Huntingtin Interacting Protein 1: Interactions with Receptor Tyrosine Kinases.

Abstract

The receptor tyrosine kinase (RTK) signaling pathway is strongly activated in most forms of cancer. As a result, manipulation of the RTK pathway is a common means of promoting oncogenesis. Recently, proteins in the RTK endocytosis pathway have been added to the list of oncoproteins that affect RTK signaling. Among these proteins is Huntingtin Interacting Protein 1 (HIP1), an endocytic adaptor protein that interacts with clathrin, actin, adaptor protein 2 (AP2) and cargo proteins, including RTKs. HIP1 is expressed at high levels in many forms of cancer in a manner that also correlates with RTK overexpression. Additionally, HIP1 overexpression transforms fibroblasts by increasing RTK expression to enable anchorage-independent growth. Though HIP1 has been found to interact with and stabilize multiple RTKs following stimulation, the precise role(s) of HIP1 during the endocytic process remain unclear. This thesis examines the interaction between HIP1 and RTKs. First, high HIP1 levels were found in a rare, deadly, form of cancer, Merkel cell carcinoma (MCC), which also expresses the RTK c-Kit at high levels. Patients with metastatic MCC also demonstrated high levels of circulating antibody produced against HIP1. Additionally, HIP1 was found to interact with c-Kit and also stabilize this receptor following stem cell factor (SCF) ligand stimulation. These data collectively indicate that targeting HIP1 in MCC may improve patient prognosis. To further investigate a means of targeting HIP1 interaction with RTKs, we found that the EGFR kinase domain was necessary for HIP1 association with this receptor. HIP1 was also found to be tyrosine phosphorylated when HIP1 and EGFR are co-expressed. This phosphorylation is maintained for 15 to 30 minutes after stimulation, at which time HIP1 and EGFR localize to Rab5 sorting endosomes. Furthermore, a novel form of EGFR was identified that strongly interacts with HIP1 and structurally resembles the oncoprotein EGFRvIII. These findings all provide insight into possible mechanisms of how HIP1-mediates RTK stabilization in cancer.