Hormones are chemical messengers that travel throughout the body to effect responses in specific tissues. Targeted cells have receptors that, upon binding the hormone, transduce the signal into a cellular response. Insect hormones regulate development by activation of either intracellular receptors or receptors at the cell membrane.
Ecdysteroids and Juvenile Hormones Activate Intracellular Receptors
Ecdysteroids and juvenile hormones are relatively lipophilic signaling molecules able to easily traverse the cell membrane. Upon entry, they bind to intracellular proteins called nuclear receptors or nuclear transcription factors, which reside either in the cytoplasm or in the nucleus. Regardless of their initial location, hormone binding triggers passage to the nucleus, where the receptor forms a complex with other proteins and then binds directly to DNA, inducing or repressing gene expression.
Several factors govern diverse, stage-specific responses of target cells to ecdysteroids. First, ecdysteroid receptors occur as multiple subtypes, including EcR-A, EcR-B1, and EcR-B2. The response to ecdysteroids is governed by the subtype expressed by target cells, as well as which subtype of USP (USP-1 or USP-2), the EcR partner, is expressed. The transient availability of receptors in target cells leads to sensitive periods at specific stages of development. For example, both EcR-A and EcR-B1 are present throughout larval life, but the ratio of the two favors EcR-B1. In contrast, levels of EcR-A
increase during metamorphosis. Second, the affinity of EcR subtypes can vary, as can the kinetics of a given response to an ecdysteroid peak. These patterns of receptor and dimer partner expression appear to mediate different cellular responses to ecdysteroids at different developmental times.
Most peptide hormones bind to a class of integral membrane proteins on the plasma membrane of the target cell. The majority of these are G-protein-coupled receptors that trigger intracellular second messenger cascades.
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