Serum Response Factor Is Required for Immediate-Early Gene Activation yet Is Dispensable for Proliferation of Embryonic Stem Cells

Abstract

Addition of serum to mitogen-starved cells activates the cellular immediate-early gene (IEG) response. Serum response factor (SRF) contributes to such mitogen-stimulated transcriptional induction of many IEGs during the G₀-G₁ cell cycle transition. SRF is also believed to be essential for cell cycle progression, as impairment of SRF activity by specific antisera or antisense RNA has previously been shown to block mammalian cell proliferation. In contrast, Srf^−/− mouse embryos grow and develop up to E6.0. Using the embryonic stem (ES) cell system, we demonstrate here that wild-type ES cells do not undergo complete cell cycle arrest upon serum withdrawal but that they can mount an efficient IEG response. This IEG response, however, is severely impaired in Srf^−/− ES cells, providing the first genetic proof that IEG activation is dependent upon SRF. Also, Srf^−/− ES cells display altered cellular morphology, reduced cortical actin expression, and an impaired plating efficiency on gelatin. Yet, despite these defects, the proliferation rates of Srf^−/− ES cells are not substantially altered, demonstrating that SRF function is not required for ES cell cycle progression.

ACKNOWLEDGMENTS

G. Schratt and B. Weinhold contributed equally to this work.

We thank Marianne Petry for expert technical assistance. We acknowledge the gift of anti-E-cadherin antiserum from H. Beug (IMP, Vienna, Austria). We appreciate the comments on this manuscript by O. Heidenreich.

This work was supported through grants of the VolkswagenStiftung (I/74039 to U.R. and I/74043 to A.N.) and the DFG (NO 120/10-1 and NO 120/7-4 to A.N.). A.S.L. and R.A.W. acknowledge financial support through NIH grant R35CA39826.