ERK, JNK, and P38 MAPK signaling mediate the regulation of Notch4 overexpression on A549 and H1299 cells proliferation and apoptosis. (A) The MAPK signaling pathway were detected in A549 and H1299 cells transfected with siRNA against Notch4 or negative control siRNA. (B) Cell viability were examined in A549 and H1299 cells transfected with Notch4 plasmid or negative control plasmid co-treatment with ERK (U0126), JNK (SP600125), or P38 (SB203580) MAPK pathway inhibitors by using CCK-8 assay. (n = 3–4). (C) Colony formation assay was performed in A549 and H1299 cells transfected with Notch4 plasmid or negative control plasmid co-treatment with U0126, SP600125, or SB203580 for 24 h (n = 4). (D) The Annexin-V/PI assay was performed in A549 and H1299 cells transfected with Notch4 plasmid or negative control plasmid co-treatment with U0126, SP600125, or SB203580 for 24 h (n = 3–6). Analyses of apoptosis including early apoptosis (Annexin-V positive and PI negative) and late apoptosis (Annexin-V positive and PI positive) were shown. Data were presented as means ± SD. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001. CCK-8, Cell Counting Kit-8; PI, propidium iodide; si-NC, negative control short interfering RNAs (siRNA); si-Notch4, the siRNA against Notch4. Vector, negative control plasmid; Notch4, Notch4 plasmid. ERK, extracellular signal-regulated kinase; JNK, c-Jun N-terminal kinase; MAPK, mitogen-activated protein kinase.