REGULATION OF CYCLIN D1 BY CALPAIN PROTEASE

Cyclin D1, a critical positive regulator of G(1) progression, has been implicated in the pathogenesis of certain cancers. Regulation of cycl in D1 occurs at the transcriptional and posttranscriptional level. Her e we present evidence that cyclin D1 levels are regulated at the postt ranscriptional level by the Ca2+-activated protease calpain. Serum sta rvation of NIH 3T3 cells resulted in rapid loss of cyclin D1 protein t hat was completely reversible by calpain inhibitors. Actinomycin D and lovastatin induced rapid loss of cyclin D1 in prostate and breast can cer cells that was reversible by calpain inhibitors and not by phenylm ethylsulfonyl fluoride, caspase inhibitors, or lactacystin, a specific inhibitor of the 26 S proteasome. Treatment of intact NIH 3T3, prosta te, and breast cancer cells with a calpain inhibitor dramatically incr eased the half-life of cyclin D1 protein, Addition of purified calpain to PC-3-M lysates resulted in Ca2+-dependent cyclin D1 degradation. T ransient expression of the calpain inhibitor calpastatin increased cyc lin D1 protein in serum-starved NIR 3T3 cells. Cyclins A, E, and El ha ve been reported to be regulated by proteasome-associated proteolysis. The data presented here implicate calpain in cyclin D1 posttranslatio nal regulation.