SN1, a system N amino acid transporter specific for astrocytes, is mainly responsible for export of newly synthesized L-glutamine from the cells. Astrocytic retention of L-glutamine which plays a critical role in ammonia-induced astrocytic swelling resulting in brain edema, could be tentatively attributed to the impaired L-glutamine export from astrocytes. The present study demonstrates that treatment of cultured mouse cortical astrocytes for 24 h with 5 mM ammonium chloride (“ammonia”) inhibits the system N-mediated L-glutamine transport out of the cell, and that this inhibition is related to the reduced presence of the SN1 transporter on the cell membrane. Ammonia decreased total protein kinase C (PKC) activity in the absence but not in the presence of PKC activator, phorbol 12-myristate 13-acetate (PMA), and activation of PKC by PMA reversed both the ammonia-induced decrease of system N-mediated L-glutamine release and ammonia-induced SN1 deficit in the membrane fraction. However, while ammonia did not change the protein level of PKCα isoform, it decreased the protein content of PKCδ. Moreover, ammonia treatment increased the cell surface expression of SN1 in cells with silenced PKCα and PKCδ. Silencing of PKCδ abrogated the decrease of system N (SN1)-mediated L-glutamine release by ammonia. The results implicate the involvement of PKCδ in the inhibition of SN1 membrane expression and activity by ammonia.

SN1是星形胶质细胞特有的系统N氨基酸转运蛋白，主要负责从细胞中输出新合成的L-谷氨酰胺。L-谷氨酰胺的星形胶质细胞滞留在氨引起的星形细胞肿胀导致脑水肿中起关键作用，可以暂时归因于星形胶质细胞的L-谷氨酰胺输出受损。本研究表明，用5 mM氯化铵（“氨水”）处理培养的小鼠皮质星形胶质细胞24小时可抑制系统N介导的L-谷氨酰胺转运出细胞，并且这种抑制作用与减少的N-谷氨酰胺存在有关。 SN1转运蛋白在细胞膜上。在不存在但不存在PKC激活剂，佛波12-肉豆蔻酸酯13-乙酸酯（PMA）的情况下，氨会降低总蛋白激酶C（PKC）活性，和PMA激活PKC既可以逆转氨诱导的系统N介导的L-谷氨酰胺释放的减少，又可以逆转氨诱导的膜部分中SN1的缺乏。然而，尽管氨并没有改变PKCα同工型的蛋白质水平，但是却降低了PKCδ的蛋白质含量。此外，氨处理增加了PKCα和PKCδ沉默的细胞中SN1的细胞表面表达。PKCδ沉默消除了系统N（SN1）介导的L-谷氨酰胺释放的减少。该结果暗示PKCδ参与氨对SN1膜表达和活性的抑制。此外，氨处理增加了PKCα和PKCδ沉默的细胞中SN1的细胞表面表达。PKCδ沉默消除了系统N（SN1）介导的L-谷氨酰胺释放的减少。该结果暗示PKCδ参与氨对SN1膜表达和活性的抑制。此外，氨处理增加了PKCα和PKCδ沉默的细胞中SN1的细胞表面表达。PKCδ沉默消除了系统N（SN1）介导的L-谷氨酰胺释放的减少。该结果暗示PKCδ参与氨对SN1膜表达和活性的抑制。