Hyperglycemia-induced p66shc inhibits insulin-like growth factor I-dependent cell survival via impairment of Src kinase-mediated phosphoinositide-3 kinase/AKT activation in vascular smooth muscle cells

The p66Shc Redox Protein and the Emerging Complications of Diabetes

Diabetes mellitus is a chronic metabolic disease, the prevalence of which is constantly increasing worldwide. It is often burdened by disabling comorbidities that reduce the quality and expectancy of life of the affected individuals. The traditional complications of diabetes are generally described as macrovascular complications (e.g., coronary heart disease, peripheral arterial disease, and stroke), and microvascular complications (e.g., diabetic kidney disease, retinopathy, and neuropathy). Recently, due to advances in diabetes management and the increased life expectancy of diabetic patients, a strong correlation between diabetes and other pathological conditions (such as liver diseases, cancer, neurodegenerative diseases, cognitive impairments, and sleep disorders) has emerged. Therefore, these comorbidities have been proposed as emerging complications of diabetes. P66Shc is a redox protein that plays a role in oxidative stress, apoptosis, glucose metabolism, and cellular aging. It can be regulated by various stressful stimuli typical of the diabetic milieu and is involved in various types of organ and tissue damage under diabetic conditions. Although its role in the pathogenesis of diabetes remains controversial, there is strong evidence regarding the involvement of p66Shc in the traditional complications of diabetes. In this review, we will summarize the evidence supporting the role of p66Shc in the pathogenesis of diabetes and its complications, focusing for the first time on the emerging complications of diabetes.

p66Shc in Cardiovascular Pathology

p66Shc is a widely expressed protein that governs a variety of cardiovascular pathologies by generating, and exacerbating, pro-apoptotic ROS signals. Here, we review p66Shc’s connections to reactive oxygen species, expression, localization, and discuss p66Shc signaling and mitochondrial functions. Emphasis is placed on recent p66Shc mitochondrial function discoveries including structure/function relationships, ROS identity and regulation, mechanistic insights, and how p66Shc-cyt c interactions can influence p66Shc mitochondrial function. Based on recent findings, a new p66Shc mitochondrial function model is also put forth wherein p66Shc acts as a rheostat that can promote or antagonize apoptosis. A discussion of how the revised p66Shc model fits previous findings in p66Shc-mediated cardiovascular pathology follows.

The Role of p66Shc in Diabetes: A Comprehensive Review from Bench to Bedside

It is well-documented that diabetes is an inflammatory and oxidative disease, with an escalating global burden. Still, there is no definite treatment for diabetes or even prevention of its harmful complications. Therefore, understanding the molecular pathways associated with diabetes might help in finding a solution. p66Shc is a member of Shc family proteins, and it is considered as an oxidative stress sensor and regulator in cells. There are inconsistent data about the role of p66Shc in inducing diabetes, but accumulating evidence supports its role in the pathogenesis of diabetes-related complications, including macro and microangiopathies. There is growing hope that by understanding and targeting molecular pathways involved in this network, prevention of diabetes or its complications would be achievable. This review provides an overview about the role of p66Shc in the development of diabetes and its complications.

P66Shc and vascular endothelial function

Dysfunctional endothelium is an early change in vasculature known to be associated with atherosclerosis. Among many regulators of vascular endothelial function, p66Shc has consistently been shown to mediate endothelial dysfunction. Over more than three decades of active research in the field of the physiological function of p66Shc, regulation of vascular endothelial functions has emerged as one of the most robust effects in a broad range of pathological conditions including hyperlipidemia, diabetes, and aging. A significant understanding has been developed with respect to the molecular signaling regulating the oxidative function of p66Shc in endothelial cells and its targets and regulators. In addition, novel regulatory modifications of p66Shc controlling its oxidative function, subcellular distribution, and stability have also been reported. This review will focus on summarizing the molecular signaling regulating the oxidative function of p66Shc and its role in vascular endothelium.

Activated protein C reverses epigenetically sustained p66Shc expression in plaque-associated macrophages in diabetes

Impaired activated protein C (aPC) generation is associated with atherosclerosis and diabetes mellitus. Diabetes-associated atherosclerosis is characterized by the hyperglycaemic memory, e.g., failure of disease improvement despite attenuation of hyperglycaemia. Therapies reversing the hyperglycaemic memory are lacking. Here we demonstrate that hyperglycaemia, but not hyperlipidaemia, induces the redox-regulator p66Shc and reactive oxygen species (ROS) in macrophages. p66Shc expression, ROS generation, and a pro-atherogenic phenotype are sustained despite restoring normoglycemic conditions. Inhibition of p66Shc abolishes this sustained pro-atherogenic phenotype, identifying p66Shc-dependent ROS in macrophages as a key mechanism conveying the hyperglycaemic memory. The p66Shc-associated hyperglycaemic memory can be reversed by aPC via protease-activated receptor-1 signalling. aPC reverses glucose-induced CpG hypomethylation within the p66Shc promoter by induction of the DNA methyltransferase-1 (DNMT1). Thus, epigenetically sustained p66Shc expression in plaque macrophages drives the hyperglycaemic memory, which-however-can be reversed by aPC. This establishes that reversal of the hyperglycaemic memory in diabetic atherosclerosis is feasible.

Role of p66shc in skeletal muscle function

p66shc is a growth factor adaptor protein that contributes to mitochondrial ROS production. p66shc is involved in insulin signaling and its deletion exerts a protective effect against diet-induced obesity. In light of the role of skeletal muscle activity in the control of systemic metabolism and obesity, we investigated which is the contribution of p66shc in regulating muscle structure and function. Here, we show that p66shc^−/− muscles are undistinguishable from controls in terms of size, resistance to denervation-induced atrophy, and force. However, p66shc^−/− mice perform slightly better than wild type animals during repetitive downhill running. Analysis of the effects after placing mice on a high fat diet (HFD) regimen demonstrated that running distance is greatly reduced in obese wild type animals, but not in overweight-resistant p66shc^−/− mice. In addition, muscle force measured after exercise decreases upon HFD in wild type mice while p66shc^−/− animals are protected. Our data indicate that p66shc affect the response to damage of adult muscle in chow diet, and it determines the maintenance of muscle force and exercise performance upon a HFD regimen.

Nutrient-sensing mTORC1: Integration of metabolic and autophagic signals

The ability of adult cardiomyocytes to regenerate is limited, and irreversible loss by cell death plays a crucial role in heart diseases. Autophagy is an evolutionarily conserved cellular catabolic process through which long-lived proteins and damaged organelles are targeted for lysosomal degradation. Autophagy is important in cardiac homeostasis and can serve as a protective mechanism by providing an energy source, especially in the face of sustained starvation. Cellular metabolism is closely associated with cell survival, and recent evidence suggests that metabolic and autophagic signaling pathways exhibit a high degree of crosstalk and are functionally interdependent. In this review, we discuss recent progress in our understanding of regulation of autophagy and its crosstalk with metabolic signaling, with a focus on the nutrient-sensing mTOR complex 1 (mTORC1) pathway.

Contribution of neural cell death to depressive phenotypes of streptozotocin-induced diabetic mice

Major depression disorder (MDD) or depression is highly prevalent in individuals with diabetes, and the depressive symptoms are more severe and less responsive to antidepressant therapies in these patients. The underlying mechanism is little understood. We hypothesized that the pathophysiology of comorbid depression was more complex than that proposed for MDD and that neural cell death played a role in the disease severity. To test this hypothesis, we generated streptozotocin (STZ)-induced diabetic mice. These mice had blood glucose levels threefold above controls and exhibited depressive phenotypes as judged by a battery of behavioral tests, thus confirming the comorbidity in mice. Immunohistological studies showed markedly increased TUNEL-positive cells in the frontal cortex and hippocampus of the comorbid mice, indicating apoptosis. This finding was supported by increased caspase-3 and decreased Bcl-2 proteins in these brain regions. In addition, the serum brain-derived neurotrophic factor (BDNF) level of comorbid mice was reduced compared with controls, further supporting the neurodegenerative change. Mechanistic analyses showed an increased expression of mitochondrial fission genes fission protein 1 (Fis1) and dynamin-related protein 1 (Drp1), and a decreased expression of mitochondrial fusion genes mitofusin 1 (Mfn1), mitofusin 2 (Mfn2) and optical atrophy 1 (Opa1). Representative assessment of the proteins Drp1 and Mfn2 mirrored the mRNA changes. The data demonstrated that neural cell death was associated with the depressive phenotype of comorbid mice and that a fission-dominant expression of genes and proteins mediating mitochondrial dynamics played a role in the hyperglycemia-induced cell death. The study provides new insight into the disease mechanism and could aid the development of novel therapeutics aimed at providing neuroprotection by modulating mitochondrial dynamics to treat comorbid depression with diabetes.

The adaptor protein p66Shc inhibits mTOR-dependent anabolic metabolism

Adaptor proteins link surface receptors to intracellular signaling pathways and potentially control the way cells respond to nutrient availability. Mice deficient in p66Shc, the most recently evolved isoform of the Shc1 adaptor proteins and a mediator of receptor tyrosine kinase signaling, display resistance to diabetes and obesity. Using quantitative mass spectrometry, we found that p66Shc inhibited glucose metabolism. Depletion of p66Shc enhanced glycolysis and increased the allocation of glucose-derived carbon into anabolic metabolism, characteristics of a metabolic shift called the Warburg effect. This change in metabolism was mediated by the mammalian target of rapamycin (mTOR) because inhibition of mTOR with rapamycin reversed the glycolytic phenotype caused by p66Shc deficiency. Thus, unlike the other isoforms of Shc1, p66Shc appears to antagonize insulin and mTOR signaling, which limits glucose uptake and metabolism. Our results identify a critical inhibitory role for p66Shc in anabolic metabolism.

Src tyrosine kinase signaling antagonizes nuclear localization of FOXO and inhibits its transcription factor activity

Biochemical experiments in mammalian cells have linked Src family kinase activity to the insulin signaling pathway. To explore the physiological link between Src and a central insulin pathway effector, we investigated the effect of different Src signaling levels on the Drosophila transcription factor dFOXO in vivo. Ectopic activation of Src42A in the starved larval fatbody was sufficient to drive dFOXO out of the nucleus. When Src signaling levels were lowered by means of loss-of-function mutations or pharmacological inhibition, dFOXO localization was shifted to the nucleus in growing animals, and transcription of the dFOXO target genes d4E-BP and dInR was induced. dFOXO loss-of-function mutations rescued the induction of dFOXO target gene expression and the body size reduction of Src42A mutant larvae, establishing dFOXO as a critical downstream effector of Src signaling. Furthermore, we provide evidence that the regulation of FOXO transcription factors by Src is evolutionarily conserved in mammalian cells.

The adaptor proteins p66Shc and Grb2 regulate the activation of the GTPases ARF1 and ARF6 in invasive breast cancer cells

Signals downstream of growth factor receptors play an important role in mammary carcinogenesis. Recently, we demonstrated that the small GTPases ARF1 and ARF6 were shown to be activated downstream of the epidermal growth factor receptor (EGFR) and act as a key regulator of growth, migration, and invasion of breast cancer cells. However, the mechanism via which the EGFR recruits and activates ARF1 and ARF6 to transmit signals has yet to be fully elucidated. Here, we identify adaptor proteins Grb2 and p66Shc as important regulators mediating ARF activation. We demonstrate that ARF1 can be found in complex with Grb2 and p66Shc upon EGF stimulation of the basal-like breast cancer MDA-MB-231 cell line. However, we report that these two adaptors regulate ARF1 activation differently, with Grb2 promoting ARF1 activation and p66Shc blocking this response. Furthermore, we show that Grb2 is essential for the recruitment of ARF1 to the EGFR, whereas p66Shc hindered ARF1 receptor recruitment. We demonstrate that the negative regulatory role of p66Shc stemmed from its ability to block the recruitment of Grb2/ARF1 to the EGFR. Conversely, p66Shc potentiates ARF6 activation as well as the recruitment of this ARF isoform to the EGFR. Interestingly, we demonstrate that Grb2 is also required for the activation and receptor recruitment of ARF6. Additionally, we show an important role for p66Shc in modulating ARF activation, cell growth, and migration in HER2-positive breast cancer cells. Together, our results highlight a central role for adaptor proteins p66Shc and Grb2 in the regulation of ARF1 and ARF6 activation in invasive breast cancer cells.

Receptor tyrosine kinase (RTK) signalling in the control of neural stem and progenitor cell (NSPC) development

Important developmental responses are elicited in neural stem and progenitor cells (NSPC) by activation of the receptor tyrosine kinases (RTK), including the fibroblast growth factor receptors, epidermal growth factor receptor, platelet-derived growth factor receptors and insulin-like growth factor receptor (IGF1R). Signalling through these RTK is necessary and sufficient for driving a number of developmental processes in the central nervous system. Within each of the four RTK families discussed here, receptors are activated by sets of ligands that do not cross-activate receptors of the other three families, and therefore, their activation can be independently regulated by ligand availability. These RTK pathways converge on a conserved core of signalling molecules, but differences between the receptors in utilisation of signalling molecules and molecular adaptors for intracellular signal propagation become increasingly apparent. Intracellular inhibitors of RTK signalling are widely involved in the regulation of developmental signalling in NSPC and often determine developmental outcomes of RTK activation. In addition, cellular responses of NSPC to the activation of a given RTK may be significantly modulated by signal strength. Cellular propensity to respond also plays a role in developmental outcomes of RTK signalling. In combination, these mechanisms regulate the balance between NSPC maintenance and differentiation during development and in adulthood. Attribution of particular developmental responses of NSPC to specific pathways of RTK signalling becomes increasingly elucidated. Co-activation of several RTK in developing NSPC is common, and analysis of co-operation between their signalling pathways may advance knowledge of RTK role in NSPC development.

1α,25(OH)2D3-dependent modulation of Akt in proliferating and differentiating C2C12 skeletal muscle cells

We previously reported that 1α,25-dihydroxy-vitamin D(3) [1α,25(OH)(2)D(3)] induces non-transcriptional rapid responses through activation of Src and MAPKs in the skeletal muscle cell line C2C12. In the present study we investigated the modulation of Akt by the secosteroid hormone in C2C12 cells at proliferative stage (myoblasts) and at early differentiation stage. In proliferating cells, 1α,25(OH)(2)D(3) activates Akt by phosphorylation in Ser473 in a time-dependent manner (5-60 min). When these cells were pretreated with methyl-beta-cyclodextrin to disrupt caveolae microdomains, hormone-induced activation of Akt was suppressed. Similar results were obtained by siRNA silencing of caveolin-1 expression, further indicating that hormone effects on cell membrane caveolae are required for downstream signaling. PI3K and p38 MAPK, but not ERK1/2, participate in 1α,25(OH)(2)D(3) activation of Akt in myoblasts. The involvement of p38 MAPK in Akt phosphorylation by the hormone probably occurs through MAPK-activated protein kinase 2 (MK2), which is activated by the steroid. In addition, the participation of Src in Akt phosphorylation by 1α,25(OH)(2)D(3) was demonstrated using the inhibitor PP2 and antisense oligodeoxynucleotides that suppress Src expression. We also observed that PI3K participates in hormone-induced proliferation. During the early phase of C2C12 cell differentiation 1α,25(OH)(2)D(3) also increases Akt phosphorylation and activates Src. Of relevance, Src and PI3K are involved in Akt activation and in MHC and myogenin increased expression by 1α,25(OH)(2)D(3). Altogether, these data suggest that 1α,25(OH)(2)D(3) upregulates Akt through Src, PI(3)K, and p38 MAPK to stimulate myogenesis in C2C12 cells.

Hyperglycemia enhances IGF-I-stimulated Src activation via increasing Nox4-derived reactive oxygen species in a PKCζ-dependent manner in vascular smooth muscle cells

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.

IGF-1 and atherothrombosis: relevance to pathophysiology and therapy

IGF-1 (insulin-like growth factor-1) plays a unique role in the cell protection of multiple systems, where its fine-tuned signal transduction helps to preserve tissues from hypoxia, ischaemia and oxidative stress, thus mediating functional homoeostatic adjustments. In contrast, its deprivation results in apoptosis and dysfunction. Many prospective epidemiological surveys have associated low IGF-1 levels with late mortality, MI (myocardial infarction), HF (heart failure) and diabetes. Interventional studies suggest that IGF-1 has anti-atherogenic actions, owing to its multifaceted impact on cardiovascular risk factors and diseases. The metabolic ability of IGF-1 in coupling vasodilation with improved function plays a key role in these actions. The endothelial-protective, anti-platelet and anti-thrombotic activities of IGF-1 exert critical effects in preventing both vascular damage and mechanisms that lead to unstable coronary plaques and syndromes. The pro-survival and anti-inflammatory short-term properties of IGF-1 appear to reduce infarct size and improve LV (left ventricular) remodelling after MI. An immune-modulatory ability, which is able to suppress 'friendly fire' and autoreactivity, is a proposed important additional mechanism explaining the anti-thrombotic and anti-remodelling activities of IGF-1. The concern of cancer risk raised by long-term therapy with IGF-1, however, deserves further study. In the present review, we discuss the large body of published evidence and review data on rhIGF-1 (recombinant human IGF-1) administration in cardiovascular disease and diabetes, with a focus on dosage and safety issues. Perhaps the time has come for the regenerative properties of IGF-1 to be assessed as a new pharmacological tool in cardiovascular medicine.