In vivo mechanisms by which tumors producing thrombospondin 1 bypass its inhibitory effects

Thrombospondin 1 (TSP1) is a multifunctional protein able to activate TGF b eta and to inhibit angiogenesis in vivo. Although usually thought of as an inhibitor of tumor growth, TSP1 may sometimes be present at high levels dur ing tumor progression, suggesting that tumors can eventually overcome their anti-tumor effects. Using a tet-repressible expression system, we demonstr ate that murine TSP1 delayed the onset of tumor growth when produced in the tumor bed by rat fibrosarcoma tumor cells or by stromal fibroblasts coinje cted with unmodified Cb glioma tumor cells. Yet upon prolonged exposure to TSP1, tumors came to grow at the same rate in the presence as in the absenc e of TSP1 and transplantation experiments showed that they had become insen sitive to inhibition by TSP1 in both syngeneic and immune compromised hosts . Tumor resistance to TSP1 developed as a result of the in vivo outgrowth o f pre-existing tumor cell variants that (1) secreted increased amounts of a ngiogenic factors that counterbalanced the inhibitory effect of TSP1 on neo vascularization and (2) grew more efficiently in the presence of TSP1-activ ated TGE beta. These results indicate that prolonged and continuous local d elivery of a single multifunctional angiogenesis inhibitor like TSP1 to fas t-growing tumors can lead to tumor resistance in vivo by fostering the outg rowth of subpopulations that are a by-product of the genetic instability of the tumor cells themselves.