Central inhibition of HDAC6 re-sensitizes leptin signaling during obesity to induce profound weight loss.

Leptin resistance during excess weight gain significantly contributes to the recidivism of obesity to leptin-based pharmacological therapies. The mechanisms underlying the inhibition of leptin receptor (LepR) signaling during obesity are still elusive. Here, we report that histone deacetylase 6 (HDAC6) interacts with LepR, reducing the latter's activity, and that pharmacological inhibition of HDAC6 activity disrupts this interaction and augments leptin signaling. Treatment of diet-induced obese mice with blood-brain barrier (BBB)-permeable HDAC6 inhibitors profoundly reduces food intake and leads to potent weight loss without affecting the muscle mass. Genetic depletion of Hdac6 in Agouti-related protein (AgRP)-expressing neurons or administration with BBB-impermeable HDAC6 inhibitors results in a lack of such anti-obesity effect. Together, these findings represent the first report describing a mechanistically validated and pharmaceutically tractable therapeutic approach to directly increase LepR activity as well as identifying centrally but not peripherally acting HDAC6 inhibitors as potent leptin sensitizers and anti-obesity agents.

FKBP11 rewires UPR signaling to promote glucose homeostasis in type 2 diabetes and obesity.

Chronic endoplasmic reticulum (ER) stress and sustained activation of unfolded protein response (UPR) signaling contribute to the development of type 2 diabetes in obesity. UPR signaling is a complex signaling pathway, which is still being explored in many different cellular processes. Here, we demonstrate that FK506-binding protein 11 (FKBP11), which is transcriptionally regulated by XBP1s, is severely reduced in the livers of obese mice. Restoring hepatic FKBP11 expression in obese mice initiates an atypical UPR signaling pathway marked by rewiring of PERK signaling toward NRF2, away from the eIF2α-ATF4 axis of the UPR. This alteration in UPR signaling establishes glucose homeostasis without changing hepatic ER stress, food consumption, or body weight. We conclude that ER stress during obesity can be beneficially rewired to promote glucose homeostasis. These findings may uncover possible new avenues in the development of novel approaches to treat diseases marked by ER stress.

Antisense-mediated senseful regulation of orchestrated metabolic response.

Accumulation of fat in the liver predisposes affected individuals to a variety of diseases, yet the molecular mechanisms leading to steatosis still remain elusive. Matsushita et al. (2022) propose a novel mechanism that interconnects insulin resistance and fatty liver formation by an orchestrated regulation of Irs2 and its natural antisense transcript.

Transcription- and phosphorylation-dependent control of a functional interplay between XBP1s and PINK1 governs mitophagy and potentially impacts Parkinson disease pathophysiology.

Parkinson disease (PD)-affected brains show consistent endoplasmic reticulum (ER) stress and mitophagic dysfunctions. The mechanisms underlying these perturbations and how they are directly linked remain a matter of questions. XBP1 is a transcription factor activated upon ER stress after unconventional splicing by the nuclease ERN1/IREα thereby yielding XBP1s, whereas PINK1 is a kinase considered as the sensor of mitochondrial physiology and a master gatekeeper of mitophagy process. We showed that XBP1s transactivates PINK1 in human cells, primary cultured neurons and mice brain, and triggered a pro-mitophagic phenotype that was fully dependent of endogenous PINK1. We also unraveled a PINK1-dependent phosphorylation of XBP1s that conditioned its nuclear localization and thereby, governed its transcriptional activity. PINK1-induced XBP1s phosphorylation occurred at residues reminiscent of, and correlated to, those phosphorylated in substantia nigra of sporadic PD-affected brains. Overall, our study delineated a functional loop between XBP1s and PINK1 governing mitophagy that was disrupted in PD condition.Abbreviations: 6OHDA: 6-hydroxydopamine; baf: bafilomycin A1; BECN1: beclin 1; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CASP3: caspase 3; CCCP: carbonyl cyanide chlorophenylhydrazone; COX8A: cytochrome c oxidase subunit 8A; DDIT3/CHOP: DNA damage inducible transcript 3; EGFP: enhanced green fluorescent protein; ER: endoplasmic reticulum; ERN1/IRE1α: endoplasmic reticulum to nucleus signaling 1; FACS: fluorescence-activated cell sorting; HSPD1/HSP60: heat shock protein family D (Hsp60) member 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MFN2: mitofusin 2; OPTN: optineurin; PD: Parkinson disease; PINK1: PTEN-induced kinase 1; PCR: polymerase chain reaction:; PRKN: parkin RBR E3 ubiquitin protein ligase; XBP1s [p-S61A]: XBP1s phosphorylated at serine 61; XBP1s [p-T48A]: XBP1s phosphorylated at threonine 48; shRNA: short hairpin RNA, SQSTM1/p62: sequestosome 1; TIMM23: translocase of inner mitochondrial membrane 23; TM: tunicamycin; TMRM: tetramethyl rhodamine methylester; TOMM20: translocase of outer mitochondrial membrane 20; Toy: toyocamycin; TP: thapsigargin; UB: ubiquitin; UB (S65): ubiquitin phosphorylated at serine 65; UPR: unfolded protein response, XBP1: X-box binding protein 1; XBP1s: spliced X-box binding protein 1.

Yin Yang 1 protein ameliorates diabetic nephropathy pathology through transcriptional repression of TGFß1.

Transforming growth factor-ß1 (TGFß1) has been identified as a major pathogenic factor underlying the development of diabetic nephropathy (DN). However, the current strategy of antagonizing TGFß1 has failed to demonstrate favorable outcomes in clinical trials. To identify a different therapeutic approach, we designed a mass spectrometry-based DNA-protein interaction screen to find transcriptional repressors that bind to the TGFB1 promoter and identified Yin Yang 1 (YY1) as a potent repressor of TGFB1. YY1 bound directly to TGFB1 promoter regions and repressed TGFB1 transcription in human renal mesangial cells. In mouse models, YY1 was elevated in mesangial cells during early diabetic renal lesions and decreased in later stages, and knockdown of renal YY1 aggravated, whereas overexpression of YY1 attenuated glomerulosclerosis. In addition, although their duration of diabetic course was comparable, patients with higher YY1 expression developed diabetic nephropathy more slowly compared to those who presented with lower YY1 expression. We found that a small molecule, eudesmin, suppressed TGFß1 and other profibrotic factors by increasing YY1 expression in human renal mesangial cells and attenuated diabetic renal lesions in DN mouse models by increasing YY1 expression. These results suggest that YY1 is a potent transcriptional repressor of TGFB1 during the development of DN in diabetic mice and that small molecules targeting YY1 may serve as promising therapies for treating DN.

Lipocalin 2 Does Not Play A Role in Celastrol-Mediated Reduction in Food Intake and Body Weight.

Celastrol is a leptin-sensitizing agent with profound anti-obesity effects in diet-induced obese (DIO) mice. However, the genes and pathways that mediate celastrol-induced leptin sensitization have not been fully understood. By comparing the hypothalamic transcriptomes of celastrol and vehicle-treated DIO mice, we identified lipocalin-2 (Lcn2) as the gene most strongly upregulated by celastrol. LCN2 was previously suggested as an anorexigenic and anti-obesity agent. Celastrol increased LCN2 protein levels in hypothalamus, liver, fat, muscle, and bone marrow, as well as in the plasma. However, genetic deficiency of LCN2 altered neither the development of diet-induced obesity, nor the ability of celastrol to promote weight loss and improve obesity-associated dyshomeostasis. We conclude that LCN2 is dispensable for both high fat diet-induced obesity and its therapeutic reduction by celastrol.

IL1R1 is required for celastrol's leptin-sensitization and antiobesity effects.

Celastrol, a pentacyclic triterpene, is the most potent antiobesity agent that has been reported thus far1. The mechanism of celastrol's leptin-sensitizing and antiobesity effects has not yet been elucidated. In this study, we identified interleukin-1 receptor 1 (IL1R1) as a mediator of celastrol's action by using temporally resolved analysis of the hypothalamic transcriptome in celastrol-treated DIO, lean, and db/db mice. We demonstrate that IL1R1-deficient mice are completely resistant to the effects of celastrol in leptin sensitization and treatment of obesity, diabetes, and nonalcoholic steatohepatitis. Thus, we conclude that IL1R1 is a gatekeeper for celastrol's metabolic actions.

PGC-1α functions as a co-suppressor of XBP1s to regulate glucose metabolism.

OBJECTIVE: Peroxisome proliferator-activated receptor γ (PPARγ) coactivator-1α (PGC-1α) promotes hepatic gluconeogenesis by activating HNF4α and FoxO1. PGC-1α expression in the liver is highly elevated in obese and diabetic conditions, leading to increased hepatic glucose production. We previously showed that the spliced form of X-box binding protein 1 (XBP1s) suppresses FoxO1 activity and hepatic gluconeogenesis. The shared role of PGC-1α and XBP1s in regulating FoxO1 activity and gluconeogenesis led us to investigate the probable interaction between PGC-1α and XBP1s and its role in glucose metabolism. METHODS: We investigated the biochemical interaction between PGC-1α and XBP1s and examined the role of their interaction in glucose homeostasis using animal models. RESULTS: We show that PGC-1α interacts with XBP1s, which plays an anti-gluconeogenic role in the liver by suppressing FoxO1 activity. The physical interaction between PGC-1α and XBP1s leads to suppression of XBP1s activity rather than its activation. Upregulating PGC-1α expression in the liver of lean mice lessens XBP1s protein levels, and reducing PGC-1α levels in obese and diabetic mouse liver restores XBP1s protein induction. CONCLUSIONS: Our findings reveal a novel function of PGC-1α as a suppressor of XBP1s function, suggesting that hepatic PGC-1α promotes gluconeogenesis through multiple pathways as a co-activator for HNF4α and FoxO1 and also as a suppressor for anti-gluconeogenic transcription factor XBP1s.

Selective Chemical Inhibition of PGC-1α Gluconeogenic Activity Ameliorates Type 2 Diabetes.

Type 2 diabetes (T2D) is a worldwide epidemic with a medical need for additional targeted therapies. Suppression of hepatic glucose production (HGP) effectively ameliorates diabetes and can be exploited for its treatment. We hypothesized that targeting PGC-1α acetylation in the liver, a chemical modification known to inhibit hepatic gluconeogenesis, could be potentially used for treatment of T2D. Thus, we designed a high-throughput chemical screen platform to quantify PGC-1α acetylation in cells and identified small molecules that increase PGC-1α acetylation, suppress gluconeogenic gene expression, and reduce glucose production in hepatocytes. On the basis of potency and bioavailability, we selected a small molecule, SR-18292, that reduces blood glucose, strongly increases hepatic insulin sensitivity, and improves glucose homeostasis in dietary and genetic mouse models of T2D. These studies have important implications for understanding the regulatory mechanisms of glucose metabolism and treatment of T2D.

Inflammation Improves Glucose Homeostasis through IKKß-XBP1s Interaction.

It is widely believed that inflammation associated with obesity has an important role in the development of type 2 diabetes. IκB kinase beta (IKKß) is a crucial kinase that responds to inflammatory stimuli such as tumor necrosis factor α (TNF-α) by initiating a variety of intracellular signaling cascades and is considered to be a key element in the inflammation-mediated development of insulin resistance. We show here, contrary to expectation, that IKKß-mediated inflammation is a positive regulator of hepatic glucose homeostasis. IKKß phosphorylates the spliced form of X-Box Binding Protein 1 (XBP1s) and increases the activity of XBP1s. We have used three experimental approaches to enhance the IKKß activity in the liver of obese mice and observed increased XBP1s activity, reduced ER stress, and a significant improvement in insulin sensitivity and consequently in glucose homeostasis. Our results reveal a beneficial role of IKKß-mediated hepatic inflammation in glucose homeostasis.

Withaferin A is a leptin sensitizer with strong antidiabetic properties in mice.

The increasing global prevalence of obesity and its associated disorders points to an urgent need for the development of novel and effective therapeutic strategies that induce healthy weight loss. Obesity is characterized by hyperleptinemia and central leptin resistance. In an attempt to identify compounds that could reverse leptin resistance and thus promote weight loss, we analyzed a library of small molecules that have mRNA expression profiles similar to that of celastrol, a naturally occurring compound that we previously identified as a leptin sensitizer. Through this process, we identified another naturally occurring compound, withaferin A, that also acts as a leptin sensitizer. We found that withaferin-A treatment of mice with diet-induced obesity (DIO) resulted in a 20-25% reduction of body weight, while also decreasing obesity-associated abnormalities, including hepatic steatosis. Withaferin-A treatment marginally affected the body weight of ob/ob and db/db mice, both of which are deficient in leptin signaling. In addition, withaferin A, unlike celastrol, has beneficial effects on glucose metabolism that occur independently of its leptin-sensitizing effect. Our results show that the metabolic abnormalities of DIO can be mitigated by sensitizing animals to endogenous leptin, and they indicate that withaferin A is a potential leptin sensitizer with additional antidiabetic actions.

XBP1s Is an Anti-lipogenic Protein.

Endoplasmic reticulum (ER) stress has been shown to contribute to various metabolic diseases, including non-alcoholic fatty liver disease and type 2 diabetes. Reduction of ER stress by treatment with chemical chaperones or overexpression of ER chaperone proteins alleviates hepatic steatosis. Nonetheless, X-box binding protein 1s (XBP1s), a key transcription factor that reduces ER stress, has been proposed as a lipogenic transcription factor. In this report, we document that XBP1s leads to suppression of lipogenic gene expression and reduction of hepatic triglyceride and diacylglycerol content in livers of diet-induced obese and genetically obese and insulin-resistant ob/ob mice. Furthermore, we also show that PKCϵ activity, which correlates with fatty liver and which causes insulin resistance, was significantly reduced in diet-induced obese mice. Finally, we have shown that XBP1s reduces the hepatic fatty acid synthesis rate and enhances macrolipophagy, an initiating step in lipolysis. Our results reveal that XBP1s reduces hepatic lipogenic gene expression and improves hepatosteatosis in mouse models of obesity and insulin resistance, which leads us to conclude that XBP1s has anti-lipogenic properties in the liver.

Treatment of obesity with celastrol.

Despite all modern advances in medicine, an effective drug treatment of obesity has not been found yet. Discovery of leptin two decades ago created hopes for treatment of obesity. However, development of leptin resistance has been a big obstacle, mitigating a leptin-centric treatment of obesity. Here, by using in silico drug-screening methods, we discovered that Celastrol, a pentacyclic triterpene extracted from the roots of Tripterygium Wilfordi (thunder god vine) plant, is a powerful anti-obesity agent. Celastrol suppresses food intake, blocks reduction of energy expenditure, and leads to up to 45% weight loss in hyperleptinemic diet-induced obese (DIO) mice by increasing leptin sensitivity, but it is ineffective in leptin-deficient (ob/ob) and leptin receptor-deficient (db/db) mouse models. These results indicate that Celastrol is a leptin sensitizer and a promising agent for the pharmacological treatment of obesity.

BRD7 regulates XBP1s' activity and glucose homeostasis through its interaction with the regulatory subunits of PI3K.

Bromodomain-containing protein 7 (BRD7) is a member of the bromodomain-containing protein family that is known to play a role as tumor suppressors. Here, we show that BRD7 is a component of the unfolded protein response (UPR) signaling through its ability to regulate X-box binding protein 1 (XBP1) nuclear translocation. BRD7 interacts with the regulatory subunits of phosphatidylinositol 3-kinase (PI3K) and increases the nuclear translocation of both p85α and p85ß and the spliced form of XBP1 (XBP1s). Deficiency of BRD7 blocks the nuclear translocation of XBP1s. Furthermore, our in vivo studies have shown that BRD7 protein levels are reduced in the liver of obese mice, and reinstating BRD7 levels in the liver restores XBP1s nuclear translocation, improves glucose homeostasis, and ultimately reduces the blood glucose levels in the obese and diabetic mouse models.

Mom's milk molds neural wiring for metabolism.

Vogt et al. demonstrate that, in mice, maternal high-fat feeding during lactation is sufficient to program the offspring for impaired energy and glucose homeostasis throughout their lifetime. They reveal that the resulting abnormal insulin signaling in the offspring interferes with the formation of hypothalamic neural circuits that contribute to metabolic status.

Unfolded protein response signaling and metabolic diseases.

The endoplasmic reticulum (ER) is a central organelle for protein biosynthesis, folding, and traffic. Perturbations in ER homeostasis create a condition termed ER stress and lead to activation of the complex signaling cascade called the unfolded protein response (UPR). Recent studies have documented that the UPR coordinates multiple signaling pathways and controls various physiologies in cells and the whole organism. Furthermore, unresolved ER stress has been implicated in a variety of metabolic disorders, such as obesity and type 2 diabetes. Therefore, intervening in ER stress and modulating signaling components of the UPR would provide promising therapeutics for the treatment of human metabolic diseases.

IRS1Ser³°7 phosphorylation does not mediate mTORC1-induced insulin resistance.

Increased mammalian target of rapamycin complex 1 (mTORC1) activity has been suggested to play important roles in development of insulin resistance in obesity. mTORC1 hyperactivity also increases endoplasmic reticulum (ER) stress, which in turn contributes to development of insulin resistance and glucose intolerance. Increased IRS1 phosphorylation at Ser307 in vitro is correlated with mTORC1- and ER stress-induced insulin resistance. This phosphorylation site correlates strongly with impaired insulin receptor signaling in diabetic mice and humans. In contrast, evidence from knock-in mice suggests that phosphorylation of IRS1 at Ser307 is actually required to maintain insulin sensitivity. To study the involvement of IRS1(Ser307) phosphorylation in mTORC1-mediated glucose intolerance and insulin sensitivity in vivo, we investigated the effects of liver specific TSC1 depletion in IRS1(Ser307Ala) mice and controls. Our results demonstrate that blockade of IRS1(Ser307) phosphorylation in vivo does not prevent mTORC1-mediated glucose intolerance and insulin resistance.

Mitofusins: mighty regulators of metabolism.

Mitochondria are central regulators of cellular metabolism but how their function in a subset of cells affects whole-body energy balance is less understood. Two studies in this issue of Cell identify how diet-dependent modulation of mitochondrial fusion in specific neuronal circuits impact the metabolic status of an animal.

Potential for therapeutic manipulation of the UPR in disease.

Increased endoplasmic reticulum (ER) stress and the activated unfolded protein response (UPR) signaling associated with it play key roles in physiological processes as well as under pathological conditions. The UPR normally protects cells and re-establishes cellular homeostasis, but prolonged UPR activation can lead to the development of various pathologies. These features make the UPR signaling pathway an attractive target for the treatment of diseases whose pathogenesis is characterized by chronic activation of this pathway. Here, we focus on the molecular signaling pathways of the UPR and suggest possible ways to target this response for therapeutic purposes.

Epinephrine deficiency results in intact glucose counter-regulation, severe hepatic steatosis and possible defective autophagy in fasting mice.

Epinephrine is one of the major hormones involved in glucose counter-regulation and gluconeogenesis. However, little is known about its importance in energy homeostasis during fasting. Our objective is to study the specific role of epinephrine in glucose and lipid metabolism during starvation. In our experiment, we subject regular mice and epinephrine-deficient mice to a 48-h fast then we evaluate the different metabolic responses to fasting. Our results show that epinephrine is not required for glucose counter-regulation: epinephrine-deficient mice maintain their blood glucose at normal fasting levels via glycogenolysis and gluconeogenesis, with normal fasting-induced changes in the peroxisomal activators: peroxisome proliferator activated receptor γ coactivator α (PGC-1α), fibroblast growth factor 21 (FGF-21), peroxisome proliferator activated receptor α (PPAR-α), and sterol regulatory element binding protein (SREBP-1c). However, fasted epinephrine-deficient mice develop severe ketosis and hepatic steatosis, with evidence for inhibition of hepatic autophagy, a process that normally provides essential energy via degradation of hepatic triglycerides during starvation. We conclude that, during fasting, epinephrine is not required for glucose homeostasis, lipolysis or ketogenesis. Epinephrine may have an essential role in lipid handling, possibly via an autophagy-dependent mechanism.