Up-regulation of connexin43 correlates with increased synthetic activity and enhanced contractile differentiation in TGF-β-treated human aortic smooth muscle cells

Up-regulation of the gap-junctional protein connexin43 (Cx43) in arterial smooth muscle cells (SMCs) features in response to injury and in atherosclerosis, in parallel with phenotypic transition to the synthetic state. TGF-β1 is known to have a role in SMC differentiation and extracellular matrix (ECM) synthesis, key characteristics of phenotypic state. Here, we set out to examine the effects of TGF-β1 on Cx43-gap junction expression in relation to SMC differentiation, ECM synthesis and growth. Cx43 expression was analysed by immunoconfocal microscopy and Western blotting in primary human aortic SMCs treated with TGF-β1 over a 48-h period, with assessment of gap-junctional communication by cell-to-cell transfer of microinjected ethidium bromide. In parallel, synthetic activity was analysed by Northern blotting for ECM components α-1(I) and α1(III) procollagen transcripts, contractile differentiation was assessed by immunoconfocal microscopy and Western blotting of the markers smooth muscle α-actin, calponin and smooth muscle heavy chain isoform 1 (SM1), and growth was measured by BrdU incorporation. Our results demonstrate that TGF-β1 significantly up-regulates Cx43 expression and intercellular communication, in concert with increased expression of α-actin, calponin and SM1. Concomitant with contractile protein expression, ECM synthesis was increased rather than decreased, TGF-β1 inducing a significant up-regulation of both procollagen transcripts. These effects were independent of growth. We conclude that in human aortic SMCs, TGF-β1 treatment leads to up-regulation of Cx43-mediated gap-junctional communication and increased synthetic activity yet, somewhat paradoxically, also enhanced contractile differentiation.