Hepatic Free Cholesterol Accumulation Research Articles

Investigating microRNAs to Explain the Link between Cholesterol Metabolism and NAFLD in Humans: A Systematic Review.

Non-Alcoholic Fatty Liver Disease (NAFLD) is characterized by hepatic free cholesterol accumulation. In addition, microRNAs (miRNAs) might be involved in NAFLD development. Therefore, we systematically reviewed the literature to examine the link between miRNAs and cholesterol metabolism in NAFLD. Nineteen studies were retrieved by a systematic search in September 2022. From these papers, we evaluated associations between 13 miRNAs with NAFLD and cholesterol metabolism. Additionally, their diagnostic potential was examined. Four miRNAs (miR122, 34a, 132 and 21) were associated with cholesterol metabolism and markers for NAFLD. MiR122 was upregulated in serum of NAFLD patients, increased with disease severity and correlated with HDL-C, TAG, VLDL-C, AST, ALT, ALP, lobular inflammation, hepatocellular ballooning and NAFLD score. Serum and hepatic levels also correlated. Serum and hepatic miR34a levels were increased in NAFLD, and correlated with VLDL-C and TAG. Serum miR379 was also higher in NAFLD, especially in early stages, while miR21 gave ambiguous results. The diagnostic properties of these miRNAs were comparable to those of existing biomarkers. However, serum miR122 levels appeared to be elevated before increases in ALT and AST were evident. In conclusion, miR122, miR34a, miR21 and miR132 may play a role in the development of NAFLD via effects on cholesterol metabolism. Furthermore, it needs to be explored if miRNAs 122, 34a and 379 could be used as part of a panel in addition to established biomarkers in early detection of NAFLD.

A high-fat/high-cholesterol diet, but not high-cholesterol alone, increases free cholesterol and apoE-rich HDL serum levels in rats and upregulates hepatic ABCA1 expression

A high-fat/high-cholesterol (HFC) diet, but not a high-cholesterol (HC) diet, is known to induce elevated serum apolipoprotein E (apoE)-rich high-density lipoprotein (HDL) levels in animal models. However, the exact mechanisms by which the combination of dietary fat and cholesterol induces apoE-rich HDL production is not well understood. Here, we investigated the effects of dietary fat and cholesterol on serum lipoprotein profiles and hepatic mRNA expression that are associated with HDL production, cholesterol transport, and bile acid metabolism. Male Sprague-Dawley rats were fed HFC, HC, high-fat, or control diets and then evaluated. The HFC diet induced significant increases in hepatic free-cholesterol accumulation (1.4-fold, p < 0.01) and serum apoE-rich HDL cholesterol (8.7-fold, p < 0.001) levels compared with the HC diet. The apoE-rich HDL induced by the HFC diet was remarkably rich in free cholesterol. Liver gene-expression was mostly similar between the HC and HFC diet groups. However, there was a significant increase of ATP-binding cassette transporter A1 (ABCA1) levels in the HFC group compared to the HC group for both mRNA (1.9-fold, p < 0.001) and protein (6.6-fold, p < 0.01). These results suggest that an increase in apoE-rich HDL induced by dietary fat and cholesterol may be involved in cholesterol efflux from the liver through increased ABCA1-mediated free-cholesterol efflux.

Cholesterol Homeostasis: An In Silico Investigation into How Aging Disrupts Its Key Hepatic Regulatory Mechanisms.

Simple SummaryCardiovascular disease is the leading cause of death worldwide. High blood cholesterol levels are associated with an increased risk of this condition. How high levels of blood cholesterol result in cardiovascular disease during aging is a difficult question to answer. Computational models have been used over the years to help understand this problem, as they are capable of representing complex systems such as this. The aim of this work was to use a computational model of cholesterol metabolism to better understand why, in certain studies involving the oldest old (persons ≥ 85 years), high blood cholesterol levels have been associated with a decreased risk of death. Using our computational model, we found that key age-associated changes to how cholesterol is processed in the liver could be responsible for this observation. The findings from our work contribute to our understanding of cholesterol metabolism in older people and how treatments could be developed in the future to promote healthy aging.The dysregulation of intracellular cholesterol homeostasis is associated with several age-related diseases, most notably cardiovascular disease (CVD). Research in this area has benefitted from using computational modelling to study the inherent complexity associated with the regulation of this system. In addition to facilitating hypothesis exploration, the utility of modelling lies in its ability to represent an array of rate limiting enzymatic reactions, together with multiple feedback loops, which collectively define the dynamics of cholesterol homeostasis. However, to date no model has specifically investigated the effects aging has on this system. This work addresses this shortcoming by explicitly focusing on the impact of aging on hepatic intracellular cholesterol homeostasis. The model was used to investigate the experimental findings that reactive oxygen species induce the total activation of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (HMGCR). Moreover, the model explored the impact of an age-related decrease in hepatic acetyl-CoA acetyltransferase 2 (ACAT2). The model suggested that an increase in the activity of HMGCR does not have as significant an impact on cholesterol homeostasis as a decrease in hepatic ACAT2 activity. According to the model, a decrease in the activity of hepatic ACAT2 raises free cholesterol (FC) and decreases low-density lipoprotein cholesterol (LDL-C) levels. Increased acetyl CoA synthesis resulted in a reduction in the number of hepatic low-density lipoprotein receptors, and increased LDL-C, FC, and cholesterol esters. The rise in LDL-C was restricted by elevated hepatic FC accumulation. Taken together these findings have important implications for healthspan. This is because emerging clinical data suggest hepatic FC accumulation is relevant to the pathogenesis of non-alcoholic fatty liver disease (NAFLD), which is associated with an increased risk of CVD. These pathophysiological changes could, in part, help to explain the phenomenon of increased mortality associated with low levels of LDL-C which have been observed in certain studies involving the oldest old (≥85 years).

Dicer1/miR-29/HMGCR axis contributes to hepatic free cholesterol accumulation in mouse non-alcoholic steatohepatitis.

Dicer1 is an enzyme essential for microRNA (miRNA) maturation. The loss of miRNAs resulted from Dicer1 deficiency greatly contributes to the progression of many diseases, including lipid dysregulation, but its role in hepatic accumulation of free cholesterol (FC) that is critical in the development of non-alcoholic steatohepatitis (NASH) remains elusive. In this study, we used the liver-specific Dicer1-knockout mice to identify the miRNAs involved in hepatic FC accumulation. In a widely used dietary NASH model, mice were fed a methionine-choline-deficient (MCD) diet for 3 weeks, which resulted in significant increase in hepatic FC levels as well as decrease of Dicer1 mRNA levels in livers. The liver-specific Dicer1-knockout induced hepatic FC accumulation at 5-6 weeks, accompanied by increased mRNA and protein levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR), a rate-limiting enzyme of cholesterol synthesis in livers. Eleven predicted miRNAs were screened, revealing that miR-29a/b/c significantly suppressed HMGCR expression by targeting the HMGCR mRNA 3'-UTR. Overexpression of miR-29a in SMMC-7721 cells, a steatosis hepatic cell model, significantly decreased HMGCR expression and the FC level. Furthermore, the expression levels of miR-29a were inversely correlated with HMGCR expression levels in the MCD diet mouse model in vivo and in 2 steatosis hepatic cell models (SMMC-7721 and HL-7702 cells) in vitro. Our results show that Dicer1/miR-29/HMGCR axis contributes to hepatic free cholesterol accumulation in mouse NASH, and miR-29 may serve as an important regulator of hepatic cholesterol homeostasis. Thus, miR-29a could be utilized as a potential therapeutic target for the treatment of non-alcoholic fatty liver disease as well as for other liver diseases associated with FC accumulation.

Sortilin 1 Modulates Hepatic Cholesterol Lipotoxicity in Mice via Functional Interaction with Liver Carboxylesterase 1

The liver plays a key role in cholesterol metabolism. Impaired hepatic cholesterol homeostasis causes intracellular free cholesterol accumulation and hepatocyte injury. Sortilin 1 (SORT1) is a lysosomal trafficking receptor that was identified by genome-wide association studies (GWAS) as a novel regulator of cholesterol metabolism in humans. Here we report that SORT1 deficiency protected against cholesterol accumulation-induced liver injury and inflammation in mice. Using an LC-MS/MS-based proteomics approach, we identified liver carboxylesterase 1 (CES1) as a novel SORT1-interacting protein. Mechanistic studies further showed that SORT1 may regulate CES1 lysosomal targeting and degradation and that SORT1 deficiency resulted in higher liver CES1 protein abundance. Previous studies have established an important role of hepatic CES1 in promoting intracellular cholesterol mobilization, cholesterol efflux, and bile acid synthesis. Consistently, high cholesterol atherogenic diet-challenged Sort1 knock-out mice showed less hepatic free cholesterol accumulation, increased bile acid synthesis, decreased biliary cholesterol secretion, and the absence of gallstone formation. SORT1 deficiency did not alter hepatic ceramide and fatty acid metabolism in high cholesterol atherogenic diet-fed mice. Finally, knockdown of liver CES1 in mice markedly increased the susceptibility to high cholesterol diet-induced liver injury and abolished the protective effect against cholesterol lipotoxicity in Sort1 knock-out mice. In summary, this study identified a novel SORT1-CES1 axis that regulates cholesterol-induced liver injury, which provides novel insights that improve our current understanding of the molecular links between SORT1 and cholesterol metabolism. This study further suggests that therapeutic inhibition of SORT1 may be beneficial in improving hepatic cholesterol homeostasis in metabolic and inflammatory liver diseases.

ALTERACIONES EN LA HOMEOSTASIS DEL COLESTEROL HEPÁTICO Y SUS IMPLICACIONES EN LA ESTEATOHEPATITIS NO ALCOHÓLICA

Several studies have shown that hepatic free cholesterol (FC) has an important role in the pathogenesis of nonalcoholic steatohepatitis (NASH). These studies have provided evidence that hepatic FC accumulation is toxic at different levels including: mitochondrial oxidative injury, endoplasmic reticulum (ER) stress, and activation of Kupffer cells (KCs) and hepatic stellate cell (HSCs). Altogether, this suggests that hepatic FC content is important for the initiation, maintenance and modulation of the inflammatory response associated with NASH. In this review several mechanisms that participate in the regulation of cholesterol homeostasis and their possible implications in the development and progression of nonalcoholic fatty liver disease (NAFLD) are discussed.

Targeting the Enterohepatic Bile Acid Signaling to Modulate Hepatic Autophagic Activity in Non‐alcoholic steatohepatitis

Non‐alcoholic steatohepatitis is associated with hepatic free cholesterol accumulation and defective autophagy activity, which sensitize hepatocytes to injury and inflammation. Bile acid synthesis is the only quantitatively significant mechanism for hepatic cholesterol catabolism. Stimulating bile acid synthesis prevented hypercholesterolemia and hepatic steatosis in mice. Autophagy delivers intracellularly stored cholesterol to the lysosome where cholesterol ester is hydrolyzed for subsequent cellular distribution, efflux and bile acid synthesis. The objective of this study is to investigate the molecular link between cholesterol and bile acid metabolism and autophagic activity in hepatocytes and in mouse models of fatty liver disease. Confocal microscopy, immunoblot and cathespin B activity assay showed that excessive intracellular free cholesterol accumulation caused by free cholesterol loading and inhibition of cholesterol esterification in the endoplasmic reticulum (ER) with an ACAT inhibitor blocked autophagosome/lysosome fusion and impaired lysosome function in cholesterol‐laden HepG2 cells. In contrast, cholesterol ester accumulation induced by purified human low‐density lipoprotein treatment increased lysosome function without inhibiting autophagy activity. In addition, bile acid treatment blocked autophagosome/lysosome fusion without affecting key autophagy gene expression in HepG2 cells. Interestingly, over‐expression of CYP7A1 by adenoviral gene delivery, which induced ER cholesterol catabolism and caused relative ER cholesterol depletion, strongly induced autophagy flux in both human hepatocytes and HepG2 cells. Based on this novel regulatory relationship between ER cholesterol sensing and autophagy activity, confocal microscopy, electron microscopy and immunoblot further revealed that cholestyramine administration strongly induced hepatic autophagy in lean mice, and restored hepatic autophagy activity and improved hepatic steatosis in obese mice. The cholestyramine‐mediated hepatic autophagy induction can be attributed to CYP7A1‐mediated ER cholesterol catabolism and decreased hepatic bile acid signaling. In conclusion, these results suggest that hepatic autophagy is regulated by an intracellular cholesterol sensing mechanism, and is an integral part of the complex regulatory network that maintains cholesterol homeostasis in hepatocytes. This study supports a new concept of targeting the enterohepatic bile acid‐CYP7A1‐cholesterol‐autophagy axis to selectively modulate hepatic autophagy activity in fatty liver disease.Support or Funding InformationNIDDK and NIAAA