Sulfation of 25-hydroxycholesterol regulates lipid metabolism, inflammatory responses, and cell proliferation

Abstract

Intracellular lipid accumulation, inflammatory responses, and subsequent apoptosis are the major pathogenic events of metabolic disorders, including atherosclerosis and nonalcoholic fatty liver diseases. Recently, a novel regulatory oxysterol, 5-cholesten-3b, 25-diol 3-sulfate (25HC3S), has been identified, and hydroxysterol sulfotransferase 2B1b (SULT2B1b) has been elucidated as the key enzyme for its biosynthesis from 25-hydroxycholesterol (25HC) via oxysterol sulfation. The product 25HC3S and the substrate 25HC have been shown to coordinately regulate lipid metabolism, inflammatory responses, and cell proliferation in vitro and in vivo. 25HC3S decreases levels of the nuclear liver oxysterol receptor (LXR) and sterol regulatory element-binding proteins (SREBPs), inhibits SREBP processing, subsequently downregulates key enzymes in lipid biosynthesis, decreases intracellular lipid levels in hepatocytes and THP-1-derived macrophages, prevents apoptosis, and promotes cell proliferation in liver tissues. Furthermore, 25HC3S increases nuclear PPARγ and cytosolic IκBα and decreases nuclear NF-κB levels and proinflammatory cytokine expression and secretion when cells are challenged with LPS and TNFα. In contrast to 25HC3S, 25HC, a known LXR ligand, increases nuclear LXR and decreases nuclear PPARs and cytosol IκBα levels. In this review, we summarize our recent findings, including the discovery of the regulatory oxysterol sulfate, its biosynthetic pathway, and its functional mechanism. We also propose that oxysterol sulfation functions as a regulatory signaling pathway.

Keywords: 25HC3S, LXR, SREBPs, IκB, NF-κB, PPARs; cholesterol and triglyceride metabolism; nuclear receptor; oxysterol sulfate.

Fig. 1.

Possible role of 5-cholesten-3b, 25-diol 3-sulfate (25HC3S), oxysterol sulfation, in lipid metabolism. 25HC activates LXRα, and the complex binds to LXR response elements in promoter regions and activates transcription of its targeting genes, including SREBP-1 expression, which increases the biosynthesis of fatty acids and cholesterol. Thus, activation of LXRα increases lipids in the circulation by increasing lipid synthesis and efflux. 25HC3S, the product of 25HC sulfation, inhibits SREBP-1/2 expression and its activation, subsequently inhibiting lipid biosynthesis. LXRx and RXRx represent isomers (α and β); STS, sulfosteroid sulfatase.

Fig. 2.

Possible role of 25HC3S (oxysterol sulfation) in inflammatory responses in THP-1 macrophages. 25HC is sulfated by hydroxycholesterol sulfotransferase 2B1b (SULT2B1) to form 25HC3S. 25HC3S increases PPARγ translocation to the nucleus where it upregulates IκB expression and suppresses TNFα expression. As an inactive form, NF-κB is bound by members of IκBs and sequestered in the cytoplasm. When TNFα levels are increased, it removes IκBs from NF-κB by ubiquitination and degradation and subsequently activates NF-κB. The free active NF-κB translocates to the nucleus for stimulation of inflammatory responses. Thus, 25HC3S represses inflammatory responses by increasing nuclear PPARγ protein levels, suppressing TNFα and stimulating IκB expression. However, its precursor 25HC decreases PPARγ levels and increases IκB degradation, which favors proinflammatory responses.

Fig. 3.

Possible role of oxysterol sulfation in maintenance of lipid homeostasis, inflammatory responses, and cell proliferation via regulation of nuclear receptor activities. 1) When intracellular cholesterol levels are increased, mitochondrial cholesterol delivery protein StarD1 delivers cholesterol into mitochondria, where regulatory oxysterols, such as 25-OH cholesterol (25HC), are synthesized by CYP27A1. These oxysterols can in turn activate LXRα and inactivate PPARγ and subsequently regulate expression of its target genes, which increases fatty acid and triglyceride biosynthesis and induces apoptosis, as shown in light gray lines. 25HC can be sulfated by SULT2B1b to 25HC3S when cholesterol delivery to mitochondria is increased. 2) 25HC3S inactivates LXRα, suppresses SREBP-1c expression, inhibits SREBP-1c/2 processing, activates PPARγ, and induces cell proliferation, leading to transcriptional upregulation of a number of genes responsible for lipid degradation and anti-inflammation. Thus, 25HC3S decreases intracellular lipid levels by inhibiting its synthesis and stimulating its degradation, and suppresses inflammation by activating PPARγ as, shown in dark gray lines.

Fig. 4.

Novel regulatory pathway of intracellular lipid homeostasis. Diet and synthesized cholesterol using acetate as substrate are two sources for intracellular cholesterol. There are two pathways, neutral and acidic, for cholesterol degradation and bile acid synthesis. The neutral pathway is dominant for bile acid biosynthesis; the acidic serves as a regulatory pathway involved in lipid metabolism and cell proliferation. When cholesterol levels increase by exogenous diet or endogenous biosynthesis in the cells, 1) StarD1 delivers cholesterol into mitochondria where CYP27A1 is located; 2) cholesterol is 25-hydroxylated by CYP27A1 to 25-hydroxycholesterol and sulfated at its 3β-position to form 25HC3S by SULT2B1b; 3) the sulfated oxysterol modulates nuclear receptors by activating PPARs and inactivating LXRs through forming a complex that goes to nuclei; and 4) the complexes play important roles in maintenance of lipid homeostasis, inflammatory responses, antiapoptosis, and cell proliferation.