Steroid hormone receptors ecdysone receptor

Secretion of ecdysone starts the many biochemical processes that are necessary for the molting. The cells in the epidermis are stimulated to produce a new cuticle, and when ready, the insect will creep out of its old skin. The molecular mechanism of ecdysone has been studied in some detail. The molecular target of ecdysone and other ecdysteroids consists of at least two proteins, the ecdysteroid receptor (EcR) and ultraspiracle (USP). Both EcR and USP are members of the steroid hormone receptor superfamily with characteristic ligand-binding domains. An EcR-USP-ecdysteroid complex is formed, which activates several genes that code for transcription factors, i.e., proteins that activate or repress the activity of other genes, and the appropriate amounts of proteases and other enzymes necessary to degrade old structures and rebuild new ones are formed in a time-controlled sequence. 

Eukaryote organisms primarily respond to external signals by an initial signal perception by receptors. In general, such receptors can be either cytosolic or located on the plasma membrane [13-15]. The former mechanism applies to thyroid hormones (triiodothyronine and tetraiodothyronine or thyroxine), retinoids (e.g. retinoic acid), the insect developmental hormones such as ecdysone, steroid hormones (such as... 

The cDNAs for the glucocorticoid and the oestrogen receptors were isolated more than 15 years ago. They were among the first genes, coding for transcriptional gene activators, to be identified. The family of nuclear receptors is the largest family of transcription factors. Until now, more than 150 different members of the superfamily of nuclear receptors, from worms to insects to humans, have been described. The discovery of an insect receptor for a steroid hormone, ecdysone, indicated that this kind of receptor must have evolved prior to the separation of vertebrates and invertebrates. 

Nuclear receptors for the steroid hormone ecdysone in Drosophila deserve special consideration, for several reasons. First, the insect hormone ecdysone (Fig. 11.2) was the first steroid hormone shown to act at the level of the gene, because it induced puffs in the giant chromosomes of the fruitfly. Secondly, ecdysone activates the developmental programme of Drosophila, and, finally, the sequences of the DNA-binding domain (DBD) of the Drosophila nuclear ecdysone receptor and of receptor homologues (for example, the COUP-TFs chicken ovalbumin upstream promoter transcription factors) are highly conserved and nearly identical in vertebrates and humans. 

Although ecdysone is the only steroid hormone in Drosophila of which the function is known, the haemolymph of the fly contains many other ecdysteroids, the function of which remains to be clarified. The haemolymph also contains the sesquiterpenoid, juvenile hormone (JH). In many insects, JH r ulates ecdysone release. JH has structural features in common with the retinoids and may signal through a nuclear RXR receptor. 

---