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139 INDUCTION OF SERUM AND GLUCOCORTICOID-INDUCED PROTEIN KINASE 1 AND EPITHELIAL SODIUM CHANNELS BY cAMP IN EPITHELIAL CELLS.
  1. M. M. Vasquez,
  2. R. Castro,
  3. S. R. Seidner,
  4. B. M. Henson,
  5. D. J. Ashton,
  6. S. B. Mustafa
  1. University of Texas Health Science Center at San Antonio, San Antonio, TX

Abstract

Fluid reabsorption in various epithelia is regulated by Na+ transport via apically located epithelial sodium channels (ENaC). Submandibular gland epithelial (SMG-C6) cells cultured in the absence of hydrocortisone or cAMP demonstrate diminished a-ENaC expression and Na+ transport properties similar to fetal distal lung epithelial cells. Increases in the abundance of the serum and glucocorticoid-regulated protein kinase 1 (sgk1), a phosphatidylinositol 3-kinase (PI3K) activated protein, can suppress ENaC degradation. Since treatment of SMG-C6 cells with dibutyryl cAMP (DbcAMP) for 24 hours increases a-ENaC expression and function, we hypothesized that the up-regulation of a-ENaC expression and amiloride-sensitive Na+ transport by DbcAMP involves an increase in sgk1 via the protein kinase A (PKA) and PI3K pathways. SMG-C6 cells treated with DbcAMP (1 mM) increased both sgk1 RNA levels (peaking at 30 minutes) and protein expression (peaking after 4 h and remained elevated at 24 h). DbcAMP-stimulated sgk1 mRNA expression was decreased by actinomycin D and protein expression was decreased by PKA inhibitors (H-89 and KT5720). Both pharmacological inhibition of PI3K with LY294002 or transfection with dominant negative PI3K construct reduced stimulated sgk1 protein and mRNA levels, attenuated the phosphorylation of CREB (a cAMP-activated transcription factor), and, after 24 h, decreased a-ENaC protein levels and amiloride-sensitive Na+ transport. In addition, the combination of PKA inhibitors with dominant negative PI3K synergistically attenuated all amiloride-sensitive Na+ transport activity. DbcAMP induction of sgk1 acts through the PI3K and PKA pathways and sgk1 may potentiatea-ENaC.

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