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
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ARTICLE |
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
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).
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