Inhibition of human betaine-homocysteine methyltransferase expression by S-adenosylmethionline and methylthioadenosine
Inhibition of human betaine-homocysteine methyltransferase expression by S-adenosylmethionline and methylthioadenosine
复制标题
DOI：
10.1042/bj20061119
复制
发表时间：
2007-01-01
影响因子：
4.1
通讯作者：
Lu, Shelly C.
中科院分区：
文献类型：
--
作者：
Ou, Xiaopeng;Yang, Heping;Lu, Shelly C.
BHMT（甜菜碱-同型半胱氨酸甲基转移酶）将同型半胱氨酸再甲基化形成蛋氨酸。SAM（S-腺苷三硫代磷酸）抑制BHMT活性，但SAM是否调节BHMT基因表达尚不清楚。人类BHMT的转录调控也是未知的。本研究检测了SAM及其代谢产物MTA（5 '-甲硫基腺苷）对人BHMT基因的调控。为了便于这些研究，我们克隆了人BHMT基因的2.7kb 5 ′-侧翼区（GenBank登录号AY 325901）。SAM和MTA处理的HepG 2细胞导致BHMT mRNA水平的剂量和时间依赖性降低，这证实了它们对BHMT启动子活性的影响。用BHMT启动子构建体-347/+33观察到最大抑制。其含有许多NF-κ B（核因子κ B）结合位点。SAM和MTA处理增加NF-κ B核结合和NF-κ B驱动的荧光素酶活性，并增加多个组蛋白脱乙酰酶辅阻遏物与NF-κ B位点的核结合活性。p50和p65的过表达降低了BHMT启动子的活性，而阻断NF-κ B的激活增加了BHMT的表达和启动子的活性，并阻止了SAM而不是MTA抑制BHMT表达的能力。NF-κ B B在-301位点的结合位点至少部分地负责这种效应。低BHMT表达可损害同型半胱氨酸代谢，这可诱导ER（内质网）应激。事实上，NITA处理导致ER应激标志物表达增加。总之，SAM和MTA下调BHMT在HepG 2细胞中的表达，部分是通过诱导NF-κ B，其作为人BHMT基因的阻遏物。虽然SAM的机制是NF-κ B依赖性的，MTA具有NF-κ B依赖性和非依赖性机制。
BHMT (betaine-homocysteine methyltransferase) remethylates homocysteine to form methionine. SAM (S-adenosyltnethionine) inhibits BHMT activity, but whether SAM modulates BHMT gene expression is unknown. Transcriptional regulation of the human BHMT is also unknown. The present study examined regulation of the human BHMT gene by SAM and its metabolite, MTA (5'-methylthioadenosine). To facilitate these studies, we cloned the 2.7 kb 5'-flanking region of the human BHMT gene (GenBank (R) accession number AY325901). Both SAM and MTA treatment of HepG2 cells resulted in a dose- and time-dependent decrease in BHMT mRNA levels, which paralleled their effects on the BHMT promoter activity. Maximal suppression was observed with the BHMT promoter construct -347/+33. which contains a number of NF-kappa B (nuclear factor kappa B) binding sites. SAM and MTA treatment increased NF-kappa B nuclear binding and NF-kappa B-driven luciferase activities, and increased nuclear binding activity of multiple histone deacetylase co-repressors to the NF-kappa B sites. Overexpression of p50 and p65 decreased BHMT promoter activity, while blocking NF-kappa B activation increased BHMT expression and promoter activity, and prevented SAM but not MTA's ability to inhibit BHMT expression. The NF-kappa B binding site at -301 is responsible, at least in part, for this effect. Lower BHMT expression can impair homocysteine metabolism, which can induce ER (endoplasmic reticulum) stress. Indeed, NITA treatment resulted in increased expression ER stress markers. In conclusion, SAM and MTA down-regulate BHMT expression in HepG2 cells in part by inducing NF-kappa B, which acts as a repressor for the human BHMT gene. While SAM's mechanism is NF-kappa B-dependent, MTA has both NF-kappa B-dependent and -independent mechanisms.