Oxidative and nitrosative stress is increasingly associated with the pathology of neurodegeneration and aging. The molecular mechanisms underlying oxidative/nitrosative stress-induced neuronal damage are emerging and appear to involve a mode of death in which mitogen-activated protein kinase (MAPK) signaling pathways are strongly implicated. Thus, attention is turning towards the modulation of intracellular signaling as a therapeutic approach against neurodegeneration. Both endogenous and dietary agents have been suggested as potent modulators of intracellular signal transduction, e.g. nitric oxide and flavonoids, respectively. This review addresses recent findings on the biological effects of flavonoids and nitric oxide in neurodegeneration and aims to elucidate the rationale for their prospective use as modulators of cellular signal transduction. © 2002 Elsevier Science Inc. All rights reserved.Keywords: Apoptosis; Oxidative stress; Neuron; Flavonoids; MAP kinase; JNK; ERK; Epicatechin; Nitric oxide; Mitochondria; Redox status; Polyphenols Abbreviations: AKT, serine/threonine kinase (also known as protein kinase B); AP-1, activator protein-1; Apaf-1, apoptosis protease activating factor-1; ASK1, apoptosis signal-regulating kinase-1; Bad, Bcl-2/BclX Lassociated death promoting protein; Bax, pro-apoptotic protein of the Bcl-2 family; Bcl-2, B-cell lymphoma 2: protein with anti-apoptotic properties; BclX L , long form of Bclx: anti-apoptotic protein of the Bcl-2 family; Caspase, cysteinyl aspartic acid-protease; c-Jun, mammalian equivalent of Jun: part of the AP-1 transcription complex; CREB, cAMP response element binding protein; DIABLO/smac, mitochondria-related pro-apoptotic protein: inhibits XIAP; ERK1/2, extracellular signal-related kinase; Fas, CD95: member of the TNF receptor family; GSHST, glutathione Stransferase; JNK, c-Jun amino-terminal kinase; MAPK, mitogen-activated protein kinase; MAPKK, MAPK kinase; MAPKKK, MAPKK kinase; MEK1/2, ERK1/2-specific MAPKK; MKK4/7, JNK-specific MAPKK; MSK1, mitogen-and stress-activated kinase-1; NF-B, nuclear factor of immunoglobulin k locus in B-cells; ONOO − , peroxynitrite anion; p53, pro-apoptotic tumor suppressor gene product; PI3-kinase, phosphoinositol 3-kinase; Ras, small G-protein; RNS, reactive nitrogen species; ROS, reactive oxygen species; RSK, pp90 ribosomal S6 kinase; SAPK, stressactivated protein kinase; XIAP, X-linked inhibitor of apoptosis: inhibits the activation of caspase-9

Nitric oxide, generated by endogenous nitric oxide synthases or nitric oxide donors, can promote or prevent apoptosis induced by diverse pro-apoptotic stimuli in cell culture models. Both mitochondrial-dependent and -independent apoptotic signaling pathways mediate this dichotomous cellular response to nitric oxide. The molecular mechanisms behind these effects are complex and involve a number of nitrogen oxide-related species that are more reactive than nitric oxide itself. The local cellular environment plays a dynamic role in determining the nature and concentration of these species. Important components of the microenvironment include: the cellular redox state, glutathione, transition metals and the presence of other oxygen- and nitrogen-centered radicals. In particular, redox-sensitive nitrosating species are favorably generated under physiological conditions and capable of modifying multiple cell signaling pathways through reversible S-nitrosation reactions. Cytochrome c release from mitochondria is an important mechanism for the activation of caspase-3 and the initiation of cell death in response to 'intrinsic' pro-apoptotic stimuli, including oxidative and nitrosative stress. In turn, caspases and mitogen associated protein kinases may modulate cytochrome c release through their effects on the Bcl-2 family of proteins. This review will focus on (i) the importance of the cellular environment in determining the fate of nitric oxide and (ii) the ability of S-nitrosation to regulate mitochondrial-dependent apoptosis at the level of mitochondrial bioenergetics, cytochrome c release, caspases, mitogen associated protein kinases, and the Bcl-2 family of proteins.

The molecular mechanisms underlying the initiation and control of the release of cytochrome c during mitochondrion-dependent apoptosis are thought to involve the phosphorylation of mitochondrial Bcl-2 and Bcl-x(L). Although the c-Jun N-terminal kinase (JNK) has been proposed to mediate the phosphorylation of Bcl-2/Bcl-x(L) the mechanisms linking the modification of these proteins and the release of cytochrome c remain to be elucidated. This study was aimed at establishing interdependency between JNK signalling and mitochondrial apoptosis. Using an experimental model consisting of isolated, bioenergetically competent rat brain mitochondria, these studies show that (i) JNK catalysed the phosphorylation of Bcl-2 and Bcl-x(L) as well as other mitochondrial proteins, as shown by two-dimensional isoelectric focusing/SDS/PAGE; (ii) JNK-induced cytochrome c release, in a process independent of the permeability transition of the inner mitochondrial membrane (imPT) and insensitive to cyclosporin A; (iii) JNK mediated a partial collapse of the mitochondrial inner-membrane potential (Deltapsim) in an imPT- and cyclosporin A-independent manner; and (iv) JNK was unable to induce imPT/swelling and did not act as a co-inducer, but as an inhibitor of Ca-induced imPT. The results are discussed with regard to the functional link between the Deltapsim and factors influencing the permeability transition of the inner and outer mitochondrial membranes. Taken together, JNK-dependent phosphorylation of mitochondrial proteins including, but not limited to, Bcl-2/Bcl-x(L) may represent a potential of the modulation of mitochondrial function during apoptosis.