Genetic Demonstration of a Redundant Role of Extracellular Signal-Regulated Kinase 1 (ERK1) and ERK2 Mitogen-Activated Protein Kinases in Promoting Fibroblast Proliferation

Abstract

The extracellular signal-regulated kinase 1 and 2 (ERK1/2) mitogen-activated protein (MAP) kinase signaling pathway plays an important role in the proliferative response of mammalian cells to mitogens. However, the individual contribution of the isoforms ERK1 and ERK2 to cell proliferation control is unclear. The two ERK isoforms have similar biochemical properties and recognize the same primary sequence determinants on substrates. On the other hand, analysis of mice lacking individual ERK genes suggests that ERK1 and ERK2 may have evolved unique functions. In this study, we used a robust genetic approach to analyze the individual functions of ERK1 and ERK2 in cell proliferation using genetically matched primary embryonic fibroblasts. We show that individual loss of either ERK1 or ERK2 slows down the proliferation rate of fibroblasts to an extent reflecting the expression level of the kinase. Moreover, RNA interference-mediated silencing of ERK1 or ERK2 expression in cells genetically disrupted for the other isoform similarly reduces cell proliferation. We generated fibroblasts genetically deficient in both Erk1 and Erk2. Combined loss of ERK1 and ERK2 resulted in a complete arrest of cell proliferation associated with G₁ arrest and premature replicative senescence. Together, our findings provide compelling genetic evidence for a redundant role of ERK1 and ERK2 in promoting cell proliferation.

We thank Kim Lévesque for animal care and maintenance, Sébastien Harton for expert help with tetraploid aggregation experiments, Danièle Gagné for help with FACS analysis, and Pierre Chagnon and Raphaële Lambert for quantitative PCR analysis. We thank Bernard Thorens and Trang Hoang for reagents. We are grateful to Gilles Pagès and Jacques Pouysségur for originally providing the Erk1^−/− mice. We thank Philippe Lenormand for communicating results prior to publication and for discussions.

This work was supported by a grant from the National Cancer Institute of Canada. C. Frémin is recipient of a fellowship from the Cole Foundation. S. Meloche holds the Canada Research Chair in Cellular Signaling.