The role of Sprouty4 in development and cancer and functional consequences of Sprouty interacting with Caveolin-1

intracellular signaling pathways that ultimately determine cell fate. The coordinated action of both positive and negative modulators is essential for spatial and temporal control of these pathways to avoid developmental defects and malignant transformation. Sprouty (Spry) proteins are a family of ligand-induced regulators of RTK-mediated intracellular cascades. In mammals, there are four isoforms. Their activity targets signal transduction induced by various growth factors, suggesting that their activities have broad biological consequences. Spry proteins have the capacity to attenuate or potentiate different pathways depending on the Spry isoform, the growth factor and the cellular context. In this study I investigate the functional consequences of Spry interacting with Caveolin-1. I demonstrate that in the context of FGF2 and EGF signaling, Caveolin-1 differentially modulates the function of the four isoforms. This suggests that Caveolin-1 might contribute to the divergence among the four isoforms and, depending on its own expression pattern, could contribute to the cell context-dependency of Spry proteins. In a second part of my studies I assess the role of mSpry4 in murine pancreas development. Analyzing this organ for the presence of Spry proteins, I detect very specific expression of mSpry4 in one population of endocrine cells, namely in the α cells. In the pancreas, different endocrine cell types are organized within islets of Langerhans. The insulin-secreting β cells occupy the core of the islets and are surrounded by α, δ and PP cells, secreting glucagon, somatostatin and pancreatic polypeptide, respectively. Using a doxycycline-inducible transgenic system I conditionally express mSpry4 in the insulinproducing β cells and demonstrate that ectopic mSpry4 interferes with proper segregation of the endocrine cells. Employing the endocrine precursor cell line PANC-1 as an in vitro model for islet formation I find that mSpry4 inhibits migration and adhesion of these cells, probably by interfering with the localization PTP1B, a mediator of integrin signaling. Furthermore, the role of mSpry4 in tumorigenesis was assessed using the well established Rip1Tag2 mouse model. In these mice, transgenic expression of large T antigen results in β cell carcinogenesis. We find that expression of mSpry4 in β cells has a moderate effect on tumor formation. Experiments with isolated tumor cells imply that transformation of β cells by means of large T antigen results in constitutive activation of p42/44 ERK signaling by a mechanism that renders mSpry4 ineffective as an inhibitor of this pathway.