Apolipoprotein A-I activates cellular cAMP signaling through the ABCA1 transporter.
It has been suggested that the signal transduction pathway initiated by apoA-I activates key proteins involved in cellular lipid efflux. We investigated apoA-I-mediated cAMP signaling in cultured human fibroblasts induced with (22R)-hydroxycholesterol and 9-cis-retinoic acid (stimulated cells). Treatment of stimulated fibroblasts with apoA-I for short periods of time (<or=45 min) increased ATP binding cassette A1 (ABCA1) phosphorylation in a concentration-dependent manner. Concomitantly, apoA-I increased the intracellular level of cAMP in a concentration- and time-dependent manner. The maximal cAMP level was reached within 10 min at 10 microg/ml apoA-I representing a 1-fold increase. The ability of apoA-I to mediate cAMP production was only observed in stimulated fibroblasts. Furthermore, overexpression of ABCA1 in Chinese hamster ovary cells resulted in a 1.5-fold increase in apoA-I-mediated cAMP accumulation as compared with untransfected cells. In contrast, forskolin increased cAMP production significantly in unstimulated fibroblasts as well as in untransfected Chinese hamster ovary cells. Pharmacological inhibition of protein kinase A (H89) completely blocked apoA-I-mediated ABCA1 phosphorylation. Naturally occurring mutations of ABCA1 associated with Tangier disease (C1477R, 2203X, and 2145X) severely reduced apoA-I-mediated cAMP production, ABCA1 phosphorylation, (125)I-apoA-I binding, and lipid efflux, without affecting forskolin-mediated cAMP elevation. In contrast, the protein kinase A catalytic subunit was able to phosphorylate ABCA1 similarly from mutant and normal cell lines in vitro. Together, our results indicate that apoA-I activates ABCA1 phosphorylation through the cAMP/protein kinase A-dependent pathway, apoA-I-mediated cAMP production required high level expression of functional ABCA1, and Tangier disease mutants have defective apoA-I-mediated cAMP signaling. These findings suggest that apoA-I may activate cAMP signaling through G protein-coupled ABCA1 transporter.
Haidar
Bassam
B
Cardiovascular Genetics Laboratory, Division of Cardiology, McGill University Health Centre/Royal Victoria Hospital, Montréal, Québec H3A 1A1, Canada.
Denis
Maxime
M
Marcil
Michel
M
Krimbou
Larbi
L
Genest
Jacques
J
D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D000077556:Alitretinoin; D000483:Alleles; D000818:Animals; D016632:Apolipoprotein A-I / Q000378:metabolism*; D019921:Biotinylation; D016466:CHO Cells; D020134:Catalytic Domain; D002462:Cell Membrane / Q000378:metabolism; D005576:Colforsin / Q000494:pharmacology; D006224:Cricetinae; D000242:Cyclic AMP / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000737:chemistry; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D006888:Hydroxycholesterols / Q000378:metabolism; D007546:Isoquinolines / Q000494:pharmacology; D050356:Lipid Metabolism; D009154:Mutation; D010766:Phosphorylation; D011233:Precipitin Tests; D011485:Protein Binding; D015398:Signal Transduction; D013449:Sulfonamides; D013631:Tangier Disease / Q000378:metabolism; D013997:Time Factors; D014162:Transfection; D014212:Tretinoin / Q000378:metabolism
C577836:ABCA1 protein, human; D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters; D016632:Apolipoprotein A-I; D004791:Enzyme Inhibitors; D006888:Hydroxycholesterols; D007546:Isoquinolines; D013449:Sulfonamides; C003585:22-hydroxycholesterol; D005576:Colforsin; D000077556:Alitretinoin; D014212:Tretinoin; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; C063509:N-(2-(4-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide