A critical event during tumorigenesis is the conversion from a static primary tumor to an invasive, disseminating metastasis. Moreover, tumor cells show an increased capacity to migrate. Numerous intracellular signaling molecules have been implicated in migratory processes. Among them, the Rho GTPase family plays a pivotal role in regulating the biochemical and cytoskeletal pathways relevant to cell migration. The Rho GTPases Rac, Cdc42, and Rho control cell protrusions during migration. Aberrant regulation of Rho proteins is believed to associate with metastasis by promoting cell motility (Clark et al., 2000
; Evers et al., 2000
; Jaffe and Hall, 2002
; Steeg, 2003
The bona fide environment for migrating cells is the extracellular matrix, which permits movement in a 3D scaffold that is biochemically complex and shows dynamic flexibility. 3D tissue culture models reconstitute an environment that resembles the in vivo situation in regard to cell shape and movement. This model provides important insights into the mechanisms of cell motion during carcinogenesis. Recent advances have identified two modes of cell motility in 3D matrices. The elongated mode of migration is a consequence of Rac activity and generates membrane protrusions, the lamellipodia that drive motility. In contrast, a novel rounded mode of motility depends on RhoA and its main effector, Rho-associated coil-containing protein kinase (ROCK), and resembles the amoeboid movement. This involves a rounded bleb-associated movement that generates propulsive motion through the matrix independently of proteolysis (Sahai and Marshall, 2003
Curiously, genes encoding Rho GTPases are rarely found mutated in human cancers, in which only their functional activities seem to be deregulated (Nakamoto et al., 2001
; Rihet et al., 2001
). This suggests that alterations in others genes account for functional modifications of Rho GTPases accompanying actin cytoskeleton remodeling during metastasis. We hypothesize that this set of genes controls the cell cycle. Their mutation during the initiation phase in cancer would modify the behavior of proteins involved in actin cytoskeleton dynamics, such as Rho GTPases, leading to a migratory and invasive phenotype.
Beyond these genes is the tumor suppressor p53, whose mutations occurs in >50% of human tumors (Hollstein et al., 1994
). p53 protects cells from malignant transformation by regulating cell cycle arrest or by promoting apoptosis (Levine, 1997
; Giaccia and Kastan, 1998
). We and others have recently shown that p53 modulates cell migration: p53 negatively modulates Rho GTPases and regulates cell polarization and migration (Gadea et al., 2002
; Guo et al., 2003
). However, little is known about the role of p53 in cells moving in a 3D matrix that mimics the in vivo microenvironment of tumor cells.
Identifying the mechanisms by which p53 modulates cell migration is important to understand how invasive cells arise. In this study, we show that the elongated spindle-shaped fibroblastoid mode of motility can be converted to a rounded blebbing movement by blocking p53 function. This amoeboid mode of motility requires RhoA–ROCK signaling and confers higher velocity and invasive properties to p53-deficient cells. Thus, the range of p53 tumor suppressor activity extends to the control of the mode of migration of invasive cells.