Mutant P53 induces MELK expression by release of wild-type P53-dependent suppression of FOXM1

Lakshmi Reddy Bollu,Corey Speers,Jonathan Shepherd,Dekuang Zhao,Yanxia Ma,Abhijit Mazumdar,Powel H Brown,William Tahaney,Gordon B Mills

doi:10.1038/s41523-019-0143-5

Abstract

Triple-negative breast cancer (TNBC) is the most aggressive form of breast cancer, and is associated with a poor prognosis due to frequent distant metastasis and lack of effective targeted therapies. Previously, we identified maternal embryonic leucine zipper kinase (MELK) to be highly expressed in TNBCs as compared with ER-positive breast cancers. Here we determined the molecular mechanism by which MELK is overexpressed in TNBCs. Analysis of publicly available data sets revealed that MELK mRNA is elevated in p53-mutant breast cancers. Consistent with this observation, MELK protein levels are higher in p53-mutant vs. p53 wild-type breast cancer cells. Furthermore, inactivation of wild-type p53, by loss or mutation of the p53 gene, increases MELK expression, whereas overexpression of wild-type p53 in p53-null cells reduces MELK promoter activity and MELK expression. We further analyzed MELK expression in breast cancer data sets and compared that with known wild-type p53 target genes. This analysis revealed that MELK expression strongly correlates with genes known to be suppressed by wild-type p53. Promoter deletion studies identified a p53-responsive region within the MELK promoter that did not map to the p53 consensus response elements, but to a region containing a FOXM1-binding site. Consistent with this result, knockdown of FOXM1 reduced MELK expression in p53-mutant TNBC cells and expression of wild-type p53 reduced FOXM1 expression. ChIP assays demonstrated that expression of wild-type p53 reduces binding of E2F1 (a critical transcription factor controlling FOXM1 expression) to the FOXM1 promoter, thereby, reducing FOXM1 expression. These results show that wild-type p53 suppresses FOXM1 expression, and thus MELK expression, through indirect mechanisms. Overall, these studies demonstrate that wild-type p53 represses MELK expression by inhibiting E2F1A-dependent transcription of FOXM1 and that mutation-driven loss of wild-type p53, which frequently occurs in TNBCs, induces MELK expression by suppressing FOXM1 expression and activity in p53-mutant breast cancers.

Highlights

Triple-negative breast cancers (TNBCs), a breast cancer subtype, are highly aggressive tumors occurring frequently in young women and in African American women who have a very poor prognosis
To investigate the mechanism by which maternal embryonic leucine zipper kinase (MELK) expression is highly elevated in TNBCs, we analyzed The Cancer Genome Atlas (TCGA) and Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) breast cancer patient data sets to determine whether upregulated MELK expression correlates with gene copy number, promoter methylation, or any specific oncogenic mutations
Knockdown of p53 increased MELK mRNA expression in Wild-type p53 (WT p53) cells (Fig. 2e) but not in mutant p53 cells (Fig. 2f). These results suggest that loss of WT activity of p53 is the key mechanism for elevated MELK expression in TNBC cells

Summary

Introduction

Triple-negative breast cancers (TNBCs), a breast cancer subtype, are highly aggressive tumors occurring frequently in young women and in African American women who have a very poor prognosis. We identified several kinases overexpressed in TNBCs as compared with ER-positive breast cancers and showed that inhibition of the expression of several of these kinases suppressed the growth of TNBC cells.[1] One such critical kinase is the maternal embryonic leucine zipper kinase (MELK).[1] Several reports have identified that MELK expression is highly elevated in many human cancers and high MELK expression is associated with poor prognosis.[2,3,4,5,6,7]. The tumor suppressor protein, p53, is a transcription factor that controls both activation and repression of gene expression in eukaryotic cells.[33,34] Wild-type p53 (WT p53)

Methods

Results

Conclusion