Functional cholecystokinin receptors are distinguished kinetically by biotinyl-Tyr-Gly-(Thr28,Nle31)CCK(25-33) in rat pancreatic acini

Redistribution of protein kinase C isoforms in rat pancreatic acini during lactation and weaning

Freshly enzymatically isolated pancreatic acini from lactating and weaning Wistar rats were used to investigate the role of protein kinase C (PKC) isoforms during these physiologically relevant pancreatic secretory and growth processes. The combination of immunoblot and immunohistochemical analysis shows that the PKC isoforms alpha, delta, and epsilon are present in pancreatic acini from control, lactating and weaning rats. A vesicular distribution of PKC-alpha, -delta, and -epsilon was detected by immunohistochemical analysis in the pancreatic acini from all the experimental groups. PKC-delta showed the strongest PKC immunoreactivity (PKC-IR). In this vesicular distribution, PKC-IR was located at the apical region of the acinar cells. No differences were observed between control, lactating and weaning rats. However, the immunoblot analysis of pancreatic PKC isoforms during lactation and weaning showed a significant translocation of PKC-delta from the cytosol to the membrane fraction when compared with control animals. Translocation of PKC isoforms (alpha, delta and epsilon) in response to 12-O-tetradecanoyl phorbol 13-acetate (TPA) 1 microM (15 min, 37 degrees C) was comparable in pancreatic acini from control, lactating and weaning rats. In the control group, a significant translocation of all the isoforms (alpha, delta and epsilon) from the cytosol to the membrane was observed. The PKC isoform most translocated by TPA was PKC-delta. In contrast, no statistically significant increase in PKC-delta translocation was detected in pancreatic acini isolated from lactating or weaning rats. These results suggest that the PKC isoforms are already translocated to the surface of the acinar cells from lactating or weaning rats. In addition, they suggest that isoform specific spatial PKC distribution and translocation occur in association with the growth response previously described in the rat exocrine pancreas during lactation and weaning.

Biochemical regulation of the three different states of the cholecystokinin (CCK) receptor in pancreatic acini

We used rat pancreatic acini and measured binding of [125I]CCK-8 and [3H]L-364,718 to the three different states of the CCK receptor to examine potential biochemical regulation of ligand binding for each receptor state. Binding of [125I]CCK-8 to the high affinity state of the receptor was measured as carbachol-inhibitable binding of [125I]CCK-8, whereas binding of [125I]CCK-8 to the low affinity state was measured as carbachol-resistant binding of [125I]CCK-8. Interaction of CCK-8 with the very low affinity state of the CCK receptor was measured as CCK-8-inhibitable binding of [3H]L-364,718. [125I]CCK-8 that was bound to the high affinity state dissociated slowly at a rate of 0.20%/min and this dissociation was not altered by 30 mM NaF. Dissociation of [125I]CCK-8 bound to the low affinity state was biphasic--22% of the bound radioactivity dissociated completely within 3 min and the remaining 78% dissociated slowly at a rate of 0.19%/min. Dissociation of [125I]CCK-8 from the low affinity state was not altered by 30 mM NaF. The pattern of dissociation of bound [125I]CCK-8 from the pancreatic CCK receptor expressed in COS cells was also biphasic and closely resembled that observed in pancreatic acini. CCK-8 that was bound to the very low affinity state dissociated completely during a 20-min period of washing and resuspension of acini that had been first incubated with CCK-8. We found extensive biochemical regulation of the different states of the CCK receptor in pancreatic acini. Bombesin, TPA, NaF, CCCP and trifluoperazine each altered binding of [125I]CCK-8 to the high affinity state and to the low affinity state, and except for bombesin each agent was more potent in affecting the high affinity state than the low affinity state. No agent tested affected the low affinity state but not the high affinity state. In contrast, a number of agents affected the high affinity state but not the low affinity state. These included receptor-mediated agonists (carbachol, secretin, VIP), 8Br-cAMP, NEM, agents that affect microtubules or microfilaments (cytochalasin B, vinblastine), calmodulin inhibitors (W-7, chlorpromazine) and genistein. Experiments with EGTA, A23187 and thapsigargin indicated that none of the three receptor states was influenced by intracellular or extracellular calcium. No agent tested altered the interaction of CCK-8 with the very low affinity state of the CCK receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

New evidence for a membrane-bound pathway in hormone receptor binding

The fully active cholecystokinin analog (Thr,Nle)-CCK-9 was lipo-derivatized by N-terminal grafting of a dimyristoylglycerol moiety to induce tight interdigitation with cell membrane bilayers. While the parent CCK peptide was shown to interact only transiently with small unilamellar phospholipid vesicles, the lipo-CCK peptide, although self-aggregating into vesicles, inserts rapidly and quantitatively into phospholipid bilayers. Fluorescence and, even more so, NMR data are supportive for a chain reversal of the CCK moiety of the lipo derivative with embedment of the C-terminus into hydrophobic compartments of the bilayer. MD simulations allowed for a proposal of the folded form of CCK in bilayers with a helical array parallel to the interface and an amphipathic display of the side chains. In this model, the phenylalanine aromatic ring is heading the peptide molecule and may thus play a decisive role in the lateral penetration of the receptor at the water/lipid interface. In fact, despite the membrane-bound state, its binding affinity for rat pancreatic acini is comparable to that of the CCK peptide when tested after a 3-h equilibration period but 5-6-fold lower at 45 min, suggesting that the association rate is significantly lower than that of the unmodified CCK peptide. This can rationally be attributed to the tight interdigitation of the double-tailed lipo moiety with the membrane bilayer. Moreover, an escape of the lipopeptide into the extracellular aqueous phase is energetically highly unfavored; therefore, the receptor can only be reached by a membrane-bound two-dimensional migration. The observed difference in amplification between binding and amylase secretion may result from inadequate occupation of low-affinity CCK receptors, which leads then to poor couplings to G-proteins. Nevertheless the data confirm that lateral penetration of receptor structures is possible, and thus, preadsorption of peptide (neuro)hormones at the cell membrane bilayer may indeed represent the first step in the receptor recognition process.

Lipophilic derivatization and its effect on the interaction of cholecystokinin (CCK) nonapeptide with phospholipids

N-terminal lipophilic derivatization of the fully active cholecystokinin analogue (Thr,Nle)-CCK-9 with the di-myristoyl-raz-thioglyceryl moiety leads to spontaneous self-aggregation of the lipopeptide into polydispersed vesicles at the liquid state. The high degree of fluidification of the vesicles favors a fast and net transfer of monomers to phospholipid bilayers even below the phase-transition temperature of the acceptor vesicles. Surprisingly, the process is accompanied by formation of peptide clusters. The peptide-rich domains exhibit a high affinity for Ca2+, a fact which may be correlated to the biological function of CCK as this hormone is known to stimulate calcium outflux from reserves in the cell membranes. Moreover, induced membrane affinity allowed to study more precisely the interaction of the CCK peptide with lipid bilayers as mimicry of cell membranes. Differently from what was observed with a similar lipogastrin derivative in which the peptide tail was found to be permanently exposed to the aqueous phase, in the case of the lipo-CCK compound insertion of its C-terminal active site into more hydrophobic compartments of the bilayer is occurring, as well as a folding into beta-type structures, thus confirming the role of cell membranes in displaying peptide hormones for specific receptor recognition.

Study of the states and populations of the rat pancreatic cholecystokinin receptor using the full peptide antagonist JMV 179

The full peptide antagonist of the pancreatic cholecystokinin (CCK) receptor, JMV 179, [Boc-Tyr(SO3H)-Ahx-Gly-dTrp-Ahx-Asp phenylethyl ester, where Tyr(SO3H) = sulfated tyrosine, Ahx = 6-aminohexanoic acid] was modified at its N-terminus by incorporation of p-hydroxyphenyl propionate (Bolton-Hunter reagent, BH) and was subsequently radioiodinated. After HPLC purification, 125I-BH-JMV-179, a CCK antagonist radioligand of high specific activity (2000 Ci/mmol) was obtained. 125I-BH-JMV-179 bound to a single population of sites on rat pancreatic plasma membranes, (Kd = 3.9 nM, Bmax = 40 pmol/mg protein). Binding was dependent on time, temperature, and protein concentration, and was fully reversible. JMV 179 radioligand detected four times as many sites as an agonist radioligand [C. Hadjiivanova, M. Dufresne, S. Poirot, P. Sozzani, N. Vaysse, L. Moroder and D. Fourmy (1992) Eur. J. Biochem. 204, 273-279]. Agonists and antagonists of the A- and B-subtype CCK/gastrin receptors inhibited 125I-BH-JMV-179 binding with an order of potency compatible with the A-subtype CCK receptor pharmacology. Moreover, the sulfate group on the tyrosine residue of the CCK peptides appeared to be of much less importance for antagonist affinity than for agonist affinity. Inhibition of 125I-BH-JMV-179 binding by agonists (except JMV 180), demonstrated the presence of two affinity classes of binding sites. The population of sites having an apparent high affinity for CCK represented 30 pmol/mg protein and threefold the number of high-affinity sites previously identified by an agonist radioligand. In presence of non-hydrolyzable GTP, all the sites bound CCK agonists with a low affinity. Moreover, saturation analysis of JMV 179 radioligand binding in the presence of CCK indicated that CCK interacted competitively with all JMV 179 sites and demonstrated binding of JMV 179 radioligand to two distinct affinity classes of sites. In the presence of GTP[S] a single affinity class of sites for JMV 179 radioligand was found as in the control experiments without CCK. This study, with the first CCK peptide antagonist radioligand, demonstrates that CCK receptors exist in two interconvertible affinity states regulated by guanine-nucleotide-binding regulatory protein(s) in rat pancreatic plasma membranes. JMV 179 radioligand does not induce receptor coupling but distinguishes the two affinity states of the CCK receptors. JMV 179 reveals the existence of populations of high-affinity and low-affinity sites for CCK which had not previously been detected by agonist radioligand binding, thus suggesting heterogeneity of CCK receptor sites in membranes.

Induction and detection of anti-peptide antibody specificity is critically affected by the mode of hapten presentation

C-Terminal cholecystokinin (CCK)-peptides of increasing chain lengths were all linked at their N-termini to the single surface-exposed cysteine residue 107 of yeast iso-1-cytochrome c by the maleimide/thiol reaction. The resulting CCK/cytochrome 1:1 conjugates with the haptenic peptides in the identical protein environment were used to immunize outbred guinea pigs in order to assess the critical size of CCK peptides required for the expression of a CCK-specific epitope and the induction of antibodies not crossreacting with the homologous gastrin sequence. By using standard ELISA techniques with polystyrene-adsorbed antigen to evaluate the specificity of the antisera, none of the conjugates were found to induce anti-CCK antisera not crossreacting with gastrin. However, when the biotinyl-CCK-antigen was immobilized by polystyrene-adsorbed avidin, i.e. via a procedure which assures maximum accessibility of the bound antigen, we were able to demonstrate that with CCK-12 and particularly CCK-13, linked through their N-termini to the carrier, the critical length for the expression and recognition of a CCK-specific epitope was reached. The related polyclonal antisera did not crossreact with the homologous gastrin in the modified ELISA.