Receptor-mediated effect of thrombin on plasminogen activator inhibitor-1 synthesis in rat osteoblast-like cells

The effect of thrombin on components of the plasminogen activator (PA)/plasmin pathway in osteoblast-like cells has been investigated. Thrombin was found to increase PA inhibitor-1 (PAI-I) mRNA and protein in a time- and dose-dependent manner. Tissue-type plasminogen activator (t-PA) activity was inhibited in a time- and dose-dependent manner but there was no alteration of t-PA steady-state mRNA levels. Therefore it is likely that the inhibition of t-PA activity was due entirely to the inhibition of t-PA by PAI-1. Thrombin induced a small but consistent transient increase in both urokinase-type plasminogen activator (u-PA) and u-PA receptor mRNA levels but no u-PA activity was detectable by the amidolytic assay used here. The thrombin and u-PAI inhibitor protease nexin (PN-I), present in high levels constitutively in these cells, was found to be unaltered by thrombin treatment. The effects of thrombin on PAI-1 mRNA and protein and t-PA activity appear to be mediated by the thrombin receptor since the thrombin receptor activating peptide, SFLLRN (single letter code for amino acids), was able to mimic the effects of thrombin. Thrombin has been reported to induce prostaglandin synthesis in rat osteoblasts, and several actions of thrombin on osteoblasts have been reported to be partially dependent on prostaglandin synthesis. However, the effects of both thrombin and SFLLRN on PAI-I protein were found to be independent of prostaglandin synthesis.