Martínez-Chantar María L, Vázquez-Chantada Mercedes, Garnacho Marta, Latasa M Ujue, Varela-Rey Marta, Dotor Javier, Santamaria Monica, Martínez-Cruz Luis A, Parada Luis A, Lu Shelly C, Mato José M
CIC bioGUNE, Technological Park of Bizkaia, Bizkaia, Spain.
Gastroenterology. 2006 Jul;131(1):223-32. doi: 10.1053/j.gastro.2006.04.019.
BACKGROUND & AIMS: After liver injury, hepatic S-adenosylmethionine (SAM) content decreases, and the blockage this molecule imposes on hepatocyte proliferation is released, facilitating liver regeneration. This activity of SAM is important for normal liver function because mice deficient in hepatic SAM display abnormal liver regeneration and develop hepatocellular carcinoma. How SAM regulates hepatocyte growth is unclear, but because SAM blocks hepatocyte growth factor (HGF)-induced cyclin D1 expression and DNA synthesis without affecting HGF-induced extracellular signal-regulated kinase phosphorylation, the mitogen-activated protein kinase (MAPK) pathway is probably not the target.
The effects of SAM on AMPK, HuR localization were assessed in rat hepatocytes after HGF, AICAR, and SAM treatment.
We show here that HGF and 5-aminoimidazole-4-carboxamide-riboside (AICAR), an activator of AMP-activated protein kinase (AMPK), induce the phosphorylation of AMPK in hepatocytes and that SAM blocks this process. We also show that HGF- and AICAR-induced AMPK activation stimulate the transport from nucleus to cytoplasm of HuR, an RNA-binding protein that increases the half-life of target mRNA such as cyclin A2, and that SAM blocks this process. We found that, in hepatocytes, AICAR increases HuR binding to cyclin A2 messenger RNA (mRNA) as well as the expression and stability of this mRNA and that SAM blocks these events. Consistently, we found that AICAR induces hepatocyte proliferation and that SAM blocks this effect. Finally, we found that liver AMPK phosphorylation, cytoplasmic HuR, and binding of HuR to HuR-target mRNA and the steady-state levels of these mRNA are increased in knockout mice deficient in hepatic SAM.
Our results yield novel insights about the mechanism by which SAM inhibits cell-cycle progression in the liver.
肝损伤后，肝脏S-腺苷甲硫氨酸（SAM）含量降低，该分子对肝细胞增殖的阻滞作用解除，从而促进肝脏再生。SAM的这一活性对正常肝功能很重要，因为肝脏SAM缺乏的小鼠表现出肝脏再生异常并发展为肝细胞癌。SAM如何调节肝细胞生长尚不清楚，但由于SAM可阻断肝细胞生长因子（HGF）诱导的细胞周期蛋白D1表达和DNA合成，而不影响HGF诱导的细胞外信号调节激酶磷酸化，因此丝裂原活化蛋白激酶（MAPK）途径可能不是其作用靶点。
在HGF、AICAR和SAM处理后的大鼠肝细胞中评估SAM对AMPK、HuR定位的影响。
我们在此表明，HGF和5-氨基咪唑-4-甲酰胺核苷（AICAR），一种AMP活化蛋白激酶（AMPK）的激活剂，可诱导肝细胞中AMPK的磷酸化，而SAM可阻断这一过程。我们还表明，HGF和AICAR诱导的AMPK激活会刺激HuR从细胞核转运至细胞质，HuR是一种RNA结合蛋白，可增加细胞周期蛋白A2等靶mRNA的半衰期，而SAM可阻断这一过程。我们发现，在肝细胞中，AICAR可增加HuR与细胞周期蛋白A2信使RNA（mRNA）的结合以及该mRNA的表达和稳定性，而SAM可阻断这些事件。一致地，我们发现AICAR可诱导肝细胞增殖，而SAM可阻断这一作用。最后，我们发现肝脏SAM缺乏的基因敲除小鼠肝脏中AMPK磷酸化、细胞质HuR、HuR与HuR靶mRNA的结合以及这些mRNA的稳态水平均升高。
我们的结果为SAM抑制肝脏细胞周期进程的机制提供了新的见解。
