Why is microRNA action tissue specific? A putative defense mechanism against growth disorders, tumor development or progression mediated by circulating microRNA?

MicroRNAs as endogenous mediators of RNA interference and epigenetic regulation are involved in the regulation of numerous basic physiological processes. Both their expression and action is tissue specific, as microRNA target different messenger RNA molecules in different tissues and have various actions. MicroRNAs are major players in tumor development and act as oncogenes and tumor suppressors that also depend on the cellular context. MicroRNA are secreted and are present in the circulation, and circulating microRNA might affect gene expression in various cells. We present a hypothesis on the relevance of tissue specific microRNA action supposing that it might be a putative defense mechanism preventing secreted microRNA-mediated uniform gene expression changes (e.g. inducing cell proliferation or inhibiting apoptosis) and thus growth disorders, tumor development or progression that would occur if all cells and tissues would respond in the same way to circulating microRNA.