SEQUENTIAL DEVELOPMENT OF AN ANGIOGENIC PHENOTYPE BY HUMAN FIBROBLASTS PROGRESSING TO TUMORIGENICITY

As normal cells progress to malignancy they must acquire an angiogenic phenotype that will enable them to attract the blood vessels necessar y to support their progressive growth. Here we define the mechanism by which human fibroblasts cultured from Li Fraumeni patients and progre ssing to tumorigenicity in vitro become angiogenic. Initially cells we re anti-angiogenic due to the secretion of high levels of inhibitory t hrombospondin that overrode the modest amounts of the major inducer, v ascular endothelial cell growth factor (VEGF), that were also produced . Cells became fully angiogenic in two steps, the first dependent on t he loss of both alleles of wild-type p53 which caused a drop of at lea st 20-fold in secreted thrombospondin and a fourfold increase in secre ted VEGF. Angiogenic activity increased again upon transformation by a ctivated ras due to a further twofold increase in VEGF. Changes in rel ative levels of VEGF mRNA were sufficient to account for changes in se creted protein levels and in overall angiogenic activity. These studie s demonstrate that an angiogenic phenotype able to support tumorigenic ity can arise in a step-wise fashion in response to both oncogene acti vation and tumor suppressor gene loss and involve both a decrease in t he secretion of inhibitors and the sequential ratcheting up of the sec retion of inducers of angiogenesis.