okadaic-acid and Cell-Transformation--Viral

Lithium has long been used for the treatment and prophylaxis of bipolar mood disorder. However, nerve cells are not the sole targets of lithium. Indeed, lithium was reported to target numerous cell types, and affect cell proliferation, differentiation and death. Thus, the idea has been raised that lithium may act on signaling pathways involved in neoplastic transformation. Indeed, the effect of lithium on tumor progression is currently being tested in a limited number of clinical trials. However, the molecular mechanisms by which lithium affects neoplastic transformation remain to be characterized. Here, using mouse fibroblasts transformed by the v-src oncogene as a model, we show that lithium drastically inhibits cell motility and compromises the invasive phenotype of v-src-transformed cells. In addition, we show that this effect is mediated by the activation of phosphotyrosine phosphatases, but not by the direct inhibition of the v-Src tyrosine kinase. Finally, we show that lithium activates phosphotyrosine phosphatases by the modulation of the redox status of the cell, independently of the Wnt and the inositol phosphate canonical pathways. Thus, this study supports the idea that lithium, acting similar to an antioxydizer, may have antimetastatic properties in vivo.

Topics: Animals; Blotting, Western; Cell Movement; Cell Proliferation; Cell Transformation, Viral; Chick Embryo; Chorioallantoic Membrane; Dose-Response Relationship, Drug; Enzyme Activation; Enzyme Inhibitors; Glutathione; Lithium Chloride; Lithium Compounds; Matrix Metalloproteinase 2; Mice; Microscopy, Fluorescence; Neoplasms, Experimental; NIH 3T3 Cells; Okadaic Acid; Oncogene Protein pp60(v-src); Phosphorylation; Protein Tyrosine Phosphatases; Reactive Oxygen Species; Vanadates

We previously reported the generation of a library of hydrophobic oxazole-based small molecules designed as inhibitors of phosphatases involved in cellular signaling and cell cycle control. One member of the targeted array library, 4-(benzyl-(2-[(2, 5-diphenyl-oxazole-4-carbonyl)-amino]-ethyl)-carbamoyl)-2-decanoylami no butyric acid (SC-alphaalphadelta9), inhibited cell growth in the G0/G1 phase of the cell cycle. To investigate potential mechanisms for SC-alphaalphadelta9 antiproliferative activity, we have used mouse embryonic fibroblasts transformed with simian virus 40 large T antigen mouse embryonic fibroblasts as a model system for a malignant phenotype that depends on overexpression of cell cycle regulators and autocrine stimulation by insulin-like growth factor-1. Structure-activity relationship studies with SC-alphaalphadelta9 and four library congeners demonstrated that antiproliferative activity was not a result of overall hydrophobicity. Rather, SC-alphaalphadelta9 decreased insulin-like growth factor-1 receptor tyrosine phosphorylation, receptor expression, mitogen-activated protein kinase activation and levels of the cyclin-dependent kinase Cdc2. Less toxic congeners only partially affected receptor expression, receptor tyrosine phosphorylation and Cdc2 levels. Thus SC-alphaalphadelta9, which is structurally distinct from other known small molecules that decrease intracellular Cdc2 levels, has profound effects on intracellular signaling. Furthermore, SC-alphaalphadelta9, but not vanadate or okadaic acid, selectively inhibited the growth of simian virus 40 large T antigen mouse embryonic fibroblasts compared to the parental cells. These results suggest that overexpression of Cdc2 and increased dependence on insulin-like growth factor-1 autocrine stimulation are responsible for the increased sensitivity of simian virus 40 large T antigen mouse embryonic fibroblasts to SC-alphaalphadelta9. The SC-alphaalphadelta9 pharmacophore could be a useful platform for the development of novel antisignaling agents.

Topics: Animals; CDC2 Protein Kinase; cdc25 Phosphatases; Cell Cycle Proteins; Cell Division; Cell Line; Cell Transformation, Viral; Down-Regulation; Enzyme Inhibitors; Female; Insulin-Like Growth Factor I; Mice; Okadaic Acid; Oxazoles; Phosphoprotein Phosphatases; Phosphorylation; Pregnancy; Protein Tyrosine Phosphatase, Non-Receptor Type 1; Receptor, IGF Type 1; Signal Transduction; Structure-Activity Relationship; Tyrosine; Vanadates

To study the mechanism by which v-mos induces cell transformation, we generated a transformed rat cell line (DTM) containing two functional copies of mos, one encoding the p37v-mos of the m1 wild-type strain of Moloney murine sarcoma virus (Mo-MuSV) and the other the p85gag-mos fusion protein of the ts110 mutant of Moloney murine sarcoma virus. Subsequently, we isolated a revertant cell line (F-1) following transfection of DTM with a mutant retroviral construct (pIC4Neo) carrying a selectable marker. Like DTM, the F-1 revertant contained two integrated copies of v-mos, expressed mos containing viral RNA, and contained rescuable transforming viruses. The revertant did not grow in soft agar, showed a greatly reduced ability to form tumors in nude mice, and exhibited organized tubulin and actin structures similar to those found in normal cells. Revertant cells were resistant to retransformation by v-mos and v-raf but could be retransformed by v-ras. MAP kinase (ERK-2) and MAP kinase kinase (MKK-1) activity, which are constitutively elevated in v-mos- and v-raf-transformed cells, exhibits levels in the F-1 revertant similar to those seen in nontransformed cells. F-1 and normal REF-1 cells express elevated levels of protein phosphatases in comparison to DTM cells. In vivo treatment with okadaic acid, a potent protein phosphatase inhibitor, leads to an increase in MKK-1 and MAP kinase activity in F-1 cells but not in REF-1. The results support the hypothesis that mos acts through the MAP kinase cascade (MKK-1 and ERK-2) to induce cell transformation and that blocking v-mos activation of that cascade (possibly because of increased levels of phosphatase) prevents transformation.

Topics: Animals; Calcium-Calmodulin-Dependent Protein Kinases; Cell Line, Transformed; Cell Transformation, Neoplastic; Cell Transformation, Viral; Cytoskeletal Proteins; DNA Probes; Enzyme Activation; Ethers, Cyclic; Genes, ras; Mitogen-Activated Protein Kinase Kinases; Okadaic Acid; Oncogene Proteins v-mos; Oncogene Proteins v-raf; Phosphoprotein Phosphatases; Protein Kinases; ras Proteins; Rats; Retroviridae Proteins, Oncogenic; Transcription, Genetic; Transfection

The tumor promoter okadaic acid is a potent inhibitor of the serine/threonine protein phosphatases 1 and 2A. Addition of okadaic acid to v-Src-transformed BALB/c 3T3 cells reverted them to a flat morphology, increased fibronectin levels in the extracellular matrix, reduced saturation density, and inhibited the formation of colonies in soft agar. The ability of v-Src-transformed cells to proliferate in low serum was also inhibited by okadaic acid. These data implicate serine/threonine phosphatases in v-Src-induced transformation.

Topics: 3T3 Cells; Animals; Cell Adhesion; Cell Division; Cell Transformation, Neoplastic; Cell Transformation, Viral; Ethers, Cyclic; Genes, src; Mice; Mice, Inbred BALB C; Okadaic Acid; Phosphoprotein Phosphatases

Papovavirus tumor antigens have been shown to associate with the cellular phosphoserine/threonine-specific protein phosphatase 2A (PP2A). We were interested in the consequences that T-antigen association might have on PP2A activity and so studies of the phosphatase activity in immunoprecipitates, prepared from polyoma virus-transformed or polyoma virus-infected mouse 3T3 fibroblasts, were performed. The phosphoserine/threonine phosphatase activity, measured with phosphorylase a as the substrate, showed all the characteristics of PP2A. It was stimulated by polycations, inhibited by fluoride or p-nitrophenyl phosphate, sensitive to okadaic acid and microcystin and insensitive to inhibitor-1 and inhibitor-2. Phosphotyrosyl phosphatase (PTPase) activity was associated with the middle-T/small-T-associated complex when reduced, carboxamidomethylated and maleylated lysozyme, phosphorylated exclusively on tyrosyl residues, was used as the substrate. This PTPase activity was as sensitive to okadaic acid as was the phosphorylase phosphatase activity; it could be inhibited by phosphorylase a and did not dephosphorylate poly(Glu80Tyr20). The level of middle-T/small-T-associated PTPase activity relative to the phosphorylase phosphatase activity was tenfold higher than that of the purified dimeric PP2A. A similar activity ratio was observed with the purified phosphatase after stimulation with a cellular protein, designated phosphotyrosyl phosphatase activator. These results suggest that the same enzyme may possess dual specificity. In contrast to the cellular trimeric PP2A, containing the 55-kDa putative regulatory subunit, the middle-T/small-T-associated enzyme had low activity towards a retinoblastoma peptide phosphorylated by p34cdc2. These results indicate how middle-T/small-T might effect the activity of PP2A in polyoma virus-transformed cells.

Topics: 3T3 Cells; Animals; Antigens, Polyomavirus Transforming; Cell Transformation, Viral; Ethers, Cyclic; Mice; Okadaic Acid; Phosphorylase Phosphatase; Polyomavirus; Protein Phosphatase 2; Protein Tyrosine Phosphatases; Substrate Specificity

Previous results indicated that SV40 small t is essential for SV40-induced transformation of diploid cells but dispensable for the transformation of cells with a deletion on the short arm of chromosome 11 (del-11 cells). From these results we concluded that del-11 cells contain a cellular 'SV40 small t-like' factor, which is able to transactivate the HPV16 long control region (LCR) and to complement SV40 large T in transformation. Since SV40 small t and the regulatory 55 kDa subunit (PR55) of protein phosphatase 2A (PP2A), have been shown to inhibit the enzyme activity of PP2A, the PR55 beta subunit could be the putative 'small t-like' factor. In accordance with this hypothesis, we show that the PR55 beta subunit is highly expressed in del-11 but not in diploid cells and is able to trans-activate the HPV16 LCR in diploid cells. Moreover, inhibition of PP2A by okadaic acid resulted in trans-activation of the HPV16 LCR in diploid cells. Alignment of PR55 and SV40 small t showed a common four amino acid motif DKGG. We present evidence that the integrity of this motif is necessary for the PP2A-mediated ability of SV40 small t to trans-activate the HPV16 LCR.

Topics: Amino Acid Sequence; Blotting, Western; Cell Transformation, Viral; Cells, Cultured; Chromosomes, Human, Pair 11; Diploidy; Enhancer Elements, Genetic; Ethers, Cyclic; Gene Deletion; Genes, Viral; Humans; Molecular Sequence Data; Okadaic Acid; Papillomaviridae; Phosphoprotein Phosphatases; Plasmids; Promoter Regions, Genetic; Protein Phosphatase 2; RNA, Messenger; Simian virus 40; Transcriptional Activation

The simian virus 40 large T antigen (T) is a multifunctional phosphoprotein. We found that T-dependent simian virus 40 DNA replication is substantially inhibited by okadaic acid. This result suggests that DNA replication is activated by dephosphorylation in vitro. We show here that the target activated by dephosphorylation, which stimulates DNA replication, is T and that the phosphatase involved is protein phosphatase 2A.

Topics: Alkaline Phosphatase; Antigens, Polyomavirus Transforming; Cell Transformation, Viral; DNA Replication; Ethers, Cyclic; HeLa Cells; Humans; Kinetics; Okadaic Acid; Phosphoprotein Phosphatases; Protein Phosphatase 2; Simian virus 40