microcystin and Cell-Transformation--Neoplastic

It has been shown by epidemiological and animal studies that microcystin is an important exogenous factor involved in the carcinogenesis of colorectal cancer (CRC). However, details of the mechanism remain unclear. Transformation of colorectal cells is an important initial step in carcinogenesis. Whether microcystin is capable of transforming immortalized colorectal crypt cells, and what the mechanism might be, was investigated. In the present study, we demonstrated that immortalized colorectal crypt cells could be transformed by microcystin. Transformed colorectal crypt cells showed an anchorage-independent growth phenotype, and the proliferation activities of microcystin-transformed cells were also greater than that of immortalized colorectal crypt cells. The Akt and the p38, JNK of mitogen-activated protein kinase (MAPK) pathways in microcystin-transformed cells were found to be constitutively activated. In microcystin-transformed cells, PI3K, MAPKAPK2, Akt, cyclin D1 and cyclin D3 in the Akt pathway; IQGAP-2, RabGTPase, Rap1GAP, RasGAP, R-Ras, Krev-1 and TC21 of the Ras GTP/GDP protein family; and A-Raf, B-Raf and PAK in the Ras/MAPK pathway were all markedly upregulated. However, in positive control cells, dimethylhydrazine-transformed cells, only the Akt pathway was activated by PI3K, and no evidence of alteration of any molecules of the Ras superfamily was observed. Inhibition of Akt, p38 and JNK activation led to a reduced proliferation of microcystin-transformed cells. This implies that the constitutive activation of Akt and the p38, JNK of MAPK pathways in microcystin-transformed cells may be the mechanism by which this important external factor acts in the carcinogenesis of CRC.

Topics: Cell Culture Techniques; Cell Proliferation; Cell Transformation, Neoplastic; Colon; Colorectal Neoplasms; Humans; JNK Mitogen-Activated Protein Kinases; MAP Kinase Kinase 4; Microcystins; Mitogen-Activated Protein Kinase Kinases; p38 Mitogen-Activated Protein Kinases; Peptides, Cyclic; Precancerous Conditions; Protein Serine-Threonine Kinases; Proto-Oncogene Proteins; Proto-Oncogene Proteins c-akt; Rectum

The authors studied the combined carcinogenic activities of MCYST and organic pollutants in water of Dianshan Lake, using in vitro Syrian hamster embryo (SHE) cell transformation and immunohistochemical assays. The results showed that MCYST alone could not induce SHE cell transformation, but it increased the effects of low dose organic pollutants in cell transformation in a dose-dependent manner. In the cells isolated from type II transformed foci, ras P21 protein showed a positive immunohistochemical staining. It suggests MCYST may be a tumor promoter, which has a synergistic carcinogenic activity organic pollutants in water and the activation of ras oncogene may be one of the factors inducing malignant transformation.

Topics: Animals; Bacterial Toxins; Carcinogens; Cell Transformation, Neoplastic; Cocarcinogenesis; Cricetinae; Drug Synergism; Embryo, Mammalian; Mesocricetus; Microcystins; Oncogene Protein p21(ras); Peptides, Cyclic; Water Pollutants, Chemical

Microcystis aeruginosa is the dominant algae in most of the eutrophicated lakes in China. It can produce cyclic heptapeptides, known as microcystins, which can cause liver damage in wild and domestic animals. In this paper, a two-stage transformation assay for demonstrating the carcinogenic effects of the algal toxins is reported. The cell strain used in this assay was derived from embryos of Syrian golden hamster and the algal toxins were extracted from Microcystis aeruginosa, termed microcystis raw toxin (MRT). To elucidate its promoting activity, the target cells were first exposed to a low dosage of 3-methylcholanthrene (MCA) and then to MRT. The results showed that MRT significantly enhanced the MCA initiated cell transformation, and a dose-response relationship was observed, but it failed to induce transformation of SHE cells not pretreated by MCA. These results suggest that the MRT play an important role in the malignant transformation of SHE cells. MRT may thus be a tumor promoter, and this transformation assay with SHE cells may be used to predict tumor prompting activity of environmental chemicals, before long-term in vivo two-stage carcinogenesis experiments are carried out.

Topics: Animals; Cell Transformation, Neoplastic; Cells, Cultured; Cricetinae; Dose-Response Relationship, Drug; Eukaryota; Microcystins; Mutagenicity Tests; Peptides, Cyclic; Plant Extracts