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The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP and protein kinase A (PKA)-regulated Cl(-) channel in the apical membrane of epithelial cells. The metabolically regulated and adenosine monophosphate-stimulated kinase (AMPK) is colocalized with CFTR and attenuates its function. However, the sites for CFTR phosphorylation and the precise mechanism of inhibition of CFTR by AMPK remain obscure. We demonstrate that CFTR normally remains closed at baseline, but nevertheless, opens after inhibition of AMPK. AMPK phosphorylates CFTR in vitro at two essential serines (Ser(737) and Ser(768)) in the R domain, formerly identified as "inhibitory" PKA sites. Replacement of both serines by alanines (i) reduced phosphorylation of the R domain, with Ser(768) having dramatically greater impact, (ii) produced CFTR channels that were partially open in the absence of any stimulation, (iii) significantly augmented their activation by IBMX/forskolin, and (iv) eliminated CFTR inhibition post AMPK activation. Attenuation of CFTR by AMPK activation was detectable in the absence of cAMP-dependent stimulation but disappeared in maximally stimulated oocytes. Our data also suggest that AMP is produced by local phosphodiesterases in close proximity to CFTR. Thus we propose that CFTR channels are kept closed in nonstimulated epithelia with high baseline AMPK activity but CFTR may be basally active in tissues with lowered endogenous AMPK activity.

Previous in vitro studies suggested that Cl(-) currents produced by the cystic fibrosis transmembrane conductance regulator (CFTR; ABCC7) are inhibited by the alpha1 isoform of the adenosine monophosphate (AMP)-stimulated kinase (AMPK). AMPK is a serine/threonine kinase that is activated during metabolic stress. It has been proposed as a potential mediator for transport-metabolism coupling in epithelial tissues. All previous studies have been performed in vitro and thus little is known about the regulation of Cl(-) secretion by AMPK in vivo. Using AMPKalpha1(-/-) mice and wild-type littermates, we demonstrate that phenformin, an activator of AMPK, strongly inhibits cAMP-activated Cl(-) secretion in mouse airways and colon, when examined in ex vivo in Ussing chamber recordings. However, phenformin was equally effective in AMPKalpha1(-/-) and wild-type animals, suggesting additional AMPK-independent action of phenformin. Phenformin inhibited CFTR Cl(-) conductance in basolaterally permeabilized colonic epithelium from AMPKalpha1(+/+) but not AMPKalpha1(-/-) mice. The inhibitor of AMPK compound C enhanced CFTR-mediated Cl(-) secretion in epithelial tissues of AMPKalpha1(-/-) mice, but not in wild-type littermates. There was no effect on Ca(2+)-mediated Cl(-) secretion, activated by adenosine triphosphate or carbachol. Moreover CFTR-dependent Cl(-) secretion was enhanced in the colon of AMPKalpha1(-/-) mice, as indicated in Ussing chamber ex vivo and rectal PD measurements in vivo. Taken together, these data suggest that epithelial Cl(-) secretion mediated by CFTR is controlled by AMPK in vivo.