Homocysteine Stimulates Human T Cell Effector Cell

叶酸和维生素B12缺乏时，高同型半胱氨酸（Hcy）形成蛋氨酸的甲基化受损会导致高同型半胱氨酸血症。Hcy是氨基酸蛋氨酸的直接前体。在人类中，Hcy引起了极大的兴趣，因为它已被确定为几种与年龄相关的疾病状态的假定危险因素，包括动脉硬化、心肌梗死、外周动脉闭塞性疾病、皮质下血管性脑病、阿尔茨海默病和神经管缺陷。血浆Hcy升高也见于艾滋病、关节炎和多发性硬化症患者。叶酸和维生素B12缺乏也与免疫功能下降、骨髓造血祖细胞凋亡和外周循环中DNA低甲基化的白细胞的出现独立相关。这些观察结果的机制几乎完全是根据叶酸和维生素B12在基因组DNA甲基化中的专性作用来描述的。Hcy或其代谢物在这些过程中的具体作用尚未被描述。我们的初步结果显示，用无血清培养基和Hcy （250 uM至1000 uM）联合处理静息的人T细胞导致凋亡细胞死亡的剂量依赖性增加。在这方面，Hcy比Hcy硫内酯更有效，而s -腺苷型Hcy则无活性。这种hcy诱导的T细胞死亡不能通过叶酸或环己亚胺预处理来预防。然而，用人血清、半胱天冬酶抑制剂和聚adp核糖聚合酶抑制剂进行预处理可以部分或完全抑制这种作用。此外，hcy诱导的细胞凋亡可通过原钒酸盐预处理、细胞内钙螯合或抗cd3 /CD28抗体活化T细胞来消除。在含血清的培养基中，Hcy （250 μ m至1000 μ m）也会增强热休克诱导的T细胞死亡，但γ辐照（140 μ m至1400 μ m）则不会。最后，我们还发现Hcy在这些培养物中诱导Th1细胞因子的产生，并促进这些细胞活化诱导的细胞死亡。总之，这些结果表明Hcy可能是人类T细胞的促凋亡刺激物。

During folic acid and vitamin B12 deficiencies, hyperhomocystenemia occurs due to impaired methylation of homocysteine (Hcy) to form methionine. Hcy is the immediate precursor of the amino acid, methionine. In humans, Hcy is of great interest because it has been identified as a putative risk factor for several age-related disease states including arteriosclerosis, myocardial infarction, peripheral arterial occlusive disease, subcortical vascular encephelopathy, Alzheimer's disease and neural tube defects. Increased plasma Hcy has also been found in patients with AIDS, arthritis, and multiple sclerosis. Folic acid and vitamin B12 deficiency have also been independently associated with decreased immune function, the apoptosis of bone marrow hematopoietic progenitor cells and the appearance of leukocytes with hypomethylated DNA in the peripheral circulation. Mechanisms for these observations have been described almost exclusively in terms of the obligate role for folic acid and vitamin B12 in the methylation of genomic DNA. A specific role for Hcy or its metabolites in these processes has not been described. Our initial results reveal that treatment of resting human T cells with the combination of serum-free media and Hcy (250 uM to 1000 uM) resulted in a dose-dependent increase in apoptotic cell death. Hcy was more potent than Hcy thiolactone in this respect while S-adenosyl Hcy was inactive. This Hcy-induced T cell death could not be prevented by pretreatment with either folic acid or cycloheximide. However, this effect was either partially or fully inhibited by pretreatment with human serum, caspase inhibitors and poly-ADP-ribose polymerase inhibitors. In addition, Hcy-induced apoptosis was abrogated by pretreatment of T cells by orthovanadate, intracellular calcium chelation or cellular activation via anti-CD3/CD28 antibodies. Hcy (250 uM to 1000 uM) also potentiated T cell death induced by heat shock but not by gamma-irradiation (140 to 1400 rad) in serum-containing media. Finally, we have also found that Hcy induces Th1 cytokine production within these cultures and facilitates enhanced actvation-induced cell death of these cells. Together, these results suggest a possible role for Hcy as a pro-apototic stimulator of human T cells.