Ras triggers acidosis-induced activation of the extracellular-signal-regulated kinase pathway in cardiac myocytes

In cardiac myocytes, sustained (3 min) intracellular acidosis activates the ERK1/2 (extracellular-signal-regulated kinase 1/2) pathway and, through this pathway, increases sarcolemmal NHE (Na⁺/H⁺ exchanger) activity [Haworth, McCann, Snabaitis, Roberts and Avkiran (2003) J. Biol. Chem. 278, 31676–31684]. In the present study, we aimed to determine the time-dependence, pH-dependence and upstream signalling mechanisms of acidosis-induced ERK1/2 activation in ARVM (adult rat ventricular myocytes). Cultured ARVM were subjected to intracellular acidosis for up to 20 min by exposure to NH₄Cl, followed by washout with a bicarbonate-free Tyrode solution containing the NHE1 inhibitor cariporide. After the desired duration of intracellular acidosis, the phosphorylation status of ERK1/2 and its downstream effector p90^RSK (90 kDa ribosomal S6 kinase) were determined by Western blotting. This revealed a time-dependent transient phosphorylation of both ERK1/2 and p90^RSK by intracellular acidosis (intracellular pH ∼6.6), with maximum activation occurring at 3 min and a return to basal levels by 20 min. When the degree of intracellular acidosis was varied from ∼6.8 to ∼6.5, maximum ERK1/2 phosphorylation was observed at an intracellular pH of 6.64. Inhibition of MEK1/2 [MAPK (mitogen-activated protein kinase)/ERK kinase 1/2) by pre-treatment of ARVM with U0126 or adenoviral expression of dominant-negative D208A-MEK1 protein prevented the phosphorylation of ERK1/2 by sustained intracellular acidosis, as did inhibition of Raf-1 with GW 5074 or ZM 336372. Interference with Ras signalling by the adenoviral expression of dominant-negative N17-Ras protein or with FPT III (farnesyl protein transferase inhibitor III) also prevented acidosis-induced ERK1/2 phosphorylation, whereas inhibiting G-protein signalling [by adenoviral expression of RGS4 or Lsc, the RGS domain of p115 RhoGEF (guanine nucleotide-exchange factor)] or protein kinase C (with bisindolylmaleimide I) had no effect. Our data show that, in ARVM, sustained intracellular acidosis activates ERK1/2 through proximal activation of the classical Ras/Raf/MEK pathway.