A novel LXRα- or RX-independent mechanism for increasing brain APOE levels

Abstract

Apolipoprotein E (apoE) is a key lipid transport protein in brain. The e 4 allele is strongly linked to late onset and sporadic forms of Alzheimer's disease (AD). Since apoE plays an important role in neuronal repair, synaptogenesis, and clearance of toxic Aß fragments from brain, understanding how brain apoE levels are regulated is critical. In addition to known LXRα and RXR mediated mechanisms, we identified a novel mechanism mediated through pan-class I HDAC inhibition (HDAC1i) for increasing apoE levels in astrocytes. Using MS275, a pan-class I HDAC inhibitor as a tool compound, we compared the effect of HDAC1i on lipogenic gene expression in cells lines of hepatic or astrocytic origin. We also investigated the expression of these same lipogenic genes and apoE following exposure to a pan LXR agonist, T0901317 and a RXR agonist, bexarotene. Human hepatoblastoma (HepG2) cells and astrocytoma (CCF-STTG1) cells were treated with varying concentrations (0–10 μM) of MS275, TO0901317, or Bexarotene for 24 hours and mRNA expression was assessed using RT-qPCR. The amount of apoE secreted by CCF-STTG1 cells was also measured using alphaLISA after 48 hours of compound treatment. Following treatment of HepG2 cells with T0901317 we observed a dose-dependent increase in mRNA e xpression for lipogenic genes: LXRα, ABCA1, ABCG5 and ABCG8, SREPB1c and FASN. In CCF-STTG1 cells, following treatment with T0901317, we observed a dose-dependent increase in LXRα, apoE and apoE lipidating genes: ABCA1 and ABCG1. In contrast, a bell-shaped dose-response was observed in HepG2 as well as CCF-STTG1 cells for all end-points measured following treatment with bexarotene, Interestingly, treatment of HepG2 cells with MS275 did not elevate the expression of LXRα and its target genes (ABCA1, ABCG5 and ABCG8), but resulted in a significant dose-dependent increase in apoE, SREBP1c and FASN mRNA levels. Following treatment of CCF-STTG1 cells with MS275, we observed a significant increase in both apoE mRNA expression and protein secretion, as well as an increase in ABCA1 and ABCG1 mRNA levels without any significant effects on LXRα mRNA. Using MS275, T0901317 and Bexarotene as tool compounds, we demonstrate differential regulation of apoE and lipogenic gene expression that is cell type-dependent.