Analysis of the role of microsomal triglyceride transfer protein in the liver of tissue-specific knockout mice

Ovarian expression of functional MTTP and apoB for VLDL assembly and secretion in chickens

In mammals, tissues other than liver and intestine are known to possess functional MTTP (microsomal triglyceride transfer protein) and apoB (apolipoprotein B) capable of VLDL (very low-density lipoprotein) assembly. Birds are oviparous and possess unique capabilities in lipid biology to accommodate yolk formation through massive deposition of hepatically assembled yolk-targeted VLDLy into ovarian follicles. Following identifications of MTTP and ApoB expression within chicken ovarian stroma, granulosa, theca, and epithelial cells of various classes of follicles, we sought to define the functionality of ovarian MTTP and ApoB in VLDL assembly. In situ hybridization analysis found that ApoB transcripts are most abundant in thecal layers, whereas immunohistochemistry showed that MTTP predominates in the granulosa layers. MTTP lipid transfer activity was greater in small yellow follicles than in hierarchical follicles. Metabolic labeling, electron microscopy, and Western blot studies confirmed the functionality of ovarian apoB and MTTP as newly assembled VLDL around 50-200 nm in diameter and lacking ApoVLDL-II dissimilar to VLDLy, were secreted from cultured follicular cells. Lomitapide and the ApoB-antisense oligonucleotide Mipomersen dose-dependently decreased MTTP activity and VLDL-apoB secretion from cultured follicular cells, while oleate addition or acute heat stress enhanced VLDL-apoB secretion. Ultrastructural images showed VLDL assembly and trafficking toward the secretion route. The findings support the notion that VLDL assembly and secretion within avian ovarian tissues functions as a protective mechanism against fuel and physical stressors to secure follicle development and/or nutritional quality control of yolk for embryo development.

Depletion of Hepatic SREBP2 Protects Against Hypercholesterolemia and Atherosclerosis through the ANGPTL3-LPL Axis

Lipolysis of triglyceride-rich lipoproteins by peripheral lipoprotein lipase (LPL) plays an essential role in maintaining systemic cholesterol/lipid homeostasis. Human genetic studies have unequivocally demonstrated that activation of LPL pathway reduces risks for both coronary artery disease (CAD) and type 2 diabetes (T2D). Although sterol regulatory element-binding protein 2 (SREBP2) is well established as the master transcription factor that regulates the hepatic biosynthesis of both cholesterol and fatty acids, whether and how its activity in liver interacts with peripheral LPL pathway remains unknown. Here, it is demonstrated that acute liver-specific depletion of SREBP2 results in divergent effects on the regulation of peripheral LPL activity in mice, depending on the presence or absence of low-density lipoprotein receptors (LDLR). SREBP2 deficiency drastically elevates peripheral LPL activity through downregulation of plasma angiopoietin-related protein 3 (ANGPTL3) levels in LDLR-deficient mice. Moreover, in addition to SREBP2's transcriptional regulation of ANGPTL3, it is found that SREBP2 promotes proteasome-based degradation of ANGPTL3 in the presence of LDLR. Remarkably, acute depletion of hepatic SREBP2 protects against hypercholesterolemia and atherosclerosis, in which atherosclerotic lesions are reduced by 45% compared to control littermates. Taken together, these findings outline a liver-peripheral crosstalk mediated by SREBP2-ANGPTL3-LPL axis and suggest that SREBP2 inhibition can be an effective strategy to tackle homozygous familial hypercholesterolemia (HoFH).

Dynamic Proteome Changes in Cuprizone-Induced Demyelination and Remyelination in the Mouse Brain

This study aimed to gain insights into the dynamic proteome changes and underlying molecular mechanisms of de/remyelination in a cuprizone model, a widely used preclinical model of multiple sclerosis (MS). Longitudinal sampling of control or cuprizone-treated mouse brains was executed at 6 time points over 6 weeks. Data analysis included 8489 quantified proteins. Differential proteomic and GO analyses revealed that 5.9% of the quantified proteome was altered, including reported and novel de/remyelination-relevant protein changes and underlying pathways. We found that oligodendrocyte proteins (Fa2h and Ugt8) were significantly changed during demyelination, suggesting that dysregulated sphingolipid metabolism in MS may stem from oligodendrocyte pathology. Importantly, we showed that the cholesterol biosynthesis pathway was the most enriched biological process in a subset of significantly changed proteins, where myelination was highly enriched. We further validated the changes in the cholesterol biosynthesis pathway through targeted GC-MS analysis of intermediate sterols, supporting the critical role of cholesterol biosynthesis in de/remyelination. Unexpectedly, changes of myelin-associated proteins, Mbp and Plp1, were minimal, while Ermn showed significant reduction tracking with demyelination, indicating that some myelin protein changes are more sensitive to demyelination. Together with a list of significantly altered proteins, the results of this study could benefit future remyelination research.

Distinct yet Overlapping Functions of VMP1 and TMEM41B in Modulating Hepatic Lipoprotein Secretion and Autophagy in MASH

Background

Transmembrane protein 41B (TMEM41B) and vacuolar membrane protein 1 (VMP1) are endoplasmic reticulum (ER) transmembrane scramblase proteins that have been recently identified to have important roles in autophagy and hepatic lipoprotein secretion. While TMEM41B and VMP1 are structurally and functionally similar, the nature of their interactions and how they coordinately regulate hepatic lipoprotein secretion and autophagy in metabolic-associated steatotic liver disease (MASLD) and metabolic-associated steatohepatitis (MASH) remains unclear.

Methods

Liver-specific and hepatocyte-specific Tmem41b knockout (KO) mice as well as Tmem41b knock-in (KI) mice were generated from Tmem41b flox or Tmem41b KI mice by crossing with albumin-Cre mice or by injecting AAV8-TBG-cre, respectively. Lipid metabolism in these mice was characterized by lipidomic analyses. Mice with hepatic overexpression of TMEM41B that were fed a MASH diet were also characterized. To explore the relationship between TMEM41B and VMP1, Tmem41b/Vmp1 double KO (DKO), Tmem41b KO/ Vmp1 KI, and Vmp1 KO/ Tmem41b KI mice were generated, and steatosis and autophagy were characterized.

Results

The loss of hepatic Tmem41b severely impaired very low-density lipoprotein (VLDL) secretion, resulting in significant microsteatosis, increased hepatic triglycerides, inflammation, fibrosis, and ultimately the MASH development. TMEM41B protein was decreased in human MASLD livers. Overexpression of TMEM41B mitigated the effects of diet-induced MASLD. Mice lacking both Vmp1 and Tmem41b (DKO) showed further impairment in VLDL secretion compared to single Tmem41b KO, but were similar that of Vmp1 KO mice. Lipidomic analysis of liver tissues revealed decreased levels of phosphatidylcholine and phosphatidylethanolamine, along with increased neutral lipids. Cellular fractionation studies indicated that VMP1 and TMEM41B localize at the mitochondrial-associated membrane (MAM). Electron microscopy analysis showed reduced contact between mitochondria and the ER in hepatocytes deficient in either VMP1 or TMEM41B. The loss of hepatic VMP1 or TMEM41B led to markedly increased levels of LC3B-II and p62/SQSTM1, which were not further affected by double deletion of VMP1 and TMEM41B. Restoring VMP1 in Tmem41b KO mice partially improved defective VLDL secretion, though autophagy was only partially corrected by overexpression of VMP1 at a low but not high level. In contrast, restoring TMEM41B in Vmp1 KO mice dose-dependently improved both defective VLDL secretion and autophagy.

Conclusion

Loss of hepatic VMP1 or TMEM41B decreases MAM and phospholipid content and reduces VLDL secretion, resulting in the development of MASH. TMEM41B and VMP1 may have overlapping but distinct mechanisms in regulating lipoprotein secretion and autophagy.

The biogenesis and transport of triglyceride-rich lipoproteins

Triglyceride-rich lipoproteins (TRLs) play essential roles in human health and disease by transporting bulk lipids into the circulation. This review summarizes the fundamental mechanisms and diverse factors governing lipoprotein production, secretion, and regulation. Emphasizing the broader implications for human health, we outline the intricate landscape of lipoprotein research and highlight the potential coordination between the biogenesis and transport of TRLs in physiology, particularly the unexpected coupling of metabolic enzymes and transport machineries. Challenges and opportunities in lipoprotein biology with respect to inherited diseases and viral infections are also discussed. Further characterization of the biogenesis and transport of TRLs will advance both basic research in lipid biology and translational medicine for metabolic diseases.

Differential transport pathways of saturated and unsaturated fatty acid esters in male mouse hepatocytes

Saturated fatty acid (SFA) and unsaturated fatty acid (UFA) have distinct impacts on health. Whether SFA and UFA are differentially transported in liver remains elusive. Here, we find the secretion of UFA but not SFA esters is retarded in a male mouse hepatic endoplasmic reticulum (ER) stress model. Among 13 members of protein disulfide isomerase (PDI) family, only PDIA1 (PDI) deficiency leads to hepatosteatosis and hypolipidemia. In PDI-deficient male mouse liver, there is a severe accumulation but secretory blockade of UFA esters, whereas the accumulation and secretion of SFA esters remain normal. PDI catalyzes the oxidative folding of microsomal triglyceride transfer protein (MTP). In addition, PDI deficiency in hepatocytes abolishes Apolipoprotein B-100 (ApoB-100) very low-density lipoprotein (VLDL) secretion while maintaining partial ApoB-48 VLDL secretion. In summary, we find that the secretion of UFA esters is PDI-MTP indispensable, while SFA esters could be transferred out of liver via ApoB-48 VLDL through a PDI-MTP-independent pathway.

Recent advances in MASLD genetics: Insights into disease mechanisms and the next frontiers in clinical application

Metabolic dysfunction-associated steatotic liver disease (MASLD) is the most common chronic liver disease in the world and a growing cause of liver-related morbidity and mortality. Yet, at the same time, our understanding of the pathophysiology and genetic underpinnings of this increasingly common yet heterogeneous disease has increased dramatically over the last 2 decades, with the potential to lead to meaningful clinical interventions for patients. We have now seen the first pharmacologic therapy approved for the treatment of MASLD, and multiple other potential treatments are currently under investigation-including gene-targeted RNA therapies that directly extend from advances in MASLD genetics. Here we review recent advances in MASLD genetics, some of the key pathophysiologic insights that human genetics has provided, and the ways in which human genetics may inform our clinical practice in the field of MASLD in the near future.

Unraveling Mechanisms of Genetic Risks in Metabolic Dysfunction-Associated Steatotic Liver Diseases: A Pathway to Precision Medicine

Metabolic dysfunction-associated steatotic liver disease (MASLD) is a growing global health problem, affecting ∼1 billion people. This condition is well established to have a heritable component with strong familial clustering. With the extraordinary breakthroughs in genetic research techniques coupled with their application to large-scale biobanks, the field of genetics in MASLD has expanded rapidly. In this review, we summarize evidence regarding genetic predisposition to MASLD drawn from family and twin studies. Significantly, we delve into detailed genetic variations associated with diverse pathogenic mechanisms driving MASLD. We highlight the interplay between these genetic variants and their connections with metabolic factors, the gut microbiome, and metabolites, which collectively influence MASLD progression. These discoveries are paving the way for precise medicine, including noninvasive diagnostics and therapies. The promising landscape of novel genetically informed drug targets such as RNA interference is explored. Many of these therapies are currently under clinical validation, raising hopes for more effective MASLD treatment.

MicroRNA-206-3p suppresses hepatic lipogenesis and cholesterol synthesis while driving cholesterol efflux

BACKGROUND AND AIMS

Hepatosteatosis, hypertriglyceridemia, and hypercholesterolemia are interconnected metabolic disorders. This study is designed to characterize how microRNA-206-3p (miR-206) simultaneously prevents de novo lipogenesis (DNL), cholesterol synthesis, and VLDL production in hepatocytes while promoting cholesterol efflux in macrophages.

APPROACH AND RESULTS

MiR-206 levels were reduced in hepatocytes and macrophages of mice subjected to a high-fat, high-cholesterol diet. A negative feedback between LXRα (liver X receptor alpha) and miR-206 is formed to maintain high LXRα and low miR-206 in hepatocytes. Systemic administration of miR-206 alleviated hepatosteatosis, hypertriglyceridemia, and hypercholesterolemia in mice. A significant reduction in LDL cholesterol and VLDL cholesterol but unaltered HDL cholesterol was observed in miR-206-treated mice. Mirroring these findings, miR-206 reprogrammed the transcriptome of hepatocytes towards the inhibition of DNL, cholesterol synthesis, and assembly and secretion of VLDL. In macrophages, miR-206 activated the expression of genes regulating cholesterol efflux. Hepatocyte-specific expression of miR-206 reduced hepatic and circulating triglycerides and cholesterol, as well as VLDL production, while transplantation of macrophages bearing miR-206 facilitated cholesterol efflux. Mechanistically, miR-206 directly targeted Lxrα and Hmgcr in hepatocytes but facilitated expression of Lxrα in macrophages by targeting macrophage-specific tricho-rhino-phalangeal syndrome 1 (TRPS1), a transcription repressor of Lxrα . By targeting Hmgc r and Lxrα , miR-206 inhibited DNL, VLDL production, and cholesterol synthesis in hepatocytes, whereas it drove cholesterol efflux by activating the TRPS1-LXRα axis.

CONCLUSIONS

MiR-206, through differentially modulating LXRα signaling in hepatocytes and macrophages, inhibits DNL, promotes cholesterol efflux, and concurrently hinders cholesterol synthesis and VLDL production. MiR-206 simulates the functions of lipid-lowering medications, statins, and LXRα agonists.

FITM2 deficiency results in ER lipid accumulation, ER stress, and reduced apolipoprotein B lipidation and VLDL triglyceride secretion in vitro and in mouse liver

OBJECTIVE

Triglycerides (TGs) associate with apolipoprotein B100 (apoB100) to form very low density lipoproteins (VLDLs) in the liver. The repertoire of factors that facilitate this association is incompletely understood. FITM2, an integral endoplasmic reticulum (ER) protein, was originally discovered as a factor participating in cytosolic lipid droplet (LD) biogenesis in tissues that do not form VLDL. We hypothesized that in the liver, in addition to promoting cytosolic LD formation, FITM2 would also transfer TG from its site of synthesis in the ER membrane to nascent VLDL particles within the ER lumen.

METHODS

Experiments were conducted using a rat hepatic cell line (McArdle-RH7777, or McA cells), an established model of mammalian lipoprotein metabolism, and mice. FITM2 expression was reduced using siRNA in cells and by liver specific cre-recombinase mediated deletion of the Fitm2 gene in mice. Effects of FITM2 deficiency on VLDL assembly and secretion in vitro and in vivo were measured by multiple methods, including density gradient ultracentrifugation, chromatography, mass spectrometry, stimulated Raman scattering (SRS) microscopy, sub-cellular fractionation, immunoprecipitation, immunofluorescence, and electron microscopy.

MAIN FINDINGS

1) FITM2-deficient hepatic cells in vitro and in vivo secrete TG-depleted VLDL particles, but the number of particles is unchanged compared to controls; 2) FITM2 deficiency in mice on a high fat diet (HFD) results in decreased plasma TG levels. The number of apoB100-containing lipoproteins remains similar, but shift from VLDL to low density lipoprotein (LDL) density; 3) Both in vitro and in vivo, when TG synthesis is stimulated and FITM2 is deficient, TG accumulates in the ER, and despite its availability this pool is unable to fully lipidate apoB100 particles; 4) FITM2 deficiency disrupts ER morphology and results in ER stress.

CONCLUSION

The results suggest that FITM2 contributes to VLDL lipidation, especially when newly synthesized hepatic TG is in abundance. In addition to its fundamental importance in VLDL assembly, the results also suggest that under dysmetabolic conditions, FITM2 may be an important factor in the partitioning of TG between cytosolic LDs and VLDL particles.

Molecular Regulation and Therapeutic Targeting of VLDL Production in Cardiometabolic Disease

There exists a complex relationship between steatotic liver disease (SLD) and atherosclerotic cardiovascular disease (CVD). CVD is a leading cause of morbidity and mortality among individuals with SLD, particularly those with metabolic dysfunction-associated SLD (MASLD), a significant proportion of whom also exhibit features of insulin resistance. Recent evidence supports an expanded role of very low-density lipoprotein (VLDL) in the pathogenesis of CVD in patients, both with and without associated metabolic dysfunction. VLDL represents the major vehicle for exporting neutral lipid from hepatocytes, with each particle containing one molecule of apolipoproteinB100 (APOB100). VLDL production becomes dysregulated under conditions characteristic of MASLD including steatosis and insulin resistance. Insulin resistance not only affects VLDL production but also mediates the pathogenesis of atherosclerotic CVD. VLDL assembly and secretion therefore represents an important pathway in the setting of cardiometabolic disease and offers several candidates for therapeutic targeting, particularly in metabolically complex patients with MASLD at increased risk of atherosclerotic CVD. Here we review the clinical significance as well as the translational and therapeutic potential of key regulatory steps impacting VLDL initiation, maturation, secretion, catabolism, and clearance.

PNPLA3 is a triglyceride lipase that mobilizes polyunsaturated fatty acids to facilitate hepatic secretion of large-sized very low-density lipoprotein

The I148M variant of PNPLA3 is closely associated with hepatic steatosis. Recent evidence indicates that the I148M mutant functions as an inhibitor of PNPLA2/ATGL-mediated lipolysis, leaving the role of wild-type PNPLA3 undefined. Despite showing a triglyceride hydrolase activity in vitro, PNPLA3 has yet to be established as a lipase in vivo. Here, we show that PNPLA3 preferentially hydrolyzes polyunsaturated triglycerides, mobilizing polyunsaturated fatty acids for phospholipid desaturation and enhancing hepatic secretion of triglyceride-rich lipoproteins. Under lipogenic conditions, mice with liver-specific knockout or acute knockdown of PNPLA3 exhibit aggravated liver steatosis and reduced plasma VLDL-triglyceride levels. Similarly, I148M-knockin mice show decreased hepatic triglyceride secretion during lipogenic stimulation. Our results highlight a specific context whereby the wild-type PNPLA3 facilitates the balance between hepatic triglyceride storage and secretion, and suggest the potential contribution of a loss-of-function by the I148M variant to the development of fatty liver disease in humans.

StarD5 levels of expression correlate with onset and progression of steatosis and liver fibrosis

Insufficient expression of steroidogenic acute regulatory-related lipid transfer protein 5 (StarD5) on liver cholesterol/lipid homeostasis is not clearly defined. The ablation of StarD5 was analyzed in mice on a normal or Western diet (WD) to determine its importance in hepatic lipid accumulation and fibrosis compared with wild-type (WT) mice. Rescue experiments in StarD5-/- mice and hepatocytes were performed. In addition to increased hepatic triglyceride (TG)-cholesterol levels, global StarD5-/- mice fed a normal diet displayed reduced plasma triglycerides and liver very low-density lipoprotein (VLDL) secretion as compared with WT counterparts. Insulin levels and homeostatic model assessment for insulin resistance (HOMA-IR) scoring were elevated, demonstrating developing insulin resistance (IR). WD-fed StarD5-/- mice upregulated WW domain containing transcription regulator 1 (TAZ or WWTR1) expression with accelerated liver fibrosis when compared with WD-fed WT mice. Suppression of oxysterol 7α-hydroxylase (CYP7B1) coupled with chronic accumulation of toxic oxysterol levels correlated with presentation of fibrosis. "Hepatocyte-selective" StarD5 overexpression in StarD5-/- mice restored expression, reduced hepatic triglycerides, and improved HOMA-IR. Observations in two additional mouse and one human metabolic dysfunction-associated steatotic liver disease (MASLD) model were supportive. The downregulation of StarD5 with hepatic lipid excess is a previously unappreciated physiological function appearing to promote lipid storage for future needs. Conversely, lingering downregulation of StarD5 with prolonged lipid-cholesterol excess accelerates fatty liver's transition to fibrosis; mediated via dysregulation in the oxysterol signaling pathway.NEW & NOTEWORTHY We have found that deletion of the cholesterol transport protein StarD5 in mice leads to an increase in insulin resistance and lipid accumulation due to the upregulation of lipid synthesis and decrease VLDL secretion from the liver. In addition, deletion of StarD5 increased fibrosis when mice were fed a Western diet. This represents a novel pathway of fibrosis development in the liver.

Pla2g12b drives expansion of triglyceride-rich lipoproteins

Vertebrates transport hydrophobic triglycerides through the circulatory system by packaging them within amphipathic particles called Triglyceride-Rich Lipoproteins. Yet, it remains largely unknown how triglycerides are loaded onto these particles. Mutations in Phospholipase A2 group 12B (PLA2G12B) are known to disrupt lipoprotein homeostasis, but its mechanistic role in this process remains unclear. Here we report that PLA2G12B channels lipids within the lumen of the endoplasmic reticulum into nascent lipoproteins. This activity promotes efficient lipid secretion while preventing excess accumulation of intracellular lipids. We characterize the functional domains, subcellular localization, and interacting partners of PLA2G12B, demonstrating that PLA2G12B is calcium-dependent and tightly associated with the membrane of the endoplasmic reticulum. We also detect profound resistance to atherosclerosis in PLA2G12B mutant mice, suggesting an evolutionary tradeoff between triglyceride transport and cardiovascular disease risk. Here we identify PLA2G12B as a key driver of triglyceride incorporation into vertebrate lipoproteins.

Conditional hepatocyte ablation of PDIA1 uncovers indispensable roles in both APOB and MTTP folding to support VLDL secretion

OBJECTIVES

The assembly and secretion of hepatic very low-density lipoprotein (VLDL) plays pivotal roles in hepatic and plasma lipid homeostasis. Protein disulfide isomerase A1 (PDIA1/P4HB) is a molecular chaperone whose functions are essential for protein folding in the endoplasmic reticulum. Here we investigated the physiological requirement in vivo for PDIA1 in maintaining VLDL assembly and secretion.

METHODS

Pdia1/P4hb was conditionally deleted in adult mouse hepatocytes and the phenotypes characterized. Mechanistic analyses in primary hepatocytes determined how PDIA1 ablation alters MTTP synthesis and degradation as well as altering synthesis and secretion of Apolipoprotein B (APOB), along with complementary expression of intact PDIA1 vs a catalytically inactivated PDIA1 mutant.

RESULTS

Hepatocyte-specific deletion of Pdia1/P4hb inhibited hepatic MTTP expression and dramatically reduced VLDL production, leading to severe hepatic steatosis and hypolipidemia. Pdia1-deletion did not affect mRNA expression or protein stability of MTTP but rather prevented Mttp mRNA translation. We demonstrate an essential role for PDIA1 in MTTP synthesis and function and show that PDIA1 interacts with APOB in an MTTP-independent manner via its molecular chaperone function to support APOB folding and secretion.

CONCLUSIONS

PDIA1 plays indispensable roles in APOB folding, MTTP synthesis and activity to support VLDL assembly. Thus, like APOB and MTTP, PDIA1 is an obligatory component of hepatic VLDL production.

Aster-B-dependent estradiol synthesis protects female mice from diet-induced obesity

Aster proteins mediate the nonvesicular transport of cholesterol from the plasma membrane (PM) to the endoplasmic reticulum (ER). However, the importance of nonvesicular sterol movement for physiology and pathophysiology in various tissues is incompletely understood. Here we show that loss of Aster-B leads to diet-induced obesity in female but not in male mice, and that this sex difference is abolished by ovariectomy. We further demonstrate that Aster-B deficiency impairs nonvesicular cholesterol transport from the PM to the ER in ovaries in vivo, leading to hypogonadism and reduced estradiol synthesis. Female Aster-B-deficient mice exhibit reduced locomotor activity and energy expenditure, consistent with established effects of estrogens on systemic metabolism. Administration of exogenous estradiol ameliorates the diet-induced obesity phenotype of Aster-B-deficient female mice. These findings highlight the key role of Aster-B-dependent nonvesicular cholesterol transport in regulating estradiol production and protecting females from obesity.

PCSK7: A novel regulator of apolipoprotein B and a potential target against non-alcoholic fatty liver disease

BACKGROUND

Epidemiological evidence links the proprotein convertase subtilisin/kexin 7 (PCSK7) to triglyceride (TG) metabolism. We associated the known PCSK7 gain-of-function non-coding SNP rs236918 with higher levels of plasma apolipoprotein B (apoB) and the loss-of-function coding variant p.Pro777Leu (SNP rs201598301) with lower apoB and TG. Herein, we aimed to unravel the in vivo role of liver PCSK7.

METHODS

We biochemically defined the functional role of PCSK7 in lipid metabolism using hepatic cell lines and Pcsk7-/- mice. Our findings were validated following subcutaneous administration of hepatocyte-targeted N-acetylgalactosamine (GalNAc)-antisense oligonucleotides (ASOs) against Pcsk7.

RESULTS

Independent of its proteolytic activity, membrane-bound PCSK7 binds apoB100 in the endoplasmic reticulum and enhances its secretion. Mechanistically, the loss of PCSK7/Pcsk7 leads to apoB100 degradation, triggering an unfolded protein response, autophagy, and β-oxidation, eventually reducing lipid accumulation in hepatocytes. Non-alcoholic fatty liver disease (NAFLD) was induced by a 12-week high fat/fructose/cholesterol diet in wild type (WT) and Pcsk7-/- mice that were then allowed to recover on a 4-week control diet. Pcsk7-/- mice recovered more effectively than WT mice from all NAFLD-related liver phenotypes. Finally, subcutaneous administration of GalNAc-ASOs targeting hepatic Pcsk7 to WT mice validated the above results.

CONCLUSIONS

Our data reveal hepatic PCSK7 as one of the major regulators of apoB, and its absence reduces apoB secretion from hepatocytes favoring its ubiquitination and degradation by the proteasome. This results in a cascade of events, eventually reducing hepatic lipid accumulation, thus supporting the notion of silencing PCSK7 mRNA in hepatocytes for targeting NAFLD.

Loss of hepatic SMLR1 causes hepatosteatosis and protects against atherosclerosis due to decreased hepatic VLDL secretion

BACKGROUND AND AIMS

The assembly and secretion of VLDL from the liver, a pathway that affects hepatic and plasma lipids, remains incompletely understood. We set out to identify players in the VLDL biogenesis pathway by identifying genes that are co-expressed with the MTTP gene that encodes for microsomal triglyceride transfer protein, key to the lipidation of apolipoprotein B, the core protein of VLDL. Using human and murine transcriptomic data sets, we identified small leucine-rich protein 1 ( SMLR1 ), encoding for small leucine-rich protein 1, a protein of unknown function that is exclusively expressed in liver and small intestine.

APPROACH AND RESULTS

To assess the role of SMLR1 in the liver, we used somatic CRISPR/CRISPR-associated protein 9 gene editing to silence murine Smlr1 in hepatocytes ( Smlr1 -LKO). When fed a chow diet, male and female mice show hepatic steatosis, reduced plasma apolipoprotein B and triglycerides, and reduced VLDL secretion without affecting microsomal triglyceride transfer protein activity. Immunofluorescence studies show that SMLR1 is in the endoplasmic reticulum and Cis-Golgi complex. The loss of hepatic SMLR1 in female mice protects against diet-induced hyperlipidemia and atherosclerosis but causes NASH. On a high-fat, high-cholesterol diet, insulin and glucose tolerance tests did not reveal differences in male Smlr1 -LKO mice versus controls.

CONCLUSIONS

We propose a role for SMLR1 in the trafficking of VLDL from the endoplasmic reticulum to the Cis-Golgi complex. While this study uncovers SMLR1 as a player in the VLDL assembly, trafficking, and secretion pathway, it also shows that NASH can occur with undisturbed glucose homeostasis and atheroprotection.

Manganese regulation of COPII condensation controls circulating lipid homeostasis

Precise control of circulating lipids is instrumental in health and disease. Bulk lipids, carried by specialized lipoproteins, are secreted into the circulation, initially via the coat protein complex II (COPII). How the universal COPII machinery accommodates the abundant yet unconventional lipoproteins remains unclear, let alone its therapeutic translation. Here we report that COPII uses manganese-tuning, self-constrained condensation to selectively drive lipoprotein delivery and set lipid homeostasis in vivo. Serendipitously, adenovirus hijacks the condensation-based transport mechanism, thus enabling the identification of cytosolic manganese as an unexpected control signal. Manganese directly binds the inner COPII coat and enhances its condensation, thereby shifting the assembly-versus-dynamics balance of the transport machinery. Manganese can be mobilized from mitochondria stores to signal COPII, and selectively controls lipoprotein secretion with a distinctive, bell-shaped function. Consequently, dietary titration of manganese enables tailored lipid management that counters pathological dyslipidaemia and atherosclerosis, implicating a condensation-targeting strategy with broad therapeutic potential for cardio-metabolic health.

Functional role of SAP18 protein: From transcriptional repression to splicing regulation

Sin3 associated protein 18 (SAP18) is an evolutionary conserved protein, originally discovered in a complex with the transcriptional regulatory protein, Sin3. Subsequent investigations revealed SAP18 as an integral splicing component of the exon junction complex (EJC)-associated apoptosis-and splicing-associated protein (ASAP)/PNN-RNPS1-SAP18 (PSAP) complex. In association with Sin3, SAP18 contributes toward transcriptional repression of genes implicated in embryonic development, stress response, human immunodeficiency virus type 1 replication, and tumorigenesis. As a part of EJC, SAP18 mediates alternative splicing events and suppresses the cryptic splice sites present within flanking regions of exon-exon junctions. In this review, we provide a thorough discussion on SAP18, focussing on its conserved dual role in transcriptional regulation and messenger RNA splicing. Recent research on the involvement of SAP18 in the emergence of cancer and human disorders has also been highlighted. The potential of SAP18 as a therapeutic target is also discussed in these recent studies, particularly related to malignancies of the myeloid lineage.

Substrate-Specific Function of PNPLA3 Facilitates Hepatic VLDL-Triglyceride Secretion During Stimulated Lipogenesis

The I148M variant of PNPLA3 is strongly linked to hepatic steatosis. Evidence suggests a gain-of-function role for the I148M mutant as an ATGL inhibitor, leaving the physiological relevance of wild-type PNPLA3 undefined. Here we show that PNPLA3 selectively degrades triglycerides (TGs) enriched in polyunsaturated fatty acids (PUFAs) independently of ATGL in cultured cells and mice. Lipidomics and metabolite tracing analyses demonstrated that PNPLA3 mobilizes PUFAs from intracellular TGs for phospholipid desaturation, supporting hepatic secretion of TG-rich lipoproteins. Consequently, mice with liver-specific knockout or acute knockdown of PNPLA3 both exhibited aggravated liver steatosis and concomitant decreases in plasma VLDL-TG, phenotypes that manifest only under lipogenic conditions. I148M-knockin mice similarly displayed impaired hepatic TG secretion during lipogenic stimulation. Our results highlight a specific context whereby PNPLA3 facilitates the balance between hepatic TG storage and secretion and suggest the potential contributions of I148M variant loss-of-function to the development of hepatic steatosis in humans.

Summary Statement

We define the physiological role of wild type PNPLA3 in maintaining hepatic VLDL-TG secretion.

Lipid droplet biogenesis and functions in health and disease

Ubiquitous yet unique, lipid droplets are intracellular organelles that are increasingly being recognized for their versatility beyond energy storage. Advances uncovering the intricacies of their biogenesis and the diversity of their physiological and pathological roles have yielded new insights into lipid droplet biology. Despite these insights, the mechanisms governing the biogenesis and functions of lipid droplets remain incompletely understood. Moreover, the causal relationship between the biogenesis and function of lipid droplets and human diseases is poorly resolved. Here, we provide an update on the current understanding of the biogenesis and functions of lipid droplets in health and disease, highlighting a key role for lipid droplet biogenesis in alleviating cellular stresses. We also discuss therapeutic strategies of targeting lipid droplet biogenesis, growth or degradation that could be applied in the future to common diseases, such as cancer, hepatic steatosis and viral infection.

RASAL2 Deficiency Attenuates Hepatic Steatosis by Promoting Hepatic VLDL Secretion via the AKT/TET1/MTTP Axis

BACKGROUND AND AIMS

RAS protein activator like 2 (RASAL2) is a newly discovered metabolic regulator involved in energy homeostasis and adipogenesis. However, whether RASAL2 is involved in hepatic lipid metabolism remains undetermined. This study explored the function of RASAL2 and elucidated its potential mechanisms in nonalcoholic fatty liver disease (NAFLD).

METHODS

NAFLD models were established either by feeding mice a high-fat diet or by incubation of hepatocytes with 1 mM free fatty acids (oleic acid:palmitic acid=2:1). Pathological changes were observed by hematoxylin and eosin staining. Lipid accumulation was assessed by Oil Red O staining, BODIPY493/503 staining, and triglyceride quantification. The in vivo secretion rate of very low-density lipoprotein was determined by intravenous injection of tyloxapol. Gene regulation was analyzed by chromatin immunoprecipitation assays and hydroxymethylated DNA immunoprecipitation combined with real-time polymerase chain reaction.

RESULTS

RASAL2 deficiency ameliorated hepatic steatosis both in vivo and in vitro. Mechanistically, RASAL2 deficiency upregulated hepatic TET1 expression by activating the AKT signaling pathway and thereby promoted MTTP expression by DNA hydroxymethylation, leading to increased production and secretion of very low-density lipoprotein, which is the major carrier of triglycerides exported from the liver to distal tissues.

CONCLUSIONS

Our study reports the first evidence that RASAL2 deficiency ameliorates hepatic steatosis by regulating lipid metabolism through the AKT/TET1/MTTP axis. These findings will help understand the pathogenesis of NAFLD and highlight the potency of RASAL2 as a new molecular target for NAFLD.

A human iPSC-derived hepatocyte screen identifies compounds that inhibit production of Apolipoprotein B

Familial hypercholesterolemia (FH) patients suffer from excessively high levels of Low Density Lipoprotein Cholesterol (LDL-C), which can cause severe cardiovascular disease. Statins, bile acid sequestrants, PCSK9 inhibitors, and cholesterol absorption inhibitors are all inefficient at treating FH patients with homozygous LDLR gene mutations (hoFH). Drugs approved for hoFH treatment control lipoprotein production by regulating steady-state Apolipoprotein B (apoB) levels. Unfortunately, these drugs have side effects including accumulation of liver triglycerides, hepatic steatosis, and elevated liver enzyme levels. To identify safer compounds, we used an iPSC-derived hepatocyte platform to screen a structurally representative set of 10,000 small molecules from a proprietary library of 130,000 compounds. The screen revealed molecules that could reduce the secretion of apoB from cultured hepatocytes and from humanized livers in mice. These small molecules are highly effective, do not cause abnormal lipid accumulation, and share a chemical structure that is distinct from any known cholesterol lowering drug.

In vivo functional characterization of EGFR variants identifies novel drivers of glioblastoma

BACKGROUND

Glioblastoma is the most common and aggressive primary brain tumor. Large-scale sequencing initiatives have cataloged its mutational landscape in hopes of elucidating mechanisms driving this deadly disease. However, a major bottleneck in harnessing this data for new therapies is deciphering "driver" and "passenger" events amongst the vast volume of information.

METHODS

We utilized an autochthonous, in vivo screening approach to identify driver, EGFR variants. RNA-Seq identified unique molecular signatures of mouse gliomas across these variants, which only differ by a single amino acid change. In particular, we identified alterations to lipid metabolism, which we further validated through an unbiased lipidomics screen.

RESULTS

Our screen identified A289I as the most potent EGFR variant, which has previously not been characterized. One of the mechanisms through which A289I promotes gliomagenesis is to alter cellular triacylglycerides through MTTP. Knockout of Mttp in mouse gliomas, reduces gliomagenesis in multiple models.

CONCLUSIONS

EGFR variants that differ by a single amino acid residue differentially promote gliomagenesis. Among the identified mechanism that drives glioma growth include lipid metabolism through MTTP. Understanding triacylglyceride accumulation may present a prospective therapeutic pathway for this deadly disease.

Surf4 (Surfeit Locus Protein 4) Deficiency Reduces Intestinal Lipid Absorption and Secretion and Decreases Metabolism in Mice

BACKGROUND

Postprandial dyslipidemia is a causative risk factor for cardiovascular disease. The majority of absorbed dietary lipids are packaged into chylomicron and then delivered to circulation. Previous studies showed that Surf4 (surfeit locus protein 4) mediates very low-density lipoprotein secretion from hepatocytes. Silencing hepatic Surf4 markedly reduces the development of atherosclerosis in different mouse models of atherosclerosis without causing hepatic steatosis. However, the role of Surf4 in chylomicron secretion is unknown.

METHODS

We developed inducible intestinal-specific Surf4 knockdown mice (Surf4^IKO) using Vil1Cre-ER^T2 and Surf4^flox mice. Metabolic cages were used to monitor mouse metabolism. Enzymatic kits were employed to measure serum and tissue lipid levels. The expression of target genes was detected by qRT-PCR and Western Blot. Transmission electron microscopy and radiolabeled oleic acid were used to assess the structure of enterocytes and intestinal lipid absorption and secretion, respectively. Proteomics was performed to determine changes in protein expression in serum and jejunum.

RESULTS

Surf4^IKO mice, especially male Surf4^IKO mice, displayed significant body weight loss, increased mortality, and reduced metabolism. Surf4^IKO mice exhibited lipid accumulation in enterocytes and impaired fat absorption and secretion. Lipid droplets and small lipid vacuoles were accumulated in the cytosol and the endoplasmic reticulum lumen of the enterocytes of Surf4^IKO mice, respectively. Surf4 colocalized with apoB and co-immunoprecipitated with apoB48 in differentiated Caco-2 cells. Intestinal Surf4 deficiency also significantly reduced serum triglyceride, cholesterol, and free fatty acid levels in mice. Proteomics data revealed that diverse pathways were altered in Surf4^IKO mice. In addition, Surf4^IKO mice had mild liver damage, decreased liver size and weight, and reduced hepatic triglyceride levels.

CONCLUSIONS

Our findings demonstrate that intestinal Surf4 plays an essential role in lipid absorption and chylomicron secretion and suggest that the therapeutic use of Surf4 inhibition requires highly cell/tissue-specific targeting.

Surf4, cargo trafficking, lipid metabolism, and therapeutic implications

Surfeit 4 is a polytopic transmembrane protein that primarily resides in the endoplasmic reticulum (ER) membrane. It is ubiquitously expressed and functions as a cargo receptor, mediating cargo transport from the ER to the Golgi apparatus via the canonical coat protein complex II (COPII)-coated vesicles or specific vesicles. It also participates in ER-Golgi protein trafficking through a tubular network. Meanwhile, it facilitates retrograde transportation of cargos from the Golgi apparatus to the ER through COPI-coated vesicles. Surf4 can selectively mediate export of diverse cargos, such as PCSK9 very low-density lipoprotein (VLDL), progranulin, α1-antitrypsin, STING, proinsulin, and erythropoietin. It has been implicated in facilitating VLDL secretion, promoting cell proliferation and migration, and increasing replication of positive-strand RNA viruses. Therefore, Surf4 plays a crucial role in various physiological and pathophysiological processes and emerges as a promising therapeutic target. However, the molecular mechanisms by which Surf4 selectively sorts diverse cargos for ER-Golgi protein trafficking remain elusive. Here, we summarize the most recent advances in Surf4, focusing on its role in lipid metabolism.

Expression of Synj2bp in mouse liver regulates the extent of wrappER-mitochondria contact to maintain hepatic lipid homeostasis

BACKGROUND

In mouse liver hepatocytes, nearly half of the surface area of every mitochondrion is covered by wrappER, a wrapping-type of ER that is rich in fatty acids and synthesizes lipoproteins (VLDL) (Anastasia et al. in Cell Rep 34:108873, 2021; Hurtley in Science (80- ) 372:142-143, 2021; Ilacqua et al. in J Cell Sci 135:1-11, 2021). A disruption of the ultrastructure of the wrappER-mitochondria contact results in altered fatty acid flux, leading to hepatic dyslipidemia (Anastasia et al. 2021). The molecular mechanism that regulates the extent of wrappER-mitochondria contacts is unknown.

METHODS

We evaluated the expression level of the mitochondrial protein Synj2bp in the liver of normal and obese (ob/ob) mice. In addition, we silenced its expression in the liver using an AAV8 vector. We coupled quantitative EM morphometric analysis to proteomics and lipid analyses on these livers.

RESULTS

The expression level of Synj2bp in the liver positively correlates with the extent of wrappER-mitochondria contacts. A 50% reduction in wrappER-mitochondria contacts causes hepatic dyslipidemia, characterized by a gross accumulation of lipid droplets in the liver, an increased hepatic secretion of VLDL and triglycerides, a curtailed ApoE expression, and an increased capacity of mitochondrial fatty acid respiration.

CONCLUSION

Synj2bp regulates the extent of wrappER-mitochondria contacts in the liver, thus contributing to the control of hepatic lipid flux.

Impact of NAFLD and its pharmacotherapy on lipid profile and CVD

Atherosclerotic cardiovascular disease (ASCVD) remains the leading cause of death worldwide. Increasing evidence suggests that, in addition to traditional metabolic risk factors such as obesity, hypercholesterolemia, hypertension, diabetes mellitus, and insulin resistance (IR), nonalcoholic fatty liver disease (NAFLD) is an emerging driver of ASCVD via multiple mechanisms, mainly by disrupting lipid metabolism. The lack of pharmaceutical treatment has spurred substantial investment in the research and development of NAFLD drugs. However, many reagents with promising therapeutic potential for NAFLD also have considerable impacts on the circulating lipid profile. In this review, we first summarize the mechanisms linking lipid dysregulation in NAFLD to the progression of ASCVD. Importantly, we highlight the potential risks of/benefits to ASCVD conferred by NAFLD pharmaceutical treatments and discuss potential strategies and next-generation drugs for treating NAFLD without the unwanted side effects.

The role of hepatic Surf4 in lipoprotein metabolism and the development of atherosclerosis in apoE-/- mice

Elevated plasma levels of low-density lipoprotein-C (LDL-C) increase the risk of atherosclerotic cardiovascular disease. Circulating LDL is derived from very low-density lipoprotein (VLDL) metabolism and cleared by LDL receptor (LDLR). We have previously demonstrated that cargo receptor Surfeit 4 (Surf4) mediates VLDL secretion. Inhibition of hepatic Surf4 impairs VLDL secretion, significantly reduces plasma LDL-C levels, and markedly mitigates the development of atherosclerosis in LDLR knockout (Ldlr^-/-) mice. Here, we investigated the role of Surf4 in lipoprotein metabolism and the development of atherosclerosis in another commonly used mouse model of atherosclerosis, apolipoprotein E knockout (apoE^-/-) mice. Adeno-associated viral shRNA was used to silence Surf4 expression mainly in the liver of apoE^-/- mice. In apoE^-/- mice fed a regular chow diet, knockdown of Surf4 expression significantly reduced triglyceride secretion and plasma levels of non-HDL cholesterol and triglycerides without causing hepatic lipid accumulation or liver damage. When Surf4 was knocked down in apoE^-/- mice fed the Western-type diet, we observed a significant reduction in plasma levels of non-HDL cholesterol, but not triglycerides. Knockdown of Surf4 did not increase hepatic cholesterol and triglyceride levels or cause liver damage, but significantly diminished atherosclerosis lesions. Therefore, our findings indicate the potential of hepatic Surf4 inhibition as a novel therapeutic strategy to reduce the risk of atherosclerotic cardiovascular disease.

TM6SF2: A Novel Genetic Player in Nonalcoholic Fatty Liver and Cardiovascular Disease

Transmembrane 6 superfamily member 2 (TM6SF2) is located on chromosome 19 (19p12) and encodes for a protein of undetermined function. Genetic studies have reported the association between a nonsynonymous variant in TM6SF2 (E167K, rs58542926) with hepatic triglyceride content and its impact on the cardiovascular system. Clinical and epidemiological studies have confirmed the role of TM6SF2 in the development of nonalcoholic fatty liver disease (NAFLD). Recently, TM6SF2 was also shown to play an important role in promoting hepatic fibrosis and hepatocellular cancer in mouse models. This review aims to capture the physiological role of TM6SF2 in the regulation of lipid metabolism and its involvement in cardiometabolic diseases.

A three-organelle complex made by wrappER contacts with peroxisomes and mitochondria responds to liver lipid flux changes

Hepatic lipid homeostasis depends on intracellular pathways that respire fatty acid in peroxisomes and mitochondria, and on systemic pathways that secrete fatty acid into the bloodstream, either free or condensed in very-low-density lipoprotein (VLDL) triglycerides. These systemic and intracellular pathways are interdependent, but it is unclear whether and how they integrate into a single cellular circuit. Here, we report that mouse liver wrappER, a distinct endoplasmic reticulum (ER) compartment with apparent fatty acid- and VLDL-secretion functions, connects peroxisomes and mitochondria. Correlative light electron microscopy, quantitative serial section electron tomography and three-dimensional organelle reconstruction analysis show that the number of peroxisome-wrappER-mitochondria complexes changes throughout fasting-to-feeding transitions and doubles when VLDL synthesis stops following acute genetic ablation of Mttp in the liver. Quantitative proteomic analysis of peroxisome-wrappER-mitochondria complex-enriched fractions indicates that the loss of Mttp upregulates global fatty acid β-oxidation, thereby integrating the dynamics of this three-organelle association into hepatic fatty acid flux responses. Therefore, liver lipid homeostasis occurs through the convergence of systemic and intracellular fatty acid-elimination pathways in the peroxisome-wrappER-mitochondria complex.

Enterocyte-specific ATGL overexpression affects intestinal and systemic cholesterol homeostasis

Enterocytes of the small intestine (SI) play an important role in maintaining systemic lipid levels by regulating dietary lipid absorption and postprandial lipoprotein secretion. An excessive amount of dietary-derived triglycerides (TGs) taken up by the apical side of enterocytes or basolaterally internalized lipoprotein remnants can be transiently stored in cytosolic lipid droplets (cLDs). As mice lacking adipose TG lipase (ATGL) in the SI display massive accumulation of cLDs but also delayed cholesterol absorption, we hypothesized that SI-specific overexpression of ATGL (Atgl iTg) might have beneficial effects on lipid homeostasis in the gut and possibly throughout the body. Here, we demonstrate that Atgl iTg mice had only modestly increased enzymatic activity despite drastically elevated Atgl mRNA levels (up to 120-fold) on chow diet, and was highly induced upon high-fat/high-cholesterol diet (HF/HCD) feeding. Atgl iTg mice showed markedly reduced intestinal TG concentrations after acute and chronic lipid challenge without affecting chylomicron TG secretion. Circulating plasma cholesterol levels were significantly lower in Atgl iTg mice under different feeding conditions, contrasting the accelerated uptake of dietary cholesterol into the circulation after HF/HCD feeding. In the fasted state, gene expression analysis revealed modulation of PPARα and liver X receptor (LXR) target genes by an increased fatty acid release, whereas the decreased plasma cholesterol concentrations in refed mice were more likely due to changes in HDL synthesis and secretion. We conclude that ATGL, in addition to its role in TG catabolism, plays a critical role in whole-body cholesterol homeostasis by modulating PPARα and LXR signaling in intestinal enterocytes.

Detailed Molecular Mechanisms Involved in Drug-Induced Non-Alcoholic Fatty Liver Disease and Non-Alcoholic Steatohepatitis: An Update

Non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) are some of the biggest public health challenges due to their spread and increasing incidence around the world. NAFLD is characterized by intrahepatic lipid deposition, accompanied by dyslipidemia, hypertension, and insulin resistance, leading to more serious complications. Among the various causes, drug administration for the treatment of numerous kinds of diseases, such as antiarrhythmic and antihypertensive drugs, promotes the onset and progression of steatosis, causing drug-induced hepatic steatosis (DIHS). Here, we reviewed in detail the major classes of drugs that cause DIHS and the specific molecular mechanisms involved in these processes. Eight classes of drugs, among the most used for the treatment of common pathologies, were considered. The most diffused mechanism whereby drugs can induce NAFLD/NASH is interfering with mitochondrial activity, inhibiting fatty acid oxidation, but other pathways involved in lipid homeostasis are also affected. PubMed research was performed to obtain significant papers published up to November 2021. The key words included the class of drugs, or the specific compound, combined with steatosis, nonalcoholic steatohepatitis, fibrosis, fatty liver and hepatic lipid deposition. Additional information was found in the citations listed in other papers, when they were not displayed in the original search.

Hepatic TM6SF2 Is Required for Lipidation of VLDL in a Pre-Golgi Compartment in Mice and Rats

BACKGROUND & AIMS

Substitution of lysine for glutamic acid at residu 167 in Transmembrane 6 superfamily member 2 (TM6SF2) is associated with fatty liver disease and reduced plasma lipid levels. Tm6sf2-/- mice replicate the human phenotype but were not suitable for detailed mechanistic studies. As an alternative model, we generated Tm6sf2-/- rats to determine the subcellular location and function of TM6SF2.

METHODS

Two lines of Tm6sf2-/- rats were established using gene editing. Lipids from tissues and from newly secreted very low density lipoproteins (VLDLs) were quantified using enzymatic assays and mass spectrometry. Neutral lipids were visualized in tissue sections using Oil Red O staining. The rate of dietary triglyceride (TG) absorption and hepatic VLDL-TG secretion were compared in Tm6sf2-/- mice and in their wild-type littermates. The intracellular location of TM6SF2 was determined by cell fractionation. Finally, TM6SF2 was immunoprecipitated from liver and enterocytes to identify interacting proteins.

RESULTS

Tm6sf2-/- rats had a 6-fold higher mean hepatic TG content (56.1 ± 28.9 9 vs 9.8 ± 3.9 mg/g; P < .0001) and lower plasma cholesterol levels (99.0 ± 10.5 vs 110.6 ± 14.0 mg/dL; P = .0294) than their wild-type littermates. Rates of appearance of dietary and hepatic TG into blood were reduced significantly in Tm6sf2-/- rats (P < .001 and P < .01, respectively). Lipid content of newly secreted VLDLs isolated from perfused livers was reduced by 53% (TG) and 62% (cholesterol) (P = .005 and P = .01, respectively) in Tm6sf2-/- mice. TM6SF2 was present predominantly in the smooth endoplasmic reticulum and endoplasmic reticulum-Golgi intermediate compartments, but not in Golgi. Both apolipoprotein B-48 and acyl-CoA synthetase long chain family member 5 physically interacted with TM6SF2.

CONCLUSIONS

TM6SF2 acts in the smooth endoplasmic reticulum to promote bulk lipidation of apolipoprotein B-containing lipoproteins, thus preventing fatty liver disease.

Hypomorphic ASGR1 modulates lipid homeostasis via INSIG1-mediated SREBP signaling suppression

A population genetic study identified that the asialoglycoprotein receptor 1 (ASGR1) mutation carriers had substantially lower non–HDL-cholesterol (non–HDL-c) levels and reduced risks of cardiovascular diseases. However, the mechanism behind this phenomenon remained unclear. Here, we established Asgr1-knockout mice that represented a plasma lipid profile with significantly lower non–HDL-c and triglyceride (TG) caused by decreased secretion and increased uptake of VLDL/LDL. These 2 phenotypes were linked with the decreased expression of microsomal triglyceride transfer protein and proprotein convertase subtilisin/kexin type 9, 2 key targeted genes of sterol regulatory element–binding proteins (SREBPs). Furthermore, there were fewer nuclear SREBPs (nSREBPs) on account of more SREBPs being trapped in endoplasmic reticulum, which was caused by an increased expression of insulin-induced gene 1 (INSIG1), an anchor of SREBPs. Overexpression and gene knockdown interventions, in different models, were conducted to rescue the ASGR1-deficient phenotypes, and we found that INSIG1 knockdown independently reversed the ASGR1-mutated phenotypes with increased serum total cholesterol, LDL-c, TG, and liver cholesterol content accompanied by restored SREBP signaling. ASGR1 rescue experiments reduced INSIG1 and restored the SREBP network defect as manifested by improved apolipoprotein B secretion and reduced LDL uptake. Our observation demonstrated that increased INSIG1 is a critical factor responsible for ASGR1 deficiency–associated lipid profile changes and nSREBP suppression. This finding of an ASGR1/INSIG1/SREBP axis regulating lipid hemostasis may provide multiple potential targets for lipid-lowering drug development.

Liver-Specific Deletion of Mouse Tm6sf2 Promotes Steatosis, Fibrosis, and Hepatocellular Cancer

BACKGROUND AND AIMS

Human transmembrane 6 superfamily 2 (TM6SF2) variant rs58542926 is associated with NAFLD and HCC. However, conflicting reports in germline Tm6sf2 knockout mice suggest no change or decreased very low density lipoprotein (VLDL) secretion and either unchanged or increased hepatic steatosis, with no increased fibrosis. We generated liver-specific Tm6Sf2 knockout mice (Tm6 LKO) to study VLDL secretion and the impact on development and progression of NAFLD.

APPROACH AND RESULTS

Two independent lines of Tm6 LKO mice exhibited spontaneous hepatic steatosis. Targeted lipidomic analyses showed increased triglyceride species whose distribution and abundance phenocopied findings in mice with liver-specific deletion of microsomal triglyceride transfer protein. The VLDL triglyceride secretion was reduced with small, underlipidated particles and unchanged or increased apolipoprotein B. Liver-specific adeno-associated viral, serotype 8 (AAV8) rescue using either wild-type or mutant E167K-Tm6 reduced hepatic steatosis and improved VLDL secretion. The Tm6 LKO mice fed a high milk-fat diet for 3 weeks exhibited increased steatosis and fibrosis, and those phenotypes were further exacerbated when mice were fed fibrogenic, high fat/fructose diets for 20 weeks. In two models of HCC, either neonatal mice injected with streptozotocin (NASH/STAM) and high-fat fed or with diethylnitrosamine injection plus fibrogenic diet feeding, Tm6 LKO mice exhibited increased steatosis, greater tumor burden, and increased tumor area versus Tm6 flox controls. Additionally, diethylnitrosamine-injected and fibrogenic diet-fed Tm6 LKO mice administered wild-type Tm6 or E167K-mutant Tm6 AAV8 revealed significant tumor attenuation, with tumor burden inversely correlated with Tm6 protein levels.

CONCLUSIONS

Liver-specific Tm6sf2 deletion impairs VLDL secretion, promoting hepatic steatosis, fibrosis, and accelerated development of HCC, which was mitigated with AAV8- mediated rescue.

Brain-gut-liver interactions across the spectrum of insulin resistance in metabolic fatty liver disease

Metabolic associated fatty liver disease (MAFLD), formerly named "nonalcoholic fatty liver disease" occurs in about one-third of the general population of developed countries worldwide and behaves as a major morbidity and mortality risk factor for major causes of death, such as cardiovascular, digestive, metabolic, neoplastic and neuro-degenerative diseases. However, progression of MAFLD and its associated systemic complications occur almost invariably in patients who experience the additional burden of intrahepatic and/or systemic inflammation, which acts as disease accelerator. Our review is focused on the new knowledge about the brain-gut-liver axis in the context of metabolic dysregulations associated with fatty liver, where insulin resistance has been assumed to play an important role. Special emphasis has been given to digital imaging studies and in particular to positron emission tomography, as it represents a unique opportunity for the noninvasive in vivo study of tissue metabolism. An exhaustive revision of targeted animal models is also provided in order to clarify what the available preclinical evidence suggests for the causal interactions between fatty liver, dysregulated endogenous glucose production and insulin resistance.

TMEM41B acts as an ER scramblase required for lipoprotein biogenesis and lipid homeostasis

How amphipathic phospholipids are shuttled between the membrane bilayer remains an essential but elusive process, particularly at the endoplasmic reticulum (ER). One prominent phospholipid shuttling process concerns the biogenesis of APOB-containing lipoproteins within the ER lumen, which may require bulk trans-bilayer movement of phospholipids from the cytoplasmic leaflet of the ER bilayer. Here, we show that TMEM41B, present in the lipoprotein export machinery, encodes a previously conceptualized ER lipid scramblase mediating trans-bilayer shuttling of bulk phospholipids. Loss of hepatic TMEM41B eliminates plasma lipids, due to complete absence of mature lipoproteins within the ER, but paradoxically also activates lipid production. Mechanistically, scramblase deficiency triggers unique ER morphological changes and unsuppressed activation of SREBPs, which potently promotes lipid synthesis despite stalled secretion. Together, this response induces full-blown nonalcoholic hepatosteatosis in the TMEM41B-deficient mice within weeks. Collectively, our data uncovered a fundamental mechanism safe-guarding ER function and integrity, dysfunction of which disrupts lipid homeostasis.

Metabolic-associated fatty liver disease and lipoprotein metabolism

BACKGROUND

Non-alcoholic fatty liver disease, or as recently proposed 'metabolic-associated fatty liver disease' (MAFLD), is characterized by pathological accumulation of triglycerides and other lipids in hepatocytes. This common disease can progress from simple steatosis to steatohepatitis, and eventually end-stage liver diseases. MAFLD is closely related to disturbances in systemic energy metabolism, including insulin resistance and atherogenic dyslipidemia.

SCOPE OF REVIEW

The liver is the central organ in lipid metabolism by secreting very low density lipoproteins (VLDL) and, on the other hand, by internalizing fatty acids and lipoproteins. This review article discusses recent research addressing hepatic lipid synthesis, VLDL production, and lipoprotein internalization as well as the lipid exchange between adipose tissue and the liver in the context of MAFLD.

MAJOR CONCLUSIONS

Liver steatosis in MAFLD is triggered by excessive hepatic triglyceride synthesis utilizing fatty acids derived from white adipose tissue (WAT), de novo lipogenesis (DNL) and endocytosed remnants of triglyceride-rich lipoproteins. In consequence of high hepatic lipid content, VLDL secretion is enhanced, which is the primary cause of complex dyslipidemia typical for subjects with MAFLD. Interventions reducing VLDL secretory capacity attenuate dyslipidemia while they exacerbate MAFLD, indicating that the balance of lipid storage versus secretion in hepatocytes is a critical parameter determining disease outcome. Proof of concept studies have shown that promoting lipid storage and energy combustion in adipose tissues reduces hepatic lipid load and thus ameliorates MAFLD. Moreover, hepatocellular triglyceride synthesis from DNL and WAT-derived fatty acids can be targeted to treat MAFLD. However, more research is needed to understand how individual transporters, enzymes, and their isoforms affect steatosis and dyslipidemia in vivo, and whether these two aspects of MAFLD can be selectively treated. Processing of cholesterol-enriched lipoproteins appears less important for steatosis. It may, however, modulate inflammation and consequently MAFLD progression.

Atherosclerosis-associated hepatic secretion of VLDL but not PCSK9 is dependent on cargo receptor protein Surf4

Plasma LDL is produced from catabolism of VLDL and cleared from circulation mainly via the hepatic LDL receptor (LDLR). Proprotein convertase subtilisin/kexin type 9 (PCSK9) promotes LDLR degradation, increasing plasma LDL-C levels. Circulating PCSK9 is mainly secreted by the liver, whereas VLDL is exclusively secreted by hepatocytes. However, the mechanism regulating their secretion is not completely understood. Surfeit 4 (Surf4) is a cargo receptor localized in the ER membrane. It recruits cargos into coat protein complex II vesicles to facilitate their secretion. Here, we investigated the role of Surf4 in VLDL and PCSK9 secretion. We generated Surf4 liver-specific knockout mice and found that knockout of Surf4 did not affect PCSK9 secretion, whereas it significantly reduced plasma levels of cholesterol, triglyceride, and lipid-binding protein apolipoprotein B (apoB). In cultured human hepatocytes, Surf4 coimmunoprecipitated and colocalized with apolipoprotein B100, and Surf4 silencing reduced secretion of apolipoprotein B100. Furthermore, knockdown of Surf4 in LDLR knockout (Ldlr^−/−) mice significantly reduced triglyceride secretion, plasma levels of apoB and non-HDL-C, and the development of atherosclerosis. However, Surf4 liver-specific knockout mice and Surf4 knockdown in Ldlr^−/− mice displayed similar levels of liver lipids and plasma alanine aminotransferase activity as control mice, indicating that inhibition of Surf4 does not cause notable liver damage. Expression of stearoyl-CoA desaturase-1 was also reduced in the liver of these mice, suggesting a reduction in de novo lipogenesis. In summary, hepatic deficiency of Surf4 reduced VLDL secretion and the development of atherosclerosis but did not cause significant hepatic lipid accumulation or liver damage.

Lipid Droplets as Regulators of Metabolism and Immunity

Abstract

A hallmark of sterile and nonsterile inflammation is the increased accumulation of cytoplasmic lipid droplets (LDs) in non-adipose cells. LDs are ubiquitous organelles specialized in neutral lipid storage and hydrolysis. Originating in the ER, LDs are comprised of a core of neutral lipids (cholesterol esters, triglycerides) surrounded by a phospholipid monolayer and several LD-associated proteins. The perilipin (PLIN1-5) family are the most abundant structural proteins present on the surface of LDs. While PLIN1 is primarily expressed in adipocytes, PLIN2 and PLIN3 are ubiquitously expressed. LDs also acquire a host of enzymes and proteins that regulate LD metabolism. Amongst these are neutral lipases and selective lipophagy factors that promote hydrolysis of LD-associated neutral lipid. In addition, LDs physically associate with other organelles such as mitochondria through inter-organelle membrane contact sites that facilitate lipid transport. Beyond serving as a source of energy storage, LDs participate in inflammatory and infectious diseases, regulating both innate and adaptive host immune responses. Here, we review recent studies on the role of LDs in the regulation of immunometabolism.

Effects of Edible Insect Tenebrio molitor Larva Fermentation Extract as a Substitute Protein on Hepatosteatogenesis and Proteomic Changes in Obese Mice Induced by High-Fat Diet

Mealworms (Tenebrio molitor larva) are an edible insect and a protein-rich food; however, research on mealworms as a substitute protein is insufficient. In this study, mealworm fermentation extract (TMP) was assessed as a replacement for soy protein (SP) in a control diet (CON) or a high-fat diet (HFD) of mice for 12 weeks. TMP substitution reduced body weight, body weight gain, body fat mass (perirenal and mesenteric), fat size, glucose intolerance, and insulin resistance compared to the HFD-SP group. TMP alleviated hepatic steatosis (lipid contents and lipid droplets) in high-fat-fed mice and down-regulated the PPARγ, CD36, and DGAT2 gene levels. Proteomic analysis showed that a HFD for 12 weeks up-regulated 20 proteins and down-regulated 17 proteins in mice fed SP. On the other hand, TMP reversed the protein profiles. TMP significantly down-regulated KHK, GLO1, ATP5H, SOD, and DDAH1 and up-regulated DLD, Mup1, CPS1, Ces3b, PDI, and HYOU1 compared to the HFD-SP group. These proteins are involved in the glucose, lipid, and amino acid metabolism, as well as in oxidative stress and endoplasmic reticulum stress. Thus, substituting SP for TMP helped improve HFD-induced obesity, steatosis, and insulin resistance in mice. These results suggest that TMP is a potential substitute for commonly used protein sources.

Mitochondria-rough-ER contacts in the liver regulate systemic lipid homeostasis

Contacts between organelles create microdomains that play major roles in regulating key intracellular activities and signaling pathways, but whether they also regulate systemic functions remains unknown. Here, we report the ultrastructural organization and dynamics of the inter-organellar contact established by sheets of curved rough endoplasmic reticulum closely wrapped around the mitochondria (wrappER). To elucidate the in vivo function of this contact, mouse liver fractions enriched in wrappER-associated mitochondria are analyzed by transcriptomics, proteomics, and lipidomics. The biochemical signature of the wrappER points to a role in the biogenesis of very-low-density lipoproteins (VLDL). Altering wrappER-mitochondria contacts curtails VLDL secretion and increases hepatic fatty acids, lipid droplets, and neutral lipid content. Conversely, acute liver-specific ablation of Mttp, the most upstream regulator of VLDL biogenesis, recapitulates this hepatic dyslipidemia phenotype and promotes remodeling of the wrappER-mitochondria contact. The discovery that liver wrappER-mitochondria contacts participate in VLDL biology suggests an involvement of inter-organelle contacts in systemic lipid homeostasis.

Receptor-Mediated ER Export of Lipoproteins Controls Lipid Homeostasis in Mice and Humans

Efficient delivery of specific cargos in vivo poses a major challenge to the secretory pathway, which shuttles products encoded by ∼30% of the genome. Newly synthesized protein and lipid cargos embark on the secretory pathway via COPII-coated vesicles, assembled by the GTPase SAR1 on the endoplasmic reticulum (ER), but how lipid-carrying lipoproteins are distinguished from the general protein cargos in the ER and selectively secreted has not been clear. Here, we show that this process is quantitatively governed by the GTPase SAR1B and SURF4, a high-efficiency cargo receptor. While both genes are implicated in lipid regulation in humans, hepatic inactivation of either mouse Sar1b or Surf4 selectively depletes plasma lipids to near-zero and protects the mice from atherosclerosis. These findings show that the pairing between SURF4 and SAR1B synergistically operates a specialized, dosage-sensitive transport program for circulating lipids, while further suggesting a potential translation to treat atherosclerosis and related cardio-metabolic diseases.

PRAP1 is a novel lipid-binding protein that promotes lipid absorption by facilitating MTTP-mediated lipid transport

Microsomal triglyceride transfer protein (MTTP) is an endoplasmic reticulum resident protein that is essential for the assembly and secretion of triglyceride (TG)-rich, apoB-containing lipoproteins. Although the function and structure of mammalian MTTP have been extensively studied, how exactly MTTP transfers lipids to lipid acceptors and whether there are other biomolecules involved in MTTP-mediated lipid transport remain elusive. Here we identify a role in this process for the poorly characterized protein PRAP1. We report that PRAP1 and MTTP are partially colocalized in the endoplasmic reticulum. We observe that PRAP1 directly binds to TG and facilitates MTTP-mediated lipid transfer. A single amino acid mutation at position 85 (E85V) impairs PRAP1's ability to form a ternary complex with TG and MTTP, as well as impairs its ability to facilitate MTTP-mediated apoB-containing lipoprotein assembly and secretion, suggesting that the ternary complex formation is required for PRAP1 to facilitate MTTP-mediated lipid transport. PRAP1 is detectable in chylomicron/VLDL-rich plasma fractions, suggesting that MTTP recognizes PRAP1-bound TG as a cargo and transfers TG along with PRAP1 to lipid acceptors. Both PRAP1-deficient and E85V knock-in mutant mice fed a chow diet manifested an increase in the length of their small intestines, likely to compensate for challenges in absorbing lipid. Interestingly, both genetically modified mice gained significantly less body weight and fat mass when on high-fat diets compared with littermate controls and were prevented from hepatosteatosis. Together, this study provides evidence that PRAP1 plays an important role in MTTP-mediated lipid transport and lipid absorption.

Membrane-bound sn-1,2-diacylglycerols explain the dissociation of hepatic insulin resistance from hepatic steatosis in MTTP knockout mice

Microsomal triglyceride transfer protein (MTTP) deficiency results in a syndrome of hypolipidemia and accelerated NAFLD. Animal models of decreased hepatic MTTP activity have revealed an unexplained dissociation between hepatic steatosis and hepatic insulin resistance. Here, we performed comprehensive metabolic phenotyping of liver-specific MTTP knockout (L-Mttp-/-) mice and age-weight matched wild-type control mice. Young (10-12-week-old) L-Mttp-/- mice exhibited hepatic steatosis and increased DAG content; however, the increase in hepatic DAG content was partitioned to the lipid droplet and was not increased in the plasma membrane. Young L-Mttp-/- mice also manifested normal hepatic insulin sensitivity, as assessed by hyperinsulinemic-euglycemic clamps, no PKCε activation, and normal hepatic insulin signaling from the insulin receptor through AKT Ser/Thr kinase. In contrast, aged (10-month-old) L-Mttp-/- mice exhibited glucose intolerance and hepatic insulin resistance along with an increase in hepatic plasma membrane sn-1,2-DAG content and PKCε activation. Treatment with a functionally liver-targeted mitochondrial uncoupler protected the aged L-Mttp-/- mice against the development of hepatic steatosis, increased plasma membrane sn-1,2-DAG content, PKCε activation, and hepatic insulin resistance. Furthermore, increased hepatic insulin sensitivity in the aged controlled-release mitochondrial protonophore-treated L-Mttp-/- mice was not associated with any reductions in hepatic ceramide content. Taken together, these data demonstrate that differences in the intracellular compartmentation of sn-1,2-DAGs in the lipid droplet versus plasma membrane explains the dissociation of NAFLD/lipid-induced hepatic insulin resistance in young L-Mttp-/- mice as well as the development of lipid-induced hepatic insulin resistance in aged L-Mttp-/- mice.

Maternal green tea extract intake during lactation attenuates hepatic lipid accumulation in adult male rats exposed to a continuous high-fat diet from the foetal period

Background

Maternal lipid intake in the early postnatal period has a long-term effect on the possibility of fatty liver formation in children; besides, the importance of lipid consumption during lactation for children’s health has been suggested. Green tea extract (GTE) contains abundant catechins, and it has been reported to improve lipid metabolism and prevent fatty liver.

Objective

The aim of this study was to examine the effects of maternal GTE intake during lactation on hepatic lipid accumulation in adult male rats exposed to a continuous high-fat (HF) diet from the foetal period.

Methods

Pregnant Wistar rats received diets containing 13% (control-fat, CON) or 45% (high-fat, HF) fat. CON-fed mothers received the same diet during lactation, whereas HF-fed mothers received either HF diet alone or HF diet supplemented with 0.24% GTE. At weaning, male offspring were divided into three groups, i.e. CON/CON/CON, HF/HF/HF (HF-offspring) or HF/HF+GTE/HF (GTE-offspring), and were fed until 51 weeks.

Results

A significant hepatic triglyceride (Tg) accumulation was observed in the HF-offspring when compared with the other offspring. This is presumed to be caused by the promotion of Tg synthesis derived from exogenous fatty acid due to a significant increase in diacylglycerol O-acyltransferase 1 and a decrease in Tg expenditure caused by decreasing microsomal triglyceride transfer protein (MTTP) and long-chain acyl-CoA dehydrogenase. On the other hand, attenuated hepatic Tg accumulation was observed in the GTE-offspring. The levels of the hepatic lipid metabolism-related enzymes were improved to the same level as the CON-offspring, and particularly, MTTP was significantly increased as compared with the HF-offspring.

Conclusion

This study indicates the potential protective effects of maternal GTE intake during lactation on HF diet-induced hepatic lipid accumulation in adult male rat offspring and the possible underlying mechanisms.

Defects in High Density Lipoprotein metabolism and hepatic steatosis in mice with liver-specific ablation of Hepatocyte Nuclear Factor 4A

BACKGROUND

Aberrant concentration, structure and functionality of High Density Lipoprotein (HDL) are associated with many prevalent diseases, including cardiovascular disease and non-alcoholic fatty liver disease (NAFLD). Mice with liver-specific ablation of Hnf4α (H4LivKO) present steatosis and dyslipidemia by mechanisms that are not completely understood. The aim of this study was to explore the role of liver HNF4A in HDL metabolism and the development of steatosis.

METHODS AND RESULTS

Serum and tissue samples were obtained from 6-weeks old H4LivKO mice and their littermate controls. Liver and serum lipids were measured and HDL structure and functionality were assessed. Global gene expression changes in the liver were analyzed by expression arrays, validations were performed by RT-qPCR and DNA-protein interactions were studied by chromatin immunoprecipitation (ChIP). H4LivKO mice presented liver steatosis, increased liver triglyceride content and decreased concentration of serum total cholesterol, HDL cholesterol, triglycerides, phospholipids and cholesteryl esters. Most classes of phospholipids showed significant changes in species ratio and sphingosine-1-phosphate (S1P) levels were reduced. H4LivKO serum was enriched in the smaller, denser HDL particles, devoid of APOA2 and APOM apolipoproteins, exhibiting decreased activity of paraoxonase-1 but retaining macrophage cholesterol efflux capacity and phospho-AKT activation in endothelial cells. Global gene expression analysis revealed the association of liver HNF4A with known and novel regulators of HDL metabolism as well as NAFLD-susceptibility genes.

CONCLUSIONS

HNF4A ablation in mouse liver causes hepatic steatosis, perturbations in HDL structure and function and significant global changes in gene expression. This study reveals new targets of HNF4A involved in HDL metabolism and the development of steatosis and enriches our knowledge on HDL functionality in NAFLD.