Connexin 43 loss triggers cell cycle entry and invasion in non-neoplastic breast epithelium : a role for noncanonical Wnt signaling.

Abstract

The signaling pathways through which connexin (Cx) proteins, the building blocks of gap junctions (GJs), exert their tumor suppressive roles in the mammary gland remain elusive. The Wnt signaling pathways, including the Wnt-β-catenin or the canonical Wnt pathway and the β-catenin-independent noncanonical Wnt pathway, execute key roles in the development of the mammary gland, and their deregulation is associated with impaired development and breast tumorigenesis. Evidence suggests a cross-talk between Cxs and Wnt signaling in a multitude of nonbreast tissues and biological contexts. In the breast, Cxs act as downstream transcriptional and functional targets for Wnt signaling. However, the possible role of Cxs as upstream regulators of Wnt signaling is yet to be investigated (Fostok, El-Sibai, El-Sabban, and Talhouk, 2018). Cxs exhibit spatiotemporal expression patterns that are critical for normal development and physiology of the mammary gland. Impaired expression of Cxs leads to mammary developmental defects and predisposes the mammary gland to primary tumors and lung metastases in murine models. In the human breast, Cx43 is expressed in myoepithelial and luminal epithelial cells, which additionally express Cx26. Reduced expression and altered localization of Cx43 have been reported in human breast cancer tissues and cell lines. Furthermore, Cx43 has been proposed as an independent prognostic factor in light of the positive correlation of its levels with good prognosis in breast cancer patients. Overexpression of Cx43 in human breast cancer cell lines reduces proliferation, anchorage-independent growth, migration, invasion, xenograft tumor growth, angiogenesis and metastasis. The aim of this study is to investigate possible involvement of the Wnt pathways downstream of Cx43 signaling in the homeostasis of the mammary gland. The non-neoplastic human mammary epithelial HMT-3522 S1 cells assemble into growth-arrested differentiated glandular structures, or acini, under 3-dimensional (3-D) culture condit