Regulation of actin organisation by TGF-β in Hras-transformed fibroblasts

The importance of the cytoskeleton and in particular of the actin microfilaments in the processes of malignant cell growth and transformation has received special attention recently (Stournaras et al., 1996; Assoian and Zhu, 1997; Ben-Ze’ev, 1997; Rodriguez-Viciana et al., 1997). The sensitive dynamic equilibrium of Gto F-actin is actively modulated during both malignant cell transformation and the transduction of multiple extracellular signals in normal and tumour cells (Koukouritaki et al., 1996, 1997; Papakonstanti et al., 1996, 1998; Stournaras et al., 1996). On the other hand, small GTPases of the Rho/Rac family regulate the organisation of actin microfilaments (Gutkind and Vitale-Cross, 1996; Lim et al., 1996; Hall, 1998) and have thus provided a functional link between the cytoskeleton and the process of malignant deregulation, especially in cancer models where the Ras oncoprotein is activated (Qiu et al., 1995; Zhong et al., 1997). It has been rigorously shown that Rho GTPase activation regulates actin stress fibre and focal adhesion organisation (ChrzanowskaWodnicka and Burridge, 1996; Tapon and Hall, 1997; Hall, 1998). However, the mechanism by which these small GTPases influence the intracellular dynamic equilibrium of actin monomers (G-actin) and actin polymers (F-actin) to achieve their postulated organising effects has only recently begun to be characterised (Machesky and Hall, 1997; Mackay et al., 1997). Transforming growth factor β (TGF-β) is a well studied extracellular protein that arrests cellular growth of many cell types and modulates cell shape (Massagué, 1990; Roberts and Sporn, 1990). TGFβ mediates its signal through a complex of three cell surface receptors, the type I, II and III receptors (Moustakas et al., 1993; Massagué and Weis-Garcia, 1996; ten Dijke et al., 1996) and a new family of downstream effector molecules, the tumour suppressors Smads (Heldin et al., 1997). The role of TGFβ in tumourigenesis has been primarily shown by studies in which tumour cells from progressively more advanced stages of cancer disease exhibit reduced or no responses at all to this factor due to either loss of cell surface receptors (Markowitz et al., 1995; Wang et al., 1996; DeCoteau et al., 1997) or mutation of the Smad signal transducers (Eppert et al., 1996; Riggins et al., 1997). In addition, the direct role of TGFβ in modulating the phenotypic stage of oncogenic breast epithelial cells or keratinocytes that express activated Ras oncoproteins has been reported (Cui et al., 1996; Oft et al., 1996). TGFβ thus offers good system to address the mechanistic interrelationships between regulatory pathways of malignant cell proliferation and morphology. The ability of TGFβ to modulate cell morphology and the actin cytoskeleton in a variety of cell types has been previously reported (Leof et al., 1986; Segarini et al., 1989; Coomber, 1991). However, no specific information as yet exists on the mechanism(s) by which this growth factor affects cell shape 1169 Journal of Cell Science 112, 1169-1179 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JCS9836