Let-7 miRNAs are involved in carcinogenesis and tumor progression through their roles in maintaining differentiation and normal development. However, there is little research focusing on the effects of let-7 on Wnt-activated self-renewal of breast cancer stem cells. By analyzing the expression levels of let-7 family members in clinical tissues, we found that higher expression levels of let-7b and let-7c were correlated with better clinical prognosis of patients with estrogen receptor (ER)α-positive breast tumor. Further, we found that only let-7c was inversely correlated with ERα expression, and there is corelationship between let-7c and Wnt signaling in clinical tissues. Aldehyde dehydrogenase (ALDH)1 sorting and mammosphere formation assays showed that let-7c inhibited the self-renewal of stem cells in ERα-positive breast cancer. Let-7c decreased ERα expression through directly binding to the 3'UTR (untranslated region), and let-7c inhibited the estrogen-induced activation of Wnt signaling. Depletion of ERα abolished let-7c functions in stem cell signatures, which further confirmed that let-7c inhibited estrogen-induced Wnt activity through decreasing ERα expression. Taken together, our findings identified a biochemical and functional link between let-7c with ERα/Wnt signaling in breast cancer stem cells.

The ability of the classic tumour‐suppressive let‐7 family to inhibit carcinogenesis, tumour progression, recurrence and pluripotency of cancer stem cells has generated significant interest in the field of cancer research. Through suppressing and degrading downstream‐targeted mRNA s, let‐7 affected most aspects of cell biology. It is perplexing how let‐7 affects oncogenesis, as the large influx of new mi RNA s and other kinds of non‐coding RNA s are continuously defined. In this review, we delineate the complex functions of let‐7 and discuss the future direction of let‐7 research.

The oncogenic role of estrogen receptor (ER)α and its correlation with let-7 microRNAs (miRNAs) have been studied and confirmed in breast tumors; however, this correlation has not been investigated in breast cancer stem cells (BCSCs). In the present study, we detected the expression of let-7 and ERα in ER-positive breast tumor tissues. Furthermore, we used a FACSAria cell sorter to separate side population (SP) cells from the MCF-7 and T47-D cell lines by Hoechst 33342 staining. The expression of let-7 miRNAs, ERα and its downstream genes in SP and non-SP (NSP) cells were analyzed. In additional experiments, we transfected a plasmid expressing let-7a into SP cells isolated from the MCF-7 and T47-D cell lines in order to observe changes in the expression of downstream genes (cyclin D1 and pS2). The correlation among let-7, ERα and ERα downstream genes suggested that let-7 acts as a tumor suppressor by inhibiting ERα-mediated cellular malignant growth in ER-positive breast cancer stem cells. The suppression of ERα by the upregulation of let-7 expression may be a promising strategy for the inhibition of the ER signaling pathway and for the elimination of cancer stem cells, thus aiding in the treatment of breast cancer.

Abstract. The E-cadherin gene (CDH1) is associated with poor prognosis and metastasis in patients with breast cancer, and methylation of its promoter is correlated with decreased gene expression. However, there is currently no direct evidence that CDH1 promoter methylation indicates poor prognosis in patients with breast cancer. In the present study, methylation-specific polymerase chain reaction (PCR) was applied to detect the methylation status of the CDH1 promoter in 137 primary breast cancer, 85 matched normal breast tissue and 13 lung metastasis specimens. Reverse transcription-quantitative PCR was used to assess the relative expression levels of CDH1 mRNA, and correlation analysis between CDH1 methylation status, and gene expression, clinicopathological characteristics and patient survival was performed. Methylation of CDH1 was identified in 40.9% (56/137) of primary breast cancer specimens, 61.5% (8/13) of lung metastasis specimens and none of the matched normal breast specimens. The downregulation of CDH1 mRNA and E-cadherin protein expression were identified to be significantly correlated with CDH1 methylation (P<0.05). In addition, CDH1 methylation was significantly associated with lymph node metastasis and estrogen receptor status of patients (P<0.05). In univariate analyses, patients with CDH1 methylation exhibited poor overall survival (OS) and disease-free survival (DFS; P<0.05). Furthermore, multivariate analyses revealed that CDH1 methylation was an independent prognostic factor predicting poor OS (HR, 1.737; 95% CI, 0.957-3.766; P=0.041) and DFS (HR, 2.018; 95% CI, 2.057-3.845; P=0.033) in patients with breast cancer. Therefore, the present study suggests that CDH1 promoter methylation may be correlated with breast carcinogenesis and indicates poor prognosis in patients with breast cancer.

Keywords: breast cancer stem cells, KRas, let-7a, mammosphere formation capacity, MAPK/ERK, NF-kB Abbreviations: BCSCs, breast cancer stem cells; MFE, mammosphere-forming efficiency Breast cancer stem cells (BCSCs) have the greatest potential to maintain tumorigenesis in all subtypes of tumor cells and were regarded as the key drivers of tumor. Recent evidence has demonstrated that BCSCs contributed to a high degree of resistance to therapy. However, how BCSCs self renewal and tumorigenicity are maintained remains obscure. Herein, our study illustrated that overexpression of let-7a reduced cell proliferation and mammosphere formation ability of breast cancer stem cells(BCSCs) in a KRas-dependent manner through different pathways in vitro and in vivo. To be specific, we provided the evidence that let-7a was decreased, and reversely the expression of KRas was increased with moderate expression in early stages (I/II) and high expression in advanced stages (III/IV) in breast cancer specimens. In addition, the negative correlation between let-7a and KRas was clearly observed. In vitro, we found that let-7a inhibited mammosphere-forming efficiency and the mammosphere-size via NF-kB and MAPK/ERK pathway, respectively. The inhibitory effect of let-7a on mammosphere formation efficiency and the size of mammospheres was abolished after the depletion of KRas. On the contrary, enforced expression of KRas rescued the effect of let-7a. In vivo, let-7a inhibited the growth of tumors, whereas the negative effect of let-7a was rescued after overexpressing KRas. Taken together, our findings suggested that let-7a played a tumor suppressive role in a KRas-dependent manner.