Function of proEGF cytoplasmic domain in thyroid cancer

Abstract

Epidermal Growth Factor (EGF) is a member of the EGF-like family. EGF binding to the Epidermal Growth Factor Receptor (EGFR) affects cell survival as well as proliferation, migration, and tissue differentiation. Over-expression along with ligand-induced activation of EGFR has been correlated with increased in vivo invasiveness of tumor cells and enhanced in vitro migration of cell lines. This in turn makes EGFR a very important target for cancer therapy. In our investigation we have discovered that the cytoplasmic domain of proEGF (proEGFcyt) has the ability to decrease proliferation and migration in thyroid carcinoma cell lines. We illustrated that proEGFcyt causes specific alterations in the microtubular (MT) phenotype and the composition of MT-associated proteins (MAP) in FTC-133 overexpressing proEGFcyt, a finding not observed in FTC-133 over-expressing a novel splice form of proEGFcyt with a deletion of the complete exon 23 (proEGFdel23) (Pyka et al., 2004). Here, we demonstrate that proEGFcyt suppresses motility and elastinolytic activity in human thyroid Ca cells as a result from reduced secretion of cath-L. This impaired the ability of thyroid Ca cells to penetrate elastin matrices. The reduction in cath-L secretion was as a result of an up-regulation of SNAP25, a member of the t-SNARE plasma membrane complex, which is involved in Ca2+-dependent exocytosis. Furthermore, we demonstrated that proEGFcyt-mediated silencing of UCH-L1 causes the decrease in EGFR due to enhanced EGFR ubiquitination. This correlated with altered proteasomal degradation and provides a unique new mechanism on how proEGFcyt can affect cell proliferation in human thyroid cancer cells. These studies identified novel functions of human proEGF as a bidirectional signaling molecule consisting of the known extracellular EGF domain which functions as the classical ligand and activator of EGFR-mediated cell growth and the proEGF cytoplasmic domain which has the ability to suppress migration and growth of human thyroid carcinoma cells.