CDC42 and FGD1 Cause Distinct Signaling and Transforming Activities

Abstract

Activated forms of different Rho family members (CDC42, Rac1, RhoA, RhoB, and RhoG) have been shown to transform NIH 3T3 cells as well as contribute to Ras transformation. Rho family guanine nucleotide exchange factors (GEFs) (also known as Dbl family proteins) that activate CDC42, Rac1, and RhoA also demonstrate oncogenic potential. The faciogenital dysplasia gene product, FGD1, is a Dbl family member that has recently been shown to function as a CDC42-specific GEF. Mutations within the FGD1 locus cosegregate with faciogenital dysplasia, a multisystemic disorder resulting in extensive growth impairments throughout the skeletal and urogenital systems. Here we demonstrate that FGD1 expression is sufficient to cause tumorigenic transformation of NIH 3T3 fibroblasts. Although both FGD1 and constitutively activated CDC42 cooperated with Raf and showed synergistic focus-forming activity, both quantitative and qualitative differences in their functions were seen. FGD1 and CDC42 also activated common nuclear signaling pathways. However, whereas both showed comparable activation of c-Jun, CDC42 showed stronger activation of serum response factor and FGD1 was consistently a better activator of Elk-1. Although coexpression of FGD1 with specific inhibitors of CDC42 function demonstrated the dependence of FGD1 signaling activity on CDC42 function, FGD1 signaling activities were not always consistent with the direct or exclusive stimulation of CDC42 function. In summary, FGD1 and CDC42 signaling and transformation are distinct, thus suggesting that FGD1 may be mediating some of its biological activities through non-CDC42 targets.The Rho subfamily of Ras-related GTPases (14 mammalian members) controls multiple aspects of cell behavior, including cytoskeletal rearrangement, nuclear signaling, and cell growth (reviewed in reference 67). For example, CDC42 mediates the induction of actin microspikes and filopodia by bradykinin (26, 33), whereas Rac1 is required for growth factor-induced membrane ruffling and lamellipodia formation (47). In contrast, RhoA regulates the formation of actin stress fibers (46). In Swiss 3T3 cells, the assembly of these structures involves a cascade in which CDC42 activates Rac1, which in turn activates RhoA (33). Rho family proteins also have demonstrated roles in the regulation of gene expression as measured by (i) the transcriptional activation of the serum response factor (SRF) (19), (ii) activation of c-Jun NH 2 -terminal kinase (JNK) and its downstream target c-Jun (10, 32, 35), (iii) activation of the ternary complex factor protein Elk-1 (62), (iv) activation of p38/Mpk2 (63), and (v) regulation of expression from the cyclin D1 promoter (55). Finally, there is growing evidence that the deregulated expression of Rho family members has profound effects on the proliferative potential of cells. Activated derivatives of RhoA, RhoB, Rac1, and CDC42 cause oncogenic transformation when expressed in rodent fibroblast cell lines and may contribute to Ras-mediated malignant transformation (23,39,41,42,53,...

The activation of TC10, a Rho small GTPase, contributes to v-Rel-mediated transformation

“…The dominant-negative mutant, TC10T31N, suppressed the focus formation ability of these cells. These observations are similar to those obtained for Cdc42, Rac1, RhoA and RhoB (Prendergast et al, 1995;Qiu et al, 1995Qiu et al, , 1997Osada et al, 1997;Westwick et al, 1997;Whitehead et al, 1998). Moreover, previous studies indicated that the constitutively active mutants of Cdc42 (G12V) and Rac (G12V) increased the colony formation ability of mammalian fibroblast cell lines in soft agar.…”

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

“…GTPase defective TC10 synergizes with an activated Raf-1 kinase to transform NIH3T3 cells Activated (GTPase defective) forms of Rac1, RhoA, or CDC42 show little or no focus-forming activity when assayed in NIH3T3 ®broblasts. However, activated versions of all three proteins cooperate with activated forms of the Raf-1 serine/threonine protein kinase and exhibit synergistic focus forming activity in rodent ®broblast transfection assays (Westwick et al, 1997;Whitehead et al, 1998). Therefore, we wanted to determine if activated TC10 also possesses transforming potential.…”

“…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).…”

“…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).…”

Modulation of HIV-1 Replication by a Novel RhoA Effector Activity

“…The pAX142-lacZ reporter plasmid (39) was included in the transfection mix and ␤-galactosidase activity was measured. The pAX142 promoter has been reported previously to be unaffected by Dbl family proteins (39,45).…”