u-0126 and Cell-Transformation--Viral

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma (OPA), a transmissible lung cancer of sheep. The virus can induce tumors rapidly, and we previously found that the JSRV envelope protein (Env) functions as an oncogene, because it can transform mammalian and avian fibroblast cell lines. (N. Maeda, Proc. Natl. Acad. Sci. USA 98:4449-4454, 2001). The molecular mechanisms of JSRV Env transformation are of considerable interest. Several reports suggested that the phosphatidylinositol 3-kinase/Akt pathway is important for transformation of mammalian fibroblasts but not for chicken fibroblasts. In this study, we found that Akt/mTOR is involved in JSRV transformation of mouse NIH 3T3 fibroblasts, because treatment with the mTOR inhibitor rapamycin reduced transformation. We also found that H/N-Ras inhibitor FTI-277 and MEK1/2 inhibitors PD98059 and U0126 strongly inhibited JSRV transformation of NIH 3T3 fibroblasts, suggesting that the H/N-Ras-MEK-mitogen-activated protein kinase (MAPK) p44/42 pathway is necessary for the transformation. In RK3E epithelial cells, the MEK1/2 inhibitors also eliminated transformation, but FTI-277 only partially inhibited transformation. It was noteworthy that p38 MAPK inhibitors enhanced JSRV transformation in both fibroblasts and epithelial cells. Treatment of transformed cells with p38 inhibitors both increased levels of phospho-MEK1/2 and phospho-p44/42 and induced rapid enhancement of the transformed phenotype. Immunohistochemical staining of tumor tissues from naturally and experimentally induced OPA and naturally occurring enzootic nasal adenocarcinoma revealed strong activation of MAPK p44/42 in all cases examined. However, p38 activation was not generally observed. These results indicate that signaling through two pathways (in particular, H/N-Ras-MEK-MAPK and, to a lesser extent, Akt-mTOR) is important for JSRV-induced transformation and that p38 MAPK has a negative regulatory effect on transformation, perhaps via MEK1/2 and p44/42.

Topics: Animals; Butadienes; Cell Transformation, Viral; Cells, Cultured; Enzyme Inhibitors; Epithelial Cells; Fibroblasts; Flavonoids; Jaagsiekte sheep retrovirus; MAP Kinase Kinase 1; MAP Kinase Kinase 2; Methionine; Mice; Mitogen-Activated Protein Kinase 1; Mitogen-Activated Protein Kinase 3; Nitriles; p38 Mitogen-Activated Protein Kinases; Protein Kinases; Protein Serine-Threonine Kinases; Proto-Oncogene Proteins; Proto-Oncogene Proteins c-akt; ras Proteins; Rats; Signal Transduction; Sirolimus; TOR Serine-Threonine Kinases

Myc is overexpressed in many cancers as a result of gene rearrangement or amplification, but coding sequence changes which cluster in the N-terminal transactivation domain also appear to play a role in tumour progression. The prototypic v-Myc gene of MC29 virus differs from avian c-Myc by a series of mutations, including a change at a regulatory phosphorylation site within the mutational hotspot (thr-61) which is known to potentiate transformation in vitro. We now show that the mutation at thr-61 stabilizes the v-Myc protein (turnover difference) and that this single mutation is both necessary and sufficient for the phenotype. A major involvement of the proteasome in Myc degradation was confirmed, but surprisingly, a dilysine motif adjacent to thr-61 proved not to be the ubiquitin target. Two other v-Myc genes which carry a mutation at thr-61 (avian MH2) or a large deletion encompassing this domain (feline T17) were found to be stabilized to a similar extent as MC29, showing that stabilization is a common feature of independently derived Myc oncogenes. These results suggest a common selective process in the genesis of these three viral oncoproteins and a mechanistic link with Jun, Fos and Myb oncoproteins which are also stabilized relative to their cellular counterparts.

Topics: Alleles; Amino Acid Motifs; Animals; Avian Leukosis Virus; Butadienes; Cell Transformation, Viral; Chick Embryo; Cysteine Endopeptidases; Enzyme Inhibitors; Fibroblasts; Genes, myc; Leukemia Virus, Feline; MAP Kinase Signaling System; Multienzyme Complexes; Mutation; Nitriles; Oncogene Protein p55(v-myc); Phosphorylation; Proteasome Endopeptidase Complex; Recombinant Proteins; Threonine