Understanding and Targeting Tiam1 in Cancer

Project Summary: Understanding the detailed mechanisms that underlie proliferative signal transduction pathways is of critical importance for developing strategies for diagnosis and treatment of human cancer. Because most proliferative pathways are critical for the functioning of all normal cells, the development of cancer-cell-selective interventions

without off-target effects is tremendously important yet highly challenging. The research proposed in this grant application focuses on unique ‘players’ in proliferative signaling, the guanine nucleotide exchange factors (GEFs) from the Dbl (diffuse B cell lymphoma) family. GEFs control proliferative signaling by stimulating nucleotide exchange on the small GTPases, Rho, Rac, and

Cdc42. In doing so, GEFs regulate numerous cellular activities such as gene expression, cytoskeletal rearrangements, protein synthesis, and metabolism. Gain-of-function mutations in GEFs are associated with multiple human cancers, and GEFs from the Dbl family constitute one of the largest families of proto-oncogenes.

Although different Dbl-family GEFs share similar mechanisms of action, their expression is extremely tissue- and cell-specific, and their respective mutated forms are associated with distinct and different cancers. Thus, intervention with GEF signaling may be extremely useful in multiple, seemingly unrelated malignant diseases.

The proposed work relies on our recent discovery of a novel tyrosine phosphorylation sequence motif (TEXXYVXXL) that regulates the activity of some Dbl-like GEFs implicated in human cancers. We hypothesize that selective interreference with this phosphorylation comprises a unique, novel and effective selective

intervention approach in relevant cancers. This proof-of-principle proposal focuses on Tiam1, a GEF whose dysregulation drives colorectal tumorigenesis and metastasis. We propose to decipher the molecular mechanisms and functional outcomes of TEXXYVXXL phosphorylation in Tiam1, and to test novel reagents that

target Tiam1’s tyrosine phosphorylation as selective intervention tools. In Specific Aim 1 we will decipher the role of TEXXYVXXL phosphorylation in regulating Tiam1’s key activities, i.e. GTPase activation, cell invasion and proliferation in vitro, and tumorigenesis and metastasis in vivo. In Specific Aim 2, we will evaluate the utility of

novel reagents that target Tiam1’s TEXXYVXXL phosphorylation site for intervention in a mouse model of colon cancer. These proof-of-concept experiments in cells and animals will open the door for future development and translational work in GEF-associated malignancies.