Increased hepatic synthesis and dysregulation of cholesterol metabolism is associated with the severity of nonalcoholic fatty liver disease

Abstract

Nonalcoholic fatty liver disease (NAFLD) is associated with increased cardiovascular and liver-related mortality. NAFLD is characterized by both triglyceride and free cholesterol (FC) accumulation without a corresponding increment in cholesterol esters. The aim of this study was to evaluate the expression of cholesterol metabolic genes in NAFLD and relate these to disease phenotype. NAFLD was associated with increased SREBP-2 maturation, HMG CoA reductase (HMGCR) expression and decreased phosphorylation of HMGCR. Cholesterol synthesis was increased as measured by the circulating desmosterol:cholesterol ratio. miR-34a, a microRNA increased in NAFLD, inhibited sirtuin-1 with downstream dephosphorylation of AMP kinase and HMGCR. Cholesterol ester hydrolase was increased while ACAT-2 remained unchanged. LDL receptor expression was significantly decreased and similar in NAFLD subjects on or off statins. HMGCR expression was correlated with FC, histologic severity of NAFLD and LDL-cholesterol. These data demonstrate dysregulated cholesterol metabolism in NAFLD which may contribute to disease severity and cardiovascular risks.

FIGURE 1

mRNA expression of key genes associated with hepatic cholesterol metabolism. (A): obese normal controls had the same level of expression of HMGCR, SREBP-2, LDLR and LAL as lean normal controls. However, both NAFL and NASH were associated with a significantly increased HMGCR (* p< 0.001 for both vs. either control) and LAL (* p< 0.002 for both vs. either control) expression and decreased LDLR expression (* p< 0.01 for both NAFL or NASH vs. either control), (B): NAFL and NASH were associated with significantly elevated nCEH expression (* p< 0.0001 for both vs. lean or obese controls). Obese controls, NAFL and NASH had a significantly increased CYP7A expression compared to lean controls (* p< 0.02), (C): Compared to lean and obese normal controls which were similar, NAFL and NASH were associated with a significant decrease in expression of ABCA1, ABCG1 and ABCG8 (* p< 0.05 for all). Mean ± S.D. shown for all data.

FIGURE 2

Western blot analysis and graphic summary of protein expression (mean ± S.D.) of HMGCR, LAL, SREBP-2 and LDLR in lean controls, obese controls, NAFL and NASH (panels A and B respectively), nCEH, ACAT2, CYP7A and CYP27A (panels C and D), and ABCA1, ABCG1 and ABCG8 (panels E and F). β-actin was used as a loading control for all except SREBP-2 which was performed on nuclear extracts and where lamin B was used as the loading control. NAFL and NASH were associated with a highly significant increase in HMGCR (* p< 0.0001 for both). SREBP-2 levels were significantly higher in NASH compared to controls (* p< 0.02). LDLR was significantly decreased in both NAFL and NASH (* p< 0.03 for both compared to controls) while LAL was increased (p< 0.02 for both compared to controls). NAFL and NASH were also associated with significantly increased nCEH (* p< 0.01 for both vs. controls) along with decreased expression of CYP7A (* p< 0.05 for both). There was a stepwise decrease in ABCG8 expression from normal to NAFL to NASH (p< 0.05 for NASH vs. lean and obese controls). CYP27A was decreased in NASH alone compared to controls and NAFL (p< 0.02 vs. both controls and NAFL).

FIGURE 3

Relationship of HMGCR expression versus FC (A) in 8 subjects with NAFLD and 8 controls where enough liver tissue for simultaneous FC measurement, mRNA and protein analysis was available. There was a strong and direct relationship between HMGCR and hepatic FC (p< 0.0001). The relationship of HMGCR and cytologic ballooning scores (B) and the NAFLD activity score (C) in subjects with NAFL or NASH are also shown. There was also a strong and direct relationship to cytologic ballooning and the overall NAFLD activity score (NAS). There was no relationship to other features of NASH (data not shown).

FIGURE 4

NAFL and NASH were associated with less phosphorylation of the HMGCR compared to lean or obese controls (Western blot of representative subjects (A) and graphic summary of data (n=6 for each group), p< 0.05 for both (B). In another set of experiments, miR-34a, a microRNA that is overexpressed in NASH, was over-expressed or silenced in Huh-7 cells (panels C and D). Over-expression of miR-34a suppressed Sirt1 and the phosphorylation of downstream AMP kinase and HMGCR. Silencing miR-34a had the opposite effect. These data indicate that miR-34a can modulate the phosphorylation of HMGCR. To further determine if these changes were reflected in increased cholesterol synthetic activity, the circulating desmosterol to cholesterol ratio was measured in subjects with NAFLD (n=10) versus lean controls (n=10) (E). The desmosterol:cholesterol ratio in subjects with NAFLD was almost double that in controls confirming that there was increased cholesterol synthetic activity. The sitosterol:cholesterol ratios were not significantly different indicating that cholesterol absorption was not significantly different. Mean ± S.D. shown for all graphical data.

FIGURE 5

Circulating LDL-C was directly related to HMGCR expression levels (A) and inversely related to LDLR expression (B) (data for all subjects from all groups shown). Subjects with NASH who were on statins for more than six months were compared to those who were not on statins. HMGCR and LDLR protein levels, shown as a Western blot in three representative subjects each (C), were similar in both groups (shown graphically for six subjects each in panel D). mRNA levels of HMGCR and SREBP-2 were paradoxically lower in subjects on statins (p< 0.05 for both) compared to those not on statins (E). Data for 6 subjects on statins were compared to 20 subjects not on statins. Mean ± S.D. shown for all graphical data.

FIGURE 6

A schematic showing the key molecular changes in cholesterol metabolic pathways in NASH compared to lean or obese controls. Increased HMGCR and nCEH expression would be expected to increase the FC pool. Utilization of FC for bile acid synthesis is decreased due to decreased CYP7A and CYP27A expression. ABCG1 and ABCG8 expression are also decreased which would be expected to decrease export of cholesterol from hepatocytes.