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Catalytic lectin (leczyme) from bullfrog (Rana catesbeiana) eggs
K Nitta1, K Ozaki, Y Tsukamoto
1UNIV WASHINGTON, SEATTLE, WA 98119 USA. BIOMEMBRANE INST, SEATTLE, WA 98119 USA.
Abstract:
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).
Insights
Catalytic egg lectins (leczymes) degrade RNA in tumor cells, inhibiting proliferation. These frog egg enzymes bind to cell surfaces, enter cells, and trigger signaling pathways, but are detoxified by cellular mechanisms.
Area of Science:
- Biochemistry
- Molecular Biology
- Cancer Research
Background:
- Catalytic lectins (leczymes) from frog eggs possess sialic acid-binding and RNase activities.
- These enzymes inhibit tumor cell proliferation, but their cytotoxic mechanism is not fully understood.
- RNase A lacks tumoricidal activity, suggesting a unique mechanism for leczymes.
Purpose of the Study:
- To investigate the cytotoxic mechanism of cSBL, a frog egg leczyme.
- To elucidate the role of RNA degradation and signal transduction in cSBL-induced cell death.
- To identify cellular mechanisms involved in cSBL resistance and detoxification.
Main Methods:
- Established a cSBL-resistant cell line (RC-150) from mouse leukemia P388 cells.
- Assessed RNA degradation in cSBL-treated sensitive (P388) and resistant (RC-150) cells.
- Monitored intracellular Ca2+ levels, protein kinase activities (PKA, PKG), glutathione levels, and glutathione-S-transferase (GST) activity.
Main Results:
- cSBL induced extensive RNA degradation in P388 cells within 1 hour, but not in RC-150 cells.
- cSBL treatment altered signal transduction, decreasing Ca2+ and PKA activity while increasing PKG activity in P388 cells.
- cSBL decreased glutathione levels and enhanced GST activity in P388 cells, with no effect on RC-150 cells.
Conclusions:
- cSBL induces tumor cell death through specific RNA degradation and modulation of intracellular signaling pathways.
- Internalized cSBL is detoxified by cellular mechanisms, potentially involving GST or thioltransferase, leading to resistance.
- These findings support a bifunctional model of leczymes as adhesive proteins and RNase enzymes in cytotoxicity.
