Extra-hepatic metabolism of 7-ketocholesterol occurs by esterification to fatty acids via cPLA2α and SOAT1 followed by selective efflux to HDL

Abstract

Accumulation of 7-ketocholesterol (7KCh) in tissues has been previously associated with various chronic aging diseases. Orally ingested 7KCh is readily metabolized by the liver and does not pose a toxicity threat. However, 7KCh formed in situ, usually associated with lipoprotein deposits, can adversely affect surrounding tissues by causing inflammation and cytotoxicity. In this study we have investigated various mechanisms for extra-hepatic metabolism of 7KCh (e.g. hydroxylation, sulfation) and found only esterification to fatty acids. The esterification of 7KCh to fatty acids involves the combined action of cytosolic phospholipase A2 alpha (cPLA2α) and sterol O-acyltransferase (SOAT1). Inhibition of either one of these enzymes ablates 7KCh-fatty acid ester (7KFAE) formation. The 7KFAEs are not toxic and do not induce inflammatory responses. However, they can be unstable and re-release 7KCh. The higher the degree of unsaturation, the more unstable the 7KFAE (e.g. 18:0>18:1>18:2>18:3>>20:4). Biochemical inhibition and siRNA knockdown of SOAT1 and cPLA2α ablated the 7KFAE synthesis in cultured ARPE19 cells, but had little effect on the 7KCh-induced inflammatory response. Overexpression of SOAT1 reduced the 7KCh-induced inflammatory response and provided some protection from cell death. This effect is likely due to the increased conversion of 7KCh to 7KFAEs, which reduced the intracellular 7KCh levels. Addition of HDL selectively increased the efflux of 7KFAEs and enhanced the effect of SOAT1 overexpression. Our data suggests an additional function for HDL in aiding extra-hepatic tissues to eliminate 7KCh by returning 7KFAEs to the liver for bile acid formation.

Bestrophinopathy: An RPE-photoreceptor interface disease

“…At the molecular level, excess 7KCh not only alters the biophysical properties of plasma membranes (Bach et al, 2008; Ruggiero et al, 2012), but also constitutes a ready substrate for esterification, producing a highly unstable esterified 7-keto molecule that re-releases 7KCh. The higher the degree of fatty acid unsaturation, the more unstable the esterified 7-KCh molecule is (Bretillon et al, 2008; Lee et al, 2015). Thus, the highly unsaturated fatty acids of the photoreceptor OS would provide a highly unstable variant of this compound, compromising the subretinal microenvironment, and consequently, the interplay of RPE-photoreceptor complex.…”

Circ-Tulp4 promotes β-cell adaptation to lipotoxicity by regulating soat1 expression

“…Once exogenous cholesterol is incorporated into cholesterol ester (CE), the cells could take advantage of the esterified form to avoid the toxicity of excess saturated cholesterol. Indeed, CE is found to be a strong proliferative factor for cells, including cancer cells, stem cells, and retinal cells ( Yue et al 2014 , Lee et al 2015 , Geng et al 2016 , Li et al 2016 ). CE depletion by abrogating soat1 activity significantly hinders cell growth due to the elevation of saturated cholesterol levels ( Buhman et al 2000 , Busch et al 2005 , Kedi et al 2009 , Li et al 2016 ).…”

“…CE depletion by abrogating soat1 activity significantly hinders cell growth due to the elevation of saturated cholesterol levels ( Buhman et al 2000 , Busch et al 2005 , Kedi et al 2009 , Li et al 2016 ). Additionally, CE synthesis is essential for adaptation to increased cholesterol metabolic needs and intracellular cholesterol homeostasis ( Busch et al 2005 , Lee et al 2015 ). A general decrease in cholesterol esterification, particularly concerning the toxic PA, could impair the function and survival of β cells ( Busch et al 2005 ).…”

“…As mentioned earlier, the overexpression of soat1 promotes cell proliferation, and vice versa ( Yue et al 2014 , Geng et al 2016 , Li et al 2016 , Stopsack et al 2017 ), which might be associated with increasing cyclin D1 expression ( Batetta et al 2003 ). The overexpression of soat1 enhances 7-ketocholesterol (7KCh)-fatty acid ester formation from toxic 7KCh, which significantly protects cells from 7KCh-induced cell death ( Lee et al 2015 ). Our study also proved that the soat1 enhanced β-cell proliferation under PA-induced toxic condition, possibly through regulating cyclin D1 expression.…”

“…Recently, soat1 is assumed to be a target for developing anti-atherosclerotic drugs. But neither overall nor myeloid-specific SOAT1 −/− knockout mice exhibits a hyperglycemic condition ( Lee et al 2015 ). The reason might be correlated with the complex interactions of various systems in mice models.…”