ROLE OF C-JUN IN HUMAN MYELOID-LEUKEMIA CELL APOPTOSIS INDUCED BY PHARMACOLOGICAL INHIBITORS OF PROTEIN-KINASE-C

Recent study results suggest that protein kinase C [PKC (EC 3.1.4.3)]- dependent up-regulation of c-jun may be involved in leukemic cell prog rammed cell death, or apoptosis, occurring in response to various chem otherapeutic agents. The current study was undertaken to further evalu ate the contribution of c-jun in apoptosis with the use of two highly specific pharmacological inhibitors of PKC (calphostin C and cheleryth rine). To address this issue, two human leukemic cell lines, HL-60 and U937, and a U937 subline stably expressing a dominant negative c-jun mutant (TAM67) were exposed to calphostin C and chelerythrine, and c-j un expression was monitored at both the mRNA and protein levels. Both PKC inhibitors induced the classic morphological features of apoptosis as well as internucleosomal DNA degradation in a concentration- and s chedule-dependent manner. Concomitant with these changes, unequivocal increases were observed in c-jun mRNA (U937 and HL-60) and protein (U9 37). In contrast, up-regulation of c-jun mRNA and protein in TAM67-exp ressing cells exposed to both PKC inhibitors was markedly attenuated r elative to effects observed in parental U937 cells. Importantly, despi te impaired up-regulation of c-jun at both the message and protein lev els, TAM67-expressing cells were equally susceptible to PKC inhibitor- induced apoptosis as parental and empty vector U937 cells. Collectivel y, these findings raise the possibility that c-jun up-regulation in hu man myeloid leukemia cells undergoing PKC inhibitor-associated apoptos is represents a response to, rather than a cause of, apoptotic events. They further suggest that this phenomenon involves pathways that do n ot require PKC activation.