Wattenberg E V, McNeil P L, Fujiki H, Rosner M R
Department of Applied Biological Sciences, Massachusetts Institute of Technology, Cambridge 02139.
J Biol Chem. 1989 Jan 5;264(1):213-9.
Palytoxin, a non-12-O-tetradecanoylphorbol-13-acetate type tumor promoter, has been shown to inhibit epidermal growth factor (EGF) binding to both high and low affinity receptors through a protein kinase C-independent pathway. In the present paper, we have investigated the mechanism of palytoxin action in Swiss 3T3 cells. Two lines of evidence indicate that calcium is not required for palytoxin activity. First, palytoxin can induce the loss of EGF binding sites in the absence of external calcium. Second, studies with the photosensitive protein aequorin indicate that palytoxin does not cause the influx of external calcium or the release of calcium from internal stores under the conditions used in these studies. However, palytoxin action does appear to be dependent upon the presence of sodium. When extracellular sodium is replaced by either choline, Tris, or sucrose, palytoxin is unable to decrease EGF binding to either high or low affinity receptors. Studies of sodium influx indicate that palytoxin induces rapid sodium uptake and that the rate of sodium uptake is dose-dependent. Furthermore, there appears to be a direct correspondence between the extent of inhibition of EGF binding by palytoxin and the rate of sodium uptake. Finally, the palytoxin-induced inhibition of EGF binding can be mimicked by monensin, a sodium ionophore. The specificity of this sodium dependence was tested by substituting lithium, potassium, or cesium for sodium. Although lithium is an effective substitute for sodium, palytoxin can no longer inhibit EGF binding when sodium is replaced by either potassium or cesium. Marked inhibition of palytoxin action is also obtained when 5.4 mM potassium or 5.4 mM cesium are added to the sodium-containing medium. These studies suggest that palytoxin is able to down-modulate the EGF receptor through a novel mechanism involving the activation or formation of a sodium pump or channel.
刺尾鱼毒素是一种非12 - O - 十四烷酰佛波醇 - 13 - 乙酸酯型肿瘤促进剂，已表明它可通过一条不依赖蛋白激酶C的途径抑制表皮生长因子（EGF）与高亲和力和低亲和力受体的结合。在本文中，我们研究了刺尾鱼毒素在瑞士3T3细胞中的作用机制。有两条证据表明刺尾鱼毒素的活性不需要钙。第一，在没有细胞外钙的情况下，刺尾鱼毒素能诱导EGF结合位点的丧失。第二，用光敏蛋白水母发光蛋白进行的研究表明，在这些研究使用的条件下，刺尾鱼毒素不会引起细胞外钙的内流或细胞内钙库中钙的释放。然而，刺尾鱼毒素的作用似乎确实依赖于钠的存在。当细胞外钠被胆碱、Tris或蔗糖取代时，刺尾鱼毒素无法降低EGF与高亲和力或低亲和力受体的结合。钠内流的研究表明，刺尾鱼毒素诱导快速的钠摄取，且钠摄取速率呈剂量依赖性。此外，刺尾鱼毒素对EGF结合的抑制程度与钠摄取速率之间似乎存在直接对应关系。最后，刺尾鱼毒素诱导的EGF结合抑制可被离子载体莫能菌素模拟。通过用锂、钾或铯替代钠来测试这种对钠的依赖性的特异性。尽管锂是钠的有效替代物，但当钠被钾或铯取代时，刺尾鱼毒素不再能抑制EGF结合。当向含钠培养基中添加5.4 mM钾或5.4 mM铯时，也能显著抑制刺尾鱼毒素的作用。这些研究表明，刺尾鱼毒素能够通过一种涉及钠泵或通道的激活或形成的新机制下调EGF受体。
