To determine whether Wnt proteins activate the β-catenin-dependent pathway extracellularly, ELISA analysis showed that extracellular Wnt5A, but not Wnt3A, was increased after PM2.5 exposure in both BC cell lines (T24: p = 0.023 at 24 h; TSGH: p = 0.002 at 30 min) (Fig. 4A). Here, WNT3A is linked to breast cancer.