THP‐1 macrophage membrane‐bound plasmin activity is up‐regulated by transforming growth factor‐β1 via increased expression of urokinase and the urokinase receptor

Publisher Website
Google Scholar
Abstract
Receptors for urokinase (uPA) and plasminogen provide a mechanism to direct the cellular activation of plasminogen. The regulation of these receptors is important for several macrophage functions. In these studies, the effect of transforming growth factor-b̃1 (TGF-b̃1) on uPA, uPA receptor, and plasminogen receptor expression by human THP-1 macrophage was examined. TGF-b̃1 induction of uPA expression by THP-1 cells was differentiation dependent. Suspension and adherent cultures expressed similar constitutive levels of uPA. Exposure of adherent cells to TGF-b̃1 led to a dose- and time-dependent increase in uPA activity which was paralleled by an increase in uPA antigen and uPA mRNA. In contrast, uPA expression by suspension cultures was unresponsive to TGF-b̃1. The differential response exhibited by suspension and adherent THP-1 cells may reflect differences in their expression of TGF-b̃1 receptors, since when assayed by crosslinking techniques, suspension cells primarily expressed a 65 kDa receptor; whereas, the adherent cells expressed 65 and 100 kDa receptors. TGF-b̃1-induced alterations in uPA receptor expression by adherent THP-1 cells were examined by quantitating membrane-bound uPA activity. Membrane-bound uPA activity increased three-fold when cells were incubated with TGF-b̃1. The increase in membrane-uPA activity expressed by TGF-b̃1-treated cells was not due to increased uPA receptor occupancy since incubation of either control or TGF-b̃1 primed cells with exogenous uPA did not lead to an increase in membrane-bound uPA activity. Furthermore, immunoreactive uPA receptor was increased in TGF-b̃1-treated cells. Following incubation with plasminogen, membrane-bound plasmin activity increased three-fold in TGF-b̃1-treated cells. However, no change in immunoreactive membrane-bound plasmin(ogen) was observed. In addition, binding of 125I-Lys-plasminogen to THP-1 cells was not affected by TGF-b̃1 treatment. We conclude that TGF-b̃1 stimulates membrane-bound plasmin activity, without affecting plasminogen receptor expression, through the up-regulation of uPA and the uPA receptor expression.