Anisomycin downregulates gap-junctional intercellular communication via the p38 MAP-kinase pathway

Phosphorylation of connexin 43 (Cx43) molecules (e.g. by extracellular signal-regulated kinase) leads to reductions in gap-junctional intercellular communication (GJIC). GJIC levels also appear to be lower in the presence of p38 mitogen-activated protein (MAP) kinase, for unknown reasons. In this study, we used assays of the recovery of fluorescence by photobleached WB-F344 cells to demonstrate that GJIC levels are decreased by anisomycin [a protein synthesis inhibitor as well as an activator of p38 MAP kinase and c-Jun N-terminal kinases (JNK)] as a result of time-dependent depletion of the phosphorylated forms of Cx43. Using immunohistochemistry, we also detected far less of the Cx43 proteins at cell borders. These findings agree with the photobleaching assay results. Moreover, prior treatment with SB203580 (a specific inhibitor of p38 MAP kinase) appeared to be effective in preventing the loss of phosphorylated forms of Cx43 and the loss of Cx43 proteins at cell borders. Total protein labelling with [S-35]-methionine and [P-32]-orthophosphates labelling of Cx43 showed that anisomycin enhanced the phosphorylation level of Cx43 along with inhibition of protein synthesis. SB203580 prevented the former but not the latter. The effect of anisomycin on GJIC was not dependent on the inhibition of protein synthesis because the addition of SB203580 completely maintained the level of GJIC without restoring protein synthesis. The Cx43 phosphorylation level increased by anisomycin treatment, whereas the amount of phosphorylated forms of Cx43 decreased, suggesting that activation of Cx43 phosphorylation might lead to the loss of Cx43. These results suggest that activation of p38 MAP kinase leads to reduction in the levels of phosphorylated forms of Cx43, possibly owing to accelerated degradation, and that these losses might be responsible for the reduction in numbers of gap junctions and in GJIC.