All-trans retinoic acid converts E2F into a transcriptional suppressor and inhibits the growth of normal human bronchial epithelial cells through a retinoic acid receptor-dependent signaling pathway

Retinoids, including retinol and retinoic acid derivatives, maintain the normal growth and differentiation of human bronchial epithelial (HBE) cells and are under investigation as agents for lung cancer prevention. In this study, we examined the biologic effect of retinoids on normal HBE cells and the molecular mechanisms of retinoid actions. At a dose of 10^-6 M, all- trans retinoic acid (t-RA) suppressed the proliferation of normal HBE cells, which accumulated in the G₀ phase. No evidence of programmed cell death was observed. The class of retinoid nuclear receptor that mediated the growth arrest was explored. Normal HBE cell growth was suppressed by a retinoid that selectively activates retinoic acid receptors but not by one that activates retinoid X receptors. The E2F transcription factor has demonstrated a role in G₀ entry through transcriptional suppression of genes that induce cell cycle progression. To investigate the role of E2F in retinoid signaling, transient transfection assays were performed using reporter plasmids containing E2F- binding sites. Findings from these experiments suggested that t-RA treatment converted E2F into a transcriptional suppressor. Supporting this possibility, t-RA inhibited the expression of the E2F target genes B-myb, cyclin A, and cyclin E. Further, t-RA increased the levels of nuclear E2F-4, p107, and p130 and enhanced the binding of E2F-4 to p107, which have been associated with the conversion of E2F into a transcriptional suppressor in other cells. These findings point to retinoic acid receptor- and E2F-dependent pathways as potential mediators of retinoid-induced growth arrest in normal HBE cells and have implications for the use of retinoids in clinical trials on the prevention of lung cancer.