Agonists of an ecdysone-inducible mammalian expression system inhibit Fas Ligand- and TRAIL-induced apoptosis in the human colon carcinoma cell line RKO.

The ecdysone-inducible mammalian expression system is frequently used for inducible transgene expression in vitro and in vivo. Here, we describe a strong antiapoptotic effect of ecdysone analogs in the human colon carcinoma cell line RKO, which is in contrast to published data that ecdysteroids do not influence mammalian cell physiology. Inhibition of Fas ligand- and TNF-related apoptosis-inducing ligand-induced apoptosis by muristerone A occurs at the level of caspase-8 activation and is neutralized by phosphatidylinositol-3-kinase/Akt, protein kinase C and mitogen-activated protein kinase inhibitors. Microarray, Northern and Western blot analysis revealed that incubation of RKO cells with muristerone A leads to changes in gene expression levels, including an upregulation of bcl-x(L) mRNA and protein levels. Our data imply that ecdysteroids and ecdysone mimics can induce and/or repress gene transcription in RKO and other mammalian cells, thereby influencing the apoptotic behavior. Therefore, the ecdysone-inducible mammalian expression system may not be suitable for the analysis of apoptosis-related genes.