The adenomatous polyposis coli tumor suppressor gene regulates expression of cyclooxygenase-2 by a mechanism that involves retinoic acid

Diverse actions of retinoid receptors in cancer prevention and treatment

Retinoids (retinol [vitamin A] and its biologically active metabolites) are essential signaling molecules that control various developmental pathways and influence the proliferation and differentiation of a variety of cell types. The physiological actions of retinoids are mediated primarily by the retinoic acid receptors alpha, beta, and gamma (RARs) and rexinoid receptors alpha, beta, and gamma. Although mutations in RARalpha, via the PML-RARalpha fusion proteins, result in acute promyelocytic leukemia, RARs have generally not been reported to be mutated or part of fusion proteins in carcinomas. However, the retinoid signaling pathway is often compromised in carcinomas. Altered retinol metabolism, including low levels of lecithin:retinol acyl trasferase and retinaldehyde dehydrogenase 2, and higher levels of CYP26A1, has been observed in various tumors. RARbeta(2) expression is also reduced or is absent in many types of cancer. A greater understanding of the molecular mechanisms by which retinoids induce cell differentiation, and in particular stem cell differentiation, is required in order to solve the issue of retinoid resistance in tumors, and thereby to utilize RA and synthetic retinoids more effectively in combination therapies for human cancer.

Zebrafish: an emerging model system for human disease and drug discovery

In vivo studies represent an essential step in drug development and currently rely largely on mice, yet limitations of mammalian models motivated the search for complementary vertebrate model systems. This review focuses on zebrafish, Danio rerio, as a facile model system to study human disease and drug responses. Zebrafish are particularly suited for this purpose because they represent a vertebrate species, their genome is sequenced, and a large number of synchronously developing, transparent embryos can be produced. Zebrafish embryos are permeable to drugs and can easily be manipulated using well-established genetic and molecular approaches. Here, we summarize recent work on drug discovery and toxicity in zebrafish embryos. In addition, we provide a synopsis of current efforts to establish disease models in zebrafish focusing on neoplasia. The results of these studies highlight the potential of zebrafish as a viable addition to established animal models by offering medium and, potentially, high throughput capabilities.

Cyclo-oxygenase-2 over-expression is associated with human esophageal squamous cell carcinoma

Cyclo-oxygenase (COX)-2 is not usually detectable in normal tissues but is induced in inflammation and carcinogenesis. The level of COX-2 is elevated in cancer tissues of the colon, bladder, and skin. In the esophagus, squamous cell carcinoma and adenocarcinoma are known to express COX-2. The purpose of this study was to clarify the association of COX-2 expression with clinicopathological factors of squamous cell carcinoma. The immunohistochemical expression of COX-2 was examined in 48 surgical specimens of esophageal squamous cell carcinoma. Although COX-2 over-expression was more frequently observed in tumors invading the submucosa (T1b, 76.4%), muscularis propria (T2, 57.1%), adventitia, or adjacent organs (T3 approximately 4, 83.3%), even 33.3% of mucosal cancers, such as T1a, showed COX-2 over-expression. COX-2 over-expression was present in 82.3% of lymph node-negative patients but in only 54.8% of lymph node positive patients. There was no difference in COX-2 over-expression between the earlier stages (0 and I, 60%) and more advanced stages (II approximately IV, 69.6%). COX-2 over-expression did not correlate with survival during 3 years of follow-up. These findings suggest that COX-2 is associated with the phenotype of the esophageal squamous cell carcinoma cells, including superficial cancer cells, and may be related to tumor growth in esophageal squamous cell carcinoma.

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.

Adenomatous polyposis coli control of C-terminal binding protein-1 stability regulates expression of intestinal retinol dehydrogenases

Mutations in the human adenomatous polyposis coli (APC) gene are thought to initiate colorectal tumorigenesis. The tumor suppressor function of APC is attributed primarily to its ability to regulate the WNT pathway by targeting the destruction of beta-catenin. We report here a novel role for APC in regulating degradation of the transcriptional co-repressor C-terminal-binding protein-1 (CtBP1) through a proteasome-dependent process. Further, CtBP1 suppresses the expression of intestinal retinol dehydrogenases, which are required for retinoic acid production and intestinal differentiation. In support of a role for CtBP1 in initiation of colorectal cancer, adenomas taken from individuals with familial adenomatous polyposis contain high levels of CtBP1 protein in comparison with matched, uninvolved tissue. The relationship between APC and CtBP1 is conserved between humans and zebrafish and provides a mechanistic model explaining APC control of intestinal retinoic acid biosynthesis.