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Cytotoxic compounds from Poecilocerus pictus feeding on Calotropis gigantea
Caroline Mathen and Bhagyashree Hardikar

Poecilocerus pictus, an aposematic insect, feeds on Calotropis gigantea (Asclepiadaceae) and sequesters cardiac glycosides which are used in chemical defense. The aim was to characterize the components of the methanolic extracts of P. pictus and C. gigantea and investigate preferential cytotoxicity of the insect extract, if any, on human cancer cell lines.

Comparative chemical characterization by HPTLC, UV and IR studies revealed the presence of cardenolides in both the extracts and biotransformation of some of the ingested cardenolides in the insect extract. Cytotoxicity testing of the insect extract on human and murine normal and cancer cell lines indicated a time and dose dependent inhibition of A549 and COLO205 with a 48 hour IC50 of < 5 μg/ml, but no effect on normal cell line WI38 or murine cell lines. Lymphotoxicity and genotoxicity experiments on human peripheral blood lymphocytes tested negative. DNA ploidy analysis on COLO205 indicated 36% apoptosis induction by 10 μg/ ml of insect extract.

In brief, our studies established that the methanolic extract of P. pictus contained ingested cardenolides, of which some were biotransformed. Within a range of concentrations, P. pictus extract suppressed viability in A549 and COLO205 cells but not in normal cells, lymphocytes or murine cells. It induced apoptosis in COLO205. The insect extract discriminated human cancer from normal cells, inhibited cancer cell viability specifically and may represent a vital lead for a chemotherapeutic drug.

Keywords: Poecilocerus pictus, Calotropis gigantea, cardiac glycosides, cardenolides, cytotoxicity, biotransformation, natural products

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