Protein kinase C and the opposite regulation of sodium channel α- and β1-subunit mRNA levels in adrenal chromaffin cells

Our previous [H-3]saxitoxin binding and Na-22 influx assays showed that treatment of cultured bovine adrenal chromaffin cells with 12-O-tetradecanoylphorbol 13-acetate (TPA) or phorbol 12,13-dibutyrate (PDBu), an activator of protein kinase C (PKC), decreased the number of cell surface Na channels (IC50 = 19 nM) but did not alter their pharmacological properties; Na channel downregulation developed within 3 h, reached the peak decrease of 53% at 15 h, and was mediated by transcriptional/translational events. In the present study, treatment with 100 nM TPA lowered the Na channel alpha-subunit mRNA level by 34 and 52% at 3 and 6 h, followed by restoration to the pretreatment level at 24 h, whereas 100 nM TPA elevated the Na channel beta(1)-subunit mRNA level by 13-61% between 12 and 48 h. Reduction of alpha-subunit mRNA level by TPA was concentration-dependent (IC50 = 18 nM) and was mimicked by PDBu but not by the biologically inactive 4 alpha-TPA; it was prevented by H-7, an inhibitor of PKC, but not by HA-1004, a less active analogue of H7, or by H-89, an inhibitor of cyclic AMP-dependent protein kinase. Treatment with cycloheximide, an inhibitor of protein synthesis, per se sustainingly increased the alpha-subunit mRNA level and decreased the beta(1)-subunit mRNA level for 24 h; also, the TPA-induced decrease of alpha-subunit mRNA and increase of beta(1)-subunit mRNA were both totally prevented for 24 h by concurrent treatment with cycloheximide. Nuclear run-on assay showed that TPA treatment did nor alter the transcriptional rate of the alpha-subunit gene. A stability study using actinomycin D, an inhibitor of RNA synthesis, revealed that TPA treatment shortened the t(1/2) of alpha-subunit mRNA from 18.8 to 3.7 h. These results suggest that Na channel alpha- and beta(1)-subunit mRNA levels are differentially down- and up-regulated via PKC; the process may be mediated via an induction of as yet unidentified short-lived protein(s), which may culminate in the destabilization of alpha-subunit mRNA without altering ol subunit gene transcription.