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Molecular and Cellular Biology, June 2005, p. 4676-4682, Vol. 25, No. 11
0270-7306/05/$08.00+0     doi:10.1128/MCB.25.11.4676-4682.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Graded Mitogen-Activated Protein Kinase Activity Precedes Switch-Like c-Fos Induction in Mammalian Cells

Jeffrey P. MacKeigan,{dagger} Leon O. Murphy,{dagger} Christopher A. Dimitri, and John Blenis*

Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115

Received 1 November 2004/ Returned for modification 15 November 2004/ Accepted 22 February 2005

The mitogen-activated protein kinase (MAPK) pathway is an evolutionarily conserved signaling module that controls important cell fate decisions in a variety of physiological contexts. During Xenopus oocyte maturation, the MAPK cascade converts an increasing progesterone stimulus into a switch-like, all-or-nothing response. While the importance of such switch-like behavior is widely discussed in the literature, it is not known whether the MAPK pathway in mammalian cells exhibits a switch-like or graded response. For this study, we used flow cytometry and immunofluorescence to generate single-cell measurements of MAPK signaling in Swiss 3T3 fibroblasts. In contrast to the case in Xenopus oocytes, we found that ERK activation in individual mammalian cells is not ultrasensitive and shows a graded response to changes in agonist concentration. Thus, the conserved MAPK signaling module exhibits different systems-level properties in different cellular contexts. Furthermore, the graded ERK response was converted into a more switch-like behavior at the level of immediate-early gene induction and cell cycle progression. Thus, while MAPK signaling is involved in all-or-nothing cell fate decisions for both Xenopus oocyte maturation and mammalian fibroblast proliferation, the underlying mechanisms responsible for the switch-like nature of the cellular responses are different in these two systems, with the mechanism appearing to lie downstream of the kinase cascade in mammalian fibroblasts.

* Corresponding author. Mailing address: Department of Cell Biology, Harvard Medical School, 240 Longwood Ave., Boston MA 02115. Phone: (617) 432-4848. Fax: (617) 432-1144. E-mail: john_blenis{at}hms.harvard.edu.

{dagger} These authors contributed equally to this work.

Molecular and Cellular Biology, June 2005, p. 4676-4682, Vol. 25, No. 11
0022-538X/05/$08.00+0     doi:10.1128/MCB.25.11.4676-4682.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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