Role of Cholesterol 25-Hydroxylase (Ch25h) in Mediating Innate Immune Responses to Streptococcus pneumoniae Infection

Alveolar macrophages (AM) are long-lived tissue-resident innate immune cells of the airways. AM are key effectors of recognition, initiation, and resolution of the host defense against microbes and play an essential role in mediating host responses to Streptococcus pneumoniae infection. Lipid metabolism in AM can significantly impact cellular function and biology. Dysregulated metabolism contributes to an accumulation of lipids, unfolded protein response induction, and inflammatory cytokine production. Our study was designed to investigate the impact of Ch25h on mediating innate immune responses by macrophages during S. pneumoniae infection. Using wild-type and Ch25-/- mice, we examined the role of cholesterol metabolism on inflammatory cytokine production and bacterial clearance. Our results demonstrate that Ch25h plays an important role in the initiation and intensity of cytokine and chemokine production in the lung during S. pneumoniae infection. In the absence of Ch25h, there was enhanced phagocytosis and bacterial clearance. Taken together, our findings demonstrate the important role of Ch25h in modulating host responsiveness to S. pneumoniae infection.

Rapid GPR183-mediated recruitment of eosinophils to the lung after Mycobacterium tuberculosis infection

Influx of eosinophils into the lungs is typically associated with type II responses during allergy and fungal and parasitic infections. However, we previously reported that eosinophils accumulate in lung lesions during type I inflammatory responses to Mycobacterium tuberculosis (Mtb) in humans, macaques, and mice, in which they support host resistance. Here we show eosinophils migrate into the lungs of macaques and mice as early as one week after Mtb exposure. In mice this influx is CCR3 independent and instead requires cell-intrinsic expression of the oxysterol receptor GPR183, which is highly expressed on human and macaque eosinophils. Murine eosinophils interact directly with bacilli-laden alveolar macrophages, which upregulate the oxysterol-synthesizing enzyme Ch25h, and eosinophil recruitment is impaired in Ch25h-deficient mice. Our findings show that eosinophils are among the earliest cells from circulation to sense and respond to Mtb infection of alveolar macrophages and reveal a role for GPR183 in the migration of eosinophils into lung tissue.

Eosinophils are usually associated with allergy or type II responses. Here, Bohrer et al. show that eosinophils are rapidly recruited to the lungs after respiratory infection with the intracellular pathogen Mycobacterium tuberculosis through the oxysterol sensor GPR183.

Ch25h and 25-HC prevent liver steatosis through regulation of cholesterol metabolism and inflammation

Non-alcoholic fatty liver disease (NAFLD) is currently the most prevalent metabolic disorder all over the world, and lipid metabolic disorders and inflammation are closely associated and contribute to the pathogenesis of NAFLD. Cholesterol 25-hydroxylase (Ch25h) and its product, 25-hydroxycholesterol (25-HC), play important roles in cholesterol homeostasis and inflammation, but whether Ch25h and 25-HC are involved in NAFLD remains uncertain. In this study, we use Ch25h knockout mice, hepatic cells and liver biopsies to explore the role of Ch25h and 25-HC in lipid metabolism and accumulation in liver, determine the molecular mechanism of lipid accumulation and inflammation influenced by Ch25h and 25-HC, and assess the regulatory effects of Ch25h and 25-HC on human NAFLD. Our results indicate that mice lacking Ch25h have normal cholesterol homeostasis with normal diet, but under the condition of high fat diet (HFD), the mice show higher total cholesterol and triglyceride in serum, and prone to hepatic steatosis. Ch25h deficiency reduces the cholesterol efflux regulated by liver X receptor α (LXRα), increases the synthesis of cholesterol mediated by sterol-regulatory element binding protein 2 (SREBP-2), and increases the activation of NLRP3 inflammasome, therefore promotes hepatic steatosis. Collectively, our data suggest that Ch25h and 25-HC play important roles in lipid metabolism and inflammation, thereby exerting anti-NAFLD functions.

Hepatic Reduction in Cholesterol 25-Hydroxylase Aggravates Diet-induced Steatosis

BACKGROUND & AIMS

Cholesterol 25-hydroxylase (Ch25h), converting cholesterol to 25-hydroxycholesterol (25-HC), is critical in modulating cellular lipid metabolism and anti-inflammatory and antiviral activities. However, its role in nonalcoholic fatty liver disease remains unclear.

METHODS

Ch25h expression was detected in livers of ob/ob mice and E3 rats fed a high-fat diet (HFD). Gain- or loss-of-function of Ch25h was performed using Ch25h^+/+ (wild type [WT]) mice receiving AAV8-Ch25h or Ch25h knockout (Ch25h^-/-) mice. WT mice fed an HFD were administered with 25-HC. The Ch25h-LXRα-CYP axis was measured in primary hepatocytes isolated from WT and Ch25h^-/- mice.

RESULTS

We found that Ch25h level was decreased in livers of ob/ob mice and E3 rats fed an HFD. Ch25h^-/- mice fed an HFD showed aggravated fatty liver and decreased level of cytochrome P450 7A1 (CYP7A1), in comparison with their WT littermates. RNA-seq analysis revealed that the differentially expressed genes in livers of HFD-fed Ch25h^-/- mice were involved in pathways of positive regulation of lipid metabolic process, steroid metabolic process, cholesterol metabolic process, and bile acid biosynthetic process. As gain-of-function experiments, WT mice receiving AAV8-Ch25h or 25-HC showed alleviated NAFLD, when compared with the control group receiving AAV8-control or vehicle control. Consistently, Ch25h overexpression significantly elevated the levels of primary and secondary bile acids and CYP7A1 but decreased those of small heterodimer partner and FGFR4.

CONCLUSIONS

Elevated levels of Ch25h and its enzymatic product 25-HC alleviate HFD-induced hepatic steatosis via regulating enterohepatic circulation of bile acids. The underlying mechanism involves 25-HC activation of CYP7A1 via liver X receptor. These data suggest that targeting Ch25h or 25-HC may have therapeutic advantages against nonalcoholic fatty liver disease.

Diverse Immunoregulatory Roles of Oxysterols-The Oxidized Cholesterol Metabolites

Intermediates of both cholesterol synthesis and cholesterol metabolism can have diverse roles in the control of cellular processes that go beyond the control of cholesterol homeostasis. For example, oxidized forms of cholesterol, called oxysterols have functions ranging from the control of gene expression, signal transduction and cell migration. This is of particular interest in the context of immunology and immunometabolism where we now know that metabolic processes are key towards shaping the nature of immune responses. Equally, aberrant metabolic processes including altered cholesterol homeostasis contribute to immune dysregulation and dysfunction in pathological situations. This review article brings together our current understanding of how oxysterols affect the control of immune responses in diverse immunological settings.

KLF4/Ch25h axis activated by metformin suppresses EndoMT in human umbilical vein endothelial cells

Metformin, an anti-hyperglycemia drug, protected endothelial cells (ECs) from dysfunction while high glucose (HG) caused endothelial dysfunction. Previously, we found that metformin suppressed endothelial-to-mesenchymal transition (EndoMT), a cellular process that promoted endothelial dysfunction. However, the involved mechanism is still unclear. In this study, we found that metformin increased the expression of krüppel-like factor 4 (KLF4) and cholesterol-25-hydroxylase (Ch25h) while HG decreased the expression of KLF4 and Ch25h. In addition, HG promoted EndoMT indicting by the decrease of endothelial maker genes and increase of mesenchymal maker genes. Furthermore, RNA sequence (RNA-seq) data showed that KLF4 suppressed EndoMT. Moreover, we proved that metformin increased Ch25h expression through not only KLF4 but also epigenetic modification including DNA methylation and active histone modification. Lastly, we proved that Ch25h/25 hydroxycholesterol (25 HC)/Liver X receptor α (LXRα) suppressed EndoMT. Altogether, our study demonstrated that KLF4/Ch25h/axis activated by metformin suppressed EndoMT. Therefore, KLF4/Ch25h/axis may be a new potential therapeutic target for endothelial dysfunction diseases.

Oxysterols in Autoimmunity

Cholesterol is a member of the sterol family that plays essential roles in biological processes, including cell membrane stability and myelin formation. Cholesterol can be metabolized into several molecules including bile acids, hormones, and oxysterols. Studies from the last few decades have demonstrated that oxysterols are not only active metabolites but are further involved in the modulation of immune responses. Liver X Receptors (LXRs), nuclear receptors for oxysterols, are important for cholesterol homeostasis and regulation of inflammatory response but are still poorly characterized during autoimmune diseases. Here we review the current knowledge about the role of oxysterols during autoimmune conditions and focus on the implication of LXR-dependent and LXR-independent pathways. We further highlight the importance of these pathways in particular during central nervous system (CNS) autoimmunity and inflammatory bowel diseases (IBD) in both experimental models and human studies. Finally, we discuss our vision about future applications and research on oxysterols related to autoimmunity.

Oxysterol Restraint of Cholesterol Synthesis Prevents AIM2 Inflammasome Activation

Type I interferon restrains interleukin-1β (IL-1β)-driven inflammation in macrophages by upregulating cholesterol-25-hydroxylase (Ch25h) and repressing SREBP transcription factors. However, the molecular links between lipid metabolism and IL-1β production remain obscure. Here, we demonstrate that production of 25-hydroxycholesterol (25-HC) by macrophages is required to prevent inflammasome activation by the DNA sensor protein absent in melanoma 2 (AIM2). We find that in response to bacterial infection or lipopolysaccharide (LPS) stimulation, macrophages upregulate Ch25h to maintain repression of SREBP2 activation and cholesterol synthesis. Increasing macrophage cholesterol content is sufficient to trigger IL-1β release in a crystal-independent but AIM2-dependent manner. Ch25h deficiency results in cholesterol-dependent reduced mitochondrial respiratory capacity and release of mitochondrial DNA into the cytosol. AIM2 deficiency rescues the increased inflammasome activity observed in Ch25h^-/-. Therefore, activated macrophages utilize 25-HC in an anti-inflammatory circuit that maintains mitochondrial integrity and prevents spurious AIM2 inflammasome activation.

Herpes simplex virus type 1 abrogates the antiviral activity of Ch25h via its virion host shutoff protein

Cholesterol 25-hydroxylase (Ch25h) is an interferon-inducible protein, and recent studies have demonstrated that it inhibited the replication of many enveloped viruses. However, in this study, we found that cells infected with wild-type (WT) HSV-1 reduced the expression of Ch25h, and ectopic expression of Ch25h could not inhibit the replication of WT-HSV-1. By screening assay, HSV-1 UL41 protein was found to down-regulate the expression of Ch25h. In addition, UL41 abrogated the antiviral activity of Ch25h via degrading its mRNA. Furthermore, ectopic expression of Ch25h inhibited the replication of UL41-null mutant HSV-1 (R2621), but not WT-HSV-1, and knockdown of Ch25h did not affect the replication of WT-HSV-1, but promoted the replication of the R2621. For the first time, HSV-1 UL41 was demonstrated to evade the antiviral function of Ch25h via its endonuclease activity.

Angiotensin-II-induced Muscle Wasting is Mediated by 25-Hydroxycholesterol via GSK3β Signaling Pathway

While angiotensin II (ang II) has been implicated in the pathogenesis of cardiac cachexia (CC), the molecules that mediate ang II's wasting effect have not been identified. It is known TNF-α level is increased in patients with CC, and TNF-α release is triggered by ang II. We therefore hypothesized that ang II induced muscle wasting is mediated by TNF-α. Ang II infusion led to skeletal muscle wasting in wild type (WT) but not in TNF alpha type 1 receptor knockout (TNFR1KO) mice, suggesting that ang II induced muscle loss is mediated by TNF-α through its type 1 receptor. Microarray analysis identified cholesterol 25-hydroxylase (Ch25h) as the down stream target of TNF-α. Intraperitoneal injection of 25-hydroxycholesterol (25-OHC), the product of Ch25h, resulted in muscle loss in C57BL/6 mice, accompanied by increased expression of atrogin-1, MuRF1 and suppression of IGF-1/Akt signaling pathway. The identification of 25-OHC as an inducer of muscle wasting has implications for the development of specific treatment strategies in preventing muscle loss.

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Ang II induced muscle wasting is mediated by TNF-α, which in turn up regulates Ch25h

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Knockout of TNFR1 inhibits the production of 25-OHC and blocks ang II induced muscle loss in mice

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25-OHC injection induces muscle wasting in mice by activating GSK3β

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A GSK3β inhibitor blocks ang II induced muscle atrophy, which paves the way for targeted therapy to treat muscle wasting

Cardiac cachexia (CC), a condition characterized by weight loss and muscle wasting, is a serious complication that occurs in patients with chronic heart failure. This condition impairs patient's daily physical activity and their quality of life. Specific therapy for CC is currently unavailable because the pathogenesis remains unknown. Previous studies have identified angiotensin II (ang II) as an important mediator of CC. We now report a previously unrecognized role of 25-hydroxycholesterol (25-OHC) in mediating ang II induced muscle loss. The identification of 25-OHC as a muscle wasting inducer has implications for the development of therapeutic intervention in preserving muscle mass.