MXD1 localizes in the nucleolus, binds UBF and impairs rRNA synthesis.

Maria Del Carmen Lafita-Navarro,Miguel Lafarga,María T Berciano,Judit Liaño-Pons,Olga Tapia,Jorge Mata-Garrido,Eva García-Alegría,Lucía García-Gutiérrez,Javier León,Rosa Blanco

doi:10.18632/oncotarget.11766

Abstract

MXD1 is a protein that interacts with MAX, to form a repressive transcription factor. MXD1-MAX binds E-boxes. MXD1-MAX antagonizes the transcriptional activity of the MYC oncoprotein in most models. It has been reported that MYC overexpression leads to augmented RNA synthesis and ribosome biogenesis, which is a relevant activity in MYC-mediated tumorigenesis. Here we describe that MXD1, but not MYC or MNT, localizes to the nucleolus in a wide array of cell lines derived from different tissues (carcinoma, leukemia) as well as in embryonic stem cells. MXD1 also localizes in the nucleolus of primary tissue cells as neurons and Sertoli cells. The nucleolar localization of MXD1 was confirmed by co-localization with UBF. Co-immunoprecipitation experiments showed that MXD1 interacted with UBF and proximity ligase assays revealed that this interaction takes place in the nucleolus. Furthermore, chromatin immunoprecipitation assays showed that MXD1 was bound in the transcribed rDNA chromatin, where it co-localizes with UBF, but also in the ribosomal intergenic regions. The MXD1 involvement in rRNA synthesis was also suggested by the nucleolar segregation upon rRNA synthesis inhibition by actinomycin D. Silencing of MXD1 with siRNAs resulted in increased synthesis of pre-rRNA while enforced MXD1 expression reduces it. The results suggest a new role for MXD1, which is the control of ribosome biogenesis. This new MXD1 function would be important to curb MYC activity in tumor cells.

Highlights

MXD1 is a transcription factor belonging to the MXD family of proteins [1]
It is reported that MXD1 is mostly expressed in quiescent cells or differentiated cells, we found low but detectable MXD1 protein levels in proliferating cells of the above cell lines (Figure 1A), serum deprivation resulted in increased MXD1 expression, as described [22] (Figure 1A)
The nucleolar localization of MXD1 was confirmed in preparations co-stained with propidium iodide (Figure 2A), and its preferential concentration in fibrillar centers (FCs) of neurons was demonstrated by co-localization with UBF (Figure 1A, lower panel)

Summary

Introduction

MXD1 ( known as MAD1) is a transcription factor belonging to the MXD family of proteins [1]. The MXD family (composed of MXD1, MXI1/MXD2, MXD3, MXD4 and MNT) is part of the MYC-MAXMXD network of transcription factors. These factors contain the b-HLH-LZ domain by which they bind to E-boxes in the promoter of their target genes to regulate their transcription [2,3,4].Whereas MYC-MAX activates transcription, MXD1-MAX represses transcription forming complexes with histone deacetylases. To MYC, MXD1 is readily expressed in resting and differentiated cells [6, 8, 9]. MXD1 inhibits cell proliferation and antagonize MYC transforming activity [10,11,12]

Methods

Results

Conclusion