LXR signaling pathways link cholesterol metabolism with risk for prediabetes and diabetes

Abstract

BACKGROUNDPreclinical studies suggest that cholesterol accumulation leads to insulin resistance. We previously reported that alterations in a monocyte cholesterol metabolism transcriptional network (CMTN) - suggestive of cellular cholesterol accumulation - were cross-sectionally associated with obesity and type 2 diabetes (T2D). Here, we sought to determine whether the CMTN alterations independently predict incident prediabetes/T2D risk, and correlate with cellular cholesterol accumulation.METHODSMonocyte mRNA expression of 11 CMTN genes was quantified among 934 Multi-Ethnic Study of Atherosclerosis (MESA) participants free of prediabetes/T2D; cellular cholesterol was measured in a subset of 24 monocyte samples.RESULTSDuring a median 6-year follow-up, lower expression of 3 highly correlated LXR target genes - ABCG1 and ABCA1 (cholesterol efflux) and MYLIP (cholesterol uptake suppression) - and not other CMTN genes, was significantly associated with higher risk of incident prediabetes/T2D. Lower expression of the LXR target genes correlated with higher cellular cholesterol levels (e.g., 47% of variance in cellular total cholesterol explained by ABCG1 expression). Further, adding the LXR target genes to overweight/obesity and other known predictors significantly improved prediction of incident prediabetes/T2D.CONCLUSIONThese data suggest that the aberrant LXR/ABCG1-ABCA1-MYLIP pathway (LAAMP) is a major T2D risk factor and support a potential role for aberrant LAAMP and cellular cholesterol accumulation in diabetogenesis.FUNDINGThe MESA Epigenomics and Transcriptomics Studies were funded by NIH grants 1R01HL101250, 1RF1AG054474, R01HL126477, R01DK101921, and R01HL135009. This work was supported by funding from NIDDK R01DK103531 and NHLBI R01HL119962.

Keywords: Cholesterol; Diabetes; Expression profiling; Metabolism.

Figure 1. Overview of the study design and baseline characteristics.

Prospective analyses to examine predictive effects of the CMTN on the incidence of prediabetes/T2D over a 6-year follow-up among 1,349 participants with euglycemia at Exam 5, using subsets of samples.

Figure 2. Expression of LXRα target genes predicts risk of prediabetes and T2D over a 6-year follow-up.

MESA participants with lower expression of LXRα target genes, ABCA1, ABCG1, and MYLIP (lowest tertile), in 2 independent sub-studies, the Initial Study (A) and the Replication Study (B), were more likely to develop prediabetes/T2D compared with those with greater expression (highest tertile). Cox proportional hazards regression models were used, adjusting for age, sex, race and ethnicity, cigarette smoking, physical activity level, BMI, triglycerides, HDL-cholesterol, and systolic blood pressure (SBP) in the full model. The x axis is in logarithmic scale.

Figure 3. Predictive effects of ABCG1 expression for incident prediabetes/T2D among 635 participants with euglycemia.

(A) Bar plot for hazard ratio of prediabetes/diabetes according to baseline obesity status and tertiles of ABCG1 expression, adjusting for age, sex, race and ethnicity, cigarette smoking, physical activity level, triglycerides, HDL-cholesterol, and SBP. Number of cases/number at risk is displayed for each cell. **P < 0.01, ***P < 0.001. (B) AUC–receiver operating characteristic curves for 4 models with or without ABCG1 expression. The AUC is Harrell’s C statistic from a Cox regression model. The full model includes BMI, along with age, sex, race and ethnicity, cigarette smoking, physical activity level, triglycerides, HDL-cholesterol, and SBP. Gluc, fasting glucose.

Figure 4. CMTN associations with monocyte total cellular cholesterol.

Added variable plots with adjustment of age, sex, and race and ethnicity are shown for correlation of 2 top CMTN principal components, PC1_CMTN and PC2_CMTN (A), or ABCG1 mRNA and protein expression (B), with total cholesterol in primary monocytes from a subset of 24 randomly selected participants. Partial Spearman’s r is reported. Total cholesterol was measured using gas chromatography and normalized to protein levels (Bradford assay). “Other” indicates age, sex, race and ethnicity, and batch effect. ABCG1 protein levels were measured by Western blot analysis.