Molecular Dissection of Receptor Tyrosine Kinase Regulation by Cbl Proteins

Abstract

Aberrations of receptor tyrosine kinases (RTKs) have been implicated in the etiology of human cancers. Members of the Casitas B‐lineage Lymphoma (Cbl) family of ubiquitin ligases play a role as negative regulators of RTK function and mutations of Cbl proteins are involved in human cancers such as leukemia. The purpose of studies here is to determine the molecular mechanisms of oncogenic signaling and to explore possible defects in negative regulation of RTK signaling by Cbl proteins. We hypothesize that Cbl and Cbl‐b represent redundant yet essential components to facilitate lysosomal degradation of RTKs. In initial studies, we have derived wildtype, Cbl‐null, Cbl‐b‐null and cbl/cbl‐b‐null mouse embryonic fibroblast (MEF) cell lines to demonstrate an important role of both Cbl and Cbl‐b proteins in RTK ubiquitination and degradation. Absence of Cbl and Cbl‐b expression dramatically alters actin cytoskeleton reorganization upon attachment to extracellular matrix and markedly enhances growth factor‐dependent cell migration indicating that Cbl proteins provide functionally‐important negative regulatory feedback upon RTK activation. Further studies are underway to express specific RTKs (such as human EGFR) in the characterized MEFs together with reconstitution with wildtype or mutant Cbl proteins to dissect the fundamental steps in endocytic traffic where Cbl proteins play an essential role. These studies will focus on the role of Cbl proteins in initial endocytosis vs. later endosomal sorting steps as the consequences of these mechanisms are distinct. This work was supported by: the NIH grant CA99163 to HB.