NCI Program Project in Esophageal Carcinogenesis

Project 3 - Protein Ubiquitylation in Esophageal Cancer

Project Leader:	J. Alan Diehl, Ph.D.
Post Doctoral Researchers:	Bhaskar Pongoti, Ph.D.
Research Specialist:	Margarita Romero
Research Technician:	Petia Zamfirova

The long-term objective of our research centers on elucidation of the mechanisms whereby critical cell cycle regulators contribute to the genesis and progression of neoplastic transformation.  Much of our current work focuses on how growth-signaling pathways regulate the mitogenically responsive D-type cyclins and more specifically the contribution of regulated, ubiquitin-dependent destruction of cyclin D1 for normal versus neoplastic growth.  The noted overexpression of cyclin D1 in 50% of human esophageal cancer highlights the importance of elucidating the mechanisms that regulate cyclin D1 activity in this deadly cancer.  While cyclin D1 overexpression is a consequence of gene amplification and chromosome translocation in a subset of cancers, decreased cyclin D1 protein degradation, which depends on its phosphorylation of Thr286, is thought to be the key factor in a majority of cancers.  Although our progress in the characterization of this mechanism of cyclin D1 overexpression in cancer has been hindered by the lack of information regarding the nature of the E3 ubiquitin ligase that directs cyclin D1 proteolysis, we have recently identified the SCF(Fbx4-αB crystallin) that controls cyclin D1 ubiquitination and degradation.  This critical discovery has provided essential information and tools necessary to assess the mechanisms that contribute to regulation of cyclin D1 accumulation during malignant progression.  More importantly our recent discovery of inactivating mutations in Fbx4 in esophageal cancer strongly supports our hypothesis that Fbx4 functions as a novel tumor suppressor.  The identification of this E3 ligase as well as our recent preliminary studies lead to the overarching hypothesis that the SCF(Fbx4 αB crystallin) ligase, via coordinated recognition of phosphor-cyclin D1 by Fbx4 and αB crystallin, plays a critical role in the maintenance of esophageal cell growth, proliferation, and tissue maintenance.  Experiments proposed in this grant will address post-translational regulation of Fbx4 as it pertains to ligase function, utilizing cancer derived mutations as a guide (Aim1); determine the contribution of Fbx4 to esophageal tissue homeostasis in vivo and in vitro (Aim 2); evaluate the role of Fbx4 in maintenance of genomic stability and response of tumor cells to chemotherapeutic intervention (Aim 3).