THE EFFECT OF THYROID-HORMONE TREATMENT ON THE GENE-EXPRESSION AND ENZYME-ACTIVITY OF RAT-LIVER SODIUM-POTASSIUM DEPENDENT ADENOSINE-TRIPHOSPHATASE

The effects of thyroid hormone (T-3) treatment on liver Na,K-adenosine triphosphatase (Na,K-ATPase) at the levels of subunit messenger RNA (mRNA), enzymatic activity, and enzyme content were studied in euthyroid rats injected for 5 consecutive days with T-3. Northern and slot blot analyses of polyadenylated mRNA revealed that T-3 treatment coordinately increases the level of mRNA encoding the alpha(1)- and beta(1)-subunits, approximately 4- and 3-fold, respectively, above basal levels. To determine whether this increase in the subunit mRNA consequently results in an increase in the synthesis of the enzyme, a modified liver cell fractionation procedure was developed, and the subcellular fractions from control and T-3-treated livers were examined biochemically. Western blot analysis and Na,K-ATPase assay demonstrated that T-3 treatment resulted in a 2-fold increase in both the amount and activity of the enzyme. Furthermore, the Western blot analysis of endoglycosidase-H-treated membrane fractions revealed an increase in the amount of the precursor beta-subunit in the T-3-treated liver rough microsomal fraction, suggesting that an increase in subunit synthesis contributes at least partially to the increase in the rat liver Na,K-ATPase by T-3 treatment.