Identification of Amiloride-Sensitive Na+/H+Exchange in Rat NB2 Node Lymphoma Cells. Stimulation by 12-0-Tetradecanoyl-Phorbol-13-Acetate*

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Abstract
Stimulation of mitogenesis in rat Nb2 node lymphoma cells by human (h) PRL was inhibited by inhibitors of Na+/H+ exchange (viz. amiloride and its analogs) and inhibitors of protein kinases (isoquinolinesulfonamide derivatives). The most potent were ethylisopropylamiloride (EP-Am) (IC50, 13 .mu.M) and H-7, selective for proteinase kinase C (IC50, 23 .mu.M), suggesting the possible involvement of Na+/H+ exchange and protein kinase C in mediating Nb2 cell mitogenesis. In the presence of hPRL, the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA), known to activate the Na+/H+ antiporter as well as protein kinase C in other cell systems, enhanced the hPRL-stimulated Nb2 cell mitogenesis. TPA alone caused a dose- and time-dependent stimulation of H+ efflux in stationary cultures of Nb2 cells but had no effect on cell growth. From 25-100 mM TPA, the increase in the rate of H+ efflux was detectable by 3 min, reached a maximum by 15 min, and was sustained 30 min after the addition of TPA. The TPA-stimulated H+ efflux was dependent on extracellular Na+ and was almost completely inhibited after a 10 min preincubation with 25 .mu.M EP-Am. TPA also increased the intracellular pH (pHi) of stationary cultures of Nb2 cells from 7.29 .+-. 0.02 (n = 8) to a maximum of 7.45 .+-. 0.03 (n = 10). The most rapid and greatest response was observed with 40 nM TPA which gave a detectable increase in pHi within 1 min and reached a maximum alkalinization by 6 min. Higher concentrations of TPA had no additional effect. The nontumor promoter phorbol 12,13,20-triacetate (PTA), either alone or in the presence of hPRL, had no effect on Nb2 cell proliferation or on H+ efflux or pHi in Hb2 cells. The TPA-induced increase in pHi was Na+-dependent and was inhibited by EP-Am and H-7. A preincubation with EP-Am (25 .mu.M) for 5-10 min abolished the TPA-induced increase in pHi whereas prolonged incubation with H-7 (50 .mu.M) for up to 3 h was required to decrease the stimulatory effect of TPA by approximately 50%. Although activation of the Na+/H+ exchange system is clearly an early consequence of the action of TPA on Nb2 cells, the failure of TPA to stimulate Nb2 cell proliferation suggests that stimulation of Na+/H+ exchange and protein kinase C activity are not sufficient to generate a mitogenic response in these cells.