Use of PTEN status to determine the functional outcome of combined MEK and mTOR inhibition.

Abstract

e13512 Background: PTEN is a lipid phosphatase couteracting the activity of the PI3K pathway. PTEN mutations and deficiencies are prevalent in many types of human cancers and are associated with poor prognosis and therapeutic resistance. MAPK is another key cellular network that works independently from, in parallel to, and/or through interconnections with PI3K to promote cancer development. Here we investigated whether a strategy combining MEK and mTOR inhibition may be effective in preclinical models of human cancer and the role of PTEN loss in determining sensivity/resistance to single and combined pathway inhibition. Methods: We employed in vitro assays (cell proliferation assays, cell cycle and apoptosis analysis, WB, and ELISA assays) to determine functional and molecular drug effects. Pharmacologic interactions between MEK and mTOR inhibitors were analyzed by conservative isobologram analysis using a fixed dose-ratio experimental design. Results: In cell lines with wt-PTEN (including melanoma, breast, lung, and colon cancer), combined MEK and mTOR blockade achieves synergistic effects at suboptimal drug concentrations but becomes frankly antagonistic in the presence of complete inhibition of MEK-to-ERK signaling (combination index - CI: 1.2->1000). This, in turn, led to the identification of a novel crosstalk mechanism by which MEK blockade restores PTEN expression and cross-inhibits the PI3K/AKT/mTOR pathway, thus bypassing the need for double pathway blockade. Conversely, in cancer cell lines with PTEN loss, combined MEK and mTOR blockade resulted in strongly synergistic effects (CI: 0.0005-0.4), due to cooperative induction of apoptosis. A similar synergistic growth inhibition was also observed in a patient-derived lung cancer stem cell line, which displayed low to undetectable PTEN protein levels. Conclusions: Our results suggest that combined MEK and mTOR inhibition exerts strongly synergistic effects in cancer models with PTEN loss. This notion may be helpful in selecting appropriate cellular (and possibly clinical) contexts for the design rational therapeutic strategies based on concomitant inhibition of the RAF/MEK/ERK and PI3K/AKT/mTOR pathways.