Department of Cell Biology and Anatomy, Medical University of South Carolina, Charleston, South Carolina 29425; and Mount Desert Island Biological Laboratory, Salsbury Cove, Maine 04672
THERE ARE NOW well over 50 publications documenting
regulation of epithelial function by ligands acting on the
luminal side of airway, gut, excretory, and reproductive
epithelia. This emerging field of apical membrane receptors, which
already lists almost 20 different regulatory ligands, promises to be
not only exciting but also critically important from the standpoint of
both normal and abnormal epithelial function. First, apical
responsiveness appears to be a normal characteristic of many, if not
all, epithelia. Most of the known examples of apical responsiveness to
ligands are from in vitro studies of normal tissues, either freshly
isolated or in primary culture. Second, a greater awareness of apical
receptor-related diseases is beginning to emerge. It is now known that
apical guanylin receptors are the body's vulnerable point in
life-threatening secretory diarrhea caused by the
Escherichia coli heat-stable enterotoxin: this toxin acts on gut receptors whose normal function is
to regulate salt and water transport (4). Also, mislocation of
receptors to the apical membrane is a defining characteristic in
polycystic disease of the kidney (3). Third, since caregivers have
access to a number of luminal membranes, such as those of the gut and
airways, one clinical relevance of understanding these apical receptors
is clear. A case in point is the disease cystic fibrosis: aerosolized
UTP has been tested therapeutically in an attempt to stimulate apical
membrane purinergic receptors, and thus fluid secretion, in the airway
(7).
However, we still have a great deal to learn about apical regulation of
epithelial function. From a long list of unknowns concerning this
receptor phenomenon, a central question focuses on cases in which the
locations of receptors for a given ligand are found on both sides of
the epithelium: What is the difference, if any, in the response from
the two different sides? There is only a scant literature on this
topic. Barrett et al. (1) have shown that, in the colonic epithelial
cell line T84, apical and basolateral adenosine receptors differed in
both their ability to stimulate increases in cAMP and in their
susceptibility to receptor downregulation. Denning and Welsh (2), using
primary cultures of canine trachea, have shown that basal bradykinin
exposure leads to arachidonic acid release but that apical exposure
does not, although Cl This pivotal question of apical-side vs. basolateral-side
responsiveness is the chief concern of our current article in focus (Ref. 5, see p. C1419 in this issue). Kuwada et al. (5) have utilized the powerful model system of
LLC-PK1 cells, which are polarized
and normally express epidermal growth factor (EGF) receptors
predominantly on the basolateral membrane. With EGF receptor (EGFR)
gene transfection, these cells overexpress the receptor, which is then
present in high numbers at both the apical and basolateral surfaces.
These authors have examined EGF effects on six different physiological
responses in this model system. Only two of these responses,
specifically those involving mitogenesis, were similar after
stimulation from either the apical or basal side. Cell proliferation,
as measured by
[3H]thymidine
incorporation, was similar. The authors also examined the
phosphotyrosine content of three known substrates of the
EGFR-Ras-mitogen-activated protein kinase pathway, SHC, ERK1, and ERK2,
which are known to mediate cell proliferation, and all were similar no
matter which side was stimulated with EGF. These findings demonstrate a
lack of compartmentation of EGFR substrates involved in EGF-mediated mitogenesis. In striking contrast, stimulation of endocytosis and
downregulation of EGFR were less efficient after exposure to EGF from
the apical side. Likewise, EGFR tyrosine kinase activity differed
between apical and basolateral exposures in terms of both its
stimulation and its attenuation in the following 20 h. Finally, the
EGF-induced tyrosine phosphorylation of substrates involved in cell
adhesion was measured following EGF exposure of the two cell poles.
Focal adhesion kinase was tyrosine phosphorylated more by basolateral
than apical exposure, whereas An understanding of apical membrane receptor function, particularly in
receptor gene-transfected epithelial cells, could have important
implications for future gene therapy. The strategy mentioned above for
UTP aerosol therapy for cystic fibrosis might be carried one profound
step further. Transfection of epithelial cells with the gene for an
appropriate receptor could provide supplementation or replacement for
diseased and nonfunctional receptors at either the apical or even the
basolateral surface. If this transfection-derived receptor were located
on the apical membrane, as was the case for overexpressed EGF receptors
in LLC-PK1 cells in the present study (5), the physiological function controlled by it could be
regulated luminally, from exogenous sources of ligand.
Novel and highly significant regulatory mechanisms are emerging from
studies of apical membrane signaling events. Further research into this
exciting new area of epithelial function should ultimately extend to
numerous areas of cell physiology, including membrane traffic and first
and second messenger signaling systems. Ultimately, discoveries in this
emerging field should lead to insights into normal epithelial function.
Progress in this area should provide a firm basis for understanding
aberrant cell behavior in diseases involving apical receptor
mislocation and should lead inevitably to novel clinical strategies of
receptor insertion and regulation. The study by Kuwada et al. (5)
represents a large leap forward in understanding that responses from
the two sides may be similar for some physiological responses but
different for others. It is imperative that this knowledge be factored
into future clinical strategies.
ARTICLE
Top
Article
References
secretion is stimulated by this hormone from either side. In monolayers
of polarized nasal epithelial cells, activation of purinoreceptors
regulates intracellular Ca2+
release and influx responses that are functionally confined to the
membrane of receptor activation (6). The major trend emerging from
these few studies is that there can be differences in responsiveness to
receptors located on the apical vs. the basolateral poles.
-catenin exhibited just the opposite
response. The clear message is that certain physiological mechanisms
are compartmentalized and that mislocation of a receptor such as EGFR
may contribute to the pathogenesis of certain diseases.
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ACKNOWLEDGEMENTS |
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The author is a Senior Fellow of the Salsbury Cove Research Fund of the Mt. Desert Island Biological Laboratory and is supported in part by a grant from the Department of Defense, Army Breast Cancer Program.
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REFERENCES |
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