Intraocular Expression and Release of High-Mobility Group Box 1 Protein in Retinal Detachment

Noboru Arimura; Yuya Ki-i; Teruto Hashiguchi; Ko-ichi Kawahara; Kamal K. Biswas; Makoto Nakamura; Yasushi Sonoda; Keita Yamakiri; Akiko Okubo; Taiji Sakamoto; Ikuro Maruyama

Disclosures

Lab Invest. 2009;89(3):278-289. 

In This Article

Abstract and Introduction

Abstract

High-mobility group box 1 (HMGB1) protein is a multifunctional protein, which is mainly present in the nucleus and is released extracellularly by dying cells and/or activated immune cells. Although extracellular HMGB1 is thought to be a typical danger signal of tissue damage and is implicated in diverse diseases, its relevance to ocular diseases is mostly unknown. To determine whether HMGB1 contributes to the pathogenesis of retinal detachment (RD), which involves photoreceptor degeneration, we investigated the expression and release of HMGB1 both in a retinal cell death induced by excessive oxidative stress in vitro and in a rat model of RD-induced photoreceptor degeneration in vivo. In addition, we assessed the vitreous concentrations of HMGB1 and monocyte chemoattractant protein 1 (MCP-1) in human eyes with RD. We also explored the chemotactic activity of recombinant HMGB1 in a human retinal pigment epithelial (RPE) cell line. The results show that the nuclear HMGB1 in the retinal cell is augmented by death stress and upregulation appears to be required for cell survival, whereas extracellular release of HMGB1 is evident not only in retinal cell death in vitro but also in the rat model of RD in vivo. Furthermore, the vitreous level of HMGB1 is significantly increased and is correlated with that of MCP-1 in human eyes with RD. Recombinant HMGB1 induced RPE cell migration through an extracellular signal-regulated kinase-dependent mechanism in vitro. Our findings suggest that HMGB1 is a crucial nuclear protein and is released as a danger signal of retinal tissue damage. Extracellular HMGB1 might be an important mediator in RD, potentially acting as a chemotactic factor for RPE cell migration that would lead to an ocular pathological wound-healing response.

Introduction

Cell death is the predominant event of degenerative tissue damage and can be a trigger that activates the immune system and repair program. Recently, there has been much interest in the pivotal role of endogenous danger signals released during cell death.[1] High-mobility group box 1 (HMGB1) protein is a prototypic innate danger signal, and appears to be crucial in this context because extracellular HMGB1 [2] can modulate inflammation, proliferation, and remodeling, which are involved in the wound-healing process.[3]

HMGB1 was originally described as an abundant and ubiquitous nuclear DNA-binding protein that had multiple functions dependent on its cellular location.[2,4] In the nucleus, HMGB1 binds to DNA and is critical for proper transcriptional regulation. It is also called amphoterin and accelerates cellular motility on the cell surface.[5] HMGB1 is reported to be passively released into the extracellular milieu by necrotic cells, but not by apoptotic cells,[6] or is exported actively by monocytes/macrophages[7] and neural cells[8] upon receiving appropriate stimuli. In damaged tissue, extracellular HMGB1 acts as a necrotic signal, which alerts the surrounding cells and the immune system.[2] Although extracellular HMGB1 can contribute to normal tissue development and repair, it is also implicated in the pathogenesis of several diseases (including lethal endotoxemia,[7] disseminated intravascular coagulation,[9] ischemic brain,[10] tumor,[11] atherosclerosis,[12] rheumatoid arthritis,[13] and periodontitis[14]).

Retinal detachment (RD), the physical separation of photoreceptors from the underlying retinal pigment epithelium (RPE), is one of the main causes of visual loss. Photoreceptor degeneration due to RD is thought to be executed by apoptosis[15,16] and necrosis,[17] which usually occur after tissue damage. Although retinal cell death and the following reactive responses must occur in almost all forms of retinal disease including RD,[18] data regarding the relationship among cell death, danger, and responses in the eye, have been very limited, especially in terms of danger signals. We previously reported that HMGB1 was significantly elevated in inflamed eyes with endophthalmitis, and suggested a possible link between HMGB1 and ocular inflammatory diseases.[19] On the other hand, considering the properties of HMGB1, we hypothesized that HMGB1 might have some roles in photoreceptor degeneration and subsequent damage-associated reactions in RD.

To investigate whether HMGB1 is involved in the pathogenesis of RD, we first examined the expression and release of HMGB1 both in a retinal cell death in vitro and in a rat model of RD-induced photoreceptor degeneration in vivo. To focus on human RD, we assessed the intravitreous concentrations of HMGB1 in human eyes affected by RD. Monocyte chemoattractant protein 1 (MCP-1), which was recently documented to be a potential proapoptotic mediator in RD,[20] was also measured in the same vitreous samples. We further analyzed the effects of recombinant HMGB1 (rHMGB1) on chemotactic activity in a RPE cell line in vitro. Our findings suggest that extracellular HMGB1 is evident in eyes with RD as a danger signal, potentially acting as a chemotactic factor for RPE cell migration that would lead to ocular pathological wound healing.

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