While it is known that carcinogenesis stems from unregulated epithelial proliferation and invasion, additional information is needed about the mechanistic events controlling these two critical processes. Uncontrolled Shh target gene induction promotes growth and invasion of up to 25% of human cancers. While previous studies indicate that the Ci/Gli family of zinc finger transcription factors mediates the transcriptional effects of Shh, the mechanisms for Shh-dependent growth and migration/invasion during tumorigenesis remain poorly understood. We have identified a novel Shh pathway member, Missing in Metastasis (MIM), whose levels are altered in a variety of epithelial cancers. MIM cooperates with the Gli oncogene to recapitulate Shh-mediated epithelial proliferation and invasion. MIM has two separable functions: regulation of Shh target gene induction, and control of stimuli-induced cellular migration. MIM stimulates maximal Shh target gene induction by binding to and antagonizing Sufu, the central negative regulator of the pathway. MIM regulates cellular migration by assembling protein complexes that control cortical actin filament assembly. Normal migration of Drosophila primordial germ cells (PGCs) requires precise cytoskeletal response to local tissue migratory cues that include mesoderm-derived hedgehog. We have shown that Drosophila MIM (dMIM) mutant PGCs fail to respond to these local cues and metastasize to ectopic locations. The goal of this proposal is to dissect these documented MIM functions using human and Drosophila tissue models.
We aim to 1) elucidate the mechanism of MIM potentiation of Gli transcription by assaying MIM activity in different states of the Shh pathway, determine which Sufu activity MIM antagonizes and identify the Sufu protein surface with which MIM interacts;2) establish the role of MIM in a human model of Shh-dependent tumorigenesis by determining which MIM function contributes to Shh-dependent human carcinogenesis, and how MIM contributes to Shh-dependent tumor induction and maintenance;3) analyze dMIM function in PGC migration by categorizing cytoskeletal changes in dMIM mutant PGCs, identify the genetic and biochemical interactions of dMIM with other cytoskeletal regulators, and identify dMIM functional domains through the generation of additional dMIM missense alleles. The funding of this proposal will lead to a greater understanding of the mechanisms of epithelial growth and invasion by Shh signaling and may lead to new targets for therapeutic intervention.
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