The control of ribosomal RNA synthesis in mammalian cells

Abstract

The biogenesis of ribosomes is a fundamental process that occurs in all living cells. In mammalian cells, it is a highly complex process consisting of the coordinated synthesis and assembly of four ribosomal RNAs (rRNAs) with about 80 ribosomal proteins (RPs). More than 150 non-ribosomal proteins are involved in the processing of rRNAs. The main focus of this project is to use adult rat ventricular cardiomyocytes (ARVCs) as a model to address how mTOR complex 1 (mTORC1) and other signalling pathways regulate the synthesis of rRNAs.

A new technique has been developed to monitor the synthesis of new rRNAs using 4-thiouridine (4-SU) and I have applied it in both HeLa cells and heart muscle cells to study the control of ribosome synthesis. HeLa cells were treated with different mTOR inhibitors to identify effects on the transcription and/or decay of rRNA. We analysed both the synthesis rate and the decay rate of new RNAs made by Pol I and Pol III using real-time RT-PCR. Interestingly, rapamycin not only blocked the synthesis of 18S, 28S and 5S rRNA, but also induced the decay of newly synthesized rRNAs. This demonstrates that mTORC1 regulates Pol I and Pol III transcription, as well as the decay of rRNA.

In cardiomyocytes, hypertrophic agents such as phenylephrine (PE) strongly activate protein synthesis and lead to heart cell growth. The boost of protein synthesis drives the increase of cell size and leads to hypertrophy. Cardiac hypertrophy (CH) is a major risk factor for heart failure. Therefore, it is important to understand the mechanisms that how hypertrophic agents which cause the overgrowth of heart muscle increase ribosome production. Although it is known that inhibiting mTORC1 largely blocks the rapid activation of protein synthesis by PE, here it did not affect the synthesis of new 18S rRNAs. However, inhibitors of the MEK/Erk pathway and p90RSK each block the new rRNA synthesis.

These data reveal that, in contrast to many other types of cell, ribosome biogenesis is controlled by MEK/ERK/p90RSK signalling, not mTORC1, in cardiomyocytes. Taken together, the data presented here may provide cues for potential valuable therapy of cardiac left ventricular hypertrophy.

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