Tissue inhibitor of metalloproteinase-3 is associated with neuronal death in reperfusion injury

Programmed cell death occurs in ischemia when cell surface death receptors (DRs) are stimulated by death-inducing ligands (DILs). Matrix metalloproteinases are extracellular matrix-degrading enzymes involved in the shedding of DRs and DlLs from the cell surface. Tissue inhibitor of metalloproteinase-3 (TIMP-3), which is bound to the extracellular matrix, has been shown to promote. apoptosis in cancer cell lines by inhibiting cell surface sheddases. Since apoptosis is an important mechanism of cell death in ischemia, the authors hypothesized that TIMP-3 would be expressed in ischemic neurons that are undergoing programmed cell death. Spontaneously hypertensive rats had a 90-minute middle cerebral artery occlusion with reperfusion. Transcription of TIMP-3 mRNA was measured by quantitative reverse transcription-polymerase chain reaction at 2, 6, 24 and 48 hours after reperfusion. Western blots were used to measure TIMP-3 protein expression. Spatial distribution and production of TIMP-3 was studied by immunohistochemistry at 3, 24, and 48 hours, 5 days, and 3 weeks. DNA fragmentation in cells dying by necrosis and apoptosis was identified with terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate-biotin nick end labeling (TUNEL). After 2 hours of reperfusion, TIMP-3 mRNA increased significantly in both ischemic and nonischemic hemispheres. Western blot analysis confirmed the identity of the TIMP-3, which appeared to be increased on the ischemic side. After 3 hours of reperfusion, TIMP-3 immunostaining was increased in neurons on the ischemic side, and by 24 hours the majority of the ischemic neurons were TIMP-3-positive. Dual-fluorescence staining for TUNEL and TIMP-3 showed that they were coexpressed in many neurons. The results suggest that ischemic neurons express TIMP-3, which may be inhibiting sheddases. The authors propose that TIMP-3 facilitates cell death in ischemic neurons. Further studies are needed to identify the sheddases inhibited by the TIMP-3, and on the effect of inhibition of matrix metalloproteinases on cell death mechanisms.