Emerging evidence indicates that endothelial cells (ECs) not only form the passive building blocks of blood vessels that deliver oxygen and nutrients, but also instructively participate in organ regeneration and tumorigenesis by producing tissue-specific angiocrine factors. Due to a lack of unbiased, functional angiogenic models, the role of ECs in the homeostasis of tissue-specific stem cells and propagation of malignant cells is unknown. We established a means to maintain primary EC cultures by introducing phospho-ser473 Akt, enabling their survival for weeks under serum-/cytokine-free conditions. This lentiviral-based system maintains the angiogenic repertoire without immortalization and increased tumorigenic potential. Using our novel cytokine-/serum-free in vitro EC-based culture system, we have shown that ECs are endowed with the capacity to expand and maintain bona fide hematopoietic stem cells (HSCs) and survival of leukemic cells. This unbiased system described here can serve as a platform to identify EC-derived growth and to model treatment of a wide variety of hematological and malignant conditions.