Increasing cell density down-regulates the expression of acidic FGF by human RPE cells in vitro

The retinal pigment epithelium: something more than a constituent of the blood-retinal barrier--implications for the pathogenesis of diabetic retinopathy

The retinal pigment epithelium (RPE) is an specialized epithelium lying in the interface between the neural retina and the choriocapillaris where it forms the outer blood-retinal barrier (BRB). The main functions of the RPE are the following: (1) transport of nutrients, ions, and water, (2) absorption of light and protection against photooxidation, (3) reisomerization of all-trans-retinal into 11-cis-retinal, which is crucial for the visual cycle, (4) phagocytosis of shed photoreceptor membranes, and (5) secretion of essential factors for the structural integrity of the retina. An overview of these functions will be given. Most of the research on the physiopathology of diabetic retinopathy has been focused on the impairment of the neuroretina and the breakdown of the inner BRB. By contrast, the effects of diabetes on the RPE and in particular on its secretory activity have received less attention. In this regard, new therapeutic strategies addressed to modulating RPE impairment are warranted.

The retinal pigment epithelium in visual function

Located between vessels of the choriocapillaris and light-sensitive outer segments of the photoreceptors, the retinal pigment epithelium (RPE) closely interacts with photoreceptors in the maintenance of visual function. Increasing knowledge of the multiple functions performed by the RPE improved the understanding of many diseases leading to blindness. This review summarizes the current knowledge of RPE functions and describes how failure of these functions causes loss of visual function. Mutations in genes that are expressed in the RPE can lead to photoreceptor degeneration. On the other hand, mutations in genes expressed in photoreceptors can lead to degenerations of the RPE. Thus both tissues can be regarded as a functional unit where both interacting partners depend on each other.

Retinal pigment epithelium-immune system interactions: cytokine production and cytokine-induced changes

Vision is dependent on proper function of several intraocular structures. Immune responses to eliminate invading pathogens from the eye may threat vision by causing damage to these structures. Therefore, immunological defence of the eye should be carefully balanced between efficacy and maintenance of functional integrity. The eye is equipped with several regulatory mechanisms to prevent certain immune and inflammatory responses and is, therefore, regarded as an immune privileged site. The retinal pigment epithelium (RPE) contributes to the immune privileged status of the eye as part of the blood-eye barrier and by the secretion of immunosuppressive factors inside the eye. RPE cells, however, may also play an important role in the development of immune and inflammatory responses in the posterior part of the eye. During the last decade it has become clear that RPE cells are highly sensitive to a variety of inflammatory cytokines. Under inflammatory conditions, RPE cells produce a myriad of cytokines that may activate the resident ocular cells or attract and activate leukocytes. Cytokine stimulation of RPE cells causes profound effects, including nitric oxide secretion, cell surface expression of MHC class II and adhesion molecules and abrogation of barrier function. This article provides a comprehensive review of the literature concerning RPE cells and cytokines.

drp, a novel protein expressed at high cell density but not during growth arrest

Contact is a vital mechanism used by cells to interact with their environment. Contact with living and nonliving elements adjacent to a cell is the basis for many common biological events ranging from growth regulation to metastasis to embryonic pattern formation. We describe the cloning and characterization of a novel density-regulated protein (drp) whose expression is increased in cultured cells at high density compared with cells at low density. A drp cDNA was isolated from the human teratocarcinoma cell line PA-1. Northern analysis with a drp probe revealed transcripts of 2.8 and 3.2 kb. The drp RNA was expressed in a variety of tissues, with the highest amounts in skeletal and cardiac muscle. Using antipeptide antisera, increasing amounts of a 70-kDa protein were detected using several experimental approaches in several cells lines as cell density is increased. Conditioned medium from high-density cells was unable to induce expression of drp in cells growing at low density. Similarly, growth arrest by serum starvation or transforming growth factor-beta (TGF-beta) treatment failed to elicit drp expression. We conclude that drp is a novel protein whose expression is increased at high cell density but not growth arrest.

Extracellular matrix of retinal pigment epithelium regulates choriocapillaris endothelial survival in vitro

The role of extracellular matrix of retinal pigment epithelial cells (RPE-ECM) in the regulation of the survival of choriocapillaris endothelial cells (CCE) was investigated in vitro. The CCE survival was evaluated by trypan blue staining, neutral red uptake, and the counting of viable cells. Results showed that CCE cells survived on RPE-ECM. Pre-treatment of RPE-ECM individually with neutralizing antibodies to acidic fibroblast growth factor, vascular endothelial growth factor, platelet-derived growth factor, or transforming growth factor beta(pan specific to TGFbeta1, TGFbeta1.2, TGFbeta2 and TGFbeta5), did not alter the survival rate of CCE cells on RPE-ECM, as compared to that of the control (CCE survival rates on RPE-ECM pretreated with normal rabbit IgG). However, the treatment of RPE-ECM with neutralizing antibody to basic fibroblast growth factor (bFGF) caused CCE death by 77.1+/-15.7%. The CCE death was defined as apoptosis based on the morphological markers (shrinkage in cell size with blebbing of plasma membranes, condensation and fragmentation of nuclei, and DNA fragmentation in multiples of approximately 200 bp). The addition of phorbol 12-myristate 13-acetate (PMA) (2 nM) to the culture medium was effective for complete prevention of CCE apoptosis; the protecting effect of PMA on CCE apoptosis can be abolished by H7 (25 microM), but not HA1004 (50 microM), suggesting the involvement of PKC in protecting CCE from apoptosis. The inhibition of protein synthesis of CCE cells by cycloheximide (0.1 microM) did not affect the apoptotic process of the cells. In a separate experiment, when CCE cells were cultured in a medium saturated with bFGF (5 ng ml-1) without RPE-ECM, the cells also died by apoptosis. However, this apoptotic process was not affected by PMA. Cycloheximide also failed to affect the apoptotic process. These results suggest that both RPE-ECM insoluble molecules and RPE-ECM-bound bFGF modulate choriocapillaris survival by suppressing CCE apoptosis.

Neurotrophic factors, cytokines and stress increase expression of basic fibroblast growth factor in retinal pigmented epithelial cells

Basic fibroblast growth factor (bFGF) and FGF receptors have been localized to photoreceptors and retinal pigmented epithelium (RPE), but the function of bFGF in adult retina and RPE is unknown. Exogenous bFGF has a neuroprotective effect in retina and brain and its expression is increased in some neurons in response to cytokines or stress. In this study, we investigated the effect of light, other types of stress, neurotrophic factors, and cytokines on bFGF levels in cultured human RPE. Some agents that protect photoreceptors from the damaging effects of constant light, including brainderived neurotrophic factor (BDNF), ciliary neurotrophic factor, and interleukin-1 beta, increase bFGF mRNA levels in RPE cells. Intense light and exposure to oxidizing agents also increase bFGF mRNA levels in RPE cells and cycloheximide blocks the increase. An increase in bFGF protein levels was demonstrated by ELISA in RPE cell supernatants after incubation with BDNF or exposure to intense light or oxidizing agents. These data indicate that bFGF is modulated in RPE cells by stress and by agents that provide protection from stress and support the hypothesis that bFGF functions as a survival factor in the outer retina.

Density-dependent growth regulation of pig retinal pigment epithelial cells in vitro

BACKGROUND

Transplantation of the retinal pigment epithelium (RPE) involves delivering donor RPE onto a bare area of host Bruch's membrane that is surrounded by a confluent monolayer of RPE. We investigated the effects of different plating densities and the presence of an adjacent confluent monolayer on the growth characteristics and final morphology of pig RPE in vitro.

METHODS

In single-well experiments, porcine RPE were plated in 24-well plates at densities varying from 1 to 75 cells/mm2. Triplicate plates were counted on the 3rd and 10th days after plating and at confluence. A multiwell chamber was built to allow cells plated at different densities to be bathed with conditioned media from adjoining wells.

RESULTS

In single-well experiments, plating at low densities increased the time to reach confluence and resulted in fewer, larger and more fusiform RPE at confluence. In multiwell experiments, the growth rate of cells plated at low density decreased as the amount of high-density medium increased in communicating wells and led to smaller, rounder cells at confluence. The presence of low-density RPE in adjoining wells increased the growth rate of RPE plated at high density and produced fewer, larger and more fusiform cells at confluence. Newly plated RPE grew more slowly when confluent monolayers of RPE were present in adjoining wells.

CONCLUSIONS

Plating density is a critical factor in determining the growth rate and the final morphology of RPE in tissue culture. The presence of a neighboring confluent monolayer of RPE inhibits the growth rate of newly plated RPE in vitro.

Monocyte-induced cytokine expression in cultured human retinal pigment epithelial cells

Monocytes and retinal pigment epithelial cells are intimately associated in membranes of eyes with proliferative vitreoretinopathy and in certain types of uveitis. The goal of this study was to determine whether monocytes modulate cytokine expression in retinal pigment epithelial cells, and if so, to identify the monocyte products responsible for this effect. Cultured human retinal pigment epithelial cells were exposed to varying concentrations of monocyte-conditioned medium from unstimulated human monocytes for 1-48 hr, or from monocytes prestimulated with lipopolysaccharide. mRNA expression of interleukin-1 beta, interleukin-6, interleukin-8, melanoma growth stimulating activity/gro alpha and gamma, macrophage colony stimulating factor, transforming growth factor-beta 2, basic fibroblast growth factor and activin beta A chain was determined by reverse transcription polymerase chain reaction. Protein secretion of selected cytokines, interleukin-1 beta, interleukin-6, interleukin-8, macrophage colony stimulating factor and transforming growth factor-beta 2 was measured in RPE-conditioned medium by ELISA. Retinal pigment epithelial cells constitutively expressed mRNA for interleukin-6, macrophage colony stimulating factor, transforming growth factor-beta 2, basic fibroblast growth factor and activin beta A chain. Interleukin-1 beta, melanoma growth stimulating activity/gro alpha and gamma and interleukin-8 were not expressed under basal conditions. Stimulated monocyte-conditioned medium markedly induced mRNA of all cytokines except basic fibroblast growth factor and transforming growth factor-beta 2 in a dose- and time-dependent manner. Unstimulated monocyte-conditioned medium was a less potent inducing agent, but still enhanced mRNA expression of interleukin-6, interleukin-8 and melanoma growth stimulating activity/gro alpha. Stimulated monocyte-conditioned medium also induced a time-dependent increase in interleukin-6, Interleukin-8, macrophage colony stimulation factor and transforming growth factor-beta 2, but not interleukin-1 beta protein secretion (p < 0.05 for all time points). Neutralizing antibodies to interleukin-1 beta, or tumour necrosis factor alpha, but not interleukin-1 alpha, significantly reduced cytokine mRNA expression induced by stimulated monocyte-conditioned medium. The combination of all three neutralizing antibodies almost entirely eliminated monocyte-induced mRNA expression and protein production of all cytokines studied. Activated monocytes secrete a heterogeneous mixture of products that together strongly induce expression of multiple cytokines in human retinal pigment epithelial cells. Most if not all of the inducing effect can be accounted for by interleukin-1 beta and tumour necrosis factor alpha. Because cytokines have been implicated in proliferative vitreoretinopathy and uveitis, monocyte-mediated cytokine expression by RPE cells may serve to initiate and perpetuate these diseases.