Maximizing tumor cell invasion

Abstract

The Rho family of proteins, com-prised of Rho, Rac, and Cdc42 is impli-cated actomyosin contractility andmicrotubule arrangement (Ridley, 2001;Scott, 2002). RhoA, B and C activateRho kinase (ROCK), which causes theformation of actin stress fibers by rein-forcing actomysin contractile force. Racactivation, on the other hand, results inlamellipodia formation (Ridley, 2001).Tumor cell migration is highly depen-dent on cellular actin cytoskeletondynamics. In order for cancer cells tometastasize, the primary tumor mustinvade and migrate through tissues soas to reside in a secondary location.The invasion process can either be pro-tease-dependent or -independent. Todate, cancer cells exhibit two forms ofcellular movement: mesenchymal thatinvolves elongated cells and amoeboidthat involves rounded blebbing cells.The mesenchymal mode of invasionengages the extracellular matrixremodeling by proteases, whereas theamoeboid movement is proteases-inde-pendent; instead, it relies on extracellu-lar matrix remodeling by force (Sahaiand Marshall, 2003). Ameoboid move-ment is a result of ROCK activation asit requires high actomyosin contractibil-ity. Although aberrant small Rho-GTPase activation has been linked totumor cell metastasis, little is knownabout how tumor cells switch frommesenchymal to amoeboid modes ofmovement or vice versa.Sanz-Moreno et al. (2008) unveiledthe factors that control the differentmodes of cell mobility through a sys-tematic guanine nucleotide exchangefactors (GEFs) siRNA screening. Thestudy aimed to identify GEFs that regu-late cell morphology and movement inorder to understand the interchangebetween two different modes of move-ment (mesenchymal and amoeboidmovement) in melanoma cells. Fromthe screen, DOCK3 was discovered tocontrol elongated movement. DOCK3 isa Rac specific GEF that, together withNEDD9, regulates Rac and WAVE2 toinitiate mesenchymal movement, thusinhibiting amoeboid movement. Con-versely, amoeboid movement requiresRhoA activation of ROCK and ARHGAP,which in turn inactivates mesenchymalmovement.The siRNA based screen was firsttested in A375M2 cell line, a low meta-static melanoma cells that are in amoe-boid and mesenchymal transition. Fromthe 83 GEFs that were targeted, thesuppression of DOCK3 expressionresulted in the exhibition of an ameo-boid morphology, and the inhibition ofthe transition from the amoeboid tomesenchymal mode. In this study,Sanz-Moreno et al. also found that inhi-bition of ROCK activity resulted in aconversion of the round cells to elon-gated cells, although it did not affectthe speed of cancer cell migration.DOCK3’s effect is specific, as otherDBL families do not cause cell elonga-tion. In addition, knock-down of DOCK3or expression of an internal deletionmutant, DELDOCK3, suppressed theconversion of the rounded cells to elon-gated cells. Similarly, Rac1 inhibition bysiRNA or by inhibitor also abolished theelongated morphology. Taken together