Shuai Chen

The Rab GTPase activating protein (RabGAP) AS160 translocates from the cytosol into the nucleus acting as a transcriptional co-activator of Signal Transducer and Activator of Transcription 3 (STAT3) to regulate proliferation of muscle satellite cells (MuSCs). How this AS160–STAT3 complex is regulated remains largely unclear yet. Here, we show that TBC1D1, a RabGAP related to AS160, forms a super-complex with AS160 and STAT3 to retain the AS160–STAT3 complex in the cytosol. Phosphorylation of TBC1D1-Thr⁵⁹⁶ by protein kinase B dissociates TBC1D1 from AS160 thus releasing the cytosolic retention of the AS160–STAT3 complex. A non-phosphorylatable alanine substitution of Thr⁵⁹⁶ inhibits MuSC proliferation and impairs repair of injured muscle. In contrast, TBC1D1 deficiency, but not its GAP-inactive mutation, promotes MuSC proliferation and muscle regeneration. Thus, TBC1D1 is a negative regulator of MuSC proliferation through cytosolic retention of the AS160–STAT3 complex and might be a valuable therapeutic target for muscle regeneration.

Obesity is a major pathological factor that induces insulin resistance and consequent type 2 diabetes through multiple mechanisms. Inactivation of the insulin receptor (INSR) contributes to the development of insulin resistance, whose protein level is down-regulated in obesity through as yet-undefined mechanisms. Here we show that the E3-ligase TRAF6 is a critical regulator of INSR maturation, whose inactivation prevents palmitic acid- or high-fat diet-induced diminution of the INSR. Consequently, genetic inactivation of TRAF6 enhances insulin signaling that further increases muscle glucose uptake and inhibits hepatic gluconeogenesis. TRAF6 inactivation increases the proprotein convertase FURIN that controls the processing of pro-INSR to mature INSR. Mechanistically, TRAF6 associates with the Golgi apparatus, where it ubiquitinates the cytosolic tail of FURIN, leading to its lysosomal degradation. This TRAF6-FURIN axis also regulates cholesterol metabolism via PCSK9 processing in the circulation. Collectively, our results reveal a critical role of TRAF6 in regulating proprotein processing and have therapeutic implications for metabolic control.

Dysfunction of muscle satellite cells is linked to diabetic myopathy. The mechanisms vitiating muscle satellite cell proliferative activity in diabetes are not well understood. Here, we show that AS160, a key cytosolic Rab-GTPase activating protein (RabGAP) in insulin signaling, is a moonlighting protein regulating muscle satellite cell proliferation as a transcriptional co-factor. Deletion of AS160, but not its GAP-inactive mutation, impairs muscle satellite cell proliferation and consequent muscle regeneration, and exacerbates age-related sarcopenia. Mechanistically, Thr⁶⁴² phosphorylation of AS160 promotes its translocation into the nucleus where AS160 functions as a co-factor of Signal Transducer and Activator of Transcription 3 (STAT3). AS160 binds to STAT3 to enhance the transcription of myogenic cascades and consequent muscle satellite cell proliferation. Disruption of the AS160-STAT3 interaction, or inhibition of AS160-Thr⁶⁴² phosphorylation, inhibits muscle satellite cell proliferation and impairs muscle regeneration. Together, our findings reveal a moonlighting function of AS160 as a transcriptional co-factor in the nucleus, and have therapeutic implications for muscle regeneration.

Aims: Pancreatic β cell homeostasis involving their proliferation, growth, and apoptosis is critical for postprandial insulin levels and glycaemic control. The regulatory mechanisms governing β cell homeostasis and insulin secretion are incompletely understood. This study aims to elucidate the role of Ral‐GTPase activating protein (RalGAP) complexes in β cell homeostasis and insulin secretion. Materials and Methods: Pancreatic β cell‐specific RalGAP β knockout mice and RalGAP β ‐knockdown INS‐1 cells were constructed for studying the pancreatic function of RalGAP complexes. Metabolic phenotyping, immunoblotting, RT‐QPCR, glucose‐stimulated insulin secretion (GSIS), proliferation and apoptosis assays, RNA‐sequencing and metabolomic profiling were performed. Results: High‐fat diet increased RalGAP complex and concomitantly decreased GTP‐bound active RalA in pancreatic islets. Deletion of RalGAP β , a common regulatory subunit of RalGAP complexes, destabilised RalGAP α 1/ α 2 and elevated GTP‐RalA in β cells. RalGAP β deficiency promoted proliferation of β cells, increased their sizes, and inhibited streptozotocin‐induced β cell apoptosis. Moreover, β cell‐specific deletion of RalGAP β enhanced KCl‐stimulated insulin secretion in isolated islets as well as GSIS in vivo. Consequently, RalGAP β deletion in β cells improved glucose tolerance in mice fed a chow or high‐fat diet. At the molecular level, RalGAP β deletion may reset the glucostat consisting of GLUT1 and GLUT2 to modulate glycemic set point. Furthermore, RalGAP β deficiency elevated p70 S6‐kinase in islets, whose inhibition abrogated RalGAP β deficiency‐induced β cell proliferation. Conclusion: RalGAP complexes function as a critical regulator of both β cell homeostasis and insulin secretion. Our results have implications for the discovery of drugs targeting RalGAP complexes to protect β cells in type 2 diabetes.

Background: Cholesterol crystals in hepatocytes are known to strongly associate with human metabolic dysfunction–associated steatohepatitis. However, it remains unclear which molecular pathway(s) regulates free cholesterol accumulation and the formation of cholesterol crystals in hepatocytes. In cultured cell lines, oxysterol-binding protein-related protein 2 (ORP2) functions to deliver cholesterol to the plasma membrane from endosomal compartments. Methods: Here, we generated liver-specific ORP2 knockout (ORP2-LKO) mice and characterized their metabolic phenotypes on chow and high-fat diet. Results: The ORP2-LKO mice developed much more severe hepatic steatosis than floxed control mice after high-fat diet feeding. They also demonstrated more severe liver inflammation and damage. Notably, free but not esterified cholesterol, as well as cholesterol crystals, accumulated in the ORP2-LKO liver. The expression of Cyp7a1 was significantly upregulated in the ORP2-LKO liver, accompanied by the accumulation of taurocholic acid. Our results thus unveil an important in vivo function of ORP2 in preventing free cholesterol from accumulating in the mouse liver. Conclusions: Our results suggest that impaired cholesterol trafficking may exacerbate the deposition of cholesterol crystals in hepatocytes, promoting the development of metabolic dysfunction–associated steatohepatitis.

Elevated glucose metabolism triggers two primary processes that lead to β-cell depolarization and insulin secretion: the closure of ATP-sensitive K⁺ channels via ATP-dependent mechanisms and the activation of mechanosensitive channels (MSCs) due to cell swelling. However, the identity of these MSCs remains unclear. In this study, we found that TMEM63B is a stretch-activated cation channel (SAC) crucial for regulating insulin secretion in response to elevated glucose levels. TMEM63B is abundantly expressed in β-cells, and its deletion impairs insulin secretion triggered by high glucose. High glucose levels typically increase Ca²⁺ influx and firing frequency in β-cells, a response largely eliminated when TMEM63B is deleted. Mechanistically, glucose metabolism induces cell swelling and activates TMEM63B, which, in turn, leads to β-cell depolarization and insulin secretion. In conclusion, our findings demonstrate that TMEM63B is an SAC essential for regulating insulin secretion in response to elevated glucose levels.

Insulin-like growth factor 1 (IGF1) is a regulator of both cellular hypertrophy and lipogenesis, which are two key processes for pathogenesis of obesity. However, the in vivo role of IGF1 in the development of obesity remains unclear. Here, we show that IGF1 expression is increased in adipose tissue in obese human patients and animal models. Elevation of IGF1 is associated with increased lipogenic gene expression and decreased energy expenditure. Genetic down-regulation of IGF1 normalizes lipogenic gene expression, restores aberrant energy metabolism and alleviates obese phenotype of a genetic mouse model with IGF1-hypersecretion. Importantly, genetic down-regulation of IGF1 exerts similar effects on development of diet-induced obesity. Furthermore, berberine that is an AMP-activated protein kinase (AMPK) activator in medicinal herbs inhibits IGF1 secretion, decreases lipogenic gene expression and alleviates diet-induced adiposity. Collectively, our findings demonstrate that hypersecretion of IGF1 is a critical factor for the development of obesity and can be targeted using AMPK activators to alleviate obesity.

The obese heart undergoes metabolic remodeling and exhibits impaired calcium (Ca²⁺) homeostasis, which are two critical assaults leading to cardiac dysfunction. The molecular mechanisms underlying these alterations in obese heart are not well understood. Here, we show that the Rab-GTPase activating protein AS160 is a lipid-responsive regulator of Ca²⁺ homeostasis through governing lysophosphatidylinositol metabolism and signaling. Palmitic acid/high fat diet inhibits AS160 activity through phosphorylation by NEK6, which consequently activates its downstream target Rab8a. Inactivation of AS160 in cardiomyocytes elevates cytosolic Ca²⁺ that subsequently impairs cardiac contractility. Mechanistically, Rab8a downstream of AS160 interacts with DDHD1 to increase lysophosphatidylinositol metabolism and signaling that leads to Ca²⁺ release from sarcoplasmic reticulum. Inactivation of NEK6 prevents inhibition of AS160 by palmitic acid/high fat diet, and alleviates cardiac dysfunction in high fat diet-fed mice. Together, our findings reveal a regulatory mechanism governing metabolic remodeling and Ca²⁺ homeostasis in obese heart, and have therapeutic implications to combat obesity cardiomyopathy.

Lipid droplets (LDs) serve as crucial hubs for lipid trafficking and metabolic regulation through their numerous interactions with various organelles. While the interplay between LDs and the Golgi apparatus has been recognized, their roles and underlying mechanisms remain poorly understood. Here, we reveal the role of Ras-related protein Rab-2A (Rab2A) in mediating LD-Golgi interactions, thereby contributing to very-low-density lipoprotein (VLDL) lipidation and secretion in hepatocytes. Mechanistically, our findings identify a selective interaction between Golgi-localized Rab2A and 17-beta-hydroxysteroid dehydrogenase 13 (HSD17B13) protein residing on LDs. This complex facilitates dynamic organelle communication between the Golgi apparatus and LDs, thus contributing to lipid transfer from LDs to the Golgi apparatus for VLDL2 lipidation and secretion. Attenuation of Rab2A activity via AMP-activated protein kinase (AMPK) suppresses the Rab2A-HSD17B13 complex formation, impairing LD-Golgi interactions and subsequent VLDL secretion. Furthermore, genetic inhibition of Rab2A and HSD17B13 in the liver reduces the serum triglyceride and cholesterol levels. Collectively, this study provides a new perspective on the interactions between the Golgi apparatus and LDs.