Gang Xi

Bio: Gang Xi is an academic researcher from University of North Carolina at Chapel Hill. The author has contributed to research in topics: Phosphorylation & Signal transduction. The author has an hindex of 17, co-authored 30 publications receiving 738 citations.

Suppression of AMPK Activation via S485 Phosphorylation by IGF-I during Hyperglycemia Is Mediated by AKT Activation in Vascular Smooth Muscle Cells

Abstract: As a metabolic sensor, the serine/threonine protein kinase AMP-activated protein kinase (AMPK) promotes the adaptation of cells to signals arising from nutrients, hormones, and growth factors. The ability of IGF-I to stimulate protein synthesis is suppressed by AMPK, therefore, these studies were undertaken to determine whether IGF-I modulates AMPK activity. IGF-I dose-dependently suppressed phosphorylation of AMPK T172, and it stimulated AMPK S485 phosphorylation in vascular smooth muscle cells (VSMC). To determine whether stimulation of AMPK S485 phosphorylation was mediating this response, VSMC were transduced with a mutant AMPKα (AMPK S485A). Expression of this altered form inhibited the ability of IGF-I to suppress AMPK T172 activation, which resulted in inhibition of IGF-I-stimulated phosphorylation of P70S6 kinase. In contrast, expression of an AMPK S485D mutant resulted in constitutive suppression of AMPK activity and was associated with increased IGF-I-stimulated P70S6K phosphorylation and protein synthesis. The addition of a specific AKT inhibitor or expression of an AKT1 short hairpin RNA inhibited AMPK S485 phosphorylation, and it attenuated the IGF-I-induced decrease in AMPK T172 phosphorylation. Exposure to high glucose concentrations suppressed AMPK activity and stimulated S485 phosphorylation, and IGF-I stimulated a further increase in S485 phosphorylation and AMPK T172 suppression. We conclude that AMPK S485 phosphorylation negatively regulates AMPK activity by modulating the T172 phosphorylation response to high glucose and IGF-I. IGF-I stimulates S485 phosphorylation through AKT1. The results suggest that AMPK plays an inhibitory role in modulating IGF-I-stimulated protein synthesis and that IGF-I must down-regulate AMPK activity to induce an optimal anabolic response.

Insulin-Like Growth Factor (IGF) Binding Protein 2 Functions Coordinately with Receptor Protein Tyrosine Phosphatase β and the IGF-I Receptor To Regulate IGF-I-Stimulated Signaling

Abstract: Insulin-like growth factor I (IGF-I) is a mitogen for vascular smooth muscle cells (VSMC) and has been implicated in the development and progression of atherosclerosis. IGF binding proteins (IGFBPs) modify IGF-I actions independently of IGF binding, but a receptor-based mechanism by which they function has not been elucidated. We investigated the role of IGFBP-2 and receptor protein tyrosine phosphatase β (RPTPβ) in regulating IGF-I signaling and cellular proliferation. IGFBP-2 bound RPTPβ, which led to its dimerization and inactivation. This enhanced PTEN tyrosine phosphorylation and inhibited PTEN activity. Utilization of substrate trapping and phosphatase-dead mutants showed that RPTPβ bound specifically to PTEN and dephosphorylated it. IGFBP-2 knockdown led to decreased PTEN tyrosine phosphorylation and decreased AKT Ser473 activation. IGFBP-2 enhanced IGF-I-stimulated VSMC migration and proliferation. Analysis of aortas obtained from IGFBP-2(-/-) mice showed that RPTPβ was activated, and this was associated with inhibition of IGF-I stimulated AKT Ser473 phosphorylation and VSMC proliferation. These changes were rescued following administration of IGFBP-2. These findings present a novel mechanism for coordinate regulation of IGFBP-2 and IGF-I signaling functions that lead to stimulation of VSMC proliferation. The results have important implications for understanding how IGFBPs modulate the cellular response to IGF-I.

IGF-I and IGFBP-2 Stimulate AMPK Activation and Autophagy, Which Are Required for Osteoblast Differentiation

Abstract: IGF-I/insulin-like growth factor binding protein 2 (IGFBP-2) coordinately stimulate osteoblast differentiation but the mechanisms by which they function have not been determined. AMP-activated protein kinase (AMPK) is induced during differentiation and AMPK knockout mice have reduced bone mass. IGF-I modulates AMPK in other cell types; therefore, these studies determined whether IGF-I/IGFBP-2 stimulate AMPK activation and the mechanism by which AMPK modulates differentiation. Calvarial osteoblasts and MC-3T3 cells expressed activated AMPK early in differentiation and AMPK inhibitors attenuated differentiation. However, expression of constitutively activated AMPK inhibited differentiation. To resolve this discrepancy we analyzed the time course of AMPK induction. AMPK activation was required early in differentiation (day 3-6) but down-regulation of AMPK after day 9 was also necessary. IGF-I/IGFBP-2 induced AMPK through their respective receptors and blocking-receptor activation blocked AMPK induction. To determine the mechanism by which AMPK functioned we analyzed components of the autophagosome. Activated AMPK stimulated ULK-1 S555 phosphorylation as well as beclin-1 and microtubule-associated protein 1A/1B light-chain phosphatidylethanolamine conjugate (LC3II) induction. Inhibition of AMPK attenuated these changes and direct inhibition of autophagy inhibited differentiation. Conversely, expression of activated AMPK was associated with persistence of these changes beyond day 9 and inhibited differentiation. Blocking AMPK activation after day 9 down-regulated these autophagosome components and rescued differentiation. This allowed induction of mechanistic target of rapamycin and AKT, which suppressed autophagy. The results show that early induction of AMPK in response to IGF-I/IGFBP-2 followed by suppression is required for osteoblast differentiation. AMPK functions through stimulation of autophagy. The findings suggest that these early catabolic changes are important for determining the energy source for osteoblast respiration and down-regulation of these components may be required for induction of glycolysis, which is required during the final anabolic stages of differentiation.

Hyperglycemia Enhances IGF-I–Stimulated Src Activation via Increasing Nox4-Derived Reactive Oxygen Species in a PKCζ-Dependent Manner in Vascular Smooth Muscle Cells

Abstract: IGF-I–stimulated sarcoma viral oncogene (Src) activation during hyperglycemia is required for propagating downstream signaling. The aim of the current study was to determine the mechanism by which hyperglycemia enhances IGF-I–stimulated Src activation and the role of NADPH oxidase 4 (Nox4) and protein kinase C ζ (PKCζ) in mediating this response in vascular smooth muscle cells (VSMCs). Nox4 expression was analyzed in VSMCs exposed to hyperglycemia. The role of Nox4-derived reactive oxygen species (ROS) in IGF-I–stimulated Src activation was investigated via knockdown of Nox4. Different isoforms of PKC were screened to investigate their role in hyperglycemia-induced Nox4. The oxidation of Src was shown to be a prerequisite for its activation in response to IGF-I during hyperglycemia. Hyperglycemia induced Nox4, but not Nox1, and p22 phagocyte oxidase (p22phox) expression and IGF-I stimulated Nox4/p22phox complex formation, leading to increased ROS generation. Knockdown of Nox4 prevented ROS generation and impaired the oxidation and activation of Src in response to IGF-I, whereas knockdown of Nox1 had no effect. PKCζ was shown to mediate the hyperglycemia-induced increase in Nox4 expression. The key observations in cultured VSMCs were confirmed in the diabetic mice. Nox4-derived ROS is responsible for the enhancing effect of hyperglycemia on IGF-I–stimulated Src activation, which in turn amplifies IGF-I–linked downstream signaling and biological actions.

Hyperglycemia Alters the Responsiveness of Smooth Muscle Cells to Insulin-Like Growth Factor-I

Abstract: IGF-I stimulation of smooth muscle cell (SMC) migration and proliferation requires αVβ3 ligand occupancy. We hypothesized that changes in the levels of extracellular matrix proteins induced by alterations in glucose concentrations may regulate the ability of SMCs to respond to IGF-I. IGF-I stimulated migration and proliferation of SMCs that had been maintained in 25 mm glucose containing media, but it had no stimulatory effect when tested using SMCs that had been grown in 5 mm glucose. IGF-I stimulated an increase in Shc phosphorylation and enhanced activation of the MAPK pathway in SMCs grown in 25 mm glucose, whereas in cells maintained in 5 mm glucose, IGF-I had no effect on Shc phosphorylation, and the MAPK response to IGF-I was markedly reduced. In cells grown in 25 mm glucose, the levels of αVβ3 ligands, e.g. osteopontin, vitronectin, and thrombospondin, were all significantly increased, compared with cells grown in 5 mm glucose. The addition of these αVβ3 ligands to SMCs grown in 5 mm glucose was s...

Novel mechanisms of glucocorticoid resistance in childhood acute lymphoblastic leukemia.

Abstract: 2458 Despite dramatic improvements in treatment over the past 40 years, acute lymphoblastic leukemia (ALL) remains one of the most common causes of death from disease in childhood. Glucocorticoids are among the most effective agents used in the treatment of lymphoid malignancies, and patient response to treatment is an important determinant of long-term outcome in childhood ALL. In spite of its clinical significance, the molecular basis of glucocorticoid resistance is still poorly understood. The aim of this study was to develop an experimental model system to define clinically relevant mechanisms of glucocorticoid resistance in childhood ALL. An in vivo model of childhood ALL has been developed in our laboratory, using patient biopsies established as xenografts in immune-deficient nonobese diabetic severe-combined immunodeficient (NOD/SCID) mice. This model is highly representative of the human disease (Lock et al., Blood, 99: 4100-4108, 2002). The in vivo responses of these xenografts to the glucocorticoid dexamethasone (DEX) correlated significantly with patient outcome (p 1 μM) in xenografts from six patients, five of whom died of their disease. In contrast, four DEX-sensitive xenografts (IC50 values 2-fold in sensitive xenografts within 8 hours of treatment. In contrast, Bim induction was dramatically attenuated in DEX-resistant xenografts. These results have identified a clinically significant and novel mechanism of glucocorticoid resistance in childhood ALL, which occurs downstream of receptor-ligand interactions, but upstream of the signalling pathway resulting in Bim induction and apoptosis.

Reactive Oxygen Species in Metabolic and Inflammatory Signaling.

Abstract: Reactive oxygen species (ROS) are well known for their role in mediating both physiological and pathophysiological signal transduction. Enzymes and subcellular compartments that typically produce ROS are associated with metabolic regulation, and diseases associated with metabolic dysfunction may be influenced by changes in redox balance. In this review, we summarize the current literature surrounding ROS and their role in metabolic and inflammatory regulation, focusing on ROS signal transduction and its relationship to disease progression. In particular, we examine ROS production in compartments such as the cytoplasm, mitochondria, peroxisome, and endoplasmic reticulum and discuss how ROS influence metabolic processes such as proteasome function, autophagy, and general inflammatory signaling. We also summarize and highlight the role of ROS in the regulation metabolic/inflammatory diseases including atherosclerosis, diabetes mellitus, and stroke. In order to develop therapies that target oxidative signaling, it is vital to understand the balance ROS signaling plays in both physiology and pathophysiology, and how manipulation of this balance and the identity of the ROS may influence cellular and tissue homeostasis. An increased understanding of specific sources of ROS production and an appreciation for how ROS influence cellular metabolism may help guide us in the effort to treat cardiovascular diseases.

Genetic and cell biological analysis of integrin outside-in signaling.

Abstract: Integrins are cell surface transmembrane receptors that recognize and bind to extracellular matrix proteins and counter receptors. Binding of activated integrins to their ligands induces a vast number of structural and signaling changes within the cell. Large, multimolecular complexes assemble onto the cytoplasmic tails of activated integrins to engage and organize the cytoskeleton, and activate signaling pathways that ultimately lead to changes in gene expression. Additionally, integrin-mediated signaling intersects with growth factor-mediated signaling through various levels of cross-talk. This review discusses recent work that has tremendously broadened our understanding of the complexity of integrin-mediated signaling.

AMPK as a Therapeutic Target for Treating Metabolic Diseases

Abstract: The AMP-activated protein kinase (AMPK) is a central regulator of multiple metabolic pathways and may have therapeutic importance for treating obesity, insulin resistance, type 2 diabetes (T2D), non-alcoholic fatty liver disease (NAFLD), and cardiovascular disease (CVD). Given the ubiquitous expression of AMPK, it has been a challenge to evaluate which tissue types may be most beneficially poised for mediating the positive metabolic effects of AMPK-centered treatments. In this review we evaluate the metabolic phenotypes of transgenic mouse models in which AMPK expression and function have been manipulated, and the impact this has on controlling lipid metabolism, glucose homeostasis, and inflammation. This information may be useful for guiding the development of AMPK-targeted therapeutics to treat chronic metabolic diseases.