DEATH OF TUMOR-CELLS AFTER INTRACELLULAR ACIDIFICATION IS DEPENDENT ON STRESS-ACTIVATED PROTEIN-KINASES (SAPK JNK) PATHWAY ACTIVATION AND CANNOT BE INHIBITED BY BCL-2 EXPRESSION OR INTERLEUKIN 1-BETA-CONVERTING ENZYME-INHIBITION/

The extracellular microenvironment of tumors differs from that of most normal tissues. Many tumors have relatively acidic extracellular pH, although the intracellular pH of tumor cells remains normal due to the efficient maintenance of a large proton gradient across the membrane. This difference between tumors and normal tissues might be exploited therapeutically by disruption of the mechanisms that regulate intracel lular pH, so that tumor cells are killed by intracellular acid-induced injury. To investigate the mechanisms by which intracellular acidific ation leads to cell death, we have studied the roles of the antiapopto tic gene bcl-2 and its proapoptotic binding partner bar, the stress-ac tivated protein kinases (SAPK/JNK), and the caspase proteases in media ting acid-induced cell death. Whereas the expression of bcl-2 in human bladder cancer MGH-U1 cells had no effect on acid-induced death, over expression of bar enhanced cell death, consistent with its proapoptoti c function. Inhibition of SAPK, through the expression of a dominant n egative mutant of its activator, SEK1, protected cells from acid-induc ed cell death. Caspase activation, as measured by poly(ADP-ribose) pol ymerase cleavage, was absent after lethal intracellular acidification. Consistent with this observation, inhibition of interleukin 1 beta-co nverting enzyme proteases by the peptide z-Val-Ala-Asp(OMe)-CH2F did n ot protect against acid-induced cell killing. We conclude that acid-in duced cell death depends on bar and on SAPK signaling pathways, but no t on the caspase proteases. Therapeutic manipulation of bar and SAPK m ay enhance acid-induced tumor cell killing.