mTOR-Dependent Regulation of PHLPP Expression Controls the Rapamycin Sensitivity in Cancer Cells

PHLPP belongs to a novel family of protein phosphatases that serve as negative regulators of Akt. There are two isoforms, PHLPP1 and PHLPP2, identified in this family. Our previous studies indicated a tumor suppressor role of both PHLPP isoforms in colon cancer. Here we report that the expression of PHLPP is controlled by mTOR-dependent protein translation in colon and breast cancer cells. Treating cells with rapamycin or knockdown of mTOR using RNAi results in a marked decrease of PHLPP protein expression. In contrast, stable knockdown of TSC2, a negative regulator of mTOR activity, increases PHLPP expression. The rapamycin-mediated down-regulation of PHLPP is blocked by expression of a rapamycin-insensitive mutant of p70S6K. In addition, depletion of 4E-BP1 expression by RNAi results in an increase of PHLPP expression and resistance to rapamycin-induced down-regulation. Moreover, inhibition of mTOR activity by amino acid or glucose starvation reduces PHLPP expression in cells. Functionally, we show that rapamycin-mediated inhibition of PHLPP expression contributes to rapamycin resistance in colon cancer cells. Thus, our studies identify a compensatory feedback regulation in which the activation of Akt is inhibited by up-regulation of PHLPP through mTOR, and this mTOR-dependent expression of PHLPP subsequently determines the rapamycin sensitivity of cancer cells.