Lysosomal degradation of cytoplasmic components by autophagy is essential for cellular survival and homeostasis under nutrient-deprived conditions1–4. Acute regulation of autophagy by nutrient-sensing kinases is well defined3, 5–7, but longer-term transcriptional regulation is relatively unknown. Here we show that the fed-state sensing nuclear receptor FXR8, 9 and the fasting transcriptional activator CREB10, 11 coordinately regulate the hepatic autophagy gene network. Pharmacological activation of FXR repressed many autophagy genes and inhibited autophagy even in fasted mice and feeding-mediated inhibition of macroautophagy was attenuated in FXR-knockout mice. From mouse liver ChIP-seq data12–15, FXR and CREB binding peaks were detected at 178 and 112, respectively, of 230 autophagy-related genes, and 78 genes showed shared binding, mostly in their promoter regions. CREB promoted lipophagy, autophagic degradation of lipids16, under nutrient-deprived conditions, and FXR inhibited this response. Mechanistically, CREB upregulated autophagy genes, including Atg7, Ulk1, and Tfeb, by recruiting the coactivator CRTC2. After feeding or pharmacological activation, FXR trans-repressed these genes by disrupting the functional CREB/CRTC2 complex. This study identifies the novel FXR/CREB axis as a key physiological switch regulating autophagy, resulting in sustained nutrient regulation of autophagy during feeding/fasting cycles.

SIRT1 is an NAD+-dependent deacetylase that is implicated in prevention of many age-related diseases including metabolic disorders. Since SIRT1 deacetylase activity is dependent on NAD+ levels and the development of compounds that directly activate SIRT1 has been controversial, indirectly activating SIRT1 through enhancing NAD+ bioavailability has received increasing attention. NAD+ levels are reduced in obesity and the aged, but the underlying mechanisms remain unclear. We recently showed that hepatic microRNA-34a (miR-34a), which is elevated in obesity, directly targets and decreases SIRT1 expression. Here we further show that miR-34a reduces NAD+ levels and SIRT1 activity by targeting NAMPT, the rate-limiting enzyme for NAD+ biosynthesis. A functional binding site for miR-34a is present in the 3′ UTR of NAMPT mRNA. Hepatic overexpression of miR-34a reduced NAMPT/NAD+ levels, increased acetylation of the SIRT1 target transcriptional regulators, PGC-1α, SREBP-1c, FXR, and NF-κB, and resulted in obesity-mimetic outcomes. The decreased NAMPT/NAD+ levels were independent of miR-34a effects on SIRT1 levels since they were also observed in SIRT1 liver-specific knockout mice. Further, the miR-34a-mediated decreases were reversed by treatment with the NAD+ intermediate, nicotinamide mononucleotide. Conversely, antagonism of miR-34a in diet-induced obese mice restored NAMPT/NAD+ levels and alleviated steatosis, inflammation, and glucose intolerance. Anti-miR-34a-mediated increases in NAD+ levels were attenuated when NAMPT was downregulated. Our findings reveal a novel function of miR-34a in reducing both SIRT1 expression and activity in obesity. The miR-34a/NAMPT axis presents a potential target for treating obesity- and aging-related diseases involving SIRT1 dysfunction like steatosis and type 2 diabetes.

bBrown fat generates heat through uncoupled respiration, protecting against hypothermia and obesity. Adult humans have brown fat, but the amounts and activities are substantially decreased in obesity, by unknown mechanisms. Here we show that elevated microRNA 34a (miR-34a) in obesity inhibits fat browning in part by suppressing the browning activators fibroblast growth factor 21 (FGF21) and SIRT1. Lentivirus-mediated downregulation of miR-34a in mice with diet-induced obesity reduced adiposity, improved serum profiles, increased the mitochondrial DNA copy number, and increased oxidative function in adipose tissue in both BALB/c and C57BL/6 mice. Remarkably, downregulation of miR-34a increased coexpression of the beige fat-specific marker CD137 and the browning marker UCP1 in all types of white fat, including visceral fat, and promoted additional browning in brown fat. Mechanistically, downregulation of miR-34a increased expression of the FGF21 receptor components, FGFR1 and ␤KL, and also that of SIRT1, resulting in FGF21/SIRT1-dependent deacetylation of PGC-1␣ and induction of the browning genes Ucp1, Pgc-1␣, and Prdm16. Importantly, anti-miR-34a-mediated beneficial effects, including decreased adiposity, are likely from multiple tissues, since downregulation of miR-34a also improves hepatic FGF21 signaling and lipid oxidation. This study identifies miR-34a as an inhibitor of beige and brown fat formation, providing a potential target for treating obesity-related diseases.T he global epidemic of obesity has greatly increased research interest in adipose biology, particularly that of energy-dissipating brown fat (1). While white adipose tissue (WAT) stores excess chemical energy, leading to weight gain, brown adipose tissue (BAT) expends energy as heat through uncoupled respiration via the action of mitochondrial uncoupled protein 1 (UCP1), protecting against hypothermia and obesity (2-5). Recent studies discovered a second type of brown-like fat, "beige fat," which is present in WAT and is genetically distinct from classical BAT (6). Interestingly, recent human studies using 2-fluorodeoxyglucose coupled with positron-emission tomography (PET) scanning showed that adult humans have brown fat and that the activities and amounts of brown fat are inversely related to the body mass index (BMI) and substantially decreased in obesity (3, 7). Increasing energy expenditure by promoting production of brown fat in BAT and also beige fat in WAT, particularly in obese individuals, would be an appealing option for weight reduction and for treating obesity-related diseases.Brown and beige fat depots develop in response to various activators, including cold exposure, hormones, exercise, and transcriptional regulators, such as PRDM16, PPAR␥, SIRT1, and PGC-1␣ (8-11). Fibroblast growth factor 21 (FGF21) has been shown to beneficially affect metabolism and energy balance by enhancing fatty acid ␤-oxidation during prolonged fasting and also by promoting fat browning in WAT, as well as in BAT, in response to cold exposure in mice (1...

MicroRNA-34a (miR-34a) is the most highly elevated hepatic miR in obese mice and is also substantially elevated in patients who have steatosis, but its role in obesity and metabolic dysfunction remains unclear. After a meal, FGF19 is secreted from the ileum; binds to a hepatic membrane receptor complex, FGF19 receptor 4 and coreceptor β-Klotho (βKL); and mediates postprandial responses under physiological conditions, but hepatic responses to FGF19 signaling were shown to be impaired in patients with steatosis. Here, we show an unexpected functional link between aberrantly elevated miR-34a and impaired βKL/FGF19 signaling in obesity. In vitro studies show that miR-34a down-regulates βKL by binding to the 3′ UTR of βKL mRNA. Adenoviral-mediated overexpression of miR-34a in mice decreased hepatic βKL levels, impaired FGF19-activated ERK and glycogen synthase kinase signaling, and altered expression of FGF19 metabolic target genes. Consistent with these results, βKL levels were decreased and hepatic responses to FGF19 were severely impaired in dietary obese mice that have elevated miR-34a. Remarkably, in vivo antisense inhibition of miR-34a in obese mice partially restored βKL levels and improved FGF19 target gene expression and metabolic outcomes, including decreased liver fat. Further, antimiR-34a treatment in primary hepatocytes of obese mice restored FGF19-activated ERK and glycogen synthase kinase signaling in a βKL-dependent manner. These results indicate that aberrantly elevated miR-34a in obesity attenuates hepatic FGF19 signaling by directly targeting βKL. The miR-34a/βKL/FGF19 axis may present unique therapeutic targets for FGF19-related human diseases, including metabolic disorders and cancer.etabolic disorders, such as fatty liver, obesity, and type II diabetes, due to abnormally regulated lipid and glucose levels are serious medical problems worldwide (1). The roles of pancreatic insulin in the regulation of fed-state metabolism and development of such metabolic disorders are well known, but recently discovered and relatively less understood is the role of an intestinal hormone, FGF19 (or mouse FGF15) (2). FGF19 constitutes a unique endocrine metabolic regulatory axis. After a meal, expression of FGF19 is induced by the bile acid-activated nuclear receptor, farnesoid X receptor (FXR), in the small intestine (2). Secreted FGF19 binds to a hepatic membrane receptor complex, FGF19 receptor 4 (FGFR4), and its coreceptor β-Klotho (βKL) (3-6), and it then triggers the activation of cellular kinases, including ERK and glycogen synthase kinase (GSK), to mediate postprandial metabolic responses (7,8). Interestingly, a recent study showed that the hepatic response to FGF19 is impaired in patients with nonalcoholic fatty liver disease (NAFLD) and insulin resistance (9). Despite the functional importance of βKL in transmitting FGF19 signaling, little is known about how the expression of βKL is regulated and why FGF19 signaling is impaired in patients who have fatty liver.MicroRNAs (miRs) are small, noncoding RNAs a...

Autophagy is essential for cellular survival and energy homeostasis under nutrient deprivation. Despite the emerging importance of nuclear events in autophagy regulation, epigenetic control of autophagy gene transcription remains unclear. Here, we report fasting-induced Fibroblast Growth Factor-21 (FGF21) signaling activates hepatic autophagy and lipid degradation via Jumonji-D3 (JMJD3/KDM6B) histone demethylase. Upon FGF21 signaling, JMJD3 epigenetically upregulates global autophagy-network genes, including Tfeb, Atg7, Atgl, and Fgf21, through demethylation of histone H3K27-me3, resulting in autophagy-mediated lipid degradation. Mechanistically, phosphorylation of JMJD3 at Thr-1044 by FGF21 signal-activated PKA increases its nuclear localization and interaction with the nuclear receptor PPARα to transcriptionally activate autophagy. Administration of FGF21 in obese mice improves defective autophagy and hepatosteatosis in a JMJD3-dependent manner. Remarkably, in non-alcoholic fatty liver disease patients, hepatic expression of JMJD3, ATG7, LC3, and ULK1 is substantially decreased. These findings demonstrate that FGF21-JMJD3 signaling epigenetically links nutrient deprivation with hepatic autophagy and lipid degradation in mammals.