Numb exon 9 inclusion regulates Integrinβ5 surface expression and promotes breast cancer metastasis

The endocytic adaptor protein Numb acts as a tumor suppressor through downregulation of oncogenic pathways in multiple cancer types. The identification of splicing alterations giving rise to changes in Numb protein isoform expression indicate that Numb also has tumor promoting activity, though the underlying mechanisms are unknown. Here we report that NUMB exon 9 inclusion, which results in production of a protein isoform with an additional 49 amino acids, is a feature of multiple cancer types including all subtypes of breast cancer and correlates with worse progression-free survival. Specific deletion of exon 9-included Numb isoforms (Exon9in) from breast cancer cells reduced cell growth and prevents spontaneous lung metastasis in a mouse model. Quantitative proteome profiling showed that loss of Exon9in causes downregulation of membrane receptors and adhesion molecules, as well as proteins involved in extracellular matrix organization and the epithelial-mesenchymal transition (EMT) state. In addition, exon 9 deletion caused remodeling of the endocytic network, decreased ITGβ5 surface localization, cell spreading on vitronectin and downstream signaling to ERK and SRC. Together these observations suggest that Exon9in isoform expression disrupts the endocytic trafficking functions of Numb, resulting in increased surface expression of ITGβ5 as well as other plasma membrane proteins to promote cell adhesion, EMT, and tumor metastasis.