The effect of GPR4 antagonist, NE52-QQ57, on the GPR4 and pro-apoptotic protein expressions in MPP+-treated SH-SY5Y cells that were stably GPR4-OE or GPR4-KO. 24 h serum-starved SH-SY5Y cells were pre-treated for 1 h with NE52-QQ57 (100 µM); this was followed by MPP+ (1 mM) stimulation for 24 h in serum-free culture media. (A) An immunoblot and semi-quantification of the respective protein expressions of GPR4, Bax, Bcl-2, Cleaved PARP, and β-Actin in SH-SY5Y cells. (B) An immunoblot and semi-quantification of the respective protein expressions of GPR4, Bax, Bcl-2, cleaved PARP, and β-Actin in GPR4-OE cells. (C) An immunoblot and semi-quantification of the respective protein expressions of GPR4, Bax, Bcl-2, cleaved PARP, and β-Actin in GPR4-KO cells. β-Actin was utilised as an internal control. Mean ± SEM (n = 3) was employed to express the data. Tukey’s multiple comparison test was performed using a one-way ANOVA. Each * p < 0.05 refers to the sample concentration compared with the same group of non-treated cells.