Non-redundant Role of Shc in Erk Activation by Cytoskeletal Reorganization

Insights into the Shc Family of Adaptor Proteins

The Shc family of adaptor proteins is a group of proteins that lacks intrinsic enzymatic activity. Instead, Shc proteins possess various domains that allow them to recruit different signalling molecules. Shc proteins help to transduce an extracellular signal into an intracellular signal, which is then translated into a biological response. The Shc family of adaptor proteins share the same structural topography, CH2-PTB-CH1-SH2, which is more than an isoform of Shc family proteins; this structure, which includes multiple domains, allows for the posttranslational modification of Shc proteins and increases the functional diversity of Shc proteins. The deregulation of Shc proteins has been linked to different disease conditions, including cancer and Alzheimer’s, which indicates their key roles in cellular functions. Accordingly, a question might arise as to whether Shc proteins could be targeted therapeutically to correct their disturbance. To answer this question, thorough knowledge must be acquired; herein, we aim to shed light on the Shc family of adaptor proteins to understand their intracellular role in normal and disease states, which later might be applied to connote mechanisms to reverse the disease state.

Induction of brain microvascular endothelial cell urokinase expression by Cryptococcus neoformans facilitates blood-brain barrier invasion

The invasive ability of the blood-borne fungal pathogen Cryptococcus neoformans can be enhanced through interactions with host plasma components, such as plasminogen. Previously we showed by in vitro studies that plasminogen coats the surface of C. neoformans and is converted to the active serine protease, plasmin, by host plasminogen activators. Viable, but not formaldehyde- or sodium azide-killed, cryptococcal strains undergo brain microvascular endothelial cell-dependent plasminogen-to-plasmin activation, which results in enhanced, plasmin-dependent cryptococcal invasion of primary bovine brain microvascular endothelial cells and fungal ability to degrade plasmin substrates. In the present work, brain microvascular endothelial cells cultured with viable, but not killed, cryptococcal strains led to significant increases in both urokinase mRNA transcription and cell-associated urokinase protein expression. Soluble urokinase was also detected in conditioned medium from brain microvascular endothelial cells cultured with viable, but not killed, C. neoformans. Exposure of plasminogen pre-coated viable C. neoformans to conditioned medium from strain-matched brain microvascular endothelial cell-fungal co-cultures resulted in plasminogen-to-plasmin activation and plasmin-dependent cryptococcal invasion. siRNA-mediated silencing of urokinase gene expression or the use of specific inhibitors of urokinase activity abrogated both plasminogen-to-plasmin activation on C. neoformans and cryptococcal-brain microvascular endothelial cell invasion. Our results suggest that pathogen exploitation of the host urokinase-plasmin(ogen) system may contribute to C. neoformans virulence during invasive cryptococcosis.

Deletion of the ageing gene p66(Shc) reduces early stroke size following ischaemia/reperfusion brain injury

AIMS

Stroke is a leading cause of morbidity and mortality, and its incidence increases with age. Both in animals and in humans, oxidative stress appears to play an important role in ischaemic stroke, with or without reperfusion. The adaptor protein p66(Shc) is a key regulator of reactive oxygen species (ROS) production and a mediator of ischaemia/reperfusion damage in ex vivo hearts. Hence, we hypothesized that p66(Shc) may be involved in ischaemia/reperfusion brain damage. To this end, we investigated whether genetic deletion of p66(Shc) protects from ischaemia/reperfusion brain injury.

METHODS AND RESULTS

Transient middle cerebral artery occlusion (MCAO) was performed to induce ischaemia/reperfusion brain injury in wild-type (Wt) and p66(Shc) knockout mice (p66(Shc-/-)), followed by 24 h of reperfusion. Cerebral blood flow and blood pressure measurements revealed comparable haemodynamics in both experimental groups. Neuronal nuclear antigen immunohistochemical staining showed a significantly reduced stroke size in p66(Shc-/-) when compared with Wt mice (P < 0.05, n = 7-8). In line with this, p66(Shc-/-) mice exhibited a less impaired neurological function and a decreased production of free radicals locally and systemically (P < 0.05, n = 4-5). Following MCAO, protein levels of gp91phox nicotinamide adenine dinucleotide phosphate oxidase subunit were increased in brain homogenates of Wt (P < 0.05, n = 4), but not of p66(Shc-/-) mice. Further, reperfusion injury in Wt mice induced p66(Shc) protein in the basilar and middle cerebral artery, but not in brain tissue, suggesting a predominant involvement of vascular p66(Shc).

CONCLUSION

In the present study, we show that the deletion of the ageing gene p66(Shc) protects mice from ischaemia/reperfusion brain injury through a blunted production of free radicals. The ROS mediator p66(Shc) may represent a novel therapeutical target for the treatment of ischaemic stroke.

The proximal signaling network of the BCR-ABL1 oncogene shows a modular organization

BCR-ABL1 is a fusion tyrosine kinase, which causes multiple types of leukemia. We used an integrated proteomic approach that includes label-free quantitative protein complex and phosphorylation profiling by mass spectrometry to systematically characterize the proximal signaling network of this oncogenic kinase. The proximal BCR-ABL1 signaling network shows a modular and layered organization with an inner core of three leukemia transformation-relevant adaptor protein complexes (Grb2/Gab2/Shc1 complex, CrkI complex and Dok1/Dok2 complex). We introduced an 'interaction directionality' analysis, which annotates static protein networks with information on the directionality of phosphorylation-dependent interactions. In this analysis, the observed network structure was consistent with a step-wise phosphorylation-dependent assembly of the Grb2/Gab2/Shc1 and the Dok1/Dok2 complexes on the BCR-ABL1 core. The CrkI complex demonstrated a different directionality, which supports a candidate assembly on the Nedd9 (Hef1, CasL) scaffold. As adaptor protein family members can compensate for each other in leukemic transformation, we compared members of the Dok and Crk protein families and found both overlapping and differential binding patterns. We identified an additional level of regulation for the CrkII protein via binding to 14-3-3 proteins, which was independent from its inhibitory phosphorylation. We also identified novel components of the inner core complexes, including the kinases Pragmin (Sgk223) and Lrrk1 (Lrrk2 paralog). Pragmin was found as a component of the CrkI complex and is a potential link between BCR-ABL1/CrkI and RhoA signaling. Lrrk1 is an unusual kinase with a GTPase domain. We detected Lrrk1 as a component of the Grb2/Gab2/Shc1 complex and found that it functionally interacts with the regulator of small GTPases Arap1 (Centd2) and possibly participates in the mitogen-activated protein kinase response to cellular stresses. This modular and phosphorylation-driven interaction network provides a framework for the integration of pleiotropic signaling effects of BCR-ABL1 toward leukemic transformation.

Cisplatin induces apoptosis through the ERK-p66shc pathway in renal proximal tubule cells

The extracellular signal-regulated kinase (ERK) has been shown to mediate cisplatin (CP)-induced toxicity to renal proximal tubule cells. Here, we demonstrate that ERK serves as the kinase that phosphorylates the pro-apoptotic p66shc protein at its Serine36 residue in CP-treated renal proximal tubule cells. Pharmacologic or dominant-negative inhibition of ERK mitigates cisplatin-induced Ser36 phosphorylation of p66shc. Overexpression of p66shc exacerbates while its knockdown or mutation of the Serine36 site to alanine ameliorates CP-induced nephrotoxicity in vitro. Since p66shc is Serine36 phosphorylated in the kidneys of mice after treatment with CP, a similar mechanism might exist in vivo.

p66SHC-mediated mitochondrial dysfunction in renal proximal tubule cells during oxidative injury

Mitochondrial dysfunction is involved in pathopysiology of ischemia-reperfusion-induced acute kidney injury (AKI). The p66shc adaptor protein is a newly recognized mediator of mitochondrial dysfunction, which might play a role in AKI-induced renal tubular injury. Oxidative stress-mediated Serine36 phosphorylation of p66shc facilitates its transportation to the mitochondria where it oxidizes cytochrome c and generates excessive amount of reactive oxygen species (ROS). The consequence is mitochondrial depolarization and injury. Earlier we determined that p66shc plays an essential role in injury of cultured mouse renal proximal tubule cells during oxidative stress. Here, we studied the role of p66shc in ROS generation and consequent mitochondrial dysfunction during oxidative injury in renal proximal tubule cells. We employed p66shc knockdown renal proximal tubule cells and cells that overexpress wild-type, Serine phosphorylation (S36A), or cytochrome c-binding (W134F) mutants of p66shc. Inhibition of the mitochondrial electron transport chain or the mitochondrial permeability transition revealed that hydrogen peroxide-induced injury is mitochondrial ROS and consequent mitochondrial depolarization dependent. We also found that through Ser36 phosphorylation and mitochondria/cytochrome c binding, p66shc mediates those effects. We propose a similar mechanism in vivo as we demonstrated mitochondrial binding of p66shc as well as its association with cytochrome c in the postischemic kidneys of mice. Thus, manipulating p66shc might offer a new therapeutic modality to ameliorate renal ischemic injury.

Linking cell structure to gene regulation: signaling events and expression controls on the model genes PAI-1 and CTGF

The microtubule and microfilament cytoskeletal systems as well as cell-to-cell contacts and cell-matrix interactions are critical regulators of cell structure and function. Alterations in cell shape profoundly influence signaling events and gene expression programs that impact a spectrum of biological responses including cell growth, migration and apoptosis. These same pathways also contribute to the progression of several important pathologic conditions (e.g., arteriosclerosis, vascular fibrosis, and endothelial dysfunction). Indeed, hemodynamic forces in the vascular compartment are established modifiers of endothelial and smooth muscle cell cytoarchitecture and orchestrate complex genetic and biological responses in concert with contributions from the extracellular matrix (ECM), growth factors (e.g., EGF, and TGF-beta) and cell adhesion receptors (e.g., integrins, and cadherins). The profibrotic matricellular proteins plasminogen activator inhibitor-1 (PAI-1) and connective tissue growth factor (CTGF) are prominent members of a subset of genes the expression of which is highly responsive to cell shape-altering stimuli (i.e., disruption of the actin-based and microtubule networks, shear strain and cyclic stretch). Since both PAI-1 and CTGF are major mediators of cardiovascular fibrotic disease, understanding cell structure-linked signaling cascades provides potential avenues for focused therapy. It is increasingly evident that growth factor receptors (EGFR) are activated by changes in cytoarchitecture and that the "repressive state" of certain signaling proteins (e.g., SMAD, and Rho-GEFs) is maintained by sequestration on cell structural networks. Functional repression can be relieved by cytoskeletal perturbations (e.g., in response to treatment with network-specific drugs) resulting in activation of signaling cascades (e.g., Rho, and MAPK) with associated changes in gene reprogramming. Recent studies document a complex network of both similar and unique signaling control elements leading to the induction of PAI-1 and CTGF in response to modifications in cell shape. The purpose of this review is to highlight our current understanding of "cell deformation"-responsive signaling cascades focusing on the potential value of targeting such pathways, and their model response genes (e.g., PAI-1, and CTGF), as a therapeutic option for the treatment of fibrotic diseases.

TRIM31 interacts with p52(Shc) and inhibits Src-induced anchorage-independent growth

Tripartite motif-containing protein (TRIM) family proteins are involved in a broad range of biological processes and, consistently, their alterations result in diverse pathological conditions such as genetic diseases, viral infection and cancer development. In this study, we found that one of the TRIM family proteins, TRIM31, is highly expressed in the gastrointestinal tract and interacts with p52(Shc), one of the signal transducers. We also found by a binding assay that almost the whole region other than the RING domain is required for the binding to p52(Shc) but found by pulse-chase analysis that overexpression of TRIM31 does not affect the stability of p52(Shc). Moreover, we found that overexpression of TRIM31 suppresses anchorage-independent cell growth induced by the active form of c-Src. These results suggest that TRIM31 attenuates c-Src signaling via p52(Shc) under anchorage-independent growth conditions and is potentially associated with growth activity of cells in the gastrointestinal tract.

Endothelial cell-specific molecule 2 (ECSM2) modulates actin remodeling and epidermal growth factor receptor signaling

Endothelial cell-specific molecules (ECSMs) play a pivotal role in the pathogenesis of many angiogenesis-related diseases. Since its initial discovery, the exact function of human ECSM2 has not been defined. In this study, by database mining, we identified a number of hypothetical proteins across species exhibiting substantial sequence homology to the human ECSM2. We showed that ECSM2 is preferentially expressed in endothelial cells and blood vessels. Their characteristic structures and unique expression patterns suggest that ECSM2 is an evolutionarily conserved gene and may have important functions. We further explored the potential roles of human ECSM2 at the molecular and cellular level. Using a reconstitution mammalian cell system, we demonstrated that ECSM2 mainly resides at the cell membrane, is critically involved in cell-shape changes and actin cytoskeletal rearrangement, and suppresses tyrosine phosphorylation signaling. More importantly, we uncovered that ECSM2 can cross-talk with epidermal growth factor receptor (EGFR) to attenuate the EGF-induced cell migration, possibly via inhibiting the Shc-Ras-ERK (MAP kinase) pathway. Given the importance of growth factor and receptor tyrosine kinase-mediated signaling and cell migration in angiogenesis-related diseases, our findings regarding the inhibitory effects of ECSM2 on EGF-mediated signaling and cell motility may have important therapeutic implications.

Differential requirement for MEK/ERK and SMAD signaling in PAI-1 and CTGF expression in response to microtubule disruption

Colchicine and nocodazole, both established microtubule disruptors, are useful tools to investigate cytoskeletal-dependent signaling cascades and the associated downstream transcriptional targets. Since cytoskeletal events impact pathophysiologic consequences in the vascular system, the signaling requirements underlying colchicine-stimulated expression of PAI-1 and CTGF, two prominent cell deformation-sensitive fibrosis-initiating proteins, were evaluated in vascular smooth muscle cells. Microtubule disruption rapidly induced EGFR transactivation (at the src kinase-sensitive EGFR(Y845) site) in a ROS-dependent manner. Genetic deficiency of EGFR, inhibition of EGFR signaling with AG1478 or introduction of a kinase-deficient EGFR construct effectively blocked colchicine-stimulated PAI-1 and CTGF expression. MEK/ERK involvement downstream of ROS generation was critical for PAI-1, but not CTGF, expression following cytoskeletal perturbation suggesting bifurcation of signaling pathways downstream of EGFR activation. Colchicine also stimulated SMAD2/3 phosphorylation by a Rho/ROCK-dependent mechanism independent of TGF-beta1 release or receptor activity. Rho/ROCK signaling initiated by tubulin network collapse was required for both CTGF and PAI-1 induction. Colchicine-initiated SMAD3 phosphorylation, however, was essential for PAI-1, but not CTGF, expression further highlighting divergence of signaling events downstream of Rho/ROCK that mediate microtubule deformation-associated changes in profibrotic gene transcription.

p66shc inhibits pro-survival epidermal growth factor receptor/ERK signaling during severe oxidative stress in mouse renal proximal tubule cells

The fully executed epidermal growth factor receptor (EGFR)/Ras/MEK/ERK pathway serves a pro-survival role in renal epithelia under moderate oxidative stress. We and others have demonstrated that during severe oxidative stress, however, the activated EGFR is disconnected from ERK activation in cultured renal proximal tubule cells and also in renal proximal tubules after ischemia/reperfusion injury, resulting in necrotic death. Studies have shown that the tyrosine-phosphorylated p46/52 isoforms of the ShcA family of adaptor proteins connect the activated EGFR to activation of Ras and ERK, whereas the p66(shc) isoform can inhibit this p46/52(shc) function. Here, we determined that severe oxidative stress (after a brief period of activation) terminates activation of the Ras/MEK/ERK pathway, which coincides with ERK/JNK-dependent Ser(36) phosphorylation of p66(shc). Isoform-specific knockdown of p66(shc) or mutation of Ser(36) to Ala, but not to Asp, attenuated severe oxidative stress-mediated ERK inhibition and cell death in vitro. Also, severe oxidative stress (unlike ligand stimulation and moderate oxidative stress, both of which support survival) increased binding of p66(shc) to the activated EGFR and Grb2. This binding dissociated the SOS1 adaptor protein from the EGFR-recruited signaling complex, leading to termination of Ras/MEK/ERK activation. Notably, Ser(36) phosphorylation of p66(shc) and its increased binding to the EGFR also occurred in the kidney after ischemia/reperfusion injury in vivo. At the same time, SOS1 binding to the EGFR declined, similar to the in vitro findings. Thus, the mechanism we propose in vitro offers a means to ameliorate oxidative stress-induced cell injury by either inhibiting Ser(36) phosphorylation of p66(shc) or knocking down p66(shc) expression in vivo.

Dok-3 sequesters Grb2 and inhibits the Ras-Erk pathway downstream of protein-tyrosine kinases

Adaptor proteins are essential in coordinating recruitment and, in a few cases, restraint of various effectors during cellular signaling. Dok-1, Dok-2 and Dok-3 comprise a closely related family of adaptor, which negatively regulates mitogen-activated protein kinase Erk downstream of protein-tyrosine kinases (PTKs). Recruitment of p120 rasGAP, a potent inhibitor of Ras, by Dok-1 and Dok-2 appears critical in the negative regulation of the Ras-Erk pathway. However, as Dok-3 does not bind rasGAP, it has been unclear how Dok-3 inhibits Erk downstream of PTKs. Here, we identified Grb2 as a Dok-3-binding protein upon its tyrosine phosphorylation. This interaction required the intact binding motifs of the Grb2 SH2 domain, and a mutant (Dok-3-FF) having a Tyr/Phe substitution at these motifs failed to inhibit Ras and Erk activation downstream of a cytoplasmic PTK Src. Because Grb2 forms a stable complex with Sos, a crucial activator of Ras, these data suggest that Dok-3 restrains Grb2 and inhibits the ability of the Grb2-Sos complex to activate Ras. Indeed, forced expression of Dok-3, but not Dok-3-FF, inhibited the recruitment of the Grb2-Sos complex to Shc downstream of Src, which is an essential event for activation of the Ras-Erk pathway. These findings indicate that Dok-3 sequesters Grb2 from Shc and inhibits the Ras-Erk pathway downstream of PTKs.

Induction of uPA gene expression by the blockage of E-cadherin via Src- and Shc-dependent Erk signaling

Loss of E-cadherin-mediated cell-cell adhesion and expression of proteolytic enzymes characterize the transition from benign lesions to invasive, metastatic tumor, a rate-limiting step in the progression from adenoma to carcinoma in vivo. A soluble E-cadherin fragment found recently in the serum and urine of cancer patients has been shown to disrupt cell-cell adhesion and to drive cell invasion in a dominant-interfering manner. Physical disruption of cell-cell adhesion can be mimicked by the function-blocking antibody Decma. We have shown previously in MCF7 and T47D cells that urokinase-type plasminogen activator (uPA) activity is up-regulated upon disruption of E-cadherin-dependent cell-cell adhesion. We explored the underlying molecular mechanisms and found that blockage of E-cadherin by Decma elicits a signaling pathway downstream of E-cadherin that leads to Src-dependent Shc and extracellular regulated kinase (Erk) activation and results in uPAgene activation. siRNA-mediated knockdown of endogenous Src-homology collagen protein (Shc) and subsequent expression of single Shc isoforms revealed that p46(Shc) and p52(Shc) but not p66(Shc) were able to mediate Erk activation. A parallel pathway involving PI3K contributed partially to Decma-induced Erk activation. This report describes that disruption of E-cadherin-dependent cell-cell adhesion induces intracellular signaling with the potential to enhance tumorigenesis and, thus, offers new insights into the pathophysiological mechanisms of tumor development.

Mitochondrial DNA copy number is regulated by cellular proliferation: a role for Ras and p66(Shc)

The abundance of mitochondria is regulated by biogenesis and division. These processes are controlled by cellular factors, given that, for example, mitochondria have to replicate their DNA prior to cell division. However, the mechanisms that allow a synchronization of cell proliferation with mitochondrial genome replication are still obscure. We report here our investigations on the role of proliferation and the contribution of Ras and p66Shc in the regulation of mitochondrial DNA copy number. Ras proteins mediate a variety of receptor-transduced mitogenic signals and appear to play an essential role in the cellular response to growth factors. P66Shc is a genetic determinant of life span in mammals and has been implicated in the regulation of receptor signaling and various mitochondrial functions. First, we confirmed previous reports showing that mitochondrial DNA is replicated during a specific phase of the cell cycle (the pre-S phase) and provided novel evidences that this process is regulated by mitogenic growth factors. Second, we showed that mitochondrial DNA replication is activated following Ras-induced cellular hyper-proliferation. Finally, we showed that p66Shc expression induces mitochondrial DNA replication, both in vitro and in vivo. We suggest that mitochondria are target of intracellular signaling pathways leading to proliferation, involving Ras and p66Shc, which might function to integrate cellular bio-energetic requirements and the inheritance of mitochondrial DNA in a cell cycle-dependent manner.

Cdc42 Regulates Arsenic-induced NADPH Oxidase Activation and Cell Migration through Actin Filament Reorganization

Although arsenic is a human carcinogen, the molecular mechanisms of its action remain to be understood. The present study reports that exposure to arsenic induced actin filament reorganization, resulting in lamellipodia and filopodia structures through the activation of Cdc42 in SVEC4-10 endothelial cells. It was also found that arsenic induced the formation of the superoxide anion (O2*) in SVEC4-10 cells. Immunoprecipitation and Western blotting analysis demonstrated that arsenic stimulation induced serine phosphorylation of p47phox, a key component of NADPH oxidase, indicating that arsenic induces O2* formation through NADPH oxidase activation. Inhibition of arsenic-induced actin filament reorganization by either overexpression of a dominant negative Cdc42 or pretreatment of an actin filament stabilizing regent, jasplakinolide, abrogated arsenic-induced NADPH oxidase activation, showing that the activation of NADPH oxidase was regulated by Cdc42-mediated actin filament reorganization. This study also showed that overexpression of a dominant negative Rac1 was sufficient to abolish arsenic-induced O2*- production, implying that Rac1 activities are required for Cdc42-mediated NADPH oxidase activation in response to arsenic stimulation. Furthermore, arsenic stimulation induced cell migration, which can be inhibited by the inactivation of either Cdc42 or NADPH oxidase. Taken together, the results indicate that arsenic is able to activate NADPH oxidase through Cdc42-mediated actin filament reorganization, leading to the induction of an increase in cell migration in SVEC4-10 endothelial cells.

Identification and modulation of a caveolae-dependent signal pathway that regulates plasminogen activator inhibitor-1 in insulin-resistant adipocytes

Plasminogen activator inhibitor-1 (PAI-1) plays an important role in the pathogenesis of obesity-driven type 2 diabetes mellitus and associated cardiovascular complications. Here, we show that perturbation of caveolar microdomains leads to insulin resistance and concomitant up-regulation of PAI-1 in 3T3L1 adipocytes. We present several lines of evidence showing that the phosphatidylinositol 3-kinase (PI3K) pathway negatively regulates PAI-1 gene expression. Insulin-induced PAI-1 gene expression is up-regulated by a specific inhibitor of PI3K. In addition, serum PAI-1 level is elevated in protein kinase Balpha-deficient mice, whereas it is reduced in p70 ribosomal S6 kinase 1-deficient mice. The PI3K pathway phosphorylates retinoblastoma protein (pRB), known to release free E2 (adenoviral protein) factor (E2F), which we have previously demonstrated to be a transcriptional repressor of PAI-1 gene expression. Accordingly, cell-penetrating peptides that disrupt pRB-E2F interaction, and thereby release free E2F, are able to suppress PAI-1 levels that are elevated during insulin-resistant conditions. This study identifies a caveolar-dependent signal pathway that up-regulates PAI-1 in insulin-resistant adipocytes and proposes a previously undescribed pharmacological paradigm of disrupting pRB-E2F interaction to suppress PAI-1 levels.