R.S. Ghadessy & E. Kelly
Regulation of secretin receptor responsiveness
2027
has been reported that forskolin-mediated PKA activation
can heterologously regulate A2 adenosine and prostanoid IP
receptor desensitization, however, these eects were only
observed after 17 h of forskolin treatment and were
consistent with a decrease in receptor expression (Keen et
al., 1992; Krane et al., 1994). As secretin partly exerts its
physiological eects via stimulation of adenylyl cyclase with
subsequent activation of PKA (Ulrich et al., 1998), feedback
inhibition by PKA is likely to play an important role in
physiological regulation of secretin receptor responsiveness.
From the present study, the precise role of PKA in
regulating secretin receptor desensitization in NG108-15 cells
remains unclear. Receptor desensitization by PKA may arise
from direct uncoupling of the receptor-eector signalling
pathway by receptor phosphorylation, promotion of receptor
internalization (Walker et al., 1999), and/or an involvement
of PKA in vesicular transport and resensitization (Goretzki &
Mueller, 1997). As the eect of H-89 in decreasing receptor
desensitization was only observed at longer time periods
(430 min) of agonist stimulation, PKA-mediated changes in
protein synthesis was considered as a potential mechanism.
Interestingly, treatment with the protein synthesis inhibitor
cycloheximide enhanced secretin receptor responsiveness with
a pro®le somewhat similar to that observed with PKA
inhibition. This data could be interpreted to indicate that
newly synthesized protein(s) may act as accessory molecules
to regulate secretin receptor signalling in a PKA-dependent
manner. A similar mechanism has been demonstrated for the
class II PACAP receptor which undergoes agonist-mediated
desensitization by processes involving PKA activation and
protein neosynthesis in CATH.a cells (Muller et al., 1998). In
the present study, cycloheximide also enhanced cyclic AMP
responses stimulated by NECA and iloprost, but the
increases were small in extent when compared to the eects
of cycloheximide on secretin responsiveness. The increase in
NECA and iloprost responses may re¯ect the eects of
protein synthesis inhibition at the level of the receptors or G-
proteins, since forskolin responsiveness was unchanged by
cycloheximide pretreatment.
In summary, we show that endogenous secretin receptor
responsiveness is modulated by PKA, PKC and protein
neosynthesis. Furthermore, we present evidence that com-
partmentalization of GPCR-cyclic AMP signalling occurs in
NG108-15 cells. Our future studies will seek to identify the
molecular mechanisms underlying second messenger-depen-
dent regulation of secretin receptor responsiveness, and in
particular whether the receptor itself is subject to direct
regulation by these kinases.
This work was supported by the U.K. Medical Research Council.
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British Journal of Pharmacology vol 135 (8)