Apolipoprotein A-I Activates Cellular cAMP Signaling through the ABCA1 Transporter

Abstract

It has been suggested that the signal transduction pathway initiated by apoA-I activates key proteins involved in cellular lipid efflux. We investigated apoA-Imediated 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 (<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 g/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.

Quantitative Analysis of ABCA1-dependent Compartmentalization and Trafficking of Apolipoprotein A-I

“…In light of this, apoA-I may have important implications in the activation of ABCA1, consistent with our previous study (19) which showed that brief treatment of 22OH/9CRA-stimulated fibroblasts or Chinese hamster ovary cells overexpressing ABCA1 with apoA-I triggered cAMP production and consequently induced ABCA1phosphorylationviaacAMP-dependent protein kinase-dependent mechanism. This is in agreement with the finding of Oram and Heinecke (34) that ABCA1 activity could be regulated by different signaling processes, including the JAK2 pathway.…”

“…B, normal fibroblasts stimulated with 22OH/9CRA in the presence or absence of 30-fold excess of unlabeled apoA-I, unstimulated normal fibroblasts, and ABCA1 mutant (Q597R) were incubated with 10 g/ml drastically reduced the association of 125 I-apoA-I with both the PM and ICCs. We have previously reported that the Q597R-ABCA1 mutant does not bind apoA-I but is expressed normally and localizes to the cell surface (19).…”

Inhibition of ERK1/2 and Activation of Liver X Receptor Synergistically Induce Macrophage ABCA1 Expression and Cholesterol Efflux

“…Another inhibitor of ERK1/2, U0126 (2 M, IC 50 values were 72 and 58 nM for ERK1 and ERK2, respectively) also markedly increased macrophage free cholesterol efflux (ϳ3-fold). In contrast, inhibition of other kinases, such as protein kinase C by calphostin C (1 M, IC 50 did not influence macrophage free cholesterol efflux to apoAI. Therefore, enhancement of macrophage cholesterol efflux to apoAI appears to be ERK1/2 inhibition-selective.…”

“…However, PKA activity has no effect on ABCA1 expression (48,49). ApoAI and synthetic amphiphilic helical peptides increase ABCA1 protein stability by activating PKA (50,51). ApoAI also phosphorylates ABCA1 through protein kinase C␣ to enhance ABCA1 protein stability (52).…”

A novel model of cholesterol efflux from lipid-loaded cells

“…However, when too much cholesterol enters cells, ABC-A1 forms a complex with the G protein α subunit (Gαs), which activates adenylate cyclase to produce more cAMP. This process results in the activation of PKA, which phosphorylates ABC-A1 to inhibit ABC-A1 degradation and to promote phospholipid and cholesterol efflux [50] . Recently, we found that the addition of NO-1886 (0.1 g/kg body weight/day) to the diet of high-fat/ high-sucrose/high-cholesterol-fed Chinese Bama minipigs for 5 months significantly reduced atherosclerotic lesions and significantly increased plasma HDL-c and apolipoprotein AI levels [39] .…”