okadaic-acid and Carcinoma

Serine/threonine protein phosphatase (PP) 2A is thought to dephosphorylate phosphorylated beta1 integrin to link with actin filaments (F-actin). However, whether PP2A participates in the regulation of F-actin assembly to which beta1 integrin is anchored is unclear. We report here that the core enzyme of PP2A (PP2A-AC), consisting of the regulatory subunit A (PP2A-A) and the catalytic subunit C (PP2A-C), forms a complex with beta1 integrin, a small GTPase Rac, and its effector IQGAP1 in non-malignant human mammary epithelial (HME) cells. Treatment of HME cells with okadaic acid (OA), an inhibitor of PP2A, caused cell rounding, reduction in F-actin assembly that links with beta1 integrin, and dissociation of IQGAP1-bound PP2A-AC from Rac-beta1 integrin. The dissociation of IQGAP1-PP2A-AC was accompanied by loss of F-actin gelating activity of Rac-beta1 integrin. In breast cancer MCF-7 cells, which possess PP2A-C but lack PP2A-A, IQGAP1 was not associated with Rac-beta1 integrin but with PP2A-C, with no distinct F-actin assembly that linked to Rac-beta1 integrin even before treatment with OA. We therefore propose that PP2A, especially PP2A-A, functions to maintain F-actin assembly to which beta1 integrin is anchored by recruitment of IQGAP1 to Rac-beta1 integrin.

Topics: Actins; Binding Sites; Breast Neoplasms; Carcinoma; Catalytic Domain; Cell Adhesion; Cell Line; Cell Line, Tumor; Cell Membrane; Enzyme Inhibitors; Female; Gels; Humans; Integrin beta1; Macromolecular Substances; Okadaic Acid; Phosphoprotein Phosphatases; Protein Binding; Protein Phosphatase 2; rac GTP-Binding Proteins; ras GTPase-Activating Proteins; Viscosity

Transforming growth factor beta (TGFbeta) is a tumor suppressor acting as inhibitor of cell cycle progression of epithelial cells. We show that treatment of the pancreatic carcinoma cell lines PANC-1 and BxPC-3 with TGFbeta1 inhibits both growth factor-induced activation of the extracellular signal-regulated kinase 2 (ERK2) and translocation of the kinase to the nucleus. TGFbeta1 causes a concentration-dependent reduction of cell proliferation in both cell lines. By measuring ERK activation, we can show that TGFbeta1 is able to repress ERK activation induced by mitogenic stimuli such as EGF. This inhibitory effect of TGFbeta1 is not mediated by suppression of Ras or c-Raf-1 activation, but mediated by TGFbeta1-induced activation of a serine-threonine phosphatase, as demonstrated by inhibition of phosphatases by treatment with okadaic acid. Results obtained in the Smad4-deficient pancreatic carcinoma cell line BxPC-3, demonstrate that TGFbeta1-induced growth inhibition is mediated by a Smad4-independent prevention of ERK2 activation. In contrast to the effects of TGFbeta1 on epithelial cells, mesenchymal NIH3T3 fibroblasts exhibit elevated ERK2 activation and increased cell proliferation in response to TGFbeta1 treatment. Smad4-independent phosphatase-mediated inhibition of mitogen-activated ERK2 represents a novel effector pathway contributing to suppression of epithelial pancreatic carcinoma cell proliferation by TGFbeta1, in addition to the well-known Smad-induced tumor suppressor activity of TGFbeta. Oncogene (2000) 19, 4531 - 4541.

Topics: 3T3 Cells; Animals; Biological Transport; Carcinoma; Cell Cycle; Cell Division; Cell Nucleus; DNA-Binding Proteins; Enzyme Activation; Enzyme Inhibitors; Humans; Mice; Mitogen-Activated Protein Kinase 1; Okadaic Acid; Pancreatic Neoplasms; Protein Serine-Threonine Kinases; Proto-Oncogene Proteins c-raf; ras Proteins; Receptor, Transforming Growth Factor-beta Type II; Receptors, Transforming Growth Factor beta; Signal Transduction; Smad4 Protein; Trans-Activators; Transforming Growth Factor beta; Tumor Cells, Cultured