Regulation of connexin-43 gap junctional intercellular communication by mitogen-activated protein kinase

Activation of the Ras/Raf/mitogen-activated protein kinase kinase/mitogen-activated protein (MAP) kinase signalling cascade is initiated by activation of growth factor receptors and regulates many cellular events, including cell cycle control. Our previous studies suggested that the connexin-43 gap junction protein may be a target of activated MAP kinase and that MAP kinase may regulate connexin-48 function. We identified the sites of MAP kinase phosphorylation in in vitro studies as the consensus MAP kinase recognition sites in the cytoplasmic carboxyl tail of connexin-43, Set²⁵⁵, Ser²⁷⁹, and Ser²⁸². In this study, we demonstrate that activation of MAP kinase by ligand-induced activation of the epidermal growth factor (EGF) or lysophosphatidic acid receptors or by pervanadate-induced inhibition of tyrosine phosphatases results in increased phosphorylation on connexin-43. EGF and lysophosphatidic acid-induced phosphorylation on connexin-43 and the down-regulation of gap junctional communication in EGF-treated cells were blocked by a specific mitogen-activated protein kinase kinase inhibitor (PD98059) that prevented activation of MAP kinase. These studies confirm that connexin-43 is a MAP kinase substrate in vivo and that phosphorylation on Ser²⁵⁵, Ser²⁷⁹, and/or Ser²⁸² initiates the down-regulation of gap junctional communication. Studies with connexin-43 mutants suggest that MAP kinase phosphorylation at one or more of the tandem Ser²⁷⁹/Ser²⁸² sites is sufficient to disrupt gap junctional intercellular communication.