The dark side of hippo signaling: A cancer promoter role

The Hippo signaling pathway regulates organ size and tissue homeostasis. Given this role it is unsurprising that dysregulation of this pathway has implications for cancer progression. A convincing body of literature shows that the Hippo pathway serves a tumor suppressive function with its inactivation leading to massive overgrowth. However, additional studies have also shown that activation of Hippo signaling can promote tumor progression. It remains unknown how a single pathway can produce such diametrically opposed effects. This lack of knowledge is in part due to our inability to make meaningful comparisons from studies which have taken place in a variety of cell types, tissues, and organisms. Recently however, we have published 2 studies using the Drosophila wing disk to study the Hippo pathway and have found that Hippo pathway activation can promote cell migration and invasion while Hippo pathway inactivation leads to overgrowth. Thus we propose here that Drosophila can provide a research platform with which to begin addressing how the Hippo pathway can both enhance and suppress tumor progression due to published pro- and anti-tumor functionalities of the Hippo pathway in the same tissue.

Rho1-Wnd signaling regulates loss-of-cell polarity-induced cell invasion in Drosophila

Both cell polarity and c-Jun N-terminal kinase (JNK) activity are essential to the maintenance of tissue homeostasis, and disruption of either is commonly seen in cancer progression. Despite the established connection between loss-of-cell polarity and JNK activation, much less is known about the molecular mechanism by which aberrant cell polarity induces JNK-mediated cell migration and tumor invasion. Here we show results from a genetic screen using an in vivo invasion model via knocking down cell polarity gene in Drosophila wing discs, and identify Rho1-Wnd signaling as an important molecular link that mediates loss-of-cell polarity-triggered JNK activation and cell invasion. We show that Wallenda (Wnd), a protein kinase of the mitogen-activated protein kinase kinase kinase family, by forming a complex with the GTPase Rho1, is both necessary and sufficient for Rho1-induced JNK-dependent cell invasion, MMP1 activation and epithelial-mesenchymal transition. Furthermore, Wnd promotes cell proliferation and tissue growth through wingless production when apoptosis is inhibited by p35. Finally, Wnd shows oncogenic cooperation with Ras(V12) to trigger tumor growth in eye discs and causes invasion into the ventral nerve cord. Together, our data not only provides a novel mechanistic insight on how cell polarity loss contributes to cell invasion, but also highlights the value of the Drosophila model system to explore human cancer biology.

Wallenda regulates JNK-mediated cell death in Drosophila

The c-Jun N-terminal kinase (JNK) pathway plays essential roles in regulating a variety of cellular processes including proliferation, migration and survival. Previous genetic studies in Drosophila have identified numerous cell death regulating genes, providing new insights into the mechanisms for related diseases. Despite the known role of the small GTPase Rac1 in regulating cell death, the downstream components and underlying mechanism remain largely elusive. Here, we show that Rac1 promotes JNK-dependent cell death through Wallenda (Wnd). In addition, we find that Wnd triggers JNK activation and cell death via its kinase domain. Moreover, we show that both MKK4 and Hep are critical for Wnd-induced cell death. Furthermore, Wnd is essential for ectopic Egr- or Rho1-induced JNK activation and cell death. Finally, Wnd is physiologically required for loss of scribble-induced JNK-dependent cell death. Thus, our data suggest that wnd encodes a novel essential cell death regulator in Drosophila.