Insulin-Regulated Srebp-1c and Pck1 mRNA Expression in Primary Hepatocytes from Zucker Fatty but Not Lean Rats Is Affected by Feeding Conditions

Insulin regulates the transcription of genes for hepatic glucose and lipid metabolism. We hypothesized that this action may be impaired in hepatocytes from insulin resistant animals. Primary hepatocytes from insulin sensitive Zucker lean (ZL) and insulin resistant Zucker fatty (ZF) rats in ad libitum or after an overnight fasting were isolated, cultured and treated with insulin and other compounds for analysis of gene expression using real-time PCR. The mRNA levels of one insulin-induced (Srebp-1c) and one insulin-suppressed (Pck1) genes in response to insulin, glucagon, and compactin treatments in hepatocytes from ad libitum ZL and ZF rats were analyzed. Additionally, the effects of insulin and T1317 on their levels in hepatocytes from ad libitum or fasted ZL or ZF rats were compared. The mRNA levels of Srebp-1c, Fas, and Scd1, but not that of Insr, Gck and Pck1, were higher in freshly isolated hepatocytes from ad libitum ZF than that from ZL rats. These patterns of Srebp-1c and Pck1 mRNA levels remained in primary hepatocyte cultured in vitro. Insulin's ability to regulate Srebp-1c and Pck1 expression was diminished in hepatocytes from ad libitum ZF, but not ZL rats. Glucagon or compactin suppressed Srebp-1c mRNA expression in lean, but not fatty hepatocytes. However, glucagon induced Pck1 mRNA expression similarly in hepatocytes from ad libitum ZL and ZF rats. Insulin caused the same dose-dependent increase of Akt phosphorylation in hepatocytes from ad libitum ZL and ZF rats. It synergized with T1317 to induce Srebp-1c, and suppressed Pck1 mRNA levels in hepatocytes from fasted, but not that from ad libitum ZF rats. We demonstrated that insulin was unable to regulate its downstream genes' mRNA expression in hepatocytes from ad libitum ZF rats. This impairment can be partially restored in hepatocytes from ZF rats after an overnight fasting, a phenomenon that deserves further investigation

Hypothalamic AMPK-ER Stress-JNK1 Axis Mediates the Central Actions of Thyroid Hormones on Energy Balance

Thyroid hormones (THs) act in the brain to modulate energy balance. We show that central triiodothyronine (T3) regulates de novo lipogenesis in liver and lipid oxidation in brown adipose tissue (BAT) through the parasympathetic (PSNS) and sympathetic nervous system (SNS), respectively. Central T3 promotes hepatic lipogenesis with parallel stimulation of the thermogenic program in BAT. The action of T3 depends on AMP-activated protein kinase (AMPK)-induced regulation of two signaling pathways in the ventromedial nucleus of the hypothalamus (VMH): decreased ceramide-induced endoplasmic reticulum(ER) stress, which promotes BAT thermogenesis, and increased c-Jun N-terminal kinase (JNK) activation, which controls hepatic lipid metabolism. Of note, ablation of AMPK alpha 1 in steroidogenic factor 1 (SF1) neurons of the VMH fully recapitulated the effect of central T3, pointing to this population in mediating the effect of central THs on metabolism. Overall, these findings uncover the underlying pathways through which central T3 modulates peripheral metabolism

Hepatic levels of S-adenosylmethionine regulate the adaptive response to fasting

There has been an intense focus to uncover the molecular mechanisms by which fasting triggers the adaptive cellular responses in the major organs of the body. Here, we show that in mice, hepatic S-adenosylmethionine (SAMe)—the principal methyl donor—acts as a metabolic sensor of nutrition to fine-tune the catabolic-fasting response by modulating phosphatidylethanolamine N-methyltransferase (PEMT) activity, endoplasmic reticulum-mitochondria contacts, β-oxidation, and ATP production in the liver, together with FGF21-mediated lipolysis and thermogenesis in adipose tissues. Notably, we show that glucagon induces the expression of the hepatic SAMe-synthesizing enzyme methionine adenosyltransferase α1 (MAT1A), which translocates to mitochondria-associated membranes. This leads to the production of this metabolite at these sites, which acts as a brake to prevent excessive β-oxidation and mitochondrial ATP synthesis and thereby endoplasmic reticulum stress and liver injury. This work provides important insights into the previously undescribed function of SAMe as a new arm of the metabolic adaptation to fasting