EDITORIAL FOCUS
Regulating epithelia from the apical side: new insights Focus on "Differential signaling and regulation of apical vs. basolateral EGFR in polarized epithelial cells"

Karl J. Karnaky Jr.

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

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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- 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.

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 beta -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.

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.

    ACKNOWLEDGEMENTS

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.

    REFERENCES
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1.   Barrett, K. E., P. A. Huott, S. S. Shalini, K. Dharmsathaphorn, and S. I. Wasserman. Differing effects of apical and basolateral adenosine on colonic epithelial cell line T84. Am. J. Physiol. 256 (Cell Physiol. 25): C197-C203, 1989[Abstract/Free Full Text].

2.   Denning, G. M., and M. J. Welsh. Polarized distribution of bradykinin receptors on airway epithelial cells and independent coupling to second messenger pathways. J. Biol. Chem. 266: 12932-12938, 1991[Abstract/Free Full Text].

3.   Du, J., and P. Wilson. Abnormal polarization of EGF receptors and autocrine stimulation of cyst epithelial growth in human ADPKD. Am. J. Physiol. 269 (Cell Physiol. 38): C487-C495, 1995[Abstract/Free Full Text].

4.   Forte, L. R., and F. K. Hamra. Guanylin and uroguanylin: intestinal peptide hormones that regulate epithelial transport. News Physiol. Sci. 11: 17-23, 1996.[Abstract/Free Full Text]

5.   Kuwada, S. K., K. A. Lund, X. F. Li, P. Cliften, K. Amsler, L. K. Opresko, and H. S. Wiley. Differential signaling and regulation of apical vs. basolateral EGFR in polarized epithelial cells. Am. J. Physiol. 275 (Cell Physiol. 44): C1419-C1428, 1998[Abstract/Free Full Text].

6.   Paradiso, A. M., S. J. Mason, E. R. Lazarowski, and R. C. Boucher. Membrane-restricted regulation of Ca2+ release and influx in polarized epithelia. Nature 377: 643-646, 1995[Medline].

7.   Wagner, J. A., A. C. Chao, and P. Gardner. Molecular strategies for therapy of cystic fibrosis. Annu. Rev. Pharmacol. Toxicol. 35: 257-276, 1995[Medline].


Am J Physiol Cell Physiol 275(6):C1417-C1418
0002-9513/98 $5.00 Copyright © 1998 the American Physiological Society