Expression and regulation of alarmin cytokine IL-1α in human retinal pigment epithelial cells.

Human retinal pigment epithelial (hRPE) cells play important immune-regulatory roles in a variety of retinal pathologic processes, including the production of inflammatory cytokines that are essential mediators of the innate immune response within the ocular microenvironment. The pro-inflammatory "alarmin" cytokine IL-1α has been implicated in both infectious and non-infectious retinal diseases, but its regulation in the retina is poorly understood. The purpose of this study was to elucidate the expression and regulation of IL-1α within hRPE cells. To do this, IL-1α mRNA and protein in hRPE cells was assessed by RT-PCR, qPCR, ELISA, Western blot, and immunofluorescence following treatment with a variety of stimuli and inhibitors. ER stress, LPS, IL-1ß, and TLR2 activation all significantly increased intracellular IL-1α protein. Increasing intracellular calcium synergized both LPS- and Pam3CSK4-induced IL-1α protein production. Accordingly, blocking calcium signaling and calpain activity strongly suppressed IL-1α protein expression. Significant but more moderate inhibition occurred following blockage of TLR4, caspase-4, or caspase-1. Neutralizing antibodies to IL-1ß and TLR2 partially eliminated LPS- and TLR2 ligand Pam3CSK4-stimulated IL-1α protein production. IFN-ß induced caspase-4 expression and activation, and also potentiated LPS-induced IL-1α expression, but IFN-ß alone had no effect on IL-1α protein production. Interestingly, all inhibitors targeting the PI3K/Akt pathway, with the exception of Ly294002, strongly increased IL-1α protein expression. This study improves understanding of the complex mechanisms regulating IL-1α protein expression in hRPE cells by demonstrating that TLR4 and TLR2 stimulation and exposure to IL-1ß, ER stress and intracellular calcium all induce hRPE cells to produce intracellular IL-1α, which is negatively regulated by the PI3K/Akt pathway. Additionally, the non-canonical inflammasome pathway was shown to be involved in LPS-induced hRPE IL-1α expression through caspase-4 signaling.

Distinct CD40L receptors mediate inflammasome activation and secretion of IL-1ß and MCP-1 in cultured human retinal pigment epithelial cells.

CD40L signaling occurs in several diseases with inflammatory components, including ocular and retinal diseases. However, it has never been evaluated as a pathogenic mechanism in age-related macular degeneration (AMD) or as an inducer of inflammasome formation in any cell type. mRNA and protein levels of CD40, IL-1ß, NALP1, NALP3, caspase-1, and caspase-5 were determined by RT-PCR, qPCR, and Western blot. CD40L receptor (CD40, α5ß1, and CD11b) expression was determined by Western and immunofluorescent staining. IL-1ß, IL-18, and MCP-1 secretions were determined by ELISA. NALP1 and NALP3 inflammasome formation were determined by Co-IP. Experiments were conducted on primary human retinal pigment epithelial (hRPE) cells from four different donors. Human umbilical vein endothelial (HUVEC) and monocytic leukemia (THP-1) cells demonstrated the general applicability of our findings. In hRPE cells, CD40L-induced NALP1 and NALP3 inflammasome activation, cleavage of caspase-1 and caspase-5, and IL-1ß and IL-18 secretion. Interestingly, neutralizing CD11b and α5ß1 antibodies, but not CD40, reduced CD40L-induced IL-1ß secretion in hRPE cells. Similarly, CD40L treatment also induced HUVEC and THP-1 cells to secret IL-1ß through CD11b and α5ß1. Additionally, the CD40L-induced IL-1ß secretion acted in an autocrine/paracrine manner to feed back and induce hRPE cells to secrete MCP-1. This study is the first to show that CD40L induces inflammasome activation in any cell type, including hRPE cells, and that this induction is through CD11b and α5ß1 cell-surface receptors. These mechanisms likely play an important role in many retinal and non-retinal diseases and provide compelling drug targets that may help reduce pro-inflammatory processes.

IL-8 and MCP Gene Expression and Production by LPS-Stimulated Human Corneal Stromal Cells.

Purpose. To determine time course of effect of lipopolysaccharide (LPS) on production of interleukin-8 (IL-8) and monocyte chemotactic protein (MCP) by cultured human corneal stromal cells. Methods. Human corneal stromal cells were harvested from donor corneal specimens, and fourth to sixth passaged cells were used. Cell cultures were stimulated with LPS for 2, 4, 8, and 24 hours. Northern blot analysis of IL-8 and MCP gene expression and ELISA for IL-8 and MCP secretion were performed. ELISA results were analyzed for statistical significance using two-tailed Student's t-test. Results. Northern blot analysis demonstrated significantly increased IL-8 and MCP gene expression after 4 and 8 hours of exposure to LPS. ELISA for secreted IL-8 and MCP demonstrated statistically significant increases (P < 0.05) after corneal stromal cell stimulation with LPS. Conclusions. This paper suggests that human corneal stromal cells may participate in corneal inflammation by secreting potent leukocyte chemotactic and activating proteins in a time-dependent manner when exposed to LPS.

Expression and functional roles of caspase-5 in inflammatory responses of human retinal pigment epithelial cells.

PURPOSE: To investigate the expression, activation, and functional involvement of caspase-5 in human retinal pigment epithelial (hRPE) cells. METHODS: Expression and activation of caspase-5 in primary cultured hRPE cells, telomerase-immortalized hTERT-RPE1 cells (hTERT-RPE1), or both, were measured after stimulation with proinflammatory agents IL-1ß, TNF-α, lipopolysaccharide (LPS), interferon-γ, monocyte coculture, adenosine triphosphate (ATP), or endoplasmic reticulum (ER) stress inducers. Immunomodulating agents dexamethasone (Dex), IL-10, and triamcinolone acetonide (TA) were used to antagonize proinflammatory stimulation. Cell death ELISA and TUNEL staining assays were used to assess apoptosis. RESULTS: Caspase-5 mRNA expression and protein activation were induced by LPS and monocyte-hRPE coculture. Caspase-5 activation appeared as early as 2 hours after challenge by LPS and consistently increased to 24 hours. Meanwhile, caspase-1 expression and protein activation were induced by LPS. Activation of caspase-5 was blocked or reduced by Dex, IL-10, and TA. Activation of caspase-5 and -1 was also enhanced by ATP and ER stress inducers. Expression and activation of caspase-5 were inhibited by a caspase-1-specific inhibitor. Caspase-5 knockdown reduced caspase-1 protein expression and activation and inhibited TNF-α-induced IL-8 and MCP-1. In contrast to caspase-4, the contribution of caspase-5 to stress-induced apoptosis was moderate. CONCLUSIONS: Caspase-5 mRNA synthesis, protein expression, and catalytic activation were highly regulated in response to various proinflammatory stimuli, ATP, and ER stress inducers. Mutual activation between caspase-5 and -1 suggests caspase-5 may work predominantly in concert with caspase-1 in modulating hRPE inflammatory responses.

Retinal flavoprotein fluorescence correlates with mitochondrial stress, apoptosis, and chemokine expression.

Oxidative stress and mitochondrial dysfunction occur before apoptosis in many retinal diseases. Under these conditions, a larger fraction of flavoproteins become oxidized and, when excited by blue-light, emit green flavoprotein fluorescence (FPF). In this study, we evaluated the utility of FPF as an early indicator of mitochondrial stress, pre-apoptotic cellular instability, and apoptosis of human retinal pigment epithelial (HRPE) cells subjected to hydrogen peroxide (H(2)O(2)) or monocytes (unstimulated or interferon-γ-stimulated) in vitro and of freshly-isolated pieces of human and rat neural retina subjected to H(2)O(2)ex vivo. Increased FPF of HRPE cells exposed to H(2)O(2) correlated with reduced mitochondrial membrane potential (ΔΨm) and increased apoptosis in a time- and dose-dependent manner. HRPE cells co-cultured with monocytes had increased FPF that correlated in a time-dependent manner with reduced ΔΨm, increased apoptosis, and early expression of pro-inflammatory chemokines, interleukin-8 (IL8) and monocyte chemotactic factor-1 (MCP1), which are known to be induced by oxidative stress. Increased FPF, reduced ΔΨm, and upregulation of IL8 and MCP1 occurred as early as 1-2 h after exposure to stressors, while apoptosis did not occur in HRPE cells until later time points. The antioxidant, N-acetyl-cysteine (NAC), inhibited increased FPF and apoptosis of HRPE cells subjected to H(2)O(2). Increased FPF of human and rat neural retina also correlated with increased apoptosis. This study suggests that FPF is a useful measure of mitochondrial function in retinal cells and tissues and can detect early mitochondrial dysfunction that may precede apoptosis.

Induction of interleukin-8 gene expression and protein secretion by C-reactive protein in ARPE-19 cells.

C-reactive protein (CRP) is an acute phase reactant and its level rises rapidly during inflammation. Recent studies have suggested the potential involvement of CRP in the pathogenesis of age-related macular degeneration (AMD). To delineate the functional roles of CRP in inflammatory response by the ocular posterior segments, the effects of CRP on ARPE-19, an immortalized human retinal pigment epithelia (hRPE) cell line, were investigated in the present study. Treatment of ARPE-19 cells with CRP resulted in enhanced NF-kB nuclear translocation and dose-dependent transient induction of IL-8 mRNA synthesis and protein secretion. Stimulated expression of VEGF, but not MCP-1 by CRP was also observed. The induced IL-8 expression was transient and peaked at 12h post stimulation. In the presence of inhibitors for NF-kB, p38, MEK and JNK, the CRP-induced IL-8 production was abolished by 99.5+/-2.3, 97.8+/-2.1, 55.3+/-2.5 and 37.3+/-1.3%, respectively. Neutralization of Fc gamma receptors by anti-CD32 and CD64 antibodies produced 39.9+/-1.6 and 59.5+/-2.6% reduction, respectively, of CRP-stimulated IL-8 secretion, whereas that by anti-CD16 antibody had no effect. This study suggests that the pro-inflammatory effects of CRP in ARPE-19 cells may contribute to the inflammatory retinal diseases by induction of pro-inflammatory cytokines such as IL-8. This induction is mediated by NF-kB and multiple MAPK pathways through Fc gamma receptors.

Dual involvement of caspase-4 in inflammatory and ER stress-induced apoptotic responses in human retinal pigment epithelial cells.

PURPOSE: To investigate the functional involvement of caspase-4 in human retinal pigment epithelial (hRPE) cells. METHODS: Expression and activation of caspase-4 in hRPE cells were measured after stimulation with proinflammatory agents IL-1beta (2 ng/mL), TNF-alpha (20 ng/mL), lipopolysaccharide (1000 ng/mL), interferon-gamma (500 U/mL), or monocyte coculture in the absence or presence of immunomodulating agent cyclosporine (3 or 30 ng/mL), dexamethasone (10 microM), or IL-10 (100 U/mL) and endoplasmic reticulum (ER) stress inducer thapsigargin (25 nM) or tunicamycin (3 or 10 microM). The onset of ER stress was determined by expression of GRP78. The involvement of caspase-4 in inflammation and apoptosis was further examined by treating the cells with caspase-4 inhibitor Z-LEVD-fmk, caspase-1 and -4 inhibitor Z-YVAD-fmk, and pan-caspase inhibitor Z-VAD-fmk. RESULTS: Caspase-4 mRNA expression and protein activation were induced by all the proinflammatory agents and ER stress inducers tested in this study. Caspase-4 activation was blocked or reduced by dexamethasone and IL-10. Elevated ER stress by proinflammatory agents and ER stress inducers was shown by increased expression of the ER stress marker GRP78. The induced caspase-4 and caspase-3 activities by tunicamycin and the stimulated IL-8 protein expression by IL-1beta were markedly reduced by caspase-4 inhibitor Z-LEVD-fmk. Although caspase-4 inhibitor Z-LEVD-fmk and caspase-1 and -4 inhibitor Z-YVAD-fmk reduced tunicamycin-induced hRPE apoptotic cell death by 59% and 86%, respectively, pan-caspase inhibitor Z-VAD-fmk completely abolished the induced apoptosis. CONCLUSIONS: Caspase-4 is dually involved in hRPE proinflammatory and proapoptotic responses. Various proinflammatory stimuli and ER stress induce hRPE caspase-4 mRNA synthesis and protein activation. ER stress-induced hRPE cell death is caspase and, in part, caspase-4 dependent.

Regulated expression of caspase-12 gene in human retinal pigment epithelial cells suggests its immunomodulating role.

PURPOSE: To investigate the expression and regulation of the short form of caspase-12, caspase-12S, in human retinal pigment epithelial (hRPE) cells. METHODS: hRPE cells were stimulated by the proinflammatory agents IL-1beta (2 ng/mL) and TNF-alpha (20 ng/mL); LPS (1000 ng/mL); coculture with monocytes; the immunomodulating agent cyclosporine (Cys; 30 ng/mL); the anti-inflammatory cytokine IL-10 (100 U/mL); and the endoplasmic reticulum (ER) stress inducers tunicamycin (3 or 10 muM) and thapsigargin (25 or 100 nM) for 6 hours or longer. The total RNAs were isolated and subjected to semiquantitative and quantitative real-time RT-PCR analysis. Effects of tunicamycin and thapsigargin on IL-1beta- and TNF-alpha-stimulated MCP-1 mRNA expression and protein production were further examined by RT-PCR and ELISA, respectively. RESULTS: RT-PCR results showed that caspase-12S is the predominant form of caspase-12 in the examined hRPE cells of this study, with expression at levels as high as those in many other human tissues such as pancreas, prostate, small intestine, lung, spleen, and kidney. Treatment with IL-1beta and TNF-alpha, as well as LPS and coculture with monocytes reduced hRPE caspase-12S mRNA expression within 6 hours. In contrast, hRPE exposure to cyclosporine (Cys) and the cytokine IL-10 for 6 hours increased caspase-12S mRNA expression. Compared to Cys and IL-10, the ER stress activators tunicamycin and thapsigargin were even more potent enhancers of hRPE caspase-12S gene expression. They also caused corresponding reductions in IL-1beta- and TNF-alpha-induced MCP-1 mRNA expression and protein production. CONCLUSIONS: hRPE cells express a high level of caspase-12S. The regulated expression of caspase-12S suggests that this caspase recruitment domain (CARD)-only protein may be an endogenous dominant negative regulator that modulates inflammatory responses in hRPE cells.

siRNA targeting mammalian target of rapamycin (mTOR) attenuates experimental proliferative vitreoretinopathy.

PURPOSE: To investigate the effect of mammalian target of rapamycin (mTOR) specific siRNA on proliferative vitreoretinopathy (PVR). METHODS: Cultured human retinal pigment epithelial (hRPE) cell line D407 was treated with three mTOR specific small interfering RNAs. Cell proliferation, attachment, spreading, and migration were performed. The impact of the mTOR specific siRNA on PVR was tested using a rabbit model in which PVR was induced by the injection of hRPE cells. RESULTS: Decreasing mTOR expression by about 82% using small interfering RNA resulted in a significant decrease in cell spreading and migration. Whereas retinal detachment occurred in 100% of the control group animals, co-injection of the mTOR specific siRNA substantially reduced the severity and incidence (50%) of retinal detachments. CONCLUSIONS: Gene therapy with mTOR specific siRNA attenuates PVR in a rabbit model of the disease. This may be a new approach to preventing PVR in humans.

Pro-inflammatory cytokines increase reactive oxygen species through mitochondria and NADPH oxidase in cultured RPE cells.

Reactive oxygen species (ROS) generated during inflammation are believed to play critical roles in various ocular diseases. However, the underlying mechanisms remain poorly understood. We investigated if pro-inflammatory cytokines, tumor necrosis factor (TNF)-alpha, interleukin-1 beta (IL-1 beta), and interferon-gamma (IFN-gamma), induce ROS in human retinal pigment epithelial (RPE) cells. TNF-alpha, IL-1 beta and IFN-gamma increased both intracellular and extracellular ROS production in a time- and dose-dependent manner. Thenoyltrifluoroacetone (TTFA), an inhibitor of mitochondrial respiratory chain, blocked TNF-alpha- and IFN-gamma-, but not IL-1 beta-induced ROS, whereas other two mitochondrial respiratory chain inhibitors, rotenone and antimycin A, had no effect. NADPH oxidase inhibitor (diphenylene iodinium) abolished the ROS production induced by IL-1 beta or IFN-gamma, but not by TNF-alpha, whereas 6-aminonicotinamide (6AN), an inhibitor of the hexose monophosphate shunt (HMS), had no significant effects on the ROS induced by all three cytokines. ROS scavengers, pyrrolidinedithiocarbamate (PDTC) and N-acetyl-cysteine (NAC), reduced the levels of ROS induced by TNF-alpha, IL-1 beta and IFN-gamma (P<0.05). Collectively, these results demonstrate that TNF-alpha, IL-1 beta and IFN-gamma increase mitochondrial- and NADPH oxidase-generated ROS in human RPE cells.

Thrombin-induced VEGF expression in human retinal pigment epithelial cells.

PURPOSE: The purpose of the present study was to investigate the effects of thrombin and thrombin in combination with other proangiogenic factors on VEGF expression in hRPE cells. METHODS: hRPE cells were stimulated with thrombin TNF-alpha, monocytes, and TGF-beta2. After stimulation, conditioned medium and lysed cells were subjected to ELISA, Western blot analysis, immunocytochemistry, and RT-PCR analyses. Inhibitors specific for various signal transduction pathways were used to determine the signaling pathways involved. RESULTS: Treatment of RPE cells with thrombin resulted in dose- and time-dependent increases in VEGF mRNA levels and protein production. hRPE VEGF expression is predominantly protease-activated receptor (PAR)-1 dependent. Approximately 80% of thrombin-induced VEGF secretion was abrogated by inhibitors of MAPK/ERK kinase (MEK), p38, c-Jun NH2-terminal kinase (JNK), protein tyrosine kinase (PTK), phosphatidylinositol 3-kinase (PI3K), protein kinase C (PKC), nuclear factor-kappaB (NF-kappaB), and reactive oxygen species (ROS). Analyses of VEGF protein production and mRNA synthesis revealed that VEGF induction by thrombin plus TNF-alpha or coculture with monocytes was additive, whereas that by co-incubation with TGF-beta2 was synergistic. The costimulated VEGF production by TGF-beta2 plus thrombin was an average of three times higher than the sum of that induced by each agent alone. Furthermore, BAPTA [bis-(o-aminophenoxy)ethane-N,N',N'-tetraacetic acid], a calcium chelator, blocked the VEGF secretion induced by thrombin and thrombin plus TGF-beta2 by 65% and 20%, respectively, but had no effect on that induced by TGF-beta2 alone. CONCLUSIONS: Thrombin alone and in combination with TNF-alpha, monocytes, and TGF-beta2 potently stimulated VEGF expression in hRPE cells via multiple signaling pathways. The thrombin-induced calcium mobilization may play an important permissive role in maximizing TGF-beta2-induced VEGF expression in RPE cells.

Regulation of VEGF mRNA expression and protein secretion by TGF-beta2 in human retinal pigment epithelial cells.

VEGF secretion by the human retinal pigment epithelium (hRPE) plays an important role in retinal and choroidal neovascularization. In this study, transforming growth factor-beta2 (TGF-beta2)-induced vascular endothelial growth factor (VEGF) gene expression was investigated in hRPE cells. Treatment of hRPE cells with TGF-beta2 for 24 and 48h as compared to 8h resulted in markedly increased VEGF secretion by fivefold and nine-fold, respectively. Induced VEGF mRNA peaked within 3h of stimulation and remained above the basal at 36h. Stimulation of VEGF expression by TGF-beta2 was blocked by cycloheximide, suggesting that de novo protein synthesis is required. Induced VEGF production was strongly inhibited by anti-inflammatory agents, dexamethasone and cyclosporin A. Despite of the weak stimulation of VEGF expression by TNF-alpha or bFGF alone, co-administration of either of these two cytokines synergized the effect of TGF-beta2 on VEGF mRNA expression and protein production. Quantitative RT-PCR revealed that the synergy was predominantly at the level of VEGF transcription. Moreover, TGF-beta2-induced RPE VEGF secretion was significantly reduced by inhibitors of mitogen-activated protein (MAP) kinase (MEK) (U0126), p38 (SB202190), c-Jun NH2-terminal kinase (JNK), Sp600125, protein tyrosine kinase (PTK) (Genistein), and phosphatidylinositol 3-kinase (PI3K) (Ly294002). Induced VEGF expression was completely abrogated by inhibitors of protein kinase C (PKC) (Ro318220), nuclear factor-kappaB (NF-kappaB) [caffeic acid phenethyl ester (CAPE)], and reactive oxygen species (ROS) [N-acetyl-cysteine (Nac) and diphenyleneiodonium (DPI)]. These results suggest that MEK, p38, JNK, PI3K, and NF-kappaB as well as multiple essential signaling intermediates, including PKC, PTK and ROS, are involved in hRPE VEGF up regulation by TGF-beta2.

Differential expression of retinal pigment epithelium (RPE) IP-10 and interleukin-8.

Interferon-gamma induced protein of 10 kDa (IP-10) is a C-X-C chemokine that attracts T lymphocytes and inhibits angiogenesis. In this study, we investigated the expression of IP-10 by human retinal pigment epithelial cells (HRPE) and compared IP-10 expression to that of interleukin-8 (IL-8), which is a leukocytic chemoattractant and pro-angiogenic factor. Cultured HRPE cells were incubated with either IL-1 beta (0.2-20 ng/ml) or tumor necrosis factor (TNF)-alpha (0.2-20 ng/ml) alone or in combination with interferon-gamma (IFN-gamma) (1000 U/ml). HRPE cells were also incubated with: (1) media conditioned by activated human T lymphocytes (CM), or (2) the same CM treated with neutralizing antibodies to IL-1, TNF, and/or IFN-gamma. IL-8 and IP-10 protein levels were measured by ELISA and mRNA levels by Northern blot analysis of HRPE cells. HRPE cells produced very high levels of IP-10 in response to either IL-1 beta/IFN-gamma, TNF-alpha/IFN-gamma or CD3-activated T-lymphocyte CM. The levels of IP-10 were at least tenfold higher (p<.001) than IL-8 measured in the same samples. Neutralizing antibodies to TNF and IFN-gamma, but not to IL-1, abrogated the ability of the CD3-activated T lymphocytes CM to induce HRPE IP-10 (p<.001). HRPE cells produce differential levels of IP-10 and IL-8 in response to various combinations of recombinant and T-lymphocyte-secreted pro-inflammatory cytokines. This may be important in evolving inflammatory and angiogenic ocular responses.

TLR4 mediates human retinal pigment epithelial endotoxin binding and cytokine expression.

PURPOSE: It was previously demonstrated that toll-like receptor 4 (TLR4) is involved in species-specific human retinal pigment epithelial (HRPE) photoreceptor outer segment recognition and oxidant production. This study was performed to demonstrate the classical role of TLR4 in HRPE endotoxin (lipopolysaccharide; LPS) binding leading to HRPE proinflammatory cytokine secretion. METHODS: Cultured HRPE cells were examined for TLR4 expression by immunofluorescence, Western blot analysis, and RT-PCR. HRPE cells labeled with fluorescent monoclonal antibodies (mAbs) to TLR4 and its associated adhesion molecule, CD14, were analyzed by real-time microscopy and resonance energy transfer (RET), determining associations of fluorescent LPS, TLR4, and CD14. LPS-induced HRPE secretion of interleukin (IL)-8 was measured with and without specific blocking mAb to TLR4 and CD14. HRPE TLR4 expression was measured after LPS exposure in the presence and absence of blocking antibodies to TLR4 and CD14. RESULTS: All three HRPE cell lines demonstrated constitutive TLR4 expression by immunofluorescence, Western blot analysis, and RT-PCR. Examination of HRPE cells labeled with fluorescent mAb to TLR4, CD14, and LPS demonstrated RET among the three molecules, indicating that LPS-CD14 binding preceded LPS-TLR4 binding and the close association of CD14 and TLR4. LPS-induced IL-8 was significantly reduced (P < 0.05) in the presence of blocking mAb to TLR4 or CD14. Upregulation of HRPE TLR4 gene and protein expression occurred in response to LPS exposure and was inhibited by mAb to TLR4 or CD14. CONCLUSIONS: HRPE TLR4 is a multifunctional molecule that participates in photoreceptor outer segment membrane recognition, oxidant production, LPS recognition, and cytokine production. These roles indicate potential involvement in retinal degenerative and inflammatory processes.

TLR4 mediates human retinal pigment epithelial endotoxin binding and cytokine expression.

PURPOSE: We previously demonstrated toll-like receptor 4 (TLR4) to be involved in species-specific human retinal pigment epithelial (HRPE) photoreceptor outer segment recognition and oxidant production. This study was performed to demonstrate the classic role of TLR4 in HRPE endotoxin (lipopolysaccharide [LPS]) binding leading to HRPE proinflammatory cytokine secretion. METHODS: Cultured human HRPE cells were examined for TLR4 expression by immunofluorescence, Western blot analysis, and reverse transcription polymerase chain reaction (RT-PCR). HRPE cells labeled with fluorescent monoclonal antibodies (mAb) to TLR4 and its associated adhesion molecule, CD14, were analyzed by real-time microscopy and resonance energy transfer (RET), determining associations of fluorescent LPS, TLR4, and CD14. LPS-induced HRPE secretion of interleukin-8 (IL-8) was measured with and without specific blocking mAb to TLR4 and CD14. HRPE TLR4 expression was measured after LPS exposure in the presence and absence of blocking antibodies to TLR4 and CD14. RESULTS: All three HRPE cell lines demonstrated constitutive TLR4 expression by immunofluorescence, Western blot analysis, and RT-PCR. Examination of HRPE cells labeled with fluorescent mAb to TLR4, CD14, and LPS demonstrated RET among the three molecules, indicating that LPS-CD14 binding preceded LPS-TLR4 binding and the close association of CD14 and TLR4. LPS-induced IL-8 was significantly reduced (P < .05) in the presence of blocking mAb to TLR4 or CD14. Up-regulation of HRPE TLR4 gene and protein expression occurred in response to LPS exposure and was inhibited by mAb to TLR4 or CD14. CONCLUSION: HRPE TLR4 is a multifunctional molecule that participates in photoreceptor outer segment membrane recognition, oxidant production, LPS recognition, and cytokine production. These roles indicate potential involvement in retinal degenerative and inflammatory processes.

Differential involvement of phosphoinositide 3-kinase/Akt in human RPE MCP-1 and IL-8 expression.

PURPOSE: To investigate the role of the phosphatidylinositol 3-kinase (PI3K) pathway and the signal mediator AP-1 in monocyte chemotactic protein (MCP)-1 and interleukin (IL)-8 gene expression in human retinal pigment epithelial (hRPE) cells. METHODS: hRPE cells were stimulated with IL-1beta and TNF-alpha and by coculturing with monocytes in the presence or absence of a series of kinase inhibitors. The induction of MCP-1 and IL-8 protein and mRNA was determined by ELISA and RT-PCR, respectively. Western blot analysis, kinase assays, and electrophoretic mobility shift assays were used to detect the activation of signaling mediators and transcription factors. RESULTS: Concomitant with the induction of chemokine expression by the stimuli, there was phosphorylation of PI3K and its downstream targets-namely, Akt, GSK, and FKHR. Ly294002, a specific inhibitor of PI3K, resulted in time- and dose-dependent blockade of MCP-1 mRNA expression and protein production. The IC(50) for inhibition of MCP-1 secretion induced by IL-1beta, TNF-alpha, and hRPE-monocyte binding was 16, 12, and less than 3 micro M, respectively. In contrast, Ly294002 did not inhibit the IL-8 expression induced by any of the stimuli. Ly294002 as well as U0126, SB202190, and SP600125, the selective inhibitors of MEK, p38, and JNK, respectively, strongly inhibited induced c-fos expression, whereas Ly294002 did not inhibit induction of MEK, p38, or JNK. Blockade of PI3K/Akt abolished IL-1beta-induced nuclear translocation of AP-1, whereas the induction of IkappaB degradation was unchanged. CONCLUSIONS: The Ly294002-sensitive PI3K/Akt pathway regulates MCP-1, but not IL-8 expression in hRPE cells independent of MAPK and IkappaB. PI3K-dependent induction of hRPE c-fos and AP-1 nuclear translocation may be a target for therapies aimed at modulating MCP-1 in retinal diseases.

Activated monocytes induce human retinal pigment epithelial cell apoptosis through caspase-3 activation.

Dysfunction and loss of human retinal pigment epithelial (HRPE) cells is a significant component of many ocular diseases, in which mononuclear phagocyte infiltration at the HRPE-related interface is also observed. In this study, we investigated whether HRPE cell apoptosis may be induced by overlay of IFN-gamma-activated monocytes. Human monocytes primed with IFN-gamma overlaid directly onto HRPE cells elicited significant increases in terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL)-positive HRPE cells (p < 0.0001) and decreases of proliferating cell nuclear antigen-positive (p < 0.0001) HRPE cells. The activated monocytes also induced HRPE cell caspase-3 activation, which was inhibited by the caspase-3 inhibitor, Z-DEVD-fmk. However, co-incubations in which activated monocytes were prevented from direct contact with HRPE cells or in which the monocytes were separated from the HRPE cells after 30 minutes of direct contact, did not induce significant HRPE cell apoptosis. Function-blocking anti-CD18 and anti-intercellular adhesion molecule-1 (ICAM-1) antibodies significantly reduced activated monocyte-induced TUNEL-positive HRPE cells by 48% (p = 0.0051) and 38% (p = 0.046), respectively. Anti-CD18 and anti-ICAM-1 antibodies significantly inhibited caspase-3 activity by 56% (p < 0.0001) and 45% (p < 0.0001), respectively. However, antibodies to vascular cell adhesion molecule-1, TNF-alpha, IL-1beta, or TNF-related apoptosis-inducing ligand did not inhibit apoptosis or caspase-3 activation. Direct overlay of monocytes also induced reactive oxygen metabolites (ROM) within HRPE cells. The intracellular HRPE cell ROM production was inhibited by the anti-CD18 and anti-ICAM-1 antibodies, but not by superoxide dismutase, presumably due to its failure to penetrate into HRPE cells. Accordingly, neither superoxide dismutase nor N(G)-monomethyl-L-arginine had significant effects on HRPE cell apoptosis or caspase-3 activation. Our results suggest that activated monocytes may induce ROM in HRPE cells through cell-to-cell contact, in part via CD18 and ICAM-1, and promote HRPE cell apoptosis. These mechanisms may compromise HRPE cell function and survival in a variety of retinal diseases.

Human RPE-monocyte co-culture induces chemokine gene expression through activation of MAPK and NIK cascade.

Cell-cell contact between human retinal pigment epithelium (hRPE) cells and monocytes occurs in many retinal diseases involving blood-retinal barrier breakdown. This study investigates chemokine secretion induced by co-culture of hRPE cells and monocytes and illustrates the roles of p38 kinase, ERK, JNK/SAPK and NF-kappaB-inducing kinase signaling pathways for hRPE IL-8 and MCP-1 secretion induced in hRPE by co-culture with monocytes. Co-culture of hRPE cells with monocytes increased steady-state IL-8 and MCP-1 mRNA and protein secretion. Stimulation of hRPE cells by monocytes resulted in prominent increases in p38, ERK1/2 and JNK/SAPK phosphorolation, IkappaBalpha degradation, and NF-kappaB nuclear translocation. The induced IL-8 and MCP-1 proteins were almost completely supporessed by U0126, a specific mitogen-activated protein kinase kinase (MEK) inhibitor, or by SB203580, a selective p38 inhibitor. Chemokine secretion was completely blocked by simultaneous administration of U0126 and SB203580. Induction of IL-8 and MCP-1 was abrogated by Ro318220, an inhibitor of PKC, as well as by genistein or herbimycin A, inhibitors of PTK. In addition, anti-inflammatory drugs dexamethasone (DEX) and cyclosporin A (CSA) both blocked activation of JNKS/SAPK and the cell-cell contact induced production of hRPE IL-8 and MCP-1, while activation of p38 and ERK was only inhibited by DEX, but not by CSA. These results suggest that activation of DEX-sensitive, CSA-resistant MEK/ERK and p38 pathways, and activation of NF-kappaB, PKC, and PTK are essential for IL-8 and MCP-1 expression by hRPE cells.

RPE CD14 immunohistochemical, genetic, and functional expression.

CD14 is the primary receptor for lipopolysaccharide (LPS)that plays important roles in host defense and subserves other host-related biological functions. We previously identified CD14 on cultured human retinal pigment epithelial (HRPE) cells using immunocytochemical techniques. In this study, we investigated immunoreactive HRPE CD14 expression by immunohistochemically staining HRPE cells and HRPE cells in sections of human eyes with anti-CD14 monoclonal antibodies (mAb). Constitutive HRPE gene and protein expression were confirmed by semiquantitative PCR and western blotting. ELISA for cell-associated and secreted (s) HRPE CD14 revealed that specific digestion by phosphoinositol-specific phospholipase C (PI-PLC) significantly reduced (P<0.01) cell-associated HRPE CD14 which was not modulated by LPS or gamma-IFN. ELISA of the conditioned media (CM) of HRPE cells treated with PI-PLC contained significantly more (P<0.001) sCD14, but sCD14 was not modulated by LPS or gamma-IFN. FACS analysis confirmed HRPE cell surface CD14. To show functional CD14, fluorescently-labelled LPS and CD14 were demonstrated to show significant co-localization on live, cultured HRPE cells in close proximity (<7A) as demonstrated by resonance energy transfer of the fluorescent ligands (P<0.0001). Significant inhibition (P<0.001) of LPS-induced IL-8 secretion, as measured by ELISA, occurred in the presence of function blocking anti-CD14 mAb. Significant inhibition of LPS-induced HRPE IL-8 secretion by PKC, PTK, PI3 kinase, and p38 kinase inhibitors indicated cell mediators responsible for LPS-induced HRPE chemokine secretion. This study demonstrates that HRPE cells express functional CD14 in vitro and in situ along at the outer blood-retina barrier.

Human RPE cell apoptosis induced by activated monocytes is mediated by caspase-3 activation.

PURPOSE: To determine the effects of activated monocytes on the induction of human retinal pigment epithelial (HRPE) cell reactive oxygen metabolite (ROM) production and apoptosis. METHODS: HRPE cells were co-cultured with interferon-gamma (IFN-gamma)-stimulated human monocytes. HRPE apoptosis was detected by propidium iodide, proliferating cell nuclear antigen (PCNA) and TdT-mediated dUTP nick end labeling (TUNEL) staining, caspase-3 activation, and Western blot analysis. HRPE cell ROMs were imaged using the fluorescent marker dihydrotetramethylrosamine (H2TMRos). RESULTS: IFN-gamma-activated monocytes in direct contact with HRPE cells elicited significant increases in TUNEL-positive (P < .0001) and decreases in PCNA-positive (P < .0001) HRPE cells. The activated monocytes also induced HRPE cell caspase-3 activation, which was inhibited by inhibitor Z-DEVD-fmk. Co-incubations, in which monocytes were either prevented from direct contact with HRPE cells or separated from HRPE cells after 30 minutes of direct contact, did not induce significant HRPE cell apoptosis. Anti-CD18 and anti-ICAM-1 antibodies significantly reduced activated monocyte-induced TUNEL-positive HRPE cells, by 48% (P = .0051) and 38% (P = .046), respectively, and caspase-3 activity by 56% (P < .0001) and 45% (P < .0001), respectively. Overlay of monocytes induced HRPE cell ROM that was inhibited by anti-CD18 and anti-ICAM-1 antibodies, but not by superoxide dismutase (SOD) or nitric oxide (NO) inhibitors. Accordingly, neither SOD nor NO inhibitors had significant effects on HRPE cell apoptosis or caspase-3 activation. CONCLUSIONS: We demonstrated that IFN-gamma-activated monocytes may induce ROM in HRPE cells through cell-to-cell contact, in part via CD18 and ICAM-1, and promote HRPE cell apoptosis via caspase-3 activation. These mechanisms may compromise HRPE cell function and survival in retinal diseases in which mononuclear phagocyte infiltration at the HRPE interface is observed.