Cancer is currently the second leading cause of death in the United States and, if current trends continue, will soon be the leading killer. The major reasons for cancer deaths are complications arising from metastasis. Therefore, improved morbidity and mortality will require effective treatments targeting metastatic disease. Despite many advancements in knowledge from over a century of researching metastasis, the molecular mechanisms are still not completely understood. Several reports have described the involvement of a recently discovered class of non-coding regulatory RNA, termed microRNA (miRNA), in the regulation of cancer (oncomir; (1
)). More recently, a specialized family of miRNA, which we call metastamir,
have been shown to have pro-and anti-metastatic effects.
miRNA were originally discovered because of their roles in controlling the timing of C. elegans
larval development. Less than a decade later they were identified in plant and mammalian cells (see (3
) for review. Typically, pri-miRNA are transcribed by RNA polymerase II before capping, polyadenylation and maturation of a hairpin loop structure by ribonuclease 3 (Drosha) into pre-miRNA. Following export into the cytoplasm, the pre-miRNA become associated with several ribonucleoproteins of the RISC (RNA-induced silencing complex), including Dicer and Argonaut family members from which a mature miRNA are formed. miRNA complement the 3′-UTR of mRNA in order to impair translation or alter message stability. Because of their small size, miRNA are predicted to be promiscuous and may have several hundred mRNA targets, meaning that a single miRNA can, by itself, impact the expression of hundreds of proteins (See (3
) for review).
Metastasis involves multiple steps and multiple genes in which neoplastic cells dissociate from the primary tumor, enter body cavities or, more commonly, circulatory systems (lymphatics or blood vasculature), survive during transport until they arrest at discontiguous sites, exit the circulation, and proliferate at ectopic sites (colonization) in response to local growth factors (4
). The process is extremely inefficient (of the ~4 million cells entering the vascular compartment per gram of tumor per day, much less than 0.01% develop macroscopic masses elsewhere (4
)). The inefficiency is perhaps because every step in the metastatic cascade is selective and rate-limiting (i.e., failure to complete any step precludes subsequent steps). Each step in metastasis requires coordinated temporal expression of genes and spacio-temporal expression of proteins.
Examination of mRNA expression patterns has yielded sometimes conflicting results related to roles in metastasis, prompting some to question even the existence of metastasis-regulatory genes. Yet, multiple labs, using several different human and rodent model systems, demonstrated the existence of gene products that affect metastasis without promoting or inhibiting tumorigenicity at orthotopic sites (5
). So, while tumor formation is prerequisite to metastasis, tumorigenicity and metastasis are distinct phenotypes, the latter requiring genetic changes superimposed upon those needed to make the tumor. These considerations led us and others to predict the existence of metastamirs.
The invitation to write this mini-review was prompted by our discovery that the miR-146 family of miRNA could profoundly inhibit invasion and metastasis of MDA-MB-231 human breast carcinoma cells. In that report, we further showed that miR-146a/b was downstream of the BRMS1 metastasis suppressor and intermediate to BRMS1-regulated genes (6
). Concurrently, we have shown that BRMS1 coordinately regulates entire families of metastamirs - up-regulating metastasis-suppressing miRNA and down-regulating metastasis-promoting miRNA (7
Those findings, coupled with an explosion of papers describing miRNA and metastasis-associated steps compelled us to expand the focus of this mini-review to consider the state of the field. To date, eleven miRNA have been shown to promote or inhibit metastasis in experimental models () and the number is likely to grow even further because more than 20 more have been shown to impact critical steps in the metastatic cascade, such as epithelial-mesenchymal transition (EMT), apoptosis, and angiogenesis (). Furthermore, several clinical studies have identified correlations between miRNA expression and recurrence, development of metastases and/or survival (for a recent review, see (8
)). Therefore, our goal is to focus on the evidence for metastamirs, the implications of their existence and some technical and theoretical considerations that emerge from their discovery.
Figure 1 Critical steps in metastasis altered by metastamir. Pro- and anti-metastatic metastamir are listed with the steps in the metastatic cascade of which they affect. The metastamir that have been functionally tested for metastasis in vivo are highlighted (more ...)