Transforming growth factor-beta induces plasminogen activator inhibitor type-1 in cultured human orbital fibroblasts

The level of constitutive plasminogen activator inhibitor type-1 (PAI-1) expression in cultured human orbital fibroblasts is considerably lower than that found in dermal fibroblasts. This divergence in PAI-1 expression implies differences in the pericellular proteolytic environment and, therefore, in the turnover of extracellular matrix. In this article, the authors examine the effect of transforming growth factor-beta (TGF-beta) on PAI-1 expression in orbital fibroblasts.

Human orbital and dermal fibroblasts were grown in culture. Confluent monolayers were treated with TGF-beta. PAI-1 in the extracellular matrix was quantitated by radiolabeling the cultures and electrophoresing the cellular material on SDS-PAGE. Medium content was determined by immunoprecipitation of [35S]PAI-1 with a rabbit, anti-human, polyclonal antibody. PAI-1 mRNA was determined by Northern hybridization.

TGF-beta increased PAI-1 levels in orbital fibroblasts in a dose-dependent manner, up to 35-fold. The induction was maximal after 16 hours of treatment. The increases in extracellular matrix PAI-1 paralleled those observed in the medium. The steady state levels of the mRNA encoding the protein were upregulated by TGF-beta up to 60-fold 8 hours after the addition of TGF-beta. The fractional increase in PAI-1 expression in orbital fibroblasts was consistently greater than that observed in dermal strains.

Exposure to TGF-beta consistently induces PAI-1 expression in orbital fibroblasts, cells that do not express the polypeptide constitutively at high levels. The effects are mediated at the pretranslational level and involve the upregulation of PAI-1 mRNA. These results suggest that TGF-beta may exert a profound regulatory influence on the pericellular proteolytic environment in orbital connective tissue.