SummaryMicrovesicles are generated by the outward budding and fission of membrane vesicles from the cell surface. Recent studies suggest that microvesicle shedding is a highly regulated process that occurs in a spectrum of cell types and, more frequently, in tumor cells. Microvesicles have been widely detected in various biological fluids including peripheral blood, urine and ascitic fluids, and their function and composition depend on the cells from which they originate. By facilitating the horizontal transfer of bioactive molecules such as proteins, RNAs and microRNAs, they are now thought to have vital roles in tumor invasion and metastases, inflammation, coagulation, and stem-cell renewal and expansion. This Commentary summarizes recent literature on the properties and biogenesis of microvesicles and their potential role in cancer progression.

β-Catenin has a dual function in cells: fortifying cadherin-based adhesion at the plasma membrane and activating transcription in the nucleus. We found that in melanoma cells, WNT5A stimulated the disruption of N-cadherin and β-catenin complexes by activating the guanosine triphosphatase adenosine diphosphate ribosylation factor 6 (ARF6). Binding of WNT5A to the Frizzled 4–LRP6 (low-density lipoprotein receptor–related protein 6) receptor complex activated ARF6, which liberated β-catenin from N-cadherin, thus increasing the pool of free β-catenin, enhancing β-catenin–mediated transcription, and stimulating invasion. In contrast to WNT5A, the guidance cue SLIT2 and its receptor ROBO1 inhibited ARF6 activation and, accordingly, stabilized the interaction of N-cadherin with β-catenin and reduced transcription and invasion. Thus, ARF6 integrated competing signals in melanoma cells, thereby enabling plasticity in the response to external cues. Moreover, small-molecule inhibition of ARF6 stabilized adherens junctions, blocked β-catenin signaling and invasiveness of melanoma cells in culture, and reduced spontaneous pulmonary metastasis in mice, suggesting that targeting ARF6 may provide a means of inhibiting WNT/β-catenin signaling in cancer.

The study of extracellular vesicles (EVs) is a rapidly evolving field, owing in large part to recent advances in the realization of their significant contributions to normal physiology and disease. Once discredited as cell debris, these membrane vesicles have now emerged as mediators of intercellular communication by interaction with target cells, drug and gene delivery, and as potentially versatile platforms of clinical biomarkers as a result of their distinctive protein, nucleic acid and lipid cargoes. While there are multiple classes of EVs released from almost all cell types, here we focus primarily on the biogenesis, fate and functional cargoes of microvesicles (MVs). MVs regulate many important cellular processes including facilitating cell invasion, cell growth, evasion of immune response, stimulating angiogenesis, drug resistance and many others.

Cells release multiple, distinct, forms of extracellular vesicles including structures known as microvesicles which are known to alter the extracellular environment. Despite growing understanding of microvesicle biogenesis, function, and contents, mechanisms regulating cargo delivery and enrichment remain largely unknown. Here we demonstrate that in amoeboid-like invasive tumor cell lines, the v-SNARE, VAMP3, regulates delivery of microvesicle cargo such as the membrane-type 1 matrix metalloprotease (MT1-MMP) to shedding microvesicles. MT1-MMP delivery to nascent microvesicles depends on the association of VAMP3 with the tetraspanin CD9 and facilitates the maintenance of amoeboid cell invasion. VAMP3-shRNA expression depletes shed vesicles of MT1-MMP and decreases cell invasiveness when embedded in cross-linked collagen matrices. Finally, we describe functionally similar microvesicles isolated from bodily fluids of ovarian cancer patients. Together these studies demonstrate the importance of microvesicle cargo sorting in matrix degradation and disease progression.

Tumor cell invasion requires the molecular and physical adaptation of both the cell and its microenvironment. Here we show that tumor cells are able to switch between the use of microvesicles and invadopodia to facilitate invasion through the extracellular matrix. Invadopodia formation accompanies the mesenchymal mode of migration on firm matrices and is facilitated by Rac1 activation. On the other hand, during invasion through compliant and deformable environments, tumor cells adopt an amoeboid phenotype and release microvesicles. Notably, firm matrices do not support microvesicle release, whereas compliant matrices are not conducive to invadopodia biogenesis. Furthermore, Rac1 activation is required for invadopodia function, while its inactivation promotes RhoA activation and actomyosin contractility required for microvesicle shedding. Suppression of RhoA signaling blocks microvesicle formation but enhances the formation of invadopodia. Finally, we describe Rho-mediated pathways involved in microvesicle biogenesis through the regulation of myosin light chain phosphatase. Our findings suggest that the ability of tumor cells to switch between the aforementioned qualitatively distinct modes of invasion may allow for dissemination across different microenvironments.