PKA-mediated phosphorylation and inhibition of Na⁺-K⁺-ATPase in response to β-adrenergic hormone

The activity of Na⁺-K⁺-ATPase can be regulated by hormones that activate adenosine 3',5'-cyclic monophosphate-dependent protein kinase (PKA). Here, using a site-directed phosphorylation state-specific antibody, we show that hormonal regulation of Na⁺-K⁺-ATPase can occur via phosphorylation of Ser-943 on its α-subunit. cDNAs coding for wild-type rat Na⁺-K⁺-ATPase and Na⁺-K⁺-ATPase in which the PKA phosphorylation site Ser-943 was mutated to Ala were stably and transiently transfected into COS cells. In COS cells expressing wild-type Na⁺-K⁺-ATPase the β-adrenergic agonist isoproterenol (1 μM) significantly increased the level of phosphorylation of the α- subunit. Phosphorylation was accompanied by a significant inhibition of the enzyme activity, as reflected by a decrease in ATP hydrolysis and ⁸⁶Rb+ transport. The effect of isoproterenol was reproduced by the PKA activator forskolin used in combination with the phosphodiesterase inhibitor 3- isobutyl-1-methylxanthine and was abolished by the specific PKA inhibitor H- 89. Okadaic acid, an inhibitor of protein phosphatases 1 and 2A, enhanced phosphorylation and inhibition of Na⁺-K⁺-ATPase induced by isoproterenol. The changes in activity of Na⁺-K⁺-ATPase linearly correlated with the extent of the α-subunit of Na⁺-K⁺-ATPase being phosphorylated. When Ser- 943 was replaced by alanine, stimulation of the phosphorylation and inhibition of the activity of Na⁺-K⁺-ATPase induced by isoproterenol, alone or in combination with okadaic acid, were not observed. These results indicate that, in intact cells, modulation of the activity of Na⁺-K⁺- ATPase can be achieved by regulation of the state of phosphorylation of Ser- 943. Moreover, they provide a biochemical mechanism by which β-adrenergic agonists can regulate Na⁺-K⁺-ATPase activity.