Metastasis is responsible for the vast majority of prostate cancer deaths. As such, it is essential to understand the mechanisms driving prostate cancer to this lethal stage. Deregulation of microRNAs is increasingly implicated in the progression and metastasis of prostate and other cancers. MiR-23b and miR-27b, two members of the same miR cluster (miR-23b/-27b), are down-regulated in human metastatic, castration resistant prostate cancer (CRPC) as compared to primary tumors and benign tissue. However, the role of the miR-23b/-27b cluster is not fully understood, particularly in metastatic disease. Interpretation of existing data is complicated by the analysis of the individual effects of miR-23b and miR-27b even though these miRs are co-transcribed and coordinately expressed. The primary goal of our study is to determine the effects of miR-23b/-27b on metastatic processes in vitro and in vivo.

Ectopic expression of miR-23b/-27b in two aggressive prostate cancer cell lines decreases migration, invasion and anchorage-independent growth. Conversely, inhibition of miR-23b/-27b using specific antagomirs in relatively indolent prostate cancer cells promotes increased invasion and migration. E-cadherin protein and mRNA levels were inversely related to miR-23b/-27b levels. In contrast, manipulation of miR-23b/-27b levels had no effect on prostate cancer cell proliferation in any of the tested cell lines. These findings suggest that miR-23b/-27b is specifically linked to metastasis suppression. The Rho GTPase, Rac1, is hyperactive in CRPC cell lines and patient samples. Since Rac1 promotes invasion and migration, we investigated the effects of miR-23b/-27b expression and inhibition on Rac1 activity. Ectopic expression of miR-23b/-27b in aggressive prostate cancer cell lines significantly attenuates active Rac1 while having no effect on total Rac1 levels. Conversely, inhibition of miR-23b/-27b in less aggressive prostate cancer cells increased Rac1 activity but not Rac1 levels.

We further examined the effects of miR-23b/-27b on prostate cancer metastasis in vivo. Metastatic prostate cancer cells expressing luciferase and miR-23b/-27b or a scrambled control, were injected into the ventral prostates of nude mice. Orthotopic tumor formation and metastases were assessed by bioluminescence imaging. Metastatic tumor burden was greatly decreased in the tumors derived from miR-23b/-27b expressing cells.

Taken together, these data demonstrate that expression of miR-23b/-27b exerts metastasis-suppressing effects in vitro and in vivo. The miR-23b/-27b cluster may be a useful biomarker of poor prognosis in addition to having therapeutic potential in advanced, metastatic prostate cancer.

Citation Format: Meghan A. Rice, Reema Ishteiwy, Thirupandiyur Udayakumar, Derek Dykxhoorn, Kerry L. Burnstein. The miR-23b/-27b cluster decreases metastasis of aggressive prostate cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1467. doi:10.1158/1538-7445.AM2014-1467