Abstract 3526: The expression of MELK, a growth regulator of triple-negative breast cancer, is regulated by p53

Background: Triple negative breast cancer (TNBC) is the most aggressive form of breast cancer with poor prognosis. Due to frequent distant metastasis and lack of successful targeted therapies, the overall survival rate in TNBC patients is significantly lower than estrogen receptor positive (ER) and HER2 positive breast cancers. To identify potential druggable targets, previously, we performed a gene expression analysis and identified protein kinases that are highly expressed in TNBC. Through these studies, we discovered that the expression of maternal embryonic leucine zipper kinase (MELK) is highly elevated in TNBCs and correlated with p53-mutation status and metastasis-free survival. Inhibition of MELK reduced the growth and invasiveness of p53 mutant breast cancer cells. Methods: In this study, we analyzed the association of MELK expression with p53 status using publicly available datasets, and examined the role of wild-type and mutant P53 in regulating MELK expression. MELK expression at mRNA and protein levels were determined through Q-RT-PCR and western blotting analysis respectively. The association of p53 and MELK expression was determined using Oncomine analysis. We also investigated the effect of p53 knockdown and overexpression on MELK expressing using Q-RT-PCR, western blotting and promoter luciferase assays. Results: Analysis of publicly available datasets revealed that MELK expression is highly elevated p53-mutant breast cancer independent of estrogen receptor status. Consistent with observation, high MELK expression was found in p53-mutant cells compared to p53 wild-type cells. Through luciferase, western blotting and Q-RT-PCR assays, we discovered that the increased expression of MELK in TNBC cells is due to inactivation of p53 wild-type function. Consistently, over expression of p53 wild-type in p53-Null cells, using doxycycline inducible system reduced MELK promoter activity and expression. Similarly, inhibition of p53 wild-type by over expression of p53-mutants in p53-wild-type cells also increased MELK expression through dominant-negative effect. In contrast, knockdown of p53-mutants or over expression of p53-mutants in p53-null cells did not alter MELK expression. Finally, through promoter deletion studies coupled with luciferase studies, we identified a potential p53-response region in MELK promoter. Conclusion: Our results suggest that the expression of MELK, a critical regulator of growth and invasion of TNBCs, is regulated by the tumor suppressor gene, p53 wild-type. Inactivation of p53 wild-type either gene deletion or mutations induces the expression of MELK. These results suggest that MELK is a promising target for the treatment of women with p53-mutant TNBC. This work was supported by Susan G Komen Promise Grant (PB, SH, GM), SAB Komen grant (PB) and Young Foundation grant (PB). Citation Format: Lakshmi Reddy Bollu, Powel H. Brown, Abhijit Mazumdar, Dekuang Zhao, Yanxia Ma. The expression of MELK, a growth regulator of triple-negative breast cancer, is regulated by p53 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3526. doi:10.1158/1538-7445.AM2017-3526