Inhibition of MAPK pathway signaling restores sensitivity to idelalisib in MEC1 cells overexpressing MAP2K1 mutants. (A) Representative immunoblot from MEC1 cell lines stably expressing WT and mutant MAP2K1 after 1-hour in vitro treatment with vehicle, 5 μM idelalisib, 5 μM CI-1040 (MEK1/2 inhibitor) or 5 μM SCH772984 (ERK1/2 inhibitor) alone or in combination followed by stimulation with anti-IgM for 15 minutes. (B) pERK densitometry analysis of MEC1^WT and MEC1^MUT cells after vehicle, 5 μM idelalisib, 5 μM CI-1040, or 5 μM SCH772984 treatment in vitro (n = 3) for 1 hour followed by stimulation with anti-IgM for 15 minutes. (C) pERK densitometry analysis of MEC1^WT and MEC1^MUT cells after combination treatment with 5 μM idelalisib and 5 μM CI-1040 or 5 μM idelalisib and 5 μM SCH772984 in vitro (n = 3) for 1 hour followed by stimulation with anti-IgM for 15 minutes. (D) pAKT densitometry analysis of MEC1^WT and MEC1^MUT cells after in vitro treatment with vehicle, 5 μM idelalisib, 5 μM CI-1040, or 5 μM SCH772984 (n = 3) for 1 hour followed by stimulation with anti-IgM for 15 minutes. (E) pAKT densitometry analysis of MEC1^WT and MEC1^MUT cells after combination treatment with 5 μM idelalisib with either 5 μM CI-1040 or 5 μM SCH772984 in vitro (n = 3) for 1 hour followed by stimulation with anti-IgM for 15 minutes. Data represented as mean ± SEM.***P < .001; *P < .05.