Researchers from the MD Anderson Cancer Center this month reported new data showing that the microRNA-200 family, which has previously been implicated in cancer metastasis, can regulate tumor angiogenesis.
Notably, miR-200's ability to do this was found to be dependent on tumor type, with its overexpression associated with poor clinical outcomes in certain cancers and positive outcomes in others — findings that have important translational implications for the development of miRNA-based therapeutics, according to the investigators.
To date, there have been a number of papers showing that downregulation of the miR-200 family can inhibit the epithelial-mesenchymal transition — a key step in the initiation of metastasis — and that its restoration can reverse the process. However, while forced miR-200 expression can prevent the formation of metastasis in lung cancer, it has been found to enhance metastasis in certain breast cancer models.
In light of these discrepant data, MD Anderson's Anil Sood and colleagues sought to further investigate the potential mechanisms that underlie the miRNA family's different effects on different cancer types, he told Gene Silencing News.
The team began by analyzing miR-200 expression data for four adenocarcinomas and found that low expression of a member of the miRNA family was significantly associated with worse overall survival for ovarian and renal cancers. At the same time, low expression of several miR-200 isoforms was significantly associated with poor survival in lung adenocarcinomas.
In breast cancer, however, low expression of a miR-200 family member was associated with improved clinical outcome, except in the case of luminal subtypes where worsened survival was linked to low miR-200 levels.
In combination with this work, the scientists were performing extensive bioinformatics analyses to identify the pathways regulated by the different miR-200 family members, which include miR-141, miR-200a, miR-200b, miR-200c, and miR-429, Sood explained. These studies ultimately led the researchers to angiogenesis-related pathways that "all converged" onto the angiogenesis-promoting cytokines interleukin-8 and CXCL1, he said.
Specifically, the research team found a strong inverse correlation of IL-8 expression with a number of miR-200 family members, according to their study, which appeared in Nature Communications. At the same time, the cancer types in which miR-200 regulated angiogenesis — ovarian, lung, renal, and basal-like breast — were also found to have "markedly elevated" IL-8 and CXCL1 expression when compared with luminal breast cancers.
"As elevated levels of IL-8 have been associated with worsened clinical outcome in ovarian, renal and lung adenocarcinomas, we sought to determine whether this could help explain the clinical differences seen across cancer types," they wrote.
Using microarray databases, the investigators found increased IL-8 expression in basal-like breast cancers was associated with significantly worse overall patient survival, relapse-free survival, and distant metastasis-free survival — relationships not noted in luminal subtypes of the disease.
"Differences in clinical outcome were not observed in any breast cancer subtype based on CXCL1 expression, suggesting that miR-200 regulation of IL-8 is a key mediator explaining the divergence in clinical outcome between these breast cancer subtypes," they wrote. High expression of IL-8 and CXCL1 were also associated with worse overall survival in lung and renal cancers.
Additional in vitro and in vivo experiments confirmed that IL-8 and CXCL1 are direct targets of the miR-200 family, and that the miRNA family can directly affect angiogenesis in endothelial cells by targeting production of IL-8 and CXCL1.
With an eye toward clinical applications of the miR-200 family, Sood and his colleagues tested the effects of miR-200 delivery in mouse lung cancer models.
Using DOPC as a delivery vehicle, animals were treated with either control miRNAs, miR-200a, miR-200b, or miR-200a and miR-200b. After two weeks of treatment, those mice receiving miR-200a or miR-200b alone experienced reductions in tumor volume of 25 percent and 36 percent, respectively. Those that received the combination of miR-200 family members showed a 72 percent reduction in tumor volume.
The reduced tumor volumes corresponded with significant reductions in tumor mass, and similar effects were observed in follow-on testing in models of ovarian and renal cancer.
The researchers then tested the therapeutic effects of miR-200 in orthotopic cancer models. In the case of lung cancer, "delivery of miR-200a and -200b members resulted in a significant decrease in primary tumor burden as compared with control miRNA," they wrote.
"Similarly, for the ovarian cancer model, the delivery of miR-200 members resulted in a significant decrease in total metastatic burden as compared with control miRNA," with a decrease in metastatic lesions in common metastatic sites of high-grade serous ovarian carcinoma.
When Sood and the other investigators used chitosan nanoparticles to deliver miR-200a or miR-200b into tumor vasculature in an ovarian cancer mouse model, they observed a 92 percent reduction in metastatic burden as compared to animals receiving control miRNAs.
Interestingly, when the mice were treated with a combination of the miR-200 members, their tumor vasculature was found to be significantly less permeable, with less-organized vessels, compared to those receiving control miRNAs.
No adverse effects were associated with miRNA treatment.
Overall, the findings position the miR-200 family as "novel, endogenous angiogenesis inhibitors," the researchers concluded.
"The ability of the miR-200 family to target several important cancer pathways, such as EMT and angiogenesis, is highly attractive … [and] although potential challenges with therapeutic RNA interference still remain, we believe the outlook of bringing therapeutic miRNA delivery to the clinic is promising."
Sood said that his lab is now exploring whether chemical modifications to the miR-200 mimics can improve stability and efficacy, and testing the effects of the miRNA family in other cancer models and in combination with cytotoxic agents.