Inhibition of gap-junctional intercellular communication and enhanced binding of fibronectin-coated latex beads by stimulation of DNA synthesis in quiescent 3T3-L1 cells

Connexins: sensors and regulators of cell cycling

It is nowadays well established that gap junctions are critical gatekeepers of cell proliferation, by controlling the intercellular exchange of essential growth regulators. In recent years, however, it has become clear that the picture is not as simple as originally anticipated, as structural precursors of gap junctions can affect cell cycling by performing actions not related to gap junctional intercellular communication. Indeed, connexin hemichannels also foresee a pathway for cell growth communication, albeit between the intracellular compartment and the extracellular environment, while connexin proteins as such can directly or indirectly influence the production of cell cycle regulators independently of their channel activities. Furthermore, a novel set of connexin-like proteins, the pannexins, have lately joined in as regulators of the cell proliferation process, which they can affect as either single units or as channel entities. In the current paper, these multifaceted aspects of connexin-related signalling in cell cycling are reviewed.

Quiescent mammary epithelial cells have reduced connexin43 but maintain a high level of gap junction intercellular communication

Gap junctions have been implicated in growth control, but it remains unclear whether cells that enter a quiescent state continue to express connexins and maintain a high level of gap junction intercellular communication (GJIC). To this end, MAC-T cells, a bovine mammary epithelial cell line, were serum starved for 48 h to induce a quiescent (G0) state. In quiescent cells, [3H]thymidine incorporation was reduced by 97.3% from serum-fed controls. Western blotting in conjunction with Phosphorlmager analysis revealed up to a 20-fold decrease in the expression of the gap junction protein connexin43 (Cx43 or alpha 1) and a shift toward the unphosphorylated form in quiescent cells. However, cell-to-cell transfer of the gap junction-permeable fluorescent tracer, Lucifer yellow, was only moderately reduced in quiescent cells. In control cells, Cx43 was predominantly perinuclear, although it was also present at sites of cell-cell apposition. In quiescent cells, intracellular labeling for Cx43 decreased without a corresponding reduction at areas of cell-cell contact. Recovery from serum deprivation resulted in increased thymidine incorporation that corresponded with an elevation in Cx43 protein expression and phosphorylation. In parallel studies, MAC-T cells were also induced to enter a quiescent state through contact inhibition. Despite a 20-fold reduction in 5-bromo-2'-deoxyuridine and a substantial reduction in intracellular Cx43, contact inhibited MAC-T cells also maintained gap junctions and GJIC. These experiments demonstrate that the maintenance of dye coupling in quiescent mammary cells is correlated with a redistribution of intracellular stores of Cx43.