Cell Cycle Arrest and Reversion of Ras-Induced Transformation by a Conditionally Activated Form of Mitogen-Activated Protein Kinase Kinase Kinase 3

Abstract

Signal-induced proliferation, differentiation, or stress responses of cells depend on mitogen-activated protein kinase (MAPK) cascades, the core modules of which consist of members of three successively acting kinase families (MAPK kinase kinase [MAP3K], MAPK kinase, and MAPK). It is demonstrated here that the MEKK3 kinase inhibits cell proliferation, a biologic response not commonly associated with members of the MAP3K family of kinases. A conditionally activated form of MEKK3 stably expressed in fibroblasts arrests these cells in early G₁. MEKK3 critically blocks mitogen-driven expression of cyclin D1, a cyclin which is essential for progression of fibroblasts through G₁. The MEKK3-induced block of cyclin D1 expression and of cell cycle progression may be mediated via p38 MAPK, a downstream effector of MEKK3. The MEKK3-mediated block of proliferation also reverses Ras-induced cellular transformation, suggesting possible tumor-suppressing functions for this kinase. Together, these results suggest an involvement of the MEKK3 kinase in negative regulation of cell cycle progression, and they provide the first insights into biologic activities of this kinase.

ACKNOWLEDGMENTS

We are most appreciative of technical assistance given by S. Lizarraga. We thank J. Der, C. Sherr, S. Gutkind, and E. Nishida for kindly providing plasmids. We are grateful to K. Holmes and D. Stephany of the NIAID Flow Cytometry Facility for assistance with flow cytometric analysis, to Ricardo Dreyfuss for the preparation of figures, and to M. Rust for help with preparation of the manuscript. We are indebted to Y. Ward and K. Takenaka for expert help and advice, E. Nishida for support during the final stages of this work, and K. Brown and A. Leonardi for discussions. We thank A. Fauci for continuous support and encouragement.