Abstract 1097: HIV-1 Nef-Mediated Down-Regulation of ATP-Binding Cassette Transporter A1 (ABCA1) Proceeds Through a Post-Transcriptional Mechanism That Stimulates Transporter Degradation but Does Not Induce Unfolded Protein Response (UPR) Signaling
HIV-1 infection and subsequent anti-retroviral treatment (ART) drives changes in lipid and lipoprotein metabolism. Although ART has transformed HIV infection into a chronic disease state, altered lipid metabolism in this large patient cohort associates with increased atherosclerosis and cardiovascular disease Thus, defining how HIV-1 increases CVD risk is critical to the clinical management of this disease. We have previously shown that Nef, a viral HIV-1 protein, impairs ABCA1 dependent cholesterol efflux from human macrophages, an early event in reverse cholesterol transport that may increase the risk of atherosclerosis in HIV infected individuals. Here, we show that Nef down-regulates ABCA1 in HEK 293 EBNA-T cells. Analysis of ABCA1 message levels shows Nef suppressed ABCA1 protein expression by a post-transcriptional mechanism, but did not affect the protein levels of cMIR, another transmembrane protein. Using cycloheximide to block protein translation it was found that Nef stimulates the degradation of ABCA1. Moreover, through the analysis of a panel of ABCA1 mutants, co-immunioprecipitation experiments indicate Nef can interact with ABCA1 by binding its conserved C-terminus, between amino acid 2222 and 2231. However, the Nef-mediated suppression of ABCA1 protein did not correlate with its ability to bind the ABCA1 C-terminus, indicating these two functions were dissociable and which suggested Nef may act at a distance to down-regulate ABCA1. Induction of the Unfolded Protein Response can lead to the degradation of proteins and since HIV Tat has recently been shown to stimulate UPR signaling we tested whether the Nef mediated degradation of ABCA1 was proceeding through this pathway but no evidence for this was found in that Nef did not induce the expression of CHOP or spliced XBP-1, two downstream mediators of the UPR response. In conclusion we show that Nef can bind the ABCA1 C-terminus and can stimulate the degradation of ABCA1. However, the mechanism that mediates this degradation of ABCA1 can be dissociated from the binding of Nef to the ABCA1 C-terminus and can proceed with out inducing UPR signaling.
This research has received full or partial funding support from the American Heart Association, Founders Affiliate (Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, Vermont).