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Destabilization of Nav1.7 Sodium Channel α-Subunit mRNA by Constitutive Phosphorylation of Extracellular Signal-Regulated Kinase: Negative Regulation of Steady-State Level of Cell Surface Functional Sodium Channels in Adrenal Chromaffin Cells

In cultured bovine adrenal chromaffin cells expressing Nav1.7 isoform of voltage-dependent Na+channels, treatment (≥6 h) with serum deprivation, PD98059, or U0126 increased cell surface [3H]saxitoxin ([3H]STX) binding by ∼58% ( t 1/2 = 12.5 h), with no change in the K d value. Immunoblot analysis showed that either treatment attenuated constitutive phosphorylation of extracellular signal-regulated kinase (ERK) 1 and ERK2 but not of p38 mitogen-activated protein kinase and c-Jun N-terminal kinase (JNK) 1 and JNK2. The increase of [3H]STX binding and the attenuated phosphorylation of ERK1 and ERK2 returned to the control nontreated levels after the addition of serum or the washout of PD98059- or U0126-treated cells. Simultaneous treatment of serum deprivation with PD98059 or U0126 did not produce an additional increasing effect on [3H]STX binding, compared with either treatment alone. In cells subjected to either treatment, veratridine-induced maximum 22Na+ influx was augmented by ∼47%, with no change in the EC50 value; Ptychodiscus brevis toxin-3 enhanced veratridine-induced22Na+ influx by 2-fold, as in nontreated cells. Serum deprivation, PD98059, or U0126 increased Na+ channel α- but not β1- subunit mRNA level by ∼50% between 3 and 24 h; cycloheximide, an inhibitor of protein synthesis, increased α-subunit mRNA level and nullified additional increasing effect of either treatment on α-subunit mRNA level. Either treatment prolonged half-life of α-subunit mRNA from 17.5 to ∼26.3 h without altering α-subunit gene transcription. Thus, constitutively phosphorylated/activated ERK destabilizes Na+ channel α-subunit mRNA via translational event, which negatively regulates steady-state level of α-subunit mRNA and cell surface expression of functional Na+ channels.