Department of Clinical Pharmacology, College of Basic Medical Sciences, China Medical University, Shenyang, PR China.
Neurochem Int. 2012 Jul;61(2):187-94. doi: 10.1016/j.neuint.2012.04.010. Epub 2012 Apr 27.
myo-Inositol is important for cell signaling both in cytoplasm and in intracellular organelles. It is required in the plasma membrane and cytoplasm for maintained synthesis of the second messengers, inositoltrisphosphate (IP(3)) and diacylglycerol (DAG) from phosphatidylinositol bisphosphate (PIP(2)), and in organelles as precursor for synthesis of complex signaling phospholipids and inositolphosphates from IP(3) and PIP(2). myo-Inositol must be taken up into the cell where its is used, because neither neurons nor astrocytes synthesize it. It is also an osmolyte, taken up in response to surrounding hyperosmolarity and released during hypo-osmolarity. There are three myo-inositol transporters, the Na(+)-dependent SMIT1 and SMIT2, and HMIT, which co-transports myo-inositol with H(+). Their relative expressions in astrocytes and neurons are unknown. Uptake kinetics for myo-inositol in astrocytes has repeatedly been determined, but always on the assumption of only one component, leaving kinetics for the individual transporters unknown. This paper demonstrates that astrocytes obtained directly from the brain express SMIT1 and HMIT, but little SMIT2, and that all three transporters are expressed in neurons. Cultured mouse astrocytes show a high-affinity/low-capacity myo-inositol uptake (V(max): 60.0 ± 3.0 pmol/min per mg protein; K(m): 16.7 ± 2.6 μM), mediated by SMIT1 and perhaps partly by SMIT2. It was determined in cells pre-treated with HMIT-siRNA and confirmed by specific inhibition of SMIT. However at physiologically relevant myo-inositol concentrations most uptake is by a lower-affinity/higher-capacity uptake, mediated by HMIT (V(max): 358 ± 60 pmol/min per mg protein; K(m): 143 ± 36 μM) and determined by subtraction of SMIT-mediated from total uptake. At high myo-inositol concentrations, its uptake is inhibited by incubation in medium with increased pH, and increased during intracellular acidification with NH(4)Cl. This is in agreement with literature data for HMIT alone. At low concentration, where SMIT1/2 activity gains importance, myo-inositol uptake is reduced by ammonia-induced intracellular acidification, consistent with the transporter's pH sensitivity reported in the literature.
肌醇对于细胞质和细胞内细胞器中的细胞信号转导都很重要。它在质膜和细胞质中对于从磷脂酰肌醇双磷酸（PIP(2)）合成第二信使肌醇三磷酸（IP(3)）和二酰基甘油（DAG）是必需的，在细胞器中作为合成复杂信号磷脂和从 IP(3)和 PIP(2)合成肌醇磷酸的前体是必需的。肌醇必须被细胞摄取，因为神经元和星形胶质细胞都不能合成它。它也是一种渗透调节剂，在周围高渗时被摄取，在低渗时被释放。有三种肌醇转运蛋白，Na(+)-依赖性 SMIT1 和 SMIT2，以及 HMIT，它们与 H(+)共转运肌醇。它们在星形胶质细胞和神经元中的相对表达情况尚不清楚。星形胶质细胞中肌醇摄取的动力学已被多次确定，但总是假设只有一个组成部分，而各个转运蛋白的动力学则未知。本文证明，直接从大脑获得的星形胶质细胞表达 SMIT1 和 HMIT，但表达很少的 SMIT2，而这三种转运蛋白都在神经元中表达。培养的小鼠星形胶质细胞表现出高亲和力/低容量的肌醇摄取（V(max)：60.0 ± 3.0 pmol/min per mg 蛋白；K(m)：16.7 ± 2.6 μM），由 SMIT1 和可能部分由 SMIT2 介导。在先用 HMIT-siRNA 预处理的细胞中进行了测定，并通过 SMIT 的特异性抑制得到了证实。然而，在生理相关的肌醇浓度下，大部分摄取是通过低亲和力/高容量摄取介导的，由 HMIT 介导（V(max)：358 ± 60 pmol/min per mg 蛋白；K(m)：143 ± 36 μM），通过从总摄取中减去 SMIT 介导的摄取来确定。在高肌醇浓度下，其摄取会被在 pH 升高的培养基中孵育所抑制，并在细胞内酸化用氯化铵时增加。这与文献中单独针对 HMIT 的数据一致。在低浓度下，SMIT1/2 活性变得重要时，肌醇摄取会被氨诱导的细胞内酸化所减少，这与文献中报道的转运蛋白的 pH 敏感性一致。
