Addition of recombinant TSP-1 to proliferating endothelial cells blocks proliferation and induces apoptosis. (a) Recombinant human TSP-1 was added to rapidly proliferating HMEC-1 cells grown on 96-well plates in the presence of 10% FCS, and cell numbers were assessed by performing a WST-1 assay after 24 hours of further culture. The addition of TSP-1 suppressed proliferation in a dose-dependent manner. (b) In other experiments, proliferating HMEC-1 cells were treated with TSP-1 or conditioned media from cultured HMEC-1 cells and then lysed after 16 hours for Western blot analysis for cleaved caspase 3. Exposure to both TSP-1 and hypoxia-conditioned media led to elevation of cleaved caspase 3 in the cells, consistent with the induction of apoptosis. *P < 0.04. **P < 0.03.