Desmosterol Increases Lipid Bilayer Fluidity during Hepatitis C Virus Infection

Cholesterol: A friend to viruses

Cholesterol is a key life-sustaining molecule which regulates membrane fluidity and serves as a signaling mediator. Cholesterol homeostasis is closely related to various pathological conditions including tumor, obesity, atherosclerosis, Alzheimer's disease and viral infection. Viral infection disrupts host cholesterol homeostasis, facilitating their own survival. Meanwhile, the host cells strive to reduce cholesterol accessibility to limit viral infection. This review focuses on the regulation of cholesterol metabolism and the role of cholesterol in viral infection, specifically providing an overview of cholesterol as a friend to promote viral entry, replication, assembly, release and immune evasion, which might inspire valuable thinking for pathogenesis and intervention of viral infection.

Sterol, tocopherol, and bioactive fatty acid differences between conventional, high-quality, and organic cow milk

Milk contains several components that are important for human nutrition and health. To date, studies on organic and conventional milk have focused on their gross composition and fatty acid content, but little attention has been paid to the differences between other minor components, such as sterols and vitamins that may have functional actions. The aim of this study was to investigate the nutritional differences among 3 types of milk from a dairy plant: conventional, high-quality, and organic (in compliance with European regulations) milk, focusing on minor components such as sterols of animal and plant origin (phytosterols), tocopherols, and bioactive fatty acids. Cholesterol ranged from 271.37 mg/100 g of fat in conventional milk to 278.76 mg/100 g of fat in organic milk. Lanosterol was the main minor animal sterol in cow milk (ranging from 3.41 to 4.37 mg/100 g of fat), followed by desmosterol. The amount of total plant sterols in the analyzed milk ranged from 4.43 mg/100 g of fat in organic to 4.71 mg/100 g of fat in high-quality milk. Brassicasterol was the main sterol of plant origin which varied from 2.6 mg/100 g of fat in conventional and organic milk, to 2.93 mg/100 g of fat in high-quality milk. The second most present phytosterol was β-sitosterol, which ranged from 0.86 mg/100 g of fat in conventional to 0.97 mg/100 g of fat in high-quality, and organic milk. The results of the study showed no significant differences in gross and sterol composition between the 3 types of milk. However, the only significant difference found was in the fatty acid profile, with a higher n-3 content found in high-quality milk than in conventional and organic milk. These findings suggest that the investigated product categories and labels have minimal effect on the sterol and fatty acid profile of commercial cow milk.

Sarcodonol A-D from fruiting bodies of Sarcodon imbricatus inhibits HCoV-OC43 induced apoptosis in MRC-5 cells

Four new 26-carboxylated ergostane-type sterols (Sarcodonol A-D) were isolated from 70% ethanol extracts of dried fruiting bodies of Sarcodon imbricatus. Their chemical structures were elucidated using 1D- and 2D-nuclear magnetic resonance and high-resolution electrospray ionization mass spectrometry, and confirmed by comparison with previously reported data. As far as we know, this is the first instance of isolating a 26-carboxylated ergostane-type sterol from nature. The determined antiviral efficacy of sarcodonol A-D (1-4) against HCoV-OC43 in MRC-5 cells confirmed that sarcodonol D (4) had significant antiviral activity. Notably, sarcodonol D (4) potently blocked virus infection at low-micromolar concentration and showed high SI (IC50 = 2.26 μM; CC50 > 100 μM; SI > 44.2). In addition, this research shows that the antiviral effect of sarcodonol D (4) via reduced apoptosis increased by viral infection is through mitochondrial stress regulation. This suggests that sarcodonol D (4) is a potential candidate for use as an antiviral treatment.

COVID-19 and cholesterol biosynthesis: Towards innovative decision support systems

With COVID-19 becoming endemic, there is a continuing need to find biomarkers characterizing the disease and aiding in patient stratification. We studied the relation between COVID-19 and cholesterol biosynthesis by comparing 10 intermediates of cholesterol biosynthesis during the hospitalization of 164 patients (admission, disease deterioration, discharge) admitted to the University Medical Center of Ljubljana. The concentrations of zymosterol, 24-dehydrolathosterol, desmosterol, and zymostenol were significantly altered in COVID-19 patients. We further developed a predictive model for disease severity based on clinical parameters alone and their combination with a subset of sterols. Our machine learning models applying 8 clinical parameters predicted disease severity with excellent accuracy (AUC = 0.96), showing substantial improvement over current clinical risk scores. After including sterols, model performance remained better than COVID-GRAM. This is the first study to examine cholesterol biosynthesis during COVID-19 and shows that a subset of cholesterol-related sterols is associated with the severity of COVID-19.

Dose-Response Effects of 7-Dehydrocholesterol Reductase Inhibitors on Sterol Profiles and Vesicular Stomatitis Virus Replication

Cholesterol is ubiquitous in cells; it plays a critical role in membrane structure and transport as well as in intracellular trafficking processes. There are suggestions that cholesterol metabolism is linked to innate immunity with inhibitors of DHCR7, the last enzyme in the cholesterol pathway, suggested to have potential as viral therapeutics nearly a decade ago. In fact, there are a number of highly prescribed pharmaceuticals that are off-target inhibitors of DHCR7, causing increased cellular levels of 7-dehydrodesmosterol (7-DHD) and 7-dehydrocholesterol (7-DHC). We report here dose-response studies of six such inhibitors on late-stage cholesterol biosynthesis in Neuro2a cells as well as their effect on infection of vesicular stomatitis virus (VSV). Four of the test compounds are FDA-approved drugs (cariprazine, trazodone, metoprolol, and tamoxifen), one (ifenprodil) has been the object of a recent Phase 2b COVID trial, and one (AY9944) is an experimental compound that has seen extensive use as a DHCR7 inhibitor. The three FDA-approved drugs inhibit replication of a GFP-tagged VSV with efficacies that mirror their effect on DHCR7. Ifenprodil and AY9944 have complex inhibitory profiles, acting on both DHCR7 and DHCR14, while tamoxifen does not inhibit DHCR7 and is toxic to Neuro2a at concentrations where it inhibits the Δ7-Δ8 isomerase of the cholesterol pathway. VSV itself affects the sterol profile in Neuro2a cells, showing a dose-response increase of dehydrolathosterol and lathosterol, the substrates for DHCR7, with a corresponding decrease in desmosterol and cholesterol. 7-DHD and 7-DHC are orders of magnitude more vulnerable to free radical chain oxidation than other sterols as well as polyunsaturated fatty esters, and the effect of these sterols on viral infection is likely a reflection of this fact of Nature.

Hepatitis C virus NS3-4A protease regulates the lipid environment for RNA replication by cleaving host enzyme 24-dehydrocholesterol reductase

Many RNA viruses create specialized membranes for genome replication by manipulating host lipid metabolism and trafficking, but in most cases, we do not know the molecular mechanisms responsible or how specific lipids may impact the associated membrane and viral process. For example, hepatitis C virus (HCV) causes a specific, large-fold increase in the steady-state abundance of intracellular desmosterol, an immediate precursor of cholesterol, resulting in increased fluidity of the membrane where HCV RNA replication occurs. Here, we establish the mechanism responsible for HCV's effect on intracellular desmosterol, whereby the HCV NS3-4A protease controls activity of 24-dehydrocholesterol reductase (DHCR24), the enzyme that catalyzes conversion of desmosterol to cholesterol. Our cumulative evidence for the proposed mechanism includes immunofluorescence microscopy experiments showing co-occurrence of DHCR24 and HCV NS3-4A protease; formation of an additional, faster-migrating DHCR24 species (DHCR24*) in cells harboring a HCV subgenomic replicon RNA or ectopically expressing NS3-4A; and biochemical evidence that NS3-4A cleaves DHCR24 to produce DHCR24* in vitro and in vivo. We further demonstrate that NS3-4A cleaves DHCR24 between residues Cys⁹¹ and Thr⁹² and show that this reduces the intracellular conversion of desmosterol to cholesterol. Together, these studies demonstrate that NS3-4A directly cleaves DHCR24 and that this results in the enrichment of desmosterol in the membranes where NS3-4A and DHCR24 co-occur. Overall, this suggests a model in which HCV directly regulates the lipid environment for RNA replication through direct effects on the host lipid metabolism.

Identification and characterization of prescription drugs that change levels of 7-dehydrocholesterol and desmosterol

Regulating blood cholesterol (Chol) levels by pharmacotherapy has successfully improved cardiovascular health. There is growing interest in the role of Chol precursors in the treatment of diseases. One sterol precursor, desmosterol (Des), is a potential pharmacological target for inflammatory and neurodegenerative disorders. However, elevating levels of the precursor 7-dehydrocholesterol (7-DHC) by inhibiting the enzyme 7-dehydrocholesterol reductase is linked to teratogenic outcomes. Thus, altering the sterol profile may either increase risk toward an adverse outcome or confer therapeutic benefit depending on the metabolite affected by the pharmacophore. In order to characterize any unknown activity of drugs on Chol biosynthesis, a chemical library of Food and Drug Administration-approved drugs was screened for the potential to modulate 7-DHC or Des levels in a neural cell line. Over 20% of the collection was shown to impact Chol biosynthesis, including 75 compounds that alter 7-DHC levels and 49 that modulate Des levels. Evidence is provided that three tyrosine kinase inhibitors, imatinib, ponatinib, and masitinib, elevate Des levels as well as other substrates of 24-dehydrocholesterol reductase, the enzyme responsible for converting Des to Chol. Additionally, the mechanism of action for ponatinib and masitinib was explored, demonstrating that protein levels are decreased as a result of treatment with these drugs.

Antiviral activity of N-(4-hydroxyphenyl) retinamide (4-HPR) against Zika virus

The rapid spread of Zika virus (ZIKV) in recent years has highlighted the severe diseases associated with ZIKV infection, such as Guillain-Barré syndrome in adults and microcephaly in newborns; yet no vaccines or antivirals currently exist to prevent or treat ZIKV infection. We and others have previously identified N-(4-hydroxyphenyl) retinamide (fenretinide or 4-HPR) as an antiviral compound that inhibits dengue virus 2 (DV2) and other flaviviruses by limiting the steady-state accumulation of viral RNA. Here we show that 4-HPR potently inhibits ZIKV in mammalian cell culture and significantly reduces both serum viremia and brain viral burden in a murine model of ZIKV infection. Consistent with previous observations with dengue virus, this antiviral activity is associated with a significant reduction in the steady-state abundance of viral genomic RNA. We show this reduction is due to a major decrease in the rate of viral RNA synthesis, though not via direct inhibition of the activity of the viral replicase. These results establish 4-HPR's mode of action against DV and ZIKV and, taken with previous clinical trials that established 4-HPR's safety and tolerability, illustrate the potential utility of 4-HPR as an agent for treatment of ZIKV infection.

Very Long Chain Fatty Acids Are Functionally Involved in Necroptosis

Necroptosis is a form of regulated cell death that is linked to various human diseases. Distinct membrane-related, thus lipid-dependent, alterations take place during necroptosis. However, little is known about the roles of specific lipids in this process. We used an untargeted LC-MS-based approach to reveal that distinct lipid species are regulated at the molecular level during necroptosis. We found that ceramides and very long chain fatty acids accumulate during this process. Intrigued by the specificity of very long chain fatty acid accumulation, we focused on characterizing their involvement during necroptosis. Biochemical characterizations suggested that activated fatty acid biosynthesis and elongation could be responsible for these accumulations. We further showed that inhibition of fatty acid biosynthesis and depletion of very long chain fatty acids prevented loss of plasma membrane integrity and cell death, strongly suggesting that very long chain fatty acids are functionally involved in necroptosis.