Retinoic acid plays an important role in the regulation of cell growth and differentiation. In our present study, we evaluated the effects of all-trans retinoic acid (RA) on cell proliferation and on the cell cycle regulation of human gingival fibroblasts (HGFs). Cell proliferation was assessed using the MTT assay. Cell cycle analysis was performed by flow cytometry, and cell cycle regulatory proteins were determined by western blot. Cell proliferation was increased in the presence of a 0.1 nM to 1μM RA dose range, and maximal growth stimulation was observed in cells exposed to 1 nM of RA. Exposure of HGFs to 1 nM of RA resulted in an augmented cell cycle progression. To elucidate the molecular mechanisms underlying cell cycle regulation by RA, we measured the intracellular levels of major cell cycle regulatory proteins. The levels of cyclin E and cyclin-dependent kinase (CDK) 2 were found to be increased in HGFs following 1 nM of RA treatment. However, the levels of cyclin D, CDK 4, and CDK 6 were unchanged under these conditions. Also after exposure to 1 nM of RA, the protein levels of p21 WAF1/CIP1 and p16 INK4A were decreased in HGFs compared with the control group, but the levels of p53 and pRb were similar between treated and untreated cells. These results suggest that RA increases cell proliferation and cell cycle progression in HGFs via increased cellular levels of cyclin E and CDK 2, and decreased cellular levels of p21 WAF1/CIP1 and p16 INK4A.

Endocrine disrupting chemicals (EDCs) have detrimental effects on human health. Among these EDCs, bisphenol A (BPA) binds to estrogen receptors (ERs) to stimulate estrogen-mediated responses. BPA is assumed to disrupt the reproductive and developmental system of humans. In addition, BPA has recently been suspected as a risk of carcinogenesis. Because BPA can cause abnormal estrogen-mediated response in the organism, exposure to BPA may stimulate growth of estrogen-dependent breast cancers in human. In breast cancer, cyclin E and cyclin-dependent kinase inhibitor p27 are important in G1/S phase transition during cell cycle progression. In this study, using an MTT assay, we investigated the effect of BPA on proliferation of MCF-7 breast cancer cells in vitro. In addition, we also analyzed the transcriptional levels of cyclin E and p27 following treatment with BPA using semi-quantitative RT-PCR. As a result, treatment with BPA resulted in significant induction of breast cancer cell growth, compared to a vehicle. BPA caused alterations of cyclin E and p27 mRNA expression. Expression of cyclin E was increased by BPA, while p27 was decreased at 24 h after treatment with BPA in MCF-7 breast cancer cells. Taken together, these collective results suggest that exposure to BPA induced breast cancer cell proliferation with deregulation of the cell cycle. A further study is required in order to determine the effects of BPA on the carcinogenic process in in vivo models.