Transforming growth factor-beta regulates increased ductus arteriosus endothelial glycosaminoglycan synthesis and a post-transcriptional mechanism controls increased smooth muscle fibronectin, features associated with intimal proliferation

Abstract

Background: In previous studies we established that there are developmentally regulated increases in endothelial hyaluronan (HA) and heparan sulfate (HS), and smooth muscle cell fibronectin (FN) related to the formation of intimal cushions, structures essential to the postnatal closure of the ductus arteriosus (DA). In this report, we investigated the mechanisms underlying these features to ascertain whether they were independently or coordinately regulated.

Experimental design: We determined by assessing HA polymer size and by pulse labeling with [3H]glucosamine whether the increased glycosaminoglycans (GAGs) incorporated in DA compared with aorta (Ao) endothelial cell matrices reflected increased synthesis of HA and HS rather than decreased degradation. We assessed whether transforming growth factor-beta (TGF-beta) may be responsible for the increased DA endothelial GAGs and smooth muscle FN production by confirming the presence of TGF-beta in DA tissue using immunohistochemistry and by assessing the effect of adding neutralizing antibodies to the cell cultures. We next determined whether the level of regulation of the increase in FN in DA smooth muscle cells was transcriptional or post-transcriptional by relating protein synthesis to steady state mRNA levels and to mRNA levels after serum stimulation. Using northern blot analyses with specific probes, we also explored the possibility that the FN produced by the DA and Ao was qualitatively different in the proportion of isoforms containing the V95+ region associated with secretion or the EIIIB+ region that has been related to migration.

Results: We observed that HA polymer size produced by DA and Ao endothelial cells was similar and we further verified using pulse labeling that the increase in DA compared with Ao endothelial GAGs reflected increased synthesis of HA and HS rather than decreased degradation. There was increased immunostaining for TGF-beta in DA compared with Ao tissue and we showed that TGF-beta neutralizing antibodies reduced synthesis of GAGs by the DA endothelial cells to the level of that seen in the Ao cells, but did not reduce DA smooth muscle cell FN synthesis. The increase in FN synthesis by DA compared to Ao smooth muscle cells was not associated with increased levels of steady-state mRNA for FN. Furthermore, following serum-stimulated increases in FN mRNA, the DA yielded greater amounts of FN protein compared to Ao smooth muscle cells. The increased FN production by the DA smooth muscle cells could not be attributed to a relative lack of degradation of FN protein or mRNA, or to qualitative differences which might influence secretion, as both cell types contained similar proportions of EIIIB+ and V95+ isoforms of FN.

Conclusions: These results would suggest that, in contrast to the TGF-beta dependent increase in DA endothelial GAG synthesis, the increase in DA smooth muscle FN synthesis arises through differences in post-transcriptional regulation that are likely independent of TGF-beta.