Cyclin D1 proteolysis: A retinoid chemoprevention signal in normal, immortalized, and transformed human bronchial epithelial cells

Background: Retinoids (derivatives of vitamin A) are reported to reduce the occurrence of some second primary cancers, including aerodigestive tract t umors. In contrast, beta-carotene does not reduce the occurrence of primary aerodigestive tract cancers. Mechanisms explaining these effective retinoi d and ineffective carotenoid chemoprevention results are poorly defined. Re cently, the all-trans-retinoic acid (RA)-induced proteolysis of cyclin D1 t hat leads to the arrest of cells in G(1), phase of the cell cycle was descr ibed in human bronchial epithelial cells and is a promising candidate for s uch a mechanism. In this study, we have investigated this proteolysis as a common signal used by carotenoids or receptor-selective and receptor-nonsel ective retinoids, Methods: We treated cultured normal human bronchial epith elial cells, immortalized human bronchial epithelial cells (BEAS-2B), and t ransformed human bronchial epithelial cells (BEAS-2B(NNK)) with receptor-se lective or receptor-nonselective retinoids or with carotenoids and studied the effects on cell proliferation by means of tritiated thymidine incorpora tion and on cyclin D1 expression by means of immunoblot analysis. We also e xamined whether calpain inhibitor I, an inhibitor of the 26S proteasome deg radation pathway, affected the decline (i,e., proteolysis) of cyclin D1, Re sults: Receptor-nonselective retinoids were superior to the carotenoids stu died in mediating the decline in cyclin D1 expression and in suppressing th e growth of bronchial epithelial cells. Retinoids that activated retinoic a cid receptor beta or retinoid X receptor pathways preferentially led to a d ecrease in the amount of cyclin D1 protein and a corresponding decline in g rowth. The retinoid-mediated degradation of cyclin D1 was blocked by cotrea tment with calpain inhibitor I. Conclusions: Retinoid-dependent cyclin D1 p roteolysis is a common chemoprevention signal in normal and neoplastic huma n bronchial epithelial cells. In contrast, carotenoids did not affect cycli n D1 expression, Thus, the degradation of cyclin D1 is a candidate intermed iate marker for effective retinoid-mediated cancer chemoprevention in the a erodigestive tract.