ALTERATIONS IN DIFFERENTIATION AND APOPTOSIS INDUCED BY BUFALIN IN CJUN OVEREXPRESSING U-937 CELLS

Recent evidence suggests that c-jun plays a pivotal role in macrophage differentiation induced by multiple agents and that cJun overexpressi on induces partial macrophage differentiation in the leukemic cell lin e U-937. The novel differentiating agent bufalin, a Na+/K+ ATPase inhi bitor, has also been shown to induce macrophage differentiation in U-9 37. In order to further define the role of c-jun in macrophage differe ntiation, we examined the function of c-jun/AP-1 during bufalin induce d differentiation in both c-jun transfected and parental U-937 cells. In contrast to phorbol esters, bufalin does not significantly stimulat e c-jun or c-fos mRNA expression or AP-1 transactivation. However, buf alin treatment leads to markedly greater morphologic and functional ch anges indicative of more extensive differentiation in the cJun overexp ressing cells compared to the non-transfected controls. Furthermore, c Jun overexpressing cells maintain greater viability in the presence of bufalin and arrest in a different phase of the cell cycle than do con trol cell lines (G0/G1 versus S/G2M, respectively), although some apop tosis occurs in all cell lines treated with bufalin. These data sugges t that while bufalin can induce some degree of differentiation in U-93 7 cells independent of c-jun/AP-1 controlled pathways, the involvement of these pathways by enforced cJun expression enhances the extent of differentiation and shifts the balance between differentiation and apo ptosis.