Nicotine promotes M2 microglial polarization via STAT3 activation. (A–C) Expressions of M1/M2 microglia markers were examined after the treatment of human microglia (HMC3) (A), mouse microglia (SIM-A9; B), and primary mouse microglia (C) with either vehicle or nicotine (1 µM). The value of qRT-PCR in each figure was normalized using actin as a control. The y axis indicates arbitrary unit (n = 3/group). (D) Human microglia (HMC3) were treated with or without nicotine (1 µM), and the expressions of JNK and STAT3 were examined by Western blot. (E) Human microglia (HMC3) were treated with or without nicotine and/or STATTIC (0.5 µM) for 24 h, and the expression of STAT3 was examined by Western blot. (F) Mouse microglia cells (SIM-A9) were treated with or without nicotine and/or STATTIC (0.5 µM) for 24 h followed by examining the luciferase-promoter activity of Arg1 (M2; n = 4/group). (G and H) Mouse microglia (SIM-A9) were treated with or without nicotine (1 µM) and/or the STAT3 inhibitor (0.5 µM) for 24 h. Cells were then subjected to FACS analysis for analysis of Iba1⁺/F4/80⁺ (M1 cell; G) and Iba1⁺/CD206⁺ (M2 cell; H; n = 9/group). The data are presented as the mean ± SD. *, P < 0.05; **, P < 0.01; and ***, P < 0.001.