Reactive oxygen intermediates (ROI) are partial reduction products of oxygen: 1 electron reduces O2 to form superoxide (O2−), and 2 electrons reduce O2 to form hydrogen peroxide (H2O2). ROI are generated as a byproduct of aerobic metabolism and by toxicological mechanisms. There is growing evidence for regulated enzymatic generation of O2− and its conversion to H2O2 in a variety of cells. The conversion of O2− to H2O2 occurs spontaneously, but is markedly accelerated by superoxide dismutase (SOD). High levels of ROI are associated with damage to biomolecules such as DNA, biomembranes and proteins. Recent evidence indicates generation of ROI under normal cellular conditions and points to signaling roles for O2− and H2O2.
Several biological systems generate reactive oxygen. Phagocytic cells such as neutrophils generate large quantities of ROI as part of their battery of bactericidal mechanisms. Exposure of neutrophils to bacteria or to various soluble mediators such as formyl-Met-Leu-Phe or phorbol esters activates a massive consumption of oxygen, termed the respiratory burst, to initially generate superoxide, with secondary generation of H2O2, HOCl and hydroxyl radical. The enzyme responsible for this oxygen consumption is the respiratory burst oxidase (nicotinamide adenine dinucleotide phosphate-reduced form (NADPH) oxidase).
There is growing evidence for the generation of ROI by non-phagocytic cells, particularly in situations related to cell proliferation. Significant generation of H2O2, O2−, or both have been noted in some cell types. Fibroblasts and human endothelial cells show increased release of superoxide in response to cytokines such as interleukin-1 or tumor necrosis factor (TNF) (Meier et al. (1989) Biochem J. 263, 539–545.; Matsubara et al. (1986) J. Immun. 137, 3295–3298). Ras-transformed fibroblasts show increased superoxide release compared with control fibroblasts (Irani, et al. (1997) Science 275, 1649–1652). Rat vascular smooth muscle cells show increased H2O2 release in response to PDGF (Sundaresan et al. (1995) Science 270, 296–299) and angiotensin II (Griendling et al. (1994) Circ. Res. 74, 1141–1148; Fukui et al. (1997) Circ. Res. 80, 45–51; Ushio-Fukai et al. (1996) J. Biol. Chem. 271, 23317–23321), and H2O2 in these cells is associated with increased proliferation rate. The occurrence of ROI in a variety of cell types is summarized in Table 1 (adapted from Burdon, R. (1995) Free Radical Biol. Med. 18, 775–794).
TABLE 1SuperoxideHydrogen Peroxidehuman fibroblastsBalb/3T3 cellshuman endothelial cellsrat pancreatic islet cellshuman/rat smooth muscle cellsmurine keratinocyteshuman fat cellsrabbit chondrocyteshuman osteocyteshuman tumor cellsBHK-21 cellsfat cells, 3T3 L1 cellshuman colonic epithelial cells
ROI generated by the neutrophil have a cytotoxic function. While ROI are normally directed at the invading microbe, ROI can also induce tissue damage (e.g., in inflammatory conditions such as arthritis, shock, lung disease, and inflammatory bowel disease) or may be involved in tumor initiation or promotion, due to damaging effects on DNA. Nathan (Szatrowski et al. (1991) Canc. Res. 51, 794–798) proposed that the generation of ROI in tumor cells may contribute to the hypermutability seen in tumors, and may therefore contribute to tumor heterogeneity, invasion and metastasis.
In addition to cytotoxic and mutagenic roles, ROI have ideal properties as signal molecules: 1) they are generated in a controlled manner in response to upstream signals; 2) the signal can be terminated by rapid metabolism of O2− and H2O2 by SOD and catalase/peroxidases; 3) they elicit downstream effects on target molecules, e.g., redox-sensitive regulatory proteins such as NF kappa B and AP-1 (Schreck et al. (1991) EMBO J. 10, 2247–2258; Schmidt et al. (1995) Chemistry & Biology 2, 13–22). Oxidants such as O2− and H2O2 have a relatively well defined signaling role in bacteria, operating via the SoxI/II regulon to regulate transcription.
ROI appear to have a direct role in regulating cell division, and may function as mitogenic signals in pathological conditions related to growth. These conditions include cancer and cardiovascular disease. O2− is generated in endothelial cells in response to cytokines, and might play a role in angiogenesis (Matsubara et al. (1986) J. Immun. 137, 3295–3298). O2− and H2O2 are also proposed to function as “life-signals”, preventing cells from undergoing apoptosis (Matsubara et al. (1986) J. Immun. 137, 3295–3298). As discussed above, many cells respond to growth factors (e.g., platelet derived growth factor (PDGF), epidermal derived growth factor (EGF), angiotensin II, and various cytokines) with both increased production of O2−/H2O2 and increased proliferation. Inhibition of ROI generation prevents the mitogenic response. Exposure to exogenously generated. O2− and H2O2 results in an increase in cell proliferation. A partial list of responsive cell types is shown below in Table 2 (adapted from Burdon, R. (1995) Free Radical Biol. Med. 18, 775–794).
TABLE 2SuperoxideHydrogen peroxidehuman, hamster fibroblastsmouse osteoblastic cellsBalb/3T3 cellsBalb/3T3 cellshuman histiocytic leukemiarat, hamster fibroblastsmouse epidermal cellshuman smooth muscle cellsrat colonic epithelial cellsrat vascular smooth musclecellsrat vascular smooth muscle cells
While non-transformed cells can respond to growth factors and cytokines with the production of ROI, tumor cells appear to produce ROI in an uncontrolled manner. A series of human tumor cells produced large amounts of hydrogen peroxide compared with non-tumor cells (Szatrowski et al. (1991) Canc. Res. 51, 794–798). Ras-transformed NIH 3T3 cells generated elevated amounts of superoxide, and inhibition of superoxide generation by several mechanisms resulted in a reversion to a “normal” growth phenotype.
O2− has been implicated in maintenance of the transformed phenotype in cancer cells including melanoma, breast carcinoma, fibrosarcoma, and virally transformed tumor cells. Decreased levels of the manganese form of SOD (MnSOD) have been measured in cancer cells and in vitro-transformed cell lines, predicting increased O2− levels (Burdon, R. (1995) Free Radical Biol. Med. 18, 775–794). MnSOD is encoded on chromosome 6q25 which is very often lost in melanoma. Overexpression of MnSOD in melanoma and other cancer cells (Church et al. (1993) Proc. of Natl. Acad. Sci. 90, 3113–3117; Fernandez-Pol et al. (1982) Canc. Res. 42, 609–617; Yan et al. (1996) Canc. Res. 56, 2864–2871) resulted in suppression of the transformed phenotype.
ROI are implicated in growth of vascular smooth muscle associated with hypertension, atherosclerosis, and restenosis after angioplasty. O2− generation is seen in rabbit aortic adventitia (Pagano et al. (1997) Proc. Natl. Acad. Sci. 94, 14483–14488). Vascular endothelial cells release O2− in response to cytokines (Matsubara et al. (1986) J. Immun. 137, 3295–3298). O2− is generated by aortic smooth muscle cells in culture, and increased O2− generation is stimulated by angiotensin II which also induces cell hypertrophy. In a rat model system, infusion of angiotensin II leads to hypertension as well as increased O2− generation in subsequently isolated aortic tissue (Ushio-Fukai et al. (1996) J. Biol. Chem. 271, 23317–23321.; Yu et al. (1997) J. Biol. Chem. 272, 27288–27294). Intravenous infusion of a form of SOD that localizes to the vasculature or an infusion of an O2− scavenger prevented angiotensin II induced hypertension and inhibited ROI generation (Fukui et al. (1997) Circ. Res. 80, 45–51).
The neutrophil NADPH oxidase, also known as phagocyte respiratory burst oxidase, provides a paradigm for the study of the specialized enzymatic ROI-generating system. This extensively studied enzyme oxidizes NADPH and reduces oxygen to form O2−. NADPH oxidase consists of multiple proteins and is regulated by assembly of cytosolic and membrane components. The catalytic moiety consists of flavocytochrome b558, an integral plasma membrane enzyme comprised of two components: gp91phox (gp refers to glycoprotein; phox is an abbreviation of the words phagocyte and oxidase) and p22phox (p refers to protein). gp91phox contains 1 flavin adenine dinucleotide (FAD) and 2 hemes as well as the NADPH binding site. p22phox has a C-terminal proline-rich sequence which serves as a binding site for cytosolic regulatory proteins. The two cytochrome subunits, gp91phox and p22phox appear to stabilize one another, since the genetic absence of either subunit, as in the inherited disorder chronic granulomatous disease (CGD), results in the absence of the partner subunit (Yu et al. (1997) J. Biol. Chem. 272, 27288–27294). Essential cytosolic proteins include p47phox, p67phox and the small GTPase Rac, of which there are two isoforms. p47phox and p67phox both contain SH3 regions and proline-rich regions which participate in protein interactions governing assembly of the oxidase components during activation. The neutrophil enzyme is regulated in response to bacterial phagocytosis or chemotactic signals by phosphorylation of p47phox, and perhaps other components, as well as by guanine nucleotide exchange to activate the GTP-binding protein Rac.
The origin of ROI in non-phagocytic tissues is unproven, but the occurrence of phagocyte oxidase components has been evaluated in several systems by immunochemical methods, Northern blots and reverse transcriptase-polymerase chain reaction (RT-PCR). The message for p22phox is expressed widely, as is that for Rac1. Several cell types that are capable of O2− generation have been demonstrated to contain all of the phox components including gp91phox, as summarized below in Table 3. These cell types include endothelial cells, aortic adventitia and lymphocytes.
TABLE 3Tissuegp91phoxp22phoxp47phoxp67phoxneutrophil+1,2+1,2+1,2+1,2aortic adventitia+1+1+1+1lymphocytes+2+2+1,2+1,2endothelial cells+2+2+1,2+1.2glomerular mesangial—+1,2+1,2+1,2cellsfibroblasts—+2+1,2+2aortic sm. muscle—+1,2??1= protein expression shown.2= mRNA expression shown.
However, a distinctly different pattern is seen in several other cell types shown in Table 3 including glomerular mesangial cells, rat aortic smooth muscle and fibroblasts. In these cells, expression of gp91phox is absent while p22phox and in some cases cytosolic phox components have been demonstrated to be present. Since gp91phox and p22phox stabilize one another in the neutrophil, there has been much speculation that some molecule, possibly related to gp91phox, accounts for ROI generation in glomerular mesangial cells, rat aortic smooth muscle and fibroblasts (Ushio-Fukai et al. (1996) J. Biol. Chem. 271, 23317–23321). Investigation of fibroblasts from a patient with a genetic absence of gp91phox provides proof that the gp91phox subunit is not involved in ROI generation in these cells (Emmendorffer et al. (1993) Eur. J. Haematol. 51, 223–227). Depletion of p22phox from vascular smooth muscle using an antisense approach indicated that this subunit participates in ROI generation in these cells, despite the absence of detectable gp91phox (Ushio-Fukai et al. (1996) J. Biol. Chem. 271, 23317–23321). At this time the molecular candidates possibly related to gp91phox and involved in ROI generation in these cells are unknown.
Accordingly, what is needed is the identity of the proteins involved in ROI generation, especially in non-phagocytic tissues and cells. What is also needed are the nucleotide sequences encoding for these proteins, and the primary sequences of the proteins themselves. Also needed are vectors designed to include nucleotides encoding for these proteins. Probes and PCR primers derived from the nucleotide sequence are needed to detect, localize and measure nucleotide sequences, including mRNA, involved in the synthesis of these proteins. In addition, what is needed is a means to transfect cells with these vectors. What is also needed are expression systems for production of these molecules. Also needed are antibodies directed against these molecules for a variety of uses including localization, detection, measurement and passive immunization.