Buformin inhibits the stemness of erbB-2-overexpressing breast cancer cells and premalignant mammary tissues of MMTV-erbB-2 transgenic mice

Background: metformin,an fda-approved drug for the treatment of type ii diabetes,has emerged as a promising anti-cancer agent. other biguanide analogs,including buformin and phenformin,are suggested to have similar properties. although buformin was shown to reduce mammary tumor burden in carcinogen models,the anti-cancer effects of buformin on different breast cancer subtypes and the underlying mechanisms remain unclear. therefore,we aimed to investigate the effects of buformin on erbb-2-overexpressing breast cancer with in vitro and in vivo models. methods: mtt,cell cycle,clonogenic/cfc,aldefluor,tumorsphere,and western blot analyses were used to determine the effects of buformin on cell growth,stem cell populations,stem cell-like properties,and signaling pathways in skbr3 and bt474 erbb-2-overexpressing breast cancer cell lines. a syngeneic tumor cell transplantation model inoculating mmtv-erbb-2 mice with 78617 mouse mammary tumor cells was used to study the effects of buformin (1.2 g buformin/kg chow) on tumor growth in vivo. mmtv-erbb-2 mice were also fed buformin for 10 weeks,followed by analysis of premalignant mammary tissues for changes in morphological development,mammary epithelial cell (mec) populations,and signaling pathways. results: buformin significantly inhibited skbr3 and bt474 cell growth,and in vivo activity was demonstrated by considerable growth inhibition of syngeneic tumors derived from mmtv-erbb-2 mice. in particular,buformin suppressed stem cell populations and self-renewal in vitro,which was associated with inhibited receptor tyrosine kinase (rtk) and mtor signaling. consistent with in vitro data,buformin suppressed mammary morphogenesis and reduced cell proliferation in mmtv-erbb-2 mice. importantly,buformin decreased mec populations enriched with mammary reconstitution units (mrus) and tumor-initiating cells (tics) from mmtv-erbb-2 mice,as supported by impaired clonogenic and mammosphere formation in primary mecs. we further demonstrated that buformin-mediated in vivo inhibition of mec stemness is associated with suppressed activation of mtor,rtk,er,and β-catenin signaling pathways. conclusions: overall,our results provide evidence for buformin as an effective anti-cancer drug that selectively targets tics,and present a novel prevention and/or treatment strategy for patients who are genetically predisposed to erbb-2-overexpressing breast cancer. © 2017 the author(s).