Phosphatase inhibition and cell survival after DNA damage induced by radiation

The activity of certain kinases can promote cell survival after DNA damage, but the role of phosphatases in determining cell fate, although documented, is much less well defined. We sought to define a role for phosphatases in radiation survival and identify potential targets for intervention. By using naturally occurring inhibitors and siRNA we have assessed inhibition of four serine/threonine phosphatases PP1, PP2A, PHLPP and PHLPPL in a panel of tumor cell lines with H-, K- or N-ras mutations or with EGFR activation for effects on tumor cell radiosensitivity. Calyculin A, which inhibits both PP1 and PP2A reduced radiation survival in SQ20B cells (overexpressing EGFR). Okadaic acid, which preferentially inhibits PP2A showed less effect in SQ20B cells suggesting a greater involvement of PP1 in modulating radiosensitivity of these cells. T24 cells (H-Ras mutant) appeared equally sensitive to both inhibitors. The suggestion from inhibitors that PP1 might be important in radiosensitivity was supported by the greater sensitization obtained after knocking down expression of the catalytic sub-unit of PP1 over that seen after PP2A knockdown. Knocking down the PP2C like phosphatase PHLPPL also increased radiosensitivity in all cell lines tested where a second isoform PHLPP had little effect. These data suggest that targeted inhibition of phosphatase activity may be an alternative to kinase inhibition to enhance radiosensitivity in tumors.