Studies of the regulatory mechanisms of human threonyl-tRNA synthetase in the biosynthesis of angiogenic factors and mucin

Abstract

Aminoacyl-tRNA synthetases (ARSs) are essential enzymes for protein synthesis to link specific amino acids to their cognate tRNAs. Recent studies have shown that ARSs, considered as a sort of housekeeping enzyme, are now involved in a variety of functions such as transcription, translation, proliferation, inflammation, angiogenesis and cell death. This study is focused on the human threonyl-tRNA synthetase (TRS) and its potential role. In chapter I, the results show that human TRS functions as a translational initiation factor to regulate vertebrate-specific translation initiation via eIF4E homologous protein (4EHP). TRS selectively interacts with 4EHP in a manner similar to the eIF4G interaction with eIF4E. In this way, TRS acts as a scaffold protein to assemble eIF4A, consequently forming eIF4F-like complex. Importantly, complex formation is evolutionary gain-of-function to control protein synthesis of a subset of mRNAs necessary for development of the vertebrate system, verified by endothelial cell migration and vessel formation as well as in vivo zebrafish embryo vascularization assays. In chapter II, the results show that TRS specifically regulates biosynthesis of mucin1 (MUC1) through catalytic activity. The levels of MUC1 protein are affected by threonine and biosynthesis of MUC1 in pancreatic cancer is sensitive to the activity and expression of TRS that incorporates threonine to MUC1. TRS catalytic inhibitors and threonine starvation attenuate MUC1-dependent pancreatic cancer cell migration. In addition, tissue levels of TRS and MUC1 are positively correlated in clinical tumor specimen and expression of both proteins at high level is associated with poor survival outcome of the patients. To summarize, chapter I study discovers an unexpected role of TRS in regulating translation initiation in vertebrates and uncovers a previously unidentified cap-dependent translation initiation mechanism that represents an evolutionary gain of function in vertebrates. Chapter II study provides several evidences showing the potential role of TRS in the migration of human pancreatic cancer cells by enhancing MUC1 biosynthesis, suggesting a novel insight into targeting TRS as a new way to pancreatic cancer.