Ecdysteroids in insects

The biosynthesis and metabolism of the insect moulting hormone, ecdysone have been the subject of recent reviews [184-186]. Ecdysone (107) was first isolated in a crystalline form from the silkworm, Bombyx mori, by Butenandt and Karlson in 1954, and the structure was determined in 1965. Soon after, the second moulting hormone was isolated and the structure was elucidated as 20-hydroxyecdysone (108). Ibe same hormone was also isolated from the sea-water crayfish, Jasus lalandei by Horn in 1966. Many ecdysone analogues were isolated from arthropods and certain 

Ecdysteroids are the most widespread and probably oldest steroidal hormone. Recent investigations suggest that the lower animals, namely Annelida, Nematoda and Mollusca might also use ecdysteroids to control certain vital processes in their life cycle. Ecdysteroids have been found in helminth parasites of humans and livestock (trematodes, filarial nematodes and cestodes [188]. 

In insects, ecdysone is secreted from the prothoracic glands (PG) at every moulting and pupal stage. Conversion of ecdysone to 20-hydroxyecdysone occurred in fat body, Malpighian tubules, gut and body wall tissues [189]. Although ecdysone may have direct hormonal effects, it is generally held that ecdysone serves as a prehormone that is converted to 20-hydroxyecdysone which functions as the active hormone. This is because in many bioassay systems, 20-hydroxyecdysone appears to be much more active than ecdysone. 

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Hundreds of distinct steroids have been found in plants, animals, and microorganisms, respectively, for example, phytosterols and brassinosteroids in plant, ecdysteroids in insect, cholesterol, sex steroids, corticosteroids and anabolic steroids in vertebrate, and ergosterols in microorganism. Up to now, more than 40 steroid drugs have been marketed since the research and development to steroid drugs was triggered in 1950s [6, 7]. 

Blomquist, G.J. (2003). Ecdysteroid regulation of pheromone production in the housefly, Musca domestica. In Insect Pheromone Biochemistry and Molecular Biology, ed. G.J. Blomquist and R.G. Vogt. London Elsevier, pp. 231-252. 

Two main hypotheses have been put forward to account for the occurrence of ecdysteroids in the plant world. The first is that PEs have a hormonal role within the plant, but there is very little hard evidence in support of this hypothesis (reviewed in [26]). Alternatively, PEs participate in the defence of plants against non-adapted phytophagous invertebrates. Deterrent effects of 20E on non-adapted insect species are... 

Horn and Bergamasco [120] summarised their main conclusions regarding ecdysteroid structure-activity relationships in insect systems as follows ... 

The steroids known to play major regulatory roles in insect development and metamorphosis all fall into the class of polyhydroxylated ketosteroids called ecdysones (19-22). With the exception of Makisterone A (a C2 ecdysteroid identified from the milkweed bug Oncopeltus fasciatus) the known insect ecdysteroids constitute a group of eight or nine steroids that differ from one another... 

To date, interest in the ecdysteroids has focused primarily on their role in insect molting and metamorphosis and it is perhaps somewhat unfortunate that bioassays for ecdysteroid activity have been restricted almost exclusively to evaluating their effect on the molting process. It now appears probable that molting is just one of several important regulatory functions (e.g., embryogenesis, reproduction) performed by the ecdysteroids and that we are only just beginning to scratch the surface of an insect steroid system that is equally as complex as that in vertebrate species. 

Balance between the active and inactive forms of ecdysone and/or other ecdysteroids may be accomplished by the formation of conjugates such as sulfate esters or glucosides (20,27). The sulfation of phenols and a variety of sterols has been 3emonstrated in insect tissues and this, in close association with an appropriate sulfatase, would constitute a readily reversible mechanism whereby the required balance between active and inactive forms of ecdysteroids could be regulated (27). 

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