Kidney injury molecule–1 is a phosphatidylserine receptor that confers a phagocytic phenotype on epithelial cells (original) (raw)
Kim-1–expressing tubule epithelial cells bind and internalize apoptotic bodies and necrotic debris in injured kidneys. Using specific anti–Kim-1 antibodies (9), we localized Kim-1 directly adjacent to apoptotic cells and necrotic debris in the lumens of rat kidney tubules in vivo. Kim-1 also surrounds phagocytosed apoptotic bodies within tubule cells in rat kidney 24 and 48 hours after the kidney had been subjected to ischemic injury (Figure 1, A and B). Confocal images (Figure 1C) confirm internalization of apoptotic bodies within Kim-1–expressing epithelial cells. There are phagocytic cups on the apical surface of tubular cells that are lined with Kim-1 (Figure 1C). Proximal tubules in the outer medulla of the kidney, where injury after ischemia is maximal, were scored for colocalization of Kim-1 staining and the presence of apoptotic bodies (confirmed by TUNEL staining). While 34.6% ± 11.8% of Kim-1–positive tubules contained cell-internalized apoptotic cells, only 9.4% ± 3.7% of Kim-1–negative tubules contained apoptotic cells (Figure 1D). TUNEL-positive nuclei were present in intracellular phagosomes and adherent to the luminal surface of Kim-1–expressing tubules (Figure 1E). Importantly, although macrophages, and to a lesser extent other leukocytes, are recruited to the interstitium of the injured kidney, they are rarely seen within the tubule lumen, where many apoptotic and necrotic cells are seen (Figure 1F) (11). Thus, Kim-1–expressing kidney epithelial cells avidly phagocytose apoptotic and necrotic cells. Note also that Kim-1–expressing cells lack the macrophage marker CD68 and macrophages in the kidney do not express Kim-1 (Figure 1F).
Kim-1–expressing tubule epithelial cells bind and internalize apoptotic bodies and necrotic debris in rat kidneys following ischemic injury. (A) By light microscopy, necrotic cellular debris (seen by differential interference contrast [DIC]) binds to apically located Kim-1 (dark brown) of surviving tubule epithelial cells (large arrows). An apoptotic body is seen in 1 Kim-1–positive epithelial cell (small arrow). (B) By fluorescence microscopy many DAPI-positive (blue) apoptotic bodies (large arrows) can be seen binding to the surface of Kim-1–positive (red) epithelial cells, and Kim-1–expressing cells have processes (red, small arrow) internalizing an apoptotic body (blue). In addition, apoptotic bodies have been internalized by Kim-1–positive epithelial cells (blue, arrowhead). (C) Confocal image of apoptotic cells (blue) localized in phagocytic cups (small arrows) on the Kim-1–positive (red) apical surface of tubular cells. An internalized apoptotic cell is indicated by a large arrow. L denotes the tubule lumen. (D) The proportion of Kim-1–positive tubules containing internalized apoptotic bodies was greater than adjacent Kim-1–negative tubules in the postischemic kidney. Neither Kim-1 nor apoptotic bodies were identified in the normal kidneys. *P = 0.01 (error bars indicate SD). (E) Kim-1–positive epithelial cells (red) binding many TUNEL-positive apoptotic bodies (green, large arrows) and internalizing other small apoptotic bodies (green, small arrow). (F) Kim-1–positive epithelial cells (red, large arrows) are surrounded by CD68-positive macrophages (green, small arrows), which do not express Kim-1 and are found only in the interstitium. Scale bar: 10 μm.
To evaluate the phagocytic properties of Kim-1 in more detail, we cultured primary proximal tubule epithelial cells from mouse and rat kidneys using established methods (12). Primary cultures of rat tubule cells expressed Kim-1 in approximately 25% of cells after 4 days in culture (Figure 2A). Kim-1 was expressed in cytokeratin-expressing cells, confirming their identity as primary tubule epithelial cells (Figure 2B). Monolayers were cocultured for 1 hour at 37°C with 5-chloromethylfluorescein diacetate (CMFDA) fluorescent labeled apoptotic thymocytes, after which noningested apoptotic cells were washed away (Figure 2, A and C). The rat epithelial monolayer was colabeled with antibodies to Kim-1. There was marked binding and phagocytosis of apoptotic cells in Kim-1–expressing but not adjacent Kim-1–negative primary cells. We also observed Kim-1 localized at the phagocytic cup and in the phagosome (Figure 2A). The apoptotic cells associated with Kim-1–positive and Kim-1–negative epithelial cells were determined (Figure 2C). In this assay some apoptotic cells were cell surface bound in addition to others that were phagocytosed. To determine the role of Kim-1 in the phagocytic process directly, coculture with apoptotic cells was carried out in the presence of either monoclonal anti–Kim-1 antibodies with affinity for the ectodomain or isotype control antibodies (Figure 2D). After coincubations and washing, the adherent primary cells were lifted into a single-cell suspension with EDTA and trypsin, and individual suspended epithelial cells were counted by fluorescence microscopy. Kim-1 antibodies markedly inhibited phagocytosis in this assay, indicating that Kim-1 itself played a direct role in phagocytosis (Figure 2D). Further analysis in primary cells could not be undertaken because primary cultured epithelial cells do not survive passaging in vitro and variably exhibit high levels of autofluorescence. In order to study the role of KIM-1 in epithelial cell phagocytosis further, we stably expressed full-length human KIM-1 in porcine LLC-PK1 renal tubular epithelial cell lines (KIM1-PK1 cells) that do not express the native protein (Supplemental Figure 1A; supplemental material available online with this article; doi:10.1172/JCI34487DS1) and also established MDCK II canine kidney epithelial cell lines that could be induced to express KIM-1 by removal of doxycycline (Supplemental Figure 1B). After coculture of KIM1-PK1 cells with apoptotic thymocytes that had been labeled with fluorescent CMFDA, KIM-1 localized initially to the phagocytic cup where apoptotic cells became attached to the epithelial cell surface (Figure 3A, left panel). Later KIM-1 localized to phagosomes that contained apoptotic cells (Figure 3A, right panel). When confocal images were taken of KIM1-PK1 cells labelled with phalloidin to show the actin cytoskeleton, it was clear that apoptotic cells were in phagosomes (Figure 3B). In addition, we generated full-length KIM-1–GFP C terminus–tagged protein and expressed this protein stably in COS-7 cells. Time-lapse images of KIM-1–GFP cells ingesting apoptotic cells showed early recruitment of KIM-1 to the phagocytic cup where initial tethering of the apoptotic cell occurs, followed by further enhancement of the KIM-1 fluorescence in the phagosome after successful engulfment (Supplemental Figure 2 and Supplemental Video 1).
Primary cultured kidney epithelial cells express Kim-1 and phagocytose apoptotic cells by a Kim-1–dependent mechanism. (A) Following coculture with apoptotic thymocytes, Kim-1–positive (Kim-1+) (red with blue nuclei [N]) but not Kim-1–negative (Kim-1–) (blue nuclei only [n]) epithelial cells show avid binding (arrowheads) and internalization of fluorescently labeled apoptotic thymocytes (green and blue) (arrows). Note marked ring enhancement of phagosomes with Kim-1, and Kim-1 at the phagocytic cup of bound apoptotic cells. (B) Image of primary cultured rat epithelial cells all expressing cytokeratin (green) but showing heterogenous expression of Kim-1 (red). Scale bars: 10 μm. (C) The number of apoptotic cells bound or phagocytosed per 100 Kim-1–positive or 100 Kim-1–negative epithelial cells following coculture with labeled apoptotic cells and washing to remove bound cells. Note Kim-1–positive cells show avid phagocytosis. **P < 0.001. (D) Phagocytic index (number apoptotic cells/100 phagocytes) of Kim-1–positive primary epithelial cell cultures pretreated with monoclonal anti-rat Kim-1 affinity purified antibodies (15 μg/ml) followed by coculture with labeled apoptotic cells. Epithelial cells were lifted from plates and single epithelial cells in suspension scored for phagocytic index. Note that anti–Kim-1 antibodies directed at the extracellular domain block phagocytosis when compared with cells preincubated with isotype control antibodies. *P < 0.01.
KIM-1–expressing kidney epithelial cell lines avidly bind and phagocytose apoptotic and necrotic material. (A) KIM-1 (red) in a KIM1-PK1 cell (left panel) is expressed at high levels (arrows) at the point of binding of multiple apoptotic thymocytes (green and blue) and is part of the initial phagocytic cup (arrowhead). Scale bar: 5 μm. At later time points (right panel), KIM-1 remains associated with the internalized apoptotic cell, resulting in ring enhancement (arrow) of the apoptotic body. The cell border is highlighted by broken lines. Scale bar: 10 μm. (B) Multiple apoptotic thymocytes labeled with CMFDA (green) were localized intracellularly in KIM-1–expressing cells after coculture. Internalized apoptotic thymocytes are visualized in the confocal plane of cortical actin filaments (red) in this confocal image confirming internalization. Cell nuclei (N) are highlighted. Scale bar: 10 μm. (C) DIC with fluorescence (green) microscopic images of KIM1-PK1 cells confirm ingestion of CMFDA-labeled (green) apoptotic LLC-PK1 cells (left panel) or sonicated LLC-PK1 cell debris (middle panel). pCDNA-PK1 cells in the same experiment showing no phagocytosis of apoptotic cells (right panel). These microscopic studies confirm internalization of fluorescent apoptotic or necrotic cell debris (arrowheads). Original magnification, ×60.
To quantify phagocytosis using an automated system, KIM-1–expressing KIM1-PK1 cells were cocultured for 1 hour at 37°C with fluorescently labeled apoptotic LLC-PK1 cells or apoptotic thymocytes, after which noningested apoptotic cells were washed away and live cells were lifted into a single-cell suspension with EDTA and trypsin. Bound noninternalized apoptotic cells are removed by this procedure. Compared with LLC-PK1 cells stably transfected with empty vector (pcDNA-PK1), KIM1-PK1 cells more avidly internalized apoptotic cells (Figure 3). When quantitatively assessed by a flow cytometric assay that measures increased fluorescence of epithelial cells in suspension following ingestion of fluorescently labeled apoptotic cells, KIM1-PK1 cells showed approximately 10-fold greater phagocytosis of apoptotic LLC-PK1 cells (14.8% ± 4.2% were positive for apoptotic cells vs. 1.5% ± 0.4% in control cells) and 2-fold greater phagocytosis of apoptotic thymocytes (24.2% ± 1.2% KIM1-PK1 cells vs. 12.0% ± 0.7% pcDNA-PK1 cells) (Figure 4B) (13, 14). Direct fluorescence microscopy of the coincubations that had been analyzed by flow cytometry confirmed marked increases in internalized apoptotic cells; when scored by blinded microscopic counting of random fields (15, 16), 21.5% ± 2.8% of KIM1-PK1 cells internalized apoptotic thymocytes compared with 4.6% ± 1.4% in pcDNA-PK1 cells. Likewise, 30.9% ± 4.8% of KIM1-PK1 cells were observed to phagocytose apoptotic LLC-PK1 cells as compared with 4.4% ± 1.0% of pcDNA-PK1 cells. The phagocytic index was also increased as determined by microscopic scoring (KIM1-PK1 cells, 34.48 ± 2.74 vs. pcDNA-PK1 cells, 4.88 ± 1.13). To be certain that the flow cytometric assays measured phagocytosis and not merely binding, internalization was examined in cocultures incubated at 4°C, a temperature which prevents internalization (17), and at 37°C (Figure 5A). At 4°C, 0.07% ± 0.11% of KIM1-PK1 cells had increased fluorescence, indicative of phagocytosis, whereas at 37°C, 11.34% ± 4.51% of the KIM1-PK1 cells exhibited enhanced fluorescence, indicative of phagocytosis of fluorescent apoptotic cells (Figure 5A). To confirm the specificity of these findings, we used a tetracycline-dependent (tet-off), conditional KIM-1 expression cell system in MDCK canine kidney tubular epithelial cells (Supplemental Figure 1). Phagocytosis of apoptotic epithelial cells was dependent on the conditional expression of KIM-1 in the MDCK cells (induced, 11.60% ± 2.75% of cells ingested apoptotic cells vs. not induced, 2.04% ± 0.23%; Figure 4D). Phagocytosis but not binding of apoptotic cells by KIM1-PK1 cells was inhibited by pretreatment with cytochalasin D and nocodazole, inhibitors of actin filament polymerization and microtubule formation, respectively (Figure 5B). These findings in stably transfected kidney epithelial cell lines supported the role we had identified for KIM-1 in vivo (Figure 1) and in primary epithelial cell cultures (Figure 2).
Quantitative analysis of KIM-1–mediated apoptotic cell and necrotic material phagocytosis. Flow cytometric plots of green fluorescence against side scatter (SSC) for KIM1-PK1 and pcDNA-PK1 epithelial cells that have ingested fluorescently labeled (CMFDA) apoptotic thymocytes (A), apoptotic LLC-PK1 cells (B), or necrotic debris (necrotic LLC-PK1 cells) (C) in a phagocytosis assay. Percentages represent the proportion of epithelial cells that have ingested fluorescently labeled material. KIM1-PK1 cells that have not ingested apoptotic cells or debris were used to define the gated area. Without coculture with necrotic cells, only 0.93% of KIM-1–expressing epithelial cells were identified in the gated area. (D) Flow cytometric plots of green fluorescence against side scatter for KIM1–tet-off MDCK epithelial cells that have ingested fluorescently labeled (CMFDA) apoptotic LLC-PK1 in a phagocytosis assay. MDCK cells were either treated with doxycycline (100 ng/ml) to inhibit expression of the KIM-1 (left panel) or no doxycycline was used (5 days), permitting high-level expression of KIM-1 (right panel). Values represent the percentage of epithelial cells that have ingested fluorescently labeled apoptotic cells. KIM1–tet-off MDCK cells that have not ingested apoptotic cells or debris were used to define no ingestion.
KIM-1 mediates phagocytosis of apoptotic necrotic cells but not other phagocytotic targets, zymosan or latex beads. (A) Graph showing percentage of KIM1-PK1 or pcDNA-PK1 cells that have internalized fluorescent apoptotic LLC-PK1 cells after 1 hour incubation with apoptotic fluorescent green labeled cells at 37°C or 4°C (on ice). At 4°C, binding but not internalization occurs. KIM1-PK1 cells showed much less fluorescence at 4°C than at 37°C (**P = 0.006; n = 3 per condition; error bars indicate SD). (B) Graph showing phagocytosis (black bars) by KIM1-PK1 cells as assessed by flow cytometry (left axis; percentage fluorescent cells) or binding plus phagocytosis (white bars) as assessed by spectrophotometry (right axis; relative fluorescence intensity). Labeled apoptotic thymocytes were incubated with KIM1-PK1 cells that had been pretreated with cytochalasin D (30 μM), nocodazole (30 μM), or vehicle. Total (bound plus phagocytosed) thymocytes were equivalent in each group; however, phagocytosis was inhibited by cytochalasin D and nocodazole. (C) Fluorescence images of KIM1-PK1 cells following coincubation with fluorescence-labeled zymosan particles (left, 0.5 mg/ml), latex beads (center, Fluorosphere; 1:800 dilution), or heparin (right, 25 μg/ml) for 1 hour at 37°C. Note no uptake of any of these particles. (D) Preincubation of KIM1-PK1 cells with anti–Kim-1 antibodies (AWE2), but not IgG, reduced phagocytic index (number of apoptotic LLC-PK1 cells/KIM1-PK1 phagocyte) as assessed by flow cytometry (50 μg/ml, left panel). Right panels show photomicrographs of KIM1-PK1 cells after internalization and binding of CMFDA-labeled apoptotic LLC-PK1 cells that had been pretreated (1 hour) with soluble mKIM1-Fc (bottom right panel, 0.8 μg/ml) or IgG-treated (top right panel). Pretreatment with mKIM1-Fc inhibited phagocytosis. Original magnification, ×40 (C); ×10 (D).
KIM1-PK1 cells did not bind or internalize fluorescently labeled zymosan–Texas Red (0.5 mg/ml), latex beads–FITC (1:800 dilution), or heparin-FITC (25 μg/ml) when compared to pcDNA-PK1 cells (Figure 5C). This likely reflects lack of expression of receptors such as scavenger receptor A (SRA) as well as other components of the myeloid phagocytic machinery by the epithelial cell.
To determine whether KIM-1 expression also conferred enhanced capacity to ingest necrotic cellular debris, we prepared necrotic fluorescently labeled LLC-PK1 cells by sonication. Fifteen times more KIM1-PK1 cells ingested necrotic cell debris (32.9% ± 8.4%) as compared with control pcDNA-PK1 epithelial cells (1.9% ± 1.0%) as assessed by flow cytometry and confirmed by microscopy (Figure 3C and Figure 4C). Thus, KIM1-PK1 cells internalize apoptotic and necrotic cells.
Matrix proteoglycans such as heparan sulfate have been reported to act as opsonins for the clearance of apoptotic cells and necrotic debris (18), but in these studies, heparin, a heparan sulfate, did not bind to KIM1-PK1 cells. To determine the role of other opsonins, such as complement proteins and thrombospondin, we prepared apoptotic cells in serum free media and performed phagocytosis assays in the absence of serum. There was no difference in phagocytosis of apoptotic thymocytes by KIM1-PK1 cells in the presence (20.9% ± 1.6% KIM1-PK1 cells phagocytosed apoptotic cells) or absence (28.6% ± 5.2%) of serum, suggesting that serum-derived opsonins play no role in KIM-1 mediated phagocytosis. To show further specificity of KIM-1 in phagocytosis, we expressed the archetypal scavenger receptor SRA in LLC-PK1 cells. Although SRA promoted avid binding of apoptotic thymocytes, there was only a small increase in phagocytosis, 5.1 ± 0.3% versus 4.2 ± 0.3% (control), confirming the uniqueness of KIM-1 as a phagocytic receptor.
Our studies of rat primary epithelial cells indicated that rat Kim-1 was functioning directly as a phagocytic receptor (Figure 2). We next evaluated whether the enhanced phagocytosis conferred by human KIM-1 expression in KIM1-PK1 cells was directly due to KIM-1 functioning as an apoptotic receptor or indirectly due to upregulation of the intrinsic phagocytic machinery. Pretreatment of apoptotic cells with soluble KIM-1 ectodomain-Fc fusion protein (KIM1-Fc) (0.8 μg/ml) inhibited uptake into KIM1-PK1 cells (Figure 5D). Likewise, pretreatment of KIM1-PK1 cells with an anti–Kim-1 antibody that binds to the Ig domain (AWE2) also inhibited phagocytosis of apoptotic cells (Figure 5D), which together with the rat primary studies indicates KIM-1 binds to apoptotic cells via the Ig-domain. To further enhance the certainty that the KIM-1 ectodomain in cells was directly responsible for binding and internalization by acting as a phagocytic receptor, we generated a truncation mutant of KIM-1 (ΔKIM1-ecto), which expressed only 5 amino acids extracellularly but expressed the transmembrane and intracellular domain normally in cells. Compared with pcDNA-PK1 cells, the ΔKIM1-ecto–PK1 cells showed no enhancement of phagocytosis in our phagocytosis assays (data not shown).
To further underscore that the phagocytic effect of KIM-1 was a direct interaction and not due to the induction of other phagocytic receptors, we assessed KIM1-PK1 and control cells for expression of CD36 (protein) and lox-1 (transcript) but found no expression of these phagocytic receptors. Interestingly, scavenger receptor B1 (protein) was expressed by both control and KIM1-PK1 cells at equal levels (data not shown).
The KIM-1 ectodomain binds specifically to the surface of apoptotic kidney epithelial cells and PS. To investigate the biology of KIM-1 further, we assessed binding of KIM1-Fc to cells (Figure 6). KIM1-Fc bound specifically to the surface of apoptotic cells but not the surface of normal cells (Figure 6, A and B). Control c-Ret–Fc and human IgG proteins did not adhere to apoptotic or normal cells (Figure 6, A and B). KIM1-Fc binding was abolished by addition of the calcium and magnesium chelators, EGTA and EDTA, and restored in the presence of excess calcium, indicating that binding is dependent on divalent cations (Figure 6C). Divalent cation dependence has been reported for binding of other receptors of the myeloid lineage scavenger receptor family that bind apoptotic and necrotic cells (17, 19, 20).
The KIM-1 ectodomain binds specifically to the surface of apoptotic epithelial cells and binds specifically to PS and PE. (A) Flow cytometric histogram plots of fluorescence of normal live LLC-PK1 epithelial cells (left panel) or apoptotic LLC-PK1 epithelial cells (right panel), labeled with KIM1-Fc followed by anti–hIgG-FITC (green), anti–hIgG-FITC alone (red), or no reagents (blue). Note an approximately 50-fold increase in binding of KIM1-Fc to apoptotic cells. (B) Representative photomicrographs of an apoptotic LLC-PK1 cell labeled with KIM1-Fc (green, right panel) and a normal, live LLC-PK1 cell labeled identically (left panel). Nuclei were faintly stained with DAPI (blue) in both cells. Original magnification, ×60. (C) Graph of mean peak fluorescence for binding of KIM1-Fc to apoptotic cells in the absence or presence of the calcium chelators EDTA/EGTA, assessed by flow cytometry. KIM1-Fc binding was abolished by calcium chelators, and binding was restored by the addition of an excess of calcium to the chelators (*P = 0.006). (D) Graph of phagocytosis inhibition by PS liposomes. Pretreatment of KIM-1–expressing cells with PS liposomes almost completely abolished KIM-1–mediated phagocytosis of apoptotic cells (A.C.) (**P = 0.007), while equimolar PC liposomes had no effect. Error bars indicate SD. (E) In vitro binding curves for purified KIM1-Fc binding to equimolar phospholipid coated ELISA plates. KIM1-Fc binding was detected by anti-human IgG — HRP conjugated antibody followed by a colorimetric assay. Note KIM1-Fc binds to aminophospholipids PS and PE but not PC or anionic phosphatidic acid (PA), whereas control Fc proteins, human IgG and c-Ret–Fc do not bind.
Unlike live cells, apoptotic cells expose membrane aminophospholipids PS and phosphatidylethanolamine (PE) on the outer leaflet of the plasma membrane (15). The exposed lipids are targets that professional phagocytes use in recognition of apoptotic cells (15). In a cell-free binding assay, we tested whether KIM-1 recognized PS, PE, phosphatidylcholine (PC), or anionic phosphatidic acid (PA) adherent to plastic wells. KIM-1 ectodomain bound specifically to PS and to a lesser extent PE but not other membrane phospholipids, PC, or PA (Figure 6E). The K_d_ for KIM1-Fc–binding to PS was calculated from the binding curve to be 6.3 nM. To validate this assay and these observations, we confirmed that PS, PE, and PC bound equally to plastic plate wells by elution of adsorbed lipids in ethanol and analysis of eluates by TLC (data not shown). These studies suggest KIM-1 is a PS receptor but may also be a receptor for PE, albeit with a lower affinity. To confirm these properties in cells, we used liposomes containing PS to competitively block uptake of apoptotic cells by KIM1-PK1 cells (15). Pretreatment of KIM1-PK1 cells with PS liposomes (0.2 mM) almost completely abolished KIM-1 mediated phagocytosis, while PC liposomes (0.2 mM) had no effect (Figure 6D). Thus, we believe KIM-1 is a novel PS receptor. In the kidney, we did not identify any interstitial cells that expressed Kim-1 (Figure 1). In particular neither resident, inflammatory macrophages, nor neutrophils expressed Kim-1. Cultured bone marrow macrophages also did not express Kim-1 by transcript or protein assessment. Furthermore, activation by LPS or dexamethasone did not induce expression of Kim-1 in macrophages that express Emr-1 (F4/80 antigen) (Figure 7). Therefore, KIM-1 appears to be unlike other epithelial apoptotic cell uptake receptors, which are similar or identical to those used by macrophages (4).
Macrophages do not express Kim-1/Tim-1. RT-PCR (left panel) for Kim1 mRNA from day 7 mouse bone marrow macrophages cultured with LPS or dexamethasone. Emr1 (F4/80 antigen) and Gapdh were used as controls, and mouse kidney cDNA 48 hours following ischemia was used as a positive control. Immunoblot (right panel) for Kim-1 in protein lysates from cultured macrophages and postischemic mouse kidney. Note that mouse bone marrow–derived macrophages generate neither kim1 transcript nor Kim-1 protein, in quiescent or activated states. BMMf, bone marrow–derived macrophages.
KIM-1–expressing epithelial cells bind and internalize oxidized LDL through specific interactions with the KIM-1 ectodomain. Scavenger receptors, originally named by Brown and Goldstein for their capacity to bind oxidized lipids and play a role in foam cell formation (21), participate as pattern recognition molecules in host defense and innate immunity. They have also been implicated in the tethering and internalization of apoptotic and necrotic cells (20, 22–26). The plasma membrane of healthy cells is continually oxidized, but oxidized lipids are actively transported to the inner leaflet of the membrane (27). Only in dying or necrotic cells does the outer leaflet plasma membrane retain oxidized phospholipids (28). Fluorescently labeled (DiI) oxidized LDL (ox-LDL) bound to KIM-1–expressing cells and was internalized at 37°C (Figure 8, A and B). Both binding to and internalization of DiI-labeled ox-LDL by KIM1-PK1 cells was totally prevented by coincubation with a 40-fold excess of unlabeled ox-LDL (Figure 8C). KIM1-PK1 cells also bound and internalized native LDL, whereas control pcDNA-PK1 cells did not (Figure 8A). Internalized ox-LDL and native LDL were both predominantly localized in intracellular vesicles (Figure 8B). Using the tet-off conditional KIM-1 expression system (Figure 8D), we confirmed that binding and internalization of ox-LDL was dependent on expression of KIM-1 (Figure 8D). It is noteworthy that while scavenger receptors class A, C, and D do not bind native LDL, scavenger receptors class B have been reported to bind native LDL as well as PS efficiently. Thus, KIM-1 shares some functional similarity with this latter class of receptors (20, 29–32).
KIM-1–expressing epithelial cells bind and internalize ox-LDL, and the KIM-1 ectodomain binds specifically to ox-LDL. (A) Graph showing internalization of DiI-labeled ox-LDL or native LDL by KIM1-PK1 cells or pcDNA-PK1 cells over 1 hour at 37°C, as quantified by spectrofluorometry of lysed cells. (B) Photomicrographs of KIM1-PK1 cells and pcDNA-PK1 cells showing internalized ox-LDL or native LDL in intracellular vesicles. Original magnification, ×40. (C) Graph showing the effect of a 40-fold excess of unlabeled ox-LDL on internalization of DiI-labeled ox-LDL by KIM1-PK1 cells or pcDNA-PK1 cells incubated at 37°C for 1 hour. Uptake of fluorescent lipoprotein was quantified by spectrofluorometry of lysed cells. (D) Graph showing the effect of doxycycline on KIM1–tet-off MDCK cells’ capacity to internalize DiI-labeled ox-LDL and DiI-labeled native LDL. In the presence of doxycycline (DOX+), KIM-1 expression is suppressed. In these conditions, there is little uptake of labeled lipoprotein after 1 hour. In the absence of doxycycline (DOX-), KIM-1 expression is not suppressed, and there is marked uptake of both lipoproteins. (E) In vitro binding curves for purified KIM1-Fc, human IgG1, or c-Ret–Fc proteins to ox-LDL coated ELISA plates. KIM1-Fc binding was detected by anti-human IgG – HRP-conjugated antibody followed by a colorimetric assay. Note KIM1-Fc strongly binds to ox-LDL but not uncoated plastic, whereas control proteins human IgG and c-Ret–Fc do not bind ox-LDL or plastic. (F) Graph showing the effect of pretreatment of KIM1-PK1 cells with either 40 or 50 μg/ml of ox-LDL on phagocytosis of fluorescently labeled apoptotic LLC-PK1 cells as assessed by flow cytometry. (*P = 0.0009 [40 μg/ml ox-LDL]; **P = 0.0003 [50 μg/ml ox-LDL] compared with no ox-LDL pretreatment. Error bars indicate SD).
To demonstrate that internalization of ox-LDL was mediated through direct interaction with KIM-1 ectodomain, plastic wells, coated with ox-LDL or left uncoated, were incubated with purified KIM1-Fc fusion protein or control Fc-proteins (human IgG, c-Ret–Fc). After washing away unbound Fc proteins, specific binding of KIM1-Fc was identified by colorimetric assay using horseradish peroxidase–conjugated secondary antibodies against human Fc (IgG) (Figure 8E). KIM1-Fc, but not control c-Ret–Fc or human IgG proteins, bound to ox-LDL with a K_d_ of 9.4 nM. We believe the ability to specifically bind to ox-LDL in addition to PS confirms KIM-1 to be a novel scavenger receptor. Since KIM-1 is not only a PS and PE receptor but also a scavenger receptor, we evaluated whether engulfment of apoptotic cells was prevented by pretreatment of KIM1-PK1 cells with ox-LDL (Figure 8F). Phagocytosis was suppressed by 76.4% (12.3% ± 1.7% vs. 2.9% ± 1.4% with 40 μg/ml ox-LDL). Some scavenger receptors bind microbial cell surface epitopes. Although KIM1-PK1 cells did not bind yeast cell wall, zymosan (Figure 5C), KIM1-PK1 cells bound and internalized both Gram negative (E. coli) and positive (S. aureus) bacteria (Figure 9).
KIM1-PK1 cells but not pcDNA-PK1 cells bind and internalize gram negative (E. coli) and gram positive (S. aureus) bacteria. KIM-1 increases the capacity of epithelial cells to phagocytose both E. coli and S. aureus bacteria. % Phagocytosis, percentage of cells with internalized bacteria. Error bars indicate SD.