Redox-dependent and ligand-independent trans-activation of insulin receptor by globular adiponectin

From diabetes to renal aging: the therapeutic potential of adiponectin

Nowadays, the complications related to diabetes, such as nephropathy, cardiovascular problems, and aging, are highly being considered. Renal cell aging is affected by various mechanisms of inflammation, oxidative stress, and basement membrane thickening, which are significant causes of renal dysfunction in diabetes. Due to recent studies, adiponectin plays a key role in diabetes-related kidney diseases as a fat-derived hormone. In diabetes, reduced adiponectin levels are associated to renal cell aging. Oxidative stress and related signaling pathways are the main routes in which adiponectin may be effective to decline diabetes-associated aging. Therefore, adiponectin signaling in target tissues becomes one of the research areas of interest in metabolism and clinical medicine. Studies on adiponectin signaling will increase our understanding of adiponectin role in diabetes-linked diseases as well as shortening life span conditions which may guide the design of antidiabetic and anti-aging drugs.

Adiponectin Signaling Pathways in Liver Diseases

In the liver, adiponectin regulates both glucose and lipid metabolism and exerts an insulin-sensitizing effect. The binding of adiponectin with its specific receptors induces the activation of a proper signaling cascade that becomes altered in liver pathologies. This review describes the different signaling pathways in healthy and diseased hepatocytes, also highlighting the beneficial role of adiponectin in autophagy activation and hepatic regeneration.

Adiponectin-Resistance in Obesity

The decrease in adiponectin levels are negatively correlated with chronic subclinical inflammation markers in obesity. The hypertrophic adipocytes cause obesity-linked insulin resistance and metabolic syndrome. Furthermore, macrophage polarization is a key determinant regulating adiponectin receptor (AdipoR1/R2) expression and differential adiponectin-mediated macrophage inflammatory responses in obese individuals. In addition to decrease in adiponectin concentrations, the decline in AdipoR1/R2 mRNA expression leads to a decrement in adiponectin binding to cell membrane, and this turns into attenuation in the adiponectin effects. Within the receptor complex, adaptor protein-containing pleckstrin homology domain, phosphotyrosine-binding domain, and leucine zipper motif 1 (APPL1) is the intracellular binding partner of AdipoR1 and AdipoR2. The expression levels of APPL1 or APPL2 lead to an altered adiponectin activity. Despite normal or high adiponectin levels, an impaired post receptor signaling due to APPL1/APPL2 may alter adiponectin efficiency and activity. However, APPL2 blocks adiponectin signaling through AdipoR1 and AdipoR2 by competitive inhibition of APPL1. APPL1 is also an important mediator of adiponectin dependent insulin sensitization. In this context, adiponectin resistance is associated with insulin resistance and is thought to be partly due to the down-regulation of the AdipoRs in high-fat diet fed subjects. Actually, adiponectin resistance occurs very rapidly after saturated fatty acid feeding, this metabolic disturbance is not due to a decrease in AdipoR1 protein content. Intra-abdominal adipose tissue-AdipoR2 expression is reduced in obesity, whereas AdipoR1 expression is not changed. Adiponectin resistance together with insulin resistance forms a vicious cycle. The elevated adiponectin levels with adiponectin resistance is a compensatory response in the condition of an unusual discordance between insulin resistance and adiponectin unresponsiveness.Additionally, different mechanisms are involved in vascular adiponectin resistance at different stages of obesity. Nevertheless, diet-induced hyperlipidemia is the leading cause of vascular adiponectin resistance. Leptin/adiponectin imbalance may also be an important marker of the elevated risk of developing abdominal obesity-associated cardiovascular diseases.

Oxidative Stress in the Healthy and Wounded Hepatocyte: A Cellular Organelles Perspective

Accurate control of the cell redox state is mandatory for maintaining the structural integrity and physiological functions. This control is achieved both by a fine-tuned balance between prooxidant and anti-oxidant molecules and by spatial and temporal confinement of the oxidative species. The diverse cellular compartments each, although structurally and functionally related, actively maintain their own redox balance, which is necessary to fulfill specialized tasks. Many fundamental cellular processes such as insulin signaling, cell proliferation and differentiation and cell migration and adhesion, rely on localized changes in the redox state of signal transducers, which is mainly mediated by hydrogen peroxide (H₂O₂). Therefore, oxidative stress can also occur long before direct structural damage to cellular components, by disruption of the redox circuits that regulate the cellular organelles homeostasis. The hepatocyte is a systemic hub integrating the whole body metabolic demand, iron homeostasis and detoxification processes, all of which are redox-regulated processes. Imbalance of the hepatocyte's organelles redox homeostasis underlies virtually any liver disease and is a field of intense research activity. This review recapitulates the evolving concept of oxidative stress in the diverse cellular compartments, highlighting the principle mechanisms of oxidative stress occurring in the healthy and wounded hepatocyte.

Adiponectin as a tissue regenerating hormone: more than a metabolic function

The great interest that scientists have for adiponectin is primarily due to its central metabolic role. Indeed, the major function of this adipokine is the control of glucose homeostasis that it exerts regulating liver and muscle metabolism. Adiponectin has insulin-sensitizing action and leads to down-regulation of hepatic gluconeogenesis and an increase of fatty acid oxidation. In addition, adiponectin is reported to play an important role in the inhibition of inflammation. The hormone is secreted in full-length form, which can either assemble into complexes or be converted into globular form by proteolytic cleavage. Over the past few years, emerging publications reveal a more varied and pleiotropic action of this hormone. Many studies emphasize a key role of adiponectin during tissue regeneration and show that adiponectin deficiency greatly inhibits the mechanisms underlying tissue renewal. This review deals with the role of adiponectin in tissue regeneration, mainly referring to skeletal muscle regeneration, a process in which adiponectin is deeply involved. In this tissue, globular adiponectin increases proliferation, migration and myogenic properties of both resident stem cells (namely satellite cells) and non-resident muscle precursors (namely mesoangioblasts). Furthermore, skeletal muscle could be a site for the local production of the globular form that occurs in an inflamed environment. Overall, these recent findings contribute to highlight an intriguing function of adiponectin in addition to its well-recognized metabolic action.

Succinate dehydrogenase subunit B mutations modify human neuroblastoma cell metabolism and proliferation

Paragangliomas (PGLs) are rare neuroendocrine tumours. About 30-40 % of these tumours are mutated in one of the different susceptibility genes, including those encoding the different subunits of the succinate dehydrogenase, a complex involved both in the tricarboxylic acid cycle and in the oxygen transport chain. The aim of this work was to investigate whether SDHB mutations may account for alterations in cell metabolism and functions. Since human PGL cell lines are not available, we used the neuroblastoma cell line (SK-N-AS) stably transfected with the wild-type human SDHB or different SDHB-mutated constructs carrying some significant mutations found in our patients affected by PGLs. Similarly to succinate dehydrogenase (SDH)-mutated tumour cells, mutated SK-N-AS clones showed reduced SDH enzyme activity. All clones showed normal citrate synthase activity, reduced oxygen consumption and reduced carbonic anhydride production, thus demonstrating a decreased in mitochondrial metabolism. In two of the three mutated SK-N-AS, we also found an increase in HIF1α expression. Surprisingly and unexpectedly, in all the SDHB-mutated clones, we found a significant decrease in glucose uptake and in lactate culture medium concentration, suggesting also a decrease of cytosolic metabolism. Finally, we found that these energetic changes were associated to an increase in cell proliferation and migration. Overall, these data demonstrate that although SDHB mutations significantly downregulate both mitochondrial and cytoplasmic cellular metabolism, these mutations are associated to an upregulation of some cellular functions, such as growth rate and invasiveness.

An Intimate Relationship between ROS and Insulin Signalling: Implications for Antioxidant Treatment of Fatty Liver Disease

Oxidative stress damages multiple cellular components including DNA, lipids, and proteins and has been linked to pathological alterations in nonalcoholic fatty liver disease (NAFLD). Reactive oxygen species (ROS) emission, resulting from nutrient overload and mitochondrial dysfunction, is thought to be a principal mediator in NAFLD progression, particularly toward the development of hepatic insulin resistance. In the context of insulin signalling, ROS has a dual role, as both a facilitator and inhibitor of the insulin signalling cascade. ROS mediate these effects through redox modifications of cysteine residues affecting phosphatase enzyme activity, stress-sensitive kinases, and metabolic sensors. This review highlights the intricate relationship between redox-sensitive proteins and insulin signalling in the context of fatty liver disease, and to a larger extent, the importance of reactive oxygen species as primary signalling molecules in metabolically active cells.

Hepatitis B virus X protein mutant HBxΔ127 promotes proliferation of hepatoma cells through up-regulating miR-215 targeting PTPRT

The mutant of virus is a frequent event. Hepatitis B virus X protein (HBx) plays a vital role in the development of hepatocellular carcinoma (HCC). Therefore, the identification of potent mutant of HBx in hepatocarcinogenesis is significant. Previously, we identified a natural mutant of the HBx gene (termed HBxΔ127). Relative to wild type HBx, HBxΔ127 strongly enhanced cell proliferation and migration in HCC. In this study, we aim to explore the mechanism of HBxΔ127 in promotion of proliferation of hepatoma cells. Our data showed that both wild type HBx and HBxΔ127 could increase the expression of miR-215 in hepatoma HepG2 and H7402 cells. However, HBxΔ127 was able to significantly increase miR-215 expression relative to wild type HBx in the cells. We identified that protein tyrosine phosphatase, receptor type T (PTPRT) was one of the target genes of miR-215 through targeting 3'UTR of PTPRT mRNA. In function, miR-215 was able to promote the proliferation of hepatoma cells. Meanwhile anti-miR-215 could partially abolish the enhancement of cell proliferation mediated by HBxΔ127 in vitro. Knockdown of PTPRT by siRNA could distinctly suppress the decrease of cell proliferation mediated by anti-miR-215 in HepG2-XΔ127/H7402-XΔ127 cells. Moreover, we found that anti-miR-215 remarkably inhibited the tumor growth of hepatoma cells in nude mice. Collectively, relative to wild type HBx, HBxΔ127 strongly enhances proliferation of hepatoma cells through up-regulating miR-215 targeting PTPRT. Our finding provides new insights into the mechanism of HBx mutant HBxΔ127 in promotion of proliferation of hepatoma cells.

Globular adiponectin activates motility and regenerative traits of muscle satellite cells

Regeneration of adult injured skeletal muscle is due to activation of satellite cells, a population of stem cells resident beneath the basal lamina. Thus, information on soluble factors affecting satellite cell activation, as well as migration towards injury and fusion into new myofibers are essential. Here, we show that globular adiponectin (gAd), positively affects several features of muscle satellite cells. gAd activates satellite cells to exit quiescence and increases their recruitment towards myotubes. gAd elicits in satellite cells a specific motility program, involving activation of the small GTPase Rac1, as well as expression of Snail and Twist transcription factors driving a proteolytic motility, useful to reach the site of injury. We show that satellite cells produce autocrine full length adiponectin (fAd), which is converted to gAd by activated macrophages. In turns, gAd concurs to attract to the site of injury both satellite cells and macrophages and induces myogenesis in muscle satellite cells. Thus, these findings add a further role for gAd in skeletal muscle, including the hormone among factors participating in muscle regeneration.

Regulation of protein tyrosine phosphatases by reversible oxidation

Oxidation of the catalytic cysteine of protein-tyrosine phosphatases (PTP), which leads to their reversible inactivation, has emerged as an important regulatory mechanism linking cellular tyrosine phosphorylation and signalling by reactive-oxygen or -nitrogen species (ROS, RNS). This review focuses on recent findings about the involved pathways, enzymes and biochemical mechanisms. Both the general cellular redox state and extracellular ligand-stimulated ROS production can cause PTP oxidation. Members of the PTP family differ in their intrinsic susceptibility to oxidation, and different types of oxidative modification of the PTP catalytic cysteine can occur. The role of PTP oxidation for physiological signalling processes as well as in different pathologies is described on the basis of well-investigated examples. Criteria to establish the causal involvement of PTP oxidation in a given process are proposed. A better understanding of mechanisms leading to selective PTP oxidation in a cellular context, and finding ways to pharmacologically modulate these pathways are important topics for future research.

Adipokines and redox signaling: impact on fatty liver disease

Adipokines (adipose tissue cytokines) are polypeptide factors secreted by adipose tissue in a highly regulated manner. The 'classical' adipokines (leptin, adiponectin, and resistin) are expressed only by adipocytes, but other adipokines have been shown to be released by resident and infiltrating macrophages, as well as by components of the vascular stroma. Indeed, adipose tissue inflammation is known to be associated with a modification in the pattern of adipokine secretion. Several studies indicate that adipokines can interfere with hepatic injury associated with fatty infiltration, differentially modulating steatosis, inflammation, and fibrosis. Moreover, plasma levels of adipokines have been investigated in patients with nonalcoholic fatty liver disease in order to establish correlations with the underlying state of insulin resistance and with the type and severity of hepatic damage. In this Forum article, we provide a review of recent data that suggest a significant role for oxidative stress, reactive oxygen species, and redox signaling in mediating actions of adipokines that are relevant in the pathogenesis of nonalcoholic fatty liver disease, including hepatic insulin resistance, inflammation, and fibrosis.

Adiponectin resistance and vascular dysfunction in the hyperlipidemic state

Insulin plays an important role in the stimulation of vascular nitric oxide production, with both short term (vasomotility and anti-thrombotic effects) and long term (smooth muscle cell growth and migration inhibition) benefits. Impaired vasodilatory response to insulin, the hallmark of vascular insulin resistance (IR), has important implications for circulatory pathophysiology. An association between adipokines and IR has been observed in both diabetic and nondiabetic states. Adiponectin (APN) is an insulin-sensitizing adipokine known to stimulate skeletal muscle fatty acid (FA) oxidation and reduce lipid accumulation. Recent demonstrations of potential cross-talk between APN and insulin in vascular function regulation are particularly interesting. The lipid accumulation observed after chronic high-fat (HF) diets and in the obese state may reduce vascular response to APN, a pathologic state termed as APN resistance. This review highlights the importance of insulin sensitivity and APN activity in the maintenance of endothelial function. It explores the relationships between vascular IR and APN resistance in the hyperlipidemic pathological condition, representative of the metabolic syndrome. The investigation of vascular insulin and APN resistance provides not only better understanding of vascular pathophysiology, but also an opportunity for therapeutic targeting in individuals affected by the metabolic syndrome.

Globular adiponectin as a complete mesoangioblast regulator: role in proliferation, survival, motility, and skeletal muscle differentiation

This article shows that globular adiponectin regulates vital cues of mesoangioblast, such as proliferation, survival, and migration toward myotubes and the myogenic properties. In vivo experiments confirm that globular adiponectin increases the survival, engraftment, and localization to muscle of mesoangioblasts in α-sarcoglycan-null mice.

Mesoangioblasts are progenitor endowed with multipotent mesoderm differentiation ability. Despite the promising results obtained with mesoangioblast transplantation in muscle dystrophy, an improvement of their efficient engrafting and survival within damaged muscles, as well as their ex vivo activation/expansion and commitment toward myogenic lineage, is highly needed and should greatly increase their therapeutic potential. We show that globular adiponectin, an adipokine endowed with metabolic and differentiating functions for muscles, regulates vital cues of mesoangioblast cell biology. The adipokine drives mesoangioblasts to entry cell cycle and strongly counteracts the apoptotic process triggered by growth factor withdrawal, thereby serving as an activating and prosurvival stem cell factor. In addition, adiponectin provides a specific protection against anoikis, the apoptotic death due to lack of anchorage to extracellular matrix, suggesting a key protective role for these nonresident stem cells after systemic injection. Finally, adiponectin behaves as a chemoattractive factor toward mature myotubes and stimulates their differentiation toward the skeletal muscle lineage, serving as a positive regulator in mesoangioblast homing to injured or diseased muscles. We conclude that adiponectin exerts several advantageous effects on mesoangioblasts, potentially valuable to improve their efficacy in cell based therapies of diseased muscles.

Sphingosine 1-phosphate increases glucose uptake through trans-activation of insulin receptor

Sphingosine 1-phosphate (S1P) is a bioactive lipid that acts through a family of G-protein-coupled receptors. Herein, we report evidence of a novel redox-based cross-talk between S1P and insulin signaling pathways. In skeletal muscle cells S1P, through engagement of its S1P(2) receptor, is found to produce a transient burst of reactive oxygen species through a calcium-dependent activation of the small GTPase Rac1. S1P-induced redox-signaling is sensed by protein tyrosine phosphatase-1B, the main negative regulator of insulin receptor phosphorylation, which undergoes oxidation and enzymatic inhibition. This redox-based inhibition of the phosphatase provokes a ligand-independent trans-phosphorylation of insulin receptor and a strong increase in glucose uptake. Our results propose a new role of S1P, recognizing the lipid as an insulin-mimetic cue and pointing at reactive oxygen species as critical regulators of the cross-talk between S1P and insulin pathways. Any possible implication of S1P-directed insulin signaling in diabetes and insulin resistance remains to be established.

Adipokines in liver diseases

Adipokines are polypeptides secreted in the adipose tissue in a regulated manner. While some of these molecules are expressed only by adipocytes, resident and infiltrating macrophages and components of the vascular stroma markedly contribute to expression of other adipokines. As a result, adipose tissue inflammation is associated with a modification in the pattern of adipokine secretion. Leptin, adiponectin, and resistin are the best-studied molecules in this class, but cytokines such as tumor necrosis factor or interleukin-6 are also secreted at high levels by the adipose tissue. Several other molecules have been recently identified and are actively investigated. Adipokines interfere with hepatic injury associated with fatty infiltration, differentially modulating steatosis, inflammation, and fibrosis. Several studies have investigated plasma levels of adiponectin in patients with nonalcoholic fatty liver disease, to establish correlations with the underlying state of insulin resistance and with the type and severity of hepatic damage. Hepatitis C is another disease where adipokines may represent a link between viral infection, steatosis, and metabolic disturbances. Identification of the mediators secreted by expanded adipose tissue and their pathogenic role is pivotal in consideration of the alarming increase in the prevalence of obesity and of the detrimental role that this condition exerts on the course of liver diseases.

Regulation of globular adiponectin-induced apoptosis by reactive oxygen/nitrogen species in RAW264 macrophages

Adiponectin, produced predominantly by differentiating adipocytes, is a protein hormone with antidiabetic and immunosuppressive properties. Here, we report evidence that treatment with globular adiponectin (gAd) induces apoptosis in murine macrophage-like RAW264 cells through the generation of reactive oxygen and/or nitrogen species (ROS/RNS). Treatment with gAd induced apoptosis and enhanced the activities of caspase-3 and -9, but not caspase-8. The gAd stimulation increased ROS generation and significantly reduced the ratio of NADPH to total NADP. Pretreatment with diphenyleneiodonium or apocynin reduced ROS and apoptosis in gAd-treated cells. In addition, transfection with p47(phox)- or gp91(phox)-specific small interfering RNA (siRNA) partially reduced ROS and apoptosis in response to gAd treatment. These results suggest that the administration of gAd induces apoptosis after ROS generation involving activation of NADPH oxidases. The gAd stimulation increased the release of NO into the culture medium, the activity of nitric oxide synthase (NOS), and the expression of inducible NOS (iNOS) mRNA in RAW264 cells. l-NAME reduced gAd-induced apoptotic cell death. In addition, transfection with an iNOS-specific siRNA markedly reduced the generation of NO and the population of apoptotic cells. Taken together, these results demonstrate that the gAd-induced apoptotic process in RAW264 cells involves ROS and RNS, which are generated by NADPH oxidases and iNOS, respectively.

The chemistry of cell signaling by reactive oxygen and nitrogen species and 4-hydroxynonenal

During the past several years, major advances have been made in understanding how reactive oxygen species (ROS) and nitrogen species (RNS) participate in signal transduction. Identification of the specific targets and the chemical reactions involved still remains to be resolved with many of the signaling pathways in which the involvement of reactive species has been determined. Our understanding is that ROS and RNS have second messenger roles. While cysteine residues in the thiolate (ionized) form found in several classes of signaling proteins can be specific targets for reaction with H(2)O(2) and RNS, better understanding of the chemistry, particularly kinetics, suggests that for many signaling events in which ROS and RNS participate, enzymatic catalysis is more likely to be involved than non-enzymatic reaction. Due to increased interest in how oxidation products, particularly lipid peroxidation products, also are involved with signaling, a review of signaling by 4-hydroxy-2-nonenal (HNE) is included. This article focuses on the chemistry of signaling by ROS, RNS, and HNE and will describe reactions with selected target proteins as representatives of the mechanisms rather attempt to comprehensively review the many signaling pathways in which the reactive species are involved.

The PIM1 kinase is a critical component of a survival pathway activated by docetaxel and promotes survival of docetaxel-treated prostate cancer cells

A defining characteristic of solid tumors is the capacity to divide aggressively and disseminate under conditions of nutrient deprivation, limited oxygen availability, and exposure to cytotoxic drugs or radiation. Survival pathways are activated within tumor cells to cope with these ambient stresses. We here describe a survival pathway activated by the anti-cancer drug docetaxel in prostate cancer cells. Docetaxel activates STAT3 phosphorylation and transcriptional activity, which in turns induces expression of the PIM1 gene, encoding a serine-threonine kinase activated by many cellular stresses. Expression of PIM1 improves survival of docetaxel-treated prostate cancer cells, and PIM1 knockdown or expression of a dominant-negative PIM1 protein sensitize cells to the cytotoxic effects of docetaxel. PIM1 in turn mediates docetaxel-induced activation of NFkappaB transcriptional activity, and PIM1 depends in part on RELA/p65 proteins for its prosurvival effects. The PIM1 kinase plays a critical role in this STAT3 --> PIM1 --> NFkappaB stress response pathway and serves as a target for intervention to enhance the therapeutic effects of cytotoxic drugs such as docetaxel.

Activation of peroxisome proliferator-activated receptor-alpha inhibits the injurious effects of adiponectin in rat steatotic liver undergoing ischemia-reperfusion

Hepatic steatosis is a major risk factor in ischemia-reperfusion (I/R). Adiponectin acts as an antiobesity and anti-inflammatory hormone. Adiponectin activates peroxisome proliferator-activated receptor-alpha (PPAR-alpha), a transcription factor that regulates inflammation in liver disease. Ischemic preconditioning (PC) based on brief periods of I/R protects steatotic livers against subsequent sustained I/R injury, but just how this is achieved is poorly understood. This study explains the role of PPAR-alpha and adiponectin in the vulnerability shown by steatotic livers to I/R and the benefits of PC in this situation. PPAR-alpha and adiponectin levels in nonsteatotic livers undergoing I/R were similar to those found in the sham group. However, reduced PPAR-alpha and increased adiponectin levels, particularly the high molecular weight isoform, were observed in steatotic livers as a consequence of I/R. Our results suggest that mitogen-activated protein kinases (MAPKs) may be positive regulators of adiponectin accumulation in steatotic livers. The addition of adiponectin small interfering RNA (siRNA) before I/R protected steatotic livers against oxidative stress and hepatic injury. The induction of PC before I/R increased PPAR-alpha and reduced adiponectin levels in steatotic livers. PC, which increased PPAR-alpha, as well as PPAR-alpha agonist pretreatment reduced MAPK expression, adiponectin, oxidative stress, and hepatic injury that follows I/R. In addition, the administration of a PPAR-alpha antagonist in preconditioned steatotic livers eliminated the beneficial effects of PC on MAPKs, adiponectin, oxidative stress, and hepatic injury.

CONCLUSION

Steatotic livers are more predisposed to down-regulate PPAR-alpha and overexpress adiponectin when subjected to I/R. PPAR-alpha agonists and adiponectin siRNA are promising candidates to protect steatotic livers. PPAR-alpha agonists as well as PC, through PPAR-alpha, inhibited MAPK expression following I/R. This in turn inhibited adiponectin accumulation in steatotic livers and adiponectin-worsening effects on oxidative stress and hepatic injury.