Posttranslational mechanisms contribute to the suppression of specific cyclin : CDK complexes by all-trans retinoic acid in human bronchial epithelial cells

Retinoids have demonstrated activity in the chemoprevention of aerodigestiv e tract cancer. Potentially contributing to their lung cancer chemopreventi ve effects, retinoids inhibit the growth of human bronchial epithelial (HBE ) cells. We observed previously that all-trans retinoic acid (t-RA) arrests the growth of HBE cells in the G(0) phase of the cell cycle through activa tion of retinoic acid receptor-dependent pathways, which enhances the assoc iation of E2F-4 with retinoblastoma protein family members, converting E2F into a transcriptional suppressor. In this study, we examined the mechanism by which t-RA blocks cell cycle progression in HBE cells and the possibili ty that this signaling event is blocked in non-small cell lung cancer (NSCL C) cells that are refractory to the growth inhibitory effects of t-RA. t-RA suppressed the expression and activity of cyclin D1, cyclin E, and cyclin- dependent kinases (CDK)-2 and CDK-4, increased expression of the CDK inhibi tor p27, and shifted the retinoblastoma protein to a hypophosphorylated for m. Posttranslational mechanisms contributed to the changes in CDK-2, CDK-4, and p27 levels, which, in the case of CDK-4, involved the ubiquitin-protea some pathway. In contrast, despite retinoic acid receptor transcriptional a ctivation, these signaling events did not occur in a NSCLC cell line that i s refractory to growth inhibition by t-RA. These findings provide the first evidence that t-RA activates degradation of CDK-4 through the ubiquitin-pr oteasome pathway, a novel mechanism by which t-RA causes HBE cells to exit the cell cycle, and blockade of these signaling events may contribute to th e development of retinoid resistance in NSCLC cells.