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.

Increased levels of homocysteine have been established as a risk factor for cardiovascular disease (CVD) by mechanisms still incompletely defined. S-Adenosylhomocysteine (SAH) is the metabolic precursor of homocysteine that accumulates in the setting of hyperhomocysteinemia and is a negative regulator of most cell methyltransferases. Several observations, summarized in the current review, support the concept that SAH, rather than homocysteine, may be the culprit in the CVD risk that has been associated with hyperhomocysteinemia. This review examines the biosynthesis and catabolism of homocysteine and how these pathways regulate accumulation of SAH. In addition, the epidemiological and experimental links between hyperhomocysteinemia and CVD are discussed, along with the evidence suggesting a role for SAH in the disease. Finally, the effects of SAH on the hypomethylation of DNA, RNA, and protein are examined, with an emphasis on how specific molecular targets may be mediators of homocysteine-associated vascular disease.

Background: Methylation of tRNASec facilitates the incorporation of selenocysteine at a UGA codon during translation. Results: Accumulation of the homocysteine precursor S-adenosylhomocysteine decreases tRNA Sec methylation, reducing glutathione peroxidase 1 expression and increasing oxidative stress-induced inflammatory activation of endothelial cells. Conclusion: Methylation modulates the expression of selenoproteins to regulate redox-dependent inflammatory pathways. Significance: Hypomethylation stress promotes a proatherogenic endothelial cell phenotype.

S-Adenosylhomocysteine (SAH) can induce endothelial dysfunction and activation, contributing to atherogenesis; however, its role in the activation of the inflammatory mediator NFkB has not been explored. Our aim was to determine the role of NFkB in SAH-induced activation of endothelial cells. Furthermore, we examined whether SAH, as a potent inhibitor of S-adenosylmethionine-dependent methyltransferases, suppresses the function of EZH2 methyltransferase to contribute to SAH-induced endothelial cell activation. We found that excess SAH increases the expression of adhesion molecules and cytokines in human coronary artery endothelial cells. Importantly, this up-regulation was suppressed in cells expressing a dominant negative form of the NFkB inhibitor, IkB. Moreover, SAH accumulation triggers the activation of both the canonical and non-canonical NFkB pathways, decreases EZH2, and reduces histone 3 lysine 27 trimethylation. EZH2 knockdown recapitulated the effects of excess SAH on endothelial activation, i.e., it induced NFkB activation and the subsequent up-regulation of adhesion molecules and cytokines. Our findings suggest that suppression of the epigenetic regulator EZH2 by excess SAH may contribute to NFkB activation and the consequent vascular inflammatory response. These studies unveil new targets of SAH regulation, demonstrating that EZH2 suppression and NFkB activation mediated by SAH accumulation may contribute to its adverse effects in the vasculature.

Bone marrow stromal cells (BMSC) transplanted into the contused spinal cord may support repair by improving tissue sparing. We injected allogeneic BMSC into the moderately contused adult rat thoracic spinal cord at 15 min (acute) and at 3, 7, and 21 days (delayed) post-injury and quantified tissue sparing and BMSC survival up to 4 weeks post-transplantation. BMSC survival within the contusion at 7 days post-transplantation was significantly higher with an acute injection (32%) and 3-day delayed injection (52%) than with a 7- or 21-day delayed injection (9% both; p < 0.01). BMSC survival at 28 days post-transplantation was close to 0 in all paradigms, indicating rejection. In contused rats without a BMSC transplant (controls), the volume of spared tissue gradually decreased until 46% (p < 0.001) of the volume of a comparable uninjured spinal cord segment at 49 days post-injury. In rats with BMSC, injected at 15 min, 3, or 7 days post-injury, spared tissue volume was significantly higher in grafted rats than in control rats at the respective endpoints (i.e., 28, 31, and 35 days post-injury). Acute and 3-day delayed but not 7- and 21-day delayed injection of BMSC significantly improved tissue sparing, which was strongly correlated (r = 0.79-1.0) to BMSC survival in the first week after injection into the contusion. Our data showed that neuroprotective effects of BMSC transplanted into a moderate rat spinal cord contusion depend strongly on their survival during the first week post-injection. Acutely injected BMSC elicit more tissue sparing than delayed injected BMSC.

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Natural laminin matrices are formed on cell membranes by a cooperative process involving laminin self-polymerization and binding to cognate cellular receptors. In a cell-free system, laminin can self-polymerize, given that a minimal critical concentration is achieved. We have previously described that pH acidification renders self-polymerization independent of protein concentration. Here we studied the ultrastructure of acid-induced laminin polymers using electron and atomic force microscopies. Polymers presented the overall appearance of natural matrices and could be described as homogeneous polygonal sheets, presenting struts of 21 ؎ 5 and 86 ؎ 3 nm of height, which approximately correspond to the sizes of the short and the long arms of the molecule, respectively. The addition of fragment E3 (the distal two domains of the long arm) did not affect the polymerization in solution nor the formation of adsorbed matrices. On the other hand, the addition of fragment E1, which contains two intact short arms, completely disrupted polymerization. These results indicate that acid-induced polymers, like natural ones, involve only interactions between the short arms. The electrostatic surface map of laminin ␣1 LG4 -5 shows that acidification renders the distal end in the long arms exclusively positive, precluding homophylic interactions between them. Therefore, acidification reproduces in vitro, and at a physiological protein concentration, what receptor interaction does in the cellular context, namely, it prevents the long arm from disturbing formation of the homogeneous matrix involving the short arms only. We propose that acid-induced polymers are the best tool to study cellular response to laminin in the future.