p16INK4A Represses Breast Stromal Fibroblasts Migration/Invasion and Their VEGF-A-dependent Promotion of Angiogenesis through Akt Inhibition

Abstract

Stromal fibroblasts, the most abundant and probably the most active cellular component of breast cancer-associated stroma, become active and promote angiogenesis through paracrine effects. However, it still unclear how these processes are regulated. Here, we have shown that down-regulation of the tumor suppressor p16INK4A protein enhances the migration/invasion abilities of breast stromal fibroblasts, which form dendritic network of extensions into matrigel. Furthermore, we present clear evidence that p16INK4A represses the expression/secretion of the proangiogenesis protein vascular endothelial growth factor A (VEGF-A). Consequently, p16INK4A-deficient breast stromal fibroblasts and mouse embryonic fibroblasts enhanced endothelial cell differentiation into capillary-like structures in a paracrine manner. This effect was suppressed by adding bevacizumab, a specific VEGF-A inhibitor. Additionally, p16INK4A-defective mouse embryonic fibroblasts enhanced angiogenesis in breast cancer xenografts in mice. Furthermore, we have shown that p16INK4A suppresses the Akt/mammalian target of rapamycin (mTOR) signaling pathway and its downstream effector hypoxia-inducible factor 1-alpha (HIF-1α), which transactivates VEGF-A. Consequently, Akt inactivation suppressed both the p16INK4A-dependent autocrine effect on fibroblast migration/invasion and the paracrine effect on angiogenesis, showing the important role of this protein kinase in mediating the various effects related to p16INK4A deficiency. These results indicate that p16INK4A is an efficient inhibitor of the migration/invasion abilities of breast stromal fibroblasts and also their paracrine proangiogenic effects, through inhibition of Akt. Therefore, pharmacologic restoration of p16INK4A level in stromal fibroblasts may be exploited as therapeutic strategy to help eradicate tumor cells and/or prevent their recurrence, through suppressing cell non-autonomous procarcinogenic mediators.