Abstract
BackgroundThe RNA-binding protein Musashi-2 (MSI2) has been implicated in the tumorigenesis and tumor progression of some human cancers. MSI2 has also been reported to suppress tumor epithelial-to-mesenchymal transition (EMT) progression in breast cancer, and low MSI2 expression is associated with poor outcomes for breast cancer patients; however, the underlying mechanisms have not been fully investigated. This study investigated the expression and phenotypic functions of two major alternatively spliced MSI2 isoforms (MSI2a and MSI2b) and the potential molecular mechanisms involved in triple-negative breast cancer (TNBC) progression.MethodsThe Illumina sequencing platform was used to analyze the mRNA transcriptomes of TNBC and normal tissues, while quantitative reverse transcription-polymerase chain reaction and immunohistochemistry validated MSI2 isoform expression in breast cancer tissues. The effects of MSI2a and MSI2b on TNBC cells were assayed in vitro and in vivo. RNA immunoprecipitation (RIP) and RNA sequencing were performed to identify the potential mRNA targets of MSI2a, and RIP and luciferase analyses were used to confirm the mRNA targets of MSI2.ResultsMSI2 expression in TNBC tissues was significantly downregulated compared to that in normal tissues. In TNBC, MSI2a expression was associated with poor overall survival of patients. MSI2a overexpression in vitro and in vivo inhibited TNBC cell invasion as well as extracellular signal-regulated kinase 1/2 (ERK1/2) activity. However, MSI2b overexpression had no significant effects on TNBC cell migration. Mechanistically, MSI2a expression promoted TP53INP1 mRNA stability by its interaction with the 3′-untranslated region of TP53INP1 mRNA. Furthermore, TP53INP1 knockdown reversed MSI2a-induced suppression of TNBC cell invasion, whereas ectopic expression of TP53INP1 and inhibition of ERK1/2 activity blocked MSI2 knockdown-induced TNBC cell invasion.ConclusionsThe current study demonstrated that MSI2a is the predominant functional isoform of MSI2 proteins in TNBC, that its downregulation is associated with TNBC progression and poor prognosis and that MSI2a expression inhibited TNBC invasion by stabilizing TP53INP1 mRNA and inhibiting ERK1/2 activity. Overall, our study provides new insights into the isoform-specific roles of MSI2a and MSI2b in the tumor progression of TNBC, allowing for novel therapeutic strategies to be developed for TNBC.
Highlights
The RNA-binding protein Musashi-2 (MSI2) has been implicated in the tumorigenesis and tumor progression of some human cancers
Cohort 1 contained 25 triple-negative breast cancer (TNBC) tissues and 5 adjacent normal breast tissues (ANTs) collected between January 2013 and May 2015 and were used for transcriptome profiling analysis (Table S1); cohort 2 included 27 paired TNBC tissues and ANTs (Table S1) for quantitative realtime polymerase chain reaction analysis; cohort 3 consisted of 129 invasive ductal breast cancer tissue samples that were collected between February 2007 and October 2009 and were used for qRT-Polymerase chain reaction (PCR) analysis of MSI2a and MSI2b expression; and cohort 4 with 388 paraffin-embedded tissue blocks between September 2001 and January 2008 to construct tissue microarrays was used for immunohistochemical analysis of the MSI2a protein
We further assessed the expression of MSI2a and MSI2b by qRT-PCR in another group of 129 frozen breast cancer tissues and found that the expression of the former in TNBC tissues was downregulated compared with that in luminal A and luminal B breast cancer tissues (Fig. 1e), which is consistent with The Cancer Genome Atlas (TCGA) database
Summary
The RNA-binding protein Musashi-2 (MSI2) has been implicated in the tumorigenesis and tumor progression of some human cancers. MSI2 has been reported to suppress tumor epithelial-to-mesenchymal transition (EMT) progression in breast cancer, and low MSI2 expression is associated with poor outcomes for breast cancer patients; the underlying mechanisms have not been fully investigated. This study investigated the expression and phenotypic functions of two major alternatively spliced MSI2 isoforms (MSI2a and MSI2b) and the potential molecular mechanisms involved in triple-negative breast cancer (TNBC) progression. Triple-negative breast cancer (TNBC) is characterized by the lack of estrogen receptor, progesterone receptor expression, and human epidermal growth factor receptor 2 amplification [3]. RNA-binding proteins (RBPs) are key molecules that regulate the splicing, polyadenylation, localization, translation, and decay of target mRNAs [7]. Accumulating evidence indicates that RBP dysfunction or aberrant expression contributes to cancer initiation and progression; RBPs might serve as a good therapeutic target for cancer therapy [8]
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