Connexin26-mediated gap junctional communication reverses the malignant phenotype of MCF-7 breast cancer cells

A growing body of evidence indicates that the gap junction (GJ) plays a pivotal role in tumor suppression by exerting cell-cell communication. It has, however, been reported that expression of connexin26 (Cx26) protein is induced in human ductal carcinomas of the breast and that its amount increases in proportion to the grade of malignancy. We thus examined the effects of over-expressed Cx26 on growth characteristics in GJ-deficient human MCF-7 breast cancer cells that maintain the phenotype of early-stage cancers. MCF-7 cells were transfected with Cx26 cDNA, and several clones of stable transformants exhibiting a high level of cell-cell communication were established. When they were examined in terms of various growth characteristics in vitro , the proliferation rate and the saturation density were drastically reduced in Cx26-transfected clones compared with the mock-transfectant. The anchorage-independent growth capacity was also decreased by 50–75% after transfection of Cx26. Furthermore, the cell migration toward growth factors and cell invasion into Matrigel in a Boyden chamber were suppressed to 5–10% and 20–60%, respectively, of the control in Cx26-transfected clones. When implanted into the mammary fat pads of nude mice in the presence of an excess of 17β-estradiol, Cx26-transfected clones tended to show slower tumor growth than the mock-transfectant, although the difference was not statistically significant. Our results strongly suggest that the induction of Cx26 protein observed in human breast cancers, reported previously, may not be very relevant to the development of breast cancers, and that Cx26 can function as a tumor suppressor in breast cancer cells.