POSTTRANSLATIONAL REGULATION OF CYCLIN D1 BY RETINOIC ACID - A CHEMOPREVENTION MECHANISM

The retinoids are reported to reduce incidence of second primary aerod igestive cancers. Mechanisms for this chemoprevention are previously l inked to all-trans retinoic acid (RAJ signaling growth inhibition at G (1) in carcinogen-exposed immortalized human bronchial epithelial cell s, This study investigated how RA suppresses human bronchial epithelia l cell growth at the G(1)-S cell cycle transition, RA signaled growth suppression of human bronchial epithelial cells and a decline in cycli n D1 protein but not mRNA expression. Exogenous cyclin D1 protein also declined after RA treatment of transfected, immortalized human bronch ial epithelial cells, suggesting that posttranslational mechanisms wer e active in this regulation of cyclin D1 expression, Findings were ext ended by showing treatment with ubiquitin-dependent proteasome inhibit ors: calpain inhibitor I and lactacystin each prevented this decreased cyclin D1 protein expression, despite RA treatment, Treatment with th e cysteine proteinase inhibitor, E-64, did not prevent this cyclin D1 decline, High molecular weight cyclin D1 protein species appeared afte r proteasome inhibitor treatments, suggesting that ubiquitinated speci es were present, To learn whether RA directly promoted degradation of cyclic D1 protein, studies using human bronchial epithelial cell prote in extracts and in vitro-translated cyclin D1 were performed, In vitro -translated cyclin D1 degraded more rapidly when incubated with extrac ts from RA treated vs, untreated cells, Notably, this RA-signaled cycl in DI proteolysis depended on the C-terminal PEST sequence, a region r ich in proline (P), glutamate (E), serine (S), and threonine (T), Take n together, these data highlight RA-induced cyclin D1 proteolysis as a mechanism signaling growth inhibition at G(1) active in the preventio n of human bronchial epithelial cell transformation.