miR-23b and miR-27b are oncogenic microRNAs in breast cancer: evidence from a CRISPR/Cas9 deletion study

BACKGROUND

Altered expression of microRNAs (miRNAs) is known to contribute to cancer progression. miR-23b and miR-27b, encoded within the same miRNA cluster, are reported to have both tumor suppressive and oncogenic activity across human cancers, including breast cancer.

METHODS

To clarify this dichotomous role in breast cancer, miR-23b and miR-27b were knocked out using CRISPR/Cas9 gene knockout technology, and the role of endogenous miR-23b and miR-27b was examined in a breast cancer model system in vitro and in vivo.

RESULTS

Characterization of the knockout cells in vitro demonstrated that miR-23b and miR-27b are indeed oncogenic miRNAs in MCF7 breast cancer cells. miR-23b and miR-27b knockout reduced tumor growth in xenograft nude mice fed a standard diet, supporting their oncogenic role in vivo. However, when xenograft mice were provided a fish-oil diet, miR-27b depletion, but not miR-23b depletion, compromised fish-oil-induced suppression of xenograft growth, indicating a context-dependent nature of miR-27b oncogenic activity.

CONCLUSIONS

Our results demonstrate that miR-23b and miR-27b are primarily oncogenic in MCF7 breast cancer cells and that miR-27b may have tumor suppressive activity under certain circumstances.

Metastasis-associated protein 1 (MTA1) is transferred by exosomes and contributes to the regulation of hypoxia and estrogen signaling in breast cancer cells

BACKGROUND

Exosomes are small membrane-bound vesicles that contribute to tumor progression and metastasis by mediating cell-to-cell communication and modifying the tumor microenvironment at both local and distant sites. However, little is known about the predominant factors in exosomes that contribute to breast cancer (BC) progression. MTA1 is a transcriptional co-regulator that can act as both a co-activator and co-repressor to regulate pathways that contribute to cancer development. MTA1 is also one of the most up-regulated proteins in cancer, whose expression correlates with cancer progression, poor prognosis and increased metastatic potential.

METHODS

We identified MTA1 in BC exosomes by antibody array and confirmed expression of exosome-MTA1 across five breast cancer cells lines. Ectopic expression of tdTomato-tagged MTA1 and exosome transfer were examined by fluorescent microscopy. CRISPR/Cas9 genetic engineering was implemented to knockout MTA1 in MCF7 and MDA-MB-231 breast cancer cells. Reporter assays were used to monitor hypoxia and estrogen receptor signaling regulation by exosome-MTA1 transfer.

RESULTS

Ectopic overexpression of tdTomato-MTA1 in BC cell lines demonstrated exosome transfer of MTA1 to BC and vascular endothelial cells. MTA1 knockout in BC cells reduced cell proliferation and attenuated the hypoxic response in these cells, presumably through its co-repressor function, which could be rescued by the addition of exosomes containing MTA1. On the other hand, consistent with its co-activator function, estrogen receptor signaling was enhanced in MTA1 knockout cells and could be reversed by addition of MTA1-exosomes. Importantly, MTA1 knockout sensitized hormone receptor negative cells to 4-hydroxy tamoxifen treatment, which could be reversed by the addition of MTA1-exosomes.

CONCLUSIONS

This is the first report showing that BC exosomes contain MTA1 and can transfer it to other cells resulting in changes to hypoxia and estrogen receptor signaling in the tumor microenvironment. These results, collectively, provide evidence suggesting that exosome-mediated transfer of MTA1 contributes to BC progression by modifying cellular responses to important signaling pathways and that exosome-MTA1 may be developed as a biomarker and therapeutic target for BC.

Plasma exosome microRNAs are indicative of breast cancer

BACKGROUND

microRNAs are promising candidate breast cancer biomarkers due to their cancer-specific expression profiles. However, efforts to develop circulating breast cancer biomarkers are challenged by the heterogeneity of microRNAs in the blood. To overcome this challenge, we aimed to develop a molecular profile of microRNAs specifically secreted from breast cancer cells. Our first step towards this direction relates to capturing and analyzing the contents of exosomes, which are small secretory vesicles that selectively encapsulate microRNAs indicative of their cell of origin. To our knowledge, circulating exosome microRNAs have not been well-evaluated as biomarkers for breast cancer diagnosis or monitoring.

METHODS

Exosomes were collected from the conditioned media of human breast cancer cell lines, mouse plasma of patient-derived orthotopic xenograft models (PDX), and human plasma samples. Exosomes were verified by electron microscopy, nanoparticle tracking analysis, and western blot. Cellular and exosome microRNAs from breast cancer cell lines were profiled by next-generation small RNA sequencing. Plasma exosome microRNA expression was analyzed by qRT-PCR analysis.

RESULTS

Small RNA sequencing and qRT-PCR analysis showed that several microRNAs are selectively encapsulated or highly enriched in breast cancer exosomes. Importantly, the selectively enriched exosome microRNA, human miR-1246, was detected at significantly higher levels in exosomes isolated from PDX mouse plasma, indicating that tumor exosome microRNAs are released into the circulation and can serve as plasma biomarkers for breast cancer. This observation was extended to human plasma samples where miR-1246 and miR-21 were detected at significantly higher levels in the plasma exosomes of 16 patients with breast cancer as compared to the plasma exosomes of healthy control subjects. Receiver operating characteristic curve analysis indicated that the combination of plasma exosome miR-1246 and miR-21 is a better indicator of breast cancer than their individual levels.

CONCLUSIONS

Our results demonstrate that certain microRNA species, such as miR-21 and miR-1246, are selectively enriched in human breast cancer exosomes and significantly elevated in the plasma of patients with breast cancer. These findings indicate a potential new strategy to selectively analyze plasma breast cancer microRNAs indicative of the presence of breast cancer.