic cells over the whole cell population was not affected by both retinoids. Instead, the proportion of S phase cells was significantly increased by retinoids. Under this condition, bcl-2 mRNA levels were significantly increased over time by retinoid treatment, whereas bax mRNA levels were not affected. This suggests that retinoids increase viable cells by enhancing proliferation rather than by suppressing apoptosis. In an attempt to dissect the molecular mechanism underlying retinoid-induced HiB5 cell proliferation, we examined the expression patterns of retinoid receptors following retinoid treatment. Retinoids induced RARgamma mRNA, which paralleled the increase in the transactivation of strong retinoic acid response element (RARE) reporter construct. Accordingly, treatment of HiB5 cells with RARgamma-selective agonist (CD666) increased HiB5 cell number in a dose-dependent manner, which was blocked by co-treatment with RARgamma-selective antagonist (CD2665). Taken together, these data clearly indicate that activation of RARgamma increases proliferation of immortalized hippocampal progenitor cells.