Nuclear Oxysterols, 25HC and 25HC3S, Regulate Nuclear Orphan Receptor Activities and Attenuate Intracellular Lipid Levels

Abstract

We recently identified the sulfated oxysterol, 5‐cholesten‐3beta, 25‐diol 3‐sulfate (25HC3S), as an important regulatory molecule. When cholesterol delivered to mitochondria was increased by overexpression of the transport protein StarD1 in primary rat hepatocytes, 25‐Hydroxycholesterol (25HC) and 25HC3S were accumulated in the nucleus, suggesting a role in transcriptional regulation. Nuclear orphan receptors are key transcriptional regulators of hepatocyte lipid metabolism. We now have studied the effects of 25HC and 25HC3S on the activation of nuclear receptors, LXR, RXR, PPAR, and SREBPs. Addition of 25HC3S to hepatocytes in culture markedly decreased nuclear LXR but increased PPAR protein levels, followed by dose‐ and time‐dependent decreases in SREBP‐1 protein and mRNA levels. 25HC3S administration also led to a decrease in the expression of a number of SREBP‐1 responsive genes including acetyl CoA carboxylase‐1 (ACC‐1) and fatty acid synthase (FAS), key enzymes involved in fatty acid biosynthesis, and subsequently, decreased the intracellular free fatty acids. 25HC, a LXR ligand and the precursor of 25HC3S, had opposite effects: increase nuclear LXR protein as well as SREBP‐1 and FAS mRNA, and decrease nuclear PPAR protein levels. This work implies that sulfation of 25HC does more than simple inactivation of 25HC. It provides a new regulatory mechanism involved in lipid metabolism.