Adipocyte Thyroid Hormone ß Receptor-Mediated Hormone Action Fine-tunes Intracellular Glucose and Lipid Metabolism and Systemic Homeostasis.

Thyroid hormone (TH) has a profound effect on energy metabolism and systemic homeostasis. Adipose tissues are crucial for maintaining whole-body homeostasis; however, whether TH regulates systemic metabolic homeostasis through its action on adipose tissues is unclear. Here, we demonstrate that systemic administration of triiodothyronine (T3), the active form of TH, affects both inguinal white adipose tissue (iWAT) and whole-body metabolism. Taking advantage of the mouse model lacking adipocyte TH receptor (TR) α or TRß, we show that TRß is the major TR isoform that mediates T3 action on the expression of genes involved in multiple metabolic pathways in iWAT, including glucose uptake and use, de novo fatty acid synthesis, and both UCP1-dependent and -independent thermogenesis. Moreover, our results indicate that glucose-responsive lipogenic transcription factor in iWAT is regulated by T3, thereby being critically involved in T3-regulated glucose and lipid metabolism and energy dissipation. Mice with adipocyte TRß deficiency are susceptible to diet-induced obesity and metabolic dysregulation, suggesting that TRß in adipocytes may be a potential target for metabolic diseases. ARTICLE HIGHLIGHTS: How thyroid hormone (TH) achieves its diverse biological activities in the regulation of metabolism is not fully understood. Whether TH regulates systemic metabolic homeostasis via its action on white adipose tissue is unclear. Adipocyte TH receptor (TR) ß mediates the triiodothyronine effect on multiple metabolic pathways by targeting glucose-responsive lipogenic transcription factor in white adipose tissue; mice lacking adipocyte TRß are susceptible to high-fat diet-induced metabolic abnormalities. TRß in white adipocytes controls intracellular and systemic metabolism and may be a potential target for metabolic diseases.

Aberrant elevation of FTO levels promotes liver steatosis by decreasing the m6A methylation and increasing the stability of SREBF1 and ChREBP mRNAs.

Previous studies have indicated an association of fat mass and obesity-associated (FTO) with nonalcoholic fatty liver disease (NAFLD), the most common chronic liver disease worldwide. This study aimed to decipher the complex role of FTO in hepatic lipid metabolism. We found that a decrease in N6-methyladenosine (m6A) RNA methylation in the liver of mice fed with a high-fat diet (HFD) was accompanied by an increase in FTO expression. Overexpression of FTO in the liver promoted triglyceride accumulation by upregulating the expression of lipogenic genes. Mechanistical studies revealed that FTO could stabilize the mRNAs of sterol regulatory element binding transcription factor 1 (SREBF1) and carbohydrate responsive element binding protein (ChREBP), two master lipogenic transcription factors, by demethylating m6A sites. Knockdown of either SREBF1 or ChREBP attenuated the lipogenic effect of FTO, suggesting that they are bona fide effectors for FTO in regulating lipogenesis. Insulin could stimulate FTO transcription through a mechanism involving the action of intranuclear insulin receptor beta, while knockdown of FTO abrogated the lipogenic effect of insulin. Inhibition of FTO by entacapone decreased the expression of SREBF1, ChREBP, and downstream lipogenic genes, ameliorating liver steatosis in HFD-fed mice. Thus, our study established a critical role of FTO in both the insulin-regulated hepatic lipogenesis and the pathogenesis of NAFLD and provided a potential strategy for treating NAFLD.

Hepatic thyroid hormone signalling modulates glucose homeostasis through the regulation of GLP-1 production via bile acid-mediated FXR antagonism.

Thyroid hormones (TH) regulate systemic glucose metabolism through incompletely understood mechanisms. Here, we show that improved glucose metabolism in hypothyroid mice after T3 treatment is accompanied with increased glucagon-like peptide-1 (GLP-1) production and insulin secretion, while co-treatment with a GLP-1 receptor antagonist attenuates the effects of T3 on insulin and glucose levels. By using mice lacking hepatic TH receptor ß (TRß) and a liver-specific TRß-selective agonist, we demonstrate that TRß-mediated hepatic TH signalling is required for both the regulation of GLP-1 production and the insulinotropic and glucose-lowering effects of T3. Moreover, administration of a liver-targeted TRß-selective agonist increases GLP-1 and insulin levels and alleviates hyperglycemia in diet-induced obesity. Mechanistically, T3 suppresses Cyp8b1 expression, resulting in increased the levels of Farnesoid X receptor (FXR)-antagonistic bile acids, thereby potentiating GLP-1 production and insulin secretion by repressing intestinal FXR signalling. T3 correlates with both plasma GLP-1 and fecal FXR-antagonistic bile acid levels in people with normal thyroid function. Thus, our study reveals a role for hepatic TH signalling in glucose homeostasis through the regulation of GLP-1 production via bile acid-mediated FXR antagonism.

Hepatic P38 Activation Modulates Systemic Metabolism Through Fgf21-Mediated Interorgan Communication.

The mechanisms underlying the pathogenesis of steatosis and insulin resistance in nonalcoholic fatty liver disease remain elusive. Increased phosphorylation of hepatic p38 has long been noticed in fatty liver; however, whether the activation of hepatic p38 is a cause or consequence of liver steatosis is unclear. Here, we demonstrate that hepatic p38 activation by MKK6 overexpression in the liver of mice induces severe liver steatosis, reduces fat mass, and elevates circulating fatty acid levels in a hepatic p38α- and FGF21-dependent manner. Mechanistically, through increasing the FGF21 production from liver, hepatic p38 activation increases the influx of fatty acids from adipose tissue to liver, leading to hepatic ectopic lipid accumulation and insulin resistance. Although hepatic p38 activation exhibits favorable effects in peripheral tissues, it impairs the hepatic FGF21 action by facilitating the ubiquitination and degradation of FGF21 receptor cofactor ß-Klotho. Consistently, we show that p38 phosphorylation and FGF21 expffression are increased, ß-Klotho protein levels are decreased in the fatty liver of either mice or patients. In conclusion, our study reveals previously undescribed effects of hepatic p38 activation on systemic metabolism and provides new insights into the roles of hepatic p38α, FGF21, and ß-Klotho in the pathogenesis of nonalcoholic fatty liver disease.

Hepatic P38 Activation Modulates Systemic Metabolism Through Fgf21-Mediated Interorgan Communication.

The mechanisms underlying the pathogenesis of steatosis and insulin resistance in nonalcoholic fatty liver disease remain elusive. Increased phosphorylation of hepatic p38 has long been noticed in fatty liver; however, whether the activation of hepatic p38 is a cause or consequence of liver steatosis is unclear. Here, we demonstrate that hepatic p38 activation by MKK6 overexpression in the liver of mice induces severe liver steatosis, reduces fat mass, and elevates circulating fatty acid levels in a hepatic p38α- and FGF21-dependent manner. Mechanistically, through increasing the FGF21 production from liver, hepatic p38 activation increases the influx of fatty acids from adipose tissue to liver, leading to hepatic ectopic lipid accumulation and insulin resistance. Although hepatic p38 activation exhibits favorable effects in peripheral tissues, it impairs the hepatic FGF21 action by facilitating the ubiquitination and degradation of FGF21 receptor cofactor ß-Klotho. Consistently, we show that p38 phosphorylation and FGF21 expffression are increased, ß-Klotho protein levels are decreased in the fatty liver of either mice or patients. In conclusion, our study reveals previously undescribed effects of hepatic p38 activation on systemic metabolism and provides new insights into the roles of hepatic p38α, FGF21, and ß-Klotho in the pathogenesis of nonalcoholic fatty liver disease.

Saringosterol from Sargassum fusiforme Modulates Cholesterol Metabolism and Alleviates Atherosclerosis in ApoE-Deficient Mice.

Dysregulation of cholesterol homeostasis is a major risk factor of atherosclerosis, which can lead to serious health problems, including heart attack and stroke. Liver X receptor (LXR) α and ß are transcription factors belonging to the nuclear receptor superfamily, which play important roles in cholesterol homeostasis. Selectively activating LXRß provides a promising strategy for the treatment of atherosclerosis. Here, we employed atherosclerotic apoE-knockout mice to evaluate the effects of saringosterol, a phytosterol with potent and selective action for LXRß, which we identified previously in edible marine seaweed Sargassum fusiforme. We found that saringosterol treatment reduced the atherosclerotic plaque burden without having undesirable adverse hepatic effects in apoE-deficient mice fed an atherogenic diet. Meanwhile, reduced serum levels of cholesterol, accompanied by altered expression of LXR-regulated genes involved in cholesterol absorption, transport, efflux, excretion, and elimination, were observed in apoE-knockout mice after saringosterol treatment. Together, our study not only establishes saringosterol as an effective cholesterol-lowering and anti-atherogenic phytosterol but also provides insights into the underlying mechanism.