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Am J Physiol Lung Cell Mol Physiol (May 9, 2003). doi:10.1152/ajplung.00376.2002
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Submitted on November 4, 2002
Accepted on April 4, 2003

Thyroid Hormone Stimulates Na,K-ATPase Activity and Its Plasma Membrane Insertion in Rat Alveolar Epithelial Cells

Jianxun Lei1, Sogol Nowbar1, Cary N. Mariash1, and David H. Ingbar1*

1 Department of Medicine, University of Minnesota, Minneapolis, MN, USA

* To whom correspondence should be addressed. E-mail: ingba001{at}umn.edu.

The Na,K-ATPase protein is critical for maintaining cellular ion gradients and volume and for transepithelial ion transport in the kidney and lung. Thyroid hormone, 3,3',5-triiodo-L-thyronine (T3), given for 2 days to adult rats increases alveolar fluid resorption by 65%, but the mechanism is undefined. We tested the hypothesis that T3 stimulates Na,K-ATPase in adult rat alveolar epithelial cells (AEC) including primary rat alveolar type II (ATII) cells and determined the mechanisms of T3 effect on the Na,K-ATPase enzyme using two adult rat AEC cell lines (MP48 and RLE-6TN). T3 increased significantly the hydrolytic activity of Na,K-ATPase at 10-8 M and 10-5 M in primary ATII cells and in both AEC cell lines. The increased activity was dose dependent in the cell lines (10-9 to 10-4 M) and was detected within 30 minutes and peaked at 6 hours. The maximal increases in Na,K-ATPase activity were 2-fold in MP48 and RLE-6TN cells at pharmacological T3 of 10-5 M and 10-4 M, respectively, but statistically significant increases occurred at physiologic concentrations of T3 as low as 10-9 M. This effect was T3 specific since reverse T3 (3,3',5'-triiodo-L-thyronine) (10-9 - 10-4 M) had no effect. The T3-induced increase in the Na,K-ATPase hydrolytic activity was not blocked by actinomycin D. No significant change in mRNA and total cell protein levels of Na,K-ATPase were detected with 6 hours of T3 (10-9 - 10-5 M) exposure. However, T3 increased the cell surface expression of Na,K-ATPase {alpha}1 or {beta}1 subunit proteins by 1.7 and 2 fold, respectively, and the increases in Na,K-ATPase activity and the cell surface expression were abolished by brefeldin A. These data indicate that T3 specifically stimulates Na,K-ATPase activity in adult rat AEC. Surprisingly the upregulation involves the translocation of Na,K-ATPase to plasma membrane, not increased gene transcription. These results suggest a novel non-transcriptional mechanism for the regulation of Na,K-ATPase by thyroid hormone.




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