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Department of Physiology, Tufts University School of Medicine, Boston, Massachusetts 02111
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 p110
in both cell lines. The
effect of cAMP was KT-5720 sensitive, suggesting involvement of protein
kinase A. Expression of a dominant-negative -adrenergic receptor
kinase COOH terminus (-ARKct), which blocks G
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 G. 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.
bile secretion; heterotrimeric G protein; G; protein kinase
A; Ras
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