Induction of Na(+)-K(+)-ATPase and its subunit mRNAs by serum in a rat liver cell line

Addition of serum to confluent Clone 9 cells increased the protein, RNA, and DNA content per plate of cells; the increments became manifest within 3-6 h and were sustained for the 48-h duration of study. After the addition of serum, Na(+)-K(+)-ATPase activity was stimulated 1.25- and 1.45-fold at 6 and 12 h, respectively. In cells preincubated in the absence of serum for 24 h, addition of serum increased the abundances of Na(+)-K(+)-ATPase subunit mRNAs, mRNA alpha 1 and mRNA beta, coordinately by approximately 2- and 2.7-fold at 3 h, an effect that preceded the stimulation of Na(+)-K(+)-ATPase activity. The serum-induced increments in subunit mRNA abundances were further enhanced by the combined presence of serum and cycloheximide; mRNA alpha 1 and mRNA beta abundances were also augmented by cycloheximide alone (approximately 2.5- and 9.2-fold at 6 h, respectively). In cells incubated in the absence of serum, the half-lives of mRNA alpha 1 and mRNA beta, estimated from decrements in their abundances after the addition of actinomycin D, were 12 and 10 h, respectively. These data demonstrate that serum enhances Na(+)-K(+)-ATPase subunit mRNA abundance and enzyme activity in Clone 9 cells. Comparison of the estimated half-lives of Na(+)-K(+)-ATPase subunit mRNAs with the observed increments in their abundances at 3 h suggests that the serum-induced increases in mRNA alpha 1 and mRNA beta abundances are in large part due to enhanced synthesis of these mRNAs.