Michela Pellegrini

Extensive temporally regulated reorganization of the lipid raft proteome following T-cell antigen receptor triggering.

Signalling by immunoreceptors is orchestrated at specific plasma membrane microdomains, referred to as lipid rafts. Here we present a proteomics approach to the temporal analysis of protein association with lipid rafts following T-cell antigen receptor (TCR) triggering. We show that TCR engagement promotes the temporally regulated recruitment of proteins participating in the TCR signalling cascade to lipid rafts. Furthermore, TCR triggering results in profound modifications in the composition of lipid rafts involving a number of proteins associated either directly or indirectly with both plasma and intracellular membranes. Raft-associated proteins can be clustered according to their temporal profile of raft association. The data identify lipid rafts as highly dynamic structures and reveal a dramatic impact of surface TCR triggering not only on components of the TCR signalling machinery but also on proteins implicated in a number of diverse cellular processes.

Simvastatin inhibits T-cell activation by selectively impairing the function of Ras superfamily GTPases

Statins are widely used hypocholesterolemic drugs that inhibit 3‐hydroxy‐3‐methylglutaryl‐coenzyme A (HMG‐CoA) reductase, a rate‐limiting enzyme of the mevalonate pathway whose biosynthetic end product is cholesterol. In addition to lowering circulating cholesterol, statins perturb the composition of cell membranes, resulting in disruption of lipid rafts, which function as signaling platforms in immunoreceptor signaling. Furthermore, by inhibiting protein prenylation, a process also dependent on mevalonate, statins block membrane targeting and hence activity of small GTPases, which control multiple pathways triggered by these receptors. T‐cell activation is crucially dependent on Ras, Rho and Rab GTPases. Furthermore TCR signaling is orchestrated at lipid rafts, identifying T‐cells as potential cellular targets of statins. Here we report that simvastatin suppresses T‐cell activation and proliferation as the result of its capacity to inhibit HMG‐CoA reductase. T‐cell treatment with simvastatin does not affect intracellular cholesterol levels or raft integrity nor, accordingly, the initial tyrosine phosphorylation‐dependent cascade. Conversely, inhibition of protein prenylation by simvastatin results in a dramatic impairment in the pathways regulated by small GTPases, including the Ras/MAP kinase pathway, the Rac/stress kinase pathway, and the Rab‐dependent pathway of receptor endocytosis. The results identify Ras superfamily GTPases as strategic molecular targets in T‐cell immunosuppression by statins.

p66SHC Promotes Apoptosis and Antagonizes Mitogenic Signaling in T Cells

ABSTRACT Of the three Shc isoforms, p66Shc is responsible for fine-tuning p52/p46Shc signaling to Ras and has been implicated in apoptotic responses to oxidative stress. Here we show that human peripheral blood lymphocytes and mouse thymocytes and splenic T cells acquire the capacity to express p66Shc in response to apoptogenic stimulation. Using a panel of T-cell transfectants and p66Shc−/− T cells, we show that p66Shc expression results in increased susceptibility to apoptogenic stimuli, which depends on Ser36 phosphorylation and correlates with an altered balance in apoptosis-regulating gene expression. Furthermore, p66Shc blunts mitogenic responses to T-cell receptor engagement, at least in part by transdominant inhibition of p52Shc signaling to Ras/mitogen-activated protein kinases, in an S36-dependent manner. The data highlight a novel interplay between p66Shc and p52Shc in the control of T-cell fate.

p66SHC: The apoptotic side of Shc proteins

Initially identified as components of the signaling pathways triggered by receptor tyrosine kinases and leading to Ras activation, Shc proteins have been more recently implicated in the regulation of signals controlling not only cell proliferation, but also cell survival and apoptosis. Here we briefly review the current understanding of Shc proteins as promoters of apoptosis. Specifically, we focus on the 66 kDa isoform of ShcA, whose paramount importance in the regulation of oxidative stress responses leading to cell apoptosis and ageing has been by now firmly established.

p66SHC promotes T cell apoptosis by inducing mitochondrial dysfunction and impaired Ca2+ homeostasis.

p66Shc, a redox enzyme that enhances reactive oxygen species (ROS) production by mitochondria, promotes T cell apoptosis. We have addressed the mechanisms regulating p66Shc-dependent apoptosis in T cells exposed to supraphysiological increases in [Ca2+]c. p66Shc expression resulted in profound mitochondrial dysfunction in response to the Ca2+ ionophore A23187, as revealed by dissipation of mitochondrial transmembrane potential, cytochrome c release and decreased ATP levels. p66Shc expression also caused a dramatic alteration in the cells’ Ca2+-handling ability, which resulted in Ca2+ overload after A23187 treatment. The impairment in Ca2+ homeostasis was ROS dependent and caused by defective Ca2+ extrusion due at least in part to decreased plasma membrane ATPase (PMCA) expression. Both effects of p66Shc required Ca2+-dependent serine-36 phosphorylation. The mitochondrial effects of p66Shc were potentiated by but not strictly dependent on the rise in [Ca2+]c. Thus, Ca2+-dependent p66Shc phosphorylation causes both mitochondrial dysfunction and impaired Ca2+ homeostasis, which synergize in promoting T cell apoptosis.

The proapoptotic and antimitogenic protein p66SHC acts as a negative regulator of lymphocyte activation and autoimmunity

The ShcA locus encodes 3 protein isoforms that differ in tissue specificity, subcellular localization, and function. Among these, p66Shc inhibits TCR coupling to the Ras/MAPK pathway and primes T cells to undergo apoptotic death. We have investigated the outcome of p66Shc deficiency on lymphocyte development and homeostasis. We show that p66Shc(-/-) mice develop an age-related lupus-like autoimmune disease characterized by spontaneous peripheral T- and B-cell activation and proliferation, autoantibody production, and immune complex deposition in kidney and skin, resulting in autoimmune glomerulonephritis and alopecia. p66Shc(-/-) lymphocytes display enhanced proliferation in response to antigen receptor engagement in vitro and more robust immune responses both to vaccination and to allergen sensitization in vivo. The data identify p66Shc as a negative regulator of lymphocyte activation and show that loss of this protein results in breaking of immunologic tolerance and development of systemic autoimmunity.

p66Shc is involved in promoting HIF‐1α accumulation and cell death in hypoxic T cells

Hypoxia results in adaptationally appropriate alterations of gene expression through the activation of hypoxia‐inducible factor (HIF)‐1 to overcome any shortage of oxygen. Peripheral blood mononuclear cells may be exposed to low oxygen tensions for different times as they migrate between blood and various tissues. We and others have previously shown that T‐cell adaptation to hypoxia is characterized by a modulation of cytokine expression and an inhibition of T‐cell activation. We have recently demonstrated that the adaptor protein p66Shc negatively regulates T‐cell activation and survival. We here show that hypoxia enhances HIF‐1α accumulation and vascular endothelial growth factor production in T cells. Hypoxic T cells expressed high levels of p21WAF1/CIP1, of the pro‐apoptotic molecules BNIP3, a classic HIF target gene, and BAX, as well as low levels of the anti‐apoptotic molecule BCLxl, associated with an induction of cell death. We found out that hypoxic T cells expressed p66Shc. Furthermore, using T‐cell transfectants expressing p66Shc, as well as T cells derived from mice p66Shc−/−, we defined a role of p66Shc in T‐cell responses to hypoxia. Of interest, hypoxic p66Shc‐positive transfectants expressed higher level of HIF‐1α than negative controls. Thus, p66Shc may play an important role in downstream hypoxic signaling, involving HIF‐1α protein accumulation and cell death in T lymphocytes. J. Cell. Physiol. 211: 439–447, 2007.

Apoptosis and Oxidative Stress-Related Diseases: The p66Shc Connection

p66Shc is the only known proapoptotic member of the Shc protein family of molecular adaptors. Through its redox activity, p66Shc oxidates cytochrome-c, leading to increased ROS production and, eventually, to apoptosis. p66Shc has been implicated in the control of oxidative stress and life span in mammals. In this review the multifaceted role of p66Shc in redox regulation will be discussed, with a focus on the mechanisms underlying p66Shc-dependent apoptosis and its role in oxidative stress-related diseases.

Cooperation and selectivity of the two Grb2 binding sites of p52Shc in T-cell antigen receptor signaling to Ras family GTPases and Myc-dependent survival.

Shc proteins participate in a variety of processes regulating cell proliferation, survival and apoptosis. The two ubiquitously expressed isoforms, p52Shc/p46Shc, couple tyrosine kinase receptors to Ras by recruiting Grb2/Sos complexes to a membrane-proximal localization. Tyrosine residues 239/240 and 317 become phosphorylated following receptor engagement and, as such, form two Grb2 binding sites, which have been proposed to be differentially coupled to Myc-dependent survival and to fos-dependent proliferation, respectively. Here, we have addressed the individual function of YY239/240 and Y317 in T-cell antigen receptor (TCR) signaling. We show that p52Shc is phosphorylated on both YY239/240 and Y317 following TCR engagement. Mutation of either YY239/240 or Y317 results in impaired interaction with Grb2 and inhibition of Ras/MAP kinase activation and CD69 induction, supporting a role for both Grb2 binding sites in this function. Substitution of either YY239/240 or Y317 also results in a defective activation of Rac and the coupled stress kinases JNK and p38. Furthermore, mutation of Y317 or, to a larger extent, of YY239/240, results in increased activation-induced cell death, which in cells expressing the FF239/240 mutant is accompanied by impaired TCR-dependent c-myc transcription. The data underline a pleiotropic and nonredundant role of Shc, mediated by both YY239/240 and Y317, in T-cell activation and survival.

The adaptor protein p66shc is a positive regulator in the angiogenic response induced by hypoxic T cells

Immune cells play an important role in the onset of angiogenesis. Here, we report that VEGF represents the major proangiogenic factor expressed by T cells exposed to hypoxia, a common feature of inflammation and tumor microenvironment. The supernatants of hypoxic T cells were highly angiogenic when delivered on the chick embryo CAM. The angiogenic response was abrogated by a neutralizing anti‐VEGF antibody and mimicked by rVEGF. Interestingly, VEGF induction by hypoxia was up‐regulated in Jurkat T cells overexpressing the adaptor protein p66Shc but not the inactive S36 p66Shc mutant, and it was abolished in p66Shc−/− mouse splenocytes. Accordingly, the angiogenic response induced by the supernatants from hypoxic p66Shc−/− splenocytes was reduced dramatically when compared with the wild‐type controls. In conclusion, hypoxic T cells may contribute to the onset of angiogenesis through a novel VEGF‐mediated mechanism, where p66Shc acts as a positive regulator.