Crucial step in cholesterol homeostasis: sterols promote binding of SCAP to INSIG-1, a membrane protein that facilitates retention of SREBPs in ER

Structure and mechanism of vitamin-K-dependent γ-glutamyl carboxylase

γ-Glutamyl carboxylase (GGCX) is the sole identified enzyme that uses vitamin K (VK) as a cofactor in humans. This protein catalyses the oxidation of VK hydroquinone to convert specific glutamate residues to γ-carboxyglutamate residues in VK-dependent proteins (VDPs), which are involved in various essential biological processes and diseases1-3. However, the working mechanism of GGCX remains unclear. Here we report three cryogenic electron microscopy structures of human GGCX: in the apo state, bound to osteocalcin (a VDP) and bound to VK. The propeptide of the VDP binds to the lumenal domain of GGCX, which stabilizes transmembrane helices 6 and 7 of GGCX to create the VK-binding pocket. After binding of VK, residue Lys218 in GGCX mediates the oxidation of VK hydroxyquinone, which leads to the deprotonation of glutamate residues and the construction of γ-carboxyglutamate residues. Our structural observations and results from binding and cell biological assays and molecular dynamics simulations show that a cholesterol molecule interacts with the transmembrane helices of GGCX to regulate its protein levels in cells. Together, these results establish a link between cholesterol metabolism and VK-dependent pathways.

Elocalcitol, a fluorinated vitamin D derivative, prevents high-fat diet-induced obesity via SCAP downregulation and miR-146a-associated mechanisms

Background

Obesity is an emerging health problem worldwide as it is associated with increased risk of cardiovascular, metabolic, mental disorders, and cancer. Therapeutic weight management remains one of the options for the treatment of excess weight and associated comorbidities. In this study, the therapeutic potential of elocalcitol, a fluorinated derivative of vitamin D, was studied on the model of high-fat diet (HFD)-induced obesity in mice.

Results

It was demonstrated that co-administration of elocalcitol in the doses 15 ug/kg (i.p.) twice a week for 16 weeks prevented body weight gain by approximately 15%. The significant retardation in the body weight gain was observed already on the second week of elocalcitol treatment. Administration of elocalcitol also reduced visceral and epididymal fat accumulation by 55% and 35%, respectively, metabolic syndrome development, and lipid droplets accumulation in the liver of mice exposed to HFD. In contrast, the administration of cholecalciferol (vitamin D)-a precursor to calcitriol, the biologically active form of vitamin D, did not affect significantly the signs of obesity and metabolic syndrome, suggesting that the anti-obese effects of elocalcitol are not related to the canonical vitamin D receptor (VDR). Further studies have demonstrated that the preventive effect of elocalcitol is associated with the decreased levels of sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP) and upregulation insulin-inducing gene-1 (Insig1) mRNA expression suggesting that the anti-obese effect of elocalcitol is mediated via inhibition of SREBP-mediated lipogenesis. We also demonstrated that elocalcitol prevents an increase in the expression of proinflammatory cytokines such as interleukin-1 beta (Il1b), tumor necrosis factor-alpha (Tnf), and interleukin-18 (Il18), and this effect was associated with upregulation of microRNA-146a (miR-146a). Deletion of the miR-146a gene reduced the anti-obese effects of elocalcitol and prevented its actions on the SCAP levels. The data indicate that elocalcitol's reduction of SCAP is at least partly mediated by miR-146a modulation.

Conclusion

The study demonstrates that elocalcitol prevents HFD-induced obesity and metabolic syndrome in mice, likely by inhibiting SREBP-mediated lipogenesis and upregulating miR-146a. These findings provide valuable insights into the anti-obesity mechanisms of fluorinated D-vitamin analogs and suggest potential therapeutic strategies for obesity prevention.

Biallelic variants in SREBF2 cause autosomal recessive spastic paraplegia

Hereditary spastic paraplegias (HSPs) refer to a genetically and clinically heterogeneous group of neurodegenerative disorders characterized by the degeneration of motor neurons. To date, a significant number of patients still have not received a definite genetic diagnosis. Therefore, identifying unreported causative genes continues to be of great importance. Here, we perform whole exome sequencing in a cohort of Chinese HSP patients. Three homozygous variants (p.L604W, p.S517F, and p.T984A) within the sterol regulatory element-binding factor 2 (SREBF2) gene are identified in one autosomal recessive family and two sporadic patients, respectively. Co-segregation is confirmed by Sanger sequencing in all available members. The three variants are rare in the public or in-house database and are predicted to be damaging. The biological impacts of variants in SREBF2 are examined by functional experiments in patient-derived fibroblasts and Drosophila. We find that the variants upregulate cellular cholesterol due to the overactivation of SREBP2, eventually impairing the autophagosomal and lysosomal functions. The overexpression of the mature form of SREBP2 leads to locomotion defects in Drosophila. Our findings identify SREBF2 as a causative gene for HSP and highlight the impairment of cholesterol as a critical pathway for HSP.

Sigma 1 Receptor and Its Pivotal Role in Neurological Disorders

Sigma 1 receptor (S1R) is a multifunctional, ligand-activated protein located in the membranes of the endoplasmic reticulum (ER). It mediates a variety of neurological disorders, including epilepsy, amyotrophic lateral sclerosis, Alzheimer's disease, Huntington's disease. The wide neuroprotective effects of S1R agonists are achieved by a variety of pro-survival and antiapoptotic S1R-mediated signaling functions. Nonetheless, relatively little is known about the specific molecular mechanisms underlying S1R activity. Many studies on S1R protein have highlighted the importance of maintaining normal cellular homeostasis through its control of calcium and lipid exchange between the ER and mitochondria, ER-stress response, and many other mechanisms. In this review, we will discuss S1R different cellular localization and explain S1R-associated biological activity, such as its localization in the ER-plasma membrane and Mitochondrion-Associated ER Membrane interfaces. While outlining the cellular mechanisms and important binding partners involved in these processes, we also explained how the dysregulation of these pathways contributes to neurodegenerative disorders.

Noncanonical PI(4,5)P2 coordinates lysosome positioning through cholesterol trafficking

In p53-deficient cancers, targeting cholesterol metabolism has emerged as a promising therapeutic approach, given that p53 loss dysregulates sterol regulatory element-binding protein 2 (SREBP-2) pathways, thereby enhancing cholesterol biosynthesis. While cholesterol synthesis inhibitors such as statins have shown initial success, their efficacy is often compromised by the development of acquired resistance. Consequently, new strategies are being explored to disrupt cholesterol homeostasis more comprehensively by inhibiting its synthesis and intracellular transport. In this study, we investigate a previously underexplored function of PI5P4Ks, which catalyzes the conversion of PI(5)P to PI(4,5)P2 at intracellular membranes. Our findings reveal that PI5P4Ks play a key role in facilitating lysosomal cholesterol transport, regulating lysosome positioning, and sustaining growth signaling via the mTOR pathway. While PI5P4Ks have previously been implicated in mTOR signaling and tumor proliferation in p53-deficient contexts, this work elucidates an upstream mechanism that unifies these earlier observations.

Glutamine sensing licenses cholesterol synthesis

The mevalonate pathway produces essential lipid metabolites such as cholesterol. Although this pathway is negatively regulated by metabolic intermediates, little is known of the metabolites that positively regulate its activity. We found that the amino acid glutamine is required to activate the mevalonate pathway. Glutamine starvation inhibited cholesterol synthesis and blocked transcription of the mevalonate pathway-even in the presence of glutamine derivatives such as ammonia and α-ketoglutarate. We pinpointed this glutamine-dependent effect to a loss in the ER-to-Golgi trafficking of SCAP that licenses the activation of SREBP2, the major transcriptional regulator of cholesterol synthesis. Both enforced Golgi-to-ER retro-translocation and the expression of a nuclear SREBP2 rescued mevalonate pathway activity during glutamine starvation. In a cell model of impaired mitochondrial respiration in which glutamine uptake is enhanced, SREBP2 activation and cellular cholesterol were increased. Thus, the mevalonate pathway senses and is activated by glutamine at a previously uncharacterized step, and the modulation of glutamine synthesis may be a strategy to regulate cholesterol levels in pathophysiological conditions.

Porcine reproductive and respiratory syndrome virus activates lipid synthesis through a ROS-dependent AKT/PCK1/INSIG/SREBPs axis

The porcine reproductive and respiratory syndrome virus (PRRSV) is a highly contagious pathogen in pigs. This study aimed to investigate the impact of PRRSV infection on cellular metabolism, particularly focusing on lipid metabolism to understand its role in promoting viral replication. We conducted a metabolic analysis on MARC-145 cells before and after PRRSV infection. Our results demonstrated that the most significant alterations in cellular metabolism, accounting for 40.8 % of total changes, were related to lipid metabolism. These changes were primarily driven by the activation of sterol regulatory-element binding proteins (SREBPs), critical regulators of lipid biosynthesis. To understand the mechanisms behind SREBPs activation by PRRSV, we investigated the involvement of upstream effectors, specifically protein kinase B (AKT) and phosphoenolpyruvate carboxykinase 1 (PCK1). Our findings indicated that PRRSV infection triggered AKT activation, leading to the subsequent activation of PCK1. Activated PCK1 then phosphorylated insulin-induced genes (INSIGs), resulting in their degradation. This degradation facilitated the translocation of SREBPs from the endoplasmic reticulum to the nucleus. Additionally, we observed that PRRSV infection stimulated the production of reactive oxygen species (ROS), which played a critical role in activating AKT. Collectively, our findings demonstrate that PRRSV enhances lipid synthesis through a ROS-dependent AKT/PCK1/INSIG/SREBPs signaling axis, which provides new insights into the metabolic strategies employed by PRRSV.

A receptor kinase senses sterol by coupling with elicitins in auxotrophic Phytophthora

Sterols are vital nutrients and signals for eukaryotic organisms. Mammalian cells are known to sense and respond to sterol status changes to maintain them within strict limits, a process associated with various human diseases. However, this process is not understood in oomycete pathogens, most of which are sterol auxotrophic and must obtain sterols from host plants. Here, we report that Phytophthora sojae SSRK1 (sterol-sensing receptor kinase 1) detects host sterols by coupling with elicitins, thereby controlling signaling and sterol absorption. Sterols are recruited by extracellular soluble elicitins, and these complexes then bind to SSRK1 to form trimolecular complexes. These complexes subsequently trigger downstream calcium influx, activation of mitogen-activated protein kinase, and transcriptome reprogramming through the receptor's kinase activity. Our data demonstrate a unique sterol sensing pathway where elicitins and a transmembrane receptor kinase SSRK1 act as coreceptors for extracellular sterols. These findings also portray a sterol-based war between oomycetes and plants, providing a unique perspective on how a pathogen detects host signals during infection.

ARMC5 selectively degrades SCAP-free SREBF1 and is essential for fatty acid desaturation in adipocytes

SREBF1 plays the central role in lipid metabolism. It has been known that full-length SREBF1 that did not associate with SCAP (SCAP-free SREBF1) is actively degraded, but its molecular mechanism and its biological meaning remain unclear. ARMC5-CUL3 complex was recently identified as E3 ubiquitin ligase of full-length SREBF. Although ARMC5 was involved in SREBF pathway in adrenocortical cells, the role of ARMC5 in adipocytes has not been investigated. In this study, adipocyte-specific Armc5 KO mice were generated. In the white adipose tissue of these mice, all the stearoyl-CoA desaturase (Scd) were drastically downregulated. Consistently, unsaturated fatty acids were decreased and saturated fatty acids were increased. The protein amount of full-length SREBF1 was increased, but ATAC-Seq peaks at the SREBF1-binding sites were markedly diminished around the Scd1 locus in the WAT of Armc5 KO mice. Armc5-deficient 3T3-L1 adipocytes also exhibited downregulation of Scd. Mechanistically, disruption of Armc5 restored decreased full-length SREBF1 in CHO cells deficient for Scap. Overexpression of Scap inhibited ARMC5-mediated degradation of full-length SREBF1, and overexpression of Armc5 increased nuclear SREBF1/full-length SREBF1 ratio and SREBF1 transcriptional activity in the presence of exogenous SCAP. These results demonstrated that ARMC5 selectively removes SCAP-free SREBF1 and stimulates SCAP-mediated SREBF1 processing, hence is essential for fatty acid desaturation in vivo.

Beyond conventional treatment: ASGR1 Leading the new era of hypercholesterolemia management

Cardiovascular disease (CVD) remains a leading cause of mortality worldwide, with hypercholesterolemia being a major risk factor. Although various lipid-lowering therapies exist, many patients fail to achieve optimal cholesterol control, highlighting the need for novel therapeutic approaches. ASGR1 (asialoglycoprotein receptor 1), predominantly expressed on hepatocytes, has emerged as a key regulator of cholesterol metabolism and low-density lipoprotein (LDL) clearance. This receptor's ability to regulate lipid homeostasis positions it as a promising target for therapeutic intervention in hypercholesterolemia and related cardiovascular diseases. This review critically examines the biological functions and regulatory mechanisms of ASGR1 in cholesterol metabolism, with a focus on its potential as a therapeutic target for hypercholesterolemia and related cardiovascular diseases. By analyzing recent advances in ASGR1 research, this article explores its role in liver-specific pathways, the implications of ASGR1 variants in CVD risk, and the prospects for developing ASGR1-targeted therapies. This review aims to provide a foundation for future research and clinical applications in hypercholesterolemia management.

25-Hydroxycholesterol inhibits Hantavirus infection by reprogramming cholesterol metabolism

Hantavirus causes two types of acute diseases: hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome. It is a major health concern due to its high mortality and lack of effective treatment. Type I interferon treatment has been suggested to be effective against hantavirus when treated in advance. Interferons induce multiple interferon-stimulated genes (ISGs), whose products are highly effective at resisting and controlling viruses. A product of ISGs, the enzyme cholesterol 25-hydroxylase (CH25H), catalyzes the oxidation of cholesterol to 25-hydroxycholesterol (25HC). 25HC can inhibit multiple enveloped-virus infections, but the mechanism is largely unknown, and whether 25HC plays an important role in regulating hantavirus remains unexplored. In this study, we show that Hantaan virus (HTNV), the prototype hantavirus, induced CH25H gene in infected cells. Overexpression of CH25H and treatment with 25HC, inhibited HTNV infection, possibly by lowering 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMG-CoA reductase, HMGCR), which inhibits cholesterol biosynthesis. In addition, cholesterol-lowering drugs such as HMGCR-targeting statins have potent hantavirus inhibitory effects. Our results indicate that 25HC and some statins are potential antiviral agents effective against hantavirus infections. This study provides evidence that targeting cholesterol metabolism is promising in developing specific hantavirus antivirals and indicates the possibility of repurposing FDA-approved cholesterol-lowering drug, statins for treating hantavirus infection.

The lipid side of unfolded protein response

Although our current knowledge of the molecular crosstalk between the ER stress, the unfolded protein response (UPR), and lipid homeostasis remains limited, there is increasing evidence that dysregulation of either protein or lipid homeostasis profoundly affects the other. Most research regarding UPR signaling in human diseases has focused on the causes and consequences of disrupted protein folding. The UPR itself consists of very complex pathways that function to not only maintain protein homeostasis, but just as importantly, modulate lipid biogenesis to allow the ER to adjust and promote cell survival. Lipid dysregulation is known to activate many aspects of the UPR, but the complexity of this crosstalk remains a major research barrier. ER lipid disequilibrium and lipotoxicity are known to be important contributors to numerous human pathologies, including insulin resistance, liver disease, cardiovascular diseases, neurodegenerative diseases, and cancer. Despite their medical significance and continuous research, however, the molecular mechanisms that modulate lipid synthesis during ER stress conditions, and their impact on cell fate decisions, remain poorly understood. Here we summarize the current view on crosstalk and connections between altered lipid metabolism, ER stress, and the UPR.

Infiltrating treg reprogramming in the tumor immune microenvironment and its optimization for immunotherapy

Immunotherapy has shown promising anti-tumor effects across various tumors, yet it encounters challenges from the inhibitory tumor immune microenvironment (TIME). Infiltrating regulatory T cells (Tregs) are important contributors to immunosuppressive TIME, limiting tumor immunosurveillance and blocking effective anti-tumor immune responses. Although depletion or inhibition of systemic Tregs enhances the anti-tumor immunity, autoimmune sequelae have diminished expectations for the approach. Herein, we summarize emerging strategies, specifically targeting tumor-infiltrating (TI)-Tregs, that elevate the capacity of organisms to resist tumors by reprogramming their phenotype. The regulatory mechanisms of Treg reprogramming are also discussed as well as how this knowledge could be utilized to develop novel and effective cancer immunotherapies.

Phosphorylation of Insig-2 mediates inhibition of fatty acid synthesis by polyunsaturated fatty acids

Insig-1 and Insig-2 are endoplasmic reticulum (ER) proteins that inhibit lipid synthesis by blocking transport of sterol regulatory element-binding proteins (SREBP-1 and SREBP-2) from ER to Golgi. In the Golgi, SREBPs are processed proteolytically to release their transcription-activating domains, which enhance the synthesis of fatty acids, triglycerides, and cholesterol. Heretofore, the two Insigs have redundant functions, and there is no rationale for two isoforms. The current data identify a specific function for Insig-2. We show that eicosapentaenoic acid (EPA), a polyunsaturated fatty acid, inhibits fatty acid synthesis in human fibroblasts and rat hepatocytes by activating adenylate cyclase, which induces protein kinase A (PKA) to phosphorylate serine-106 in Insig-2. Phosphorylated Insig-2 inhibits the proteolytic processing of SREBP-1, thereby blocking fatty acid synthesis. Phosphorylated Insig-2 does not block the processing of SREBP-2, which activates cholesterol synthesis. Insig-1 lacks serine-106 and is not phosphorylated at this site. EPA inhibition of SREBP-1 processing was reduced by the replacement of serine-106 in Insig-2 with alanine or by treatment with KT5720, a PKA inhibitor. Inhibition did not occur in mutant human fibroblasts that possess Insig-1 but lack Insig-2. These data provide an Insig-2-specific mechanism for the long-known inhibition of fatty acid synthesis by polyunsaturated fatty acids.

Research progress of SREBP and its role in the pathogenesis of autoimmune rheumatic diseases

Autoimmune rheumatic diseases comprise a group of immune-related disorders characterized by non-organ-specific inflammation. These diseases include systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), ankylosing spondylitis (AS), gout, among others. Typically involving the hematologic system, these diseases may also affect multiple organs and systems. The pathogenesis of autoimmune rheumatic immune diseases is complex, with diverse etiologies, all associated with immune dysfunction. The current treatment options for this type of disease are relatively limited and come with certain side effects. Therefore, the urgent challenge remains to identify novel therapeutic targets for these diseases. Sterol regulatory element-binding proteins (SREBPs) are basic helix-loop-helix-leucine zipper transcription factors that regulate the expression of genes involved in lipid and cholesterol biosynthesis. The expression and transcriptional activity of SREBPs can be modulated by extracellular stimuli such as polyunsaturated fatty acids, amino acids, glucose, and energy pathways including AKT-mTORC and AMP-activated protein kinase (AMPK). Studies have shown that SREBPs play roles in regulating lipid metabolism, cytokine production, inflammation, and the proliferation of germinal center B (GCB) cells. These functions are significant in the pathogenesis of rheumatic and immune diseases (Graphical abstract). Therefore, this paper reviews the potential mechanisms of SREBPs in the development of SLE, RA, and gout, based on an exploration of their functions.

A multi-layered integrative analysis reveals a cholesterol metabolic program in outer radial glia with implications for human brain evolution

The definition of molecular and cellular mechanisms contributing to brain ontogenetic trajectories is essential to investigate the evolution of our species. Yet their functional dissection at an appropriate level of granularity remains challenging. Capitalizing on recent efforts that have extensively profiled neural stem cells from the developing human cortex, we develop an integrative computational framework to perform trajectory inference and gene regulatory network reconstruction, (pseudo)time-informed non-negative matrix factorization for learning the dynamics of gene expression programs, and paleogenomic analysis for a higher-resolution mapping of derived regulatory variants in our species in comparison with our closest relatives. We provide evidence for cell type-specific regulation of gene expression programs during indirect neurogenesis. In particular, our analysis uncovers a key role for a cholesterol program in outer radial glia, regulated by zinc-finger transcription factor KLF6. A cartography of the regulatory landscape impacted by Homo sapiens-derived variants reveals signals of selection clustering around regulatory regions associated with GLI3, a well-known regulator of radial glial cell cycle, and impacting KLF6 regulation. Our study contributes to the evidence of significant changes in metabolic pathways in recent human brain evolution.

A genome-wide CRISPR/Cas9 screen identifies calreticulin as a selective repressor of ATF6α

Activating transcription factor 6 (ATF6) is one of three endoplasmic reticulum (ER) transmembrane stress sensors that mediate the unfolded protein response (UPR). Despite its crucial role in long-term ER stress adaptation, regulation of ATF6 alpha (α) signalling remains poorly understood, possibly because its activation involves ER-to-Golgi and nuclear trafficking. Here, we generated an ATF6α/Inositol-requiring kinase 1 (IRE1) dual UPR reporter CHO-K1 cell line and performed an unbiased genome-wide CRISPR/Cas9 mutagenesis screen to systematically profile genetic factors that specifically contribute to ATF6α signalling in the presence and absence of ER stress. The screen identified both anticipated and new candidate genes that regulate ATF6α activation. Among these, calreticulin (CRT), a key ER luminal chaperone, selectively repressed ATF6α signalling: Cells lacking CRT constitutively activated a BiP::sfGFP ATF6α-dependent reporter, had higher BiP levels and an increased rate of trafficking and processing of ATF6α. Purified CRT interacted with the luminal domain of ATF6α in vitro and the two proteins co-immunoprecipitated from cell lysates. CRT depletion exposed a negative feedback loop implicating ATF6α in repressing IRE1 activity basally and overexpression of CRT reversed this repression. Our findings indicate that CRT, beyond its known role as a chaperone, also serves as an ER repressor of ATF6α to selectively regulate one arm of the UPR.

Control of innate immunity and lipid biosynthesis in neurodegeneration

The cGAS-STING innate immunity pathway and the SREBP-activated cholesterol and fatty acid synthesis pathway are abnormally co-regulated in neurodegenerative disease. Activation of STING signaling occurs at the endoplasmic reticulum (ER) membrane with STING anchored by INSIG1 along with SREBP and the sterol-bound SREBP cleavage activating protein (SCAP) when sterols are in abundance. When sterols are low, the INSIG-dependent STING pathway is inactivated and the SREBP-SCAP complex is translocated to the Golgi where SREBP is cleaved and translocated to the nucleus to transactivate genes for cholesterol and fatty acid synthesis. Thus, there is inverse activation of STING vs. SREBP: when innate immunity is active, pathways for cholesterol and fatty acid synthesis are suppressed, and vice versa. The STING pathway is stimulated by foreign viral cytoplasmic nucleic acids interacting with the cyclic GMP-AMP synthase (cGAS) DNA sensor or RIG-I and MDA5 dsRNA sensors, but with neurodegeneration innate immunity is also activated by self-DNAs and double-stranded RNAs that accumulate with neuronal death. Downstream, activated STING recruits TBK1 and stimulates the transactivation of interferon stimulated genes and the autophagy pathway, which are both protective. However, chronic activation of innate immunity contributes to microglia activation, neuroinflammation and autophagy failure leading to neurodegeneration. STING is also a proton channel that when activated stimulates proton exit from STING vesicles leading to cell death. Here we review the salient features of the innate immunity and cholesterol and fatty acid synthesis pathways, observations of abnormal STING and SREBP signaling in neurodegenerative disease, and relevant therapeutic approaches.

N-SREBP2 Provides a Mechanism for Dynamic Control of Cellular Cholesterol Homeostasis

Cholesterol is required to maintain the functional integrity of cellular membrane systems and signalling pathways, but its supply must be closely and dynamically regulated because excess cholesterol is toxic. Sterol regulatory element-binding protein 2 (SREBP2) and the ER-resident protein HMG-CoA reductase (HMGCR) are key regulators of cholesterol biosynthesis. Here, we assessed the mechanistic aspects of their regulation in hepatic cells. Unexpectedly, we found that the transcriptionally active fragment of SREBP2 (N-SREBP2) was produced constitutively. Moreover, in the absence of an exogenous cholesterol supply, nuclear N-SREBP2 became resistant to proteasome-mediated degradation. This resistance was paired with increased occupancy at the HMGCR promoter and HMGCR expression. Inhibiting nuclear N-SREBP2 degradation did not increase HMGCR RNA levels; this increase required cholesterol depletion. Our findings, combined with previous physiological and biophysical investigations, suggest a new model of SREBP2-mediated regulation of cholesterol biosynthesis in the organ that handles large and rapid fluctuations in the dietary supply of this key lipid. Specifically, in the nucleus, cholesterol and the ubiquitin-proteasome system provide a short-loop system that modulates the rate of cholesterol biosynthesis via regulation of nuclear N-SREBP2 turnover and HMGCR expression. Our findings have important implications for maintaining cellular cholesterol homeostasis and lowering blood cholesterol via the SREBP2-HMGCR axis.

Tubular insulin-induced gene 1 deficiency promotes NAD+ consumption and exacerbates kidney fibrosis

Profibrotic proximal tubules (PT) were identified as a unique phenotype of proximal tubule cells (PTCs) in renal fibrosis by single-cell RNA sequencing (scRNA-seq). Controlling the process of renal fibrosis requires understanding how to manage the S1 subset's branch to the S3 subset rather than to the profibrotic PT subset. Insulin-induced gene 1 (Insig1) is one of the branch-dependent genes involved in controlling this process, although its role in renal fibrosis is unknown. Here, we discovered that tubular Insig1 deficiency, rather than fibroblast Insig1 deficiency, plays a detrimental role in the pathogenesis of renal fibrosis in vivo and in vitro. Overexpression of Insig1 profoundly inhibited renal fibrosis. Mechanistically, Insig1 deletion in PTCs boosted SREBP1 nuclear localization, increasing Aldh1a1 transcriptional activity, causing excessive NAD+ consumption and ER enlargement, as well as accelerating renal fibrosis. We also identified nicardipine as a selective inhibitor of Aldh1a1, which could restore NAD+ and maintain ER homeostasis, as well as improve renal fibrosis. Together, our findings support tubular Insig1 as a new therapeutic target for chronic kidney disease (CKD).

ALOX15B controls macrophage cholesterol homeostasis via lipid peroxidation, ERK1/2 and SREBP2

Macrophage cholesterol homeostasis is crucial for health and disease and has been linked to the lipid-peroxidizing enzyme arachidonate 15-lipoxygenase type B (ALOX15B), albeit molecular mechanisms remain obscure. We performed global transcriptome and immunofluorescence analysis in ALOX15B-silenced primary human macrophages and observed a reduction of nuclear sterol regulatory element-binding protein (SREBP) 2, the master transcription factor of cellular cholesterol biosynthesis. Consequently, SREBP2-target gene expression was reduced as were the sterol biosynthetic intermediates desmosterol and lathosterol as well as 25- and 27-hydroxycholesterol. Mechanistically, suppression of ALOX15B reduced lipid peroxidation in primary human macrophages and thereby attenuated activation of mitogen-activated protein kinase ERK1/2, which lowered SREBP2 abundance and activity. Low nuclear SREBP2 rendered both, ALOX15B-silenced and ERK1/2-inhibited macrophages refractory to SREBP2 activation upon blocking the NPC intracellular cholesterol transporter 1. These studies suggest a regulatory mechanism controlling macrophage cholesterol homeostasis based on ALOX15B-mediated lipid peroxidation and concomitant ERK1/2 activation.

Endocrine Control of Lipid Metabolism

Lipids are essential in insects and play pleiotropic roles in energy storage, serving as a fuel for energy-driven processes such as reproduction, growth, development, locomotion, flight, starvation response, and diapause induction, maintenance, and termination. Lipids also play fundamental roles in signal transduction, hormone synthesis, forming components of the cell membrane, and thus are essential for maintenance of normal life functions. In insects, the neuroendocrine system serves as a master regulator of most life activities, including growth and development. It is thus important to pay particular attention to the regulation of lipid metabolism through the endocrine system, especially when considering the involvement of peptide hormones in the processes of lipogenesis and lipolysis. In insects, there are several lipogenic and lipolytic hormones that are involved in lipid metabolism such as insulin-like peptides (ILPs), adipokinetic hormone (AKH), 20-hydroxyecdysone (20-HE), juvenile hormone (JH), and serotonin. Other neuropeptides such as diapause hormone-pheromone biosynthesis activating neuropeptide (DH-PBAN), CCHamide-2, short neuropeptide F, and the cytokines Unpaired 1 and 2 may play a role in inducing lipogenesis. On the other hand, neuropeptides such as neuropeptide F, allatostatin-A, corazonin, leukokinin, tachykinins, limostatins, and insulin-like growth factor (ILP6) stimulate lipolysis. This chapter briefly discusses the current knowledge of the endocrine regulation of lipid metabolism in insects that could be utilized to reveal differences between insects and mammalian lipid metabolism which may help understand human diseases associated with dysregulation of lipid metabolism. Physiological similarities of insects to mammals make them valuable model systems for studying human diseases characterized by disrupted lipid metabolism, including conditions like diabetes, obesity, arteriosclerosis, and various metabolic syndromes.

Transcriptomics analysis of the bovine endometrium during the perioestrus period

During the oestrous cycle, the bovine endometrium undergoes morphological and functional changes, which are regulated by alterations in the levels of oestrogen and progesterone and consequent changes in gene expression. To clarify these changes before and after oestrus, RNA-seq was used to profile the transcriptome of oestrus-synchronized beef heifers. Endometrial samples were collected from 29 animals, which were slaughtered in six groups beginning 12 h after the withdrawal of intravaginal progesterone releasing devices until seven days post-oestrus onset (luteal phase). The groups represented proestrus, early oestrus, metoestrus and early dioestrus (luteal phase). Changes in gene expression were estimated relative to gene expression at oestrus. Ingenuity Pathway Analysis (IPA) was used to identify canonical pathways and functional processes of biological importance. A total of 5,845 differentially expressed genes (DEGs) were identified. The lowest number of DEGs was observed at the 12 h post-oestrus time point, whereas the greatest number was observed at Day 7 post-oestrus onset (luteal phase). A total of 2,748 DEGs at this time point did not overlap with any other time points. Prior to oestrus, Neurological disease and Organismal injury and abnormalities appeared among the top IPA diseases and functions categories, with upregulation of genes involved in neurogenesis. Lipid metabolism was upregulated before oestrus and downregulated at 48h post-oestrus, at which point an upregulation of immune-related pathways was observed. In contrast, in the luteal phase the Lipid metabolism and Small molecule biochemistry pathways were upregulated.

Dimethyloxalylglycine Suppresses SREBP1c and Lipogenic Gene Expressions in Hepatocytes Independently of HIF1A

Dimethyloxalylglycine (DMOG) is a representative inhibitor of the prolyl hydroxylase domain (PHD), which mediates the degradation of hypoxia-inducible factor-1-alpha (HIF1A). DMOG exerts its pharmacological effects via the canonical pathway that involves PHD inhibition; however, it remains unclear whether DMOG affects lipogenic gene expression in hepatocytes. We aimed to elucidate the effects of DMOG on sterol regulatory element-binding protein-1c (SREBP1c), a master regulator of fatty acid synthesis in hepatocytes. DMOG treatment inhibited SREBP1c mRNA and protein expression in HepG2 and AML12 hepatocytes and reduced the transcript levels of SREBP1c-regulated lipogenic genes. A luciferase reporter assay revealed that DMOG inhibited the transcriptional activity of SREBP1c. Moreover, DMOG suppressed SREBP1c expression in mice liver. Mechanistically, treatment with DMOG enhanced the expression of HIF1A and insulin-induced gene 2 (INSIG2), which inhibits the activation of SREBP1c. However, HIF1A or INSIG2 knockdown failed to reverse the inhibitory effect of DMOG on SREBP1c expression, suggesting a redundant role of HIF1A and INSIG2 in terms of repressing SREBP1c. DMOG did not function through the canonical pathway involving inhibition of SREBP1c by PHD, highlighting the presence of non-canonical pathways that mediate its anti-lipogenic effect.

DGAT2 inhibition blocks SREBP-1 cleavage and improves hepatic steatosis by increasing phosphatidylethanolamine in the ER

Diacylglycerol acyltransferase 2 (DGAT2) catalyzes the final step of triglyceride (TG) synthesis. DGAT2 deletion in mice lowers liver TGs, and DGAT2 inhibitors are under investigation for the treatment of fatty liver disease. Here, we show that DGAT2 inhibition also suppressed SREBP-1 cleavage, reduced fatty acid synthesis, and lowered TG accumulation and secretion from liver. DGAT2 inhibition increased phosphatidylethanolamine (PE) levels in the endoplasmic reticulum (ER) and inhibited SREBP-1 cleavage, while DGAT2 overexpression lowered ER PE concentrations and increased SREBP-1 cleavage in vivo. ER enrichment with PE blocked SREBP-1 cleavage independent of Insigs, which are ER proteins that normally retain SREBPs in the ER. Thus, inhibition of DGAT2 shunted diacylglycerol into phospholipid synthesis, increasing the PE content of the ER, resulting in reduced SREBP-1 cleavage and less hepatic steatosis. This study reveals a new mechanism that regulates SREBP-1 activation and lipogenesis that is independent of sterols and SREBP-2 in liver.

Hepatic glycogenesis antagonizes lipogenesis by blocking S1P via UDPG

The identification of mechanisms to store glucose carbon in the form of glycogen rather than fat in hepatocytes has important implications for the prevention of nonalcoholic fatty liver disease (NAFLD) and other chronic metabolic diseases. In this work, we show that glycogenesis uses its intermediate metabolite uridine diphosphate glucose (UDPG) to antagonize lipogenesis, thus steering both mouse and human hepatocytes toward storing glucose carbon as glycogen. The underlying mechanism involves transport of UDPG to the Golgi apparatus, where it binds to site-1 protease (S1P) and inhibits S1P-mediated cleavage of sterol regulatory element-binding proteins (SREBPs), thereby inhibiting lipogenesis in hepatocytes. Consistent with this mechanism, UDPG administration is effective at treating NAFLD in a mouse model and human organoids. These findings indicate a potential opportunity to ameliorate disordered fat metabolism in the liver.

PEDV inhibits HNRNPA3 expression by miR-218-5p to enhance cellular lipid accumulation and promote viral replication

Porcine epidemic diarrhea virus (PEDV) requires complete dependence on the metabolic system of the host cell to complete its life cycle. There is a strong link between efficient viral replication and cellular lipid synthesis. However, the mechanism by which PEDV interacts with host cells to hijack cellular lipid metabolism to promote its replication remains unclear. In this study, PEDV infection significantly enhanced the expression of lipid synthesis-related genes and increased cellular lipid accumulation. Furthermore, using liquid chromatography-tandem mass spectrometry, we identified heterogeneous nuclear ribonucleoprotein A3 (HNRNPA3) as the interacting molecule of PEDV NSP9. We demonstrated that the expression of HNRNPA3 was downregulated by PEDV-induced miR-218-5p through targeting its 3' untranslated region. Interestingly, knocking down HNRNPA3 facilitated the PEDV replication by promoting cellular lipid synthesis. We next found that the knockdown of HNRNPA3 potentiated the transcriptional activity of sterol regulatory element-binding transcription factor 1 (SREBF1) through zinc finger protein 135 (ZNF135) as well as PI3K/AKT and JNK signaling pathways. In summary, we propose a model in which PEDV downregulates HNRNPA3 expression to promote the expression and activation of SREBF1 and increase cellular lipid accumulation, providing a novel mechanism by which PEDV interacts with the host to utilize cellular lipid metabolism to promote its replication.IMPORTANCEAs the major components and structural basis of the viral replication complexes of positive-stranded RNA viruses, lipids play an essential role in viral replication. However, how PEDV manipulates host cell lipid metabolism to promote viral replication by interacting with cell proteins remains poorly understood. Here, we found that SREBF1 promotes cellular lipid synthesis, which is essential for PEDV replication. Moreover, HNRNPA3 negatively regulates SREBF1 activation and specifically reduces lipid accumulation, ultimately inhibiting PEDV dsRNA synthesis. Our study provides new insight into the mechanisms by which PEDV hijacks cell lipid metabolism to benefit viral replication, which can offer a potential target for therapeutics against PEDV infection.

Direct binding to sterols accelerates endoplasmic reticulum-associated degradation of HMG CoA reductase

The maintenance of cholesterol homeostasis is crucial for normal function at both the cellular and organismal levels. Two integral membrane proteins, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) and Scap, are key targets of a complex feedback regulatory system that operates to ensure cholesterol homeostasis. HMGCR catalyzes the rate-limiting step in the transformation of the 2-carbon precursor acetate to 27-carbon cholesterol. Scap mediates proteolytic activation of sterol regulatory element-binding protein-2 (SREBP-2), a membrane-bound transcription factor that controls expression of genes involved in the synthesis and uptake of cholesterol. Sterol accumulation triggers binding of HMGCR to endoplasmic reticulum (ER)-localized Insig proteins, leading to the enzyme's ubiquitination and proteasome-mediated ER-associated degradation (ERAD). Sterols also induce binding of Insigs to Scap, which leads to sequestration of Scap and its bound SREBP-2 in the ER, thereby preventing proteolytic activation of SREBP-2 in the Golgi. The oxygenated cholesterol derivative 25-hydroxycholesterol (25HC) and the methylated cholesterol synthesis intermediate 24,25-dihydrolanosterol (DHL) differentially modulate HMGCR and Scap. While both sterols promote binding of HMGCR to Insigs for ubiquitination and subsequent ERAD, only 25HC inhibits the Scap-mediated proteolytic activation of SREBP-2. We showed previously that 1,1-bisphosphonate esters mimic DHL, accelerating ERAD of HMGCR while sparing SREBP-2 activation. Building on these results, our current studies reveal specific, Insig-independent photoaffinity labeling of HMGCR by photoactivatable derivatives of the 1,1-bisphosphonate ester SRP-3042 and 25HC. These findings disclose a direct sterol binding mechanism as the trigger that initiates the HMGCR ERAD pathway, providing valuable insights into the intricate mechanisms that govern cholesterol homeostasis.

SpoVAF and FigP assemble into oligomeric ion channels that enhance spore germination

Bacterial spores can remain dormant for decades yet rapidly germinate and resume growth in response to nutrients. GerA family receptors that sense and respond to these signals have recently been shown to oligomerize into nutrient-gated ion channels. Ion release initiates exit from dormancy. Here, we report that a distinct ion channel, composed of SpoVAF (5AF) and its newly discovered partner protein, YqhR (FigP), amplifies the response. At high germinant concentrations, 5AF/FigP accelerate germination; at low concentrations, this complex becomes critical for exit from dormancy. 5AF is homologous to the channel-forming subunit of GerA family receptors and is predicted to oligomerize around a central pore. 5AF mutations predicted to widen the channel cause constitutive germination during spore formation and membrane depolarization in vegetative cells. Narrow-channel mutants are impaired in germination. A screen for suppressors of a constitutively germinating 5AF mutant identified FigP as an essential cofactor of 5AF activity. We demonstrate that 5AF and FigP interact and colocalize with GerA family receptors in spores. Finally, we show that 5AF/FigP accelerate germination in B. subtilis spores that have nutrient receptors from another species. Our data support a model in which nutrient-triggered ion release by GerA family receptors activates 5AF/FigP ion release, amplifying the response to germinant signals.

RNA sequencing unravels novel L cell constituents and mechanisms of GLP-1 secretion in human gastric bypass-operated intestine

AIMS/HYPOTHESIS

Roux-en-Y gastric bypass surgery (RYGB) frequently results in remission of type 2 diabetes as well as exaggerated secretion of glucagon-like peptide-1 (GLP-1). Here, we assessed RYGB-induced transcriptomic alterations in the small intestine and investigated how they were related to the regulation of GLP-1 production and secretion in vitro and in vivo.

METHODS

Human jejunal samples taken perisurgically and 1 year post RYGB (n=13) were analysed by RNA-seq. Guided by bioinformatics analysis we targeted four genes involved in cholesterol biosynthesis, which we confirmed to be expressed in human L cells, for potential involvement in GLP-1 regulation using siRNAs in GLUTag and STC-1 cells. Gene expression analyses, GLP-1 secretion measurements, intracellular calcium imaging and RNA-seq were performed in vitro. OGTTs were performed in C57BL/6j and iScd1-/- mice and immunohistochemistry and gene expression analyses were performed ex vivo.

RESULTS

Gene Ontology (GO) analysis identified cholesterol biosynthesis as being most affected by RYGB. Silencing or chemical inhibition of stearoyl-CoA desaturase 1 (SCD1), a key enzyme in the synthesis of monounsaturated fatty acids, was found to reduce Gcg expression and secretion of GLP-1 by GLUTag and STC-1 cells. Scd1 knockdown also reduced intracellular Ca2+ signalling and membrane depolarisation. Furthermore, Scd1 mRNA expression was found to be regulated by NEFAs but not glucose. RNA-seq of SCD1 inhibitor-treated GLUTag cells identified altered expression of genes implicated in ATP generation and glycolysis. Finally, gene expression and immunohistochemical analysis of the jejunum of the intestine-specific Scd1 knockout mouse model, iScd1-/-, revealed a twofold higher L cell density and a twofold increase in Gcg mRNA expression.

CONCLUSIONS/INTERPRETATION

RYGB caused robust alterations in the jejunal transcriptome, with genes involved in cholesterol biosynthesis being most affected. Our data highlight SCD as an RYGB-regulated L cell constituent that regulates the production and secretion of GLP-1.

Cholesterol metabolism: physiological regulation and diseases

Cholesterol homeostasis is crucial for cellular and systemic function. The disorder of cholesterol metabolism not only accelerates the onset of cardiovascular disease (CVD) but is also the fundamental cause of other ailments. The regulation of cholesterol metabolism in the human is an extremely complex process. Due to the dynamic balance between cholesterol synthesis, intake, efflux and storage, cholesterol metabolism generally remains secure. Disruption of any of these links is likely to have adverse effects on the body. At present, increasing evidence suggests that abnormal cholesterol metabolism is closely related to various systemic diseases. However, the exact mechanism by which cholesterol metabolism contributes to disease pathogenesis remains unclear, and there are still unknown factors. In this review, we outline the metabolic process of cholesterol in the human body, especially reverse cholesterol transport (RCT). Then, we discuss separately the impact of abnormal cholesterol metabolism on common diseases and potential therapeutic targets for each disease, including CVD, tumors, neurological diseases, and immune system diseases. At the end of this review, we focus on the effect of cholesterol metabolism on eye diseases. In short, we hope to provide more new ideas for the pathogenesis and treatment of diseases from the perspective of cholesterol.

The lipid metabolism remodeling: A hurdle in breast cancer therapy

Lipids, as one of the three primary energy sources, provide energy for all cellular life activities. Lipids are also known to be involved in the formation of cell membranes and play an important role as signaling molecules in the intracellular and microenvironment. Tumor cells actively or passively remodel lipid metabolism, using the function of lipids in various important cellular life activities to evade therapeutic attack. Breast cancer has become the leading cause of cancer-related deaths in women, which is partly due to therapeutic resistance. It is necessary to fully elucidate the formation and mechanisms of chemoresistance to improve breast cancer patient survival rates. Altered lipid metabolism has been observed in breast cancer with therapeutic resistance, indicating that targeting lipid reprogramming is a promising anticancer strategy.

The Role of SCAP/SREBP as Central Regulators of Lipid Metabolism in Hepatic Steatosis

The prevalence of metabolic dysfunction-associated steatotic liver disease (MASLD) is rapidly increasing worldwide at an alarming pace, due to an increase in obesity, sedentary and unhealthy lifestyles, and unbalanced dietary habits. MASLD is a unique, multi-factorial condition with several phases of progression including steatosis, steatohepatitis, fibrosis, cirrhosis, and hepatocellular carcinoma. Sterol element binding protein 1c (SREBP1c) is the main transcription factor involved in regulating hepatic de novo lipogenesis. This transcription factor is synthesized as an inactive precursor, and its proteolytic maturation is initiated in the membrane of the endoplasmic reticulum upon stimulation by insulin. SREBP cleavage activating protein (SCAP) is required as a chaperon protein to escort SREBP from the endoplasmic reticulum and to facilitate the proteolytic release of the N-terminal domain of SREBP into the Golgi. SCAP inhibition prevents activation of SREBP and inhibits the expression of genes involved in triglyceride and fatty acid synthesis, resulting in the inhibition of de novo lipogenesis. In line, previous studies have shown that SCAP inhibition can resolve hepatic steatosis in animal models and intensive research is going on to understand the effects of SCAP in the pathogenesis of human disease. This review focuses on the versatile roles of SCAP/SREBP regulation in de novo lipogenesis and the structure and molecular features of SCAP/SREBP in the progression of hepatic steatosis. In addition, recent studies that attempt to target the SCAP/SREBP axis as a therapeutic option to interfere with MASLD are discussed.

Enzymatically Formed Oxysterols and Cell Death

The side-chain hydroxylation of cholesterol by specific enzymes produces 24(S)-hydroxycholesterol, 25-hydroxycholesterol, 27-hydroxycholesterol, and other products. These enzymatically formed side-chain oxysterols act as intermediates in the biosynthesis of bile acids and serve as signaling molecules that regulate cholesterol homeostasis. Besides these intracellular functions, an imbalance in oxysterol homeostasis is implicated in pathophysiology. Furthermore, growing evidence reveals that oxysterols affect cell proliferation and cause cell death. This chapter provides an overview of the pathophysiological role of side-chain oxysterols in developing human diseases. We also summarize our understanding of the molecular mechanisms underlying the induction of various forms of cell death by side-chain oxysterols.

Lipid Metabolism Regulators Are the Possible Determinant for Characteristics of Myopic Human Scleral Stroma Fibroblasts (HSSFs)

The purpose of the current investigation was to elucidate what kinds of responsible mechanisms induce elongation of the sclera in myopic eyes. To do this, two-dimensional (2D) cultures of human scleral stromal fibroblasts (HSSFs) obtained from eyes with two different axial length (AL) groups, <26 mm (low AL group, n = 2) and >27 mm (high AL group, n = 3), were subjected to (1) measurements of Seahorse mitochondrial and glycolytic indices to evaluate biological aspects and (2) analysis by RNA sequencing. Extracellular flux analysis revealed that metabolic indices related to mitochondrial and glycolytic functions were higher in the low AL group than in the high AL group, suggesting that metabolic activities of HSSF cells are different depending the degree of AL. Based upon RNA sequencing of these low and high AL groups, the bioinformatic analyses using gene ontology (GO) enrichment analysis and ingenuity pathway analysis (IPA) of differentially expressed genes (DEGs) identified that sterol regulatory element-binding transcription factor 2 (SREBF2) is both a possible upstream regulator and a causal network regulator. Furthermore, SREBF1, insulin-induced gene 1 (INSIG1), and insulin-like growth factor 1 (IGF1) were detected as upstream regulators, and protein tyrosine phosphatase receptor type O (PTPRO) was detected as a causal network regulator. Since those possible regulators were all pivotally involved in lipid metabolisms including fatty acid (FA), triglyceride (TG) and cholesterol (Chol) biosynthesis, the findings reported here indicate that FA, TG and Chol biosynthesis regulation may be responsible mechanisms inducing AL elongation via HSSF.

Lipid compartments and lipid metabolism as therapeutic targets against coronavirus

Lipids perform a series of cellular functions, establishing cell and organelles' boundaries, organizing signaling platforms, and creating compartments where specific reactions occur. Moreover, lipids store energy and act as secondary messengers whose distribution is tightly regulated. Disruption of lipid metabolism is associated with many diseases, including those caused by viruses. In this scenario, lipids can favor virus replication and are not solely used as pathogens' energy source. In contrast, cells can counteract viruses using lipids as weapons. In this review, we discuss the available data on how coronaviruses profit from cellular lipid compartments and why targeting lipid metabolism may be a powerful strategy to fight these cellular parasites. We also provide a formidable collection of data on the pharmacological approaches targeting lipid metabolism to impair and treat coronavirus infection.

SARS-CoV-2 papain-like protease plays multiple roles in regulating cellular proteins in the endoplasmic reticulum

Nsp3s are the largest nonstructural proteins of coronaviruses. These transmembrane proteins include papain-like proteases (PLpro) that play essential roles in cleaving viral polyproteins into their mature units. The PLpro of SARS-CoV viruses also have deubiquitinating and deISGylating activities. As Nsp3 is an endoplasmic reticulum (ER)-localized protein, we asked if the deubiquitinating activity of SARS-CoV-2 PLpro affects proteins that are substrates for ER-associated degradation (ERAD). Using full-length Nsp3 as well as a truncated transmembrane form we interrogated, by coexpression, three potential ERAD substrates, all of which play roles in regulating lipid biosynthesis. Transmembrane PLpro increases the level of INSIG-1 and decreases its ubiquitination. However, different effects were seen with SREBP-1 and SREBP-2. Transmembrane PLpro cleaves SREBP-1 at three sites, including two noncanonical sites in the N-terminal half of the protein, resulting in a decrease in precursors of the active transcription factor. Conversely, cleavage of SREBP-2 occurs at a single canonical site that disrupts a C-terminal degron, resulting in increased SREBP-2 levels. When this site is mutated and the degron can no longer be interrupted, SREBP-2 is still stabilized by transmembrane PLpro, which correlates with a decrease in SREBP-2 ubiquitination. All of these observations are dependent on PLpro catalytic activity. Our findings demonstrate that, when anchored to the ER membrane, SARS-CoV-2 Nsp3 PLpro can function as a deubiquitinating enzyme to stabilize ERAD substrates. Additionally, SARS-CoV-2 Nsp3 PLpro can cleave ER-resident proteins, including at sites that could escape analyses based on the established consensus sequence.

Hypoxia activates SREBP2 through Golgi disassembly in bone marrow-derived monocytes for enhanced tumor growth

Bone marrow-derived cells (BMDCs) infiltrate hypoxic tumors at a pre-angiogenic state and differentiate into mature macrophages, thereby inducing pro-tumorigenic immunity. A critical factor regulating this differentiation is activation of SREBP2-a well-known transcription factor participating in tumorigenesis progression-through unknown cellular mechanisms. Here, we show that hypoxia-induced Golgi disassembly and Golgi-ER fusion in monocytic myeloid cells result in nuclear translocation and activation of SREBP2 in a SCAP-independent manner. Notably, hypoxia-induced SREBP2 activation was only observed in an immature lineage of bone marrow-derived cells. Single-cell RNA-seq analysis revealed that SREBP2-mediated cholesterol biosynthesis was upregulated in HSCs and monocytes but not in macrophages in the hypoxic bone marrow niche. Moreover, inhibition of cholesterol biosynthesis impaired tumor growth through suppression of pro-tumorigenic immunity and angiogenesis. Thus, our findings indicate that Golgi-ER fusion regulates SREBP2-mediated metabolic alteration in lineage-specific BMDCs under hypoxia for tumor progression.

Inhibition of the SREBP pathway prevents SARS-CoV-2 replication and inflammasome activation

SARS-CoV-2 induces major cellular lipid rearrangements, exploiting the host's metabolic pathways to replicate. Sterol regulatory element binding proteins (SREBPs) are a family of transcription factors that control lipid metabolism. SREBP1 is associated with the regulation of fatty acids, whereas SREBP2 controls cholesterol metabolism, and both isoforms are associated with lipid droplet (LD) biogenesis. Here, we evaluated the effect of SREBP in a SARS-CoV-2-infected lung epithelial cell line (Calu-3). We showed that SARS-CoV-2 infection induced the activation of SREBP1 and SREBP2 and LD accumulation. Genetic knockdown of both SREBPs and pharmacological inhibition with the dual SREBP activation inhibitor fatostatin promote the inhibition of SARS-CoV-2 replication, cell death, and LD formation in Calu-3 cells. In addition, we demonstrated that SARS-CoV-2 induced inflammasome-dependent cell death by pyroptosis and release of IL-1β and IL-18, with activation of caspase-1, cleavage of gasdermin D1, was also reduced by SREBP inhibition. Collectively, our findings help to elucidate that SREBPs are crucial host factors required for viral replication and pathogenesis. These results indicate that SREBP is a host target for the development of antiviral strategies.

Adverse outcome pathway for pregnane X receptor-induced hypercholesterolemia

Pharmaceuticals and environmental contaminants contribute to hypercholesterolemia. Several chemicals known to cause hypercholesterolemia, activate pregnane X receptor (PXR). PXR is a nuclear receptor, classically identified as a sensor of chemical environment and regulator of detoxification processes. Later, PXR activation has been shown to disrupt metabolic functions such as lipid metabolism and recent findings have shown PXR activation to promote hypercholesterolemia through multiple mechanisms. Hypercholesterolemia is a major causative risk factor for atherosclerosis and greatly promotes global health burden. Metabolic disruption by PXR activating chemicals leading to hypercholesterolemia represents a novel toxicity pathway of concern and requires further attention. Therefore, we constructed an adverse outcome pathway (AOP) by collecting the available knowledge considering the molecular mechanisms for PXR-mediated hypercholesterolemia. AOPs are tools of modern toxicology for systematizing mechanistic knowledge to assist health risk assessment of chemicals. AOPs are formalized and structured linear concepts describing a link between molecular initiating event (MIE) and adverse outcome (AO). MIE and AO are connected via key events (KE) through key event relationships (KER). We present a plausible route of how PXR activation (MIE) leads to hypercholesterolemia (AO) through direct regulation of cholesterol synthesis and via activation of sterol regulatory element binding protein 2-pathway.

Carbon dioxide regulates cholesterol levels through SREBP2

In mammals, O2 and CO2 levels are tightly regulated and are altered under various pathological conditions. While the molecular mechanisms that participate in O2 sensing are well characterized, little is known regarding the signaling pathways that participate in CO2 signaling and adaptation. Here, we show that CO2 levels control a distinct cellular transcriptional response that differs from mere pH changes. Unexpectedly, we discovered that CO2 regulates the expression of cholesterogenic genes in a SREBP2-dependent manner and modulates cellular cholesterol accumulation. Molecular dissection of the underlying mechanism suggests that CO2 triggers SREBP2 activation through changes in endoplasmic reticulum (ER) membrane cholesterol levels. Collectively, we propose that SREBP2 participates in CO2 signaling and that cellular cholesterol levels can be modulated by CO2 through SREBP2.

METTL3 drives NAFLD-related hepatocellular carcinoma and is a therapeutic target for boosting immunotherapy

Non-alcoholic fatty liver disease (NAFLD) is an emerging risk factor of hepatocellular carcinoma (HCC). However, the mechanism and target therapy of NAFLD-HCC are still unclear. Here, we identify that the N6-methyladenosine (m6A) methyltransferase METTL3 promotes NAFLD-HCC. Hepatocyte-specific Mettl3 knockin exacerbated NAFLD-HCC formation, while Mettl3 knockout exerted the opposite effect in mice. Single-cell RNA sequencing revealed that METTL3 suppressed antitumor immune response by reducing granzyme B (GZMB+) and interferon gamma-positive (IFN-γ+) CD8+ T cell infiltration, thereby facilitating immune escape. Mechanistically, METTL3 mediates sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP) mRNA m6A to promote its translation, leading to the activation of cholesterol biosynthesis. This enhanced secretion of cholesterol and cholesteryl esters that impair CD8+ T cell function in the tumor microenvironment. Targeting METTL3 by single-guide RNA, nanoparticle small interfering RNA (siRNA), or pharmacological inhibitor (STM2457) in combination with anti-programmed cell death protein 1 (PD-1) synergized to reinvigorate cytotoxic CD8+ T cells and mediate tumor regression. Together, METTL3 is a therapeutic target in NAFLD-HCC, especially in conjunction with immune checkpoint blockade (ICB) therapy.

A HIF independent oxygen-sensitive pathway for controlling cholesterol synthesis

Cholesterol biosynthesis is a highly regulated, oxygen-dependent pathway, vital for cell membrane integrity and growth. In fungi, the dependency on oxygen for sterol production has resulted in a shared transcriptional response, resembling prolyl hydroxylation of Hypoxia Inducible Factors (HIFs) in metazoans. Whether an analogous metazoan pathway exists is unknown. Here, we identify Sterol Regulatory Element Binding Protein 2 (SREBP2), the key transcription factor driving sterol production in mammals, as an oxygen-sensitive regulator of cholesterol synthesis. SREBP2 degradation in hypoxia overrides the normal sterol-sensing response, and is HIF independent. We identify MARCHF6, through its NADPH-mediated activation in hypoxia, as the main ubiquitin ligase controlling SREBP2 stability. Hypoxia-mediated degradation of SREBP2 protects cells from statin-induced cell death by forcing cells to rely on exogenous cholesterol uptake, explaining why many solid organ tumours become auxotrophic for cholesterol. Our findings therefore uncover an oxygen-sensitive pathway for governing cholesterol synthesis through regulated SREBP2-dependent protein degradation.

Low abundance of insulin-induced gene 1 contributes to SREBP-1c processing and hepatic steatosis in dairy cows with severe fatty liver

Fatty liver is a major metabolic disorder of high-producing dairy cows during the transition period. In nonruminants, it is well established that insulin-induced gene 1 (INSIG1) plays a crucial role in regulating hepatic lipogenesis by controlling the anchoring of sterol regulatory element-binding protein 1 (SREBP-1) on the endoplasmic reticulum along with SREBP cleavage-activating protein (SCAP). Whether the INSIG1-SCAP-SREBP-1c transport axis is affected in cows experiencing fatty liver is unknown. Thus, the aim of this study was to investigate the potential role of INSIG1-SCAP-SREBP-1c axis in the progression of fatty liver in dairy cows. For in vivo experiments, 24 dairy cows at the start of their fourth lactation (median; range 3-5) and 8 d in milk (median; range 4-12 d) were selected into a healthy group [n = 12; triglyceride (TG) content <1%] and a severe fatty liver group (n = 12; TG content >10%) according to their hepatic TG content. Blood samples were collected for detecting serum concentrations of free fatty acids, β-hydroxybutyrate, and glucose. Compared with healthy cows, cows with severe fatty liver had higher serum concentrations of β-hydroxybutyrate and free fatty acids and lower concentration of glucose. Liver biopsies were used to detect the status of INSIG1-SCAP-SREBP-1c axis, and the mRNA expression of SREBP-1c-target lipogenic genes acetyl-CoA carboxylase α (ACACA), fatty acid synthase (FASN), and diacylglycerol acyltransferase 1 (DGAT1). Cows with severe fatty liver had lower protein expression of INSIG1 in the hepatocyte endoplasmic reticulum fraction, greater protein expression of SCAP and precursor SREBP-1c in the hepatocyte Golgi fraction, and greater protein expression of mature SREBP-1c in the hepatocyte nuclear fraction. In addition, the mRNA expression of SREBP-1c-target lipogenic genes ACACA, FASN, and DGAT1 was greater in the liver of dairy cows with severe fatty liver. In vitro experiments were conducted on hepatocytes isolated from 5 healthy 1-d-old female Holstein calves, and hepatocytes from each calf were run independently. First, hepatocytes were treated with 0, 200, or 400 μM palmitic acid (PA) for 12 h. Exogenous PA treatment decreased INSIG1 protein abundance, enhanced the endoplasmic reticulum to Golgi export of SCAP-precursor SREBP-1c complex and the nuclear translocation of mature SREBP-1c, all of which was associated with increased transcriptional activation of lipogenic genes and TG synthesis. Second, hepatocytes were transfected with INSIG1-overexpressing adenovirus for 48 h and treated with 400 μM PA 12 h before the end of transfection. Overexpressing INSIG1 inhibited PA-induced SREBP-1c processing, upregulation of lipogenic genes, and TG synthesis in hepatocytes. Overall, the present in vivo and in vitro results indicated that the low abundance of INSIG1 contributed to SREBP-1c processing and hepatic steatosis in dairy cows. Thus, the INSIG1-SCAP-SREBP-1c axis may be a novel target for treatment of fatty liver in dairy cows.

Decoding the crosstalk between mevalonate metabolism and T cell function

The mevalonate pathway is an essential metabolic pathway in T cells regulating development, proliferation, survival, differentiation, and effector functions. The mevalonate pathway is a complex, branched pathway composed of many enzymes that ultimately generate cholesterol and nonsterol isoprenoids. T cells must tightly control metabolic flux through the branches of the mevalonate pathway to ensure sufficient isoprenoids and cholesterol are available to meet cellular demands. Unbalanced metabolite flux through the sterol or the nonsterol isoprenoid branch is metabolically inefficient and can have deleterious consequences for T cell fate and function. Accordingly, there is tight regulatory control over metabolic flux through the branches of this essential lipid synthetic pathway. In this review we provide an overview of how the branches of the mevalonate pathway are regulated in T cells and discuss our current understanding of the relationship between mevalonate metabolism, cholesterol homeostasis and T cell function.

Progesterone signaling in the regulation of luteal steroidogenesis

The corpus luteum is the major source of progesterone, the essential hormone for female reproductive function. While progesterone activity has been the subject of extensive research for decades, characterization of non-canonical progesterone receptor/signaling pathways provided a new perspective for understanding the complex signal transduction mechanisms exploited by the progesterone hormone. Deciphering these mechanisms has significant implications in the management of luteal phase disorders and early pregnancy complications. The purpose of this review is to highlight the complex mechanisms through which progesterone-induced signaling mediates luteal granulosa cell activity in the corpus luteum. Here, we review the literature and discuss the up-to-date evidence on how paracrine and autocrine effects of progesterone regulate luteal steroidogenic activity. We also review the limitations of the published data and highlight future research priorities.

Apolipoprotein E ε4 triggers neurotoxicity via cholesterol accumulation, acetylcholine dyshomeostasis, and PKCε mislocalization in cholinergic neuronal cells

The Apolipoprotein E (ApoE) has been known to regulate cholesterol and β-amyloid (Aβ) production, redistribution, and elimination, in the central nervous system (CNS). The ApoE ε4 polymorphic variant leads to impaired brain cholesterol homeostasis and amyloidogenic pathway, thus representing the major risk factor for Alzheimer's Disease (AD). Currently, less is known about the molecular mechanisms connecting ApoE ε4-related cholesterol metabolism and cholinergic system degeneration, one of the main AD pathological features. Herein, in vitro cholinergic neuron models were developed in order to study ApoE neuronal expression and investigate the possible interplay between cholesterol metabolism and cholinergic pathway impairment prompted by ε4 isoform. Particularly, alterations specifically occurring in ApoE ε4-carrying neurons (i.e. increased intracellular ApoE, amyloid precursor protein (APP), and Aβ levels, elevated apoptosis, and reduced cell survival) were recapitulated. ApoE ε4 expression was found to increase intracellular cholesterol accumulation, by regulating the related gene expression, while reducing cholesterol precursor acetyl-CoA, which in turn fuels the acetylcholine (ACh) synthesis route. In parallel, although the ACh intracellular signalling was activated, as demonstrated by the boosted extracellular ACh as well as increased IP₃ and Ca²⁺, the PKCε activation via membrane translocation was surprisingly suppressed, probably explained by the cholesterol overload in ApoE ε4 neuron-like cells. Consequently, the PKC-dependent anti-apoptotic and neuroprotective roles results impaired, reliably adding to other causes of cell death prompted by ApoE ε4. Overall, the obtained data open the way to further critical considerations of ApoE ε4-dependent cholesterol metabolism dysregulation in the alteration of cholinergic pathway, neurotoxicity, and neuronal death.

Cholesterol metabolism in cancer and cell death

SIGNIFICANCE

Cholesterol is a type of lipid that plays a crucial role in building and maintaining cell membranes, producing certain hormones, and aiding in digestion. The two main types of cholesterol are low-density lipoprotein and high-density lipoprotein, and maintaining a healthy balance between them is essential for cellular function and organism health.

RECENT ADVANCES

Cholesterol metabolism is a complex and dynamic process that involves biosynthesis, uptake, efflux, transport, and esterification. Disruptions in cholesterol metabolism are implicated in all stages of cancer, contributing to drug resistance, immune evasion, and autophagy dysfunction. These disruptions have also been linked to various types of regulated cell death, such as apoptosis, anoikis, lysosome-dependent cell death, pyroptosis, NETosis, necroptosis, entosis, ferroptosis, alkaliptosis, immunogenic cell death, and paraptosis.

CRITICAL ISSUES

Understanding the complex interplay between cholesterol metabolism and cell death and their impact on cancer development and progression is still a significant challenge. Additionally, there is currently a lack of reliable biomarkers that can accurately reflect cholesterol metabolism dysregulation in cancer.

FUTURE DIRECTIONS

To develop more specific and effective cholesterol metabolism-targeted therapies, a better understanding of the mechanisms by which cholesterol metabolism dysregulation contributes to cell death and cancer progression is needed. Additionally, improving the accuracy and reliability of biomarkers will be crucial for monitoring and diagnosing cholesterol-related cancer subtypes and evaluating the effectiveness of cholesterol metabolism-targeted therapies. These efforts will require ongoing research and collaboration among multidisciplinary teams of scientists and clinicians.

In vivo PAR-CLIP (viP-CLIP) of liver TIAL1 unveils targets regulating cholesterol synthesis and secretion

System-wide cross-linking and immunoprecipitation (CLIP) approaches have unveiled regulatory mechanisms of RNA-binding proteins (RBPs) mainly in cultured cells due to limitations in the cross-linking efficiency of tissues. Here, we describe viP-CLIP (in vivo PAR-CLIP), a method capable of identifying RBP targets in mammalian tissues, thereby facilitating the functional analysis of RBP-regulatory networks in vivo. We applied viP-CLIP to mouse livers and identified Insig2 and ApoB as prominent TIAL1 target transcripts, indicating an important role of TIAL1 in cholesterol synthesis and secretion. The functional relevance of these targets was confirmed by showing that TIAL1 influences their translation in hepatocytes. Mutant Tial1 mice exhibit altered cholesterol synthesis, APOB secretion and plasma cholesterol levels. Our results demonstrate that viP-CLIP can identify physiologically relevant RBP targets by finding a factor implicated in the negative feedback regulation of cholesterol biosynthesis.