Kapus A, Grinstein S, Wasan S, Kandasamy R, Orlowski J
Division of Cell Biology, Hospital for Sick Children, Toronto, Ontario, Canada.
J Biol Chem. 1994 Sep 23;269(38):23544-52.
Four distinct isoforms of the mammalian Na+/H+ exchanger (NHE) have been identified by molecular cloning. Three of these (NHE-1, NHE-2, and NHE-3) have been shown to be functionally active by heterologous expression. Their kinetic and pharmacological properties are well documented, yet comparatively little is known about their regulation. In this report, rat NHE-1, NHE-2, and NHE-3 were stably transfected into antiporter-deficient Chinese hamster ovary cells to study their role in cellular proliferation and their regulation by nucleotides and cell volume. Their ability to influence cell proliferation was assessed by measuring the growth of antiporter-deficient cells and of the different transfectants in media of varying pH. While antiporter-deficient cells were unable to grow at acidic pH levels, all three isoforms supported proliferation under these conditions. Therefore, while the epithelia-specific isoforms (NHE-2 and NHE-3) are thought to play primarily a role in transcellular ion transport, they can also contribute to intracellular pH (pHi) homeostasis and have a permissive role in cell growth. The activity of the three isoforms was markedly inhibited by depletion of cellular ATP. In the pHi 6.0-7.2 range, decreases in the affinity for internal H+ and/or the maximal rate of transport accounted for the inhibitory effect, depending on the isoform. The osmotic responsiveness of the three isoforms was also compared. As reported earlier, NHE-1 was stimulated by hypertonicity. Under similar conditions, NHE-2 was also stimulated to a comparable extent. Conversely, both isoforms were inhibited in hypotonic media. In contrast, NHE-3 was markedly inhibited by hypertonic cell shrinking but was unaffected by hypotonicity. Osmotic inhibition of NHE-3 was rapid, reversible, and apparent throughout the pH range studied. Osmotic inhibition of NHE-3 may play a role in the physiology and pathophysiology of epithelia.
通过分子克隆已鉴定出哺乳动物钠氢交换体（NHE）的四种不同亚型。其中三种（NHE-1、NHE-2和NHE-3）已通过异源表达显示具有功能活性。它们的动力学和药理学特性已有充分记录，但对其调节的了解相对较少。在本报告中，将大鼠NHE-1、NHE-2和NHE-3稳定转染到缺乏反向转运体的中国仓鼠卵巢细胞中，以研究它们在细胞增殖中的作用以及核苷酸和细胞体积对它们的调节。通过测量缺乏反向转运体的细胞以及不同转染体在不同pH值培养基中的生长情况，评估它们影响细胞增殖的能力。虽然缺乏反向转运体的细胞在酸性pH水平下无法生长，但在这些条件下所有三种亚型都支持细胞增殖。因此，虽然上皮细胞特异性亚型（NHE-2和NHE-3）主要被认为在跨细胞离子转运中起作用，但它们也有助于细胞内pH（pHi）稳态，并在细胞生长中起允许作用。细胞ATP耗竭会显著抑制这三种亚型的活性。在pHi 6.0 - 7.2范围内，根据亚型不同，对胞内H⁺亲和力的降低和/或转运最大速率的降低导致了抑制作用。还比较了这三种亚型的渗透反应性。如先前报道，NHE-1受到高渗刺激。在类似条件下，NHE-2也受到类似程度的刺激。相反，在低渗培养基中这两种亚型均受到抑制。相比之下，NHE-3受到高渗细胞收缩的显著抑制，但不受低渗影响。NHE-3的渗透抑制作用迅速、可逆，且在所研究的整个pH范围内都很明显。NHE-3的渗透抑制作用可能在上皮细胞的生理和病理生理中起作用。
