Cell coupling mediated by connexin 26 selectively contributes to reduced adhesivity and increased migration.

Gap junction proteins (connexins) have crucial effects on cell motility in many systems, from migration of neural crest cells to promotion of metastatic invasiveness. Here, we show that expression of Cx26 (also known as GJB2) in HeLa cells specifically enhances cell motility in scrape wounding and sparse culture models. This effect is dependent on gap junction channels and is isotype specific [Cx26 enhances motility, whereas Cx43 (also known as GJA1) does not and Cx32 (also known as GJB1) has an intermediate effect]. The increased motility is associated with reduced cell adhesiveness, caused by loss of N-cadherin protein and RNA at the wound edge. This in turn causes a redistribution of N-cadherin-binding proteins (p120 catenin and ß-catenin) to the cytosol and nucleus, respectively. The former activates Rac-1, which mediates cytoskeletal rearrangements needed for filopod extension. The latter is associated with increased expression of urokinase plasminogen activating receptor (an activator of extracellular proteases) and secretion of extracellular matrix components like collagen. Although these effects were dependent on Cx26-mediated coupling of the cells, they are not mediated by the same signal (i.e. cAMP) through which Cx26 has been shown to suppress proliferation in the same system.

Intercellular redistribution of cAMP underlies selective suppression of cancer cell growth by connexin26.

Connexins (Cx), which constitute gap junction intercellular channels in vertebrates, have been shown to suppress transformed cell growth and tumorigenesis, but the mechanism(s) still remain largely speculative. Here, we define the molecular basis by which Cx26, but less frequently Cx43 or Cx32, selectively confer growth suppression on cancer cells. Functional intercellular coupling is shown to be required, producing partial blocks of the cell cycle due to prolonged activation of several mitogenic kinases. PKA is both necessary and sufficient for the Cx26 induced growth inhibition in low serum and the absence of anchorage. Activation of PKA was not associated with elevated cAMP levels, but appeared to result from a redistribution of cAMP throughout the cell population, eliminating the cell cycle oscillations in cAMP required for efficient cell cycle progression. Cx43 and Cx32 fail to mediate this redistribution as, unlike Cx26, these channels are closed during the G2/M phase of the cell cycle when cAMP levels peak. Comparisons of tumor cell lines indicate that this is a general pattern, with growth suppression by connexins occurring whenever cAMP oscillates with the cell cycle, and the gap junction remain open throughout the cell cycle. Thus, gap junctional coupling, in the absence of any external signals, provides a general means to limit the mitotic rate of cell populations.