Ⅰ. Introduction

Many researchers working on oxidative stress, followed by Esterbauer et al.¹⁾ who defined the role of 4-hydroxynonenal (HNE), would agree that NHE is a toxic product of lipid peroxidation and second messenger of free radicals. Peroxidative degradation of lipids produces the oxidized low-density lipoprotein (LDL) or the bioactive aldehyde HNE that are intimately involved in the pathogenesis of cardiovascular diseases like atherosclerosis^2,3) because of its consistent detection in atherosclerotic lesion^4,5) and fibrotic plaque in humans.⁶⁾ The formation of atherosclerotic lesions is a complex process that is primarily mediated by inflammatory and oxidative process with its accompanying vascular dysfunction including smooth muscle cell (VSMC).⁷⁾

Oxidative stress is one of the factors that can promote both VSMC proliferation/migration in atherosclerotic lesions and VSMC apoptosis, which is potentially involved in atherosclerotic plaque instability and rupture.^8,9) In contrast to the stable atherosclerotic plaque according to VSMC proliferation, the unstable variety may become enlarged and lead to plaque rupture and thrombosis.¹⁰⁾ Unstable plaque is characterized by the presence of a sustained inflammatory stimulus, which in turn is maintained by sustained partial cell death.^11,12) HNE consistently presents in significant amounts in unstable atherosclerotic plaque, and most likely provides a primary contribution to the whole process through their pronounced proinflammatory, profibrogenic and proapoptotic effects.^13,14) It requires the cooperation of several cell types such as endothelial cells(ECs), VSMCs, which should be activated, proliferate and migrate to invade the extracellular matrix and cause vascular remodeling.^15,16)

Thus, the apparent ability of HNE to favour the activation of VSMC points to a probably significant contribution to the promotion and progression of atherosclerosis. In this review, we will examine the current evidence of linking reactive aldehyde, HNE, to VSMC activation leading to oxidative stress-related retinal degeneration and vision loss.

Ⅱ. Biochemical action of HNE

Major sources of reactive aldehydes in vivo are lipid peroxidation, glycation, and acid oxidation.¹⁷⁾ Lipid peroxidation proceeds by a free radical chain reaction.¹⁾ Termination of lipid peroxidation occurs when two radical species react with each other to form a non-radical product.¹⁸⁾ Overall, lipid peroxidation is a self-propagating process that will proceed until a substrate is consumed or termination occurs.

There are broad outcomes to lipid peroxidation, structural damage to membranes and generation of bioactive secondary products fragmentation of lipid hydroperoxides. In addition, lipid peroxidation produces abnormal fatty acid esters and liberates a number of diffusible products, some of which are potent electrophiles.¹⁹⁾ Chemical reactive aldehydes, such as malondialdehyde(MDA), acrolein, NHE, and 4-hydroxylhexenal(HHE) is known as the most abundant products of lipid peroxidation.¹⁾ Among these reactive aldehydes, HNE quantitatively represents a major aldehydic product of the nonenzymatic oxidative breakdown of polyunsaturated fatty acids (FUFAs), namely linoleic and arachidonic acids.^1,20)

HNE contains three functional groups making it highly reactive: a C=C double bond, a carbonyl group, and a hydroxyl group. HNE can react with the thiol and amino groups of macromolecules including proteins, peptides, lipids, and nucleic acids.^20,21) Primary reactants for HNE are the amino acids cysteine, histidine and lysine^22-24) to crosslink proteins via reaction of the adduct aldehyde with amines. The mechanism of modification of these amino acids with HNE that it primarily forms adducts having a hemiacetal structure via the Michael reaction.¹⁷⁾ As a strong electrophile, HNE can also significantly alter cellular redox status by depleting cellular sulfhydryl compounds, such as glutathione (GSH).^22,23)

As noted in a review,^11,25) HNE-protein adducts have been detected in a variety of animal models of oxidative stress, as well as in tissues prepared from humans having clinically divers diseases associated with oxidative stress linked directly or indirectly with chronic inflammation. However, the action mechanism of HNE-modified protein in the cellular damage accompanying oxidative stress is not well described.

Ⅲ. HNE as a mediator of oxidative stress in vasculature

In human venous plasma, normal HNE levels were estimated between 0.3-1.0 μM.¹⁸⁾ However, under pathological conditions, concentrations of HNE can be markedly enhanced and accumulated at concentrations up to 5 mM in response to oxidative insult.^1,19,24) HNE have been shown to induce intracellular oxygen species (ROS) production in cultured VSMC.¹⁶⁾ Moreover, HNE can produce the nitro oxide (NO) in VSMC (Fig. 1). HNE are also known to cause cellular structural perturbations and various redox-related degenerative processes, including vascular dysfunction.²⁶⁾ NO seems to be cause diabetic angiopathy.^27,28)

Previous studies demonstrated molecular course by which, in endothelial cells, oxidative stressmediated the nuclear factor-κB (NF-κB) activation mechanism was elicited, especially by reactive aldehydes (Fig. 2). It seems that HNE mediating redox disturbances and/or oxidative stress are intricately involved in vascular dysfunction.^24,26,29)

Compared to free radicals, the aldehydes are biologically active compounds, and it is known as long-lived and diffusible compounds. Aldehydes also have a capacity to attacking macromolecules distant from the initial oxidative event.¹⁾ Aldehydes, HNE and HHE may have no cell surface receptors but directly react with matrix tissue or cell surface proteins which may resulted in causing alteration of the structure and function of matrix proteins.

Ⅳ. HNE as signaling molecule occurring proinflammatory process

During progression of atherosclerosis, inflammation certainly plays a pivotal role.^30,31) Although the initial phase of atherogenesis involves endothelial activation and immune cell recruitment, the evolution of this lesion is critically modulated by VSMCs. Through effects on VSMC migration, proliferation, and elaboration of cytokines, proinflammatory stimuli can alter atherosclerotic lesion characteristics and attendant complication.^11,12) Validation of VSMC response to proinflammatory stimuli is, thus, of considerable interest.

Aldehydes can cause redox disturbances and various degenerative processes. The redox sensitive transcription factor NF-κB has an important function in the regulation of many genes involved in the inflammatory and proliferative responses of cells and apoptotic cell death. Recent studies indicate that NF-κB is involved not only in pathogenesis of atherosclerosis.^32,33) Our data definitely proved the ability of HHE to activation of NF-κ B, which can lead to vascular dysfunction by the activation of various proinflammatory genes such as nitric oxide synthase (iNOS).³⁴⁾ iNOS is reported to be found in a number of different cell types in the vessel wall, including VSMCs after arterial balloon injury or exposure to inflammatory cytokines, such as interleukin-1β, tumor necrosis factor, and γ-interferon.^35-37)

Our other report provides new insights into the molecular events of HHE-induced vascular dysfunction and the insidious vascular inflammatory process that NF-κB activation through inhibitory-κB kinase/ NF-κB-inducing kinase (IKK/NIK) pathway and/or p38 Mitogen-activated protein kinase (p38 MAPK) and extracellular signal-regulated kinase (ERK) activation.³⁸⁾ Therefore, it is suggested that upregulation of NF-κB-regulated gene expression by reactive aldehydes contributes at certain stages of atherosclerosis to chronic inflammation finally leading to retinal degeneration (Fig. 2). Previous studies underlay molecular course by which oxidative stress-mediated the NF-κB activation mechanism elicited by reactive aldehydes. The mechanism might lead to vascular dysfunction by the activation of various proinflammatiry genes such as inducible iNOS, cyclooxygenase-2 (COX-2), and xanthine oxidase (XOD).^34,38,39)

Ⅴ. Dual effects of HNE on proliferation and apoptosis in VSMC

VSMC proliferation and apoptosis are plays essential roles in vascular development and atherosclerosis.^11-13) The proliferation and migration of VSMC from the media to the intima layer of the arterial wall represent one of the end stage events in atherogenesis as it contributes to the formation of fibrous cap over the atherosclerotic plaques.⁴⁰⁾ Death of VSMCs has been demonstrated in vessel development and in disease, most notably in atherosclerosis, but also after injury and remodeling.

HNE has been widely accepted as an inducer and mediator of oxidative stress of events leading to pathogenesis like atherosclerosis involved VSMC proliferation and apoptosis.^13,36) Excess oxidative stress may be toxic, exerting cytostatic effects, causing membrane damage, and activating pathways of cell death (apoptosis and/or necrosis). According to the previous study, HNE and HHE induce the cell apoptosis associated with oxidative stress in vascular cells.^13,26) Produced HNE with relatively large amounts is believed to be responsible for the key mediator of oxidative stress-induced cell death.

However, recent studies show that HNE/HHE under nontoxic levels can stimulate the expression of early growth-response genes in some cell type. This study found that HNE, at low levels, induced the Akt phosphorylation (Fig. 3). It may act via attenuating apoptosis and increasing synthesis of NO through both the phosphatidylinositol 3-kinase (PI3K)/Akt and MAPK signaling pathway to reduce oxidative stress and cell apoptosis.⁴¹⁾ While oxidative stress mediated induction of Akt phosphorylation and cell proliferation are known,^42,43) it appears that kinases play an important role on downstreaming of the oxidative stress, which is integrating ROS signals into general cellular responses.^44,45) This study indicates an oxidative stress in VSMCs subjected to vascular dysfunction where HNE could mediate these effects influencing the cellular signals upstream of Akt phosphorylation. Even the hyperproliferation of VSMCs that occurs in vascular disease may, at least in part, be mediated by HNE.

Ⅵ. Conclusions

Taking all available data together, it is seems reasonable to consider HNE as one of the molecules consistently involved in the vascular disorder leading to oxidative stress-related retinal degeneration and vision loss, both in the formation of the atheroma and in the fibrotic transformation of the arterial wall. HNE induced by oxidative stress involved in the initiation of the atherosclerotic lesion through the VSMC activation.