Abstract Studies demonstrated loss of connexin 43 (Cx43) expression in breast cancer cells in-vitro and in-vivo. Our previous studies have shown that silencing Cx43 in HMT-3522 S1 cells, non-neoplastic human mammary epithelial cells (S1 cells), altered apical polarity and mitotic spindle orientation under 3-dimensional (3-D) culture conditions. The purpose of this study is to investigate the effects of Cx43 loss on proliferation and invasion of non-neoplastic breast epithelium. Cx43 shRNA-transfected and control S1 cells were cultured under 2-D and 3-D conditions. Cx43-knockdown S1 cells exhibited enhanced proliferation both in 2-D and 3-D throughout the culture period, as shown by the increased cell counts and acinar size, respectively. In addition, cell cycle entry was enhanced, as indicated by the increased percentages of cells in S and G2/M phases and the reduced percentage of cells in G0/G1 phase. Cx43-silenced S1 cells upregulated the expression of proliferation genes, c-Myc and cyclin D1, in 2-D and 3-D, but maintained β-catenin levels. Furthermore, immunoprecipitation studies showed that Cx43 associated with β-catenin and ZO-2 in 2-D and 3-D cultures of S1 cells, and co-localized with β-catenin in 3-D, suggesting assembly of gap junction complexes in S1 cells. The loss of Cx43 caused mis-localization of β-catenin in 3-D cultures of S1 cells, suggesting involvement of the Wnt/β-catenin pathway downstream of Cx43 signaling. β-Catenin primarily exhibited apico-lateral distribution in S1 cells and re-localized to basolateral membrane domains in Cx43 knockdown cells. Silencing Cx43 enhanced migration of S1 cells by 40%, as shown by time-lapse imaging. In addition, Cx43-silenced S1 cells exhibited loss of the characteristic spheroid morphology and acquired a migratory phenotype in acini. Matrigel-invasion and MMP-9 activity were also enhanced. Cx43-silenced S1 cells were capable of invading the matrigel by two-fold compared to control cells. This was concomitant with enhanced expression and activity levels of Rho GTPases (RhoA, Rac1 and Cdc42), suggesting that Cx43 regulates non-canonical Wnt signaling, as a mechanism to induce migration and invasion. We propose a role for Cx43 loss in triggering proliferation and invasion events in non-neoplastic breast epithelium downstream of disrupted Wnt signaling. Citation Format: Sabreen F. Fostok, Dana Bazzoun, Sophie A. Lelievre, Mirvat El-Sibai, Rabih S. Talhouk. Silencing connexin 43 induces cell cycle entry and invasion in non-neoplastic mammary epithelial cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4505.