Basis for selective action of a synthetic molting hormone agonist, RH-5992 on lepidopteran insects

The non-steroidal ecdysone agonist, RH-5992, induces a precocious incomplete molt in lepidopteran insects but is refractory to insects of other orders. We used two lepidopteran cell lines, FPMI-CF-203 (CF-203) and IPRI-MD-66 (MD-66) and two dipteran cell lines, DM-2 and Kc, to investigate the lepidopteran specificity of RH-5992. The mRNAs for hormone receptor 3 homologues cloned from Drosophila (DHR3) and Choristoneura (CHR3) are directly induced by 20-hydroxyecdysone (20E) and serve as suitable markers for studying ecdysone action. Dose response experiments showed that 10(-7) M 20E induced CHR3 mRNA in CF-203 cell and DHR3 mRNA in DM-2 cells. Concentrations of RH-5992 as low as 10(-10) M induced CHR3 mRNA in CF-203 cells, whereas concentrations as high as 10(-6) M induced only very low levels of DHR3 mRNA in DM-2 cells. Studies using 14C-RH-5992 revealed that lepidopteran cell lines (CF-203 and MD-66) retained more of this compound within the cells than the dipteran cell lines (DM-2 and Kc). The clearance of RH-5992 from DM-2 cells was temperature dependent and was blocked by 10(-5) M ouabain, an inhibitor of Na+, K(+)-ATPase suggesting that the efflux was due to active transport.