Lily Q. Dong

Here, we use an ex vivo approach compatible with the circadian timescale to interrogate protein secretion from liver, revealing several findings. Proteomic analyses in male and female mice identify hundreds of proteins that exhibit time-of-day-dependent or clock-dependent secretion involved in extracellular matrix, immune, redox, xenobiotic, and fatty acid functions. Among these, the liver secretes more endostatin, a cleavage product of collagen type XVIII alpha 1 (COL18A1), during the inactive, fasting phase of the diurnal cycle. Temporal regulation of COL18A1/endostatin is dysregulated upon loss of Bmal1 through combined effects on Col18a1 transcriptional repression and proteolytic processing. Functional experiments in vivo and in vitro reveal that endostatin suppresses mitochondrial gene expression in white adipose tissue in a time-dependent manner and reduces mitochondrial respiration in adipocytes, while enhancing lipolysis. These results support a mechanism of inter-organ crosstalk whereby hepatically derived, temporally-restricted endostatin tunes adipocytes toward metabolic activities required during the fasting phase.

Whereas mitochondrial dysfunction is implicated in metabolic dysfunction-associated steatohepatitis (MASH), the precise underlying mechanisms remain obscure. Here, we identify the Sirtuin 3 (SIRT3)-Disulfide-bond-A oxidoreductase-like protein (DsbA-L)-Mitochondrial Transcription Factor A (TFAM) axis as a crucial suppressor for mitochondrial stress-induced cyclic GMP-AMP synthase (cGAS) activation in hepatocytes, thereby alleviating MASH in mice. SIRT3 facilitates the deacetylation of DsbA-L at lysine residues (Lys ¹⁶⁵ , Lys ¹⁶⁷ , and Lys ¹⁷⁷ ), which promotes the interaction between DsbA-L and TFAM, essential for the preservation of mitochondrial integrity and function. Hepatocyte-specific knockout of SIRT3 or DsbA-L in male mice promoted mitochondrial DNA release into the cytosol, resulting in activation of the cGAS pathway and exacerbation of MASH characteristics. Conversely, hepatocyte-specific knockout of cGAS or overexpression of DsbA-L mitigated diet- and SIRT3 deficiency-induced MASH progression. Our study underscores the clinical significance of targeting SIRT3 and cGAS as pivotal therapeutic avenues to inhibit the progression of MASH.

Ataxia-telangiectasia is a rare genetic disorder characterized by neurological defects, immunodeficiency, cancer predisposition, radiosensitivity, decreased blood vessel integrity, and diabetes. ATM, the protein mutated in Ataxia-telangiectasia, responds to DNA damage and oxidative stress, but its functional relationship to the progressive clinical manifestation of this disorder is not understood. CD98HC chaperones cystine/glutamate and cationic/neutral amino acid antiporters to the cell membrane, and CD98HC phosphorylation by ATM accelerates membrane localization to acutely increase amino acid transport. Loss of ATM impacts tissues reliant on heterodimeric amino acid transporters relevant to Ataxia-telangiectasia phenotypes, such as endothelial cells (telangiectasia) and pancreatic α-cells (fatty liver and diabetes), with toxic glutamate accumulation. Bypassing the antiporters restores intracellular metabolic balance in ATM-deficient cells and mouse models. These findings provide insight into the long-known benefits of N-acetyl cysteine in Ataxia-telangiectasia cells beyond oxidative stress through removing glutamate excess by producing glutathione.

Aims/hypothesis Upregulation of serum leucine-rich α-2-glycoprotein 1 (LRG1) has been implicated in diet-induced obesity and metabolic disorders. However, its specific hormonal actions remain unclear. This study aimed to determine whether diet-enhanced serum LRG1 levels promote hyperinsulinaemia by directly stimulating insulin secretion from pancreatic beta cells. Methods Human serum samples were obtained from individuals (both male and female) undergoing plastic surgery. Male C57BL/6 wild-type and Lrg1 whole-body knockout (Lrg1^KO) mice were fed a 45% high-fat diet, with serum samples collected every 2 weeks to monitor LRG1 and insulin levels throughout diet-induced obesity. MIN6 beta cells were used to investigate the effects of LRG1 on insulin secretion and intracellular Ca²⁺ release. Antibodies targeting various LRG1 epitopes were used to neutralise LRG1 stimulation in MIN6 cells, and their effectiveness was tested in vivo to assess their ability to prevent LRG1-induced hyperinsulinaemia. Results We observed a significant positive association between human serum LRG1 levels and both age and BMI, with elevated levels observed in individuals with vs without type 2 diabetes. In mice fed a high-fat diet, LRG1 upregulation in serum was associated with hyperinsulinaemia. Lrg1 knockout protected mice from diet-induced islet hyperplasia and the loss of beta cell mass. Furthermore, neutralising LRG1 activity prevented the onset of diet-induced hyperinsulinaemia and preserved glucose tolerance. Mechanistically, LRG1 induces inositol triphosphate (IP₃) production and intracellular Ca²⁺ release from the endoplasmic reticulum (ER) in a phospholipase C (PLC)-dependent manner, leading to excessive insulin secretion and ER stress in MIN6 beta cells. Conclusions/interpretation In summary, this study identifies LRG1 as a significant contributor to hyperinsulinaemia and beta cell dysfunction. Targeting LRG1 activity emerges as a promising therapeutic approach for addressing diet-induced beta cell dysfunction and managing type 2 diabetes. Graphical Abstract

Aims/hypothesis Glucagon-stimulated hepatic gluconeogenesis contributes to endogenous glucose production during fasting. Recent studies suggest that TGF-β is able to promote hepatic gluconeogenesis in mice. However, the physiological relevance of serum TGF-β levels to human glucose metabolism and the mechanism by which TGF-β enhances gluconeogenesis remain largely unknown. As enhanced gluconeogenesis is a signature feature of type 2 diabetes, elucidating the molecular mechanisms underlying TGF-β-promoted hepatic gluconeogenesis would allow us to better understand the process of normal glucose production and the pathophysiology of this process in type 2 diabetes. This study aimed to investigate the contribution of upregulated TGF-β1 in human type 2 diabetes and the molecular mechanism underlying the action of TGF-β1 in glucose metabolism. Methods Serum levels of TGF-β1 were measured by ELISA in 74 control participants with normal glucose tolerance and 75 participants with type 2 diabetes. Human liver tissue was collected from participants without obesity and with or without type 2 diabetes for the measurement of TGF-β1 and glucagon signalling. To investigate the role of Smad3, a key signalling molecule downstream of the TGF-β1 receptor, in mediating the effect of TGF-β1 on glucagon signalling, we generated Smad3 knockout mice. Glucose levels in Smad3 knockout mice were measured during prolonged fasting and a glucagon tolerance test. Mouse primary hepatocytes were isolated from Smad3 knockout and wild-type (WT) mice to investigate the underlying molecular mechanisms. Smad3 phosphorylation was detected by western blotting, levels of cAMP were detected by ELISA and levels of protein kinase A (PKA)/cAMP response element-binding protein (CREB) phosphorylation were detected by western blotting. The dissociation of PKA subunits was measured by immunoprecipitation. Results We observed higher levels of serum TGF-β1 in participants without obesity and with type 2 diabetes than in healthy control participants, which was positively correlated with HbA_1c and fasting blood glucose levels. In addition, hyperactivation of the CREB and Smad3 signalling pathways was observed in the liver of participants with type 2 diabetes. Treating WT mouse primary hepatocytes with TGF-β1 greatly potentiated glucagon-stimulated PKA/CREB phosphorylation and hepatic gluconeogenesis. Mechanistically, TGF-β1 treatment induced the binding of Smad3 to the regulatory subunit of PKA (PKA-R), which prevented the association of PKA-R with the catalytic subunit of PKA (PKA-C) and led to the potentiation of glucagon-stimulated PKA signalling and gluconeogenesis. Conclusions/interpretation The hepatic TGF-β1/Smad3 pathway sensitises the effect of glucagon/PKA signalling on gluconeogenesis and synergistically promotes hepatic glucose production. Reducing serum levels of TGF-β1 and/or preventing hyperactivation of TGF-β1 signalling could be a novel approach for alleviating hyperglycaemia in type 2 diabetes. Graphical abstract

Dysregulation in adipokine biosynthesis and function contributes to obesity-induced metabolic diseases. However, the identities and functions of many of the obesity-induced secretory molecules remain unknown. Here, we report the identification of leucine-rich alpha-2-glycoprotein 1 (LRG1) as an obesity-associated adipokine that exacerbates high fat diet–induced hepatosteatosis and insulin resistance. Serum levels of LRG1 were markedly elevated in obese humans and mice compared with their respective controls. LRG1 deficiency in mice greatly alleviated diet-induced hepatosteatosis, obesity, and insulin resistance. Mechanistically, LRG1 bound with high selectivity to the liver and promoted hepatosteatosis by increasing de novo lipogenesis and suppressing fatty acid β-oxidation. LRG1 also inhibited hepatic insulin signaling by downregulating insulin receptor substrates 1 and 2. Our study identified LRG1 as a key molecule that mediates the crosstalk between adipocytes and hepatocytes in diet-induced hepatosteatosis and insulin resistance. Suppressing LRG1 expression and function may be a promising strategy for the treatment of obesity-related metabolic diseases.

A dynamically regulated microenvironment, which is mediated by crosstalk between adipocytes and neighboring cells, is critical for adipose tissue homeostasis and function. However, information on key molecules and/or signaling pathways regulating the crosstalk remains limited. In this study, we identify adipocyte miRNA-182-5p (miR-182-5p) as a crucial antiobesity molecule that stimulated beige fat thermogenesis by promoting the crosstalk between adipocytes and macrophages. miR-182-5p was highly enriched in thermogenic adipocytes, and its expression was markedly stimulated by cold exposure in mice. In contrast, miR-182-5p expression was significantly reduced in adipose tissues of obese humans and mice. Knockout of miR-185-5p decreased cold-induced beige fat thermogenesis whereas overexpression of miR-185-5p increased beiging and thermogenesis in mice. Mechanistically, miR-182-5p promoted FGF21 expression and secretion in adipocytes by suppressing nuclear receptor subfamily 1 group D member 1 (Nr1d1) at 5′-UTR, which in turn stimulates acetylcholine synthesis and release in macrophages. Increased acetylcholine expression activated the nicotine acetylcholine receptor in adipocytes, which stimulated PKA signaling and consequent thermogenic gene expression. Our study reveals a key role of the miR-182-5p/FGF21/acetylcholine/acetylcholine receptor axis that mediates the crosstalk between adipocytes and macrophages to promote beige fat thermogenesis. Activation of the miR-182-5p–induced signaling pathway in adipose tissue may be an effective approach to ameliorate obesity and associated metabolic diseases.

Adiponectin is an adipokine that exerts insulin-sensitizing and anti-inflammatory roles in insulin target tissues including liver. While the insulin-sensitizing function of adiponectin has been extensively investigated, the precise mechanism by which adiponectin alleviates diet-induced hepatic inflammation remains elusive. Here, we report that hepatocyte-specific knockout (KO) of the adaptor protein APPL2 enhanced adiponectin sensitivity and prevented mice from developing high-fat diet–induced inflammation, insulin resistance, and glucose intolerance, although it caused fatty liver. The improved anti-inflammatory and insulin-sensitizing effects in the APPL2 hepatocyte–specific KO mice were largely reversed by knocking out adiponectin. Mechanistically, hepatocyte APPL2 deficiency enhances adiponectin signaling in the liver, which blocks TNF-α–stimulated MCP-1 expression via inhibiting the mTORC1 signaling pathway, leading to reduced macrophage infiltration and thus reduced inflammation in the liver. With results taken together, our study uncovers a mechanism underlying the anti-inflammatory role of adiponectin in the liver and reveals the hepatic APPL2–mTORC1–MCP-1 axis as a potential target for treating overnutrition-induced inflammation in the liver.

Background and Purpose: Adiponectin (APN) is an adipokine secreted from adipocytes that binds to APN receptors AdipoR1 and AdipoR2 and exerts an anti‐inflammatory response through mechanisms not fully understood. There is a need to develop small molecules that activate AdipoR1 and AdipoR2 and to be used to inhibit the inflammatory response in lipopolysaccharide (LPS)‐induced endotoxemia and other inflammatory disorders.Experimental Approach: We designed 10 new structural analogues of an AdipoR agonist, AdipoRon (APR), and assessed their anti‐inflammatory properties. Bone marrow‐derived macrophages (BMMs) and peritoneal macrophages (PEMs) were isolated from mice. Levels of pro‐inflammatory cytokines were measured by reverse transcription and real‐time quantitative polymerase chain reaction (qRT‐PCR), enzyme‐linked immunosorbent assay (ELISA) and microarray in LPS‐induced endotoxemia mice and diet‐induced obesity (DIO) mice in which systemic inflammation prevails. Western blotting, immunohistochemistry (IHC), siRNA interference and immunoprecipitation were used to detect signalling pathways.Key Results: A novel APN receptor agonist named adipo anti‐inflammation agonist (AdipoAI) strongly suppresses inflammation in DIO and endotoxemia mice, as well as in cultured macrophages. We also found that AdipoAI attenuated the association of AdipoR1 and APPL1 via myeloid differentiation marker 88 (MyD88) signalling, thus inhibiting activation of nuclear factor kappa B (NF‐κB), mitogen‐activated protein kinase (MAPK) and c‐Maf pathways and limiting the production of pro‐inflammatory cytokines in LPS‐induced macrophages.Conclusion and Implications: AdipoAI is a promising alternative therapeutic approach to APN and APR to suppress inflammation in LPS‐induced endotoxemia and other inflammatory disorders via distinct signalling pathways.

Obesity is a global epidemic that is caused by excessive energy intake or inefficient energy expenditure. Brown or beige fat dissipates energy as heat through non-shivering thermogenesis by their high density of mitochondria. However, how the mitochondrial stress-induced signal is coupled to the cellular thermogenic program remains elusive. Here, we show that mitochondrial DNA escape-induced activation of the cGAS-STING pathway negatively regulates thermogenesis in fat-specific DsbA-L knockout mice, a model of adipose tissue mitochondrial stress. Conversely, fat-specific overexpression of DsbA-L or knockout of STING protects mice against high-fat diet-induced obesity. Mechanistically, activation of the cGAS-STING pathway in adipocytes activated phosphodiesterase PDE3B/PDE4, leading to decreased cAMP levels and PKA signaling, thus reduced thermogenesis. Our study demonstrates that mitochondrial stress-activated cGAS-STING pathway functions as a sentinel signal that suppresses thermogenesis in adipose tissue. Targeting adipose cGAS-STING pathway may thus be a potential therapeutic strategy to counteract overnutrition-induced obesity and its associated metabolic diseases.