Ecdysteroid signaling in ecdysteroid-resistant cell lines from the polyphagous noctuid pest Spodoptera exigua

Although dibenzoylhydrazine-type non-steroidal ecdysone agonists such as methoxyfenozide (RH-2485) have an excellent performance record, the emergence of resistance could severely compromise the efficacy of these compounds in integrated pest management programs. To investigate possible mechanisms of resistance, cell lines derived from the polyphagous noctuid pest Spodoptera exigua (Se4 cells) were selected for continuous growth in the presence of high concentrations of 20-hydroxyecdysone (20E) or methoxyfenozide. Here we describe an analysis of ecdysteroid receptor signaling in the ecdysteroid-resistant Se4 cell lines. In contrast to other ecdysteroid-resistant cell lines described in literature, our data support the existence of a normal functioning ecdysteroid receptor complex in the resistant Se4 cell lines: (1) using a recombinant BmNPV baculovirus as a transduction tool, activation of an ecdysone-responsive luciferase cassette was demonstrated; (2) the early gene HR3 is constitutively expressed in the resistant cell lines that are grown in the presence of 20E or methoxyfenozide. Quantitative RT-PCR experiments indicated that expression levels of SeEcR mRNA were comparable among sensitive and resistant cell lines. Sequencing of PCR fragments also revealed the presence of SeEcR mRNA with a wild-type ligand-binding domain in resistant cells. Finally, a possible role for the gene FTZ-F1, whose expression correlates with the absence of circulating ecdysteroids during insect development, in the resistance mechanism was investigated. However, it was observed that FTZ-F1, in contrast to what is observed during insect development, is constitutively expressed in Se4 cells and that its expression is not regulated by the addition of ecdysteroid. It is proposed that the resistance mechanism in Se4 cells resides at the coupling between the conserved hierarchical cascade of early and early-late gene expression and the differentiation program in the Se4 cell line. The use of insect cell lines for the investigation of resistance against dibenzoylhydrazine ecdysone agonists and their relevance for uncovering resistance mechanisms in insects during pest control programs is discussed.