Title: Contributions of phosphoinositide-3-kinase to tumorigenesis both inside and outside the tumor
Description: The goal of personalized medicine is to identify the mutated gene that drives a tumor and to treat the patient with a drug that targets the corresponding protein. To achieve this goal, targeted inhibitors of many oncogenes are in development, including inhibitors of phosphoinositide-3-kinase (PI3K). Overactivation of the PI3K pathway is one of the most frequent events in cancer. PI3K generates phosphatidylinositol-3,4,5-triphosphate (PI-3,4,5-P 3 or PIP 3 ) at cell membranes, which acts as a docking site for many proteins including the nodal kinase, AKT. At the membrane, AKT facilitates cell proliferation, growth, survival and metabolism. Upregulation of this pathway in cancer cells thus facilitates tumorigenesis in multiple ways. PI3K is also an important enzyme in non-cancerous stromal cells in the tumor microenvironment. The stromal compartment contains endothelial cells, immune cells and fibroblasts, all of which have been shown to utilize the PI3K-AKT pathway. Activation of this pathway both inside and outside of the tumor offers the unique prospect of using a single agent to attack tumors on multiple fronts.

To investigate this possibility, we depleted endothelial cells of class IA PI3K activity using a conditional mouse model. We assessed the effects of PI3K-loss on development and tumor angiogenesis and found that PI3K signaling is critical for the maintenance of vessel integrity. This study thus reveals potential antiangiogenic benefits of using PI3K inhibitors to treat solid tumors. Single agent therapies generally will not be effective in most tumors. Genetic and non-genetic heterogeneity have been observed in many cell populations and results in differential sensitivity to anti-cancer agents. Recent preclinical studies demonstrate that PI3K inhibition leads to cytostasis and delayed growth but not tumor regression. To address the possibility that cell-to-cell variability in PI3K activity contributes to this incomplete response, we undertook an analysis of PI3K signaling on the single cell level in normal and oncogenic mammary epithelial cells. We found robust heterogeneity in PI3K activation, which is regulated by modulation of PI3K protein levels. These results begin to explain why PI3K inhibitors as single agents do not cause complete tumor regression and emphasize the need to optimize dosing strategies and employ combination therapies.