Molecular dissection of the eukaryotic initiation factor 4E (eIF4E) export-competent RNP

LRPPRC and SLIRP interact in a ribonucleoprotein complex that regulates posttranscriptional gene expression in mitochondria

Mutations in LRPPRC are responsible for the French Canadian variant of Leigh syndrome (LSFC), a neurodegenerative disorder caused by a tissue-specific deficiency in cytochrome c oxidase (COX). To investigate the pathogenic mechanism of disease, we studied LRPPRC function in LSFC and control fibroblasts. The level of mutated LRPPRC is reduced in LSFC cells, and this results in decreased steady-state levels of most mitochondrial mRNAs, but not rRNAs or tRNAs, a phenotype that can be reproduced by siRNA-mediated knockdown of LRPPRC in control cells. Processing of the primary transcripts appears normal. The resultant defect in mitochondrial protein synthesis in LSFC cells disproportionately affects the COX subunits, leading to an isolated COX assembly defect. Further knockdown of LRPPRC produces a generalized assembly defect in all oxidative phosphorylation complexes containing mtDNA-encoded subunits, due to a severe decrease in all mitochondrial mRNAs. LRPPRC exists in a high-molecular-weight complex, and it coimmunoprecipitates with SLIRP, a stem-loop RNA-binding protein. Although this interaction does not depend on mitochondrial mRNA, both proteins show reduced stability in its absence. These results implicate LRPPRC in posttranscriptional mitochondrial gene expression as part of a ribonucleoprotein complex that regulates the stability and handling of mature mRNAs.

Understanding and Targeting the Eukaryotic Translation Initiation Factor eIF4E in Head and Neck Cancer

The eukaryotic translation initiation factor eIF4E is elevated in about 30% of human malignancies including HNSCC where its levels correlate with poor prognosis. Here, we discuss the biochemical and molecular underpinnings of the oncogenic potential of eIF4E. Studies in human leukemia specimens, and later in a mouse model of prostate cancer, strongly suggest that cells with elevated eIF4E develop an oncogene dependency to it, making them more sensitive to targeting eIF4E than normal cells. We describe several strategies that have been suggested for eIF4E targeting in the clinic: the use of a small molecule antagonist of eIF4E (ribavirin), siRNA or antisense oligonucleotide strategies, suicide gene therapy, and the use of a tissue-targeting 4EBP fusion peptide. The first clinical trial targeting eIF4E indicates that ribavirin effectively targets eIF4E in poor prognosis leukemia patients and more importantly leads to striking clinical responses including complete and partial remissions. Finally, we discuss the relevance of these findings to HNSCC.

Molecular targeting of the oncogene eIF4E in acute myeloid leukemia (AML): a proof-of-principle clinical trial with ribavirin

The eukaryotic translation initiation factor eIF4E is elevated in 30% of malignancies including M4/M5 subtypes of acute myeloid leukemia (AML). The oncogenic potential of eIF4E arises from its ability to bind the 7-methyl guanosine (m(7)G) cap on mRNAs, thereby selectively enhancing eIF4E-dependent nuclear mRNA export and translation. We tested the clinical efficacy of targeting eIF4E in M4/M5 AML patients with a physical mimic of the m(7)G cap, ribavirin. Among 11 evaluable patients there were 1 complete remission (CR), 2 partial remissions (PRs), 2 blast responses (BRs), 4 stable diseases (SDs), and 2 progressive diseases (PDs). Ribavirin-induced relocalization of nuclear eIF4E to the cytoplasm and reduction of eIF4E levels were associated with clinical response. Lack of response or relapse coincided with continued or renewed nuclear localization of eIF4E. This first clinical study to target eIF4E in human malignancy demonstrates clinical activity and associated molecular responses in leukemic blasts. This trial is registered at ClinicalTrials.gov (NCT00559091).