Progression of breast cancer to metastatic bone disease is associated with an aberrantly elevated expression of Runx2, which promotes disease progression through transcriptional activation of genes involved in metastasis. Inhibition of Runx2 in metastatic breast cancer cells prevents metastatic bone disease, thus providing a basis for Runx2 as a potential therapeutic target. Since transcription factors are challenging to target for therapeutic intervention, our goal was to evaluate the potential clinical use of Runx2-targeting microRNAs (miRNAs) to reduce tumour growth and bone metastatic burden. Expression analysis of a panel of miRNAs regulating Runx2 revealed a reciprocal relationship between the abundance of Runx2 protein and two miRNAs, miR-135 and miR-203. These miRNAs are highly expressed in normal breast epithelial cells where Runx2 is not detected, and conversely are absent in metastatic breast cancer cell lines and importantly, in tissue biopsies that express Runx2. Reconstituting metastatic MDA-MB-231-luc cells with miR-135 and miR-203 reduced the abundance of Runx2 and the expression of the metastasis-promoting Runx2 target genes. Additionally, tumour cell viability was decreased and migration suppressed in vitro. In vivo implantation of MDA-MB-231-luc cells reconstituted with miR-135 or miR-203 into the mammary gland, followed by additional intratumoural administration of the synthetic miRNAs reduced tumour growth and importantly, spontaneous metastasis to bone. Furthermore, intratibial injection of these cells impaired tumour growth in the bone environment, inhibited bone resorption and secondary metastasis to lung. Importantly, reconstitution of Runx2 in MDA-MB-231-luc cells delivered with miR-135 and miR-203 reversed the inhibitory effect of the miRNAs on tumour growth and metastasis. We conclude that aberrant expression of Runx2 in aggressive tumour cells is related to the loss of specific Runx2-targeting miRNAs and that a clinically relevant replacement strategy by delivery of synthetic miRNAs is a viable therapeutic approach to target transcription factors for the prevention of metastatic bone disease.
Disclosure: The authors declared no competing interests.