The role of eIF4F-driven mRNA translation in regulating the tumour microenvironment

eIF4E orchestrates mRNA processing, RNA export and translation to modify specific protein production

The eukaryotic translation initiation factor eIF4E acts as a multifunctional factor that simultaneously influences mRNA processing, export, and translation in many organisms. Its multifactorial effects are derived from its capacity to bind to the methyl-7-guanosine cap on the 5'end of mRNAs and thus can act as a cap chaperone for transcripts in the nucleus and cytoplasm. In this review, we describe the multifactorial roles of eIF4E in major mRNA-processing events including capping, splicing, cleavage and polyadenylation, nuclear export and translation. We discuss the evidence that eIF4E acts at two levels to generate widescale changes to processing, export and ultimately the protein produced. First, eIF4E alters the production of components of the mRNA processing machinery, supporting a widescale reprogramming of multiple mRNA processing events. In this way, eIF4E can modulate mRNA processing without physically interacting with target transcripts. Second, eIF4E also physically interacts with both capped mRNAs and components of the RNA processing or translation machineries. Further, specific mRNAs are sensitive to eIF4E only in particular mRNA processing events. This selectivity is governed by the presence of cis-acting elements within mRNAs known as USER codes that recruit relevant co-factors engaging the appropriate machinery. In all, we describe the molecular bases for eIF4E's multifactorial function and relevant regulatory pathways, discuss the basis for selectivity, present a compendium of ~80 eIF4E-interacting factors which play roles in these activities and provide an overview of the relevance of its functions to its oncogenic potential. Finally, we summarize early-stage clinical studies targeting eIF4E in cancer.

TSC2 loss in neural progenitor cells suppresses translation of ASD/NDD-associated transcripts in an mTORC1- and MNK1/2-reversible fashion

Tuberous sclerosis complex (TSC), an inherited neurodevelopmental (ND) disorder with frequent manifestations of epilepsy and autism spectrum disorder (ASD). TSC is caused by mutations in TSC1 or TSC2 tumor suppressor genes, with encoded proteins hamartin/TSC1 and tuberin/TSC2 forming a functional complex inhibiting mechanistic target of rapamycin complex-1 (mTORC1) signaling, leading to FDA-approved allosteric mTORC1-selective rapamycin analogs for TSC tumors. Rapalogs are effective for TSC-associated hamartomas, however, they are not effective for treating ND manifestations. mTORC1 signaling plays an essential role in protein synthesis through mTORC1-eIF4F and MNK-eIF4E-mediated mRNA translation. Further, the effects on mRNA translation by specific mTORC1 and MNK inhibitors such as RMC-6272 and eFT-508 in TSC have never been explored. Here, employing CRISPR-modified, isogenic TSC2 patient-derived neural progenitor cells (NPCs), we have examined mRNA translation upon loss of TSC2 . Our results reveal dysregulated translation in TSC2 -Null NPCs, which significantly overlap with the translatome from TSC1 -Null NPCs, which we reported recently. Most notably, numerous non-monogenic ASD-NDD- and epilepsy-associated genes identified in patients harboring putative loss-of-function mutations, including protein truncating, or damaging missense variants, were translationally suppressed in TSC2 -Null NPCs, and their translation were reversed upon RMC-6272 or eFT-508 treatment. Our study here establishes the importance of mTORC1-eIF4F and MNK-eIF4E-mediated mRNA translation in TSC, ASD and other neurodevelopmental disorders and lay the groundwork for evaluating drugs in clinical development that target these pathways as a treatment strategy for TSC as well as ASD/NDD.

The molecular basis of translation initiation and its regulation in eukaryotes

The regulation of gene expression is fundamental for life. Whereas the role of transcriptional regulation of gene expression has been studied for several decades, it has been clear over the past two decades that post-transcriptional regulation of gene expression, of which translation regulation is a major part, can be equally important. Translation can be divided into four main stages: initiation, elongation, termination and ribosome recycling. Translation is controlled mainly during its initiation, a process which culminates in a ribosome positioned with an initiator tRNA over the start codon and, thus, ready to begin elongation of the protein chain. mRNA translation has emerged as a powerful tool for the development of innovative therapies, yet the detailed mechanisms underlying the complex process of initiation remain unclear. Recent studies in yeast and mammals have started to shed light on some previously unclear aspects of this process. In this Review, we discuss the current state of knowledge on eukaryotic translation initiation and its regulation in health and disease. Specifically, we focus on recent advances in understanding the processes involved in assembling the 43S pre-initiation complex and its recruitment by the cap-binding complex eukaryotic translation initiation factor 4F (eIF4F) at the 5' end of mRNA. In addition, we discuss recent insights into ribosome scanning along the 5' untranslated region of mRNA and selection of the start codon, which culminates in joining of the 60S large subunit and formation of the 80S initiation complex.

The Effects of Deregulated Ribosomal Biogenesis in Cancer

Ribosomes are macromolecular ribonucleoprotein complexes assembled from RNA and proteins. Functional ribosomes arise from the nucleolus, require ribosomal RNA processing and the coordinated assembly of ribosomal proteins (RPs), and are frequently hyperactivated to support the requirement for protein synthesis during the self-biosynthetic and metabolic activities of cancer cells. Studies have provided relevant information on targeted anticancer molecules involved in ribosome biogenesis (RiBi), as increased RiBi is characteristic of many types of cancer. The association between unlimited cell proliferation and alterations in specific steps of RiBi has been highlighted as a possible critical driver of tumorigenesis and metastasis. Thus, alterations in numerous regulators and actors involved in RiBi, particularly in cancer, significantly affect the rate and quality of protein synthesis and, ultimately, the transcriptome to generate the associated proteome. Alterations in RiBi in cancer cells activate nucleolar stress response-related pathways that play important roles in cancer-targeted interventions and immunotherapies. In this review, we focus on the association between alterations in RiBi and cancer. Emphasis is placed on RiBi deregulation and its secondary consequences, including changes in protein synthesis, loss of RPs, adaptive transcription and translation, nucleolar stress regulation, metabolic changes, and the impaired ribosome biogenesis checkpoint.