Heterogeneous vascular dependence of tumor cell populations

Cells within a tumor are highly heterogeneous with respect to a wide range of genotypic and phenotypic characteristics. The latter include such proper ties as growth, survival, invasion, and metastasis, We asked whether the de gree to which individual tumor cells rely on a tumor's vasculature might al so be heterogeneous. By adapting an intravital Hoechst 33342 staining techn ique, we labeled and isolated tumor cells based on their relative proximity to perfused vessels. Because tumor regions distal to the vasculature are l ikely hypoxic, we examined cells deficient for hypoxia-inducible factor-1 a lpha (HIF-1 alpha), a transcription factor that has been shown to mediate h ypoxia-induced responses, including apoptosis. Despite reduced vascularizat ion in HIF-1 alpha (-/-) embryonic stem cell-derived tumors, their growth i n vivo was found to be accelerated relative to HIF-1 alpha (+/+) tumor coun terparts. We hypothesized that this paradoxical observation is because of d ecreased apoptotic rate, resulting in diminished vascular dependence of HIF -1 alpha (-/-) cells. Analysis of heterogeneous tumors established from mix tures of HIF-1 alpha (-/-) with HIF-1 alpha (-/-) cells revealed that the p roportion of cells expressing wildtype HIF-1 alpha was increased in perivas cular areas and decreased in distal tumor regions. Thus, cells expressing H IF-1 alpha were found to be highly dependent on proximity to blood vessels for their growth and survival in vivo, whereas cells that had lost HIF-1 al pha expression were much less so. Heterogeneity in angiogenesis dependence was also observed among cell subpopulations Isolated from human melanoma xe nografts. This potential for selection of less vascular-dependent tumor cel l variants throughout the course of disease progression may have important implications for the long-term efficacy of anti-angio-genic therapy.