Sonia R. Cerda

Influence of oxygen radical injury on DNA methylation

One of the most prevalent products of oxygen radical injury in DNA is 8-hydroxyguanosine. Cells must be able to withstand damage by oxygen radicals and possess specific repair mechanisms that correct this oxidative lesion. However, when these defenses are oversaturated, such as under conditions of high oxidative stress, or when repair is inefficient, the miscoding potential of this lesion can result in mutations in the mammalian genome. In addition to causing genetic changes, active oxygen species can lead to epigenetic alterations in DNA methylation, without changing the DNA base sequence. Such changes in DNA methylation patterns can strongly affect the regulation of expression of many genes. Although DNA methylation patterns have been found to be altered during carcinogenesis, little is known about the mechanism(s) that produce this loss of epigenetic controls of gene expression in tumors. Replacement of guanine with the oxygen radical adduct 8-hydroxyguanine profoundly alters methylation of adjacent cytosines, suggesting a role for oxidative injury in the formation of aberrant DNA methylation patterns during carcinogenesis. In this paper, we review both the genetic and epigenetic mechanisms of oxidative DNA damage and its association with the carcinogenic process, with special emphasis on the influence of free radical injury on DNA methylation.

Epidermal growth factor receptor signaling is up-regulated in human colonic aberrant crypt foci.

Aberrant crypt foci (ACF) are collections of abnormal colonic crypts with heterogeneous molecular and pathologic characteristics. Large and dysplastic ACF are putative precursors of colon cancer with neoplastic risk related to increased proliferation. In this study, we examined the role of epidermal growth factor receptor (EGFR) signaling in regulating ACF proliferation. Using magnification chromoendoscopy, we collected large ACF with endoscopic features of dysplasia and separately biopsied adjacent mucosa. Transcript levels were measured by real-time PCR, proteins were assessed by Western blotting, and levels were expressed as fold changes of adjacent mucosa. K-ras and B-Raf mutations were assessed by PCR and Ras activation by the ratio Ras-GTP / (Ras-GTP + Ras-GDP). At the RNA level, 38% of ACF were hyperproliferative, with proliferating cell nuclear antigen (PCNA) mRNA >/=2-fold of adjacent mucosa. Hyperproliferative ACF had significantly increased mRNA levels of EGFR (6.0 +/- 1.7-fold), transforming growth factor-alpha (14.4 +/- 5.0-fold), heparin-binding EGF-like growth factor (4.5 +/- 1.4-fold), cyclin D1 (4.6 +/- 0.7-fold), and cyclooxygenase-2 (COX-2; 9.3 +/- 4.2-fold; P < 0.05). At the protein level, 46% of ACF were hyperproliferative (PCNA, 3.2 +/- 1.2-fold). In hyperproliferative ACF, 44% possessed significant increases in four EGFR signaling components: EGFR (9.5 +/- 1.3-fold), phosphoactive ErbB2 (2.6 +/- 0.4-fold), phosphoactive extracellular signal-regulated kinase (3.7 +/- 1.1-fold), and cyclin D1 (3.4 +/- 0.8-fold; P < 0.05). Ras was activated in 46% of ACF (3.2 +/- 0.4-fold; P < 0.05), but K-ras mutations were present in only 7% of ACF. In contrast to COX-2 mRNA, the protein was not increased in hyperproliferative ACF. In summary, we have shown that ACF with up-regulated PCNA possess increased EGFR signaling components that likely contribute to the enhanced proliferative state of dysplastic-appearing ACF.

Epidermal Growth Factor Receptor Signaling Is Required for Microadenoma Formation in the Mouse Azoxymethane Model of Colonic Carcinogenesis

Colonic carcinogenesis involves the progressive dysregulation of homeostatic mechanisms that control growth. The epidermal growth factor (EGF) receptor (EGFR) regulates colonocyte growth and differentiation and is overexpressed in many human colon cancers. A requirement for EGFR in colonic premalignancy, however, has not been shown. In the current study, we used a specific EGFR antagonist, gefitinib, to investigate this role of the receptor in azoxymethane colonic premalignancy. The azoxymethane model shares many clinical, histologic, and molecular features of human colon cancer. Mice received azoxymethane i.p. (5 mg/kg/wk) or saline for 6 weeks. Animals were also gavaged with gefitinib (10 mg/kg body weight) or vehicle (DMSO) thrice weekly for 18 weeks, a dose schedule that inhibited normal receptor activation by exogenous EGF. Compared with control colonocytes [bromodeoxyuridine (BrdUrd), 2.2+/-1.2%], azoxymethane significantly increased proliferation (BrdUrd, 12.6+/-2.8%), whereas gefitinib inhibited this hyperproliferation (BrdUrd, 6.2+/-4.0%; <0.005). Azoxymethane significantly induced pro-transforming growth factor-alpha (6.4+/-1.3-fold) and increased phospho-(active) EGFR (5.9+/-1.1-fold), phospho-(active) ErbB2 (2.3+/-0.2-fold), and phospho-(active) extracellular signal-regulated kinase (3.3+/-0.4-fold) in premalignant colonocytes. Gefitinib inhibited activations of these kinases by >75% (P<0.05). Gefitinib also significantly reduced the number of large aberrant crypt foci and decreased the incidence of colonic microadenomas from 75% to 33% (P<0.05). Gefitinib concomitantly decreased cell cycle-regulating cyclin D1 and prostanoid biosynthetic enzyme cyclooxygenase-2 in microadenomas, suggesting that these regulators are key targets of EGFR in colonic carcinogenesis. These results show for the first time that EGFR signaling is required for early stages of colonic carcinogenesis. Our findings suggest, moreover, that inhibitors of EGFR might be useful in chemopreventive strategies in individuals at increased risk for colonic malignancies.

Epidermal Growth Factor Receptor Controls Flat Dysplastic Aberrant Crypt Foci Development and Colon Cancer Progression in the Rat Azoxymethane Model

Purpose: Colonic carcinogenesis deranges growth-regulating epidermal growth factor receptors (EGFR). We previously showed that EGFR signals were up-regulated in human aberrant crypt foci (ACF), putative colon cancer precursors. The azoxymethane model of colon cancer recapitulates many aspects of human colonic tumors. Recent studies indicate that flat dysplastic ACF with increased β-catenin are tumor precursors in this model. We asked, therefore, if EGFR signals are required for flat dysplastic ACF development and cancer progression. Experimental Design: Rats received azoxymethane or saline, and standard chow or chow supplemented with gefitinib, an EGFR inhibitor, for 44 weeks. EGFR signals were quantified in normal colon, flat ACF, and tumors by computerized analysis of immunostains and Western blots. K-ras mutations were assessed by PCR and mRNA for egfr ligands by quantitative real-time PCR. Results: EGFR inhibition with gefitinib decreased the incidence of flat dysplastic ACF from 66% to 36% and tumors from 71% to 22% (P < 0.05). This inhibitor also reduced the overexpressions of cyclin D1 and Cox-2 in flat ACF. Furthermore, in flat ACF, EGFR blockade decreased the up-regulation of c-Jun, FosB, phosphorylated active signal transducers and activators of transcription 3, and CCAAT/enhancer binding protein-β, potential regulators of cyclin D1 and Cox-2. In colonic tumors, EGFR blockade significantly decreased angiogenesis, proliferation, and progression while also increasing apoptosis (P < 0.05). Gefitinib also inhibited the activations of extracellular signal–regulated kinase, Src, and AKT pathways in tumors. Conclusions: We have shown for the first time that EGFR promotes the development of flat dysplastic ACF and the progression of malignant colonic tumors. Furthermore, we have mechanistically identified several transcription factors and their targets as EGFR effectors in colonic carcinogenesis.

Lithocholic acid down-regulation of NF-κB activity through vitamin D receptor in colonic cancer cells

Lithocholic acid (LCA), a secondary bile acid, is a vitamin D receptor (VDR) ligand. 1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), the hormonal form of vitamin D, is involved in the anti-inflammatory action through VDR. Therefore, we hypothesize that LCA acts like 1,25(OH)(2)D(3) to drive anti-inflammatory signals. In present study, we used human colonic cancer cells to assess the role of LCA in regulation of the pro-inflammatory NF-kappaB pathway. We found that LCA treatment increased VDR levels, mimicking the effect of 1,25(OH)(2)D(3). LCA pretreatment inhibited the IL-1beta-induced IkappaBalpha degradation and decreased the NF-kappaB p65 phosphorylation. We also measured the production of IL-8, a well-known NF-kappaB target gene, as a read-out of the biological effect of LCA expression on NF-kappaB pathway. LCA significantly decreased IL-8 secretion induced by IL-1beta. These LCA-induced effects were very similar to those of 1,25(OH)(2)D(3.) Thus, LCA recapitulated the effects of 1,25(OH)(2)D(3) on IL-1beta stimulated cells. Mouse embryonic fibroblast (MEF) cells lacking VDR have intrinsically high NF-kappaB activity. LCA pretreatment was not able to prevent TNFalpha-induced IkappaBalpha degradation in MEF VDR (-/-), whereas LCA stabilized IkappaBalpha in MEF VDR (+/-) cells. Collectively, our data indicated that LCA activated the VDR to block inflammatory signals in colon cells.

Protein kinase C delta inhibits Caco-2 cell proliferation by selective changes in cell cycle and cell death regulators

PKC-δ is a serine/threonine kinase that mediates diverse signal transduction pathways. We previously demonstrated that overexpression of PKC-δ slowed the G1 progression of Caco-2 colon cancer cells, accelerated apoptosis, and induced cellular differentiation. In this study, we further characterized the PKC-δ dependent signaling pathways involved in these tumor suppressor actions in Caco-2 cells overexpressing PKC-δ using a Zn2+ inducible expression vector. Consistent with a G1 arrest, increased expression of PKC-δ caused rapid and significant downregulation of cyclin D1 and cyclin E proteins (50% decreases, P<0.05), while mRNA levels remained unchanged. The PKC agonist, phorbol 12-myristate 13-acetate (TPA, 100 nM, 4 h), induced two-fold higher protein and mRNA levels of p21Waf1, a cyclin-dependent kinase (cdk) inhibitor in PKC-δ transfectants compared with empty vector (EV) transfected cells, whereas the PKC-δ specific inhibitor rottlerin (3 μM) or knockdown of this isoenzyme with specific siRNA oligonucleotides blocked p21Waf1 expression. Concomitantly, compared to EV control cells, PKC-δ upregulation decreased cyclin D1 and cyclin E proteins co-immunoprecipitating with cdk6 and cdk2, respectively. In addition, overexpression of PKC-δ increased binding of cdk inhibitor p27Kip1 to cdk4. These alterations in cyclin-cdks and their inhibitors are predicted to decrease G1 cyclin kinase activity. As an independent confirmation of the direct role PKC-δ plays in cell growth and cell cycle regulation, we knocked down PKC-δ using specific siRNA oligonucleotides. PKC-δ specific siRNA oligonucleotides, but not irrelevant control oligonucleotides, inhibited PKC-δ protein by more than 80% in Caco-2 cells. Moreover, PKC-δ knockdown enhanced cell proliferation (∼1.4-2-fold, P<0.05) and concomitantly increased cyclin D1 and cyclin E expression (∼1.7-fold, P<0.05). This was a specific effect, as nontargeted PKC-ζ was not changed by PKC-δ siRNA oligonucleotides. Consistent with accelerated apoptosis in PKC-δ transfectants, compared to EV cells, PKC-δ upregulation increased proapoptotic regulator Bax two-fold at mRNA and protein levels, while antiapoptotic Bcl-2 protein was decreased by 50% at a post-transcriptional level. PKC-δ specific siRNA oligonucleotides inhibited Bax protein expression by more than 50%, indicating that PKC-δ regulates apoptosis through Bax. Taken together, these results elucidate two critical mechanisms regulated by PKC-δ that inhibit cell cycle progression and enhance apoptosis in colon cancer cells. We postulate these antiproliferative pathways mediate an important tumor suppressor function for PKC-δ in colonic carcinogenesis.

Altered expression of the DNA repair protein, N‐methylpurine‐DNA glycosylase (MPG), in breast cancer

We examined expression of N‐methylpurine‐DNA glycosylase (MPG), a DNA repair enzyme that removes N‐alkylpurine damage, in normal, malignant, and immortalized breast epithelial cells, and breast cancer cell lines (MDA‐MB‐231, MCF7, T47D). Northern analysis showed increased expression in cancer versus normal breast epithelial cells (2–24‐fold). Southern blots revealed no gene amplification or polymorphisms. Immunofluorescence, immunohistochemistry, and Western blot analysis demonstrated increased MPG protein expression in the tumor cells that correlated with elevated glycosylase activity. Since MPG overexpression has been shown to be paradoxically associated with increased susceptibility to DNA damage, up‐regulation of this gene may suggest a functional role in breast carcinogenesis.

Ursodeoxycholic Acid Suppresses Cox-2 Expression in Colon Cancer: Roles of Ras, p38, and CCAAT/Enhancer-Binding Protein

In the azoxymethane (AOM) model of experimental rodent colon cancer, cholic acid and its colonic metabolite deoxycholic acid (DCA) strongly promote tumorigenesis. In contrast, we showed that ursodeoxycholic acid (UDCA), a low abundance bile acid, inhibited AOM tumorigenesis. Dietary UDCA also blocked the development of tumors with activated Ras and suppressed cyclooxygenase-2 (Cox-2) upregulation in AOM tumors. In this study, we compared the effect of dietary supplementation with tumor-promoting cholic acid to chemopreventive UDCA on Cox-2 expression in AOM tumors. Cholic acid enhanced Cox-2 upregulation in AOM tumors, whereas UDCA inhibited this increase and concomitantly decreased CCAAT/enhancer binding protein β (C/EBPβ), a transcriptional regulator of Cox-2. In HCA-7 colon cancer cells, DCA activated Ras and increased C/EBPβ and Cox-2 by a mechanism requiring the mitogen-activated protein kinase p38. UDCA inhibited DCA-induced p38 activation and decreased C/EBPβ and Cox-2 upregulation. Using transient transfections, UDCA inhibited Cox-2 promoter and C/EBP reporter activation by DCA. Transfection with dominant-negative 17N-Ras abolished DCA-induced p38 activation and C/EBPβ and Cox-2 upregulation. Taken together, these studies have identified a transcriptional pathway regulating Cox-2 expression involving Ras, p38, and C/EBPβ that is inhibited by UDCA. These signal transducers are novel targets of UDCAs chemopreventive actions.

Protein kinase-ζ inhibits collagen I-dependent and anchorage-independent growth and enhances apoptosis of human caco-2 cells

Colonic carcinogenesis is accompanied by abnormalities in multiple signal transduction components, including alterations in protein kinase C (PKC). The expression level of PKC-ζ, an atypical PKC isoform, increases from the crypt base to the luminal surface and parallels crypt cell differentiation in normal colon. In prior studies in the azoxymethane model of colon cancer, we showed that PKC-ζ was down-regulated in rat colonic tumors. In this study, we showed that PKC-ζ is expressed predominantly in colonic epithelial and not stromal cells, and loss of PKC-ζ occurs as early as the adenoma stage in human colonic carcinogenesis. To assess the regulation of growth and differentiation by PKC-ζ, we altered this isoform in human Caco-2 colon cancer cells using stable constitutive or inducible expression vectors, specific peptide inhibitors or small interfering RNA. In ecdysone-regulated transfectants grown on collagen I, ponasterone A significantly induced PKC-ζ expression to 135% of empty vector cells, but did not alter nontargeted PKC isoforms. This up-regulation was accompanied by a 2-fold increase in basal and 4-fold increase in insulin-stimulated PKC-ζ biochemical activity. Furthermore, PKC-ζ up-regulation caused >50% inhibition of cell proliferation on collagen I (P < 0.05). Increased PKC-ζ also significantly enhanced Caco-2 cell differentiation, nearly doubling alkaline phosphatase activity, while inducing a 3-fold increase in the rate of apoptosis (P < 0.05). In contrast, knockdown of this isoform by small interfering RNA or kinase inhibition by myristoylated pseudosubstrate significantly and dose-dependently increased Caco-2 cell growth on collagen I. In transformation assays, constitutively up-regulated wild-type PKC-ζ significantly inhibited Caco-2 cell growth in soft agar, whereas a kinase-dead mutant caused a 3-fold increase in soft agar growth (P < 0.05). Taken together, these studies indicate that PKC-ζ inhibits colon cancer cell growth and enhances differentiation and apoptosis, while inhibiting the transformed phenotype of these cells. The observed down-regulation of this growth-suppressing PKC isoform in colonic carcinogenesis would be predicted to contribute to tumorigenesis. (Mol Cancer Res 2006;4(9):683–94)

Epidermal Growth Factor Receptor Is Required for Colonic Tumor Promotion by Dietary Fat in the Azoxymethane/Dextran Sulfate Sodium Model: Roles of Transforming Growth Factor-α and PTGS2

Purpose: Colon cancer is a major cause of cancer deaths. Dietary factors contribute substantially to the risk of this malignancy. Western-style diets promote development of azoxymethane-induced colon cancer. Although we showed that epidermal growth factor receptors (EGFR) controlled azoxymethane tumorigenesis in standard fat conditions, the role of EGFR in tumor promotion by high dietary fat has not been examined. Experimental Design: A/J C57BL6/J mice with wild-type Egfr (Egfrwt) or loss-of-function waved-2 Egfr (Egfrwa2) received azoxymethane followed by standard (5 fat) or western-style (20 fat) diet. As F1 mice were resistant to azoxymethane, we treated mice with azoxymethane followed by one cycle of inflammation-inducing dextran sulfate sodium to induce tumorigenesis. Mice were sacrificed 12 weeks after dextran sulfate sodium. Tumors were graded for histology and assessed for EGFR ligands and proto-oncogenes by immunostaining, Western blotting, and real-time PCR. Results:Egfrwt mice gained significantly more weight and had exaggerated insulin resistance compared with Egfrwa2 mice on high-fat diet. Dietary fat promoted tumor incidence (71.2 versus 36.7; P < 0.05) and cancer incidence (43.9 versus 16.7; P < 0.05) only in Egfrwt mice. The lipid-rich diet also significantly increased tumor and cancer multiplicity only in Egfrwt mice. In tumors, dietary fat and Egfrwt upregulated transforming growth factor-, amphiregulin, CTNNB1, MYC, and CCND1, whereas PTGS2 was only increased in Egfrwt mice and further upregulated by dietary fat. Notably, dietary fat increased transforming growth factor- in normal colon. Conclusions: EGFR is required for dietary fat-induced weight gain and tumor promotion. EGFR-dependent increases in receptor ligands and PTGS2 likely drive diet-related tumor promotion. (Clin Cancer Res 2009;15(22):67809)