EXPRESSION AND FUNCTION OF THE LEUCINE-ZIPPER PROTEIN PAR-4 IN APOPTOSIS

The prostate apoptosis response-4 (par-4) gene was identified by diffe rential screening for genes that are upregulated when prostate cancer cells are induced to undergo apoptosis. The par-4 gene is induced by a poptotic signals but not by growth-arresting, necrotic, or growth-stim ulatory signals. The deduced amino acid sequence of par-4 predicts a p rotein with a leucine zipper domain at its carboxy terminus. We have r ecently shown that the Par-4 protein binds, via its leucine zipper dom ain, to the zinc finger domain of Wilms' tumor protein WT1 (R. W. John stone et al., Mel. Cell. Biol. 16:6945-6956, 1996). In experiments aim ed at determining the functional role of par-4 in apoptosis, an antise nse par-ii oligomer abrogated pal-ii expression and activator-driven a poptosis in rat prostate cancer cell line AT-3, suggesting that par-4 is required for apoptosis in these cells. Consistent with a functional role for par-4 in apoptosis, ectopic overexpression of par-4 in prost ate cancer cell line PC-3 and melanoma cell line A375-C6 conferred sup ersensitivity to apoptotic stimuli. Transfection studies with deletion mutants of Par-4 revealed that full-length Par-4, but not mutants tha t lacked the leucine zipper domain of Par-4, conferred enhanced sensit ivity to apoptotic stimuli. Most importantly, ectopic coexpression of the leucine zipper domain of Par-4 inhibited the ability of Par-ii to enhance apoptosis. Finally, ectopic expression of WT1 attenuated apopt osis, and coexpression of Par-4 but not a leucine zipperless mutant of Par-ii rescued the cells from the antiapoptotic effect of WT1. These findings suggest that the leucine zipper domain is required for the Pa r-4 protein to function in apoptosis.