Oxidative Processes in Glomerulonephritides: Additional Facts

Soon after it was realized that oxidative processes are an integral part of inflammation, principally because the phagocytes (neutrophils or macrophages) produce reactive oxygen species (ROS) [1], there was a focus on the role of oxidation in glomerular inflammation [2, 3, 4, 5]. Oxidants cause the liberation from glomerular cells of eicosanoids, chemokines, cytokines, and growth factors [4, 6]. Hydrogen peroxide can be used to activate signalling components for cellular activation like Ras and the mitogen-activated protein kinases, and activation of the transcription factor NF-kappaB [7, 8]that determines release of chemokines and cytokines, of proteolytic enzymes from phagocytes, and expression of tissue factor thromboplastin. The transcription factor AP-1, which is formed from C-jun and c-fos products, is redox regulated, and it is required for release of growth factors. Indeed, there is now a vast literature on how gene expression is regulated by reactive oxygen [9, 10].Sources of ROS within cells include the mitochondrial respiratory chain, the NADPH oxidases, nitric oxide synthetases, cyclo-oxygenases and the lipoxygenases, and enzymes like xanthine oxidase. Consider that up to 5% of the electrons entering the mitochondrial respiratory chain can leak out and form superoxide anions. The renal cytochrome P-450 system [11, 12]is an additional source of oxygen radicals. This means that when kidneys are exposed to nephrotoxic and immunotoxic chemicals, like the hydrocarbons, damage will ensue [13]. Certainly such considerations apply to induction of renal interstitial fibroses [14].Oxidants damage the heparan sulphates on glomerular endothelial cells and within basement membranes, so that proteinuria ensues [15]. Once there is a hint of oxidation of cell membrane phospholipids, phospholipase A2 is activated, and platelet-activating factor is liberated. ROS arise not only from phagocytes [1, 3], but also from the intrinsic cells of the glomeruli [4]that are subject to complement-inflicted injury [16]. Of course, phagocytes will also be releasing lysosomal proteases, and they will exert unmitigated damage, if local antiproteases are inhibited concurrently by oxidation [17]. Products of the myeloperoxidase-H2O2-halide system released by polymorphonuclear neutrophils (PMNs) result in direct injury to intrinsic cells, as well as causing halogenation and oxidation of the glomerular basement membrane (GBM) that results in permeability. Glomerular epithelial cells make oxidants as well as releasing proteases that digest the GBM. Kerjaschki [18]has analyzed in detail how in passive Heymann nephritis the onset of proteinuria is determined by generation of ROS following insertion of C5b-9 into the cell membranes of podocytes. There is then lipid peroxidation of the surfaces of the epithelial cells and oxidation of the GBM. Whichever form of experimental or clinical proliferative nephritis one examines, one can find illustrations of the damage caused by ROS. Thus Kashem et al. [19]have demonstrated how aggregated IgA-induced Fc-α receptor expression on PMNs in IgA nephropathy correlated with superoxide anion formation and with the degree of proteinuria. One assumes that mesangial cells would behave similarly.Damage in the medulla that leads to tubulo-interstitial fibrosis is integral to all glomerulonephritides. Interaction between macrophages and fibroblasts can lead to release of oxygen radicals that presages augmented release of chemokines like MIP1-α [20, 21].I would like to draw attention to other ways by which oxidation will influence mechanisms implicated in glomerulonephritides and vasculitis.Oxidation of proteins allows unfolding and enhanced proteolytic processing [22], so that antigenic peptide epitopes become exposed and so will elicit specific T cells. Oxidants lead to modification of amino acid residues like cysteine, methionine, histidine, tryptophan, tyrosine, and lysine, and thus the carboxyl content is increased and the surface charge of proteins is altered [23]. Oxidatively modified proteins are preferred substrates for proteolytic degradation [24].During the course of inflammation release of myeloperoxidase from PMNs leads to the formation of hypochlorite that alters structure and antigenicity of proteins [25]. OCl– reacts with SH and NH2 groupings.α2-Macroglobulin is found at high concentrations in plasma. It has been recognized for years as a plasma and inflammatory fluid proteinase inhibitor. Now it is known that α2-macroglobulin binds cytokines like interleukins 1β, 2, and 6 and tumour necrosis factor alpha and the growth factors basic fibroblast growth factor, platelet-derived growth factor, and transforming growth factor beta. Hence no free platelet-derived growth factor or transforming growth factor beta exists in the circulation. Yet when hypochlorite is released by PMNs, there is oxidation of α2-macroglobulin, so that its tetramers become dimers [26]. The result is enhanced binding of the cytokines interleukins 2 and 6 and tumour necrosis factor alpha and conversely release of the growth factors basic fibroblast growth factor, platelet-derived growth factor, and transforming growth factor beta. Hence local cellular proliferation and formation of collagen will be enhanced, just as occurs in glomeruloscleroses.When α1-proteinase inhibitor is inactivated by oxidants, the activity of various proteases like the elastase from PMNs will be enhanced at inflammatory sites [17]. The various white cells and platelets release heparanase. Heparanase helps remove the anticoagulant surface that is provided by the glycocalyx of the endothelial cells of capillaries. Heparanase modifies significantly the state and composition of the extracellular matrix [27]. By removing sites for heparin-binding growth factors, it will enhance the immediate actions of chemokines, cytokines, and growth factors. Furthermore, by clearing heparan sulphate moieties from the extracellular matrix, heparanase facilitates movements of cells (leucocytes or fibroblasts) through the extracellular tissues.All this will explain why there must be other means of limiting the influx of PMNs and macrophages into tissues [28]. Otherwise any inflammatory reaction might result in gross tissue destruction [17], as phagocytes turn into pro-inflammatory cells. In renal terminology, this refers to necrotizing glomerulonephritis.With respect to vasculitides it is now realized that ROS, especially hydroxyl radicals, act as intracellular messengers for induction of the chemokine interleukin 8, a potent chemo-attractant and activator of neutrophils [29]. Furthermore, reaction of superoxide anions formed by activated leucocytes with nitric oxide arising from inducible nitric oxide synthetase will give rise to peroxynitrite ONOO–. Peroxynitrite can stimulate interleukin 8 production too [30].I have indicated that ROS can be involved in the activation of transcription factors NF-kappaB and AP-1 and thus in the release of cytokines and chemokines. However, oxidative induction of NF-kappaB or AP-1 is not a universal phenomenon. It applies to particular tissues and situations [9]. A recent paper has shown how application of oxidative stress to hepatocytes induces release of chemokines and cytokines [31]. Renal tubules could behave likewise. The induction of both CXC and C-C chemokines can be helped by oxidative stress [32]. Since this is relatively new information, there are no specific examples from the kidney. However, when Ou et al. [33]used purine aminonucleoside, which we know creates oxidative stress [18], to produce tubulo-interstitial nephritis in rats, that process was heralded by rapid release of the chemokines MCP-1, MCP-3, and TCA-3. Hydrogen peroxide can cross lipid plasma membranes with ease, and it is known that oxidants can activate the mitogen-activated protein kinases involved in cell activation. Indeed, oxidant stress can determine expression of ICAM-1 [34]and VCAM-1 on endothelial cell surfaces, albeit physiological regulation is by other means.Formation of oxidants during the course of inflammation will lead to apoptosis of glomerular cells and of some of the infiltrating leucocytes [35]. It has been demonstrated that interleukin 1 sensitizes glomerular cells to oxidant-induced apoptosis [36]by its lowering of the protective chaperonin HSP 70. Lipid hydroperoxides have been shown to cause apoptosis of cells [37]. Any lowering of intracellular glutathione facilitates apoptosis [38, 39]. If there is formation of peroxynitrite from hydroxyl radicals, that will promote apoptosis [40]. So will the action of hypochlorite [25]. ROS alone will do that [41].In addition to the process just discussed [26], lipid peroxides can enhance release of the fibrosing cytokine transforming growth factor beta [42]. Furthermore, transforming growth factor beta is able to suppress expression of antioxidant enzymes [43]. Hence the formation of ROS during the acute phase of a glomerulonephritis is often followed by persistent activation of transforming growth factor beta 1 which leads to subsequent glomerular scarring [44]. No doubt, similar logic could be applied to the renal medulla.