Reactive oxygen species (ROS) homeostasis and redox regulation in cellular signaling

Byproducts of normal mitochondrial metabolism and homeostasis include the buildup of potentially damaging levels of reactive oxygen species (ROS), Ca2+, etc., which must be normalized. Evidence suggests that brief mitochondrial permeability transition pore (mPTP) openings play an important physiological role maintaining healthy mitochondria homeostasis. Adaptive and maladaptive responses to redox stress may involve mitochondrial channels such as mPTP and inner membrane anion channel (IMAC). Their activation causes intra- and intermitochondrial redox-environment changes leading to ROS release. This regenerative cycle of mitochondrial ROS formation and release was named ROS-induced ROS release (RIRR). Brief, reversible mPTP opening-associated ROS release apparently constitutes an adaptive housekeeping function by the timely release from mitochondria of accumulated potentially toxic levels of ROS (and Ca2+). At higher ROS levels, longer mPTP openings may release a ROS burst leading to destruction of mitochondria, and if propagated from mitochondrion to mitochondrion, of the cell itself. The destructive function of RIRR may serve a physiological role by removal of unwanted cells or damaged mitochondria, or cause the pathological elimination of vital and essential mitochondria and cells. The adaptive release of sufficient ROS into the vicinity of mitochondria may also activate local pools of redox-sensitive enzymes involved in protective signaling pathways that limit ischemic damage to mitochondria and cells in that area. Maladaptive mPTP- or IMAC-related RIRR may also be playing a role in aging. Because the mechanism of mitochondrial RIRR highlights the central role of mitochondria-formed ROS, we discuss all of the known ROS-producing sites (shown in vitro) and their relevance to the mitochondrial ROS production in vivo.

Mitochondrial Reactive Oxygen Species (ROS) and ROS-Induced ROS Release

Electron Transfer between Cytochrome c and p66Shc Generates Reactive Oxygen Species that Trigger Mitochondrial Apoptosis

Membrane lipids were once thought to be homogenously distributed in the 2D surface of a membrane, but the lipid raft theory suggests that cholesterol and sphingolipids partition away from other membrane lipids. Lipid raft theory further implicates these cholesterol-rich domains in many processes such as signaling and vesicle traffic. However, direct characterization of rafts has been difficult, because they cannot be isolated in pure form. In the first functional proteomic analysis of rafts, we use quantitative high-resolution MS to specifically detect proteins depleted from rafts by cholesterol-disrupting drugs, resulting in a set of 241 authentic lipid raft components. We detect a large proportion of signaling molecules, highly enriched versus total membranes and detergent-resistant fractions, which thus far biochemically defined rafts. Our results provide the first large-scale and unbiased evidence, to our knowledge, for the connection of rafts with signaling and place limits on the fraction of plasma membrane composed by rafts.

https://www.pnas.org/content/pnas/100/10/5813.full.pdf

Unbiased quantitative proteomics of lipid rafts reveals high specificity for signaling factors

Aging is an inherently complex process that is manifested within an organism at genetic, molecular, cellular, organ, and system levels. Although the fundamental mechanisms are still poorly understo...

https://www.physiology.org/doi/pdf/10.1152/ajpregu.00327.2006

An integrated view of oxidative stress in aging: basic mechanisms, functional effects, and pathological considerations

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.

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

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.

/pdf/extensive-temporally-regulated-reorganization-of-the-lipid-db6nh58d6g.pdf

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

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 Promotes Apoptosis and Antagonizes Mitogenic Signaling in T Cells

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: the apoptotic side of Shc proteins.

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.

Michela Pellegrini

Papers

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

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

p66SHC Promotes Apoptosis and Antagonizes Mitogenic Signaling in T Cells

p66SHC: the apoptotic side of Shc proteins.

Apoptosis and oxidative stress-related diseases: the p66Shc connection.