Juvenile hormone and “retinoic acid” receptors in Manduca epidermis

Previous studies reviewed here have indicated that juvenile hormone (JH) specifically binds to a 29 kDa protein in epidermal nuclei from Manduca sexta larvae. Also, a 29 kDa nuclear protein that showed the same developmental pattern bound specifically to a larval cuticle gene LCP14. These results indicate that the 29 kDa nuclear protein is likely a JH receptor. Two retinoids (SRI 5942-64 and Ro 13-6298) were found to be weak JH mimics with ED₅₀s 60–100 times higher than that of JH III in the black Manduca larval bioassay. A genomic clone (Manduca “RAR”) then was isolated using the human retinoic acid receptor (hRAR) cDNA and the homologous region was sequenced. Thirteen out of 14 amino acids constituting the C-terminal half of the second zinc finger were identical in Manduca “RAR” and hRAR. Manduca “RAR” selected two mRNAs (3.8 and 4.5 kb) that are expressed at the peaks of the ecdysteroid titer during both the larval and the pupal molts, but not during the intermolt periods. When pieces of integument from day two 4th instar larvae were cultured with 4 × 10⁻⁶ M 20-hydroxyecdysone (20HE), Manduca “RAR” mRNA increased 13–15-fold by 6 h, then decreased after 12 h in the continuous presence of 20HE. The presence of 3 × 10⁻⁶ M JH slowed the rate of induction by 20HE. Thus, the “RAR” gene product is likely not the 29 kDa JH receptor but rather a transcriptional regulatory factor whose presence during a molt is modulated by JH.