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Activation of G1 progression, JNK mitogen-activated protein kinase, and actin filament assembly by the exchange
K Nagata1, M Driessens, N Lamarche
1Medical Research Council Laboratory for Molecular Cell Biology, Cancer Research Campaign Oncogene and Signal Transduction Group, University College London, Gower Street, London WC1E 6BT, United Kingdom.
Abstract:
Cdc42 has been shown to control bifurcating pathways leading to filopodia formation/G1 cell cycle progression and to JNK mitogen-activated protein kinase activation. To dissect these pathways further, the cellular effects induced by a Cdc42 guanine nucleotide exchange factor, FGD1, have been examined. All exchange factors acting on the Rho GTPase family have juxtaposed Dbl homology (DH) and pleckstrin homology (PH) domains. We report here that FGD1 triggers G1 cell cycle progression and filopodia formation in Swiss 3T3 fibroblasts as well as JNK mitogen-activated protein kinase activation in COS cell transfection assays. FGD1-induced filopodia formation is Cdc42-dependent, and both the DH and PH domains are essential. Although expression of the FGD1 DH domain alone does not activate Cdc42 and induce filopodia, it does trigger both the JNK cascade in COS cells and G1 progression in quiescent Swiss 3T3 cells. We conclude that FGD1 can trigger G1 progression independently of actin polymerization or integrin adhesion complex assembly. Furthermore, since FGD1 activates JNK and G1 progression in a Cdc42-independent manner, it must have additional, as yet unidentified, targets.
Insights
FGD1, a Cdc42 guanine nucleotide exchange factor, drives cell cycle progression and filopodia formation. It also activates JNK signaling, revealing new cellular targets beyond Cdc42.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Cdc42 regulates critical cellular processes including filopodia formation, G1 cell cycle progression, and JNK mitogen-activated protein kinase (MAPK) activation.
- Guanine nucleotide exchange factors (GEFs) for Rho GTPases, like FGD1, possess Dbl homology (DH) and pleckstrin homology (PH) domains.
Purpose of the Study:
- To investigate the specific cellular effects mediated by FGD1, a GEF for Cdc42.
- To elucidate the distinct roles of FGD1's domains in regulating downstream signaling pathways.
Main Methods:
- Transfection assays in COS cells to study JNK activation.
- Stimulation of Swiss 3T3 fibroblasts to examine G1 cell cycle progression and filopodia formation.
- Analysis of FGD1 mutants, including the DH domain alone, to dissect functional requirements.
Main Results:
- FGD1 induces G1 cell cycle progression and filopodia formation in Swiss 3T3 fibroblasts.
- FGD1 activates JNK MAPK in COS cells, demonstrating its role in this signaling cascade.
- The DH and PH domains of FGD1 are essential for FGD1-induced filopodia formation, which is Cdc42-dependent.
- The FGD1 DH domain alone activates JNK signaling and G1 progression independently of Cdc42 and filopodia formation.
Conclusions:
- FGD1 can promote G1 cell cycle progression independent of actin polymerization or integrin adhesion complex assembly.
- FGD1 activates JNK signaling and G1 progression through Cdc42-independent mechanisms, suggesting the existence of additional, yet unidentified, targets.
- These findings highlight the multifaceted roles of FGD1 in cellular regulation beyond its known interaction with Cdc42.