THE ROLE OF PLASMINOGEN-ACTIVATOR INHIBITOR TYPE-1 IN THE CLEARANCE OF TISSUE-TYPE PLASMINOGEN-ACTIVATOR BY RAT HEPATOMA-CELLS

The role of plasminogen activator inhibitor type 1 (PAI-1) in the clea rance of tissue-type plasminogen activator (t-PA) by hepatocyte like c ells was studied. Rat (Novikoff) hepatoma cells were able to bind and degrade t-PA in a PAI-1 independent fashion, but PAI-I markedly increa sed the rate of degradation and t-PA/PAI-1 was a more efficient inhibi tor of I-125-t-PA or of I-125-t-PA/PAI-1 degradation than free t-PA. C ompetition studies revealed that the effect of PAI-1 is unlikely to in volve determinants located on the PAI-1 part of the complex: 1) an exc ess of fi ee PAI had no effect on the rate of degradation of I-125-t-P A/PAI-1. 2) Complexes of PAI-1 with urokinase-type PA or with a t-PA m utant lacking the finger and growth factor domains were unable to comp ete for the binding and degradation of free or PAI-l-complexed I-125-t -PA.3) t-PA KHRR296-299AAAA, a mutant which reacts 2 orders of magnitu de slower with PAI-1 than wild type t-PA, behaved similar to wild type t-PA. The clearance via both the PAI-1-dependent and the PAI-1-indepe ndent mechanisms was inhibited by the receptor-associated protein, a g eneral inhibitor of clearance mediated by the LDL receptor-related pro tein. We conclude that t-PA can be cleared by rat hepatoma cells in a PAI-1 independent fashion, but after complex formation with PAI-1, bin ding of BPA to the cells is increased and clearance accelerated. Both mechanisms seem to involve the same receptor.