Homocysteine Stimulates Human T Cell Effector Cell

在叶酸和维生素B12缺乏期间，由于同型半胱氨酸(Hcy)甲基化以形成蛋氨酸，会发生高同型半胱氨酸血症。同型半胱氨酸是蛋氨酸的直接前体。在人类中，Hcy引起了极大的兴趣，因为它被认为是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、外周动脉闭塞症、皮质下血管性脑病、阿尔茨海默病和神经管缺陷。在艾滋病、关节炎和多发性硬化症患者中也发现血浆同型半胱氨酸升高。叶酸和维生素B12缺乏也独立地与免疫功能下降、骨髓造血祖细胞凋亡和外周循环中出现带有低甲基化DNA的白细胞有关。这些观察的机制几乎完全是根据叶酸和维生素B12在基因组DNA甲基化中的专有作用来描述的。同型半胱氨酸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，用无血清培养液和Hcy(250微米至1000微米)联合处理静息的人T细胞，可导致剂量依赖性的细胞凋亡增加。同型半胱氨酸在这方面比同型半胱氨酸硫内酯更有效，而S-腺苷同型半胱氨酸活性不强。这种同型半胱氨酸诱导的T细胞死亡不能被叶酸或放线菌亚胺预处理所阻止。然而，人血清、半胱氨酸天冬氨酸氨基转移酶抑制剂和多聚ADP-核糖聚合酶抑制剂可部分或完全抑制这种作用。此外，T细胞经原钒酸盐预处理、细胞内钙离子螯合或抗CD3/CD28抗体激活细胞后，Hcy诱导的细胞凋亡也被取消。在含血清的培养液中，Hcy(250微米至1000微米)也可增强热休克所致的T细胞死亡，但不能增强伽玛射线(140至1400rad)所致的T细胞死亡。最后，我们还发现，同型半胱氨酸在这些培养物中诱导Th1细胞因子的产生，并促进这些细胞的增强激活诱导的细胞死亡。综上所述，这些结果提示同型半胱氨酸可能是人类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.