Apolipoprotein A–I stimulates cholesteryl ester transfer protein and apolipoprotein E secretion from lipid-loaded macrophages; the role of NF-κB and PKA signaling pathways

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Cholesteryl ester transfer protein (CETP) and apolipoprotein E (apoE) are secreted by macrophages. Apolipoprotein A–I (apoA–I) is a potent inducer of apoE secretion from lipid-loaded macrophages, but its effect on CETP is not known. We aimed to identify the signaling pathways involved in apoA–I and HDL-mediated regulation of CETP and apoE secretion from lipid-loaded macrophages. THP-1 macrophages were loaded with lipids by incubation with human copper-oxidized LDL. The cells were subsequently exposed to human purified apoA–I or HDL 3 with/without inhibitors of NF-κB (TPCK) or PKA (H89). CETP and apoE in the cultured cells and media were quantified by real-time PCR and Western blot. Results showed that in lipid-loaded macrophages: (i) CETP and apoE gene expression and secretion were increased in the presence of apoA–I, and further increased by inhibition of NF-kB with TPCK; (ii) CETP and apoE gene expression and secretion were reduced by the inhibition of PKA with H89; (iii) PKA-gamma subunit was activated by oxidized LDL and moreover by apoA–I. We also showed that: (i) siRNA-mediated CETP gene silencing diminished apoE secretion from both non-loaded and lipid-loaded macrophages; (ii) addition of apoA–I partially restored apoE secretion from lipid-loaded macrophages with the silenced CETP gene. In conclusion, our data suggest a new mechanism by which apoA–I stimulates CETP secretion, in addition to apoE, from lipid loaded macrophages, a process involving NF-κB inhibition and/or PKA pathway activation.