Transferrin and transferrin receptor expression in intraocular proliferative disease. APAAP-immunolabeling of retinal membranes and ELISA for vitreal transferrin

Mononuclear Phagocytes in Proliferative Vitreoretinopathy (PVR). A Specific Role of Microglial Cells in Non-Traumatic Disease?

Mononuclear phagocytes have been a focus of attention in the cellular biology of proliferative vitreoretinopathy (PVR) for more than ten years. The pattern of phagocyte participation in periretinal traction membrane formation in PVR depends on the etiology, i.e. trauma, rhegmatogenous retinal detachment, previous therapy, i.e. multiple surgical interventions, and the clinical stage of the disease. We have recently identified microglial cells as a distinct cellular population, in membranes from patients with non-traumatic PVR. Current evidence of mononuclear phagocyte function in PVR suggests a role for resident phagocytes of the vitreous and retina in PVR subsequent to rhegmatogenous detachment, and a role for blood-derived monocytes in post-traumatic PVR. The cellular biology of PVR may be much more heterogeneous than previously assumed.

Iron overload in diabetic retinopathy: a cause or a consequence of impaired mechanisms?

Iron is an essential ion for life, playing a central role in many metabolic processes. The most important property of free iron is its capacity to be reversibly oxidized and reduced, but at same time this make it highly pro-oxidant molecule. In this regard, iron is able to generate powerful reactive oxygen species (ROS). For this reason, careful control on iron availability is central to the maintenance of normal cell function in the retina. In the diabetic eye there is an impairment of iron homeostasis, thus leading to iron overload. The mechanisms involved in this process include: (1) Destruction of heme molecules induced by hyperglycemia (2) Intraretinal and vitreal hemorrhages (3) Overexpression of the renin-angiotensin system. The main consequences of iron overload are the following: (1) Retinal neurodegeneration due to the increase of oxidative stress (2) Increase of AGE-RAGE binding (3) Defective phagocytosis of retinal pigment epithelium, which generates the accumulation of autoantigens and the synthesis of proinflammatory cytokines. Further studies addressed to explore not only the role of iron in the pathogenesis of diabetic retinopathy, but also to design novel therapeutic strategies based on the regulation of iron homeostasis are needed.

Transferrin secretion by lens epithelial cells in culture

Transferrin (Tf), the plasma iron transport protein which supports cell proliferation and differentiation and has bacteriostatic, antioxidant and anti-inflammatory activity, has been found in relatively high concentrations in the intraocular fluids. Intraocular synthesis of Tf has recently been demonstrated, although the intraocular tissue(s) responsible have not been identified. We designed this study to determine whether certain ocular tissues can make and secrete transferrin. Transferrin content of aqueous and vitreous humors and whole lenses was determined by ELISA. Transferrin secretion by cultured epithelia from lens and ciliary body was also measured. In addition, Northern blots of RNA from cultured lens epithelial cells, ciliary body pigmented and non-pigmented epithelial cells, and from whole iris, ciliary body and retina were probed with riboprobes for Tf mRNA and 18S rRNA. Transferrin made up 23% and 16% of total canine aqueous and vitreous protein. All ocular tissues and cultured cells tested contained mRNA for Tf, however Tf was secreted into the bathing medium from lens epithelial cell cultures, but not from either the pigmented or non-pigmented epithelial cells of the ciliary body cultures, but not from either the pigmented or non-pigmented epithelial cells of the ciliary body Cycloheximide inhibited secretion of Tf from the lens epithelial cells. Lenses from inflamed eyes contained higher levels of Tf than their contralateral controls. This is the first experimental demonstration that an intraocular tissue can make and secrete Tf. Transferrin secretion by the lens may contribute significantly to the IOF content of this important intraocular protein.

Increased lipid peroxide levels and myeloperoxidase activity in the vitreous of patients suffering from proliferative diabetic retinopathy

Lipid peroxide (LPO) levels, as determined by high-performance liquid chromatography (HPLC) and by the thiobarbituric acid (TBA) method, and myeloperoxidase (MPO) activity in vitreous of patients vitrectomized because of proliferative diabetic retinopathy were compared with LPO levels and MPO activity in vitreous of patients with no vitreoretinal proliferation. Both LPO levels and MPO activity were significantly elevated in the vitreous of patients with fibrovascular vitreoretinal proliferations secondary to diabetes. The TBA method produced higher values for LPO levels than did the HPLC method. The correlation between the two methods was 0.94. Our results suggest that both oxygen-free radicals and inflammation-related reactions can participate in the pathogenesis of diabetic retinopathy.

Monensin enhances the cytotoxic effect of antitransferrin receptor immunotoxin on cultured RPE cells

The effect of monensin on the cytotoxic effect of antitransferrin receptor immunotoxin (IT) was determined on cultured, human retinal pigment epithelial (hRPE) cells. Human RPE cells were treated with 0.1-10,000 ng/ml IT with and without 0.01-0.1 microM monensin, a lysosomotropic reagent that can influence IT activity. Monensin (0.01 microM) shortened the onset of cell kill with IT (10,000 ng/ml) from 48 to 24 hours (p = 0.0016). Although 0.01 microM monensin alone was not cytotoxic to hRPE cells, a single 7-day treatment with monensin caused up to a 4.1-fold increase in antiproliferative potency of IT on proliferating hRPE cells (p < or = 0.0001). Enhancement was obtained with only a 1-hour exposure to 0.1 microM monensin (p = 0.0001). In contrast, IT (0.1-10,000 ng/ml) combined with monensin (0.01 microM) had minimal effect on density-arrested cells. IT with or without monensin did not inhibit proliferation of Rhesus monkey RPE cells. Our results indicate that monensin enhances the selective cytotoxic effect of IT on proliferating hRPE cells in culture.

Changes in the soluble protein of the human vitreous in vitreoretinal disease

Samples of the vitreous were analysed in order to identify changes of soluble proteins in vitreo-retinal disease. The soluble proteins of the vitreous were separated on an anion exchange column (Mono-Q). The degree of neutral proteolytic activity in vitreous body was also measured. The vitreous from cataract cases without vitreoretinal disease was characterized by its low content of soluble proteins equivalent to about 1% of that of serum. Albumin and transferrin were the major identified components and their concentrations were approximately 0.85 and 0.03 g/l, respectively. In cases with vitreoretinal disease the vitreous showed changes of total soluble protein and the appearance of additional protein peaks. In patients with PVR the albumin concentration in the vitreous was found to be three times higher as compared to the control group consisting of patients with cataract. Neutral proteolytic activity in the vitreous was relatively low in both normal and pathological vitreous.

The role of growth factors in the embryogenesis and differentiation of the eye

The vertebrate eye is composed of a variety of tissues that, embryonically, have their derivation from surface ectoderm, neural ectoderm, neural crest, and mesodermal mesenchyme. During development, these different types of cells are subjected to complex processes of induction and suppressive interactions that bring about their final differentiation and arrangement in the fully formed eye. With the changing concept of ocular development, we present a new perspective on the control of morphogenesis at the cellular and molecular levels by growth factors that include fibroblast growth factors, epidermal growth factor, nerve growth factor, platelet-derived growth factor, transforming growth factors, mesodermal growth factors, transferrin, tumor necrosis factor, neuronotrophic factors, angiogenic factors, and antiangiogenic factors. Growth factors, especially transforming growth factor-beta, have a crucial role in directing the migration and developmental patterns of the cranial neural-crest cells that contribute extensively to the structures of the eye. Some growth factors also exert an effect on the developing ocular tissues by influencing the synthesis and degradation of the extracellular matrix. The mRNAs for the growth factors that are involved in the earliest aspects of the growth and differentiation of the fertilized egg are supplied from maternal sources until embryonic tissues are able to synthesize them. Subsequently, the developing eye tissues are exposed to both endogenous and exogenous growth factors that are derived from nonocular tissues as well as from embryonic fluids and the systemic circulation. The early interaction between the surface head ectoderm and the underlying chordamesoderm confers a lens-forming bias on the ectoderm; later, the optic vesicle elicits the final phase of determination and enhances differentiation by the lens. After the blood-ocular barrier is established, the internal milieu of the eye is controlled by the interactions among the intraocular tissues; only those growth factors that selectively cross the barrier or that are synthesized by the ocular tissues can influence further development and differentiation of the cells. An understanding of the tissue interactions that are regulated by growth factors could clarify the precise mechanism of normal and abnormal ocular development.

An immunochemical quantitative analysis of the protein pattern in physiologic and pathologic vitreous

Biochemical changes in the vitreous in different vitreoretinal disorders have not yet been thoroughly studied. Using enzyme-linked immunosorbent analysis (ELISA), we established mean values and 95% confidence intervals for six proteins of physiologic human vitreous: albumin (293 +/- 18 mg/l), transferrin (73.7 +/- 6.6 mg/l), immunoglobulin G (IgG), (33.5 +/- 3 mg/l), alpha 1-antitrypsin (14.1 +/- 2.9 mg/l), alpha 1-acid glycoprotein (4 +/- 0.7 mg/l), and lactoferrin (less than 50 micrograms/l). These six proteins were also determined in vitreous aspirates from patients with idiopathic proliferative vitreoretinopathy (n = 10), traumatic proliferative vitreoretinopathy (n = 10), and proliferative diabetic retinopathy (n = 15). The pattern of protein levels varied widely within each of the disorders. An analysis of absolute protein levels showed significant differences in total protein and alpha 1-antitrypsin levels between controls and pathologic vitreous samples. We observed differences in transferrin between controls and proliferative diabetic retinopathy (PDR), and differences in alpha 1-acid glycoprotein between controls and both types of proliferative vitreoretinopathy (PVR). The single disorders themselves could not be differentiated by any of the proteins. When the relative contribution of single proteins to total vitreal protein was compared, albumin was lower in all three disorders than in controls. Transferrin was lower in traumatic PVR than in controls, in PDR, or in idiopathic PVR. Our results indicate that the three vitreoretinal disorders studied are characterized by a breakdown of blood-ocular barriers.

Vitronectin and proliferative intraocular disorders. II. Expression of cell surface receptors for fibronectin and vitronectin in periretinal membranes

Several cell types participate in the formation of vitreoretinal traction membranes in proliferative intraocular disorders. The communication between these cells involves hormones, growth factors, and the interaction with extracellular matrix molecules. We have previously demonstrated a partial colocalisation of two potent mediators of cell attachment, fibronectin and vitronectin, in periretinal membranes from patients with proliferative vitreoretinopathy (PVR). We found a similar pattern of vitronectin and fibronectin deposition in proliferative diabetic retinopathy (PDR) (n = 6). Now we show the expression of the corresponding cell surface receptors, integrins, for fibronectin and vitronectin by proliferating cells in 22 periretinal membranes, including traumatic (n = 8) and idiopathic (n = 8) PVR as well as PDR membranes (n = 6). Integrins are membrane receptors for extracellular matrix macromolecules which are involved in such basic biological phenomena as embryogenesis and metastasis. Future studies on the pathogenesis of vitreoretinal proliferation will have to focus on the initiation, maintenance, and regulation of this intercellular communication network involving attachment proteins and integrins.

Immunoglobulin G, complement factor C3 and lymphocytes in proliferative intraocular disorders

This study examines a possible immunological contribution to the development of proliferative intraocular disorders (PID) with traction retinal detachment. We analysed 24 periretinal membranes and 35 vitreous aspirates from patients with idiopathic proliferative vitreoretinopathy (PVR), traumatic PVR, and proliferative diabetic retinopathy (PDR). Lymphocytes and complement factor C3 deposits could not be detected in any of the membrane specimens. IgG was present in all but one of the PVR membranes but in less than half of the PDR specimens and there to a lesser extent. The IgG immunoreactivity was not collocalized with macrophages but instead located to the extracellular matrix. The intravitreal levels of IgG (ELISA) and protein were elevated in PID but the range of these biochemical changes was so wide that there were no significant differences of the IgG levels between the single types of PID. Using electrophoresis and Western blotting, C3 was detected in normal and pathologic vitreous but smaller C3 fragments indicative of C3 breakdown were only found in PID.

Proliferative vitreoretinopathy--is it anything more than wound healing at the wrong place?

Proliferative vitreoretinopathy (PVR) is a reactive process of the ocular tissue after perforating trauma, retinal detachment, and surgical manipulations. Although several studies, most of them experimental, have focused on the detection of specific etiologic factors in the development of PVR, there is compelling evidence that PVR is nothing more than a physiologic tissue repair process with undesirable consequences for the retina. Important features of PVR involving the role of platelets, mononuclear phagocytes, and fibroblasts parallel the chain of events observed in tissue repair elsewhere in the body. Numerous experimental models for PVR, originally designed to find specific stimuli for the generation of intraocular traction membrane formation, have shown that the process of PVR is the common pathway of the eye's reaction to vitreoretinal trauma of any kind. Accordingly, vitreoretinal surgeons could learn a lot from the work of other disciplines, e.g. surgery and dermatology, on wound healing, and the factors known to modify wound healing elsewhere in the body should be taken into consideration. The well-established impairment of tissue repair processes caused by medical treatment with corticosteroids and cytotoxic agents suggests a combined medical approach to PVR as an adjunct to surgical treatment, using refined methods of application and dosage. Steroids and cytotoxic drugs will influence the course of PVR by suppressing macrophage recruitment and the initial inflammatory reaction as well as the proliferative phase of wound healing with traction retinal detachment, respectively.

The pathogenesis of vitreoretinal proliferation and traction: a working hypothesis

Traction retinal detachment due to proliferative vitreoretinopathy (PVR) is a serious complication of ocular trauma, retinal detachment, and previous vitreoretinal surgery. The cause is the active proliferation of fibroblasts, glial cells, and retinal pigment epithelial cells in the periretinal spaces, leading to the formation of contractile cellular membranes. The generation of growth and mitosis stimulation for these cells has remained obscure. We postulate that invading macrophages and local microglia secrete growth factors, notably PDGF (platelet-derived growth factor), which in turn mediates the mitogenic effects of transferrin (TF), a protein present in huge amounts in native vitreous, in plasma and in intraocular proliferative tissue.