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 w ild-type p53, Bax, and an acidic endonuclease. We have shown recently that its cytotoxic signaling is mediated via membrane-associated SHP-1 and is de pendent on decrease in intracellular pH (pH(i)) to 6.5. Here we investigate d the relationship between intracellular acidification and SHP-1 in cytotox ic signaling. Clamping of pH(i) at 7.25 by the proton-ionophore nigericin a bolished SST-signaled apoptosis without affecting its ability to regulate S HP-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 translocatio n of SHP-1 to the membrane, enhanced in cells overexpressing SHP-1, and was abolished by its inactive mutant SHP-1C455S. Acidification caused by inhib ition of Na+/H+ exchanger and H+ ATPase (pH(i) = 6.55 and 6.65, respectivel y) also triggered apoptosis. The effect of concurrent inhibition of Na+/Hexchanger and H+-ATPase on pH(i) and apoptosis was comparable with that of SST. Acidifiration-induced, SHP-1-dependent apoptosis occurred in breast ca ncer 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 occur s subsequent to or independent of the induction of p53 and Bax; (b) SST-ind uced intracellular acidification may arise due to inhibition of Na+/H+ exch anger and H+-ATPase; and (c) SHP-1 is necessary not only for agonist-induce d acidification but also for the execution of acidification-dependent apopt osis. We suggest that combined targeting of SHP-1 and intracellular acidifi cation may lead to a novel strategy of anticancer therapy bypassing the nee d for receptor-mediated signaling.