Huang Yi-Fang, Wang Yanxin, Watford Malcolm
Department of Nutritional Sciences, Cook College, Rutgers University, New Brunswick, NJ 08901, USA.
J Nutr. 2007 Jun;137(6):1357-62. doi: 10.1093/jn/137.6.1357.
This study examined the regulation of glutamine synthetase protein levels, in response to changes in external glutamine concentration, in mouse C2C12 skeletal muscle cells. Glutamine, at concentrations as low as 0.25 mmol/L, downregulated endogenous and exogenous (plasmid encoded) glutamine synthetase with maximal effect at 2 mmol/L. Glutamine appears to act by changing the stability of the glutamine synthetase protein, and the effect was partially blocked by the proteasome inhibitor MG132. The addition of the glutamine structural analog and glutaminase inhibitor, 6-diazo-5-oxo-L-norleucine, in the presence or absence of glutamine, also resulted in low glutamine synthetase protein levels. Otherwise, the effect was specific for glutamine, and the only compounds able to mimic the effect of glutamine were amino acids, glutamate, alanine, and ornithine, which can be converted to glutamine. Other amino acids, analogs, and products of glutamine metabolism were without effect. Methionine sulfoximine, an inhibitor of glutamine synthetase, stabilized the protein and prevented the glutamine effect. Thus, in mouse C2C12 skeletal muscle cells, glutamine synthetase protein expression is regulated by glutamine through changes in the rate of degradation of the protein. The effect is specific to glutamine, which acts directly without requiring prior metabolism.
本研究检测了小鼠C2C12骨骼肌细胞中谷氨酰胺合成酶蛋白水平对细胞外谷氨酰胺浓度变化的响应。谷氨酰胺浓度低至0.25 mmol/L时即可下调内源性和外源性（质粒编码）谷氨酰胺合成酶，在2 mmol/L时作用最强。谷氨酰胺似乎通过改变谷氨酰胺合成酶蛋白的稳定性发挥作用，蛋白酶体抑制剂MG132可部分阻断该效应。无论有无谷氨酰胺存在，添加谷氨酰胺结构类似物及谷氨酰胺酶抑制剂6-重氮-5-氧代-L-正亮氨酸也会导致谷氨酰胺合成酶蛋白水平降低。此外，该效应具有谷氨酰胺特异性，唯一能模拟谷氨酰胺效应的化合物是可转化为谷氨酰胺的氨基酸、谷氨酸、丙氨酸和鸟氨酸。其他氨基酸、类似物及谷氨酰胺代谢产物均无此作用。谷氨酰胺合成酶抑制剂甲硫氨酸亚砜胺可使该蛋白稳定并阻止谷氨酰胺的作用。因此，在小鼠C2C12骨骼肌细胞中，谷氨酰胺通过改变谷氨酰胺合成酶蛋白的降解速率来调节其蛋白表达。该效应具有谷氨酰胺特异性，谷氨酰胺直接发挥作用，无需预先代谢。
