Mechanism by which cAMP activates PI3-kinase and increases bile acid secretion in WIF-B9 cells

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Abstract
Previous studies in rat bile canalicular membrane vesicles and WIF-B9 cells revealed that cAMP-induced trafficking of ATP-binding cassette (ABC) transporters to the canalicular membrane and their activation require phosphoinositide 3-kinase (PI3-K) products. In the present studies, canalicular secretion of fluorescein isothiocyanate-glycocholate in WIF-B9 cells was increased by cAMP and a decapeptide that enhances PI3-K activity; these effects were inhibited by wortmannin. To determine the mechanism(s) whereby cAMP activates PI3-K, we examined signal transduction pathways in WIF-B9 and COS-7 cells. cAMP activated PI3-K in both cell lines in a phosphotyrosine-independent manner. PI3-K activity increased in association with p110β in both cell lines. The effect of cAMP was KT-5720 sensitive, suggesting involvement of protein kinase A. Expression of a dominant-negative β-adrenergic receptor kinase COOH terminus (β-ARKct), which blocks Gβγ signaling, decreased PI3-K activation in both cell lines. cAMP increased GTP-bound Ras in COS-7 but not WIF-B9 cells. Expression of dominant-negative Ras abolished cAMP-mediated PI3-K, which suggests that the effect is downstream of Ras and Gβγ. These data indicate that cAMP activates PI3-K in a cell type-specific manner and provide insight regarding mechanisms of PI3-K activation required for bile acid secretion.

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