Hyperhomocysteinemia

Homocysteine (Hcy) is a sulfur-containing non-proteinogenic amino acid formed during the metabolism of the essential amino acid methionine. Hcy is considered a risk factor for atherosclerosis and cardiovascular disease (CVD), but the molecular basis of these associations remains elusive. The impairment of endothelial function, a key initial event in the setting of atherosclerosis and CVD, is recurrently observed in hyperhomocysteinemia (HHcy). Various observations may explain the vascular toxicity associated with HHcy. For instance, Hcy interferes with the production of nitric oxide (NO), a gaseous master regulator of endothelial homeostasis. Moreover, Hcy deregulates the signaling pathways associated with another essential endothelial gasotransmitter: hydrogen sulfide. Hcy also mediates the loss of critical endothelial antioxidant systems and increases the intracellular concentration of reactive oxygen species (ROS) yielding oxidative stress. ROS disturb lipoprotein metabolism, contributing to the growth of atherosclerotic vascular lesions. Moreover, excess Hcy maybe be indirectly incorporated into proteins, a process referred to as protein N-homocysteinylation, inducing vascular damage. Lastly, cellular hypomethylation caused by build-up of S-adenosylhomocysteine (AdoHcy) also contributes to the molecular basis of Hcy-induced vascular toxicity, a mechanism that has merited our attention in particular. AdoHcy is the metabolic precursor of Hcy, which accumulates in the setting of HHcy and is a negative regulator of most cell methyltransferases. In this review, we examine the biosynthesis and catabolism of Hcy and critically revise recent findings linking disruption of this metabolism and endothelial dysfunction, emphasizing the impact of HHcy on endothelial cell methylation status.

Hyperhomocysteinemia (HHcy) exerts a wide range of biological effects and is associated with a number of diseases, including cardiovascular disease, dementia, neural tube defects, and cancer. Although mechanisms of HHcy toxicity are not fully uncovered, there has been a significant progress in their understanding. The picture emerging from the studies of homocysteine (Hcy) metabolism and pathophysiology is a complex one, as Hcy and its metabolites affect biomolecules and processes in a tissue- and sex-specific manner. Because of their connection to one carbon metabolism and editing mechanisms in protein biosynthesis, Hcy and its metabolites impair epigenetic control of gene expression mediated by DNA methylation, histone modifications, and non-coding RNA, which underlies the pathology of human disease. In this review we summarize the recent evidence showing that epigenetic dysregulation of gene expression, mediated by changes in DNA methylation and histone N-homocysteinylation, is a pathogenic consequence of HHcy in many human diseases. These findings provide new insights into the mechanisms of human disease induced by Hcy and its metabolites, and suggest therapeutic targets for the prevention and/or treatment.

Elevated plasma homocysteine (Hcy) level, known as hyperhomocysteinemia (HHcy) has been linked to different systemic and neurological diseases, well-known as a risk factor for systemic atherosclerosis and cardiovascular disease (CVD) and has been identified as a risk factor for several ocular disorders, such as diabetic retinopathy (DR) and age-related macular degeneration (AMD). Different mechanisms have been proposed to explain HHcy-induced visual dysfunction, including oxidative stress, upregulation of inflammatory mediators, retinal ganglion cell apoptosis, and extracellular matrix remodeling. Our previous studies using in vivo and in vitro models of HHcy have demonstrated that Hcy impairs the function of both inner and outer blood retinal barrier (BRB). Dysfunction of BRB is a hallmark of vision loss in DR and AMD. Our findings highlighted oxidative stress, ER stress, inflammation, and epigenetic modifications as possible mechanisms of HHcy-induced BRB dysfunction. In addition, we recently reported HHcy-induced brain inflammation as a mechanism of blood-brain barrier (BBB) dysfunction and pathogenesis of Alzheimer's disease (AD). Moreover, we are currently investigating the activation of glutamate receptor N-methyl-d-aspartate receptor (NMDAR) as the molecular mechanism for HHcy-induced BRB dysfunction. This review focuses on the studied effects of HHcy on BRB and the controversial role of HHcy in the pathogenesis of aging neurological diseases such as DR, AMD, and AD. We also highlight the possible mechanisms for such deleterious effects of HHcy.

Hyperhomocysteinemia is a risk factor of cardiovascular disease (CVD). High-density lipoprotein (HDL) plays an important role in anti-atherosclerosis, with its anti-atherogenic function attributed to HDL-associated proteins such as apolipoprotein A-I (apoA-I) and paraoxonase 1 (PON1). Homocysteine (Hcy) thiolactone modifies lysine residues in proteins, thereby altering their function. Although dysfunction of apoA-I and PON1 has been reported, the precise modification sites and underlying mechanisms have remained unclear. In this study, we aimed to identify Hcy-thiolactone modification sites on apoA-I and PON1. In addition, we sought to clarify the effects of Hcy-thiolactone on PON1 activity and its distribution. Modification sites were analyzed using MALDI-TOF MS. The effects of Hcy-thiolactone on various specimens, including purified proteins, reconstituted HDL (rHDL), HDL collected by ultracentrifugation, and serum samples, were characterized using enzymatic assays measuring three major PON1 activities (arylesterase, paraoxonase, and lactonase) and Western blotting. Our results demonstrated that while some Hcy-thiolactone modification sites were detected on apoA-I, PON1 itself was not directly modified by Hcy-thiolactone. Thiolactonase activity was reduced by Hcy-thiolactone in large HDL particles. Furthermore, a general reduction of PON1 activity and changes in HDL remodeling and distribution were observed in serum samples treated with Hcy-thiolactone. These findings suggest that PON1 dysfunction induced by Hcy-thiolactone is influenced by alterations in HDL remodeling and the enzyme's distribution on HDL particles. Analysis of PON1's distribution dynamics under pathological conditions may provide crucial insights into the mechanism of HDL function decline in CVD.