SERUM AND GROWTH-FACTOR INDUCTION OF NA+-K+-ATPASE SUBUNIT MESSENGER-RNAS IN CLONE-9 CELLS - ROLE OF PROTEIN-KINASE-C

In a previous study, we found that addition of serum to confluent Clone 9 cells, a nontransformed rat liver cell line, increased the abundance of mRNA-alpha-1 and mRNA-beta-1 at 3 h by 2- and 2.7-fold, respectively [Bhutada et al. Am. J. Physiol. 258 (Cell Physiol. 27): C1044-C1050, 1990]. We now report that exposure of these cells to 160 nM 12-O-tetradecanoylphorbol 13-acetate (TPA) for 6 h increases mRNA-alpha-1 and mRNA-beta-1 by 1.7 +/- 0.2 and 2.1 +/- 0.3-fold, respectively. Incubation in the presence of 160 nM TPA for 24 h reduced high-affinity phorbol dibutyrate-binding sites [dissociation constant (K(d)) = 5 nM; maximum binding (B(max)) = 1.2 pmol/mg protein] to undetectable levels. In such cells, exposure to 10% serum for 6 h still resulted in two- and fourfold increment in mRNA-alpha-1 and mRNA-beta-1 abundances, respectively, while further addition of TPA to these protein kinase C (PKC)-depleted cells resulted in no change in the subunit mRNA abundances. The increments in mRNA-alpha-1 content in response to 10% serum and 160 nM TPA at 6 h were additive, whereas the increments in mRNA-beta-1 were not. The following agents increased mRNA-alpha-1 and mRNA-beta-1 abundance in both control and PKC-depleted cells: epidermal growth factor, platelet-derived growth factor, basic fibroblast growth factor, insulin, dexamethasone, and hypothyroid calf serum. In contrast, N6,2'-O-dibutyryl-adenosine 3',5'-cyclic monophosphate and aldosterone had no effect. Time course studies showed that, upon addition of serum, the subunit mRNA abundances increased earlier and to a larger extent in the nuclear than in the cytoplasmic RNA pool. It is concluded that, while TPA increases Na+-K+-ATPase mRNA-alpha-1 and mRNA-beta-1 abundances, stimulation of PKC is not necessary for the induction of the subunit mRNAs by serum and specific growth factors.