P62/SQSTM1 enhances breast cancer stem-like properties by stabilizing MYC mRNA

L-Z Xu,Q-X Zhang,C-L Wang,H-J Wang,D-P Liang,X-B Wan,Q Liu,W Zhou,M Kamran,J Xu,Z-J Kang,Z-J Hou,Z-W Zhao,E W-F Lam,S-S Li

doi:10.1038/onc.2016.202

Abstract

Aberrant p62 overexpression has been implicated in breast cancer development. Here, we found that p62 expression was elevated in breast cancer stem cells (BCSCs), including CD44+CD24− fractions, mammospheres, ALDH1+ populations and side population cells. Indeed, short-hairpin RNA (shRNA)-mediated knockdown of p62 impaired breast cancer cells from self-renewing under anchorage-independent conditions, whereas ectopic overexpression of p62 enhanced the self-renewal ability of breast cancer cells in vitro. Genetic depletion of p62 robustly inhibited tumor-initiating frequencies, as well as growth rates of BCSC-derived tumor xenografts in immunodeficient mice. Consistently, immunohistochemical analysis of clinical breast tumor tissues showed that high p62 expression levels were linked to poorer clinical outcome. Further gene expression profiling analysis revealed that p62 was positively correlated with MYC expression level, which mediated the function of p62 in promoting breast cancer stem-like properties. MYC mRNA level was reduced upon p62 deletion by siRNA and increased with p62 overexpression in breast cancer cells, suggesting that p62 positively regulated MYC mRNA. Interestingly, p62 did not transactivate MYC promoter. Instead, p62 delayed the degradation of MYC mRNA by repressing the expression of let-7a and let-7b, thus promoting MYC mRNA stabilization at the post-transcriptional level. Consistently, let-7a and let-7b mimics attenuated p62-mediated MYC mRNA stabilization. Together, these findings unveiled a previously unappreciated role of p62 in the regulation of BCSCs, assigning p62 as a promising therapeutic target for breast cancer treatments.

Highlights

As a highly heterogeneous malignancy, breast cancer is the leading cause of female cancer-related deaths.[1,2,3] the mortality rate has drastically dropped as a result of improved early diagnostic methods and therapeutic advancements,[4] a significant portion of patients eventually suffer from tumor relapse and develop drug resistance
CD44+ CD24 − /low subpopulation tumor cells are significantly associated with lymph-node involvement, estrogen receptor and progestogen receptor status and a shorter cumulative disease-free survival (DFS) and overall survival (OS).[18]
We uncover a previously unrecognized role of p62 in the regulation of breast cancer stem-like properties based on the following novel findings: (a) p62 expression is elevated in breast cancer stem cells (BCSCs)-enriched cell populations (Figure 1); (b) p62 is essential for promoting breast cancer stem-like properties both in vivo and in vitro (Figures 2 and 3, Supplementary Figure 5); (c) p62 is overexpressed in breast cancer tissues, and high p62 expression levels predict poorer clinical outcome (Figure 4); (d) p62 specially promotes MYC mRNA stability, mostly by repressing the expression of let-7a /b (Figures 5–7)

Summary

INTRODUCTION

As a highly heterogeneous malignancy, breast cancer is the leading cause of female cancer-related deaths.[1,2,3] the mortality rate has drastically dropped as a result of improved early diagnostic methods and therapeutic advancements,[4] a significant portion of patients eventually suffer from tumor relapse and develop drug resistance. We set out to investigate the potential role of p62 in the regulation of breast cancer stem-like properties and the mechanism involved

RESULTS

DISCUSSION

Findings

315 MATERIALS AND METHODS