Ecdysone
Ecdysone is one of several insect steroid hormones, known as ecdysteroids. These hormones resemble vertebrate steroid hormones such as vitamin D and estrogen. Among insects, ticks, scorpions, crustaceans, and spiders, ecdysteroids play a critical role at many times during growth and development from the egg stage through adult reproduction. For those insects which feed as larvae and then make the transition to the adult stage, large surges of ecdysteroid hormone produced inside the developing insect are responsible for triggering major changes in the insect's body plan, including the larval molts and metamorphosis. Generally, the presence of ecdysteroids along with a second hormonal class, the juvenile hormones, leads to a larval molt, whereas an ecdysteroid peak with no JH leads to metamorphosis.
Ecdysteroids act within insect cells using the same general mode of action as vertebrate steroids do in their cells. In fact, the mode of steroid action was first revealed by insect researchers in the 1950s, who found that ecdysteroids activate specific genes in the cell's DNA, which in turn, trigger cellular events such as the formation of organs and tissues and reproductive maturation. The molecular target of ecdysteroids is actually two proteins that form a dimer pair when stabilized by the presence of ecdysteroids. Both belong to a family of proteins known as the nuclear receptor superfamily, which also includes a large number of vertebrate steroid hormone and vitamin receptors. This protein pair interacts with specific DNA sequences located in several genes to regulate their activity.
Ecdysteroids of many different kinds are also produced by several plant species, which produce these chemicals as a defense against insect predation. Insects feeding on the ecdysteroid-producing plant often molt inappropriately or stop feeding as their cells are confused by the false hormonal signal that they have ingested. More recently, other synthetic compounds which mimic the action of natural ecdysteroids have also been used to activate the ecdysteroid receptor, and are sold commercially as insecticides. When tested in vertebrate cells, ecdysteroids apparently do not have any biological effect, and for this reason, the ecdysteroid receptor has been proposed as a gene therapy vector. According to this idea, the ecdysteroid receptor is genetically introduced into vertebrate cells along with a "therapy gene" that can be activated by the receptor. Then, the addition of ecdysteroid "drug" can be used to activate the therapy through the action of the receptor, and the effect will be specific, since the receptor does not seem to work on vertebrate genes. A receptor-based gene regulation system has been tested in mouse cells with some success. Nevertheless, some recent clinical studies have suggested that ecdysteroids may have a biological effect in humans, though more studies will need to be done. An ecdysteroid-receptor based gene activation system has also been developed for plants, with the goal of activating genes in crops such as corn and soybean that give them better resistance to drought and disease, while improving their crop yield.
(V.Henrich, Greensboro, NC, USA) |