Describing the role of the 40S-LARP1 complex in the synthesis of ribosomal proteins driven by oncogenic MYC

Abstract

[eng] MYC dysregulation is the most common alteration in cancers, driving several cellular processes that cooperatively promote aberrant cell growth and proliferation. Among them, the induction of protein synthesis has been demonstrated to have a functional significance in the process of tumor initiation and maintenance. MYC-driven tumors are therefore characterized by increased rates of Ribosome Biogenesis (RiBi), which sustain their overt anabolic demand. The role of MYC in RiBi is traditionally linked to its transcriptional activity, by upregulation of the RNA polymerases (Pol) I, II and III required for the transcription of ribosomes constituents, respectively the 47S precursor ribosomal RNA (pre- rRNA), ~80 ribosomal proteins (RPs) mRNAs and the 5S rRNA. With respect to RPs transcripts, the synthesis of their cognate proteins is ultimately controlled at the translational level by a cis regulatory motif located at their transcriptional start site called 5’ oligopyrimidine tract (5’TOP) and defined by an invariant cytosine at position +1 followed by a stretch of pyrimidines of variable length and composition. Our laboratory demonstrated that the RNA binding protein LARP1, together with the free 40S ribosomal subunit, forms a complex that stabilizes 5’TOP mRNAs. As RiBi is a high energy consumption process, its regulation is strictly dependent on the energetic status of the cell, which is sensed by the master regulator of cellular metabolism, the mechanistic target of rapamycin (mTOR). mTOR acts to lower the protein synthesis rate and global metabolism upon nutritional limitations, whereas it re-establishes the anabolic rate of the cell in response to proliferative signals. Consistently, our laboratory showed that upon mTOR inhibition 5’TOP transcripts are preserved from degradation by the 40S-LARP1 complex in a translationally repressed state, generating a reservoir of anabolic power in the form of mRNA that cells can unleash when external conditions return permissive. In light of the role of LARP1 in the post-transcriptional regulation of RP mRNAs and based on recent evidence pointing at the ability of MYC to promote translational efficiency of RPs, we sought to investigate the role of LARP1 on RP mRNAs translation in the context of oncogenic MYC. To this aim, we discovered a novel regulatory axis between MYC and LARP1 sustained by oncogenic MYC levels in osteosarcoma, lymphoid and colorectal cancer cells. We showed that MYC, by inhibiting the expression of the tumor suppressor microRNAs miR-26a and miR-26b, relieves their targeting on LARP1 mRNA thus sustaining LARP1 expression. In turn, the increase in LARP1 expression appeared to be necessary for the execution of MYC oncogenic program with respect to the increased global protein synthesis and cell proliferation rates. We reported indeed that ablating LARP1 under MYC overexpression leads to a strong reduction in RP mRNAs translational efficiency, thus impairing MYC-driven protein synthetic and proliferative capacity of the cell. In conclusion, these observations are unraveling a novel molecular mechanism explaining how MYC oncogenesis sustains the ribosome biogenesis at the translational level.

[spa] Con el objetivo de mantener su elevada tasa de proliferación, los tumores impulsados por la proteína MYC dependen especialmente de la biogénesis de ribosomas (RiBi), un proceso complejo que implica la síntesis de nuevos ribosomas y que requiere un alto gasto energético. La actividad transcripcional de MYC permite aumentar la cantidad de ribosomas disponibles promoviendo la transcripción de componentes ribosómicos como las proteínas ribosomales (RPs). No obstante, la RiBi depende de manera crucial del estado energético de la célula, siendo éste detectado por la molécula diana de la rapamicina (mTOR). En este contexto, nuestro laboratorio ha demostrado que la proteína de unión al ARN LARP1, fosfodiana de mTOR y que se une formando un complejo con la subunidad ribosómica pequeña 40S, estabiliza los ARNm de las RPs, activando y desactivando su traducción en función del estado de mTOR. A la luz del papel de LARP1 en la regulación postranscripcional de los ARNm de RPs y la evidencia reciente que apunta a la capacidad de MYC para promover la eficiencia traduccional de las RPs, nos propusimos investigar el papel de LARP1 en la traducción de los ARNm de RPs en el contexto de MYC oncogénico. En este estudio, demostramos que MYC mantiene los niveles de LARP1 para estimular la estabilidad y la traducción de los ARNm de las RPs. De hecho, demostramos que la reducción de LARP1 afecta al aumento de la síntesis de RPs inducida por MYC, resultando en una disminución de la síntesis global de proteínas y de la proliferación celular. Además, describimos en detalle el mecanismo molecular subyacente a este novedoso eje regulador MYC-LARP1. Al inhibir la expresión de los supresores tumorales miR-26a y miR-26b, MYC disminuye su inhibición sobre el ARNm de LARP1, provocando un aumento de su expresión. En conclusión, identificamos un nuevo eje MYC-miR-26-LARP1 responsable de la regulación postranscripcional de MYC sobre los ARNm de las RPs, necesario para sustentar el fenotipo maligno inducido por MYC, caracterizado por la síntesis anómala de proteínas y la proliferación celular desregulada.