Abstract
Plants can respond to traumatism by synthesis and secretion of defence molecules. Wound-healing and desiccation stress of pieces of potato tuber parenchyma (Solanum tuberosum) promoted the secretion of a patatin-like phospholipase A2, PLA2 (EC.3.1.1.4.) that displayed cytotoxic activity against tumour cells. The potato secretion product, an oligomeric form of patatin-like protein, was shown to contain several isoforms of PLA2 polypeptides and to be associated with other proteins, including Kunitz-type protease inhibitors. Patatin-like protein secretion was inhibited by vanadate. Secreted patatin-like proteins displayed specific features, such as extracellular function and low molecular weights, mainly 36 to 40 kDa. The 36-kDa polypeptide sequence was related to iPLA2α. Polypeptide spots of secreted patatin-like protein exhibited a nucleotide-binding consensus motif, GGGIKG that has been described in iPLA2 gene family. The cytotoxic agent caused cell death of plant crown-gall induced by Agrobacterium tumefaciens, and inhibited B16 cell proliferation, but at the same concentration did not display any toxicity against non-transformed cells. PLA2 enzyme activity was required for cytotoxicity against B16 melanoma cells. A model for such a specific activity against tumour cells is discussed in connection with asymmetric phospholipid patterns of cell membranes. In conclusion, secreted patatin-like PLA2 (e-patatin) may represent a novel therapeutic target for the development of new agents against cancer.
Highlights
In response to a stress, plants can synthesize and secrete a large variety of secondary metabolites
We show that a patatin-like phospholipase A2 (PLA2) was actively secreted from potato tuber parenchyma after cutting and water stress, and that protein is involved in cytotoxicity mainly against tumour cells in plants and animals
Using SDS-Polyacrylamide Gel Electrophoresis (PAGEs), we previously described two sets of polypeptides which comigrate in band B: a major polypeptide (MW 35 - 40 kDa) and a smaller one containing Kunitz-type protease inhibitors [3]
Summary
In response to a stress, plants can synthesize and secrete a large variety of secondary metabolites. We have previously shown that a wound healing and desiccation of plant tubers promoted secretion of cytokinin-like products. In Jerusalem artichoke tubers (Helianthus tuberosus), after the same stimuli, we have shown secretion of a cytotoxic protein related to a superoxide dismutase [2]. This protein alone or possibly associated with a co-factor was shown to induce cytotoxicity in plant and animal tumour cells, preserving non-tumour cells in both cases. In potato (Solanum tuberosum), after such a stress condition, tuber parenchyma was shown to secrete new types of Kunitz-like protease inhibitors closely associated with another unknown polypeptide of approximately 40 kDa [3]
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