<i>MED1</i> mediator subunit is a key regulator of hepatic autophagy and lipid metabolism

Abstract

Hepatic macroautophagy/autophagy and fatty acid metabolism are transcriptionally regulated by nuclear receptors (NRs); however, it is not known whether their transcriptional co-activators are involved in autophagy. We thus examined MED1 (mediator complex subunit 1), a key component of the Mediator Complex that directly interacts with NRs, on these processes. We found that <i>MED1</i> knockdown (KD) in cultured hepatic cells decreased autophagy and mitochondrial activity that was accompanied by decreased transcription of genes involved in these processes. Lipophagy and fatty acid β-oxidation also were impaired. These effects also occurred after thyroid hormone stimulation, nutrient-replete or -deplete conditions, and in liver-specific <i>Med1</i> KD (<i>Med1</i> LKD) mice under fed and fasting conditions. Together, these findings showed that <i>Med1</i> played a key role in hepatic autophagy, mitochondria function, and lipid metabolism under these conditions. Additionally, we identified downregulated hepatic genes in <i>Med1</i> LKD mice, and subjected them to ChIP Enrichment Analysis. Our findings showed that the transcriptional activity of several NRs and transcription factors (TFs), including PPARA and FOXO1, likely were affected by <i>Med1</i> LKD. Finally, <i>Med1</i> expression and autophagy also were decreased in two mouse models of nonalcoholic fatty liver disease (NAFLD) suggesting that decreased <i>Med1</i> may contribute to hepatosteatosis. In summary, <i>MED1</i> plays an essential role in regulating hepatic autophagy and lipid oxidation during different hormonal and nutrient conditions. Thus, <i>MED1</i> may serve as an integrator of multiple transcriptional pathways involved in these metabolic processes.<b>Abbreviations:</b> BAF: bafilomycin A₁; db/db mice; <i>Lepr</i>^db/db mice; ECAR: extracellular acidification rate; KD: knockdown; MED1: mediator complex subunit 1; NAFLD: nonalcoholic fatty liver disease; OCR: oxygen consumption rate; PPARA/PPARα: peroxisomal proliferator activated receptor alpha; TF: transcription factor; TFEB: transcription factor EB; tf-LC3: tandem fluorescence RFP-GFP-LC3; TG: triglyceride; TH: Thyroid hormone; TR: thyroid hormone receptors; V-ATPase: vacuolar-type H⁺-ATPase; WDF: Western diet with 15% fructose in drinking water.