UC San Diego

Breast tumors are often identified based on their apparent hardness compared to normal breast tissue, and in breast cancer patients an increase in tissue rigidity is often correlated with an increase in metastasis. When human mammary epithelial cells are grown in 3D culture with the matrix stiffness of breast tumors, they develop a more malignant phenotype. The link between increased tissue rigidity and invasion and metastasis at the molecular level is not well described and is the focus of this dissertation. The transcription factor Twist1 is a key regulator of metastasis through its ability to induce Epithelial-Mesenchymal Transition (EMT), a developmental program also used by cancer cells to invade and metastasize. Previous research in the Yang lab has described a mechanosensing pathway in which Twist1 nuclear translocation is stimulated by increases in matrix rigidity. This nuclear translocation is controlled by phosphorylation of Twist1 on a tyrosine residue which decreases the interaction between Twist1 and its cytoplasmic binding partner G3BP2, allowing Twist1 to enter the nucleus where it can induce EMT. Tyrosine kinase 2 (Tyk2) was identified in a knockdown screen, where loss of Tyk2 was shown to increase nuclear Twist at lower rigidities. This dissertation characterizes the role of Tyk2 in regulating the Twist1-mechanosensing pathway. I describe how Tyk2 is affecting known components of the Twist1-mechanosesning pathway. I also show that Tyk2 activity and subcellular localization are regulated by rigidity. Tyk2 localizes to the plasma membrane at low rigidities and becomes cytoplasmic at high rigidities. This localization change for Tyk2 is recapitulated in human breast tissue samples. Tyk2 is membranous in normal breast tissue and low grade ductal intraepithelial neoplasia (DIN) lesions, which have softer ECM signatures. In ductal carcinoma in situ (DCIS) and breast tumors samples, which have stiffer ECM signatures Tyk2 is cytoplasmic. These findings indicate that Tyk2 is a negative regulator of Twist1 translocation in response to stiffness. This work also reveals a novel role for Tyk2 loss of function in breast cancer invasion and metastasis.