Reba Mustafi

Inhibition of protein kinases sensitizes human tumor cells to ionizing radiation.

Protein kinase C (PKC) is activated rapidly and transiently following ionizing radiation exposure and is postulated to activate downstream nuclear signal transducers. Inhibition of this enzyme attenuates radiation-mediated expression of the c-jun and Egr-1/zif-268 genes which are associated with cellular proliferation. To investigate further the role of PKC in the radiation response of human tumor cell lines, two human squamous cell carcinoma cell lines, SQ-20B and JSQ-3, were exposed to graded doses of X rays in the presence of staurosporine, sangivamycin, or H7, all PKC inhibitors. The protein kinase inhibitors staurosporine and sangivamycin produced dose-dependent cytotoxicity in cells of the SQ-20B and JSQ-3 cell lines while H7 did not. Nontoxic concentrations of sangivamycin (10 nM) and staurosporine (1 nM), added to cell cultures from 1 to 7 h before X irradiation, enhanced cell killing by radiation in both cell lines. Maximal sensitization of killing occurred when inhibitors were added 1 h prior to irradiation. The enhanced radiation-induced cell killing was not due to any measurable alteration in the induction or rejoining of DNA single- or double-strand breaks as determined by alkaline and neutral filter elution assays. These data suggest that protein kinase activity is important for cell survival following radiation exposure, although the specific role of PKC in radiation responses is unknown.

miR‐143 and miR‐145 are downregulated in ulcerative colitis: Putative regulators of inflammation and protooncogenes

Background: miR‐143 and miR‐145 are believed to function as colon cancer tumor suppressors, as they inhibit colon cancer cell growth and are downregulated in sporadic colonic tumors. We speculated that miR‐143 and miR‐145 might also be downregulated and contribute to malignant transformation of colonic epithelium in longstanding ulcerative colitis (UC). Methods: Biopsies were obtained 20 cm proximal to the anus from individuals with quiescent UC and from normal controls. RNA and proteins were extracted and measured. miR‐143 and miR‐145 were quantified by real‐time polymerase chain reaction (PCR) and miR‐145 was also assessed by in situ hybridization. Putative targets of these miRNAs, K‐RAS, API5, MEK‐2 (miR‐143), and IRS‐1 (miR‐145) were determined by western blotting. To assess the effects of miR‐143 and miR‐145 on these predicted targets, HCT116 and HCA‐7 colorectal cancer cells were transfected with miR‐143 and miR‐145 and expression levels of these proteins were measured. Results: In UC, miR‐143 and miR‐145 were significantly downregulated 8.3‐fold (3.4–20.1) (P < 0.0001) and 4.3‐fold (2.3–7.8) (P < 0.0001), respectively, compared to normal colon. In contrast, IRS‐1, K‐RAS, API5, and MEK‐2 were upregulated in UC, consistent with their assignments as targets of these miRNAs. Furthermore, transfected miR‐143 and miR‐145 significantly downregulated these proteins in HCT116 or HCA‐7 cells. Conclusions: Compared to normal colonic mucosa, in chronic UC miR‐143 and miR‐145 were significantly downregulated and their predicted targets, IRS‐1, K‐RAS, API5, and MEK‐2 were upregulated. We postulate that loss of these tumor suppressor miRNAs predispose to chronic inflammation and neoplastic progression in IBD. (Inflamm Bowel Dis 2011;)

EGFR Signals Downregulate Tumor Suppressors miR-143 and miR-145 in Western Diet–Promoted Murine Colon Cancer: Role of G1 Regulators

Epidermal growth factor receptors (EGFR) contribute to colonic tumorigenesis in experimental models of colon cancer. We previously showed that EGFR was also required for colonic tumor promotion by Western diet. The goal of this study was to identify EGFR-regulated microRNAs that contribute to diet-promoted colonic tumorigenesis. Murine colonic tumors from Egfrwt and hypomorphic Egfrwa2 mice were screened using micro RNA (miRNA) arrays and miR-143 and miR-145 changes confirmed by Northern, real-time PCR, and in situ analysis. Rodent and human sporadic and ulcerative colitis (UC)-associated colon cancers were examined for miR-143 and miR-145. Effects of EGFR on miR-143 and miR-145 expression were assessed in murine and human colonic cells and their putative targets examined in vitro and in vivo. miR-143 and miR-145 were readily detected in normal colonocytes and comparable in Egfrwt and Egfrwa2 mice. These miRNAs were downregulated in azoxymethane and inflammation-associated colonic tumors from Egfrwt mice but upregulated in Egfrwa2 tumors. They were also reduced in human sporadic and UC colon cancers. EGFR signals suppressed miR-143 and miR-145 in human and murine colonic cells. Transfected miR-143 and miR-145 inhibited HCT116 cell growth in vitro and in vivo and downregulated G1 regulators, K-Ras, MYC, CCND2, cdk6, and E2F3, putative or established targets of these miRNAs. miRNA targets Ras and MYC were increased in colonic tumors from Egfrwt but not Egfrwa2 mice fed a Western diet. EGFR suppresses miR-143 and miR-145 in murine models of colon cancer. Furthermore, Western diet unmasks the tumor suppressor roles of these EGFR-regulated miRNAs. Mol Cancer Res; 9(7); 960–75. ©2011 AACR.

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.

radiation-induced DNA double-strand break frequencies in human squamous cell carcinoma cell lines of different radiation sensitivities

DNA neutral (pH 9.6) filter elution was used to measure radiation-induced DNA double-strand break (dsb) frequencies in eight human squamous cell carcinoma cell lines with radiosensitivities (D0) ranging from 1.07 to 2.66 Gy and D values ranging from 1.46 to 4.08 Gy. The elution profiles of unirradiated samples from more radiosensitive cell lines were all steeper in slope than the profiles from resistant cells. The shapes of the dsb induction curves were curvilinear and there was some variability from cell line to cell line in the dose-response for the induction of DNA dsb after exposures to 5-100 Gy 60Co gamma-rays. There was no relation between the shapes of the survival curves and the shapes of the dose-responses for the induction of DNA dsb. At low doses (5-25 Gy), three out of four of the more sensitive cell lines (D less than 2.5 Gy) had larger initial break frequencies than the more resistant lines (D greater than 3.0 Gy). Although the low-dose (5-25 Gy) elution results were variable, they do suggest that DNA neutral elution will detect differences between sensitive and resistant tumour cells in initial DNA dsb frequencies.

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.

Inactivation of the vitamin D receptor in APCmin/+ mice reveals a critical role for the vitamin D receptor in intestinal tumor growth

Emerging evidence supports an inhibitory role for vitamin D in colorectal carcinogenesis; however, the mechanism remains unclear. The adenomatous polyposis coli (APC)/β‐catenin pathway plays a critical role in colorectal carcinogenesis. The purpose of our study is to explore the interactions of vitamin D and APC/β‐catenin pathways in intestinal tumor development. APCmin/+ mice with genetic inactivation of the vitamin D receptor (VDR) were generated through breeding. Intestinal tumorigenesis was compared between APCmin/+ and APCmin/+VDR−/− mice at different ages. No differences were seen in the number of small intestinal and colonic tumors between APCmin/+ and APCmin/+VDR−/− mice aged 3, 4, 6 and 7 months. The size of the tumors, however, was significantly increased in APCmin/+VDR−/− mice in all age groups. Immunostaining showed significant increases in β‐catenin, cyclin D1, phosphorylated Stat‐3 and MSH‐2 levels and decreases in Stat‐1 in APCmin/+VDR−/− tumors compared to APCmin/+ tumors. These observations suggest that VDR signaling inhibits tumor growth rather than tumor initiation in the intestine. Thus, the increased tumor burden in APCmin/+VDR−/− mice is likely due to the loss of the growth‐inhibiting effect of VDR. This study provides strong evidence for the in vivo relevance of the interaction demonstrated in vitro between the vitamin D and β‐catenin signaling pathways in intestinal tumorigenesis.