EDITORIAL FOCUS
What molecular events underlie heterologous
desensitization? Focus on "Receptor
phosphorylation does not mediate cross talk between muscarinic
M3 and bradykinin
B2 receptors"
M. Marlene
Hosey
Department of Molecular Pharmacology and Biological Chemistry,
Northwestern University Medical School, Chicago, Illinois 60611
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ARTICLE |
G PROTEIN-COUPLED RECEPTORS (GPCRs) comprise one of the
largest families of signaling molecules and play physiologically
important roles in every cell type. Thousands of GPCRs are predicted to exist. Indeed, even in the tiny worm Caenorhabditis
elegans, there are predicted to be ~650
different GPCRs (10). The GPCRs are activated by a diverse array of
molecules, including hormones, neurotransmitters, paracrine factors,
sensory molecules, ions, cyclic nucleotides, and proteases, among
others. GPCRs signal in cells by activating heterotrimeric G proteins,
which consist of 

-subunits. Because there are
multiple genes encoding
-,
-, and
-subunits, a large number of
different heterotrimers can theoretically assemble. Once activated, the
- and 
-subunits of G proteins dissociate, and each can
modulate the activity of a growing list of effectors, including enzymes
such as adenylyl cyclases and phospholipases, as well as ion channels
(14). In contrast to the large number of GPCRs, the number of
identified effectors is considerably smaller. Because many cells have
multiple GPCRs that signal through common effectors, it is not
surprising that cross-regulation can occur in the signaling pathways
from one GPCR to another. This "cross talk" is commonly seen as
an inhibition (desensitization) induced by one GPCR onto another GPCR,
although stimulation (sensitization) of signaling of one GPCR by
another has also been frequently observed.
Desensitization of GPCRs, or an attenuation of signaling that occurs
despite the continued presence of a ligand, is a physiologically important and complex process that participates in the turning off of
the GPCRs (3, 6, 12, 17, 18). Traditionally, desensitization has been
divided into two forms. Homologous desensitization is a process whereby
only the activated GPCRs are "turned off" or desensitized,
whereas heterologous desensitization refers to processes whereby the
activation of one GPCR can result in the inhibition of another,
heterologous GPCR to signal. It is not clear why many GPCRs exhibit
desensitization while others do not or why some GPCRs may exhibit
either homologous or heterologous desensitization or both. For example,
in the current article in focus, Willars et al. (Ref. 24, see page C859
in this issue) have observed heterologous desensitization
of bradykinin B2 receptors after
activation of M3 muscarinic
receptors while the converse did not occur, i.e., the
M3 receptors were not desensitized
by the B2 receptors. Different
cellular backgrounds can also influence the type and degree of
desensitization that is observed, suggesting that multiple events are
required for the different processes.
What are the molecular events underlying desensitization of GPCRs?
Homologous desensitization has been extensively studied, and an
important hallmark of this process is that it is exquisitely dependent
on agonist occupancy of the GPCRs. Agonist-induced phosphorylation of
GPCRs is an early event that is important in the process of homologous
desensitization (3, 5, 17, 18). The processes underlying heterologous
or cross-desensitization are less well understood, and it appears that
there will be multiple mechanisms contributing to this type of
inhibition. GPCRs that undergo heterologous desensitization do not need
to be occupied by an agonist. Rather, the activation of one GPCR
generates a signal that causes inhibition of signaling by a second,
heterologous GPCR. This type of desensitization is due to cross talk in
signaling pathways that involves modifications of the activities of
GPCRs, G proteins, or effectors (8, 9, 12).
Because phosphorylation of the GPCRs plays a central role in homologous
desensitization, many have asked whether receptor phosphorylation might
play a role in heterologous desensitization. Activation of second
messenger-dependent protein kinases, such as protein kinase A and
protein kinase C (PKC), by one GPCR can result in the phosphorylation
of other GPCRs in the same cell and result in heterologous
desensitization (9, 12). Indeed, many GPCRs are phosphorylated by PKC,
and an illustration of the role of PKC in cross-desensitization is
provided from studies of chemoattractant receptors (1). Notably,
PKC-mediated phosphorylation of platelet-activating factor (PAF)
receptors is induced by activation of other chemoattractant receptors
and inhibits both coupling of the PAF receptors to G proteins and
mobilization of intracellular calcium stores (20). Interestingly, the
cross-regulation is unidirectional, because activation of wild-type PAF
receptors does not cross-regulate the chemoattractant receptors that
induced the cross-regulation of the PAF receptors (20).
A unidirectional regulation of GPCRs was also found in the study by
Willars et al. (24); however, the consequence of phosphorylation by
PKC was markedly different. Willars et al. demonstrate that activation
of bradykinin B2 receptors induced
a PKC-dependent phosphorylation of the
M3 muscarinic receptors. However,
this phosphorylation did not appear to alter the ability of the
M3 receptors to increase
phosphoinositide or calcium signaling. On the other hand, activation of
the M3 receptors did not induce phosphorylation of the bradykinin
B2 receptors but did induce cross-inhibition of phosphoinositide and calcium signaling. Thus these
data provide an example where the cross-regulation of receptor signaling did not appear to be linked to receptor
cross-phosphorylation, whereas, in other cases, such as that cited
above for the PAF receptors (1, 20), receptor phosphorylation plays an
obligatory role in cross-desensitization. It is likely that multiple
events participate in heterologous regulation of GPCRs. It is becoming increasingly clear that receptor phosphorylation is not the exclusive mediator of heterologous desensitization (1, 8) and that events
downstream are clearly involved. Although the exact nature of these
downstream events is not known in detail, many have suggested the
involvement of, and/or modification of, G proteins, downstream effectors, and/or stores of small signaling molecules (1, 8, 9, 12).
What else might be contributing to cross-desensitization of GPCRs? In
recent years, the identification and characterization of the growing
family of regulators of G protein signaling (RGS) proteins has added
another player to desensitization pathways (2, 11). Because RGS
proteins have been demonstrated to be GTPase-activating proteins (GAPs)
that accelerate the turnoff of activated G proteins, it is likely that
RGS proteins contribute to desensitization. The role of RGS proteins in
the regulation of GPCRs is under intensive study, and we are certain to
learn more about how they participate in desensitization. Of interest, there are indications that the inhibition of activated G
-subunits by
RGS proteins might allow for enhanced expression of G
-dependent pathways (4, 8, 16). Interestingly, certain effectors such as
phospholipase C-
1 possess GAP activity, and this can be blocked by
G
-subunits (7). It remains to be determined how GAPs contribute
to cross-regulation.
In addition, previously unappreciated inhibitory signaling mechanisms
are becoming more obvious contributors to heterologous desensitization.
For example, it is possible that cross-desensitization might result
when two GPCRs regulate a common effector by different signaling
pathways. Recent studies of the regulation of voltage-dependent calcium
channels by GPCRs have suggested this possibility (19). Potentially,
the ability of one GPCR to regulate one effector may be more dominant
and preclude the ability of another GPCR to exert its effect. Another
area of developing interest is the role of lipids or lipid-derived
signaling molecules. Arachidonic acid production has been linked to the
ability of endothelin A receptors to heterologously inhibit the ability
of µ-opiate receptors to stimulate G protein-activated inwardly
rectifying potassium channels (GIRKs) (21). This is interesting because
the endothelin receptors by themselves do not appear to exert effects
on the GIRK channels (21).
Other mechanisms of GPCR regulation also may involve lipid-dependent
regulation. The findings that arrestin-dependent internalization of
GPCRs may possess a requirement for phosphoinositides (13), and that
agonist-dependent internalization of GPCRs may be modulated by
depletion of phosphoinositides (22), both suggest that the status of
lipid stores may influence the desensitization of GPCRs. In addition,
there appears to be a requirement for phosphoinositides in
the GPCR-dependent regulation of GIRK channels (15, 23). Consequently, depletion of phosphatidylinositol
4,5-bisphosphate stores by GPCR-mediated activation of
phospholipases could contribute to heterologous desensitization of
GIRKs. Future studies should reveal other previously unrecognized
signaling events that have roles in the desensitization of responses
elicited by GPCRs. From what we know so far, it is likely that the
molecular events underlying cross-regulation of GPCRs will be complex
and that a universal mechanism will not be found responsible.
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FOOTNOTES |
Address for reprint requests and other correspondence: M. M. Hosey,
Dept. of Molecular Pharmacology and Biological Chemistry, Northwestern
Univ. Medical School, 303 E. Chicago Ave.-S215, Chicago, IL 60611 (E-mail: mhosey{at}nwu.edu).
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Am J Physiol Cell Physiol 277(5):C856-C858
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