Pillai S, Bikle D D
Department of Medicine, University of California, San Francisco.
J Cell Physiol. 1992 Jun;151(3):623-9. doi: 10.1002/jcp.1041510323.
Trivalent cation lanthanum (La) binds to calcium binding sites of cells and either mimics the properties of calcium or inhibits the effects of calcium by displacing calcium from its binding sites. Extracellular calcium induces differentiation of human epidermal keratinocytes in culture, in part by increasing the intracellular calcium levels (Cai). Therefore, in this study we determined the effect of La on differentiation and intracellular calcium levels of keratinocytes. We observed that La inhibited the production of cornified envelopes, a marker for terminal differentiation of keratinocytes. La inhibited the calcium requiring envelope cross-linking enzyme, transglutaminase, in a direct manner, presumably, by displacing calcium from its binding site on the enzyme. La inhibited the influx and the efflux of 45Ca from keratinocytes. Paradoxically, extracellular La appeared to increase the Cai levels of keratinocytes as measured by the fluorescent probe indo-1. However, subsequent experiments revealed that indo-1 bound La with a higher affinity than Ca and emitted fluorescence in the same wavelength as the Ca bound form. Using this probe, we observed that La enters keratinocytes in a dose-dependent fashion and achieves concentrations exceeding 80 nM when the external La concentration is raised to 300 microM. This fully accounted for the apparent increase in Cai when La was added to the cells. Treatment of cells with ionomycin increased indo-1 fluorescence maximally in the presence of La indicating influx of La via this Ca specific ionophore. Our results indicate that La enters cells and inhibits calcium mediated keratinocyte differentiation both by blocking Ca influx and by blocking calcium regulated intracellular processes such as transglutaminase directed cornified envelope formation.
三价阳离子镧（La）与细胞的钙结合位点结合，要么模拟钙的特性，要么通过将钙从其结合位点上置换下来来抑制钙的作用。细胞外钙在一定程度上通过提高细胞内钙水平（Cai）来诱导培养的人表皮角质形成细胞分化。因此，在本研究中，我们确定了La对角质形成细胞分化和细胞内钙水平的影响。我们观察到La抑制了角质形成细胞终末分化标志物——角质包膜的产生。La直接抑制了需要钙的包膜交联酶——转谷氨酰胺酶，推测是通过将钙从该酶的结合位点上置换下来。La抑制了45Ca从角质形成细胞的流入和流出。矛盾的是，用荧光探针indo-1测量时，细胞外La似乎增加了角质形成细胞的Cai水平。然而，随后的实验表明，indo-1与La的结合亲和力高于与Ca的结合亲和力，并且在与Ca结合形式相同的波长下发出荧光。使用该探针，我们观察到La以剂量依赖性方式进入角质形成细胞，当外部La浓度提高到300μM时，其浓度超过80 nM。这充分解释了向细胞中添加La时Cai的明显增加。在La存在的情况下，用离子霉素处理细胞可使indo-1荧光最大程度增加，表明La通过这种钙特异性离子载体流入细胞。我们的结果表明，La进入细胞并通过阻断Ca流入以及阻断钙调节的细胞内过程（如转谷氨酰胺酶介导的角质包膜形成）来抑制钙介导的角质形成细胞分化。
