Evidence of a bi-phasic effect of thrombospondin-1 on angiogenesis.

Contradictory results have been reported regarding the association between vascularity (used as an index of angiogenesis) and thrombospondin-1 (TSP-1) in human tumours. In previous studies, the reported association was based on the estimated average TSP-1 value per tumour, with a sufficient number of specimens collectively analysed per tumour type. Given the extent of intra-tumour heterogeneity, we determined the association between TSP-1 and vascularity within individual specimens, based on the average values of TSP-1 and vascularity in 10-20 pre-selected areas per tumour. Cells expressing TSP-1 mRNA were visualised by in situ hybridisation and quantified by point counting. Vascularity was quantified by point counting and vessel density of von Willebrand Factor-positive vessels. In 10 ductal breast carcinomas, a direct correlation between TSP-1 and vascularity was found in 4 tumours, no correlation in 3 and an inverse correlation in 3. The effect of TSP-1 on endothelial cell migration in vitro was assessed in the Boyden chamber assay. TSP-1 stimulated cell migration at low concentrations (0.1-10 microg/ml) and was inhibitory at high concentrations (25-100 microg/ml). These results suggest that TSP-1 may elicit a concentration-dependent, bi-phasic, effect on angiogenesis.