Rapid computerized analysis of Na+/H+ exchange flux

Na+/H+ exchanger-dependent intracellular alkalinization is an early event in malignant transformation and plays an essential role in the development of subsequent transformation-associated phenotypes

In this study we investigate the mechanism of intracellular pH change and its role in malignant transformation using the E7 oncogene of human papillomavirus type 16 (HPV16). Infecting NIH3T3 cells with recombinant retroviruses expressing the HPV16 E7 or a transformation deficient mutant we show that alkalinization is transformation specific. In NIH3T3 cells in which transformation can be turned on and followed by induction of the HPV16 E7 oncogene expression, we demonstrate that cytoplasmic alkalinization is an early event and was driven by stimulation of Na+/H+ exchanger activity via an increase in the affinity of the intracellular NHE-1 proton regulatory site. Annulment of the E7-induced cytoplasmic alkalinization by specific inhibition of the NHE-1, acidification of culture medium, or clamping the pHi to nontransformed levels prevented the development of later transformed phenotypes such as increased growth rate, serum-independent growth, anchorage-independent growth, and glycolytic metabolism. These findings were verified in human keratinocytes (HPKIA), the natural host of HPV. Results from both NIH3T3 and HPKIA cells show that alkalinization acts on pathways that are independent of the E2F-mediated transcriptional activation of cell cycle regulator genes. Moreover, we show that the transformation-dependent increase in proliferation is independent of the concomitant stimulation of glycolysis. Finally, treatment of nude mice with the specific inhibitor of NHE-1, DMA, delayed the development of HPV16-keratinocyte tumors. Our data confirm that activation of the NHE-1 and resulting cellular alkalinization is a key mechanism in oncogenic transformation and is necessary for the development and maintenance of the transformed phenotype.

Subcellular redistribution is involved in acute regulation of the brush border Na+/H+ exchanger isoform 3 in human colon adenocarcinoma cell line Caco-2. Protein kinase C-mediated inhibition of the exchanger

Na+/H+ exchanger isoform 3 (NHE3), an epithelial brush border isoform of the Na+/H+ exchanger gene family, plays an important role in reabsorption of Na+ in the small intestine, the colon, and the kidney. In several cell types, phorbol 12-myristate 13-acetate (PMA) acutely inhibits NHE3 activity by changes in Vmax, but the mechanism of this inhibition is unknown. We investigated the role of subcellular redistribution of NHE3 in the PMA-induced inhibition of endogenous brush border NHE3 in a model human colon adenocarcinoma cell line, Caco-2. Subcellular localization of NHE3 was examined by confocal morphometric analysis complemented with cell surface biotinylation and compared with NHE3 activity evaluated by fluorometric measurement of intracellular pH. PMA inhibited NHE3 activity by 28% (p < 0.01), which was associated with a decrease of the ratio of the brush border/subapical cytoplasmic compartment of NHE3 from approximately 4.3 to approximately 2.4. This translocation resulted in 10-15% of the total cell NHE3 being shifted from the brush border pool to the cytoplasmic pool. These effects were mediated by protein kinase C, since they were blocked by the protein kinase C inhibitor H7. We conclude that inhibition of NHE3 by protein kinase C in Caco-2 cells involves redistribution of the exchanger from brush border into a subapical cytoplasmic compartment, and that this mechanism contributes approximately 50% to the overall protein kinase C-induced inhibition of the exchanger.

Role of acid/base homeostasis in the suppression of apoptosis in haemopoietic cells by v-Abl protein tyrosine kinase

Removal of interleukin-3 from murine IC.DP pre-mast cells results in irreversible commitment to apoptosis within 18 hours. To identify early events necessary for the engagement of apoptosis we examined the regulation of intracellular pH (pH(i)). IC.DP cells acidified 2 hours after removal of interleukin-3 (before discernible signs of apoptosis) and by 18 hours pH(i) had decreased by 0.15 units. The acidification was due to both an increase in an acid-loading process which only occurs when intracellular pH is above 6.8 and a slight reduction in H+ efflux via NA+/H+ exchange. Activation of a temperature sensitive mutant of v-Abl protein tyrosine kinase suppressed apoptosis of IC.DP cells in the absence of interleukin-3 but did not stimulate proliferation, and moreover prevented cellular acidification. Acidification of the cells by 0.2 units to pH 6.86 by complete inhibition of Na+/H+ exchange by 10 microM 5′-(N-methyl-N-isobutyl)-amiloride prevented the suppression of apoptosis by v-abl protein tyrosine kinase following IL 3 withdrawal. However in the presence of interleukin-3, addition of 10 microM 5′-(N-methyl-N-isobutyl)-amiloride only resulted in a fall of pH(i) to 7.17. Apoptosis did not occur and the cells continued to proliferate. Thus, in this model intracellular pH must fall below a critical value for apoptosis to occur. Together these data point to a step in cytokine deprivation induced apoptosis (at least in some haemopoietic cell types) which is either enhanced by or dependent upon an acidic intracellular environment which is the result of an increase in acid loading and inhibition of Na+/H+ exchange activity. One of the mechanisms by which activation of v-Abl protein tyrosine kinase suppresses apoptosis is by prevention of intracellular acidification.