Figure 5.

Synergistic response of breast cancer cell lines to a combination of MEK and PI3K inhibitors correlates with their up-regulated p-AKT status induced by MEK inhibition. A, Western blot analysis of p-AKT and p-ERK expression in response to CI1040 treatment in four breast cancer cell lines: MDAMB231, MDAMB175, HS578T, and SUM149. The cells were treated with MEK inhibitor in low serum conditions, and in 30 min, they were stimulated with EGF and the protein lysates were collected at 4 h after EGF. B and C, schematic summary of the RAS-RAF-MEK-ERK and PI3K pathway interconnectivity in the absence (B) and presence (C) of MEK inhibitors. D, synergistic effect of combination of MEK and PI3K inhibitors on cell viability of cell lines displaying MEK inhibitor–induced AKT phosphorylation. Dose/effect curves for single inhibitors CI1040 and PIK90 and their combinations at fixed molar ratio are presented. Cell viability was measured at 72 h after treatment with the drugs using ATP-based cell viability assay (Promega). Relative cell viability of drug-treated cells was calculated as a fraction of control. Points, mean of triplicates; bars, SD. CIs at 50% dose response are calculated using CalcuSyn software.