The fruit fly, Drosophila melanogaster, is a well-established model system to define conserved mechanisms that control proliferation, survival, invasiveness, and stem-like qualities of developing epithelia. A number of clinically significant oncogenes and tumor suppressors have been discovered and defined in flies (e.g. Yki/Yap1, Notch and archipelago/Fbw7)1-3, and multiple Drosophila tumor models are currently being used as discovery platforms to identify potential therapeutic compounds and cocktails4. The Drosophila Taiman (Tai) and Yorkie (Yki) transcriptional co-activator proteins are the sole fly orthologs of the well-established human oncoproteins SRC-3 and Yap1. Tai is the prime coactivator for the ecdysone receptor (EcR), a functional homolog of nuclear hormone receptors, that controls many aspects of epithelial and stem cell development5. Yki is the main transcriptional effector of the Hippo pathway. Our laboratory discovered that Yki and Tai physically associate on target gene promoters and that this Tai-Yki complex enables crosstalk between the Hippo and steroid hormone pathways during developmental and pathologic growth in Drosophila6. The current project is focused on a role for the Tai-Yki axis in homeostatic regrowth of wing epithelium following wounding, and thus lies at the intersection of wound healing and cancer noted by Harold Dvorak?s prescient description of cancer as ?wounds that do not heal?7. I have found preliminary evidence that transient and local reduction in the ability of cells surrounding epithelial wounds to synthesize 20-hydroxyecdysone (20HE), the steroid ligand of EcR, inhibits wound repair and regeneration. In parallel, I have found that EcR transcriptional activity is also significantly induced within cells surrounding the wound. These data, in light of the Tai-Yki complex, could link 20HE/EcR steroid signaling to Yki/Yap1-driven growth at the site of the wound. Indeed, Tai supports expression of certain Yki pro-growth targets in uninjured discs that are upregulated during regeneration (e.g. dIlp8). Thus, I will test the hypothesis that the steroid hormone 20HE is required for imaginal wing disc regeneration through promoting transcriptional activity of Tai-dependent Yki targets.
The Specific Aims of this project are: 1) test the requirement for 20HE biosynthesis in wing disc regeneration; 2) define the pattern of EcR activity in regenerating wing discs; 3) test the local requirement for the Tai-Yki axis in regenerative growth.
In Aim 1, I will compare the extent of regeneration of wing discs depleted of 20HE biosynthesis genes to control discs, and determine the pattern of 20HE biosynthesis gene expression in regenerating imaginal wing discs.
In Aim 2, I will place EcR activation into the context with other pathways activated during regeneration.
In Aim 3, I will determine the effect of disrupting Tai-Yki association on the extent of regeneration and Tai-dependent Yki target expression in wing discs. Success in these studies will provide insight into how local hormone synthesis regulates homeostatic wound repair and, potentially, pathologic cancer growth.
Evidence suggests that molecular mechanisms that guide tissue regrowth during wound repair can become de-regulated during cancer. My goal is to develop a better understanding of how local hormone signaling supports wound repair through interactions between conserved molecular pathways. Success in these studies would provide insight into how local hormone signaling could be a key regulator of homeostatic wound repair and, potentially, pathologic cancer growth.