Differential involvement of initiator caspases in apoptotic volume decrease and potassium efflux during Fas- and UV-induced cell death

Caspase activation and apoptotic volume decrease are fundamental features o f programmed cell death; however, the relationship between these components is not well understood. Here we provide biochemical and genetic evidence f or the differential involvement of initiator caspases in the apoptotic volu me decrease during both intrinsic and extrinsic activation of apoptosis. Ap optosis induction in Jurkat T lymphocytes by Fas receptor engagement (intri nsic) or ultraviolet (UV)-C radiation (extrinsic) triggered the loss of cel l volume, which was restricted to cells with diminished intracellular K+ io ns. These characteristics kinetically coincided with the proteolytic proces sing and activation of both initiator and effector caspases. Although the p olycaspase inhibitor benzyloxycarbonyl-Val-Ala-Asp fluoromethyl ketone comp letely inhibited the Fas-mediated apoptotic volume decrease and K+ efflux, it was much less effective in preventing these processes during UV-induced cell death under conditions whereby caspase activities and DNA degradation were blocked. To define the roles of specific initiator caspases, we utiliz ed Jurkat cells genetically deficient in caspase-8 or stably transfected wi th a dominant-negative mutant of caspase-9. The results show that the activ ation of caspase-8, but not caspase-9, is necessary for Fas-induced apoptos is. Conversely, caspase-9, but not caspase-8, is important for UV-mediated shrunken morphology and apoptosis progression. Together, these findings ind icate that cell shrinkage and K+ efflux during apoptosis are tightly couple d, but are differentially regulated by either caspase-8 or caspase-9 depend ing on specific pathways of cell death.