Dexras1/AGS-1, a steroid hormone-induced guanosine triphosphate-binding protein, inhibits 3 ',5 '-cyclic adenosine monophosphate-stimulated secretionin AtT-20 corticotroph cells

Dexras1 is a novel GTP-binding protein that shares structural similarity wi th the Ras family of small molecular weight GTPases and is strongly and rap idly induced during treatment with dexamethasone. The function of Dexras1 a nd its contribution to glucocorticoid-dependent signaling in the corticotro ph cell are unknown. The present study was undertaken to examine the potent ial role of Dexras1 in the regulation of peptide hormone secretion in the A tT-20 corticotroph cell line. To determine the effects of Dexras1 expressed independently of glucocorticoid treatment, expression plasmids for wild-ty pe and constitutively active mutant Dexrasl proteins were cotransfected wit h human GH (hGH), which provides an ectopic marker for the stimulus-coupled secretory pathway. GTP binding properties and the GTP to GDP ratio of wild -type and mutant Dexrasl proteins were examined in transiently transfected AtT-20 and COS-7 cells. Stimulated and constitutive components of secretion were assessed after 2-h incubations with 5 mM 8-Br-cAMP or control. cAMP t reatment led to a a-fold increase in hGH secretion relative to control. Cot ransfection of wild-type Dexrasl had no effect on cAMP-stimulated hGH secre tion, but a constitutively active mutant, Dexras[A178V], attenuated stimula ted secretion by 86% (P < 0.01). A double-mutant containing a deletion of t he carboxyl terminus isoprenylation site, Dexras [A178V/C277term], did not inhibit cAMP-stimulated hGH secretion, indicating that the effect is prenyl ation dependent. These findings suggest that activation of Dexras1 has impo rtant functional consequences leading to inhibition of stimulus-secretion c oupling in corticotroph cells. Because Dexrasl messenger RNA is strongly an d rapidly induced during glucocorticoid treatment, these results raise the possibility that Dexrasl may participate in the signal transduction pathway s that govern the rapid regulatory effects of glucocorticoids on peptide ho rmone secretion in corticotroph cells.