Identification of Fhit as a post-transcriptional effector of Thymidine Kinase 1 expression

FHIT is a genome caretaker gene that is silenced in >50% of cancers. Loss of Fhit protein expression promotes accumulation of DNA damage, affects apoptosis and epithelial-mesenchymal transition, though molecular mechanisms underlying these alterations have not been fully elucidated. Initiation of genome instability directly follows Fhit loss and the associated reduced Thymidine Kinase 1 (TK1) protein expression. The effects on TKI of Fhit knockdown and Fhit induction in the current study confirmed the role of Fhit in regulating TKI expression. Changes in Fhit expression did not impact TKI protein turnover or transcription from the TKI promoter, nor steady-state levels of TK1 mRNA or turnover. Polysome profile analysis showed that up-regulated Fhit expression resulted in decreased TKI RNA in non-translating messenger ribonucleoproteins and increased ribosome density on TK1 mRNA. Fhit does not bind RNA but its expression increased luciferase expression from a transgene bearing the TK1 5 '-UTR. Fhit has been reported to act as a scavenger decapping enzyme, and a similar result with a mutant (H96) that binds but does not cleave nucleoside 5 ',5 '-triphosphates suggests the impact on TK1 translation is due to its ability to modulate the intracellular level of cap-like molecules. Consistent with this, cells expressing Fhit mutants with reduced activity toward cap-like dinucleotides exhibit DNA damage resulting from TK1 deficiency, whereas cells expressing wild-type Fhit or the H96N mutant do not. The results have implications for the mechanism by which Fhit regulates TKI mRNA, and more broadly, for its modulation of multiple functions as tumor suppressor/genome caretaker. (C) 2017 Elsevier B.V. All rights reserved.