EDITORIAL FOCUS
CCK receptor trafficking: a novel paradigm of travel. Focus on "Regulation of lateral mobility and cellular trafficking of the CCK receptor by a partial agonist"

John del Valle

Department of Internal Medicine, The University of Michigan Medical Center, Ann Arbor, Michigan 48109


    ARTICLE
Top
Article
References

CHOLECYSTOKININ (CCK) is a gastrointestinal peptide that plays an important role in regulating a broad array of physiological actions, extending from exocrine pancreatic secretion and gallbladder contractility to gastrointestinal motility and neural modulation of appetite. The multiple actions of CCK are mediated by the G protein-coupled CCK receptor. This receptor leads to cell regulation via activation of several signaling pathways, with the phospholipase C/phosphoinositide system being of primary importance. As with all G protein-linked receptor (GPR)-mediated processes, strict cellular and molecular paradigms are in place to safeguard against random and unabated CCK-mediated cell activation. The CCK receptor is regulated via the three principal mechanisms that are important for regulating all GPRs: desensitization, sequestration, and downregulation. Cell surface receptor internalization and trafficking are essential in the latter two forms of receptor regulation; thus it is important to understand the factors involved in this process. The work presented in the current article in focus by Roettger et al. (Ref. 16, see p. C539 in this issue) represents a novel approach toward further elucidating the cellular pathways, which direct trafficking of the CCK receptor.

Roettger and co-workers build upon a substantial body of work directed at mapping the pathways by which the CCK receptor is internalized after ligand binding. The CCK receptor undergoes phosphorylation within key intracellular structural segments (4, 12, 13), is "insulated" within the plasma membrane (17), and is internalized into the cell via both clathrin-dependent and clathrin-independent pathways (18). Moreover, recent work by this investigative team has demonstrated that antagonist binding leads to receptor trafficking that is qualitatively but not quantitatively similar to that observed with the native ligand (CCK) (15). Despite these important advances, questions regarding the cellular events important in regulating CCK receptor trafficking still remain.

In the work presented in this issue, Roettger and co-workers fluorescently labeled the partial agonist for the CCK receptor, rhodamine-Gly-[(Nle28,31)CCK-26-32]-phenethyl ester (3), and utilized this as a probe to further examine regulation of this receptor in its native cell (pancreatic acini) and in a receptor-transfected cell model (Chinese hamster ovary-CCK receptor cells). This analog, which binds to the high-affinity CCK receptor, leads to mobilization of intracellular calcium with a minimal effect on phospholipase C (3) and minimal receptor phosphorylation. Roettger and colleagues made the key observation that binding of this ligand leads the CCK receptor to follow a trafficking pattern similar to that observed with the native ligand.

The processes of receptor desensitization and internalization are traditionally linked to receptor occupancy by an agonist. The work presented in this issue and in previous studies by this group adds to this paradigm by demonstrating that both a partial agonist (16) and an antagonist (15) can lead to patterns of receptor processing that are similar to those observed with a full agonist. It appears that cell surface receptors can be internalized and processed independently of their ability to activate G proteins, stimulate classical signaling cascades, or undergo phosphorylation. Although the phosphoinositide/protein kinase C pathway is not involved in CCK receptor trafficking, it remains to be established whether the ability of CCK to regulate phospholipase A2 (20) has any impact on this process. These observations amplify the traditional understanding of basic receptor biology and may impact on the approach to the design of pharmacological tools.

Significant progress has been made toward understanding the cellular elements important in regulating the state of activity of GPRs (1, 19). GPR desensitization, or the course of becoming refractory to protracted stimuli, is mediated to a significant extent by kinases such as protein kinase C, protein kinase A, or G protein-coupled receptor kinases (GRKs). GRKs mediate desensitization by phosphorylating the receptor and facilitating recruitment of arrestins to the phosphorylated receptor (2, 8). Recent work suggests that GRKs and arrestins may also play a role in facilitating GPR endocytosis and trafficking (2, 6, 7). Studies with the beta 2-adrenergic receptor suggest that arrestins may serve as adapter molecules, which specifically target GPR/clathrin-mediated endocytosis. The observation that phosphorylation is not essential for CCK receptor internalization and insulation would suggest that the GRK/arrestin system is not involved in trafficking of this receptor. It appears that phosphorylation of serine residues found within the third intracellular loop of the CCK receptor are essential for desensitization (14), but, of interest, this process does not appear to be important for receptor trafficking. Recent work by this same group has demonstrated that a segment of the carboxy-terminal tail of the CCK receptor is important for trafficking (5), further supporting a nontraditional mechanism for receptor internalization. Studies with the angiotensin receptor illustrate that other GPRs may also undergo trafficking through a G protein/signal transduction-independent mechanism (9). These important observations lend support to the existence of a novel paradigm for receptor internalization and transport through the cell.

In summary, the elegant studies by Roettger and colleagues in this article in focus demonstrate that binding of a partial agonist can set in motion the cellular processes involved in CCK receptor trafficking. This work builds on the concept that CCK receptors can undergo internalization and insulation via a mechanism that is independent of G protein activation and receptor phosphorylation. The signal for CCK receptor trafficking may be within the conformational changes this receptor undergoes on ligand binding (5), although this needs to be proven. Future studies are needed to elucidate the molecular signals required by the CCK receptor for initiating travel through the cell. Characterization of novel paradigms for receptor trafficking becomes of greater relevance in view of the recent studies demonstrating the potential role of receptor internalization in key cellular events such as receptor resensitization (10) and activation of post-receptor cascades such as the mitogen-activated protein kinase signaling system (11).


    REFERENCES
Top
Article
References

1.   Carman, C. V., and J. L. Benovic. G-protein-coupled receptors: turn-ons and turn-offs. Curr. Opin. Neurobiol. 8: 335-344, 1998[Medline].

2.   Ferguson, S. S. G., W. E. Downey III, A. M. Colapietro, L. S. Barak, L. Menard, and M. G. Caron. Role of beta -arrestin agonist-promoted G protein-coupled receptor internalization. Science 271: 363-365, 1996[Abstract].

3.   Gaisano, H. Y., U. G. Klueppelberg, D. I. Pinon, M. A. Pfenning, S. P. Powers, and L. J. Miller. Novel tool for the study of cholecystokinin-stimulated pancreatic enzyme secretion. J. Clin. Invest. 83: 321-325, 1989[Medline].

4.   Gates, M. C., C. D. Ulrich, and L. J. Miller. Multiple kinases phosphorylate the pancreatic cholecystokinin receptor in an agonist-dependent manner. Am. J. Physiol. 264 (Gastrointest. Liver Physiol. 27): G840-G847, 1993[Abstract/Free Full Text].

5.   Go, W. Y., E. L. Holicky, E. M. Hadac, R. V. Rao, and L. J. Miller. Identification of a domain in the carboxy terminus of CCK receptor that affects its intracellular trafficking. Am. J. Physiol. 275 (Gastrointest. Liver Physiol. 38): G56-G62, 1998[Abstract/Free Full Text].

6.   Goodman, O. B., Jr., J. G. Krupnick, F. Santini, V. V. Gurevich, R. B. Penn, A. W. Gagnon, J. H. Keen, and J. L. Benovic. beta -Arrestin acts as a clathrin adaptor in endocytosis of the beta 2-adrenergic receptor. Nature 383: 447-450, 1996[Medline].

7.   Goodman, O. B., Jr., J. G. Krupnick, F. Santini, V. V. Gurevich, R. B. Penn, A. W. Gagnon, J. H. Keen, and J. L. Benovic. Role of arrestins in G-protein-coupled receptor endocytosis. Adv. Pharmacol. 42: 429-433, 1998[Medline].

8.   Gurevich, V. V., S. B. Dion, J. J. Onorato, J. Ptasienski, C. M. Kim, R. Sterne-Marr, M. M. Hosey, and J. L. Benovic. Arresting interactions with G protein-coupled receptors: direct binding studies of wild type and mutant arrestins with rhodopsin, beta 2-adrenergic, and m2 muscarinic cholinergic receptors. J. Biol. Chem. 270: 720-731, 1995[Abstract/Free Full Text].

9.   Hunyady, L., A. J. Baukal, T. Balla, and K. J. Catt. Independence of type I angiotensin II receptor endocytosis from G protein coupling and signal transduction. J. Biol. Chem. 269: 24798-24804, 1994[Abstract/Free Full Text].

10.   Kallal, L., A. W. Gagnon, R. B. Penn, and J. L. Benovic. Visualization of agonist-induced sequestration and down-regulation of a green fluorescent protein-tagged beta 2-adrenergic receptor. J. Biol. Chem. 273: 322-328, 1998[Abstract/Free Full Text].

11.   Luttrell, L. M., Y. Daaka, G. L. Della Rocca, and R. J. Lefkowitz. G protein-coupled receptors mediate two functionally distinct pathways of tyrosine phosphorylation in rat 1a fibroblasts: Shc phosphorylation and receptor endocytosis correlate with activation of Erk kinases. J. Biol. Chem. 272: 31648-31656, 1997[Abstract/Free Full Text].

12.   Ozcelebi, F., and L. J. Miller. Phosphopeptide mapping of cholecystokinin receptors on agonist-stimulated native pancreatic acinar cells. J. Biol. Chem. 270: 3434-3441, 1995.

13.   Ozcelebi, F., and L. J. Miller. Phosphorylation of cholecystokinin receptors expressed on Chinese hamster ovary cells: similarities and differences relative to native pancreatic acinar cells. J. Biol. Chem. 271: 3750-3755, 1996[Abstract/Free Full Text].

14.   Rao, R. V., B. F. Roettger, E. M. Hadac, and L. J. Miller. Roles of cholecystokinin receptor phosphorylation in agonist-stimulated desensitization of pancreatic acinar cells and receptor-bearing Chinese hamster ovary cholecystokinin receptor cells. Mol. Pharmacol. 51: 185-192, 1997[Abstract/Free Full Text].

15.   Roettger, B. F., D. Ghanekar, R. Roe, C. Toledo, J. Yingling, D. Pinon, and L. J. Miller. Antagonist-stimulated internalization of the G protein-coupled cholecystokinin receptor. Mol. Pharmacol. 51: 357-362, 1997[Abstract/Free Full Text].

16.   Roettger, B. F., D. I. Pinon, T. P. Burghardt, and L. J. Miller. Regulation of lateral mobility and cellular trafficking of the CCK receptor by a partial agonist. Am. J. Physiol. 276 (Cell Physiol. 45): C539-C547, 1999[Abstract/Free Full Text].

17.   Roettger, B. F., R. U. Rentsch, E. M. Hadac, E. H. Hellen, T. P. Burghardt, and L. J. Miller. Insulation of a G protein-coupled receptor on the plasmalemmal surface of the pancreatic acinar cell. J. Cell. Biol. 130: 579-590, 1995[Abstract].

18.   Roettger, B. F., R. U. Rentsch, D. Pinon, E. Holicky, E. Hadac, J. M. Larkin, and L. J. Miller. Dual pathways of internalization of the cholecystokinin receptor. J. Cell. Biol. 128: 1029-1042, 1995[Abstract].

19.   Sterne-Marr, R., and J. L. Benovic. Regulation of G protein-coupled receptors by receptor kinases and arrestins. Vitam. Horm. 51: 193-234, 1995[Medline].

20.   Tsunoda, Y., and C. Owyang. High-affinity cholecystokinin receptors are coupled to phospholipase A2 pathways to mediate pancreatic amylase secretion. Am. J. Physiol. 269 (Gastrointest. Liver Physiol. 32): G435-G444, 1995[Abstract/Free Full Text].


Am J Physiol Cell Physiol 276(3):C537-C538
0002-9513/99 $5.00 Copyright © 1999 the American Physiological Society




This Article
Full Text (PDF)
Alert me when this article is cited
Alert me if a correction is posted
Citation Map
Services
Email this article to a friend
Similar articles in this journal
Similar articles in PubMed
Alert me to new issues of the journal
Download to citation manager
Google Scholar
Articles by del Valle, J.
Articles citing this Article
PubMed
PubMed Citation
Articles by del Valle, J.


HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online