| The current work describes studies of the regulation of two PTPs especially important in medical research---the PTEN tumor suppressor, the lack of which due to loss of heterozygosity is a marker for glioblastoma, and PTP1B, a popular target for therapy of non-insulin-dependent diabetes. Oxidative processes leading to reversible inhibition of PTEN may play a role in signaling through the phosphatidylinositol (PI) 3-kinase/Akt growth signaling pathway, and in this study Akt phosphorylation was increased when rat astrocytes were treated with low concentrations of hydrogen peroxide (H2O2) through a process that was blocked by inhibition of PI 3-kinase. Akt stimulation also occurred when cells were treated with mitochondrial electron transport inhibitors that encourage oxidant generation, and this, too, was blocked by PI 3-kinase inhibition. These results were interpreted to indicate that major oxidative effects on this pathway probably occur upstream of Akt in these cells, but they do not rule out PTEN oxidation as a contributing factor. Labeled thiol reagents and kinetic PTP assays were employed to show that an electrophilic by-product of lipid peroxidation, 4-hydroxynonenal, reacts with the active site cysteine of recombinant PTEN and that of recombinant PTP1B in vitro and diminishes their activity. Kinetic assays using recombinant PTP1B and a fluorescent phosphatase substrate revealed that this phosphatase, a model for the PTP family, is highly sensitive to active site oxidation by H2O2 in the absence of glutathione and that phosphatase activity is increasingly protected as the glutathione concentration is increased. (Abstract shortened by UMI.). |
