Saponins show high entomotoxicity by cell membrane permeation in Lepidoptera

In this study, the effects of three saponins and one sapogenin with a triterpenoid or steroid structure in two lepidopteran insect cell lines, ovarian Bm5 and midgut CF-203 cells, were analysed with regard to cell viability, cell membrane permeation, EcR responsiveness and DNA fragmentation. In addition, the entomotoxic action of Q. saponaria saponin with primary midgut cell cultures and larval stages of the cotton leafworm Spodoptera littoralis was tested.

Both lepidopteran cell lines show a high sensitivity to all four sapo(ge)nins, with a concentration-dependent viability loss and EC₅₀ values of 25-100 µM in MTT bioassays. A trypan blue assay with Q. saponaria saponin confirmed rapid cell membrane permeation to be a cause of cytotoxicity. Saponins caused no EcR activation in Bm5 cells, but a loss of ecdysteroid signalling was observed with IC₅₀ values of 5-10 µM. Lower saponin concentrations induced DNA fragmentation, confirming their potential to induce apoptosis. Finally, Q. saponaria saponin caused cytotoxicity in primary midgut cell cultures of S. littoralis (EC(50) = 4.7 µM) and killed 70-84% of S. littoralis larvae at pupation at 30-70 mg g(-1) , while lower concentrations retarded larval weight gain and development.

The data obtained provide evidence that saponins exert a strong activity on lepidopteran cells, presumably based on a cytotoxic action due to permeation of the cell membrane. Primary midgut cell cultures and larvae of S. littoralis showed high sensitivity to Q. saponaria saponin, indicating the insect midgut as a primary target for entomotoxicity and the potential use of saponins in the control of pest Lepidoptera.