Expression of G protein alpha subunits in normal rat colon and in azoxymethane-induced colonic neoplasms

Neutrophil Extracellular Traps in Colorectal Cancer Progression and Metastasis

Neutrophils form sticky web-like structures known as neutrophil extracellular traps (NETs) as part of innate immune response. NETs are decondensed extracellular chromatin filaments comprising nuclear and cytoplasmic proteins. NETs have been implicated in many gastrointestinal diseases including colorectal cancer (CRC). However, the regulatory mechanisms of NET formation and potential pharmacological inhibitors in the context of CRC have not been thoroughly discussed. In this review, we intend to highlight roles of NETs in CRC progression and metastasis as well as the potential of targeting NETs during colon cancer therapy.

Characterization of the impact of glutaredoxin-2 (GRX2) deficiency on superoxide/hydrogen peroxide release from cardiac and liver mitochondria

Mitochondria are critical sources of hydrogen peroxide (H2O2), an important secondary messenger in mammalian cells. Recent work has shown that O2•-/H2O2 emission from individual sites of production in mitochondria is regulated by protein S-glutathionylation. Here, we conducted the first examination of O2•-/H2O2 release rates from cardiac and liver mitochondria isolated from mice deficient for glutaredoxin-2 (GRX2), a matrix-associated thiol oxidoreductase that facilitates the S-glutathionylation and deglutathionylation of proteins. Liver mitochondria isolated from mice heterozygous (GRX2+/-) and homozygous (GRX2-/-) for glutaredoxin-2 displayed a significant decrease in O2•-/H2O2 release when oxidizing pyruvate or 2-oxoglutarate. The genetic deletion of the Grx2 gene was associated with increased protein expression of pyruvate dehydrogenase (PDH) but not 2-oxoglutarate dehydrogenase (OGDH). By contrast, O2•-/H2O2 production was augmented in cardiac mitochondria from GRX2+/- and GRX2-/- mice metabolizing pyruvate or 2-oxoglutarate which was associated with decreased PDH and OGDH protein levels. ROS production was augmented in liver and cardiac mitochondria metabolizing succinate. Inhibitor studies revealed that OGDH and Complex III served as high capacity ROS release sites in liver mitochondria. By contrast, Complex I and Complex III were found to be the chief O2•-/H2O2 emitters in cardiac mitochondria. These findings identify an essential role for GRX2 in regulating O2•-/H2O2 release from mitochondria in liver and cardiac tissue. Our results demonstrate that the GRX2-mediated regulation of O2•-/H2O2 release through the S-glutathionylation of mitochondrial proteins may play an integral role in controlling cellular ROS signaling.

Probiotic Pediococcus pentosaceus strain GS4 alleviates azoxymethane-induced toxicity in mice

Probiotic treatment has been gaining attention due to its remarkable effects in alleviating toxicity and carcinogenesis. The novel strain Pediococcus pentosaceus GS4 has been reported for probiotic, survivability in simulated gastrointestinal fluid, and antioxidative and biohydrogenation properties. Therefore, we hypothesize that this specific strain might be able to assuage the effect of azoxymethane (AOM)-induced toxicity in mice. Twenty-eight Swiss albino mice were divided into 4 groups and were studied for 32 weeks. Azoxymethane (10 mg/kg body weight) was administered intraperitoneally twice (0th and 14th days), and probiotic GS4 (1.1 × 10(9) colony-forming unit/mL) was given orally for the respective groups. Mice who served as the normal control received only normal saline. GS4-intervened AOM-induced mice showed marked improvement at the histopathologic level, in the liver and kidney. Moreover, probiotic GS4 intervention in AOM-induced mice exhibited a significant reduction in the liver function biomarker when compared with the AOM-induced mice. Probiotic GS4 intervention reduced the intestinal structural deformities as evident from the elevated brush border membrane-associated disaccharidases (sucrase, lactase) and intestinal alkaline phosphatase activities, which were found disrupted by AOM intoxication. Fecal bacterial load was found to be reduced in AOM-induced mice which were subsequently replenished by the probiotic GS4 intervention as apparent from the enhanced fecal bacterial load. There were no adverse effects observed in the probiotic control group. Conclusively, novel probiotic strain GS4 exhibited safe and beneficial effects against the toxicity threats posed by AOM. Thus, GS4 could be considered as a potential food supplement/additive for therapeutic purposes in gastrointestinal disorders related to inflammation and cancer.

G-protein alpha(olf) subunit promotes cellular invasion, survival, and neuroendocrine differentiation in digestive and urogenital epithelial cells

The heterotrimeric G-protein subunits Galpha and Gbetagamma are involved in cellular transformation and tumor development. Here, we report the expression of Galpha(olf) in human digestive and urogenital epithelial cells using RT-PCR and Western blot. When the constitutively activated form of Galpha(olf)Q214L (AGalpha(olf)) was stably transfected in canine kidney MDCKts.src and human colonic HCT-8/S11 epithelial cells, it induced cellular invasion in collagen gels. AGalpha(olf)-mediated invasion was abrogated by agonists of platelet activating factor receptors (PAF-R) and protease-activated receptors -1 (PAR-1), pharmacological inhibitors of PI3'-Kinase (wortmannin), protein kinase C (Gö6976 and GF109203X), Rho GTPase (C3T exoenzyme), but was independent of protein kinase A. Accordingly, the invasive phenotype induced by AGalpha(olf) in HCT-8/S11 cells was reversed by the RhoA antagonist RhoD (G26V). Although AGalpha(olf) protected MDCKts.src cells against serum starvation-mediated apoptosis via a Rho-independent pathway, both AGalpha(olf) and Rho inhibition by C3T induced neuroendocrine-like differentiation linked to extensive neurite outgrowth and parathyroid hormone-related protein expression in human prostatic LNCaP-AGalpha(olf) cells. Since prostate tumors with a larger neuroendocrine cell population display increased invasiveness, persistent activation of the G-protein alpha(olf) may exert convergent adverse effects on cellular invasion and survival in solid tumors during the neoplastic progression towards metastasis. doi:10.1038/sj.onc.1205498

Suppression of cellular invasion by activated G-protein subunits Galphao, Galphai1, Galphai2, and Galphai3 and sequestration of Gbetagamma

It was shown previously that platelet-activating factor receptors (PAF-Rs) inhibit invasiveness of colonic and kidney epithelial cells induced by the src and Met oncogenes via a pertussis toxin-sensitive mechanism. Therefore, Madin-Darby canine kidney (MDCKts.src) cells were stably transfected with constitutively activated forms of Galphao, Galphai1, Galphai2, Galphai3 (AGalphao/i), two Gbetagamma sequestering proteins [C-terminal end of beta-adrenergic receptor kinase (ct-betaARK) and the Galphat subunit of retinal G-protein transducin], and Gbeta1-Ggamma2 subunits alone or in combination. Cellular invasion induced by src, Met, and leptin was abrogated by the AGalphao/i, ct-betaARK, and Galphat-positive clones, but was induced by coexpression of Gbeta1gamma2. In contrast, invasion stimulated by the trefoil factors (TFFs) pS2 and intestinal trefoil factor in MDCKts.src cells or human colonic epithelial cells PCmsrc and HCT8/S11 was insensitive to PAF, AGalphao, AGalphai1, and AGalphai2, but was abolished by AGalphai3 and the protease-activated receptor-1 (PAR-1) agonist thrombin receptor-activating peptide. Depletion of free Gbetagamma heterodimers by ct-betaARK resulted in a remarkable decrease of cellular adhesion and spreading on collagen matrix. Our data demonstrate the following: 1) PAF-Rs impair cellular invasion induced by src, Met, and leptin via the activation of Galphao and Galphai1 to -3; 2) invasion induced by TFFs is selectively inhibited by PAR-1 receptors and Galphai3 activation; and 3) Gbetagamma dimers are required as positive effectors of invasion pathways induced by oncogenes and epigenetic factors. Thus, redistribution of Galphao/Galphai and Gbeta/gamma heterotrimeric G-proteins by PAF-R and PAR-1 exert differential functions on positive and negative signaling pathways involved in cellular invasion and may serve as potential targets for anticancer therapy.

p16(INK4a) expression begins early in human colon neoplasia and correlates inversely with markers of cell proliferation

BACKGROUND & AIMS

p16(INK4a) is a cell cycle inhibitor and a major tumor-suppressor protein, but the regulation of p16(INK4a) is poorly understood and the physiologic settings in which it exerts its antiproliferative effects are unknown. A role for p16(INK4a) in intestinal neoplasia is suggested by the observation that the promoter region is methylated in a subset of human colon tumors. We examined the expression of the protein in specimens representing the full spectrum of neoplastic progression in the human colon and determined whether expressing cells showed evidence of cell cycle inhibition.

METHODS

We studied p16(INK4a) expression by immunoprecipitation, immunoblotting, reverse-transcription polymerase chain reaction (RT-PCR), immunohistochemistry, and immunofluorescence in matched normal and neoplastic colonic tissue from 70 patients.

RESULTS

p16(INK4a) expression was very low in normal mucosa, with staining observed in rare epithelial cells at the base of crypts. A distinctly higher expression was found in 4 of 7 aberrant crypt foci, 32 of 36 adenomas, 18 of 28 primary carcinomas, and 5 of 5 metastatic carcinomas. Within each neoplasm p16(INK4a) staining was heterogeneous, with higher expression commonly seen in areas bordering normal tissue. p16(INK4a) staining correlated inversely with that of Ki67, cyclin A, and the retinoblastoma protein, suggesting that cell cycle progression was inhibited.

CONCLUSIONS

These results suggest that p16(INK4a) expression begins in the earliest detectable stages of neoplastic progression in the human colon and exerts a continuous, piecemeal constraint on tumor growth.

Inhibition of O(6)-methylguanine-DNA methyltransferase increases azoxymethane-induced colonic tumors in rats

Azoxymethane (AOM) causes O(6)-methylguanine adduct formation which leads to G-->A transitions. Their repair is carried out by O(6)-methylguanine-DNA methyltransferase (MGMT). To evaluate the importance of this repair event in AOM-induced carcinogenesis, we examined the effect of O(6)-benzylguanine (BG), a potent inhibitor of MGMT, on colonic tumor development. Rats were treated weekly for 2 weeks at 0 and 24 h with BG (60 mg/kg body wt i.p.) or vehicle (40% polyethylene glycol, PEG-400), followed 2 h after the first dose of BG with AOM (15 mg/kg body wt) or vehicle (saline) i.p. Rats were killed 35 weeks later and tumors harvested and DNA extracted. In the AOM-treated groups, BG caused a significant increase in tumor incidence with tumors in 65.9%, versus 30.8% in the AOM/PEG-treated group (P < 0.05). In the BG/AOM group there was also a significant increase in tumor multiplicity, with 2.3 tumors/tumor-bearing rat, versus 1.6 tumors/tumor- bearing rat in the AOM/PEG group (P < 0.05). Since O(6)-methylguanine adducts can cause activating mutations in the K-ras and beta-catenin genes, we examined the effects of BG on these mutations. In the BG group there were seven mutations in codon 12 or 13 of exon 1 of the K-ras gene in 51 tumors examined, compared with no K-ras mutations in 17 tumors analyzed in the AOM/PEG group (P = 0.12). In the BG/AOM group there were 10 mutations in exon 3 of the beta-catenin gene among 48 tumors evaluated, compared with six mutations in 16 tumors analyzed in the PEG/AOM group (P = 0.16). In summary, MGMT inhibition increases AOM-induced colonic tumor incidence and multiplicity in rats.