Role of Homocysteine in the Ischemic Stroke and Development of Ischemic Tolerance

Abstract

Homocysteine (Hcy) is a toxic, sulfur-containing intermediate of methionine metabolism. Hyperhomocysteinemia (hHcy), as a consequence of impaired Hcy metabolism or defects in crucial co-factors that participate in its recycling, is assumed as an independent human stroke risk factor. Neural cells are sensitive to prolonged hHcy treatment, because Hcy cannot be metabolized either by the transsulfuration pathway or by the folate/vitamin B12 independent remethylation pathway. Its detrimental effect after ischemia-induced damage includes accumulation of reactive oxygen species (ROS) and posttranslational modifications of proteins via homocysteinylation and thiolation. Ischemic preconditioning (IPC) is an adaptive response of the CNS to sub-lethal ischemia, which elevates tissues tolerance to subsequent ischemia. The main focus of this review is on the recent data on homocysteine metabolism and mechanisms of its neurotoxicity. In this context, the review documents an increased oxidative stress and functional modification of enzymes involved in redox balance in experimentally induced hyperhomocysteinemia. It also gives an interpretation whether hyperhomocysteinemia alone or in combination with IPC affects the ischemia-induced neurodegenerative changes as well as intracellular signaling. Studies document that hHcy alone significantly increased Fluoro-Jade C- and TUNEL-positive cell neurodegeneration in the rat hippocampus as well as in the cortex. IPC, even if combined with hHcy, could still preserve the neuronal tissue from the lethal ischemic effects. This review also describes the changes in the mitogen-activated protein kinase (MAPK) protein pathways following ischemic injury and IPC. These studies provide evidence for the interplay and tight integration between ERK and p38 MAPK signaling mechanisms in response to the hHcy and also in association of hHcy with ischemia/IPC challenge in the rat brain. Further investigations of the protective factors leading to ischemic tolerance and recognition of the co-morbid risk factors would result in development of new avenues for exploration of novel therapeutics against ischemia and stroke.

Keywords: MAP kinases; brain; hyperhomocysteinemia; intracellular signaling; ischemic preconditioning; neurodegeneration.

Figure 1

Schematic pathways of homocysteine toxicity leading to brain ischemic stroke. SAM-S—Adenosyl Methionine, SAH-S—S -Adenosyl Homocysteine, hHcy—hyperhomocysteinemia. High dietary intake of diet rich in methionine and deficiency of vitamine B₆, B₁₂ and folate lead to hyperhomocysteinemia in predisposed individuals. A prolonged elevated level of homocysteine initiates complex processes which include oxidative stress, protein homocysteinylation and Ca²⁺ dysregulation. These events in parallel with epigenetic changes can culminate in apoptosis, neuronal death and blood-brain barrier dysregulation manifested as ischemic stroke (Kalani et al., ; Petras et al., ; Kovalska et al., ; Lehotsky et al., ; Škovierová et al., 2015). Adapted from Lehotsky et al. (2015).

Figure 2

Schematic overview of homocysteine metabolism and the role of dietary vitamins folate and vitamin B₆. Dietary methionine acts as a methyl donor via conversion of S-Adenosyl Methionine (SAM) to S-Adenosyl Homocysteine (SAH). SAH is converted to homocysteine by releasing adenosine. Methionine is directly converted to homocysteine in the presence of Methyl tetrahydrofolic acid (THF) and vitamin B₆. Homocysteine is converted in error editing reaction to homocysteine thiolactone. Conversion of homocysteine to cysteine requires vitamin B₆ (Kalani et al., ; Petras et al., ; Lehotsky et al., 2015). Adapted from Lehotsky et al. (2015).

Figure 3

Proposed mechanisms leading to homocysteine neurotoxicity and the protection induced by ischemic preconditioning (IPC) in hyperhomocysteinemic conditions (hHCy). (+): increased number of cells and/or activity, (−): decreased number of cells and/or activity. Homocysteine induced neurotoxicity includes dysregulation in redox balance, lipoperoxidation and protein oxidation, Ca²⁺ pump dysfunction and activation of MAPKp38 which is detected in vulnerable cells by increased Fluoro-Jade C staining (+) and TUNEL positive cells (+). Ischemic preconditioning suppresses oxidative dysregulation and activates MAPK-ERK which leads to reduced Fluoro Jade C and TUNEL positivity in sensitive cells (Pavlikova et al., ; Petras et al., ; Kovalska et al., ; Lehotsky et al., ; Škovierová et al., 2015). Adapted from Lehotsky et al. (2015).