Caspase-8-mediated intracellular acidification precedes mitochondrial dysfunction in somatostatin-induced apoptosis

Activation of initiator and effector caspases, mitochondrial changes involv ing a reduction in its membrane potential and release of cytochrome c (cyt c) into the cytosol, are characteristic features of apoptosis, These change s are associated with cell acidification in some models of apoptosis, The h ierarchical relationship between these events has, however, not been deciph ered. We have shown that somatostatin (SST), acting via the Src homology 2 bearing tyrosine phosphatase SHP-1, exerts cytotoxic action in MCF-7 cells, and triggers cell acidification and apoptosis, We investigated the tempora l sequence of apoptotic events linking caspase activation, acidification, a nd mitochondrial dysfunction in this system and report here that (i) SHP-1- mediated caspase-8 activation is required for SST-induced decrease in pH(i) , (ii) Effector caspases are induced only when there is concomitant acidifi cation. (iii) Decrease in pH, is necessary to induce reduction in mitochond rial membrane potential, cyt c release and caspase-9 activation and (iv) de pletion of ATP ablates SST-induced cyt c release and caspase-9 activation, but not its ability to induce effector caspases and apoptosis, These data r eveal that SHP-1-/caspase-8-mediated acidification occurs at a site other t han the mitochondrion and that SST-induced apoptosis is not dependent on di sruption of mitochondrial function and caspase-9 activation.