Thyroid hormone-related regulation of gene expression in human fatty liver

Abstract

Context: Fatty liver is an important complication of obesity; however, regulatory mechanisms mediating altered gene expression patterns have not been identified.

Objective: The aim of the study was to identify novel transcriptional changes in human liver that could contribute to hepatic lipid accumulation and associated insulin resistance, type 2 diabetes, and nonalcoholic steatohepatitis.

Design: We evaluated gene expression in surgical liver biopsies from 13 obese (nine with type 2 diabetes) and five control subjects using Affymetrix U133A microarrays. PCR validation was performed in liver biopsies using an additional 16 subjects. We also tested thyroid hormone responses in mice fed chow or high-fat diet.

Setting: Recruitment was performed in an academic medical center.

Participants: Individuals undergoing elective surgery for obesity or gallstones participated in the study.

Results: The top-ranking gene set, down-regulated in obese subjects, was comprised of genes previously demonstrated to be positively regulated by T(3) in human skeletal muscle (n = 399; P < 0.001; false discovery rate = 0.07). This gene set included genes related to RNA metabolism (SNRPE, HNRPH3, TIA1, and SFRS2), protein catabolism (PSMA1, PSMD12, USP9X, IBE2B, USP16, and PCMT1), and energy metabolism (ATP5C1, COX7C, UQCRB). We verified thyroid hormone regulation of these genes in the liver after injection of C57BL/6J mice with T(3) (100 microg/100 g body weight); furthermore, T(3)-induced increases in expression of these genes were abolished by high-fat diet. In agreement, expression of these genes inversely correlated with liver fat content in humans.

Conclusions: These data suggest that impaired thyroid hormone action may contribute to altered patterns of gene expression in fatty liver.

Figure 1

Thyroid-responsive gene set is enriched with down-regulated genes in liver of obese humans. A, Heatmap showing liver expression of genes in different subcategories of the T₃-regulated gene set (genes regulated by T₃ in human skeletal muscle). Each column presents data for an individual subject, whereas each row represents an individual probe set. Expression values are colored by row-normalized values. Graphs demonstrate PCR verification of representative genes in liver of lean (open bars), obese (striped), and obese diabetic (black) subjects (mean ± sd). *, P < 0.05. Data for all genes are shown in Supplementary Table 2. B, Spearman correlation of liver fat content with expression of representative genes.

Figure 2

Genes in RNA metabolism, protein degradation, and energy metabolism categories of the thyroid-responsive gene set are up-regulated by T₃ in chow-fed mice. Basal levels of expression, as well as T₃-stimulated responses, are abolished by HFD in mice. Mean ± sd shown in bar graphs. *, P < 0.05.

Figure 3

Potential regulators of thyroid hormone action. A, Expression of DIO2 and DIO3 is decreased in liver of obese subjects. B, DIO1 activity is not significantly altered in obese vs. lean humans. Expression of PPARGC1A (C) and LEPR (D) isoforms, as determined by PCR, was decreased in subjects with obesity (striped bars) and T2DM (black bars). Mean ± sd is shown in bar graphs. *, P < 0.05.

Figure 4

Expression of genes regulating glycolysis, HLA class II molecules, and TGBF1 is increased in liver of subjects with T2DM. Mean ± sd is shown in bar graphs. *, P < 0.05.