MiR-34a Replacement As a Novel Therapeutic Approach for Multiple Myeloma: Preclinical In Vitro and In Vivo Evidence

Multiple myeloma (MM) remains an incurable disease despite important therapeutic advances in the last few years. Small non-coding RNAs (miRNAs) synthetic mimics are a new class of biological agents which have recently demonstrated preclinical activity against a variety of human neoplasms. miRNA antitumor activity has been related to their capacity to interfere with mRNA stability and protein transducing activity. miR-34a has tumor suppressor activity and is transcriptionally regulated by p53. We investigated the in vitro and in vivo therapeutic potential of pre-miR-34a mimics against human MM cells. Transient expression of pre-miR-34a mimics, after electroporation of SKMM1 and RPMI-8226 MM cell lines which display low constitutive miR-34a levels, triggered antiproliferative effects, as demonstrated by MTT and long-term soft-agar colony assays. 48 hours after cell transfection, apoptotic events were detected in both cell lines exposed to miR-34a mimics. In parallel experiments, MM cells stably transduced with miR-34a gene cloned in a lentiviral vector showed significant growth reduction as compared to empty vector-transduced cell colonies, providing additional evidence of miR-34a tumor suppressor activity in MM cells. qRT-PCR analysis of treated MM cells showed that pre-miR-34a mimics induced down-regulation of mRNAs coding for Notch1 and the cell-cycle dependent kinase 6 (CDK6), validated miR-34a targets. Furthermore, decreased anti-apoptotic Bcl-2 and CDK6 proteins was detected after pre-miR-34a mimic expression, evidence by western blotting analysis. The anti-MM activity of pre-miR-34a mimics was also evaluated in vivo using xenografted SCID models of human MM. Intra-tumor delivery of pre-miR-34a was performed by a novel formulation with Neutral Lipid Emulsion (NLE). Following 4 injections (3 days apart) of pre-miR34a formulated in NLE particles, a highly significant inhibition of tumor growth was detected in SKMM1 xenografted SCID mice. At day 13 after the first treatment, tumors in mice treated with formulated pre-miR-34a were significantly smaller than tumors in mice treated with the formulated scrambled sequence (P=0.0002) or vehicle (P=0.0002) or PBS (P=0.0001). Interestingly, at day 21a three mice enrolled in the miR-34a treated group showed complete regression of tumors. Formulated synthetic pre-miR-34a also produced a significant increase of mice survival (P=0.01 versus formulated scrambled sequence). A similar in vivo tumor growth inhibition was observed in mice xenografted with SKMM1 MM cells stably transduced with a miR-34a lentiviral construct, as compared to cells transduced with the empty vector. We here provide the first proof-of-principle demonstrating that replacement of miR-34a produces therapeutic activity against MM cells with low constitutive miR-34a expression. Our findings provide a framework for development of miR-34a-based therapeutic strategies in MM.