RESUMEN
OBJECTIVE: Metabolic dysfunction-associated steatohepatitis (MASH) is characterized by inflammation, fibrosis and accumulation of fatty acids in the liver. MASH disease progression has been associated with reduced thyroid hormone (TH) signalling in the liver, including reduced expression of deiodinase type I (DIO1) and TH receptor beta (THRB). However, the underlying mechanisms mediating these effects remain elusive. Here, we hypothesized, that epigenetic mechanisms may be involved in modulating hepatic TH action. METHODS: Liver samples from patients with and without MASH were analyzed by qRT-PCR and correlated with clinical parameters. Luciferase reporter assays and overexpression of miRNA in HepG2-cells were used to validate functional binding of miRNA to predicted targets. DNA-methylation was analyzed by bisulfite-pyrosequencing. RESULTS: miR-34a-5p was upregulated in MASH patients and correlated positively with clinical parameters of MASH. Using in silico and in vitro analysis we demonstrate that miR-34a-5p is capable of targeting several modulators of local hepatic TH action, as evidenced by functional binding of miR-34a-5p to the seed sequence in the THRB and DIO1 genes. Consequently, overexpression of miR-34a-5p in HepG2-cells reduced the expression of THRA, THRB, DIO1 and SLC10A1, thus potentially mediating an acquired hepatic resistance to TH in MASH. As additional regulatory mechanism, DNA-methylation of THRB intron 1 was increased in MASH and negatively correlated with THRB expression. CONCLUSION: miR-34a-5p constitutes a possible epigenetic master regulator of hepatic TH action, which together with THRB specific DNA-methylation could explain a possible developing TH resistance in the liver during MASH progression on the molecular level.
RESUMEN
De novo lipogenesis (DNL) in visceral adipose tissue (VAT) is associated with systemic insulin sensitivity. DNL in VAT is regulated through ChREBP activity and glucose uptake through Glut4 (encoded by Slc2a4). Slc2a4 expression, ChREBP activity, and DNL are decreased in obesity, the underlying cause however remains unidentified. We hypothesize that increased DNA methylation in an enhancer region of Slc2a4 decreases Slc2a4 expression in obesity and insulin resistance. We found that SLC2A4 expression in VAT of morbidly obese subjects with high HbA1c (>6.5%, n = 35) is decreased, whereas DNA methylation is concomitantly increased compared to morbidly obese subjects with low HbA1c (≤6.5%, n = 65). In diet-induced obese (DIO) mice, DNA methylation of Slc2a4 persistently increases with the onset of obesity and insulin resistance, while gene expression progressively decreases. The regulatory impact of DNA methylation in the investigated enhancer region on SLC2A4 gene expression was validated with a reporter gene assay. Additionally, treatment of 3T3 pre-adipocytes with palmitate/oleate during differentiation decreased DNA methylation and increased Slc2a4 expression. These findings highlight a potential regulation of Slc2a4 by DNA methylation in VAT, which is induced by fatty acids and may play a role in the progression of obesity and insulin resistance in humans.