Truncation mutants of the tight junction protein ZO-1 disrupt corneal epithelial cell morphology.

The tight junction is the most apical intercellular junction of epithelial cells and regulates transepithelial permeability through the paracellular pathway. To examine possible functions for the tight junction-associated protein ZO-1, C-terminally truncated mutants and a deletion mutant of ZO-1 were epitope tagged and stably expressed in corneal epithelial cell lines. Only full-length ZO-1 and one N-terminal truncation mutant targeted to cell borders; other mutants showed variable cytoplasmic distributions. None of the mutants initially disrupted the localization of endogenous ZO-1. However, long-term stable expression of two of the N-terminal mutants resulted in a dramatic change in cell shape and patterns of gene expression. An elongated fibroblast-like shape replaced characteristic epithelial cobblestone morphology. In addition, vimentin and smooth muscle actin expression were up-regulated, although variable cytokeratin expression remained, suggesting a partial transformation to a mesenchymal cell type. Concomitant with the morphological change, the expression of the integral membrane tight junction protein occludin was significantly down-regulated. The localizations of endogenous ZO-1 and another family member, ZO-2, were disrupted. These findings suggest that ZO-1 may participate in regulation of cellular differentiation.

CD36 expression is altered in retinal pigment epithelial cells of the RCS rat.

The retinal pigment epithelial cell has several important functions, one of which is the phagocytosis of photoreceptor outer segments which are discarded diurnally. We previously provided evidence in human retinal pigment epithelium that CD36, an 88 kDa integral membrane glycoprotein, participates in the phagocytosis of photoreceptor outer segments. Since in the Royal College of Surgeons dystrophic rat, retinal pigment epithelial cells fail to perform this function and as a result the photoreceptor cells degenerate, the expression of CD36 has now been examined by retinal pigment epithelial cells of the dystrophic rat. Consistent with earlier work using human retinal pigment epithelial cells, expression of CD36 by freshly isolated retinal pigment epithelial cells of Long Evans rats was confirmed by immunoblotting and immunocytochemistry with antibody to rat CD36. The protein was also present in lysates of cultured retinal pigment epithelium. Furthermore, with an in vitro phagocytosis assay using 125I-labeled outer segments, it was demonstrated that the binding and ingestion of outer segments by rat retinal pigment epithelial cells was reduced by 64% in the presence of antibodies to rat CD36. In contrast to observations in the Long Evans rat, immunoblotting of retinal pigment epithelial cells isolated from the adult Royal College of Surgeons rat revealed that CD36 protein was not present. This appeared to be a tissue-specific absence since CD36 protein was present in peritoneal macrophages harvested from the adult Royal College of Surgeons rat. A developmental study of CD36 expression also demonstrated an absence of the protein on the day of birth and at 1 and 2 weeks postnatally. By reverse transcriptase-polymerase chain reaction, CD36 mRNA was detected in freshly harvested retinal pigment epithelial cells of the Royal College of Surgeons rat at only PN1, 1 week and 10 days. Significantly, at 2 weeks of age and in the adult Royal College of Surgeons rat. CD36 transcripts were no longer present. Nevertheless, by Northern blot analysis CD36 mRNA was detected in various other tissues shown previously to express CD36. We conclude that in RPE cells of the Royal College of Surgeons rat, CD36 protein is not expressed while CD36 mRNA is present only transiently during postnatal development.

Binding of anionic phospholipids to retinal pigment epithelium may be mediated by the scavenger receptor CD36.

The specific recognition of negatively charged phospholipids in cell membranes has been suggested to play an important role in a variety of physiological and pathophysiological processes. Recent work (Rigotti, A., Acton, S. L., and Krieger, M. (1995) J. Biol. Chem. 270, 16221-16224) has described specific and tight binding of anionic phospholipids, such as phosphatidylserine (PS) and phosphatidylinositol (PI), to the class B scavenger receptors, CD36 and SR-B1. We have previously reported that CD36 is present on retinal pigment epithelium (RPE) and plays a role in the phagocytosis of photoreceptor outer segments (ROS), a function critical to the normal visual process (Ryeom, S. W., Sparrow, J. R., and Silverstein, R. L. (1996) J. Cell Sci. 109, 387-395). We now report that phospholipid liposomes PS and PI, but not phosphatidylethanolamine, bind specifically to RPE. Cross-competition experiments suggest that PS and PI recognize the same receptor on RPE, while immunoinhibition studies indicate that the receptor is CD36. RPE cells isolated from a mutant rat strain, the RPE of which does not express CD36 ( Sparrow, J. R., Ryeom, S. W. , Abumrad, N., Ibrahimi, A., and Silverstein, R. L. (1996) Exp. Eye Res., in press), did not bind PS or PI, further confirming the role of CD36. We also showed that purified ROS blocked binding and uptake of anionic phospholipid liposomes by RPE and that PS and PI liposomes blocked ROS uptake by RPE, suggesting that PS and PI on the ROS membrane may be the ligands on ROS recognized by CD36. This is the first demonstration that CD36-phospholipid interactions may play a role in normal physiology.

CD36 participates in the phagocytosis of rod outer segments by retinal pigment epithelium.

Mechanisms of phagocytosis are complex and incompletely understood. The retinal pigment epithelium provides an ideal system to study the specific aspects of phagocytosis since an important function of this cell is the ingestion of packets of membranous discs that are normally discarded at the apical ends of rod and cone cells during outer segment renewal. Here we provide evidence that rod outer segment phagocytosis by retinal pigment epithelium is mediated by CD36, a transmembrane glycoprotein which has been previously characterized on hematopoietic cells as a receptor for apoptotic neutrophils and oxidized low density lipoprotein. Immunocytochemical staining with monoclonal and polyclonal antibodies demonstrated CD36 expression by both human and rat retinal pigment epithelium in transverse cryostat sections of normal retina and in primary cultured cells. By western blot analysis of retinal pigment epithelial cell lysates, polyclonal and monoclonal antibodies to CD36 recognized an 88 kDa protein which comigrated with platelet CD36. Furthermore, the synthesis of CD36 mRNA by retinal pigment epithelium was confirmed by reverse transcriptase-PCR using specific CD36 oligonucleotides. The addition of CD36 antibodies to cultured retinal pigment epithelial cells reduced the binding and internalization of 125I-labeled rod outer segments by 60%. Immunofluorescence confocal microscopy confirmed that outer segment uptake was significantly diminished by an antibody to CD36. Moreover, we found that transfection of a human melanoma cell line with CD36 cDNA enabled these cells to bind and internalize isolated photoreceptor outer segments as seen by double immunofluorescent staining for surface bound and total cell-associated rod outer segments, and by measurement of cell-associated 125I-labeled rod outer segments. We conclude that the multifunctional scavenger receptor CD36 participates in the clearance of photoreceptor outer segments by retinal pigment epithelium and thus, participates in the visual process.

Lipoxin recognition sites. Specific binding of labeled lipoxin A4 with human neutrophils.

Lipoxin A4 stimulates rapid lipid remodeling and a pertussis toxin-sensitive release of arachidonic acid in polymorphonuclear leukocytes (PMN) (Nigam, S., Fiore, S., Luscinskas, F.W., and Serhan, C.N. (1990) J. Cell. Physiol. 143, 512-523) and has been shown to inhibit leukocyte responses in several systems. To examine the basis underlying these actions, we have prepared [11,12-3H]lipoxin A4 (LXA4) and characterized its interactions with human PMN. Time course studies (0-90 min) with intact PMN demonstrated cell association of 3H label which was specific and reversible. PMN bound [3H]LXA4 with a Kd of 0.5 +/- 0.3 nM, representing approximately 1,830 sites/PMN, and the Hill plot value of 1.9 suggests cooperative binding. [3H]LXA4 binding was stereoselective since neither leukotriene B4 (LTB4), lipoxin B4 (LXB4), (6S)-LXA4, 11-trans-LXA4, nor SKF 104353 competed for [3H]LXA4-specific binding while LTD4 and LTC4 partially competed. Subcellular fractionation revealed that specific binding with [3H]LXA4 was associated with membrane (42.1%)-, granule (34.5%)-, and nuclear (23.3%)-enriched fractions, a distribution distinct from that of [14,15-3H] LTB4 binding. [11,12-3H]LXA4-specific binding was modulated by guanosine analogs, suggesting the involvement of G proteins. A fluorescent LXA4 derivative (methyl-7-methoxycoumarin-LXA4) competed with [3H]LXA4 binding to intact PMN and showed specific and reversible binding as monitored by flow cytometric analysis. These results indicate that PMN possess specific recognition sites for LXA4 which may mediate its actions.

Ultrastructural immunogold localization of prostaglandin endoperoxide synthase (cyclooxygenase) to non-membrane-bound cytoplasmic lipid bodies in human lung mast cells, alveolar macrophages, type II pneumocytes, and neutrophils.

Lipid bodies are non-membrane-bound, lipid-rich cytoplasmic inclusions that occur in many mammalian cell types. Because lipid bodies are more prominent in cells associated with inflammation and are repositories of arachidonyl-phospholipids, a role for lipid bodies in the oxidative metabolism of arachidonic acid to form eicosanoids has been suggested. To evaluate further whether lipid bodies, in addition to serving as non-membranous sources of substrate arachidonate, are involved in eicosanoid formation, we used cells isolated from human lung to investigate the intracellular localization of prostaglandin endoperoxide (PGH) synthase (cyclooxygenase), the key initial, rate-limiting enzyme in the formation of prostaglandins and thromboxanes. Isolated lung cells containing a mixture of mast cells, alveolar macrophages, Type II alveolar pneumocytes, and neutrophils from short-term cultures were fixed in suspension in a dilute aldehyde mixture, post-fixed in osmium tetroxide, stained en bloc with uranyl acetate, dehydrated in a graded series of alcohols, and embedded in Epon. A post-embedding immunogold procedure was used with a primary PGH synthase monoclonal antibody and 20-nm gold-conjugated secondary antibody to demonstrate enzyme locations. Specificity controls were also done. We found PGH synthase in lipid bodies of human lung mast cells, alveolar macrophages, Type II alveolar pneumocytes, and neutrophils. Specific secretory and lysosomal granules and plasma membranes did not express PGH synthase. Specificity controls, including omission of the primary antibody or substitution with an irrelevant antibody, were negative. Absorption of the specific PGH synthase antibody with purified solid-phase PGH synthase resulted in a marked reduction of label in lipid bodies of all four cell types. These findings establish the presence of PGH synthase in lipid bodies of human lung mast cells, alveolar macrophages, Type II alveolar pneumocytes, and neutrophils and, in concert with previous studies, suggest that these cytoplasmic lipid-rich organelles may be non-membrane sites of eicosanoid formation.

Cytoplasmic lipid bodies of neutrophils: formation induced by cis-unsaturated fatty acids and mediated by protein kinase C.

Lipid bodies, nonmembrane-bound cytoplasmic inclusions, serve as repositories of esterified arachidonate and are increased in cells associated with inflammatory reactions. We have evaluated stimuli and mechanisms responsible for lipid body formation within human polymorphonuclear leukocytes (PMNs). Arachidonic acid and oleic acid stimulated dose-dependent formation of lipid bodies over 0.5-1 h. Other C20 and C18 fatty acids were less active and demonstrated rank orders as follows: cis-unsaturated fatty acids were much more active than trans-fatty acids, and activity diminished with decreasing numbers of double bonds. Lipid bodies elicited in vitro with cis-fatty acids were ultrastructurally identical to lipid bodies present in PMNs in vivo. Lipid body induction was not because of fatty acid-elicited oxidants or fatty acid-induced ATP depletion. Cis-fatty acid-induced activation of protein kinase C (PKC) was involved in lipid body formation as evidenced by the capacity of other PKC activators, 1-oleoyl-2-acetyl-glycerol and two active phorbol esters, phorbol myristate acetate, and phorbol 12,13 dibutyrate, but not an inactive phorbol, to induce lipid body formation. The PKC inhibitor, 1-O-hexadecyl-2-O-methyl-glycerol, inhibited PMN lipid body formation induced by oleic and arachidonic acids and by 1-oleoyl-2-acetyl-glycerol and phorbol myristate acetate. Other PKC inhibitors (staurosporine, H-7) also inhibited lipid body formation. Formation of lipid bodies in PMNs is a specific cellular response, stimulated by cis-fatty acids and diglycerides and apparently mediated by PKC, which results in the mobilization and deposition of lipids within discrete, ultrastructurally defined cytoplasmic domains.