Exogenous CXCL12 activates protein kinase C to phosphorylate connexin 43 for gap junctional intercellular communication among confluent breast cancer cells

Despite ongoing attempts to improve the overall breast cancer (BC) survival rate, BC cells' (BCCs) predilection for metastasizing to the bone marrow has enabled BCCs to not only remain dormant, but also evade detection. BCCs are able to acquire quiescence by establishing gap junctional intercellular communication (GJIC) with the stroma through the assembly of connexins (Cxs). The chemoattractant CXCL12 also appears to play a role in GJIC based on its tendency to decrease when GJIC is formed between BCCs and bone marrow stroma. This study investigates the role CXCL12 has on Cx43 expression and PKC-mediated Cx43 phosphorylation. Cx43 gene reporter assays revealed that as the BCCs come in contact with each other and establish GJIC, there is an inverse relationship between CXCL12 level and Cx43 expression. Immunoblot analyses confirmed this relationship at the level of protein, showing decreased Cx43 and reduced Cx43 phosphorylation at higher CXCL12 concentrations. However, real-time PCR studies revealed little change in Cx43 mRNA levels, despite stimulation with different concentrations of CXCL12, indicating CXCL12's effect on Cx43 is post-translational, through phosphorylation. Immunoblot analyses and functional dye exchange studies showed activation of PKC by exogenous CXCL12 in the phosphorylation, which in turn, increased intercellular communication. These findings elucidate the importance of considering the microenvironment's role in micrometastasis in clinical studies pertaining to prospective breast cancer treatment.