Cell Migration & Metastasis
Preventing metastasis, the spread of cancer cells from a primary tumor to a secondary site, represents an important therapeutic approach to cancer treatment. To identify potential targets, LICR investigators are studying cell migration, the first step in metastasis.
Introduction
Cancer cells spread from the initial site of tumor growth by first invading the surrounding tissue (migration), then entering the blood or lymph vessels (intravasation), and finally crossing the vessel wall to exit the vasculature (extravasation) in distal organs. The cancer cells then colonize the new site and proliferate to form a second tumor mass (See also Angiogenesis & Lymphangiogenesis).
Many different signaling proteins contribute to cell migration, from transmembrane receptors to transcription factors, but a group of proteins known as the Rho GTPases act as a focal point for migration signaling. There are 22 Rho family members in humans, with RhoA, Rac1 and Cdc42 being the best-studied members. They regulate cell migration, affecting actin and microtubule dynamics, cell-cell and cell-extracellular matrix adhesion, and intracellular trafficking of proteins required for cells to move. LICR scientists are investigating how Rho GTPases and their interacting partners affect cell shape, or ‘morphology’ and the various steps of cell migration that lead to cancer metastasis.
Cell Morphology
Cell migration and morphology are determined largely by the ‘cytoskeleton’, the internal scaffolding of proteins that give the cell shape, polarity (organization of the cell’s contents into ‘front’ and ‘back’ zones) and the capacity to move. Rho GTPases are thought to modulate the cytoskeleton by regulating the polymerization of actin into filaments that form the bulk of the cytoskeleton. Although the molecular events are incompletely understood, Rac and Cdc42 are thought to promote the activation of the proteins SCAR/WAVE and WASP to generate cellular protrusions at the front of cells, which are needed for migration. SCAR and WASP then activate the Arp2/3 complex to nucleate the formation of new actin filaments that exert a protrusive force on the membrane.
LICR investigators have used genetic (RNAi) and cell biology techniques to identify genes required for the formation of protrusions in cells from the fruit-fly, Drosophila melanogaster. The team delineated an entire pathway from Cdc42 and Rac to SCAR and the Arp2/3 complex, and also identified proteins that protect SCAR from downregulation by proteasomal degradation.
Cell Migration
Rho proteins in cell migration
LICR scientists have shown that RhoA and Rac1 act together to coordinate cell migration. As described above, Rac1 acts at the front of migrating cells to stimulate actin-mediated membrane protrusion and the attachment of those protrusions to the extracellular matrix. RhoA acts primarily at the rear of cells to induce forward movement of the nucleus and cell body, and to mediate detachment of the back of the cell from the extracellular matrix (see Figure).
Cdc42 is important for cell polarization and directionality of movement, and depending on the cell type can also contribute to cell speed by enhancing Rac-mediated membrane protrusion at the front of cells. The LICR team has shown that WASP activity is not only stimulated by Cdc42, but also regulated by phosphorylation. Thus WASP acts as a node to receive signals from multiple inputs. This finding may be a paradigm for other Rho GTPase targets.
LICR scientists are also investigating the function of a less well-characterized member of the Rho family, RhoE. RhoE is unusual in that it does not hydrolyze GTP, and therefore is not technically a GTPase. Instead, RhoE function may be regulated by altering its expression; RhoE is upregulated in a number of cancers, and LICR investigators have found it is increased in response to growth factor stimulation and DNA damage. RhoE acts in opposition to RhoA, specifically interacting with ROCK I, a RhoA target, and preventing ROCK I from stimulating assembly of actin and myosin filaments to form stress fibers. Sustained upregulation of RhoE expression inhibits cell cycle progression and cell proliferation, in part by preventing accumulation of the cell cycle regulator Cyclin D1. RhoE therefore appears to have two functions, one in cell migration and one in cell cycle control. As both of these processes are crucial for metastasis, RhoE represents a particularly interesting target for therapy.
Key Publications
- M.J. Karkkainen, P. Haiko, K. Sainio, J. Partanen, J. Taipale, T.V. Petrova, M. Jeltsch, D.G. Jackson, M. Talikka, H. Rauvala, C. Betsholtz, K. Alitalo. Vascular endothelial growth factor C is required for sprouting of the first lymphatic vessels from embryonic veins. Nature Immunology 5(1):11-2, 2004.
- K. Riento, N. Totty, P. Villalonga, R. Garg, R. Guasch, A.J. Ridley. RhoE function is regulated by ROCK I-mediated phosphorylation. EMBO Journal 24(6):1170-80, 2005. [PMID: 15775972]
- P. Villalonga, M.G. Guasch, K. Riento, A.J. Ridley. RhoE inhibits cell cycle progression and Ras-induced transformation. Molecular & Cellular Biology 24:7829-7840, 2004. [PMID: 15340047]
- P. Kunda, G. Craig, V. Dominguez, B. Baum. Abi, Sra1, and Kette control the stability and localization of SCAR/WAVE to regulate the formation of actin-based protrusions. Current Biology 13(21):1867-75, 2003. [PMID: 14588242]
- G.O.C. Cory, R. Cramer, L. Blanchoin, A.J. Ridley. Phosphorylation of the WASp-VCA Domain Increases its Affinity for the Arp2/3 Complex and Enhances Actin Polymerization by WASp. Molecular Cell 11:1229-1339, 2003. [PMID: 12769847]
- B. Wojciak-Stothard, A.J.Ridley. Shear stress-induced endothelial cell polarization is mediated by Rho and Rac but not Cdc42 or PI 3-kinases. Journal of Cell Biology 161:429-439, 2003. [PMID: 12719476]
- K. Riento, R.M. Guasch, R. Garg, J. Bouquin, A.J. Ridley. RhoE binds to ROCKI and inhibits downstream signalling. Molecular Cell Biology 23:4219-4229, 2003. [PMID: 12773565]