The downregulation of the pro-apoptotic protein Par-4 is critical for Ras-induced survival and tumor progression

Inhibition of apoptosis is an important characteristic of oncogenic transfo rmation. The Par-4 gene product has recently been shown to be upregulated i n cells undergoing apoptotic cell death, and its ectopic expression was sho wn to be critical in apoptosis, We demonstrate that expression of oncogenic Pas promotes a potent reduction of Par-4 protein and mRNA levels through a MEK-dependent pathway. In addition, the expression of permanently active m utants of MEK, Raf-l or zeta protein kinase C but not of phosphatidylinosit ol 3-kinase (PI 3-kinase) is sufficient to decrease Par-4 levels. These eff ects are independent of p53, p16 and p19, and were detected not only in fib roblast primary cultures but also in NIH 3T3 and HeLa cells, indicating tha t they are not secondary to Pas actions on cell cycle regulation. Important ly, restoration of Par-4 levels to normal in Ras-transformed cells makes th ese cells sensitive to the pro-apoptotic actions of tumor necrosis factor-a lpha under conditions in which PI 3-kinase is inhibited and also severely i mpairs colony formation in soft agar and tumor development in nude mice, as well as increases the sensitivity of these tumors to camptothecin, This in dicates that the downregulation of Par-4 by oncogenic Pas is a critical eve nt in tumor progression.