Abstract 462: Activation of Akt2 by Apolipoprotein E Protects ATP-binding Cassette A1 From Degradation

We previously reported that lipid-free apolipoprotein E (apoE) is able to increase ATP-binding cassette transporter A1 (ABCA1) transcription by activation of a signaling cascade involving very low-density lipoprotein receptor, apoE receptor 2, disabled-1, phosphatidylinositol 3-kinase (PI3K), protein kinase Cζ, and specificity protein 1 (Sp1). Here we demonstrated that treatment of RAW 264.7 murine macrophages with human apoE3 enhanced Akt phosphorylation, and upregulated ABCA1 protein and mRNA expression. Inhibition of PI3K weakened apoE3-induced Akt phosphorylation, and reduced ABCA1 protein and mRNA levels. In contrast, inhibition of Akt only diminished apoE-induced ABCA1 protein but not significantly alter ABCA1 mRNA level. Inhibition of protein synthesis did not erase the ability of apoE3 to increase ABCA1 protein level. Further, apoE3 increased the resistance of ABCA1 protein to calpain-mediated degradation without affecting calpain activity. Treatment of 264.7 cells with apoE3 selectively enhanced the phosphorylation of Akt1 and Akt2 but not Akt3. Knockdown of Akt1 enhanced the phosphorylation of Akt2. Vice versa, knockdown of Akt2 enhanced Akt1 phosphorylation. Underexpression of Akt1 or Akt2, respectively, increased and decreased ABCA1 protein level; while overexpression of these Akt isoenzymes caused changes in ABCA1 protein level opposite to those induced by knockdown of the corresponding Akt. These data imply that apoE3 guards against calpain-mediated ABCA1 degradation through Akt2. PI3K might be a step in the signaling pathway for apoE3 to trigger Akt2. [This study was supported by NIH grants SC1HL101431].