Transcriptional up-regulation of cell surface Na V 1.7 sodium channels by insulin-like growth factor-1 via inhibition of glycogen synthase kinase-3β in adrenal chromaffin cells: enhancement of 22Na+ influx, 45Ca2+ influx and catecholamine secretion.

Insulin-like growth factor-1 (IGF-1) plays important roles in the regulation of neuronal development. The electrical activity of Na+ channels is crucial for the regulation of synaptic formation and maintenance/repair of neuronal circuits. Here, we examined the effects of chronic IGF-1 treatment on cell surface expression and function of Na+ channels. In cultured bovine adrenal chromaffin cells expressing NaV1.7 isoform of voltage-dependent Na+ channels, chronic IGF-1 treatment increased cell surface [3H]saxitoxin binding by 31%, without altering the Kd value. In cells treated with IGF-1, veratridine-induced 22Na+ influx, and subsequent 45Ca2+ influx and catecholamine secretion were augmented by 35%, 33%, 31%, respectively. Pharmacological properties of Na+ channels characterized by neurotoxins were similar between nontreated and IGF-1-treated cells. IGF-1-induced up-regulation of [3H]saxitoxin binding was prevented by phosphatydil inositol-3 kinase inhibitors (LY204002 or wortmannin), or Akt inhibitor (Akt inhibitor IV). Glycogen synthase kinase-3 (GSK-3) inhibitors (LiCl, valproic acid, SB216763 or SB415286) also increased cell surface [3H]saxitoxin binding by ∼33%, whereas simultaneous treatment of IGF-1 with GSK-3 inhibitors did not produce additive increasing effect on [3H]saxitoxin binding. IGF-1 (100nM) increased Ser437-phosphorylated Akt and Ser9-phosphorylated GSK-3β, and inhibited GSK-3β activity. Treatment with IGF-1, LiCl or SB216763 increased protein level of Na+ channel α-subunit; it was prevented by cycloheximide. Either treatment increased α-subunit mRNA level by ∼48% and accelerated α-subunit gene transcription by ∼30% without altering α-subunit mRNA stability. Thus, inhibition of GSK-3β caused by IGF-1 up-regulates cell surface expression of functional Na+ channels via acceleration of α-subunit gene transcription.