Interleukin-1 (IL-1) regulation of tPA in hepatocytes was studied in mouse hepatocyte line AAM12. IL-1 induced transient accumulation of tPA mRNA as high as threefold by 2 h after the start of treatment. The cytokine also induced the mRNA for serum amyloid A, a typical acute-phase protein in mice, with more sustained kinetics in a time-dependent manner. In contrast to the induction of mRNA, tPA activity and protein levels in the harvested medium were dramatically diminished by IL-1. IL-1 stimulated the uptake of I-125-tPA by AML 12. This uptake was inhibited by 39-kDa receptor-associated protein (RAP), but not by the sugar mannan. These results revealed that low-density lipoprotein receptor-related protein (LRP), which is known to be a receptor for tPA and to be blocked by RAP, was up-regulated by IL-1. We also demonstrated, for the first time, that IL-1 transiently increased the mRNA level of LRP threefold by 30 min after the start of IL-1 treatment of AML 12. The receptor-mediated endocytosis of tPA by hepatocytes may thus play a crucial role in the down-regulation of fibrinolysis during the acute-phase response.