Myeloid ACAT1 in ischemic retinopathy

Myeloid ACAT1 in Ischemic Retinopathy Studies in the mouse models of oxygen-induced retinopathy (OIR) and diabetic retinopathy (DR) have shown that myeloid cell-induced angiogenesis has a critical role in pathological retinal neovascularization (RNV) and retinal neurovascular dysfunction/injury, but the mechanisms are not yet known. We propose to investigate myeloid cell expression/activity of the cholesterol metabolizing enzyme ACAT (acyl-Coenzyme A: cholesterol acyltransferase, also known as sterol O-acyltranserase) as a novel therapeutic target for ischemic retinopathy. There are two isoforms of ACAT: ACAT1 and ACAT2. ACAT1 is widely expressed and its upregulation in macrophages and microglia has been implicated in atherosclerosis and Alzheimer’s disease, respectively. ACAT2 is mainly expressed in the intestines and liver. We will focus on ACAT1 in this project. During ischemia/hypoxia, microglia/macrophages are exposed to increased levels of oxidized low density lipoprotein cholesterol (oxLDLc). Its internalization by the LDL receptor (LDLR) increases activity of ACAT1, which esterifies cholesterol to form cholesterol esters (CE). Increases in CE promote an inflammatory microglia/macrophage phenotype characterized by increased expression of the amplifier of inflammatory signaling TREM-1 (triggering receptor expressed in myeloid cells-1), MCSF (macrophage colony stimulating factor), VEGF and TNFα. Our studies in the mouse model of OIR show marked increases in lipid accumulation, LDLR expression and CE formation in areas of RNV suggesting that dyslipidemia and ACAT activity play a key role in RNV. Furthermore, LDLR deletion or ACAT inhibition prevented RNV and decreased expression of TREM-1 and MCSF. Our studies in DR mice show similar signs of dyslipidemia and ACAT1 activation associated with DR-induced retinal inflammation and neurovascular injury. Moreover, hypoxia treatment of microglia/macrophages in vitro significantly increased their expression of TREM-1 and MCSF. Based on these results our central hypothesis is that ischemia-induced activation of ACAT1 drives retinal inflammation and promotes neurovascular injury during OIR and DR. Our aims will test this hypothesis by determining whether 1) ACAT1-mediated activation of Mg/MΦ cells is critically involved in OIR-induced RNV and neuroglial injury/dysfunction, 2) DR- induced ACAT1 activation in Mg/MΦ cells promotes retinal neurovascular injury/dysfunction.