MTOR and neoplasm: For instance, the effect of PI3K/mTOR inhibitors on tumor cells is blocked by the inhibition of MEK or knockout of ERK, and the combined inhibition of MEK and PI3K/Akt/mTOR signaling thus inhibits tumor cell growth.114 In addition, MAPK pathway signals function as second messengers to attenuate PI3K/Akt signals by decreasing reactive oxygen species (ROS) levels.