UNIV WASHINGTON, SEATTLE, WA 98119 USA. BIOMEMBRANE INST, SEATTLE, WA 98119 USA.
Int J Oncol. 1996 Jul;9(1):19-23. doi: 10.3892/ijo.9.1.19.
Catalytic lectins (leczymes) of frog eggs are sialic acid-binding lectins that have intrinsic RNase activity. They inhibit tumor cell proliferation in vitro and in vivo, although their cytotoxic mechanism remains unclear. RNase A has no tumoricidal activity. It is hypothesized that leczymes bind to cell surface sialoglycoconjugate receptors, enter the cell, and subsequently degrade RNA. In order to investigate the cytotoxic mechanism of cSBL, a leczyme from Rana catesbeiana eggs, we established cSBL-resistant clone RC-150 from mouse leukemia P388 cells. cSBL-treated P388 cells showed extensive RNA degradation over the course of 1 h, whereas cSBL-treated RC-150 cells showed no RNA degradation even over the course of 24 h. Treatment of P388 cells with cSBL led to decreased concentration of intracellular Ca2+, decreased protein kinase A activity, and increased protein kinase G activity. Incubation with cSBL decreased glutathione levels and enhanced glutathione-S-transferase (GST) activity in P388 cells, but had no effect on RC-150 cells. We conclude that cSBL-specific degradation of RNA occurs in cSBL-sensitive tumor cells, that cSBL leads to alteration of signal transduction and an intracellular protein kinase cascade reaction, and that internalized cSBL is detoxified by GST or thioltransferase. Our findings support a bifunctional model in which a leczyme is both an adhesive protein (binding to sialoglycoconjugates) and an enzyme (displaying RNnase activity).
蛙卵催化凝集素（leczymes）是一种具有内在 RNase 活性的唾液酸结合凝集素。它们在体外和体内抑制肿瘤细胞增殖，尽管其细胞毒性机制尚不清楚。RNase A 没有抗肿瘤活性。据推测，leczymes 与细胞表面的唾液酸化糖缀合物受体结合，进入细胞，随后降解 RNA。为了研究 cSBL 的细胞毒性机制，cSBL 是从牛蛙卵中提取的一种凝集素，我们从鼠白血病 P388 细胞中建立了 cSBL 抗性克隆 RC-150。cSBL 处理的 P388 细胞在 1 小时内表现出广泛的 RNA 降解，而 cSBL 处理的 RC-150 细胞即使在 24 小时内也没有 RNA 降解。用 cSBL 处理 P388 细胞导致细胞内 Ca2+浓度降低、蛋白激酶 A 活性降低和蛋白激酶 G 活性增加。cSBL 的孵育降低了 P388 细胞中的谷胱甘肽水平并增强了谷胱甘肽-S-转移酶（GST）活性，但对 RC-150 细胞没有影响。我们得出结论，cSBL 特异性地降解 RNA 发生在 cSBL 敏感的肿瘤细胞中，cSBL 导致信号转导和细胞内蛋白激酶级联反应的改变，并且内化的 cSBL 被 GST 或硫转移酶解毒。我们的发现支持了一个双功能模型，其中凝集素既是一种黏附蛋白（与唾液酸化糖缀合物结合），又是一种酶（具有 RNnase 活性）。
