Structure/Function Relationships of Calcitonin Analogues as Agonists, Antagonists, or Inverse Agonists in a Constitutively Activated Receptor Cell System
- Neurobiology Unit, St. Vincent’s Institute of Medical Research, 41 Victoria Parade, Fitzroy, Victoria 3065, Australia
Abstract
The structure/function relationship of salmon calcitonin (sCT) analogues was investigated in heterologous calcitonin receptor (CTR) expression systems. sCT analogues with progressive amino-terminal truncations intermediate of sCT-(1–32) to sCT-(8–32) were examined for their ability to act as agonists, antagonists, or inverse agonists. Two CTR cell clones, B8-H10 and G12-E12, which express ∼5 million and 25,000 C1b receptors/cell, respectively, were used for this study. The B8-H10 clone has an ∼80-fold increase in basal levels of intracellular cAMP due to constitutive activation of the overexpressed receptor. In whole-cell competition binding studies, sCT-(1–32) was more potent than any of its amino-terminally truncated analogues in competition for 125I-sCT binding. In cAMP accumulation studies, sCT-(1–32) and modified analogues sCT-(2–32) and sCT-(3–32) had agonist activities. SDZ-216–710, with an amino-terminal truncation of four amino acids, behaved as a partial agonist/antagonist, whereas amino-terminal truncations of six or seven amino acid residues produced a 16-fold reduction in basal cAMP levels and attenuated the response to the agonist sCT-(1–32) in the constitutively active CTR system. This inverse agonist effect was insensitive to pertussis toxin inhibition. In contrast, the inverse agonist activity of these peptides was not observed in the nonconstitutively active CTR system, in which sCT analogues with amino-terminal truncations of four or more amino acids behaved as neutral competitive antagonists. These results suggest that the inverse agonist activity is mediated by stabilization of the inactive state of the receptor, which does not couple to G protein, and attenuates basal signaling initiated by ligand-independent activation of the effector adenylyl cyclase.
Footnotes
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Send reprint requests to: Dr. Patrick M. Sexton, Neurobiology Unit, St. Vincent’s Institute of Medical Research, 41 Victoria Parade, Fitzroy 3065, Victoria, Australia. E-mail:pms{at}rubens.its.unimelb.edu.au
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↵1 J. M. Hilton, M. Dowton, S. Houssami, and P. M. Sexton. Ability to form an amphipathic α-helix is a critical factor in the irreversibility of salmon calcitonin binding to calcitonin receptors. Submitted for publication.
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This work was supported by grants from the National Health and Medical Research Council of Australia. P.M.S. is a Research Fellow of the Australian Research Council.
- Abbreviations:
- CT
- calcitonin
- CTR
- calcitonin receptor
- sCT
- salmon calcitonin
- PBS
- phosphate-buffered saline
- HEK
- human embryonic kidney
- BSA
- bovine serum albumin
- PTX
- pertussis toxin
- R*
- active conformation of receptor
- R
- inactive conformation of receptor
- GPCR
- G protein-coupled receptor
- GRK
- G protein-coupled receptor kinase
- βARK
- β-adrenergic receptor kinase
- SDZ-212–769
- [Ala7]salmon calcitonin-(7–32)
- SDZ-216–710
- isocaproyl-[Ala7,Aib10,17,Lys(For)11,18,Lys(fructosyl)24]salmon calcitonin-(5–32) (ASC710)
- SDZ-218–686
- [Ala7]salmon calcitonin-(3–32)
- SDZ-219–379
- Ac-[Aib10,17,Lys(For)11,18,Lys(Ip)24]-salmon calcitonin-(8–32)
- SDZ-220–235
- Ac-[Ala7,Aib17]-salmon calcitonin-(2–32)
- AC512
- Bolton-Hunter-[Arg18,Asn30,Tyr32]salmon calcitonin-(8–32)
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- Received September 3, 1996.
- Accepted January 10, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
