Plasma membrane Na+/H+ exchanger isoforms (NHE-1, -2, and -3) are differentially responsive to second messenger agonists of the protein kinase A and C pathways

Abstract

Na+/H+ exchanger (NHE) activity is regulated by several types of receptors directly coupled to distinct classes (i.e. G8, G(i), G(q), and G12) of heterotrimeric (αβγ) GTP-binding proteins (G proteins), which, upon activation, modulate production of various second messengers (e.g. cAMP, cGMP, diacylglycerol, inositol trisphosphate, and Ca2+). Recently, four isoforms of the rat Na+/H+ exchanger were identified by molecular cloning. To examine their intrinsic responsiveness to G protein and second messenger stimulation, three of these isoforms, NHE-1, -2, and -3, were stably expressed in mutant Chinese hamster ovary cells devoid of endogenous NHE activity (AP-1 cells). Incubation of cells with either AIF4/-, a general agonist of G proteins, or cholera toxin, a selective activator of Gα(s) that stimulates adenylate cyclase, accelerated the rates of amiloride-inhibitable 22Na+ influx mediated by NHE-1 and -2, whereas they inhibited that by NHE- 3. Similarly, short term treatment with phorbol 12-myristate 13-acetate, which mimics diacylglycerol activation of protein kinase C (PKC), or with agents (i.e. forskolin, 8-(4-chlorophenylthio)-cAMP, and isobutylmethylxanthine) that lead to activation of cAMP-dependent protein kinase (PKA) also stimulated transport by NHE-1 and NHE-2 but depressed that by NHE-3. The effects of phorbol 12-myristate 13-acetate were blocked by depleting cells of PKC or by inhibiting PKC using chelerythrine chloride, confirming a role for PKC in modulating NHE isoform activities. Likewise, the PKA antagonist, H-89, attenuated the effects of elevated cAMP(i) on NHE-1, - 2, and -3, further demonstrating the regulation by PKA. Unlike cAMP(i), elevation of cGMP(i) by treatment with dibutyryl-cGMP or 8-bromo-cGMP had no influence on NHE isoform activities, thereby excluding the possibility of a role for cGMP-dependent protein kinase in these cells. These data support the concept that the NHE isoforms are differentially responsive to agonists of the PKA and PKC pathways.