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Inhibition by cAMP of Ras-dependent activation of Raf
1Onyx Pharmaceuticals, Richmond, CA 94806.
Abstract:
Activation of the Raf and extracellular signal-regulated kinases (ERKs) (or mitogen-activated protein kinases) are key events in mitogenic signalling, but little is known about interactions with other signaling pathways. Agents that raise levels of intracellular cyclic adenosine 3',5'-monophosphate (cAMP) blocked DNA synthesis and signal transduction in Rat1 cells exposed to epidermal growth factor (EGF) or lysophosphatidic acid. In the case of EGF, receptor tyrosine kinase activity and association with the signaling molecules Grb2 and Shc were unaffected by cAMP. Likewise, EGF-dependent accumulation of the guanosine 5'-triphosphate-bound form of Ras was unaffected. In contrast, activation of Raf-1 and ERK kinases was inhibited. Thus, cAMP appears to inhibit signal transmission from Ras by preventing Ras-dependent activation of Raf-1.
Insights
Elevated cyclic adenosine 3
Area of Science:
- Cellular signaling pathways
- Molecular biology
- Signal transduction
Background:
- Mitogenic signaling involves Raf and extracellular signal-regulated kinases (ERKs).
- Interactions between mitogenic signaling and other pathways remain largely uncharacterized.
Purpose of the Study:
- To investigate the inhibitory effects of cyclic adenosine 3',5'-monophosphate (cAMP) on mitogenic signaling pathways.
- To elucidate the specific mechanisms by which cAMP interferes with signal transduction.
Main Methods:
- Utilized Rat1 cells treated with epidermal growth factor (EGF) or lysophosphatidic acid.
- Assessed the impact of elevated intracellular cAMP levels on DNA synthesis and signal transduction.
- Analyzed receptor tyrosine kinase activity, Grb2/Shc association, Ras-GTP levels, and Raf-1/ERK activation.
Main Results:
- Elevated cAMP blocked DNA synthesis and signal transduction induced by EGF and lysophosphatidic acid.
- cAMP did not affect EGF receptor tyrosine kinase activity or its association with Grb2 and Shc.
- cAMP did not alter EGF-dependent Ras-GTP accumulation but inhibited Raf-1 and ERK activation.
Conclusions:
- Cyclic adenosine 3',5'-monophosphate (cAMP) inhibits signal transmission from Ras to Raf-1.
- cAMP prevents Ras-dependent activation of Raf-1, thereby blocking downstream ERK activation.
- This study reveals a novel inhibitory role of cAMP in the Ras-Raf-ERK signaling cascade.