Cyclic AMP Blocks Cell Growth through Raf-1-Dependent and Raf-1-Independent Mechanisms

Abstract

It is widely accepted that cyclic AMP (cAMP) can block cell growth by phosphorylating Raf-1 on serine 43 and inhibiting signaling to extracellular signal-regulated protein kinase. We show that the suppression of Raf-1 by cAMP is considerably more complex than previously reported. When cellular cAMP is elevated, Raf-1 is phosphorylated on three residues (S43, S233, and S259), which work independently to block Raf-1. Both Ras-dependent and Ras-independent processes are disrupted. However, when cAMP-insensitive versions of Raf-1 are expressed in NIH 3T3 cells, their growth is still strongly suppressed when cAMP is elevated. Thus, although Raf-1 appears to be an important cAMP target, other pathways are also targeted by cAMP, providing alternative mechanisms that lead to suppression of cell growth.

We thank A. Chiloeches for helpful comments and for generating pMCEF-mRaf-1. We thank members of the Signal Transduction Laboratory for stimulating discussions and C. Marshall for critical reading of the manuscript. We thank J. Metcalf for peptide synthesis, H. King for DNA sequencing, and J. Cordell for technical expertise.

This work was funded by grants from the Medical Research Council (component grant: G9900391), Cancer Research UK, and The Institute of Cancer Research.