The retinoid X receptor (RXR) is activated by its often elusive cognate ligand, 9-cis-retinoic acid (9-cis-RA). In flies and moths, molting is mediated by a heterodimer ecdysone receptor consisting of the ecdysone monomer (EcR) and an RXR homolog, ultraspiracle (USP); the latter is believed to have diverged from its RXR origin. In the more primitive insect, Locusta migratoria (Lm), RXR is more similar to human RXRs than to USPs. LmRXR was detected in early embryos when EcR transcripts were absent, suggesting another role apart from ecdysone signaling. Recombinant LmRXRs bound 9-cis-RA and alltrans-RA with high affinity (IC 50 ‫؍‬ 61.2-107.7 nM; Kd ‫؍‬ 3 nM), similar to human RXR. To determine whether specific binding had functional significance, the presence of endogenous retinoids was assessed. Embryos were extracted by using modified Bligh and Dyer and solid-phase protocols to avoid the oily precipitate that makes this material unsuitable for assay. These extracts contained retinoids (5.4 nM) as assessed by RA-inducible Cyp26A1-promoter luciferase reporter cell lines. Furthermore, the use of HPLC and MS confirmed the presence of retinoids and identified in any embryo, 9-cis-RA, in addition to all-trans-RA. We estimate that whole embryos contain 3 nM RA, including 9-cis-RA at a concentration of 1.6 nM. These findings strongly argue for a functional role for retinoids in primitive insects and favor a model where signaling through the binding of 9-cis-RA to its RXR is established relatively early in evolution and embryonic development.all-trans-retinoic acid ͉ Locusta migratoria ͉ ultraspiracle I nsect development and metamorphosis are directed by two principal lipophilic hormones: 20-hydroxyecdysone (20-OH-Ec), the active molting hormone, and juvenile hormone (JH), whose titer determines the nature of the molt (1, 2). As demonstrated in the fruitfly, Drosophila melanogaster (Dm), 20-OH-Ec binds to the ecdysone receptor (EcR), which in turn is bound to its obligate heterodimerization partner ultraspiracle (USP), a homologue of the vertebrate retinoid X receptor (RXR) (3-6). As members of the nuclear receptor superfamily, EcR and USP/RXR share a common modular structure (7) comprised of a N-terminal variable domain (A/B), a DNA binding domain (C), hinge (D), and C-terminal ligand-binding domain (LBD; or domain E/F).The vertebrate RXRs are known heterodimeric partners of several members of the nuclear receptor superfamily, including the retinoid, thyroid, and vitamin D receptors (8). As demonstrated in vivo, these RXRs can also form homodimers and conceivably mediate an independent retinoid signaling pathway (9, 10). Indeed, the vertebrate RXRs are known ligand-activated transcription factors that bind 9-cis-retinoic acid (9-cis-RA), a stereoisomer of the vitamin A derivative, all-trans-RA (11, 12). RA receptors (RARs), reported only in vertebrates, are distinct in that they bind both all-trans-RA and 9-cis-RA with high affinity (13, 6). In contrast to the vertebrate RXRs, crystal structures reveal that DmUSP and t...

Antifolates, such as methotrexate (MTX), are the treatment of choice for numerous cancers. MTX inhibits dihydrofolate reductase (DHFR), which is essential for cell growth and proliferation. Mammalian cells can acquire resistance to antifolate treatment through a variety of mechanisms but decreased antifolate titers due to changes in drug efflux or influx, or alternatively, the amplification of the DHFR gene are the most commonly acquired resistance mechanisms. In Drosophila, however, a resistant phenotype has only been observed to occur by mutation resulting in a MTX-resistant DHFR. It is unclear if differences in gene structure and/or genome organization between Drosophila and mammals contribute to the observed differences in acquired drug resistance. To investigate if gene structure is involved, Drosophila Dhfr cDNA was transfected into a line of CHO cells that do not express endogenous DHFR. These transgenic cells, together with wild-type CHO cells, were selected for 19 months for resistance to increasing concentrations of MTX, from 50- to 200-fold over the initial concentration. Since Drosophila Dhfr appears to have been amplified several fold in the selected transgenic mammalian cells, a difference in genome organization may contribute to the mechanism of MTX resistance.