Selective susceptibility of transformed T lymphocytes to induction of apoptosis by PSC 833, an inhibitor of P-glycoprotein

P-glycoprotein is a cellular efflux pump. The P-glycoprotein inhibitor PSC 833 causes apoptosis of cancer cells and induces a rise in the intracellula r levels of ceramide. Our aims were to determine whether a cause and effect relationship exists between these two actions of PSC 833, and to assess wh ether the PSC 833-induced apoptosis is restricted to transformed cells. Apo ptosis was determined by flow cytometry and radioactive quantitation of DNA fragmentation. PSC 833 induced apoptosis in the human T leukemia cell line s: Molt-4 and Jurkat, Analysis of the apoptosis in Molt-4 and Jurkat cells revealed that PSC 833 induced a rise in the cellular ceramide levels (as me asured by the DG kinase assay). PSC 833-induced apoptosis was significantly reduced by specific inhibitors of ceramide de novo synthesis (i.e., fumoni sin B1 and L-cycloserine). On the other hand, PSC 833 did not induce apopto sis in normal peripheral blood T cells regardless of whether these cells we re quiescent, activated, or proliferating. Our results suggest that PSC 833 induces apoptotic death in human transformed T lymphocytes through an incr ease in ceramide de novo synthesis. In addition, normal lymphocytes are not susceptible to induction of apoptosis by PSC 833. This difference between normal lymphocytes and leukemia cells presents a potential target for chemo therapy.