Rac Regulation of Transformation, Gene Expression, and Actin Organization by Multiple, PAK-Independent Pathways

Abstract

Rac1 and RhoA are members of the Rho family of Ras-related proteins and function as regulators of actin cytoskeletal organization, gene expression, and cell cycle progression. Constitutive activation of Rac1 and RhoA causes tumorigenic transformation of NIH 3T3 cells, and their functions may be required for full Ras transformation. The effectors by which Rac1 and RhoA mediate these diverse activities, as well as the interrelationship between these events, remain poorly understood. Rac1 is distinct from RhoA in its ability to bind and activate the p65 PAK serine/threonine kinase, to induce lamellipodia and membrane ruffling, and to activate the c-Jun NH 2 -terminal kinase (JNK). To assess the role of PAK in Rac1 function, we identified effector domain mutants of Rac1 and Rac1-RhoA chimeric proteins that no longer bound PAK. Surprisingly, PAK binding was dispensable for Rac1-induced transformation and lamellipodium formation, as well as activation of JNK, p38, and serum response factor (SRF). However, the ability of Rac1 to bind to and activate PAK correlated with its ability to stimulate transcription from the cyclin D1 promoter. Furthermore, Rac1 activation of JNK or SRF, or induction of lamellipodia, was neither necessary nor sufficient for Rac1 transforming activity. Finally, the signaling pathways that mediate Rac1 activation of SRF or JNK were distinct from those that mediate Rac1 induction of lamellipodia. Taken together, these observations suggest that Rac1 regulates at least four distinct effector-mediated functions and that multiple pathways may contribute to Rac1-induced cellular transformation.

Cellular functions of TC10, a Rho family GTPase: regulation of morphology, signal transduction and cell growth

“…Speci®cally, JNK is activated in cells expressing constitutively active CDC42 or Rac. JNK activation, in turn, results in the phosphorylation and activation of the transcription factors c-Jun and Elk-1, which stimulate transcription from genes containing AP-1 and SRE elements, respectively (Joneson et al, 1996;Lamarche et al, 1996;Westwick et al, 1997; for review see Wasylyk et al, 1998). Further supporting a role for CDC42 and Rac in JNK signaling, JNK and cJun are activated in cells over-expressing GEFs that act on Rac and CDC42 (Westwick et al, 1998;Whitehead et al, 1998;Zhou et al, 1998).…”

“…These include formation of ®lopodia (CDC42), lamellipodia (Rac), stress ®bers and focal adhesion complexes (Rho), formation of other adhesive-like complexes (CDC42 and Rac), stimulation of Mitogen-Activated Protein Kinase (MAPK) cascades (CDC42 and Rac), and activation of gene expression (CDC42, Rac, and Rho). In addition, Rho family members play essential roles in G1 progression, initiation of DNA synthesis, and Rasmediated transformation (CDC42, Rac and Rho) Westwick et al, 1997). Rho, Rac and CDC42 also activate Cyclin D1 transcription, providing a direct link to cell cycle regulation (Westwick et al, 1997).…”

“…In addition, Rho family members play essential roles in G1 progression, initiation of DNA synthesis, and Rasmediated transformation (CDC42, Rac and Rho) Westwick et al, 1997). Rho, Rac and CDC42 also activate Cyclin D1 transcription, providing a direct link to cell cycle regulation (Westwick et al, 1997). The existence of a functional link between Rho family GTPases and cell cycle control in vivo is supported by the observation that several Rho family speci®c GEFs are proto-oncogenes.…”

“…Activated RhoA has no e ect on JNK activation, but has been shown to stimulate SREmediated transcription. In addition, CDC42 and Rac activate SRF, by a pathway independent of JNK (Westwick et al, 1997;Whitehead et al, 1998;Zohn et al, 1998). Rho, Rac and CDC42 have been implicated in the activation of the transcription factor Nuclear Factor kappa B (NF-kB) (Perona et al, 1997).…”

“…Regions of CDC42 and Rac1 required for these and other cellular events have been examined by introducing e ector mutations into activated proteins (Joneson et al, 1996;Lamarche et al, 1996;Westwick et al, 1997). These mutagenesis studies have centered on the region spanning amino acids 26 ± 43 of CDC42 and Rac1.…”