Inhibition of Polyamine Synthesis in Colon Cancer Cells Alters Expression of microRNAs and Cancer-Associated Genes

Polyamines are ubiquitous organic cations that are required for normal growth, development, and tissue repair. Multiple studies have demonstrated a clear association between high level of polyamines and colorectal cancer. The recent success of a randomized placebo-controlled double blind trial of -difluoromethylornithine (DFMO) in combination with sulindac validated the polyamine pathway as a target for preventing metachronous colon adenomas in patients with prior colon polyps. DFMO is an irreversible inhibitor of ornithine decarboxylase (ODC) resulting in decreased intracellular polyamine pools. Polyamines have been shown to alter translation at both the initiation and elongation steps of protein synthesis. We hypothesized that the molecular mechanisms of action of tissue polyamine depletion by DFMO may be due to modulation of microRNA expression profiles resulting in altered protein synthesis. We treated colorectal cancer cell lines with DFMO and measured microRNA changes by microarray methods, which were validated using real time PCR. Using immunoblot analysis and luciferase assay we identified specific tumor associated oncomiRs responsive to DFMO treatment. Overall, polyamine depletion alters expression of canonical oncogenes and tumor suppressors in a manner associated with changes in growth and specific cancer phenotypes. Taken together, our data suggests that part of the anti-carcinogenic actions of agents that deplete cell and tissue polyamines involves altered expression of specific microRNAs and subsequent microRNA-dependent effects on expression of cancer causing and/or suppressing genes.