Lipoprotein receptor–mediated induction of matrix metalloproteinase by tissue plasminogen activator

Although thrombolysis with tissue plasminogen activator (tPA) is a stroke therapy approved by the US Food and Drug Administration, its efficacy may be limited by neurotoxic side effects1,2. Recently, proteolytic damage involving matrix metalloproteinases (MMPs) have been implicated. In experimental embolic stroke models, MMP inhibitors decreased cerebral hemorrhage and injury after treatment with tPA3,4. MMPs comprise a family of zinc endopeptidases that can modify several components of the extracellular matrix5,6. In particular, the gelatinases MMP-2 and MMP-9 can degrade neurovascular matrix integrity. MMP-9 promotes neuronal death by disrupting cell-matrix interactions7, and MMP-9 knockout mice have reduced blood-brain barrier leakage and infarction after cerebral ischemia8. Hence it is possible that tPA upregulates MMPs in the brain, and that subsequent matrix degradation causes brain injury. Here we show that tPA upregulates MMP-9 in cell culture and in vivo. MMP-9 levels were lower in tPA knockouts compared with wild-type mice after focal cerebral ischemia. In human cerebral microvascular endothelial cells, MMP-9 was upregulated when recombinant tPA was added. RNA interference (RNAi) suggested that this response was mediated by the low-density lipoprotein receptor–related protein (LRP), which avidly binds tPA9 and possesses signaling properties10. Targeting the tPA-LRP signaling pathway in brain may offer new approaches for decreasing neurotoxicity and improving stroke therapy.