The ecdysone receptor promotes or suppresses proliferation according to ligand level
Steroid hormones control various cellular activities in a context-dependent manner. For example, ecdysone, which acts through a type II nuclear receptor, has seemingly opposite effects in Drosophila wing precursors, promoting proliferation during larval stages, and triggering proliferation arrest at pupariation. We find that wing precursors proliferate normally in the complete absence of the ecdysone receptor (EcR), whether ecdysone is present or not, suggesting that ecdysone overrides a default anti-proliferative activity of the receptor. By contrast, termination of proliferation by high concentration of 20E at the end of larval life requires ligand-induced transcriptional activation by the receptor. The switch from one mode of regulation to the other is determined by ligand level, as measured with a calibrated EcR transcriptional reporter and ex vivo proliferation assays. Accordingly, RNA Seq analysis uncovers distinct transcriptional responses to different doses of ecdysone. Some genes are only activated at high doses (high threshold targets) and likely to comprise genes that stop proliferation at pupariation, when ecdysone titres are high. We find that other target genes respond to all physiological concentrations of ecdysone. Some of these genes are known to promote proliferation and could therefore contribute to the pro-proliferation activity of low-level ecdysone. Finally, we show mathematically and with synthetic reporters that relatively simple combinations of regulatory elements can recapitulate the behaviour of both types of target genes. Overall design: Total-RNAseq from 24h AL2-L3 wing imaginal discs incubated for 2.5h at 25°C in Dye medium supplemented with different concentrations of 20E.