Prolonged Activation of Extracellular Signal-regulated Kinase by a Protein Kinase C-dependent and N17Ras-insensitive Mechanism Mediates the Proliferative Response of Gi/o-coupled Somatostatin sst4 Receptors*
- From the Glaxo Institute of Applied Pharmacology, Department of Pharmacology, University of Cambridge, Cambridge, CB2 1QJ, United Kingdom
Abstract
The human sst4 receptor, recombinantly expressed in Chinese hamster ovary cells, mediates proliferative activity of the peptide hormone somatostatin. This effect was shown to involve activation of pertussis toxin-sensitive G proteins and was inhibited by overexpression of the βγ-sequestrant, transducin. Somatostatin-induced proliferation was abolished by the MEK1 inhibitor, PD 98059, whereas the Src inhibitor, PP1, had no effect. A marked increase was observed in the phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1 and ERK2) 10 min after sst4 receptor activation, which was blocked by pertussis toxin, decreased by PP1 and the βγ-sequestrant, but unaffected by PD 98059. In contrast, the somatostatin-induced phosphorylation of ERK obtained at 4 h, although sensitive to both pertussis toxin and transducin, was unaffected by PP1 but ablated by PD 98059. Protein kinase C inhibition also abolished this somatostatin-induced sustained phosphorylation of ERK, together with the associated increase in cell proliferation. Expression of dominant negative Ras (N17) failed to significantly reduce the proliferative effect mediated by the sst4 receptor but markedly attenuated the acute phase of the somatostatin-induced phosphorylation of ERK obtained at 10 min. In contrast, the phosphorylation induced at 4 h was unaffected. We conclude that ERK activation by Gi/o-coupled sst4receptors involves a Src and Ras-dependent acute phase, but the proliferative response is dependent upon the prolonged ERK-induced activity, mediated by protein kinase C.
Footnotes
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↵* The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‡ To whom correspondence should be addressed: Glaxo Institute of Applied Pharmacology, Dept. of Pharmacology, University of Cambridge, Tennis Court Rd., Cambridge, CB2 1QJ, U.K. Tel.: 44-1223-334-177; Fax: 44-1223-334-178; E-mail: wtem15797@glaxowellcome.co.uk.
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↵2 L. A. Sellers, unpublished data.
- Abbreviations:
- ERK
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extracellular signal-regulated kinase
- MAP
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mitogen-activated protein
- bFGF
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basic fibroblast growth factor
- CHO
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Chinese hamster ovary
- PDGF
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platelet-derived growth factor
- TBS
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Tris-buffered saline
- MEK
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MAP kinase kinase
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- Received April 13, 1999.
- Revision received June 8, 1999.
- The American Society for Biochemistry and Molecular Biology, Inc.
