PlGF signaling and macrophage repolarization contribute to the anti-neoplastic effect of metformin

Placental growth factor (PlGF) related signaling pathway has been shown to have close relationship with the progression of cancers. Metformin has been reported to have an inhibitory effect on PlGF expression in a breast cancer model. However, little is known about whether the anti-neoplastic activity of metformin is contributed by its inhibitory effect on PlGF expression. Protein, mRNA and secretion levels of PlGF were tested and the proliferation of cancer cells was determined. After treatment of metformin, BALB/c mice bearing 4T1 tumors were sacrificed and immunohistochemistry staining of the tumor sections was obtained. Baseline expression of autocrine PlGF varied between different breast cancer cell lines, while the expression of vascular endothelial growth factor receptor-1 (VEGFR-1) was comparable between cell lines. Other clinical data showed that the expression of PlGF other than VEGFR-1 had a prognostic value for patients with breast cancers. Metformin significantly decreased the secretion and mRNA levels of PlGF, which greatly contributed to its inhibitory effect on the proliferation of breast cancer cells with high P1GF expression. The unresponsiveness of tumor cells with low PlGF expression to genetic silencing was reversed by the supplementation of exogenous PlGF. Systemic metformin administration apparently inhibited the in vivo growth of 4T1 carcinoma, which was accompanied by the repolarization of macrophages from M2 to M1. These findings indicated that both autocrine and paracrine PlGF signaling and macrophage repolarization are involved in the progression of breast cancer, which could be targeted by metformin.