Thangaraju M, Sharma K, Liu D, Shen S H, Srikant C B
Fraser Laboratories, McGill University and Royal Victoria Hospital, Montreal, Quebec, Canada.
Cancer Res. 1999 Apr 1;59(7):1649-54.
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 (pHi) to 6.5. Here we investigated the relationship between intracellular acidification and SHP-1 in cytotoxic signaling. Clamping of pHi 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 pHi to 6.5. Such acidification-induced apoptosis was not observed at pHi <6.0 or >6.7. pHi-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 (pHi = 6.55 and 6.65, respectively) also triggered apoptosis. The effect of concurrent inhibition of Na+/H+ exchanger and H(+)-ATPase on pHi 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.
G蛋白偶联受体激动剂生长抑素（SST）可诱导MCF-7人乳腺癌细胞凋亡。这与野生型p53、Bax和一种酸性核酸内切酶的诱导有关。我们最近发现，其细胞毒性信号是通过膜相关的SHP-1介导的，并且依赖于细胞内pH值（pHi）降至6.5。在此，我们研究了细胞内酸化与SHP-1在细胞毒性信号传导中的关系。用质子离子载体尼日利亚菌素将pHi钳制在7.25可消除SST信号诱导的凋亡，而不影响其调节SHP-1、p53和Bax的能力。将pHi用尼日利亚菌素钳制到6.5可诱导凋亡。在pHi<6.0或>6.7时未观察到这种酸化诱导的凋亡。pHi依赖性凋亡与SHP-1向膜的转位有关，在过表达SHP-1的细胞中增强，并被其无活性突变体SHP-1C455S消除。抑制Na+/H+交换体和H+ATP酶（分别使pHi为6.55和6.65）引起的酸化也触发凋亡。同时抑制Na+/H+交换体和H(+)-ATP酶对pHi和凋亡的影响与SST的影响相当。酸化诱导的、SHP-1依赖性凋亡发生在SST具有细胞毒性的乳腺癌细胞系（MCF-7和T47D）或无细胞毒性的细胞系（MDA-MB-231）中。我们得出以下结论：（a）SST诱导的SHP-1依赖性酸化发生在p53和Bax诱导之后或与之无关；（b）SST诱导的细胞内酸化可能是由于抑制Na+/H+交换体和H(+)-ATP酶所致；（c）SHP-1不仅是激动剂诱导酸化所必需的，也是酸化依赖性凋亡执行所必需的。我们建议，联合靶向SHP-1和细胞内酸化可能会产生一种无需受体介导信号传导的新型抗癌治疗策略。
