Scott Valastyan

Tumor Metastasis: Molecular Insights and Evolving Paradigms

Metastases represent the end products of a multistep cell-biological process termed the invasion-metastasis cascade, which involves dissemination of cancer cells to anatomically distant organ sites and their subsequent adaptation to foreign tissue microenvironments. Each of these events is driven by the acquisition of genetic and/or epigenetic alterations within tumor cells and the co-option of nonneoplastic stromal cells, which together endow incipient metastatic cells with traits needed to generate macroscopic metastases. Recent advances provide provocative insights into these cell-biological and molecular changes, which have implications regarding the steps of the invasion-metastasis cascade that appear amenable to therapeutic targeting.

A Pleiotropically Acting MicroRNA, miR-31, Inhibits Breast Cancer Metastasis

MicroRNAs are well suited to regulate tumor metastasis because of their capacity to coordinately repress numerous target genes, thereby potentially enabling their intervention at multiple steps of the invasion-metastasis cascade. We identify a microRNA exemplifying these attributes, miR-31, whose expression correlates inversely with metastasis in human breast cancer patients. Overexpression of miR-31 in otherwise-aggressive breast tumor cells suppresses metastasis. We deploy a stable microRNA sponge strategy to inhibit miR-31 in vivo; this allows otherwise-nonaggressive breast cancer cells to metastasize. These phenotypes do not involve confounding influences on primary tumor development and are specifically attributable to miR-31-mediated inhibition of several steps of metastasis, including local invasion, extravasation or initial survival at a distant site, and metastatic colonization. Such pleiotropy is achieved via coordinate repression of a cohort of metastasis-promoting genes, including RhoA. Indeed, RhoA re-expression partially reverses miR-31-imposed metastasis suppression. These findings indicate that miR-31 uses multiple mechanisms to oppose metastasis.

Concomitant suppression of three target genes can explain the impact of a microRNA on metastasis

It remains unclear whether a microRNA (miRNA) affects a given phenotype via concomitant down-regulation of its entire repertoire of targets or instead by suppression of only a modest subset of effectors. We demonstrate that inhibition of breast cancer metastasis by miR-31-a miRNA predicted to modulate >200 mRNAs-can be entirely explained by miR-31s pleiotropic regulation of three targets. Thus, concurrent re-expression of integrin-alpha5, radixin, and RhoA abrogates miR-31-imposed metastasis suppression. These effectors influence distinct steps of the metastatic process. Our findings have implications concerning the importance of pleiotropy for the biological actions of miRNAs and provide mechanistic insights into metastasis.

Concurrent Suppression of Integrin α5, Radixin, and RhoA Phenocopies the Effects of miR-31 on Metastasis

miR-31 inhibits breast cancer metastasis via the pleiotropic suppression of a cohort of prometastatic target genes that include integrin alpha(5) (ITGA5), radixin (RDX), and RhoA. We previously showed that the concomitant overexpression of ITGA5, RDX, and RhoA was capable of overriding the antimetastatic effects of ectopically expressed miR-31 in vivo. However, these prior studies failed to investigate whether the combined suppression of the endogenous mRNAs encoding these three proteins recapitulated the in vivo consequences of miR-31 expression on metastasis. We show here that short hairpin RNA-mediated concurrent downregulation of ITGA5, RDX, and RhoA is sufficient to phenocopy the full spectrum of described influences of miR-31 on metastasis in vivo, including the effects of this microRNA (miRNA) on local invasion, early post-intravasation events, and metastatic colonization. These findings provide mechanistic insights into the metastatic process and have implications about the importance of pleiotropy for the biological actions of miRNAs.

Activation of miR-31 function in already-established metastases elicits metastatic regression

Distant metastases, rather than the primary tumors from which these lesions arise, are responsible for >90% of carcinoma-associated mortality. Many patients already harbor disseminated tumor cells in their bloodstream, bone marrow, and distant organs when they initially present with cancer. Hence, truly effective anti-metastatic therapeutics must impair the proliferation and survival of already-established metastases. Here, we assess the therapeutic potential of acutely expressing the microRNA miR-31 in already-formed breast cancer metastases. Activation of miR-31 in established metastases elicits metastatic regression and prolongs survival. Remarkably, even brief induction of miR-31 in macroscopic pulmonary metastases diminishes metastatic burden. In contrast, acute miR-31 expression fails to affect primary mammary tumor growth. miR-31 triggers metastatic regression in the lungs by eliciting cell cycle arrest and apoptosis; these responses occur specifically in metastases and can be explained by miR-31-mediated suppression of integrin-α5, radixin, and RhoA. Indeed, concomitant re-expression of these three proteins renders already-seeded pulmonary metastases refractory to miR-31-conferred regression. Upon miR-31 activation, Akt-dependent signaling is attenuated and the proapoptotic molecule Bim is induced; these effects occur in a metastasis-specific manner in pulmonary lesions and are abrogated by concurrent re-expression of integrin-α5, radixin, and RhoA. Collectively, these findings raise the possibility that intervention strategies centered on restoring miR-31 function may prove clinically useful for combating metastatic disease.

miR-31: a crucial overseer of tumor metastasis and other emerging roles.

MicroRNAs constitute a family of pleiotropically acting short regulatory RNAs. Increasingly, specific microRNAs have been implicated as key modulators of a variety of normal physiologic processes; moreover, the aberrant activity of certain microRNAs has been linked to the pathogenesis of multiple diseases. The microRNA miR-31 has been identified as a crucial overseer of several normal and diseased phenotypes. Here, we describe current knowledge regarding the functions of miR-31, with an emphasis placed upon the role of this microRNA in neoplastic development and tumor metastasis. Additionally, we highlight a number of recent reports concerning the contributions of miR-31 to other pathological states, the role of this microRNA in normal physiology, and the upstream mechanisms by which miR-31 expression levels are regulated. Assessed collectively, existing evidence suggests that miR-31 concomitantly regulates a number of essential signaling pathways in mammalian cells. For these reasons, further elucidation of the biological actions of miR-31 may prove significant for the prognosis and remediation of various pathological states.

Roles for microRNAs in the regulation of cell adhesion molecules

Maintenance of appropriate cell adhesion is crucial for normal cellular and organismal homeostasis. Certain microRNAs have recently been found capable of regulating molecules that oversee the fundamental cell biological events that drive cellular adhesion. It is now apparent that microRNAs play crucial roles in the great majority of biochemical pathways that contribute to normal cell adhesion. In this Commentary, we describe the latest advances within this still-emerging field, and highlight connections between the deregulation of microRNAs that affect cell-adhesion-associated molecules and the pathogenesis of several human diseases. Current evidence suggests that the ability of certain microRNAs – notably miR-17, miR-29, miR-31, miR-124 and miR-200 – to pleiotropically regulate multiple molecular components of the cell adhesion machinery endows these microRNAs with the capacity to function as key modulators of adhesion-associated processes. This, in turn, holds important implications for our understanding of both the basic biology of cell adhesion and the etiology of multiple pathological conditions.

MicroRNAs: Crucial multi-tasking components in the complex circuitry of tumor metastasis

Distant metastases are the underlying cause of patient mortality in an overwhelming majority of human carcinomas. Certain microRNAs have recently been found capable of regulating the process of tumor metastasis. In this review, we highlight recent advances within this rapidly emerging field, endeavor to connect known microRNA pathways with recent conceptual advances in the larger field of metastasis research, and speculate regarding the future utility of microRNAs in the diagnosis and treatment of human cancers. Assessed collectively, current evidence suggests that the pleiotropic activities of microRNAs endow them with the capacity to function as crucial, yet previously unappreciated, nodes within already-identified metastasis regulatory circuitry. This has important implications for our understanding of the pathogenesis of high-grade malignancies.

Assaying microRNA loss-of-function phenotypes in mammalian cells: Emerging tools and their potential therapeutic utility

MicroRNAs are small, non-coding RNAs that are increasingly appreciated to play critical roles in the modulation of gene expression. In mammalian cells, our knowledge regarding the full impact of microRNAs on cellular behavior remains fragmentary. This has been due, in significant part, to the limited availability of experimental tools for studying microRNA loss-of-function phenotypes. Recently, several strategies for achieving this goal have been developed. Here, we discuss these methodologies for inhibiting specific microRNAs in mammalian cells both in vitro and in vivo, compare and contrast the strengths and weaknesses of these approaches, and speculate regarding the future impact of these antagonists on the treatment of human diseases such as cancer. These emerging techniques enable the attenuation of microRNA function in a manner that is quite sequence-specific, relatively long-lasting, and increasingly cost-effective. As such, some of these advances hold great promise in terms of their eventual utility as therapeutic agents.

Metastasis suppression: a role of the Dice(r)

Recent studies have implicated the microRNA biogenesis enzyme Dicer as a suppressor of breast carcinoma metastasis and elucidated upstream signaling pathways that control Dicer levels.