Together and apart: inhibition of DNA synthesis by connexin-43 and its relationship to transforming growth factor beta

The membrane and channel protein connexin-43 (Cx43), as well as the cytokine transforming growth factor (TGF) beta, suppress proliferative growth in cardiomyocytes and other cell types. Previously we showed that the inhibitory effect of Cx43 is canceled when Cx43 becomes phosphorylated at serine (S) 262 in response to mitogen stimulation. We have now asked if the TGF beta-triggered inhibition of DNA synthesis is associated with changes in Cx43 phosphorylation at S262. Conversely, we investigated if inhibition of DNA synthesis by overexpressed Cx43 is dependent on engaging TGF beta signal transduction. We report that TGF beta acutely prevented mitogen-induced Cx43 phosphorylation at S262, while chronic inhibition of TGF beta signal transduction raised baseline levels of endogenous phospho-S262-Cx43 without affecting total Cx43. Inhibition of baseline TGF beta signal transduction through (a) inhibiting TGF beta receptor I (TGF beta RI) with SB431542, (b) inhibiting TGF beta receptor II (TGF beta RII) by overexpressing dominant-negative (DN)TGF beta RII, (c) inhibiting the downstream signaling mediator Smad2 by overexpressing DN Smad2, each separately increased baseline cardiomyocyte DNA synthesis, but could not reverse DNA synthesis inhibition by overexpressed Cx43. It is suggested that inhibition of cardiomyocyte DNA synthesis by TGF beta/TGF beta RI/II/phospho-Smad2 signaling is mediated, at least in part, by reducing endogenous phospho-S262-Cx43 levels.