Hypoxia-inducible lipid droplet-associated induces DGAT1 and promotes lipid storage in hepatocytes

Raman analysis of lipids in cells: Current applications and future prospects

Lipids play an important role in the regulation of cell life processes. Although there are various lipid detection methods, Raman spectroscopy, a non-invasive technique, provides the detailed chemical composition of lipid profiles without a complex sample preparation procedure and possesses greater potential in basic biology, clinical diagnosis and disease therapy. In this review, we summarized the characteristics and advantages of Raman-based techniques and their primary contribution to illustrating cellular lipid metabolism.

Gut Microbiota Regulate Lipid Metabolism via the Bile Acid Pathway: Resistance to Hypoxia in Gansu Zokor (Eospalax cansus)

The Gansu zokor (Eospalax cansus), a subterranean rodent endemic to the Loess Plateau of China, exhibits remarkable adaptability to hypoxic environments. While gut microbiota are known to regulate lipid metabolism through bile acid (BA) pathways, this phenomenon has not been investigated in subterranean rodents exposed to hypoxia. This study employed 16SrRNA sequencing, targeted analysis of BA metabolites in colonic contents, and assessments of BA and lipid metabolites alongside molecular analyses in the liver and ileum under conditions of acute and chronic hypoxia in Gansu zokors. The results revealed that hypoxia altered the composition of gut microbiota and BA pools in Gansu zokors. Hypoxia-induced changes increased the abundance of gut microbiota associated with BA metabolism, thereby modulating lipid metabolism via farnesoid X receptor (FXR) signaling in the distal ileum and liver cells. Under acute hypoxia, FXR upregulated lipid synthesis and suppressed fatty acid β-oxidation by downregulating the carnitine palmitoyl-transferase1A (CPT1A) expression. Conversely, during chronic hypoxia, particularly under long-term exposure, FXR reduced lipid synthesis and enhanced fatty acid β-oxidation by upregulating acyl-CoA oxidase (ACOX1) expression. In both hypoxic conditions, FXR facilitated lipoprotein metabolism. In summary, this study elucidates that gut microbiota-mediated BA metabolic pathways contribute to the Gansu zokor's ability to maintain lipid metabolic homeostasis and adaptation to hypoxia.

Integrating larval zebrafish model and network pharmacology for screening and identification of edible herbs with therapeutic potential for MAFLD: A promising drug Smilax glabra Roxb

Metabolic-associated fatty liver disease (MAFLD) is becoming a prevalent chronic liver disease. Many medicinal and edible herbs exhibit remarkable biological activities in ameliorating MAFLD but lack a comprehensive assessment of their therapeutic efficacy. This study determined total phenolic and flavonoid contents and in vitro antioxidant properties of 34 edible herbs. Smilax glabra Roxb. (SGR), Coreopsis tinctoria Nutt., and Smilax china L. were obtained with the best bioactivity and antioxidant capacity. The high-cholesterol diet-induced larval zebrafish model was established to compare the anti-MAFLD activity of the three herb extracts mentioned above. In vivo experiments revealed that SGR intervention significantly decreased lipid accumulation, alleviated oxidative stress, and modulated intestinal microbiota composition in zebrafish. Furthermore, three potential active components in SGR and their possible mechanisms were explored through network pharmacology and molecular docking. Our study suggested that SGR is a potential candidate for developing new drugs or dietary supplements for MAFLD therapy.

HIG-2 promotes glioma stemness and radioresistance mediated by IGFBP2-rich microparticles in hypoxia

Hypoxia can weaken the efficacy of radiotherapy and decrease tumor immunogenicity leading to immune escape. Thus, a thorough understanding of the key signaling pathways regulated by hypoxia is vitally important to enhance the radiosensitivity and improve immunosuppressive microenvironment of glioma. In this study, we verified the crucial role of hypoxia-inducible gene 2 (HIG-2) in lipid droplet (LD) accumulation and demonstrated that HIG-2 binding to frizzled class receptor 10 (FZD10) activated Wnt/β-catenin signaling pathway and increased its downstream insulin-like growth factor binding protein 2 (IGFBP2) level in microparticles (MPs) derived from glioma stem cells (GSCs), leading to decreased radiosensitivity and immunogenicity of MPs-receiving cells via the cross-talk between GSCs and non-stem glioma cells (GCs). These findings suggest that HIG-2 may be a promising target in glioma radiotherapy and/or immunotherapy.

Progress in understanding of relationship of chronic hypoxia and hypoxia-inducing factors with liver cancer

At present, progress has been made in the understanding of the correlation between hypoxia and liver cancer. In recent years, the combination of hypoxia-inducible factor inhibitors and anticancer drugs in the treatment of liver cancer has achieved gratifying effects, reducing the progression and metastasis of liver cancer, and extending the survival period of patients. Liver damage can destroy the liver's vascular system, disrupting normal blood flow and oxygen supply, and creating an anoxic microenvironment. During hypoxia, liver cells deposit collagen, leading to fibrosis and cirrhosis, which further aggravate hypoxia. Studies have shown that hypoxia, mitochondrial abnormalities, oxidative stress, and liver inflammation are closely related to liver cancer. This article reviews the progress in the understanding of relationship of chronic hypoxia and hypoxia-inducing factors with liver cancer.

FRET-based analysis on the fate of liposome and cyclodextrin@liposome nanocomposites from ocular surface to the posterior segment of the eye

Investigating the structural integrity of nanocarriers in vivo is vital for exploring the fate of nanocarriers from ocular surface to the posterior segment of the eye. Most of the published studies adopted the structural integrity ratio of nanocarriers to determine the fate of them, which lacked scientific support. In this study, two methods were used to explore the factors which affected the structural integrity of liposomes. A new method with the standard curve of FRET fluorescence intensity and carbocyanine 7 (Cy7) content was drawn for the first time. Secondly, we used the traditional method of drawing the standard curve of FRET fluorescence efficiency and structural integrity ratios. The results showed that liposomes with particle size about 120 nm, positively charged, polyethyleneglycol5000 (PEG5000) and glycine sarcosine (GS) modified had the highest Cy7 content in rabbit tissues. When the dosage of Cy7 was 25 μg, at 60 min, the content of Cy7 in intact liposomes and the structural integrity ratio of liposomes in sclera was 210.5 ± 14.9 ng and 19.8 ± 5.3 %, respectively. Compared with the structural integrity ratio, the Cy7 content in the intact carrier could better estimate the fate of nanocarriers in vivo scientifically. On this basis, the fate of dual-carrier nanocomposites and the inner cyclodextrin complexes in vivo was investigated. The intact cyclodextrin complexes could reach choroid-retina with the protection of outer liposomes. The structural integrity ratios of liposomes were also studied after crossing human conjunctival epithelial cells (HConEpiC) monolayer, but in vitro cellular experiments could not simulate the real situation in vivo. Finally, the tissue distribution of nanocomposites was studied in rabbit eyes. The concentration of dexamethasone (Dex) in choroid-retina was 158 ± 23 ng/g after 3 h, which exhibited better drug delivery ability compared with our previous study. Overall, the present study provides a new scientific method to estimate the structural integrity in vivo, which is beneficial for the rational design of drug delivery systems with more structural integrity in vivo and higher drug delivery efficiency.

Cholesterol Metabolism in Neurodegenerative Diseases

Significance: Cholesterol plays a crucial role in the brain, where it is highly concentrated and tightly regulated to support normal brain functions. It serves as a vital component of cell membranes, ensuring their integrity, and acts as a key regulator of various brain processes. Dysregulation of cholesterol metabolism in the brain has been linked to impaired brain function and the onset of neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease, and Huntington's disease. Recent Advances: A significant advancement has been the identification of astrocyte-derived apoliprotein E as a key regulator of de novo cholesterol biosynthesis in neurons, providing insights into how extracellular signals influence neuronal cholesterol levels. In addition, the development of antibody-based therapies, particularly for AD, presents promising opportunities for therapeutic interventions. Critical Issues: Despite significant research, the association between cholesterol and neurodegenerative diseases remains inconclusive. It is crucial to distinguish between plasma cholesterol and brain cholesterol, as these pools are relatively independent. This differentiation should be considered when evaluating statin-based treatment approaches. Furthermore, assessing not only the total cholesterol content in the brain but also its distribution among different types of brain cells is essential. Future Direction: Establishing a causal link between changes in brain/plasma cholesterol levels and the onset of brain dysfunction/neurodegenerative diseases remains a key objective. In addition, conducting cell-specific analyses of cholesterol homeostasis in various types of brain cells under pathological conditions will enhance our understanding of cholesterol metabolism in neurodegenerative diseases. Manipulating cholesterol levels to restore homeostasis may represent a novel approach for alleviating neurological symptoms. Antioxid. Redox Signal. 41, 1051-1072.

A global view on quantitative proteomic and metabolic analysis of rat livers under different hypoxia protocols

Hypobaric hypoxia causes altitude sickness and significantly affects human health. As of now, focusing on rats different proteomic and metabolic changes exposed to different hypoxic times at extreme altitude is blank. Our study integrated in vivo experiments with tandem mass tag (TMT)- and gas chromatography time-of-flight (GC-TOF)-based proteomic and metabolomic assessments, respectively. Male Sprague-Dawley rats were exposed to long-term constant hypoxia for 40 days or short-term constant hypoxia for three days, and their responses were compared with those of a normal control group. Post-hypoxia, serum marker assays related to lipid metabolism revealed significant increases in the levels of low-density lipoprotein (LDL), triglycerides (TG), and total cholesterol (TC) in the liver. In contrast, high-density lipoprotein (HDL) levels were upregulated in the long-term constant hypoxia cohorts and were significantly reduced in the short-term constant hypoxia cohorts. Furthermore, metabolic pathway analysis indicated that glycerolipid and glycerophospholipid metabolisms were the most significantly affected pathways in long-term hypoxia group. Subsequently, RT-qPCR analyses were performed to corroborate the key regulatory elements, including macrophage galactose-type lectin (MGL) and Fatty Acid Desaturase 2 (FADS2). The results of this study provide new information for understanding the effects of different hypobaric hypoxia exposure protocols on protein expression and metabolism in low-altitude animals.

Astaxanthin Alleviates Hepatic Lipid Metabolic Dysregulation Induced by Microcystin-LR

Microcystin-LR (MC-LR), frequently generated by cyanobacteria, has been demonstrated to raise the likelihood of liver disease. Few previous studies have explored the potential antagonist against MC-LR. Astaxanthin (ASX) has been shown to possess various beneficial effects in regulating lipid metabolism in the liver. However, whether ASX could alleviate MC-LR-induced hepatic lipid metabolic dysregulation is as yet unclear. In this work, the important roles and mechanisms of ASX in countering MC-LR-induced liver damage and lipid metabolic dysregulation were explored for the first time. The findings revealed that ASX not only prevented weight loss but also enhanced liver health after MC-LR exposure. Moreover, ASX effectively decreased triglyceride, total cholesterol, aspartate transaminase, and alanine aminotransferase contents in mice that were elevated by MC-LR. Histological observation showed that ASX significantly alleviated lipid accumulation and inflammation induced by MC-LR. Mechanically, ASX could significantly diminish the expression of genes responsible for lipid generation (Srebp-1c, Fasn, Cd36, Scd1, Dgat1, and Pparg), which probably reduced lipid accumulation induced by MC-LR. Analogously, MC-LR increased intracellular lipid deposition in THLE-3 cells, while ASX decreased these symptoms by down-regulating the expression of key genes in the lipid synthesis pathway. Our results implied that ASX played a crucial part in lipid synthesis and effectively alleviated MC-LR-induced lipid metabolism dysregulation. ASX might be developed as a novel protectant against hepatic impairment and lipid metabolic dysregulation associated with MC-LR. This study offers new insights for further management of MC-LR-related metabolic diseases.

Push-Pull Fluorescent Dyes with Trifluoroacetyl Acceptor for High-Fidelity Sensing of Polarity and Heterogeneity of Lipid Droplets

Imaging and sensing of lipid droplets (LDs) attracted significant attention due to growing evidence for their important role in cell life. Solvatochromic dyes are promising tools to probe LDs' local polarity, but this analysis is biased by their non-negligible emission from intracellular membranes and capacity to emit from both the apolar core and polar interface of LDs. Here, we developed two push-pull solvatochromic dyes based on naphthalene and fluorene cores bearing an exceptionally strong electron acceptor, the trifluoroacetyl group. The latter was found to boost the optical properties of the dyes by shifting their absorption and emission to red and increasing their extinction coefficient, photostability, and sensitivity to solvent polarity (solvatochromism). In contrast to classical solvatochromic dyes, such as parent aldehydes and reference Nile Red, the new dyes exhibited strong fluorescence quenching by millimolar water concentrations in organic solvents. In live cells, the trifluoroacetyl dyes exhibited high specificity to LDs, whereas the parent aldehydes and Nile Red showed a detectable backgrounds from intracellular membranes. Experiments in model lipid membranes and nanoemulsion droplets confirmed the high selectivity of new probes to LDs in contrast to classical solvatochromic dyes. Moreover, the new probes were found to be selective to the LDs oil core, where they can sense lipid unsaturation and chain length. Their ratiometric imaging in cells revealed strong heterogeneity in polarity within LDs, which covered the range of polarities of unsaturated triglyceride oils, whereas Nile Red failed to properly estimate the local polarity of LDs. Finally, the probes revealed that LDs core polarity can be altered by fatty acid diets, which correlates with their chain length and unsaturation.

Omega-3 fatty acids prevent gestational diabetes mellitus via modulation of lipid metabolism

The incidence rate of gestational diabetes mellitus (GDM) remains high among pregnant women in the second trimester of pregnancy. However, the main clinical approach to alleviate the symptoms of GDM is to control the diet. Our study explored the therapeutic effects of omega-3 fatty acids (ω-3 FAs) on GDM at the cellular and animal levels. We found that ω-3 FAs can promote the transformation of M0 macrophages into anti-inflammatory M2 macrophages. The transformed M2 macrophages promoted β-oxidation and reduced hepatocyte lipid synthesis (P < 0.05), thereby promoting hepatic function and preventing the excessive accumulation of lipid droplets in the hepatocyte cell line HepG2. Supplementation of ω-3 FAs in pregnant GDM mice significantly reduced fasting blood glucose levels, glucose tolerance test, and insulin tolerance test indices, and lipid accumulation in the liver and effectively prevented the occurrence of liver fibrosis (P < 0.05). These therapeutic effects may be mediated through the anti-inflammatory effects of ω-3 FAs (P < 0.05). ω-3 FAs also had positive effects on the offspring of pregnant GDM mice, as demonstrated by reduced birth mortality and improved glycemic stabilization (P < 0.05). In conclusion, this study provides a possible translational medicine strategy for the treatment of GDM.

Developmental neurotoxicity of PFOA exposure on hiPSC-derived cortical neurons

PFOA is a legacy Per- and Polyfluorinated Substances (PFAS), a group of chemicals widely used in various industrial applications and consumer products. Although there has been a voluntary phase out of PFOA since 2005, it is still widely detected in various water supplies. A growing body of evidence suggests an association between PFOA exposure, particularly during developmental stages, with increased risks of neurodegenerative diseases (NDs). The neurotoxic mechanism of developmental PFOA exposure, however, remains poorly understood. Utilizing human induced-pluripotent stem cell (hiPSC)-derived cortical neurons, we investigated the effect of PFOA exposure prior to differentiation and assessed changes in neuronal characteristics, transcriptome, and neurodegeneration markers mimicking a Developmental Origin of Health and Disease (DoHAD) paradigm. Exposure to PFOA before neuron differentiation resulted in persistent alterations in nuclear morphology, neuronal network, and calcium activity. RNA sequencing analysis further revealed transcriptomic changes aligning with Alzheimer's Disease (AD) after PFOA exposure. These observations were further corroborated by alterations in tau phosphorylation markers, the presence of fibrillar tau, an increase in liquid droplets, and a decrease in RNA translational efficiency characterized using a battery of biochemical assays. Taken together, our results revealed persistent deficits of key neuronal characteristics induced by pre-differentiation PFOA exposure, suggesting impairments in several AD-related pathways that can together contribute to the elevation of AD risk after pre-differentiation PFOA exposure.

Protective Effects of Velvet Antler Methanol Extracts on Hypoxia-Induced Damage in Caenorhabditis elegans through HIF-1 and ECH-8 Mediated Lipid Accumulation

Velvet antler, a traditional tonic widely used in East Asia for its health benefits, is explored in this study for its protective effects against hypoxia-induced damage using Caenorhabditis elegans (C. elegans) as a model. Hypoxia, characterized by low oxygen availability, induces significant physiological stress and potential tissue damage. Our research demonstrates that methanol extracts from velvet antler (MEs) enhance the survival of C. elegans under hypoxic conditions. This enhancement is achieved through the stabilization of hypoxia-inducible factor-1 (HIF-1) and the promotion of lipid accumulation, both of which are crucial for mitigating cellular damage. Specifically, MEs improve mitochondrial function, increase ATP production, and aid in the recovery of physical activity in C. elegans post-hypoxia or following hypoxia-reoxygenation (HR). The pivotal role of HIF-1 is underscored by the loss of these protective effects when HIF-1 function is inhibited. Additionally, our findings reveal that the gene related to lipid metabolism, ech-8, significantly contributes to the lipid accumulation that enhances resilience to hypoxia in C. elegans treated with MEs. These results not only highlight the therapeutic potential of velvet antler in modern medical applications, particularly for conditions involving hypoxic stress, but also provide insights into the molecular mechanisms by which MEs confer protection against hypoxic damage.

Effects of dietary metabolizable energy level on hepatic lipid metabolism and cecal microbiota in aged laying hens

Lipid metabolic capacity, feed utilization, and the diversity of gut microbiota are reduced in the late laying stage for laying hens. This experiment aimed to investigate the effects of different levels of dietary metabolizable energy (ME) on hepatic lipid metabolism and cecal microbiota in late laying hens. The 216 Peking Pink laying hens (57-wk-old) were randomly assigned to experimental diets of 11.56 (HM = high ME), 11.14 (MM = medium ME), or 10.72 (LM = low ME) MJ of ME/kg, with each dietary treatment containing 6 replicates per group and 12 chickens per replicate. The HM group showed higher triglyceride (TG), total cholesterol (T-CHO), and low-density lipoprotein cholesterol (LDL-C) concentrations in the liver compared with the LM group; second, the HM group showed higher TG concentration and the LM group showed lower T-CHO concentration compared with MM group; finally, the HM group showed a lower hepatic lipase (HL) activity compared with the MM and LM groups (P < 0.05). There was a significant difference in the microbial community structure of the cecum between the HM and MM groups (P < 0.05). The decrease of dietary ME level resulted in a gradual decrease relative abundance of Proteobacteria. At the genus level, beneficial bacteria were significantly enriched in the LM group compared to the MM group, including Faecalibacterium, Lactobacillus, and Bifidobacterium, (linear discriminant analysis [LDA] >2, P <0.05). In addition, at the species level, Lactobacillus crispatus, Parabacteroides gordonii, Blautia caecimuris, and Lactobacillus johnsonii were significantly enriched in the LM group (LDA>2, P < 0.05). The HM group had a higher abundance of Sutterella spp. compared to the LM group (LDA>2, P <0.05). In conclusion, this research suggests that the reduction in dietary energy level did not adversely affect glycolipid metabolism or low dietary ME (10.72 MJ/kg). The findings can be helpful for maintaining intestinal homeostasis and increasing benefit for gut microbiota in late laying hens.

In Vitro Nonalcoholic Fatty Liver Disease Model Elucidating the Effect of Immune Environment on Disease Progression and Alleviation

Nonalcoholic fatty liver disease (NAFLD), which is a major cause of chronic liver disease, is characterized by fat accumulation in the liver. Existing models struggle to assess medication effects on liver function in the context of NAFLD's unique inflammatory environment. We address this by developing a 3D in vitro NAFLD model using HepG2 and THP-1 cells (mimicking liver and Kupffer cells) cocultured using transwell and hydrogel system. This mimics liver architecture and allows for manipulation of the immune environment. We demonstrate that the model recapitulates key NAFLD features: steatosis (induced by fatty acids), oxidative stress, inflammation, and impaired liver function embodying the interrelationship between NAFLD and the surrounding immune environment. This versatile model offers a valuable tool for preclinical NAFLD research by incorporating a disease-relevant immune environment.

The integrate profiling of single-cell and spatial transcriptome RNA-seq reveals tumor heterogeneity, therapeutic targets, and prognostic subtypes in ccRCC

Clear-cell renal cell carcinoma (ccRCC) is the most common type of RCC; however, the intratumoral heterogeneity in ccRCC remains unclear. We first identified markers and biological features of each cell cluster using bioinformatics analysis based on single-cell and spatial transcriptome RNA-sequencing data. We found that gene copy number loss on chromosome 3p and amplification on chromosome 5q were common features in ccRCC cells. Meanwhile, NNMT and HILPDA, which are associated with the response to hypoxia and metabolism, are potential therapeutic targets for ccRCC. In addition, CD8+ exhausted T cells (LAG3+ HAVCR2+), CD8+ proliferated T cells (STMN+), and M2-like macrophages (CD68+ CD163+ APOC1+), which are closely associated with immunosuppression, played vital roles in ccRCC occurrence and development. These results were further verified by whole exome sequencing, cell line and xenograft experiments, and immunofluorescence staining. Finally, we divide patients with ccRCC into three subtypes using unsupervised cluster analysis. and generated a classifier to reproduce these subtypes using the eXtreme Gradient Boosting algorithm. Our classifier can help clinicians evaluate prognosis and design personalized treatment strategies for ccRCC. In summary, our work provides a new perspective for understanding tumor heterogeneity and will aid in the design of antitumor therapeutic strategies for ccRCC.

Exploring the impact of lipid droplets on the evolution and progress of hepatocarcinoma

Over the past 10 years, the biological role of lipid droplets (LDs) has gained significant attention in the context of both physiological and pathological conditions. Considerable progress has been made in elucidating key aspects of these organelles, yet much remains to be accomplished to fully comprehend the myriad functions they serve in the progression of hepatic tumors. Our current perception is that LDs are complex and active structures managed by a distinct set of cellular processes. This understanding represents a significant paradigm shift from earlier perspectives. In this review, we aim to recapitulate the function of LDs within the liver, highlighting their pivotal role in the pathogenesis of metabolic dysfunction-associated steatotic liver disease (MASLD) (Hsu and Loomba, 2024) and their contribution to the progression towards more advanced pathological stages up to hepatocellular carcinoma (HC) (Farese and Walther, 2009). We are aware of the molecular complexity and changes occurring in the neoplastic evolution of the liver. Our attempt, however, is to summarize the most important and recent roles of LDs across both healthy and all pathological liver states, up to hepatocarcinoma. For more detailed insights, we direct readers to some of the many excellent reviews already available in the literature (Gluchowski et al., 2017; Hu et al., 2020; Seebacher et al., 2020; Paul et al., 2022).

Protein aggregation and biomolecular condensation in hypoxic environments (Review)

Due to molecular forces, biomacromolecules assemble into liquid condensates or solid aggregates, and their corresponding formation and dissolution processes are controlled. Protein homeostasis is disrupted by increasing age or environmental stress, leading to irreversible protein aggregation. Hypoxic pressure is an important factor in this process, and uncontrolled protein aggregation has been widely observed in hypoxia‑related conditions such as neurodegenerative disease, cardiovascular disease, hypoxic brain injury and cancer. Biomolecular condensates are also high‑order complexes assembled from macromolecules. Although they exist in different phase from protein aggregates, they are in dynamic balance under certain conditions, and their activation or assembly are considered as important regulatory processes in cell survival with hypoxic pressure. Therefore, a better understanding of the relationship between hypoxic stress, protein aggregation and biomolecular condensation will bring marked benefits in the clinical treatment of various diseases. The aim of the present review was to summarize the underlying mechanisms of aggregate assembly and dissolution induced by hypoxic conditions, and address recent breakthroughs in understanding the role of aggregates in hypoxic‑related diseases, given the hypotheses that hypoxia induces macromolecular assemblage changes from a liquid to a solid phase, and that adenosine triphosphate depletion and ATP‑driven inactivation of multiple protein chaperones play important roles among the process. Moreover, it is anticipated that an improved understanding of the adaptation in hypoxic environments could extend the overall survival of patients and provide new strategies for hypoxic‑related diseases.

The role of DGAT1 and DGAT2 in regulating tumor cell growth and their potential clinical implications

Lipid metabolism is widely reprogrammed in tumor cells. Lipid droplet is a common organelle existing in most mammal cells, and its complex and dynamic functions in maintaining redox and metabolic balance, regulating endoplasmic reticulum stress, modulating chemoresistance, and providing essential biomolecules and ATP have been well established in tumor cells. The balance between lipid droplet formation and catabolism is critical to maintaining energy metabolism in tumor cells, while the process of energy metabolism affects various functions essential for tumor growth. The imbalance of synthesis and catabolism of fatty acids in tumor cells leads to the alteration of lipid droplet content in tumor cells. Diacylglycerol acyltransferase 1 and diacylglycerol acyltransferase 2, the enzymes that catalyze the final step of triglyceride synthesis, participate in the formation of lipid droplets in tumor cells and in the regulation of cell proliferation, migration and invasion, chemoresistance, and prognosis in tumor. Several diacylglycerol acyltransferase 1 and diacylglycerol acyltransferase 2 inhibitors have been developed over the past decade and have shown anti-tumor effects in preclinical tumor models and improvement of metabolism in clinical trials. In this review, we highlight key features of fatty acid metabolism and different paradigms of diacylglycerol acyltransferase 1 and diacylglycerol acyltransferase 2 activities on cell proliferation, migration, chemoresistance, and prognosis in tumor, with the hope that these scientific findings will have potential clinical implications.

Elongation factor 1A1 regulates metabolic substrate preference in mammalian cells

Eukaryotic elongation factor 1A1 (EEF1A1) is canonically involved in protein synthesis but also has noncanonical functions in diverse cellular processes. Previously, we identified EEF1A1 as a mediator of lipotoxicity and demonstrated that chemical inhibition of EEF1A1 activity reduced mouse liver lipid accumulation. These findings suggested a link between EEF1A1 and metabolism. Therefore, we investigated its role in regulating metabolic substrate preference. EEF1A1-deficient Chinese hamster ovary (2E2) cells displayed reduced media lactate accumulation. These effects were also observed with EEF1A1 knockdown in human hepatocyte-like HepG2 cells and in WT Chinese hamster ovary and HepG2 cells treated with selective EEF1A inhibitors, didemnin B, or plitidepsin. Extracellular flux analyses revealed decreased glycolytic ATP production and increased mitochondrial-to-glycolytic ATP production ratio in 2E2 cells, suggesting a more oxidative metabolic phenotype. Correspondingly, fatty acid oxidation was increased in 2E2 cells. Both 2E2 cells and HepG2 cells treated with didemnin B exhibited increased neutral lipid content, which may be required to support elevated oxidative metabolism. RNA-seq revealed a >90-fold downregulation of a rate-limiting glycolytic enzyme, hexokinase 2, which we confirmed through immunoblotting and enzyme activity assays. Pathway enrichment analysis identified downregulations in TNFA signaling via NFKB and MYC targets. Correspondingly, nuclear abundances of RELB and MYC were reduced in 2E2 cells. Thus, EEF1A1 deficiency may perturb glycolysis by limiting NFKB- and MYC-mediated gene expression, leading to decreased hexokinase expression and activity. This is the first evidence of a role for a translation elongation factor, EEF1A1, in regulating metabolic substrate utilization in mammalian cells.

Lipid droplets, autophagy, and ageing: A cell-specific tale

Lipid droplets are the essential organelle for storing lipids in a cell. Within the variety of the human body, different cells store, utilize and release lipids in different ways, depending on their intrinsic function. However, these differences are not well characterized and, especially in the context of ageing, represent a key factor for cardiometabolic diseases. Whole body lipid homeostasis is a central interest in the field of cardiometabolic diseases. In this review we characterize lipid droplets and their utilization via autophagy and describe their diverse fate in three cells types central in cardiometabolic dysfunctions: adipocytes, hepatocytes, and macrophages.

A novel clinical prediction scoring system of high-altitude pulmonary hypertension

Background

High-altitude pulmonary hypertension (HAPH) is a common disease in regions of high altitude where performing right heart catheterization (RHC) is challenging. The development of a diagnostic scoring system is crucial for effective disease screening.

Methods

A total of 148 individuals were included in a retrospective analysis, and an additional 42 residents were prospectively enrolled. We conducted a multivariable analysis to identify independent predictors of HAPH. Subsequently, we devised a prediction score based on the retrospective training set to anticipate the occurrence and severity of HAPH. This scoring system was further subjected to validation in the prospective cohort, in which all participants underwent RHC.

Results

This scoring system, referred to as the GENTH score model (Glycated hemoglobin [OR = 4.5], Echocardiography sign [OR = 9.1], New York Heart Association-functional class [OR = 12.5], Total bilirubin [OR = 3.3], and Hematocrit [OR = 3.6]), incorporated five independent risk factors and demonstrated strong predictive accuracy. In the training set, the area under the curve (AUC) values for predicting the occurrence and severity of HAPH were 0.851 and 0.832, respectively, while in the validation set, they were 0.841 and 0.893. In the validation set, GENTH score model cutoff values of ≤18 or >18 points were established for excluding or confirming HAPH, and a threshold of >30 points indicated severe HAPH.

Conclusions

The GENTH score model, combining laboratory and echocardiography indicators, represents an effective tool for distinguishing potential HAPH patients and identifying those with severe HAPH. This scoring system improves the clinical screening of HAPH diseases and offers valuable insights into disease diagnosis and management.

The Potential of Lipid Droplet-associated Genes as Diagnostic and Prognostic Biomarkers in Head and Neck Squamous Cell Carcinoma

OBJECTIVE

The role of lipid droplets (LDs) and lipid droplet-associated genes (LD-AGs) remains unclear in head and neck squamous cell carcinoma (HNSCC). This study aimed to investigate LDs in HNSCC and identify LD-AGs essential for the diagnosis and prognosis of HNSCC patients.

METHODS

The LDs in the HNSCC and normal cell lines were stained with oil red O. Bioinformatic analysis was used to find LD-AGs in HNSCC that had diagnostic and prognostic significance.

RESULTS

LDs accumulation was increased in HNSCC cell lines compared with normal cell lines (P<0.05). Fifty-three differentially expressed genes, including 34 upregulated and 19 downregulated, were found in HNSCC based on the TCGA platform (P<0.05). Then, 53 genes were proved to be functionally enriched in lipid metabolism and LDs. Among them, with an AUC value > 0.7, 34 genes demonstrated a high predictive power. Six genes (AUP1, CAV1, CAV2, CAVIN1, HILPDA, and SQLE) out of 34 diagnostic genes were linked to overall survival in patients with HNSCC (P<0.05). The significant prognostic factors AUP1, CAV1, CAV2, and SQLE were further identified using the univariate and multivariate cox proportional hazard models (P<0.05). The protein expression of CAV2 and SQLE was significantly increased in the HNSCC tissue compared to normal tissues (P<0.05). Finally, the knockdown of the four LD-AGs decreased LDs accumulation, respectively.

CONCLUSIONS

Increased LDs accumulation was a hallmark of HNSCC, and AUP1, CAV1, CAV2, and SQLE were discovered as differentially expressed LD-AGs with diagnostic and prognostic potential in HNSCC.

Identification of ferroptosis-related genes in type 2 diabetes mellitus based on machine learning

BACKGROUND

Type 2 diabetes mellitus (T2DM), which has a high incidence and several harmful consequences, poses a severe danger to human health. Research on the function of ferroptosis in T2DM is increasing. This study uses bioinformatics techniques identify new diagnostic T2DM biomarkers associated with ferroptosis.

METHODS

To identify ferroptosis-related genes (FRGs) that are differentially expressed between T2DM patients and healthy individuals, we first obtained T2DM sequencing data and FRGs from the Gene Expression Omnibus (GEO) database and FerrDb database. Then, drug-gene interaction networks and competitive endogenous RNA (ceRNA) networks linked to the marker genes were built after marker genes were filtered by two machine learning algorithms (LASSO and SVM-RFE algorithms). Finally, to confirm the expression of marker genes, the GSE76895 dataset was utilized. The protein and RNA expression of some marker genes in T2DM and nondiabetic tissues was also examined by Western blotting, immunohistochemistry (IHC), immunofluorescence (IF) and quantitative real-time PCR (qRT-PCR).

RESULTS

We obtained 58 differentially expressed genes (DEGs) associated with ferroptosis. GO and KEGG enrichment analyses showed that these DEGs were significantly enriched in hypoxia and ferroptosis. Subsequently, eight marker genes (SCD, CD44, HIF1A, BCAT2, MTF1, HILPDA, NR1D2, and MYCN) were screened by LASSO and SVM-RFE machine learning algorithms, and a model was constructed based on these eight genes. This model also has high diagnostic power. In addition, based on these eight genes, we obtained 48 drugs and constructed a complex ceRNA network map. Finally, Western blotting, IHC, IF, and qRT-PCR results of clinical samples further confirmed the results of public databases.

CONCLUSIONS

The diagnosis and aetiology of T2DM can be greatly aided by eight FRGs, providing novel therapeutic avenues.

Hypoxia tolerance in fish depends on catabolic preference between lipids and carbohydrates

Hypoxia is a common environmental stress factor in aquatic organisms, which varies among fish species. However, the mechanisms underlying the ability of fish species to tolerate hypoxia are not well known. Here, we showed that hypoxia response in different fish species was affected by lipid catabolism and preference for lipid or carbohydrate energy sources. Activation of biochemical lipid catabolism through peroxisome proliferator-activated receptor alpha (Pparα) or increasing mitochondrial fat oxidation in tilapia decreased tolerance to acute hypoxia by increasing oxygen consumption and oxidative damage and reducing carbohydrate catabolism as an energy source. Conversely, lipid catabolism inhibition by suppressing entry of lipids into mitochondria in tilapia or individually knocking out three key genes of lipid catabolism in zebrafish increased tolerance to acute hypoxia by decreasing oxygen consumption and oxidative damage and promoting carbohydrate catabolism. However, anaerobic glycolysis suppression eliminated lipid catabolism inhibition-promoted hypoxia tolerance in adipose triglyceride lipase (atgl) mutant zebrafish. Using 14 fish species with different trophic levels and taxonomic status, the fish preferentially using lipids for energy were more intolerant to acute hypoxia than those preferentially using carbohydrates. Our study shows that hypoxia tolerance in fish depends on catabolic preference for lipids or carbohydrates, which can be modified by regulating lipid catabolism.

HILPDA is a lipotoxic marker in adipocytes that mediates the autocrine negative feedback regulation of triglyceride hydrolysis by fatty acids and alleviates cellular lipotoxic stress

BACKGROUND

Lipolysis is a key metabolic pathway in adipocytes that renders stored triglycerides available for use by other cells and tissues. Non-esterified fatty acids (NEFAs) are known to exert feedback inhibition on adipocyte lipolysis, but the underlying mechanisms have only partly been elucidated. An essential enzyme in adipocyte lipolysis is ATGL. Here, we examined the role of the ATGL inhibitor HILPDA in the negative feedback regulation of adipocyte lipolysis by fatty acids.

METHODS

We exposed wild-type, HILPDA-deficient and HILPDA-overexpressing adipocytes and mice to various treatments. HILPDA and ATGL protein levels were determined by Western blot. ER stress was assessed by measuring the expression of marker genes and proteins. Lipolysis was studied in vitro and in vivo by measuring NEFA and glycerol levels.

RESULTS

We show that HILPDA mediates a fatty acid-induced autocrine feedback loop in which elevated intra- or extracellular fatty acids levels upregulate HILPDA by activation of the ER stress response and the fatty acid receptor 4 (FFAR4). The increased HILPDA levels in turn downregulate ATGL protein levels to suppress intracellular lipolysis, thereby maintaining lipid homeostasis. The deficiency of HILPDA under conditions of excessive fatty acid load disrupts this chain of events, leading to elevated lipotoxic stress in adipocytes.

CONCLUSION

Our data indicate that HILPDA is a lipotoxic marker in adipocytes that mediates a negative feedback regulation of lipolysis by fatty acids via ATGL and alleviates cellular lipotoxic stress.

HILPDA-mediated lipidomic remodelling promotes radiotherapy resistance in nasopharyngeal carcinoma by accelerating mitophagy

Radiotherapy resistance is a major obstacle to nasopharyngeal carcinoma (NPC) therapy and contributes to tumour recurrence and metastasis. Lipid metabolism is a key regulatory mechanism in cancer biology; however, its role in NPC radiotherapy resistance remains unclear. In this study, we identified hypoxia-inducible lipid droplet-associated protein (HILPDA) as a newly discovered regulator of radioresistance that induces not only lipid droplet (LD) formation but also intracellular lipid remodelling, notably changing mitochondrial cardiolipin (CL) levels. Additionally, we found that the upregulation of CL promotes mitophagy in response to irradiation exposure. Mechanistically, HILPDA inhibits PINK1-mediated CLS1 ubiquitination and degradation. The combination of a mitophagy inhibitor and irradiation significantly increases the radiosensitivity of NPC cells. Human cancer-derived data confirmed that the HILPDA-CLS1 pathway promotes NPC radioresistance. Collectively, these findings suggest that HILPDA plays a critical role in promoting NPC radioresistance and might be targeted to overcome radiotherapeutic resistance in NPC patients in the clinic.

HILPDA promotes NASH-driven HCC development by restraining intracellular fatty acid flux in hypoxia

BACKGROUND & AIMS

The prevalence of non-alcoholic steatohepatitis (NASH)-driven hepatocellular carcinoma (HCC) is rising rapidly, yet its underlying mechanisms remain unclear. Herein, we aim to determine the role of hypoxia-inducible lipid droplet associated protein (HILPDA)/hypoxia-inducible gene 2 (HIG2), a selective inhibitor of intracellular lipolysis, in NASH-driven HCC.

METHODS

The clinical significance of HILPDA was assessed in human NASH-driven HCC specimens by immunohistochemistry and transcriptomics analyses. The oncogenic effect of HILPDA was assessed in human HCC cells and in 3D epithelial spheroids upon exposure to free fatty acids and either normoxia or hypoxia. Lipidomics profiling of wild-type and HILPDA knockout HCC cells was assessed via shotgun and targeted approaches. Wild-type (Hilpdafl/fl) and hepatocyte-specific Hilpda knockout (HilpdaΔHep) mice were fed a Western diet and high sugar in drinking water while receiving carbon tetrachloride to induce NASH-driven HCC.

RESULTS

In patients with NASH-driven HCC, upregulated HILPDA expression is strongly associated with poor survival. In oxygen-deprived and lipid-loaded culture conditions, HILPDA promotes viability of human hepatoma cells and growth of 3D epithelial spheroids. Lack of HILPDA triggered flux of polyunsaturated fatty acids to membrane phospholipids and of saturated fatty acids to ceramide synthesis, exacerbating lipid peroxidation and apoptosis in hypoxia. The apoptosis induced by HILPDA deficiency was reversed by pharmacological inhibition of ceramide synthesis. In our experimental mouse model of NASH-driven HCC, HilpdaΔHep exhibited reduced hepatic steatosis and tumorigenesis but increased oxidative stress in the liver. Single-cell analysis supports a dual role of hepatic HILPDA in protecting HCC cells and facilitating the establishment of a pro-tumorigenic immune microenvironment in NASH.

CONCLUSIONS

Hepatic HILPDA is a pivotal oncometabolic factor in the NASH liver microenvironment and represents a potential novel therapeutic target.

IMPACT AND IMPLICATIONS

Non-alcoholic steatohepatitis (NASH, chronic metabolic liver disease caused by buildup of fat, inflammation and damage in the liver) is emerging as the leading risk factor and the fastest growing cause of hepatocellular carcinoma (HCC), the most common form of liver cancer. While curative therapeutic options exist for HCC, it frequently presents at a late stage when such options are no longer effective and only systemic therapies are available. However, systemic therapies are still associated with poor efficacy and some side effects. In addition, no approved drugs are available for NASH. Therefore, understanding the underlying metabolic alterations occurring during NASH-driven HCC is key to identifying new cancer treatments that target the unique metabolic needs of cancer cells.

Analytical Techniques for Single-Cell Biochemical Assays of Lipids

Lipids are essential cellular components forming membranes, serving as energy reserves, and acting as chemical messengers. Dysfunction in lipid metabolism and signaling is associated with a wide range of diseases including cancer and autoimmunity. Heterogeneity in cell behavior including lipid signaling is increasingly recognized as a driver of disease and drug resistance. This diversity in cellular responses as well as the roles of lipids in health and disease drive the need to quantify lipids within single cells. Single-cell lipid assays are challenging due to the small size of cells (∼1 pL) and the large numbers of lipid species present at concentrations spanning orders of magnitude. A growing number of methodologies enable assay of large numbers of lipid analytes, perform high-resolution spatial measurements, or permit highly sensitive lipid assays in single cells. Covered in this review are mass spectrometry, Raman imaging, and fluorescence-based assays including microscopy and microseparations.

The role of hypoxia-inducible factor 1α in hepatic lipid metabolism

Chronic liver disease is a major public health problem with a high and increasing prevalence worldwide. In the progression of chronic liver disease, steatosis drives the progression of the disease to cirrhosis or even liver cancer. Hypoxia-inducible factor 1α (HIF-1α) is central to the regulation of hepatic lipid metabolism. HIF-1α upregulates the expression of genes related to lipid uptake and synthesis in the liver and downregulates the expression of lipid oxidation genes. Thus, it promotes intrahepatic lipid deposition. In addition, HIF-1α is expressed in white adipose tissue, where lipolysis releases free fatty acids (FFAs) into the blood. These circulating FFAs are taken up by the liver and accumulate in the liver. The expression of HIF-1α in the liver condenses bile and makes it easier to form gallstones. Contrary to the role of hepatic HIF-1α, intestinal HIF-1α expression can maintain a healthy microbiota and intestinal barrier. Thus, it plays a protective role against hepatic steatosis. This article aims to provide an overview of the current understanding of the role of HIF-1α in hepatic steatosis and to encourage the development of therapeutic agents associated with HIF-1α pathways. KEY MESSAGES: • Hepatic HIF-1α expression promotes lipid uptake and synthesis and reduces lipid oxidation leading to hepatic steatosis. • The expression of HIF-1α in the liver condenses bile and makes it easier to form gallstones. • Intestinal HIF-1α expression can maintain a healthy microbiota and intestinal barrier.

Mutation c.-379 C>T in DGAT1 affects intramyocellular lipid content by altering MYOD1 binding affinity

Intramuscular fat (IMF) is one of the most important indexes of pork taste quality. Diacylglycerol acyltransferase 1 (DGAT1), belonging to the acyl-coenzyme A: DGAT enzymes family, is a rate-limiting enzyme responsible for the final step of triglyceride (TG) synthesis. It is involved in TG storage in skeletal muscle; however, the underlying mechanism is not well understood. This study aimed to uncover functional mutations that can influence DGAT1 expression and consequently affect IMF deposition in pork. Two experimental groups containing individuals with high and low IMF content (6.23 ± 0.20 vs. 1.25 ± 0.05, p < 0.01) were formed from 260 Duroc × Large White × Yorkshire (D × L × Y) cross-bred pigs. A novel SNP c.-379 C>T was uncovered in the DGAT1 gene using comparative sequencing with pool DNA of high- and low-IMF groups. The IMF content of CT genotype individuals (3.19 ± 0.11%) was higher than that of CC genotype individuals (2.86 ± 0.11%) when analyzing 260 D × L × Y pigs (p < 0.05). The DGAT1 expression levels revealed a significant positive correlation with IMF content (r = 0.33, p < 0.01). Luciferase assay revealed that the DGAT1 promoter with the c.-379 T allele has a higher transcription activity than that bearing the C allele in C2C12 myoblast cells, but not in 3T3-L1 pre-adipocytes. Online prediction followed by chromatin immunoprecipitation-polymerase chain reaction assay confirmed that myogenic determination factor 1 (MYOD1) binds to the DGAT1 promoter with the c.-379 C allele but not the T allele. In vitro experiments demonstrated that MYOD1 represses DGAT1 transcription and lipogenesis. As a muscle-specific transcription factor, MYOD1 can inhibit the transcription of DGAT1 with the c.-379 C allele in muscle cells. However, in the absence of MYOD1 binding to the mutated DGAT1 promoter with the c.-379 T allele, DGAT1 expresses at a higher level in the muscle cells of the c.-379 T genotype, leading to more intramyocellular lipid accumulation than in the muscle cells of the c.-379 C genotype. The SNP c.-379 C>T in the promoter region of the DGAT1 gene provides a promising molecular marker for improving pork IMF content without affecting other fat depots.

Downregulation of fatty acid oxidation led by Hilpda increases G2/M arrest/delay-induced kidney fibrosis

Hypoxia-regulated proximal tubular epithelial cells (PTCs) G2/M phase arrest/delay was involved in production of renal tubulointerstitial fibrosis (TIF). TIF is a common pathological manifestation of progression in patients with chronic kidney disease (CKD), and is often accompanied by lipid accumulation in renal tubules. However, cause-effect relationship between hypoxia-inducible lipid droplet-associated protein (Hilpda), lipid accumulation, G2/M phase arrest/delay and TIF remains unclear. Here we found that overexpression of Hilpda downregulated adipose triglyceride lipase (ATGL) promoted triglyceride overload in the form of lipid accumulation, leading to defective fatty acid β-oxidation (FAO), ATP depletion in a human PTC cell line (HK-2) under hypoxia and in mice kidney tissue treated with unilateral ureteral obstruction (UUO) and unilateral ischemia-reperfusion injury (UIRI). Hilpda-induced lipid accumulation caused mitochondrial dysfunction, enhanced expression of profibrogenic factors TGF-β1, α-SMA and Collagen I elevation, and reduced expression of G2/M phase-associated gene CDK1, as well as increased CyclinB1/D1 ratio, resulted in G2/M phase arrest/delay and profibrogenic phenotypes. Hilpda deficiency in HK-2 cell and kidney of mice with UUO had sustained expression of ATGL and CDK1 and reduced expression of TGF-β1, Collagen I and CyclinB1/D1 ratio, resulting in the amelioration of lipid accumulation and G2/M arrest/delay and subsequent TIF. Expression of Hilpda correlated with lipid accumulation, was positively associated with tubulointerstitial fibrosis in tissue samples from patients with CKD. Our findings suggest that Hilpda deranges fatty acid metabolism in PTCs, which leads to G2/M phase arrest/delay and upregulation of profibrogenic factors, and consequently promote TIF which possibly underlie pathogenesis of CKD.

Lipid Metabolism in Glioblastoma: From De Novo Synthesis to Storage

Glioblastoma (GBM) is the most lethal primary brain tumor. With limited therapeutic options, novel therapies are desperately needed. Recent studies have shown that GBM acquires large amounts of lipids for rapid growth through activation of sterol regulatory element-binding protein 1 (SREBP-1), a master transcription factor that regulates fatty acid and cholesterol synthesis, and cholesterol uptake. Interestingly, GBM cells divert substantial quantities of lipids into lipid droplets (LDs), a specific storage organelle for neutral lipids, to prevent lipotoxicity by increasing the expression of diacylglycerol acyltransferase 1 (DGAT1) and sterol-O-acyltransferase 1 (SOAT1), which convert excess fatty acids and cholesterol to triacylglycerol and cholesteryl esters, respectively. In this review, we will summarize recent progress on our understanding of lipid metabolism regulation in GBM to promote tumor growth and discuss novel strategies to specifically induce lipotoxicity to tumor cells through disrupting lipid storage, a promising new avenue for treating GBM.

Recent Advances on the Role of ATGL in Cancer

The hypoxic state of the tumor microenvironment leads to reprogramming lipid metabolism in tumor cells. Adipose triglyceride lipase, also known as patatin-like phospholipase= domain-containing protein 2 and Adipose triglyceride lipase (ATGL), as an essential lipid metabolism-regulating enzyme in cells, is regulated accordingly under hypoxia induction. However, studies revealed that ATGL exhibits both tumor-promoting and tumor-suppressing effects, which depend on the cancer cell type and the site of tumorigenesis. For example, elevated ATGL expression in breast cancer is accompanied by enhanced fatty acid oxidation (FAO), enhancing cancer cells’ metastatic ability. In prostate cancer, on the other hand, tumor activity tends to be negatively correlated with ATGL expression. This review outlined the regulation of ATGL-mediated lipid metabolism pathways in tumor cells, emphasizing the Hypoxia-inducible factors 1 (HIF-1)/Hypoxia-inducible lipid droplet-associated (HIG-2)/ATGL axis, peroxisome proliferator-activated receptor (PPAR)/G0/G1 switch gene 2 (G0S2)/ATGL axis, and fat-specific protein 27 (FSP-27)/Early growth response protein 1 (EGR-1)/ATGL axis. In the light of recent research on different cancer types, the role of ATGL on tumorigenesis, tumor proliferation, and tumor metastasis was systemically reviewed.

Functions of Stress-Induced Lipid Droplets in the Nervous System

Lipid droplets are highly dynamic intracellular organelles that store neutral lipids such as cholesteryl esters and triacylglycerols. They have recently emerged as key stress response components in many different cell types. Lipid droplets in the nervous system are mostly observed in vivo in glia, ependymal cells and microglia. They tend to become more numerous in these cell types and can also form in neurons as a consequence of ageing or stresses involving redox imbalance and lipotoxicity. Abundant lipid droplets are also a characteristic feature of several neurodegenerative diseases. In this minireview, we take a cell-type perspective on recent advances in our understanding of lipid droplet metabolism in glia, neurons and neural stem cells during health and disease. We highlight that a given lipid droplet subfunction, such as triacylglycerol lipolysis, can be physiologically beneficial or harmful to the functions of the nervous system depending upon cellular context. The mechanistic understanding of context-dependent lipid droplet functions in the nervous system is progressing apace, aided by new technologies for probing the lipid droplet proteome and lipidome with single-cell type precision.

A new perspective on NAFLD: Focusing on lipid droplets

Lipid droplets (LDs) are complex and metabolically active organelles. They are composed of a neutral lipid core surrounded by a monolayer of phospholipids and proteins. LD accumulation in hepatocytes is the distinctive characteristic of non-alcoholic fatty liver disease (NAFLD), which is a chronic, heterogeneous liver condition that can progress to liver fibrosis and hepatocellular carcinoma. Though recent research has improved our understanding of the mechanisms linking LD accumulation to NAFLD progression, numerous aspects of LD biology are either poorly understood or unknown. In this review, we provide a description of several key mechanisms that contribute to LD accumulation in hepatocytes, favouring NAFLD progression. First, we highlight the importance of LD architecture and describe how the dysregulation of LD biogenesis leads to endoplasmic reticulum stress and inflammation. This is followed by an analysis of the causal nexus that exists between LD proteome composition and LD degradation. Finally, we describe how the increase in size of LDs causes activation of hepatic stellate cells, leading to liver fibrosis and hepatocellular carcinoma. We conclude that acquiring a more sophisticated understanding of LD biology will provide crucial insights into the heterogeneity of NAFLD and assist in the development of therapeutic approaches for this liver disease.

Lipid alterations in chronic liver disease and liver cancer

Lipids are a complex and diverse group of molecules with crucial roles in many physiological processes, as well as in the onset, progression, and maintenance of cancers. Fatty acids and cholesterol are the building blocks of lipids, orchestrating these crucial metabolic processes. In the liver, lipid alterations are prevalent as a cause and consequence of chronic hepatitis B and C virus infections, alcoholic hepatitis, and non-alcoholic fatty liver disease and steatohepatitis. Recent developments in lipidomics have also revealed that dynamic changes in triacylglycerols, phospholipids, sphingolipids, ceramides, fatty acids, and cholesterol are involved in the development and progression of primary liver cancer. Accordingly, the transcriptional landscape of lipid metabolism suggests a carcinogenic role of increasing fatty acids and sterol synthesis. However, limited mechanistic insights into the complex nature of the hepatic lipidome have so far hindered the development of effective therapies.

The Molecular Brakes of Adipose Tissue Lipolysis

Adaptation to changes in energy availability is pivotal for the survival of animals. Adipose tissue, the body’s largest reservoir of energy and a major source of metabolic fuel, exerts a buffering function for fluctuations in nutrient availability. This functional plasticity ranges from energy storage in the form of triglycerides during periods of excess energy intake to energy mobilization via lipolysis in the form of free fatty acids for other organs during states of energy demands. The subtle balance between energy storage and mobilization is important for whole-body energy homeostasis; its disruption has been implicated as contributing to the development of insulin resistance, type 2 diabetes and cancer cachexia. As a result, adipocyte lipolysis is tightly regulated by complex regulatory mechanisms involving lipases and hormonal and biochemical signals that have opposing effects. In thermogenic brown and brite adipocytes, lipolysis stimulation is the canonical way for the activation of non-shivering thermogenesis. Lipolysis proceeds in an orderly and delicately regulated manner, with stimulation through cell-surface receptors via neurotransmitters, hormones, and autocrine/paracrine factors that activate various intracellular signal transduction pathways and increase kinase activity. The subsequent phosphorylation of perilipins, lipases, and cofactors initiates the translocation of key lipases from the cytoplasm to lipid droplets and enables protein-protein interactions to assemble the lipolytic machinery on the scaffolding perilipins at the surface of lipid droplets. Although activation of lipolysis has been well studied, the feedback fine-tuning is less well appreciated. This review focuses on the molecular brakes of lipolysis and discusses some of the divergent fine-tuning strategies in the negative feedback regulation of lipolysis, including delicate negative feedback loops, intermediary lipid metabolites-mediated allosteric regulation and dynamic protein–protein interactions. As aberrant adipocyte lipolysis is involved in various metabolic diseases and releasing the brakes on lipolysis in thermogenic adipocytes may activate thermogenesis, targeting adipocyte lipolysis is thus of therapeutic interest.

Lipolysis: cellular mechanisms for lipid mobilization from fat stores

The perception that intracellular lipolysis is a straightforward process that releases fatty acids from fat stores in adipose tissue to generate energy has experienced major revisions over the last two decades. The discovery of new lipolytic enzymes and coregulators, the demonstration that lipophagy and lysosomal lipolysis contribute to the degradation of cellular lipid stores and the characterization of numerous factors and signalling pathways that regulate lipid hydrolysis on transcriptional and post-transcriptional levels have revolutionized our understanding of lipolysis. In this review, we focus on the mechanisms that facilitate intracellular fatty-acid mobilization, drawing on canonical and noncanonical enzymatic pathways. We summarize how intracellular lipolysis affects lipid-mediated signalling, metabolic regulation and energy homeostasis in multiple organs. Finally, we examine how these processes affect pathogenesis and how lipolysis may be targeted to potentially prevent or treat various diseases.

Lipid Droplet-Associated Factors, PNPLA3, TM6SF2, and HSD17B Proteins in Hepatopancreatobiliary Cancer

Simple Summary

Aberrant lipid synthesis and reprogrammed lipid metabolism are both associated with the development and progression of pancreatic and liver cancer. Most cells store fatty acids in the form of triacylglycerols in lipid droplets. Lipid droplets are intracellular organelles that not only store neutral lipids, but also play roles as molecular messengers and signaling factors. Some cancer cells accumulate massive amount of lipid droplets. Lipid droplets and lipid droplet-associated factors are further implicated to mediate proliferation, invasion, metastasis, as well as chemotherapy resistance in several types of cancer. This review dissected recent findings on the role of several lipid droplet-associated factors, patatin-like phospholipase domain-containing 3 (PNPLA3), Transmembrane 6 superfamily member 2 (TM6SF2), and 17β-hydroxysteroid dehydrogenase (HSD17B) 11 and 13 as well as their genetic variations in hepatopancreatobiliary diseases, especially cancer.

Abstract

Pancreatic and liver cancer are leading causes of cancer deaths, and by 2030, they are projected to become the second and the third deadliest cancer respectively. Cancer metabolism, especially lipid metabolism, plays an important role in progression and metastasis of many types of cancer, including pancreatic and liver cancer. Lipid droplets are intracellular organelles that store neutral lipids, but also act as molecular messengers, and signaling factors. It is becoming increasingly evident that alterations in the regulation of lipid droplets and their associated factors influence the risk of developing not only metabolic disease but also fibrosis and cancer. In the current review article, we summarized recent findings concerning the roles of lipid droplet-associated factors, patatin-like phospholipase domain-containing 3, Transmembrane 6 superfamily member 2, and 17β-hydroxysteroid dehydrogenase 11 and 13 as well as genetic variants in pancreatic and hepatic diseases. A better understanding of cancer type- and cell type-specific roles of lipid droplet-associated factors is important for establishing new therapeutic options in the future.

Acute catabolism of leukocyte lipid bodies: Characterization of a nordihydroguaiaretic acid (NDGA)-induced proteasomal-dependent model

Cytoplasmic availability of leukocyte lipid bodies is controlled by a highly regulated cycle of opposing biogenesis- and catabolism-related events. While leukocyte biogenic machinery is well-characterized, lipid body catabolic mechanisms are yet mostly unknown. Here, we demonstrated that nordihydroguaiaretic acid (NDGA) very rapidly decreases the numbers of pre-formed lipid bodies within lipid body-enriched cytoplasm of mouse leukocytes - macrophages, neutrophils and eosinophils. NDGA mechanisms driving leukocyte lipid body disappearance were not related to loss of cell viability, 5-lipoxygenase inhibition, ATP autocrine/paracrine activity, or biogenesis inhibition. Proteasomal-dependent breakdown of lipid bodies appears to control NDGA-driven leukocyte lipid body reduction, since it was Bortezomib-sensitive in macrophages, neutrophils and eosinophils. Our findings unveil an acute NDGA-triggered lipid body catabolic event - a novel experimental model for the still neglected research area on leukocyte lipid body catabolism, additionally favoring further insights on proteasomal contribution to lipid body breakdown.

Lipid droplets in the nervous system

Lipid droplets are dynamic intracellular lipid storage organelles that respond to the physiological state of cells. In addition to controlling cell metabolism, they play a protective role for many cellular stressors, including oxidative stress. Despite prior descriptions of lipid droplets appearing in the brain as early as a century ago, only recently has the role of lipid droplets in cells found in the brain begun to be understood. Lipid droplet functions have now been described for cells of the nervous system in the context of development, aging, and an increasing number of neuropathologies. Here, we review the basic mechanisms of lipid droplet formation, turnover, and function and discuss how these mechanisms enable lipid droplets to function in different cell types of the nervous system under healthy and pathological conditions.

Lipid droplet storage promotes murine pancreatic tumor growth

Hypoxia Inducible Lipid Droplet Associated (HILPDA) is frequently overexpressed in tumors and promotes neutral lipid storage. The impact of Hilpda on pancreatic ductal adenocarcinoma (PDAC) tumor growth is not known. In order to evaluate Hilpda‑dependent lipid storage mechanisms, expression of Hilpda in murine pancreatic cells (KPC) was genetically manipulated. Lipid droplet (LD) abundance and triglyceride content in vitro were measured, and model tumor growth in nu/nu mice was determined. The results showed that excess lipid supply increased triglyceride storage and LD formation in KPC cells in a HILPDA‑dependent manner. Contrary to published results, inhibition of Adipose Triglyceride Lipase (ATGL) did not ameliorate the triglyceride abundance differences between Hilpda WT and KO cells. Hilpda ablation significantly decreased the growth rate of model tumors in immunocompromised mice. In conclusion, Hilpda is a positive regulator of triglyceride storage and lipid droplet formation in murine pancreatic cancer cells in vitro and lipid accumulation and tumor growth in vivo. Our data suggest that deregulated ATGL is not responsible for the absence of LDs in KO cells in this context.