The beginning of GPX2 and 30 years later

We published the first paper to characterize GPX2 (aka GSHPx-GI) as a selenoenzyme with glutathione peroxidase activity in 1993. Among the four Se-GPX isozymes, GPX1-4, GPX1 and GPX2 are closely related in terms of structure, substrate specificities, and subcellular localization. What sets them apart are distinct patterns of gene regulation, tissue distribution and response to selenium. While we identified the digestive tract epithelium as the main site of GPX2 expression, later work has shown GPX2 is found more widely in epithelial tissues with concentration of expression in stem cell and proliferative compartments. GPX2 expression is regulated over a wide range of levels by many pathways, including NRF2, WNT, p53, RARE and this often results in attaching undue significance to GPX2 as GPX2 is only a part of a system of hydroperoxidase activities, including GPX1, peroxiredoxins and catalase. These other activities may play equal or greater roles, particularly in cell lines cultured without selenium supplementation and often with very low GPX2 levels. This could be assessed by examining levels of mRNA and protein among these various peroxidases at the outset of studies. As an example, it was found that GPX1 responds to the absence of GPX2 in mouse ileum and colon epithelium with higher expression. As such, both Gpx1 and Gpx2 had to be knocked out in mice to produce ileocolitis. However, we note that the actual role of GPX1 and GPX2 in relation to peroxiredoxin function is unclear. There may be an interdependence that requires only low amounts of GPX1 and/or GPX2 in a supporting role to maintain proper peroxiredoxin function. GPX2 levels may be prognostic for cancer progression in colon, breast, prostate and liver, however, there is no consistent trend for higher or lower levels to be favorable.

Role of the microbiota in ileitis of a mouse model of inflammatory bowel disease-Glutathione peroxide isoenzymes 1 and 2-double knockout mice on a C57BL background

C57Bl6 (B6) mice devoid of glutathione peroxidases 1 and 2 (Gpx1/2‐DKO) develop ileitis after weaning. We previously showed germ‐free Gpx1/2‐DKO mice of mixed B6.129 background did not develop ileocolitis. Here, we examine the composition of the ileitis provoking microbiota in B6 Gpx1/2‐DKO mice. DNA was isolated from the ileum fecal stream and subjected to high‐throughput sequencing of the V3 and V4 regions of the 16S rRNA gene to determine the abundance of operational taxonomic units (OTUs). We analyzed the role of bacteria by comparing the microbiomes of the DKO and pathology‐free non‐DKO mice. Mice were treated with metronidazole, streptomycin, and vancomycin to alter pathology and correlate the OTU abundances with pathology levels. Principal component analysis based on Jaccard distance of abundance showed 3 distinct outcomes relative to the source Gpx1/2‐DKO microbiome. Association analyses of pathology and abundance of OTUs served to rule out 7–11 of 24 OTUs for involvement in the ileitis. Collections of OTUs were identified that appeared to be linked to ileitis in this animal model and would be classified as commensals. In Gpx1/2‐DKO mice, host oxidant generation from NOX1 and DUOX2 in response to commensals may compromise the ileum epithelial barrier, a role generally ascribed to oxidants generated from mitochondria, NOX2 and endoplasmic reticulum stress in response to presumptive pathogens in IBD. Elevated oxidant levels may contribute to epithelial cell shedding, which is strongly associated with progress toward inflammation in Gpx1/2‐DKO mice and predictive of relapse in IBD by allowing leakage of microbial components into the submucosa.

Dexamethasone and Tofacitinib suppress NADPH oxidase expression and alleviate very-early-onset ileocolitis in mice deficient in GSH peroxidase 1 and 2

C57BL6/J (B6) mice lacking Se-dependent GSH peroxidase 1 and 2 (GPx1/2-DKO) develop mild to moderate ileocolitis around weaning. These DKO mice have a disease resembling human very-early-onset inflammatory bowel disease (VEOIBD), which is associated with mutations in NADPH oxidase genes. Drugs including dexamethasone (Dex), Tofacitinib (Tofa; a Janus kinase/JAK inhibitor) and anti-TNF antibody are effective to treat adult, but not pediatric IBD.

AIMS

To test the efficacy of hydrophobic Dex and hydrophilic Dex phosphate (Dex phos), Tofa, anti-Tnf Ab, Noxa1ds-TAT and gp91ds-TAT peptides (inhibiting NOX1 and NOX2 assembly respectively), antioxidant MJ33 and ML090, and pifithrin-α (p53 inhibitor) on alleviation of gut inflammation in DKO weanlings.

MAIN METHODS

All treatments began on 22-day-old GPx1/2-DKO mice. The mouse intestine pathology was compared between the drug- and vehicle-treated groups after six or thirteen days of treatment.

KEY FINDINGS

Among all drugs tested, Dex, Dex phos and Tofa were the strongest to suppress ileocolitis in the DKO weanlings. Dex, Dex phos and Tofa inhibited crypt apoptosis and increased crypt density. Dex or Dex phos alone also inhibited cell proliferation, exfoliation and crypt abscess in the ileum. Dex, but not Tofa, retarded mouse growth. Both Dex and Tofa inhibited ileum Nox1, Nox4 and Duox2, but not Nox2 gene expression. Noxa1ds-TAT and gp91ds-TAT peptides as well as MJ33 had subtle effect on suppressing pathology, while others had negligible effect.

SIGNIFICANCE

These findings suggest that NADPH oxidases can be novel drug targets for pediatric IBD therapy, and Tofa may be considered for treating VEOIBD.

NADPH oxidase-1 deficiency offers little protection in Salmonella typhimurium-induced typhlitis in mice

AIM

To test whether Nox1 plays a role in typhlitis induced by Salmonella enterica serovar Typhimurium (S. Tm) in a mouse model.

METHODS

Eight-week-old male wild-type (WT) and Nox1 knockout (KO) C57BL6/J (B6) mice were administered metronidazole water for 4 d to make them susceptible to S. Tm infection by the oral route. The mice were given plain water and administered with 4 different doses of S. Tm by oral gavage. The mice were followed for another 4 d. From the time of the metronidazole application, the mice were observed twice daily and weighed daily. The ileum, cecum and colon were removed for sampling at the fourth day post-inoculation. Portions of all three tissues were fixed for histology and placed in RNAlater for mRNA/cDNA preparation and quantitative real-time PCR. The contents of the cecum were recovered for estimation of S. Tm CFU.

RESULTS

We found Nox1-knockout (Nox1-KO) mice were not more sensitive to S. Tm colonization and infection than WT B6 mice. This conclusion is based on the following observations: (1) S. Tm-infection induced similar weight loss in Nox1-KO mice compared to WT mice; (2) the same S. Tm CFU was recovered from the cecal content of Nox1-KO and WT mice regardless of the inoculation dose, except the lowest inoculation dose (2 × 10⁶ CFU) for which the Nox1-KO had one-log lower CFU than WT mice; (3) there is no difference in cecal pathology between WT and Nox1-KO groups; and (4) there are no S. Tm infection-induced changes in gene expression levels (IL-1b, TNF-α, and Duox2) between WT and Nox1-KO groups. The Alpi gene expression was more suppressed by S. Tm treatment in WT than the Nox1-KO cecum.

CONCLUSION

Nox1 does not protect mice from S. Tm colonization. Nox1-KO provides a very minor protective effect against S. Tm infection. Using NOX1-specific inhibitors for colitis therapy should not increase risks in bacterial infection.

Deficiency in Duox2 activity alleviates ileitis in GPx1- and GPx2-knockout mice without affecting apoptosis incidence in the crypt epithelium

Mice deficient in glutathione peroxidase (GPx)-1 and -2 (GPx1^-/-GPx2^-/- double knockout or DKO mice) develop very-early-onset (VEO) ileocolitis, suggesting that lack of defense against reactive oxygen species (ROS) renders susceptibility to intestinal inflammation. Two members of ROS-generating NADPH oxidase family, NOX1 and DUOX2, are highly inducible in the intestinal epithelium. Previously, we reported that Nox1 deficiency ameliorated the pathology in DKO mice (Nox1-TKO). The role of Duox2 in ileocolitis of the DKO mice is evaluated here in Duoxa-TKO mice by breeding DKO mice with Duoxa^-/- mice (Duoxa-TKO), which do not have Duox2 activity. Similar to Nox1-TKO mice, Duoxa-TKO mice no longer have growth retardation, shortened intestine, exfoliation of crypt epithelium, crypt abscesses and depletion of goblet cells manifested in DKO mice by 35 days of age. Unlike Nox1-TKO mice, Duoxa-TKO mice still have rampant crypt apoptosis, elevated proliferation, partial loss of Paneth cells and diminished crypt density. Treating DKO mice with NOX inhibitors (di-2-thienyliodonium/DTI and thioridazine/THZ) and an antioxidant (mitoquinone/MitoQ) significantly reduced gut pathology. Furthermore, in the inflamed human colon, DUOX protein expression is highly elevated in the apical, lateral and perinuclear membrane along the whole length of gland. Taken together, we conclude that exfoliation of crypt epithelium, but not crypt apoptosis, is a major contributor to inflammation. Both Nox1 and Duox2 induce exfoliation of crypt epithelium, but only Nox1 induces apoptosis. NOX1 and DUOX2 may be potential therapeutic targets for treating ileocolitis in human patients suffering inflammatory bowel disease (IBD).

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Glutathione peroxidase-1/2-double knockout mice have very-early-onset ileocolitis.

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By deletion of Nox1 gene expression, the triple knockout mice are without pathology.

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By deletion of Duoxa, the mice have milder pathology without crypt exfoliation.

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The Duoxa triple knock mice still have rampant crypt epithelium apoptosis.

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Several antioxidants and NOX inhibitors reduce gut inflammation in the DKO mice.

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DKO mice are an excellent animal model for preclinical testing of NOX inhibitors.

The Gdac1 locus modifies spontaneous and Salmonella-induced colitis in mice deficient in either Gpx2 or Gpx1 gene

We previously identified the Gdac1 (Gpx-deficiency-associated colitis 1) locus, which influences the severity of spontaneous colitis in Gpx1- and Gpx2-double-knockout (Gpx1/2-DKO) mice. Congenic Gpx1/2-DKO mice in the 129S1/SvImJ (129) background but carrying the Gdac1(B6) allele have milder spontaneous colitis than 129 Gpx1/2-DKO mice carrying the Gdac1(129) allele. Here, we evaluated the effect of the Gdac1(B6) allele on 129 strain non-DKO mice that had a wild-type (WT) Gpx1 or Gpx2 allele and WT mice. We found that the congenic Gdac1(B6) Gpx2-KO, Gpx1-KO, and WT mice also had better health than the corresponding 129 mice measured by at least one of the parameters including disease signs, colon length, or weight gain. The Gdac1(B6) allele prevented loss of goblet cells and crypt epithelium exfoliation in the Gpx1/2-DKO mice, but did not affect epithelial cell apoptosis or proliferation. Because Gdac1(B6) affects gut dysbiosis in the DKO mice, we then tested its impact on bacteria-induced colitis in non-DKO mice. First, we found both Gpx1-KO and Gpx2-KO mice were susceptible to Salmonella enterica serotype typhimurium (S. Tm)-induced colitis under conditions where WT B6 and 129 mice were resistant. Second, the S. Tm-infected Gdac1(B6) Gpx1-KO mice had stronger inflammatory responses than 129 Gpx1-KO or 129 Gpx2-KO with both Gdac1 alleles and WT mice by having higher mRNA levels of Nod2, Nox2, Tnf, and Cox2. We conclude that the Gdac1 locus affects both spontaneous and S. Tm-induced colitis in 129 non-DKO mice, although in opposite directions.

Deletion of glutathione peroxidase-2 inhibits azoxymethane-induced colon cancer development

The selenoprotein glutathione peroxidase-2 (GPx2) appears to have a dual role in carcinogenesis. While it protected mice from colon cancer in a model of inflammation-triggered carcinogenesis (azoxymethane and dextran sodium sulfate treatment), it promoted growth of xenografted tumor cells. Therefore, we analyzed the effect of GPx2 in a mouse model mimicking sporadic colorectal cancer (azoxymethane-treatment only). GPx2-knockout (KO) and wild-type (WT) mice were adjusted to an either marginally deficient (−Se), adequate (+Se), or supranutritional (++Se) selenium status and were treated six times with azoxymethane (AOM) to induce tumor development. In the −Se and ++Se groups, the number of tumors was significantly lower in GPx2-KO than in respective WT mice. On the +Se diet, the number of dysplastic crypts was reduced in GPx2-KO mice. This may be explained by more basal and AOM-induced apoptotic cell death in GPx2-KO mice that eliminates damaged or pre-malignant epithelial cells. In WT dysplastic crypts GPx2 was up-regulated in comparison to normal crypts which might be an attempt to suppress apoptosis. In contrast, in the +Se groups tumor numbers were similar in both genotypes but tumor size was larger in GPx2-KO mice. The latter was associated with an inflammatory and tumor-promoting environment as obvious from infiltrated inflammatory cells in the intestinal mucosa of GPx2-KO mice even without any treatment and characterized as low-grade inflammation. In WT mice the number of tumors tended to be lowest in +Se compared to −Se and ++Se feeding indicating that selenium might delay tumorigenesis only in the adequate status. In conclusion, the role of GPx2 and presumably also of selenium depends on the cancer stage and obviously on the involvement of inflammation.

Expression of lactoperoxidase in differentiated mouse colon epithelial cells

Lactoperoxidase (LPO) is known to be present in secreted fluids, such as milk and saliva. Functionally, LPO teams up with dual oxidases (DUOXs) to generate bactericidal hypothiocyanite in the presence of thiocyanate. DUOX2 is expressed in intestinal epithelium, but there is little information on LPO expression in this tissue. To fill the gap of knowledge, we have analyzed Lpo gene expression and its regulation in mouse intestine. In wild-type (WT) C57BL/6 (B6) mouse intestine, an appreciable level of mouse Lpo gene expression was detected in the colon, but not the ileum. However, in B6 mice deficient in glutathione peroxidase (GPx)-1 and -2, GPx1/2-double-knockout (DKO), which had intestinal pathology, the colon Lpo mRNA levels increased 5- to 12-fold depending on mouse age. The Lpo mRNA levels in WT and DKO 129S1/SvlmJ (129) colon were even higher, 9- and 5-fold, than in B6 DKO colon. Higher levels of Lpo protein and enzymatic activity were also detected in the 129 mouse colon compared to B6 colon. Lpo protein was expressed in the differentiated colon epithelial cells, away from the crypt base, as shown by immunohistochemistry. Similar to human LPO mRNA, mouse Lpo mRNA had multiple spliced forms, although only the full-length variant 1 was translated. Higher methylation was found in the 129 than in the B6 strain, in DKO than in control colon, and in older than in juvenile mice. However, methylation of the Lpo intragenic CpG island was not directly induced by inflammation, because dextran sulfate sodium-induced colitis did not increase DNA methylation in B6 DKO colon. Also, Lpo DNA methylation is not correlated with gene expression.

Colitis locus on chromosome 2 impacting the severity of early-onset disease in mice deficient in GPX1 and GPX2

BACKGROUND

Genetic background has a profound effect on inflammatory bowel disease. The Gpx1 and Gpx2 double knockout (GPX1/2-DKO) mice on a mixed C57BL/6 (B6) and 129S1/SvimJ (129) background exhibit spontaneous ileocolitis. The DKO mice on a B6 background have mild ileocolitis. We characterized the 129 DKO mice to identify a genetic locus affecting disease severity.

METHODS

We backcrossed B6;129 DKO mice to 129 and analyzed for ileocolitis penetrance and severity at N5, N7, and N10. By correlating disease severity with single-nucleotide polymorphism (SNP) markers, we identified a colitis locus.

RESULTS

As early as 9 days of age, 129 DKO N5 and N10 mice showed disease signs and morbidity. The N10 DKO mice had the severest colitis with nearly complete penetrance and high morbidity compared with other generations or backgrounds. 129 DKO mice had elevated colonic KC and SAA3 expression, shorter colon length, and cecal E. coli overgrowth compared to B6 DKO mice. Analysis of the B6 loci in 129 N5, N7, and N10 cohorts pointed to a region of chromosome 2: 119 Mbp contributing to mild symptoms.

CONCLUSIONS

GPX1/2-DKO mice on 129 genetic background have the most aggressive colitis compared to B6;129 and B6 colonies. A B6 locus significantly contributing the resistance resides on chromosome 2: 119 Mbp. This region coincides with cytokine-deficiency-induced colitis susceptibility, Cdcs3, identified in the resistant B6 and sensitive C3H/HeJBir (C3Bir) with IL-10 deficiency. A three-way SNP analysis between 129, B6, and C3Bir locus points the major candidate genes to B2m, Dnajc17, Duox2, Pla2g4b, Pla2g4e, Pla2g4f and Slc30a4.

Atherogenic diets exacerbate colitis in mice deficient in glutathione peroxidase

BACKGROUND

The proinflammatory effect of high-fat diet has been observed beyond the cardiovascular system, but there is little evidence to support its role in triggering inflammatory bowel disease. GPx1/2-double-knockout (DKO) mice deficient in 2 intracellular glutathione peroxidases, GPx1 and GPx2, on a C57BL/6 (B6) background, have mild ileocolitis on a conventional chow.

METHODS

We fed B6 DKO mice 2 atherogenic diets to test the dietary effect on atherosclerosis and ileocolitis. Both atherogenic diets have high cholesterol-the Chol+/CA diet has cholic acid (CA), and the Chol+ diet has no CA.

RESULTS

The Chol+/CA diet induced severe colitis, but not ileitis, in the DKO mice compared with the Chol+ and the Chol- control diet. On the Chol+/CA diet, the wild-type (WT) mice had levels of aortic lesions and hypercholesterolemia similar to those of DKO mice but had no intestinal pathology. The diet-associated inflammatory responses in the DKO mice included increased colonic proinflammatory serum amyloid A3 expression, plasma lipopolysaccharide, and TNF-α levels. The Chol+/CA diet lowered the expression of the unfolded protein response genes ATF6, CHOP, unspliced Xbp(U) , and Grp78/Bip, in WT and DKO mice compared with mice on the Chol- diet.

CONCLUSIONS

We concluded that a cholesterol diet weakens the colon unfolded protein response, which can aggravate spontaneous colitis, leading to gut barrier breakdown. GPx has no impact on atherosclerosis without ultrahypercholesterolemia.

A Strong Impact of Genetic Background on Gut Microflora in Mice

Genetic background affects susceptibility to ileocolitis in mice deficient in two intracellular glutathione peroxidases, GPx1 and GPx2. The C57BL/6 (B6) GPx1/2 double-knockout (DKO) mice have mild ileocolitis, and 129S1/Sv (129) DKO mice have severe inflammation. We used diet to modulate ileocolitis; a casein-based defined diet with AIN76A micronutrients (AIN) attenuates inflammation compared to conventional LabDiets. Because luminal microbiota induce DKO ileocolitis, we assessed bacterial composition with automated ribosomal intergenic-spacer analysis (ARISA) on cecal DNA. We found that mouse strain had the strongest impact on the composition of microbiota than diet and GPx genotypes. In comparing AIN and LabDiet, DKO mice were more resistant to change than the non-DKO or WT mice. However, supplementing yeast and inulin to AIN diet greatly altered microflora profiles in the DKO mice. From 129 DKO strictly, we found overgrowth of Escherichia coli. We conclude that genetic background predisposes mice to colonization of potentially pathogenic E. coli.

Glutathione peroxidase-2 protects from allergen-induced airway inflammation in mice

The aim of the present study was to identify and validate the biological significance of new genes/proteins involved in the development of allergic airway disease in a murine asthma model. Gene microarrays were used to identify genes with at least a two-fold increase in gene expression in lungs of two separate mouse strains with high and low allergic susceptibility. Validation of mRNA data was obtained by western blotting and immunohistochemistry, followed by functional analysis of one of the identified genes in mice with targeted disruption of specific gene expression. Expression of two antioxidant enzymes, glutathione peroxidase-2 (GPX2) and glutathione S-transferase omega (GSTO) 1-1 was increased in both mouse strains after induction of allergic airway disease and localised in lung epithelial cells. Mice with targeted disruption of the Gpx-2 gene showed significantly enhanced airway inflammation compared to sensitised and challenged wild-type mice. Our data indicate that genes encoding the antioxidants GPX2 and GSTO 1-1 are common inflammatory genes expressed upon induction of allergic airway inflammation, and independently of allergic susceptibility. Furthermore, we provide evidence to illustrate the importance of a single antioxidant enzyme, GPX2, in protection from allergen-induced disease.

Methylation of polycomb target genes in intestinal cancer is mediated by inflammation

Epigenetic changes are strongly associated with cancer development. DNA hypermethylation is associated with gene silencing and is often observed in CpG islands. Recently, it was suggested that aberrant CpG island methylation in tumors is directed by Polycomb (PcG) proteins. However, specific mechanisms responsible for methylation of PcG target genes in cancer are not known. Chronic infection and inflammation contribute to up to 25% of all cancers worldwide. Using glutathione peroxidase, Gpx1 and Gpx2, double knockout (Gpx1/2-KO) mice as a model of inflammatory bowel disease predisposing to intestinal cancer, we analyzed genome-wide DNA methylation in the mouse ileum during chronic inflammation, aging, and cancer. We found that inflammation leads to aberrant DNA methylation in PcG target genes, with 70% of the approximately 250 genes methylated in the inflamed tissue being PcG targets in embryonic stem cells and 59% of the methylated genes being marked by H3K27 trimethylation in the ileum of adult wild-type mice. Acquisition of DNA methylation at CpG islands in the ileum of Gpx1/2-KO mice frequently correlates with loss of H3K27 trimethylation at the same loci. Inflammation-associated DNA methylation occurs preferentially in tissue-specific silent genes and, importantly, is much more frequently represented in tumors than is age-dependent DNA methylation. Sixty percent of aberrant methylation found in tumors is also present in the inflamed tissue. In summary, inflammation creates a signature of aberrant DNA methylation, which is observed later in the malignant tissue and is directed by the PcG complex.

Novel enrichment of tumor cell transfectants expressing high levels of type 4 glutathione peroxidase using 7alpha-hydroperoxycholesterol as a selection agent

A novel approach for selecting high expressing cells out of a general population that had been transfected with a construct encoding cytosolic type 4 glutathione peroxidase (GPx4) is reported. The approach is described for GPx4-null COH-BR1 breast tumor cells and is based on use of a highly specific GPx4 substrate, 7alpha-hydroperoxycholesterol (7alpha-OOH), as a selection agent. Cells recovering from a highly toxic dose of liposomal 7alpha-OOH were found to be substantially more resistant to a second 7alpha-OOH challenge than cells recovering from a less toxic dose, but were much less resistant to t-butyl hydroperoxide (t-BuOOH) or H2O2. Several clones isolated from the general transfectant population exhibited variable, relatively low GPx4 activities. However, clones from the 7alpha-OOH-selected population exhibited uniformly high GPx4 activities (each approximately 3-fold higher than that of the starting transfectant population) and elevated steady-state mRNA levels. t-BuOOH could also be used for selecting high GPx4-expressing cells, but consistent recovery from toxic doses was more difficult than with 7alpha-OOH. Compared with conventional "hit or miss" cloning procedures, the 7alpha-OOH approach we describe affords a uniform population of high GPx4-activity cells in a relatively rapid manner. This approach should prove valuable for investigators interested in the peroxide regulatory properties of GPx4, in the context of both cytoprotection and redox signaling.

Mutation Accumulation in the Intestine and Colon of Mice Deficient in Two Intracellular Glutathione Peroxidases

Mice deficient in two glutathione peroxidases (GPX), Gpx1 and Gpx2, [Gpx1/2-double knockout (DKO) mice] are prone to ileocolitis on a mixed C57BL/6 and 129S1/SvJ (B6.129) genetic background. We reported previously that approximately 25% of B6.129 Gpx1/2-DKO mice develop ileocolonic tumors by 6 to 9 months of age, when their non-DKO littermates [having at least one wild-type (WT) Gpx1 or Gpx2 allele] rarely have inflammation and none have tumors. Because genetic background affects tumor susceptibility, we have generated a B6 Gpx1/2-DKO colony and discovered that these mice have fewer inflammatory cells, milder ileocolitis, and low mortality, and only 2.5% of B6 mice developed tumors. The mutant frequency of a cII reporter gene was about 2- to 3-fold higher in 28-day-old Gpx1/2-DKO and 4-fold higher in 8-month-old Gpx1/2-DKO ileal mucosa than in controls in both genetic backgrounds. In contrast, mutant frequencies in the unaffected B6 liver were not significantly different between WT and Gpx1/2-DKO mice. The mutant frequency of 8-month-old B6.129 Gpx1/2-DKO ileum was 38.94 +/- 15.5(-5), which was not significantly higher than the age-matched B6 ileum, 25.54 +/- 10.33(-5). The mutation spectra analysis has shown that B6 Gpx1/2-DKO ileum had a 3-fold increase in small nucleotide deletions at mononucleotide repeats over control B6, which are a signature mutation associated with oxidative stress. Unexpectedly, B6 Gpx1/2-DKO mice had fewer C to T transitions at CpG dinucleotides than the WT B6 (18.0% versus 40.1%; P < 0.001). Our results suggest that inflammation drives gene mutations, which leads to neoplastic transformation of intestinal epithelium in the B6.129 Gpx1/2-DKO mice but rarely in the B6 Gpx1/2-DKO mice.

Epithelium-specific glutathione peroxidase, Gpx2, is involved in the prevention of intestinal inflammation in selenium-deficient mice

Mice deficient in 2 intracellular selenium (Se)-dependent glutathione peroxidases (Gpx1 and Gpx2), by genetically disrupting both alleles of the Gpx1 and Gpx2 genes (Gpx1(-/-)Gpx2(-/-)), develop ileocolitis around weaning. However, decreased Gpx activity in Se-depleted wild-type animals does not produce pathology in the gastrointestinal tract. Because a small percentage of Se-sufficient Gpx1(+/-)Gpx2(-/-) mice have mild ileocolitis, we hypothesized that Se-deficient Gpx1(+/-)Gpx2(-/-) mice will develop severe ileocolitis similarly to the Gpx1(-/-)Gpx2(-/-) mice, and even a trace amount of Gpx2 can protect intestinal mucosa against inflammation. To test our hypothesis, we fed mice at various stages of development with either Gpx1(+/)(-)Gpx2(-/-) or Gpx1(-/-)Gpx2(+/)(-) genotypes an Se-deficient diet for 4-5 wk and assessed the symptoms and pathology. Gpx1(+/)(-)Gpx2(-/-) mice that were deprived of Se in utero or at weaning (18-22 d of age), but not as young adults (31-51 d of age), manifested significantly worse pathology than their Se-sufficient counterparts. Both Gpx1 and Gpx2 activities and mRNA levels were significantly depressed in the ileum of Se-deprived mice. In mice deprived in utero, the pathology included acute inflammation with neutrophil and monocyte infiltration particularly in the colon and was externally manifested by perianal alopecia and ulceration. On the other hand, Gpx1(-/-)Gpx2(+/)(-) mice were unaffected by Se deprivation, regardless of the age of onset. The results show that a trace amount of Gpx2 is protective against ileocolitis, and Se-deficient young Gpx1(+/-)Gpx2(-/-) mice will develop pathology and symptoms similar to Se-adequate Gpx1(-/-)Gpx2(-/-) mice.

Role of Se-dependent glutathione peroxidases in gastrointestinal inflammation and cancer

Increase in reactive oxygen species plays an integral part in the inflammatory response, and chronic inflammation increases cancer risk. Selenium-dependent glutathione peroxidase (GPX) is well recognized for its antioxidant, and thus anti-inflammatory, activity. However, due to the multiple antioxidant families present in the gastrointestinal tract, it has been difficult to demonstrate the importance of individual antioxidant enzymes. Using genetically altered mice deficient in individual Gpx genes has provided insight into the physiological functions of these genes. Insufficient GPX activity in the mucosal epithelium can trigger acute and chronic inflammation. The presence of certain microflora, such as Helicobacter species, may affect cancer risk significantly. However, when damaged cells have progressed into a precancerous status, increased GPX activity may become procarcinogenic, presumably due to inhibition of hydroperoxide-mediated apoptosis. This review summarizes the current view of GPX in inflammation and cancer with emphasis on the GI tract.

Bacteria-induced intestinal cancer in mice with disrupted Gpx1 and Gpx2 genes

Two glutathione peroxidase (GPX) isozymes, GPX-1 and GPX-2 (GPX-GI), are the major enzymes that reduce hydroperoxides in intestinal epithelium. We have previously demonstrated that targeted disruption of both the Gpx1 and Gpx2 genes (GPX-DKO) results in a high incidence of ileocolitis in mice raised under conventional conditions, which include the harboring of Helicobacter species [non-specific-pathogen-free (non-SPF) conditions]. In this study, we have characterized GPX-DKO mice that have microflora-associated intestinal cancers, which are correlated with increased intestinal pathology/inflammation. We found that GPX-DKO mice raised under germ-free conditions have virtually no pathology or tumors. After colonizing germ-free mice with commensal microflora without any known pathogens (SPF), <9% of GPX-DKO mice develop tumors in the ileum or the colon. However, about one-fourth of GPX-DKO mice raised under non-SPF conditions from birth or transferred from SPF conditions at weaning have predominantly ileal tumors. Nearly 30% of tumors are cancerous; most are invasive adenocarcinomas and a few signet-ring cell carcinomas. On the basis of these results, we conclude that GPX-DKO mice are highly susceptible to bacteria-associated inflammation and cancer. The sensitivity exhibited in these mice suggests that peroxidative stress plays an important role in ileal and colonic pathology and inflammation, which can lead to tumorigenesis.

Microflora trigger colitis in mice deficient in selenium-dependent glutathione peroxidase and induce Gpx2 gene expression

Selenium-dependent glutathione peroxidase isoenzymes-1 and -2 are the major glutathione-dependent H2O2-reducing activities in the epithelium of the mid- to lower gastrointestinal tract. The two isoenzymes protect mice against ileocolitis. We have found that luminal microflora are required for colitis to develop in mice deficient in GPX-1 and GPX-2 activity (GPX-DKO). Within 7 days of association with microflora, previously asymptomatic germ-free GPX-DKO mice developed severe acute colitis while their littermates with at least one wild-type Gpx1 or Gpx2 gene remained virtually symptom-free. Microflora also affected Gpx2 gene expression. Gpx2, but not Gpx1, mRNA levels were elevated 4-5 fold in the ileum and colon in conventionally reared or microflora-associated adult mice compared with germ-free mice. Since the gastrointestinal tract microflora undergo major changes 2-3 weeks after birth, from relatively benign to a potentially stressful composition, we examined postnatal Gpx2 gene expression. The jejunal and ileal GPX-2 activity levels were low in two to three week-old mice and increased 5-7 fold during the next two weeks. GPX-2 activity levels were correlated with the mRNA levels. Colon Gpx2 mRNA levels held steady at about 50% of adult levels from 12-21 days of age but were several times higher than ileal levels. Our results suggest that ileal Gpx2 mRNA and GPX-2 activity levels are induced by luminal microflora. This response is consistent with a role for GPX as an anti-inflammatory activity.

Hyperresistance to cholesterol hydroperoxide-induced peroxidative injury and apoptotic death in a tumor cell line that overexpresses glutathione peroxidase isotype-4

The selenoenzyme phospholipid hydroperoxide glutathione peroxidase (PHGPX; GPX4) plays a key role in eukaryotic defense against potentially lethal peroxidative injury and also regulation of physiological peroxide tone. In this work we focused on the cytoprotective antiperoxidant effects of GPX4, using a breast tumor epithelial cell line that over-expresses the enzyme. Wild-type COH-BR1 cells, which exhibit little (if any) GPX4 activity, were transfected with a construct encoding the mitochondrion-targeted long (L) form of the enzyme. Several transfectant clones were selected which expressed relatively large amounts of GPX4, as determined by both Northern and Western analysis. Enzyme activity ranged from 15-fold to 190-fold greater than that of wild-type or null-transfected cells. The functional ramifications of GPX4 overexpression were tested by challenging cells with photochemically generated cholesterol hydroperoxides (ChOOHs) in liposomal form. Compared with vector controls, overexpressing clones were found to be substantially more resistant to ChOOH-induced killing, as determined by annexin-V (early apoptotic) and thiazolyl blue (mitochondrial dehydrogenase) reactivity. Concomitantly, the clones exhibited a striking hyper-resistance to free radical-mediated lipid peroxidation, as assessed by labeling cell membranes with [(14)C]cholesterol and measuring a family of radiolabeled oxidation products (ChOX). L-form GPX4's antiperoxidant and cytoprotective effects could reflect its ability to detoxify ChOOHs as they enter cells and/or cell-derived lipid hydroperoxides arising from ChOOH one-electron turnover.

Mice with combined disruption of Gpx1 and Gpx2 genes have colitis

Glutathione peroxidase (GPX)-1 and gastrointestinal (GI) epithelium-specific GPX (GPX-GI), encoded by Gpx1 and Gpx2, provide most GPX activity in GI epithelium. Although homozygous mice deficient in either the Gpx1 or Gpx2 gene appeared to be normal under standard housing conditions, homozygous mice deficient in both genes, double-knockout (KO) mice, had symptoms and pathology consistent with inflammatory bowel disease. These symptoms included a high incidence of perianal ulceration, growth retardation that started around weaning, and hypothermia that resembled that observed in calorie-restricted mice, even though the double-KO mice in our study were allowed to eat ad libitum. The growth retardation and hypothermia were components of cachexia, which is fatal in a high percentage of mice. Histological examination revealed that the double-KO mice had a high incidence of mucosal inflammation in the ileum and colon but not in the jejunum. Elevated levels of myeloperoxidase activity and lipid hydroperoxides were also detected in colon mucosa of these homozygous double-KO mice. These results suggest that GPX is essential for the prevention of the inflammatory response in intestinal mucosa.

Low glutathione peroxidase activity in Gpx1 knockout mice protects jejunum crypts from gamma-irradiation damage

Gpx1 knockout (KO) mice had a higher number of regenerating crypts in the jejunum than did Gpx2-KO or wild-type mice analyzed 4 days after > or =10 Gy gamma-irradiation. Without gamma-irradiation, glutathione peroxidase (GPX) activity in the jejunal and ileal epithelium of Gpx1-KO mice was <10 and approximately 35%, respectively, of that of the wild-type mice. Four days after exposure to 11 Gy, GPX activity in wild-type and Gpx1-KO ileum was doubled and tripled, respectively. However, jejunal GPX activity was not changed. Thus the lack of GPX activity in the jejunum is associated with better regeneration of crypt epithelium after radiation. Gpx2 gene expression was solely responsible for the increase in GPX activity in the ileum, since radiation did not alter GPX activity in Gpx2-KO mice. The intestinal Gpx2 mRNA levels of Gpx1-KO and wild-type mice increased up to 14- and 7-fold after radiation, respectively. Although the Gpx1-KO jejunum had higher levels of PGE(2) than the wild-type jejunum after exposure to 0 or 15 Gy, these differences were not statistically significant. Thus whether GPX inhibits PG biosynthesis in vivo remains to be established. We can conclude that the Gpx2 gene compensates for the lack of Gpx1 gene expression in the ileal epithelium. This may have abolished the protective effect in Gpx1-KO mice against the radiation damage in the ileum.

Retinoic acid induces Gpx2 gene expression in MCF-7 human breast cancer cells

We showed previously that the selenium-dependent glutathione peroxidase, GPX-GI, encoded by the Gpx2 gene, is highly expressed in the epithelium of the gastrointestinal (GI) tract and sporadically in breast tissue. To investigate whether Gpx2 gene expression is epithelium specific, we used in situ hybridization to show that Gpx2 mRNA is highly expressed in the crypt epithelium of human intestine. We also used Northern analysis to study human breast cells and found Gpx2 mRNA in human mammary epithelial cell lines as well as freshly isolated normal breast epithelial cells. Because we identified three putative retinoic acid response elements (RARE) in the Gpx2 gene, we examined the regulation of the Gpx2 gene expression by all-trans retinoic acid (RA) in RA-sensitive MCF-7 cells and RA-resistant HT29 cells. Without RA, MCF-7 cells had very low levels of Gpx2 mRNA and a low level of glutathione peroxidase (GPX) activity (17 mU/mg protein), whereas HT29 cells had a high level of Gpx2 mRNA and GPX activity (200 mU/mg protein). RA treatment increased Gpx2 mRNA level 3- to 11-fold and resulted in a fourfold increase of GPX activity (80 mU/mg protein) in MCF-7 cells. Neither Gpx2 mRNA level nor GPX activity was increased in HT29 cells. These results show that the Gpx2 gene is expressed in both breast and intestinal epithelium cells, and suggest that its expression can be highly regulated by retinoic acid, a known differentiation agent.

Glutathione peroxidase protects mice from viral-induced myocarditis

Glutathione peroxidase 1 (GPX-1) is a selenium-dependent enzyme with antioxidant properties. Previous investigations determined that mice deficient in selenium developed myocarditis when infected with a benign strain of coxsackievirus B3 (CVB3/0). To determine whether this effect was mediated by GPX-1, mice with a disrupted Gpx1 gene (Gpx1-/-) were infected with CVB3/0. Gpx1-/- mice developed myocarditis after CVB3/0 infection, whereas infected wild-type mice (Gpx1+/+) were resistant. Sequencing of viruses recovered from Gpx1(-/-)-infected mice demonstrated seven nucleotide changes in the viral genome, of which three occurred at the G residue, the most easily oxidized base. No changes were found in virus isolated from Gpx1+/+ mice. These results demonstrate that GPX-1 provides protection against viral-induced damage in vivo due to mutations in the viral genome of a benign virus.

Selenium-dependent glutathione peroxidase-GI is a major glutathione peroxidase activity in the mucosal epithelium of rodent intestine

Gpx2 mRNA, encoding a selenium-dependent glutathione peroxidase (GPX-GI), has been found to be highly expressed in the gastrointestinal tract (GI) mucosal epithelium. In this study, we show that GPX-GI is produced in the mucosal epithelium of the adult rat GI tract and that the activity levels are comparable to that from GPX-1. Post-mitochondrial supernatant GPX activity from the mucosal epithelium of the complete length of the small intestine was partially purified. A sample enriched for putative GPX-GI was fractionated by SDS-polyacrylamide gel electrophoresis. Polypeptides of 21 kDa and 22 kDa were digested with trypsin. After resolving the tryptic peptides by high pressure liquid chromatography (HPLC), the major peaks were analyzed for their amino acid sequence by Microflow-HPLC-Tandem Mass Spectrometry and automated Edman degradation sequencing. Both methods revealed that the 21-kDa sample contained rat GPX-GI determined by the sequence homology with the deduced mouse GPX-GI polypeptide sequence. Rat GPX-1 was also detected in the samples. AntiGPX-GI and antiGPX-1 antibodies were used to determine the distribution of the respective isoenzyme activities along the length of the intestine and with respect to the crypt to villus axis in rats. GPX-GI and GPX-1 activities were uniformly distributed in the middle and lower GI tract and with respect to the crypt to villus axis. GPX-GI activity accounted nearly the same percentage of the total GPX activity as GPX-1 in all of the these compartments. Studies on the distal ileum segment of wildtype and Gpx1 gene knockout mice showed that GPX-GI activity was also at parity with GPX-1 in the mucosal epithelium of this segment.

Expression and chromosomal mapping of mouse Gpx2 gene encoding the gastrointestinal form of glutathione peroxidase, GPX-GI

GPX-GI is a cytosolic tetrameric Se-dependent glutathione peroxidase, similar in properties to GPX-1. Unlike the almost ubiquitous GPX-1, GPX-GI is mainly expressed in the epithelium of gastrointestinal tract. GPX-GI contributes to at least fifty percent of GPX activity in rodent small intestinal epithelium. The total GPX activity consists of at least 70% of selenium-dependent GPX activity in this compartment. By analyzing a panel of mouse interspecies DNA from the Jackson Laboratory's backcross resource, we mapped Gpx2 gene to mouse chromosome 12 between D12Mit4 and D12Mit5, near the Ccs1 locus which contains a colon cancer susceptibility gene. A pseudogene, Gpx2-ps is mapped to mouse chromosome 7. Comparison of Gpx2 gene expression in three pairs of C57BL/6Ha and ICR/Ha mice which are respectively resistant and sensitive to dimethylhydrazine-induced colon cancer, we found a higher Gpx2 mRNA level in C57BL/6Ha colon than ICR/Ha colon. Interestingly, a lower level of GPX activity is found in the resistant strain of mice. Because GPX-1 has three times higher specific activity than GPX-GI, our data suggest that the decreased GPX activity may result from a higher level of Gpx2 gene expression in those cells co-express Gpx1 gene.

The Gpx1 gene encodes mitochondrial glutathione peroxidase in the mouse liver

Mitochondria have GPX and PHGPX activity. It has been an unsettled issue whether mitochondrial GPX is encoded by Gpx1. Unlike the Gpx4 gene which encodes PHGPX with alternative transcription and translation start sites determining the subcellular localization of PHGPX, the Gpx1 gene appears to have a single translation start site. Additionally, mitochondrial GPX has been shown to have different chromatographic and kinetic properties from the cytosolic GPX1. We studied mouse liver mitochondrial GPX activity in homozygous Gpx1-knockout mice. Mitochondria were enriched at the density of 1.10 g/ml in the Percoll gradients as shown by electron microscopy. The H2O2-reducing GPX activity in the highly enriched mitochondrial fraction of wild-type mouse liver is 2700 mU/mg which is about one-half of specific activity found in cytosol. There is less than 0.5% GPX activity in the cytosol and no GPX activity in the mitochondria of Gpx1-knockout mouse liver compared to the cytosol of wild-type mouse liver using H2O2 or cumene hydroperoxide as the substrate. The fact that the knockout mice express normal levels of plasma GPX as well as testis and liver PHGPX activity indicates that animals are not selenium-deficient. Based on these observations, we concluded that mitochondrial GPX is the product of the Gpx1 gene.

The mouse glutathione peroxidase Gpx2 gene maps to chromosome 12; its pseudogene Gpx2-ps maps to chromosome 7

The GPX2 gene codes for GSHPx-GI, a glutathione peroxidase whose mRNA is readily detectable in the gastrointestinal tract. Although GPX2 is a single gene in humans, there are two genes in the mouse genome with homology to GPX2. By analyzing a panel of mouse interspecies DNA from the Jackson Laboratory's backcross resource, we have chromosomally mapped these two genes. One was mapped to the central region of mouse chromosome 12 between D12Mit4 and D12Mit5, near fos and Tgfb3. This region is homologous to human 14q24.1, where human GPX2 has been mapped, and most likely represents the functional mouse Gpx2 gene. The other Gpx2-like gene was mapped to mouse chromosome 7 between Pcsk3 and Hbb. We have isolated the latter gene from a P1 phage library. Its pseudogene nature is revealed by the sequence analysis: (a) it is intronless; (b) it has a single nucleotide deletion in the coding region; and (c) it has a poly(A) tail at its 3'-untranslated region.

Polymorphism and chromosomal localization of the GI-form of human glutathione peroxidase (GPX2) on 14q24.1 by in situ hybridization

We have isolated a 3.3-kb DNA containing the two exons and a 2.6-kb intron of the human CPX2 gene. This gene encodes the intestinal isoenzyme of glutathione peroxidase, GSHPx-GI. Consistent signals were detected at 14q24.1 when this DNA was used as a probe for fluorescence in situ hybridization on metaphase chromosomes. Based on either single-stranded conformation polymorphism or sequencing analysis, two sites containing DNA polymorphism were found in the intron: the 5' end had a single A/T alteration, and the 3' end had a microsatellite of TC repeats.

The expression of an intestinal form of glutathione peroxidase (GSHPx-GI) in rat intestinal epithelium

We have previously identified and characterized GSHPx-GI, which is a cellular selenium-dependent glutathione peroxidase (GSHPx) distinct from the classic GSHPx-1 and phospholipid hydroperoxide glutathione peroxidase (PHGPX). We have determined the level of GSHPx-GI mRNA expression in the rat gastrointestinal tract from esophagus to colon. Although GSHPx-GI mRNA is readily detectable throughout the GI tract, the highest level is detected in the ileum and cecum. We have also determined the levels of GSHPx-GI mRNA expression and several antioxidant enzyme activities along the villus-to-crypt axis in the rat small intestine by cell fractionation. GSHPx-GI mRNA is present at a similar level in all of the epithelial fractions, whereas GSHPx-1 mRNA is detectable only in the remnant. This suggests that GSHPx-GI is the major cellular tetrameric GSHPx expressed in intestinal epithelium, and the expression of GSHPx-GI in the GI tract is not likely regulated differentially through maturation of epithelial cells. In terms of enzymatic activity, although we detected lower glutathione S-transferase activity in the crypt epithelium, there was a marginal increase of PHGPX activity, a twofold increase of GSHPx activity, and a three- to fivefold increase of catalase activity in the crypt relative to the distal villus. Thus, the crypt epithelial cells may be better protected from peroxidative damage.

Expression of selenium-dependent glutathione peroxidase in human breast tumor cell lines

In estrogen receptor (ER)-positive breast cancer cell lines, very low expression of glutathione peroxidase-1 (GPX-1) activity and hgpx1 mRNA has been observed. Such cell lines have been used as models in studies of resistance to redox cycling anticancer drugs. In particular, large increases in GPX-1 activity levels by expression of transfected GPX-1 cDNA have been shown to confer some resistance to such drugs. It has never been determined that such low GPX-1 expression is a common feature of breast cancer. Based on previous limited surveys of breast cancer cell lines, it has been suggested that there may be an inverse correlation between ER status and GPX-1 production. Here we report the results from a larger survey of breast cancer cell lines, including six recently isolated cell lines. A near absence of hgpx1 mRNA expression was observed in 3 of 13 ER-negative cell lines; 1 of 4 ER-positive cell lines had high production of GPX-1. Both observations weaken the proposed inverse correlation between ER status and GPX-1 production. We have evidence to suggest that one cell line, COH-BR-5 (ER-negative), lacked hgpx1 gene expression prior to culture. This is based on the finding of stable hgpx1 gene expression during serial culture of ER-negative breast cancer cell lines newly isolated from malignant effusion and absence of hgpx1 mRNA expression in COH-BR-5. Expression of hgpx2 mRNA (producing GPXGI, the GI tract GPX) was detected in several long and newly established, ER-negative breast cancer cell lines. Cell lines, COH-BR-5 and MDA-MB-175, expressed only hgpx2 mRNA. The hgpx2 mRNA was detected in COH-BR-5 and COH-BR-7 at low passage number, suggesting that hgpx2 gene expression occurs in breast cancer malignant effusion. Thus, studies of the role of GPX in redox drug resistance may account for changes in hgpx2 gene expression. Phospholipid hydroperoxide GPX activity was not found to be generally elevated above normal tissue levels in newly established breast cancer-derived cell lines.

Cloning and sequencing of the cDNA encoding a human testis phospholipid hydroperoxide glutathione peroxidase

A human cDNA that encodes a polypeptide that has 94% deduced amino-acid sequence identity to porcine phospholipid hydroperoxide glutathione peroxidase was cloned from a testis library. The sequence shows preservation of the UGA selenocysteine codon, putative active-site Trp and Glu residues and a Tyr residue that is phosphorylated in the porcine protein. The 3'-UTR shows some conservation of sequences implicated in the insertion of selenocysteine at an opal codon in human glutathione peroxidase-1.

Reactivity of plasma glutathione peroxidase with hydroperoxide substrates and glutathione

We studied enzyme kinetics parameters of plasma glutathione peroxidase (GSHPx-P) and the major cellular enzyme, GSHPx-1, for the substrates, H2O2, linoleic acid hydroperoxide (LinOOH), and glutathione (GSH). The major objectives were to determine whether the relatively slow GSHPx-P enzyme had a lower reactivity with hydroperoxides or with GSH and to identify favored hydroperoxide substrates. The rate constants describing the reactivity of human GSHPx-P and human GSHPx-1 with LinOOH and H2O2 are in the same range; GSHPx-P is more reactive with LinOOH and GSHPx-1 is more reactive with H2O2. GSHPx-P also has a low level of reducing activity toward cholesterol 7 alpha-OOH and no detectable activity with the 5 alpha-OOH isomer in contrast to phospholipid hydroperoxide glutathione peroxidase (PHGPx) which readily reduced both isomers. GSHPx-P catalytic activity toward phospholipid hydroperoxides is demonstrable in the absence of detergents, enhanced at low concentrations by deoxycholate, and strongly inhibited by Triton X-100 and incorporation into liposomes. These properties are the opposite of PHGPx. These results suggest that GSHPx-P largely lacks the membrane interfacial properties of PHGPx. GSHPx-P exhibits a smaller GSH rate constant than GSHPx-1. This property partially explains the slower turnover of GSHPx-P with several hydroperoxide substrates; the low reactivity with GSH is not consistent with efficient GSHPx function in the bulk plasma volume. GSHPx-P kinetic properties suggest that it would function best as a free fatty acid hydroperoxidase in GSH-rich microenvironments. Minimally, the secretion of reduced enzyme would permit it to scavenge free fatty acid hydroperoxides.

Expression, characterization, and tissue distribution of a new cellular selenium-dependent glutathione peroxidase, GSHPx-GI

We have characterized a new selenium-dependent glutathione peroxidase, GSHPx-GI, by expressing a GSHPx-GI cDNA isolated from human hepatoma HepG2 cells in human mammary carcinoma MCF-7 cells, which have virtually undetectable expression of either the classical cellular enzyme, GSHPx-1, or GSHPx-GI at the protein level. One of the G418-resistant clones, neo-D1, expresses the transfected GSHPx-GI cDNA. This is based on 1) the presence of an additional GSHPx-GI DNA restriction fragment detected by Southern analysis; 2) the presence of a 1.9-kilobase (kb) GSHPx-GI mRNA in addition to the 1.0-kb endogenous mRNA by Northern analysis; and 3) the appearance of a 22-kDa 75Se-labeled protein which is absent in parental MCF-7 cells revealed by SDS-polyacrylamide gel electrophoresis. GSHPx-GI expressed in neo-D1 is a tetrameric protein localized in cytosol. GSHPx-GI does not cross-react with antisera against human GSHPx-1 or human plasma glutathione peroxidase (GSHPx-P). Similar substrate specificities are found for GSHPx-1 and GSHPx-GI; they both catalyze the reduction of H2O2, tert-butyl hydroperoxide, cumene hydroperoxide, and linoleic acid hydroperoxide with glutathione, but not of phosphatidylcholine hydroperoxide. GSHPx-GI mRNA was readily detected in human liver and colon, and occasionally in human breast samples, but not other human tissues including kidney, heart, lung, placenta, or uterus. In rodent tissues, GSHPx-GI mRNA is only detected in the gastrointestinal tract, and not in other tissues including liver. In fact, GSHPx-GI appears to be the major glutathione-dependent peroxidase activity in rodent GI tract. This finding suggests that GSHPx-GI could play a major role in protecting mammals from the toxicity of ingested lipid hydroperoxides. In conclusion, we have demonstrated that GSHPx-GI is the fourth member in the selenium-dependent glutathione peroxidase family, in addition to GSHPx-1, GSHPx-P, and phospholipid hydroperoxide glutathione peroxidase (PHGPX).

Expression of plasma glutathione peroxidase in human liver in addition to kidney, heart, lung, and breast in humans and rodents

We analyzed the expression of plasma glutathione peroxidase (GSHPx-P) messenger RNA (mRNA) in mouse, rat, and human tissues, using a human GSHPx-P cDNA clone as the probe. Unlike the classical cellular glutathione peroxidase (GSHPx-1), GSHPx-P expression appears to be tissue-specific. In the mouse and rat, kidney expresses an mRNA at a high level detected with the human probe. A signal is also detected in mRNA isolated from mouse and rat heart, rat cardiac myocytes, mouse lung, epididymis, and the mammary gland of midpregnant mice. No signal is detected in mRNA isolated from mouse and rat liver, mouse brain, uterus, and testis. In human tissues, an mRNA hybridizing to GSHPx-P cDNA is present in liver, as well as kidney, heart, lung, breast, and placenta. We have shown that human kidney expresses a GSHPx-P mRNA, and not a GSHPx-P-like message, by isolating a cDNA clone from a human kidney library in lambda gt11. From the 412-nucleotide partial sequence of the kidney cDNA, which codes for the 40-170 amino acids of GSHPx-P including the TGA codon for selenocysteine, we found complete sequence identity of the kidney cDNA with GSHPx-P isolated from placenta. The expression of GSHPx-P mRNA in cell lines was also studied. There is some correlation of the expression of GSHPx-P in these cell lines with that in normal tissues. Cell lines that expressed GSHPx-P mRNA or protein included the human hepatocarcinoma HepG2, Hep3B cells, human kidney carcinoma A498 cells, and the human breast cancer SK-BR-3, T47D, MDA-MB-231, and AdrrMCF-7 cells. Cell lines that did not express GSHPx-P included human choriocarcinoma BeWo cells, human breast cancer MCF-7, ZR-75-1, and Hs578T cells, and mouse hepatoma Hepa-1 cells.

Characterization and partial amino acid sequence of human plasma glutathione peroxidase

Human plasma glutathione peroxidase was purified to homogeneity and partially sequenced. Overlapping peptide fragments from three endopeptidase digests permitted the determination of one sequence of 32 contiguous amino acids and one sequence of 23 contiguous amino acids. Five additional unique peptide sequences without obvious overlaps were obtained. The sequence of 32 amino acid residues aligns with positions 82-113 of human cytosolic glutathione peroxidase with nine mismatches without gaps or insertions. The sequence of 23 amino acid residues aligns with positions 157-178 with six mismatches and an insertion of one residue. Three additional peptide sequences with no obvious sequence homology to glutathione peroxidase can be aligned based on the sequence of a cDNA clone encoding plasma glutathione peroxidase that was isolated from a human placental library. The plasma enzyme is a homotetramer composed of 21-kDa subunits which cannot reduce phospholipid hydroperoxides. These results indicate that the plasma glutathione peroxidase is distinct from both the classical cytosolic enzyme and the monomeric phospholipid hydroperoxide glutathione peroxidase. Only a negligible amount of glutathione peroxidase activity was detected in bile, indicating that the liver exports plasma glutathione peroxidase exclusively to the circulation.

Phospholipid hydroperoxide glutathione peroxidase is the 18-kDa selenoprotein expressed in human tumor cell lines

Human tumor cell lines cultured in 75Se-containing media demonstrate four major 75Se-labeled cellular proteins (57, 22, 18, and 12 kDa) on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. Among these selenoproteins, an enzymatic activity is known only for the 22-kDa protein, since this protein has been identified as the monomer of glutathione peroxidase. However, all tested cell lines also contained a peroxidase activity with phospholipid hydroperoxides that is completely accounted for by the other selenoenzyme, phospholipid hydroperoxide glutathione peroxidase (PHGPX) (Ursini, F., Maiorino, M., and Gregolin, C. (1985) Biochim. Biophys. Acta 839, 62-70). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography of 75Se-labeled proteins separated by gel permeation chromatography supported the identification of PHGPX as the monomeric protein matching the 18 kDa band. This paper is the first report on the identification of PHGPX in human cells.

Modulation of glutathione peroxidase expression by selenium: effect on human MCF-7 breast cancer cell transfectants expressing a cellular glutathione peroxidase cDNA and doxorubicin-resistant MCF-7 cells

We have studied the effect of selenium on the expression of a cellular glutathione peroxidase, GSHPx-1, in transfected MCF-7 cells and in doxorubicin-resistant (Adrr) MCF-7 cells. A GSHPx-1 cDNA with a Rous Sarcoma virus promoter was transfected into a human mammary carcinoma cell line, MCF-7, which has very low endogenous cytosolic glutathione (GSH) peroxidase activity and no detectable message. The transfectant with the highest GSH peroxidase activity among the isolates, MCF-7H6, was characterized. Adrr MCF-7 cells, a subline of MCF-7 cells, also has elevated GSH peroxidase activity. GSH peroxidase expressed by MCF-7H6 and Adrr MCF-7 cells is similar to the endogenous GSHPx-1 based on molecular weight, immunoreactivity, and metabolic labeling with 75Se. MCF-7H6 and Adrr MCF-7 cells grown in Se-deficient media had 2.6 +/- 2.4 (mean +/- S.D.) and 4.2 +/- 3.6 units/mg protein of GSH peroxidase specific activity, respectively. Se supplementation increased GSH peroxidase activity in a concentration- and time-dependent fashion. Enzymatic activity reached a level of 164 +/- 62 in MCF-7H6 cells and 114 +/- 27 in Adrr MCF-7 cells within 5 days of growth in media supplemented with 30 nM Se. Northern analysis revealed that Se-deficient MCF-7H6 cells expressed 2.1 +/- 0.4-fold less GSHPx-1 mRNA than their Se-sufficient counterparts. Similarly, Se-deficient Adrr MCF-7 cells expressed 3.3 +/- 1.8-fold less GSHPx-1 mRNA than their Se-supplemented counterparts after the quantity of mRNA was normalized with beta-actin. These studies suggest that modulation of GSH peroxidase activity by Se in both MCF-7H6 transfectants expressing pRSV-GSHPx-1 and Adrr MCF-7 cells expressing endogenous GSHPx-1 occurs largely at the translational level, and to a lesser degree at the level of mRNA, possibly by stabilizing GSHPx-1 mRNA since the transfected cDNA in MCF-7H6 cells has only 5 nucleotides 5' to the AUG initiation codon.

Antioxidant and xenobiotic-metabolizing enzyme gene expression in doxorubicin-resistant MCF-7 breast cancer cells

We investigated the expression of the genes for several antioxidant and xenobiotic-detoxifying enzymes in the multidrug-resistant variant of the human breast cancer cell line MCF-7, MCF-7/Dox. MCF-7/Dox is greater than 500-fold resistant to doxorubicin by clonogenic assay. Enzyme activity determinations in the cytoplasmic compartment of MCF-7/Dox revealed a 25-fold increase in glutathione peroxidase level compared to the parent line (mean +/- SD, 10 +/- 2.8 versus 0.4 +/- 0.24 nmol/min/mg; P less than 0.005). The activity of the other major hydrogen peroxide-detoxifying enzyme, catalase, was diminished in MCF-7/Dox (2.0 +/- 0.4 versus 4.8 +/- 1.4 mumol/min/mg; P less than 0.025 compared to MCF-7). Superoxide dismutase activity did not differ between the two cell lines. The specific activity of the xenobiotic-detoxifying enzyme DT-diaphorase was 4-fold lower in MCF-7/Dox compared to MCF-7 (DT-diaphorase, 117 +/- 45 versus 509 +/- 123 nmol/min/mg; P less than 0.005). Daunorubicinol-producing carbonyl reductase activity was equal in the two lines. Northern blot analysis demonstrated a 0.9-kilobase band of glutathione peroxidase mRNA in MCF-7/Dox; no glutathione peroxidase mRNA was detected in MCF-7. A 2.4-kilobase catalase and 0.7- and 1.4-kilobase superoxide dismutase mRNAs were detectable in MCF-7/Dox and MCF-7. When normalized to 28S RNA, no difference in the mRNA levels of catalase and superoxide dismutase in MCF-7/Dox and MCF-7 could be determined. DT-diaphorase mRNAs of 1.4 and 2.7 kilobases were found in both MCF-7/Dox and MCF-7 cells. A 1.2-kilobase mRNA homologous to the putative carbonyl reductase cDNA was also easily detectable in both MCF-7 and MCF-7/Dox. The amount of mRNA for both xenobiotic-detoxifying enzymes was decreased 2- to 4-fold in the doxorubicin-resistant cells. Southern blot analysis of PstI- and MspI-restricted genomic DNA revealed no evidence for amplification or rearrangement of the glutathione peroxidase gene. These results indicate that, in addition to the previously described overexpression of anionic glutathione S-transferase in MCF-7/Dox cells, an augmented glutathione peroxidase mRNA level is the major alteration in antioxidant and xenobiotic-detoxifying enzyme expression that could contribute to doxorubicin insensitivity in these multidrug-resistant breast cancer cells.

A comparative study of ricin and diphtheria toxin-antibody-conjugate kinetics on protein synthesis inactivation

Kinetic data on toxin and antibody-toxin-conjugate inactivation of protein synthesis have been used to assess the variables which affect the transport of these toxins into the cytosol compartment. First-order inactivation rate constants of protein synthesis (ki) are compared under conditions of known receptor occupancy. The effect of inclusion of toxin B chains, both homologous and heterologous, in antibody-toxin conjugates is observed, and factors which affect toxin lag periods are studied. The results show that the inclusion of B chains in conjugates increases ki values 3-10-fold, but only if the B chain is homologous with the A chain. In spite of the augmentation of antibody-toxin-conjugate ki values by homologous toxin B chain, these ki values are only 1/20 those observed with unmodified toxins on sensitive cells. A further difference noted between toxins and antibody-toxin conjugates is the presence of a dose-dependent lag when toxins, but not antibody-toxin conjugates, effect sensitive cell types. This lag period for ricin can be shortened by alkalinizing the cell medium. The kinetic data can be fit by assuming a processing step interposed between the binding of ricin to surface receptors and the interaction of the A chain with ribosomes which is first-order in toxin concentration and pH-dependent. The time constant of this event is reflected in the dose-dependent lag period. It is proposed that antibody-toxin conjugates do not participate in this processing event and therefore fail to achieve the high entry levels exhibited by unmodified toxins.