Interdependent regulation of intracellular acidification and SHP-1 in apoptosis

The G protein-coupled receptor agonist somatostatin (SST)-induces apoptosis in MCF-7 human breast cancer cells. This is associated with induction of wild-type p53, Bax, and an acidic endonuclease. We have shown recently that its cytotoxic signaling is mediated via membrane-associated SHP-1 and is dependent on decrease in intracellular pH (pH(i)) to 6.5. Here we investigated the relationship between intracellular acidification and SHP- 1 in cytotoxic signaling. Clamping of pH(i) at 7.25 by the proton-ionophore nigericin abolished SST-signaled apoptosis without affecting its ability to regulate SHP-1, p53, and Bax. Apoptosis could be induced by nigericin clamping of pH(i) to 6.5. Such acidification-induced apoptosis was not observed at pH(i) <6.0 or >6.7. pH(i)-dependent apoptosis was associated with the translocation of SHP-1 to the membrane, enhanced in cells overexpressing SHP-1, and was abolished by its inactive mutant SHP-1C455S. Acidification caused by inhibition of Na⁺/H⁺ exchanger and H⁺ ATPase (pH(i) = 6.55 and 6.65, respectively) also triggered apoptosis. The effect of concurrent inhibition of Na⁺/H⁺ exchanger and H⁺-ATPase on pH(i) and apoptosis was comparable with that of SST. Acidification-induced, SHP-1-dependent apoptosis occurred in breast cancer cell lines in which SST was cytotoxic (MCF-7 and T47D) or not (MDA-MB-231). We conclude that: (a) SST-induced SHP-1-dependent acidification occurs subsequent to or independent of the induction of p53 and Bax; (b) SST-induced intracellular acidification may arise due to inhibition of Na⁺/H⁺ exchanger and H⁺-ATPase; and (c) SHP-1 is necessary not only for agonist-induced acidification but also for the execution of acidification- dependent apoptosis. We suggest that combined targeting of SHP-1 and intracellular acidification may lead to a novel strategy of anticancer therapy bypassing the need for receptor-mediated signaling.