Cx26 inhibits breast MDA-MB-435 cell tumorigenic properties by a gap junctional intercellular communication-independent mechanism

It has been well established that the restoration of connexin expression in tumor cells often leads to a partial reversion of tumor cell phenotypes and increased growth control. In this study, a less-aggressive variant of MDA-MB-435 cells obtained from the MDA-MB-435 cell line was engineered to express gap junctional intercellular communication (GJIC)-competent Cx26, a GJIC-incompetent cell surface transported GFP-Cx26 chimera, or a Golgi apparatus-localized, disease-linked Cx26 mutant (D66H). Collectively, these cell lines were designed to establish whether Cx26 regulates tumor properties, such as migration, invasion and growth, by (i) a GJIC-dependent pathway; (ii) a mechanism requiring Cx26 transport to the cell surface; or (iii) a mechanism where Cx26 expression alone was sufficient. The expression of Cx26 and green fluorescent protein (GFP)-Cx26 decreased cell proliferation while all three Cx26 variants inhibited anchorage-independent cell growth. All three Cx26 variants also altered the distribution of filamentous actin and significantly reduced cell migration, while only the D66H mutant failed to inhibit cell invasion through matrigel. Furthermore, expression of all the Cx26 variants reduced the levels of total beta 1 integrin, and decreased the activity of matrix metalloproteinase-9 (MMP-9) while increasing tissue inhibitors of MMP-1 (TIMP-1) activity. Interestingly, the expression of Cx43 regulated the same gene products without significantly affecting the tumorigenic properties of the MDA-MB-435 cells. Together, these results suggest that Cx26 expression, independent of the necessity for gap junctional intercellular communication, partially reverted MDA-MB-435 cell properties associated with tumorigenesis, and regulated the expression of genes important in cell migration and invasion.