Paxillin-dependent control of tumor angiogenesis

Abstract

Angiogenesis- the growth of new capillaries from existing vessels- is required for tumor growth; however, tumor vessels exhibit abnormal structure and function, which impairs the targeted delivery of anti-cancer agents. While directional migration of capillary endothelial cells is critical for normal angiogenesis, the mechanism by which oriented capillary cell migration is controlled or how it is deregulated during tumorigenesis is unknown. Recently our lab reported that the focal adhesion protein, paxillin, is required for directional migration of fibroblasts. Endothelial cells also express paxillin and localize it in their focal adhesions. Thus, I set out to analyze whether paxillin influences directional migration of endothelial cells. When the expression of paxillin is knocked down in endothelial cells, this enhances their migration but decreases their directional persistence in vitro and in vivo in migration, angiogenesis and developmental assays. Having confirmed that paxillin plays a central role in controlling oriented capillary cell migration, I then studied the mechanism by which it contributes to normal microvessel network formation and tumor angiogenesis. I found that paxillin knockdown increases microvessel density but causes loss of sprout orientation. These characteristics resemble those of tumor vasculature, and, in fact, studies revealed that tumors inhibit paxillin expression in endothelial cells in vitro and in vivo by secreting soluble factors, such as the potent angiogenic factor VEGF. Mechanistically, paxillin knockdown decreases expression of the VEGF receptor neuropilin 2 (NRP2) but not VEGF receptor 2, and this is mediated by the transcription factor GATA2. Direct knockdown of NRP2 also increases endothelial cell migration and vessel density in vitro and in vivo and these effects are rescued by over expressing paxillin. In summary, these studies have led to the discovery of a new mechanism for control of directional endothelial cell migration during angiogenesis that is mediated by paxillin-NRP2 signaling. Importantly, this previously unknown mechanism is deregulated in tumor angiogenesis, which may contribute to the enhanced, disorganized microvasculature that is hallmark of cancer. These findings also revealed a new function for the focal adhesion protein, paxillin, as a mediator of tumor angiogenesis, and elucidated a novel mechanism for control of the expression of NRP2.