Thrombin modulates vectorial secretion of extracellular matrix proteins in cultured endothelial cells

We have identified a novel cellular action of thrombin on cultured rat adrenal medullary endothelial cells (RAMEC). Five-minute incubation of Ramec with physiological concentrations of thrombin (<1 U/ml) caused within 3 h an increase in the basolateral deposition of the extracellular matrix (ECM) proteins fibronectin, laminin, and collagens IV and I, concomitant with a corresponding decrease in the apical release of these proteins into the medium. This shift in vectorial secretion of Ecm proteins, quantitated with enzyme-linked immunoassays, was time dependent. Maximal stimulation of Ecm protein deposition was observed after incubation of cells with thrombin for 5-15 min. Prolonged exposure (>1 h) to thrombin resulted in loss of proteins from the Ecm. Thrombin-stimulated Ecm protein deposition exhibited a bell- shaped dose dependence, peaking for all proteins at 0.25 U/ml of thrombin, and was independent of de novo mRNA or protein synthesis. Maximal amounts of deposited proteins increased between 2.5-fold (fibronectin) and 4-fold (collagen I) over baseline values. Similar results were obtained with thrombin receptor agonist peptide (TRAP), proteolytically active γ- thrombin, and, to a lesser extent, other serine proteases such as trypsin and plasmin. A scrambled TRAP, proteolytically inactive PPACK-thrombin, DIP- thrombin, and type IV collagenase were ineffective. Together, these results suggest that the thrombin effects are mediated by proteolytic activation of the thrombin receptor. Possible involvement of the phospholipase C-signaling pathway in thrombin-mediated ECM protein deposition was also investigated. Inhibition or downregulation of protein kinase C (PKC) and chelation of intracellular or extracellular Ca²⁺ did not suppress, but rather enhanced, basal and thrombin-stimulated ECM protein deposition. Quantitative differences in augmentation of basolateral deposition by these treatments suggest differential regulatory pathways for individual ECM proteins. Our data indicate that, in cultured RAMEC, short-term activation of the thrombin receptor causes an increase in amounts of deposited ECM protein by a cellular signaling pathway that is independent of PKC activation and/or elevation of intracellular Ca²⁺.