Figure 3. Effects of metformin pretreatment on the expression of the mitochondrial proteins TIMM23 and NDUFS3 in MPP⁺-treated differentiated SH-SY5Y cells. Cells were pretreated with metformin (500 µM) for 1 h before exposure to MPP⁺ (1000 µM) for 24 h. (a) Immunostaining for TIMM23 and NDUFS3. The nuclei were stained with DAPI. (b) Protein expression was examined by Western blotting. ß-actin was used as a normalization control. (c-d) The bar graphs show the quantifications of TIMM23 and NDUFS3 expression. The data are expressed as the means ± SD from three independent experiments. * P < 0.05, ** P < 0.01, *** P < 0.001 versus control; ### P < 0.001 versus MPP⁺-treated cells.