Ecdysteroids are a group of steroid hormones in arthropods that were originally identified as molting hormones in insects (Butenandt & Karlson, 1954; Hampshire & Horn, 1966; Huber & Hoppe, 1965). During early stages of insect development, active ecdysteroids such as 20-hydroxyecdysone (20E) induce molting, a sequence of physiological and behavioral events that begins with apolysis (i.e., detachment of the old cuticle from the epidermis) and ends with ecdysis (Riddiford, 1985; Žitňan & Adams, 2012). To coordinate multiple cellular events that accompany this highly complex process, circulating 20E enters virtually all different types of cells in the insect body and binds to the nuclear receptor complex that consists of the ecdysone receptor (EcR) and Ultraspiracle (USP) (Henrich, 2012; Riddiford, Cherbas, & Truman, 2000). Upon ligand binding, the EcR/USP complex functions as a transcription factor to induce highly stereotypic but also cell type-specific gene expression cascades (King-Jones & Thummel, 2005; Thummel, 1996), in accordance with epigenetic states and cofactor expression profiles of the responding cells (Stoiber, Celniker, Cherbas, Brown, & Cherbas, 2016; Uyehara & McKay, 2019; Uyehara et al., 2017; Uyehara, Leatham-Jensen, & McKay, 2022). Other hormones, in particular the sesquiterpenoid juvenile hormone (JH), can modify the activities of 20E and further regulate insect development. In holometabolous insects, JH functions as a “status quo” hormone during larval stages and directs 20E signaling so that it induces molting into another larval stage (Jindra, Bellés, & Shinoda, 2015; Jindra, Palli, & Riddiford, 2013; Riddiford, 1996). At the end of larval development, JH titer in the hemolymph (i.e., insect blood) declines and allows 20E to induce metamorphosis, which involves multiple cellular events required for adult development, such as apoptosis of larval cells and morphogenesis of adult-specific appendages (Truman, 2019; Truman & Riddiford, 2019).

As insect metamorphosis is a sexual maturation process through which body structures are reorganized for reproduction in the adult stage in response to steroid hormone signaling, its similarity to mammalian puberty has been identified by a number of researchers (Barredo, Gil-Marti, Deveci, Romero, & Martin, 2021; Guirado, Carranza-Valencia, & Morante, 2023; Malita & Rewitz, 2021). From this perspective, ecdysteroids can be considered as functional counterparts of sex steroids in mammals. Importantly, ecdysteroids are pleiotropic hormones with multiple roles beyond their molting hormone function, and they function as critical regulators of insect physiology throughout the life cycle (Yamanaka, 2021). In the adult stage of many insect species, ecdysteroids regulate multiple aspects of reproduction, particularly female oogenesis (Belles & Piulachs, 2006; Roy, Saha, Zou, & Raikhel, 2018), further supporting their functional resemblance with mammalian sex steroids. A recent study in the malaria mosquito Anopheles gambiae also identified a male-specific oxidized ecdysteroid that regulates female sexual receptivity and fertility (Peng et al., 2022). Additionally, ecdysteroid signaling is also induced in response to various stressors and controls stress responses in flies (Hirashima, Rauschenbach, & Sukhanova, 2000; Ishimoto & Kitamoto, 2010; Ishimoto, Sakai, & Kitamoto, 2009; Meiselman, Kingan, & Adams, 2018; Terashima, Takaki, Sakurai, & Bownes, 2005). It also controls innate immunity as well as glucose metabolism, which is reminiscent of glucocorticoid functions in mammals (Flatt et al., 2008; Han et al., 2017; Nishimura, 2020; Nunes, Koyama, & Sucena, 2021; Regan et al., 2013; Sun, Shen, Zhou, & Zhang, 2016). Ecdysteroids thus seem to be functionally analogous to multiple mammalian steroid hormones.

Both ecdysteroids and mammalian steroid hormones are derived from cholesterol or other closely related sterols. Moreover, they are both mainly synthesized by cytochrome P450 (CYP) enzymes and detected by nuclear receptors. However, despite such molecular and functional similarities between steroid hormone signaling pathways in insects and mammals, careful comparative studies have clearly shown that these pathways emerged independently (Escriva, Delaunay, & Laudet, 2000; Markov et al., 2009). When we use insect models to study steroid hormone biology, it is therefore critical to keep in mind that analogies between steroid hormone signaling pathways in insects and mammals reflect convergent evolution of functional properties. Yet, the simplicity of insect steroid hormone signaling is still a great advantage of using insects as a model system for generating higher-level insights into steroid hormones: although active ecdysteroids, such as 20E and makisterone A, can have slightly different carbon structures, they all activate EcR/USP and are considered to induce the same downstream signaling cascades (Henrich, 2012; Hill, Billas, Bonneton, Graham, & Lawrence, 2013). Our current knowledge therefore indicates that this single class of steroid hormones is responsible for all steroid-mediated physiological processes in insects. This relative simplicity, combined with other simple aspects of their physiology, such as the open circulatory system, makes insects an attractive model system to study basic aspects of steroid hormone biology.

In this chapter, we compare steroid hormone signaling pathways in insects and mammals from multiple perspectives and levels of biological organization, namely (1) phylogenetic relationships of biosynthetic pathways and nuclear receptors, (2) cholesterol homeostasis, and (3) hormone transport mechanisms. By reviewing the current state of ecdysteroid research, we aim to clarify how far, and in what ways, insect steroid hormone research can inform mammalian steroid hormone biology, so we can leverage the power of insect endocrinology to its full capacity. We also highlight a few understudied areas of steroid hormone research and discuss potential future research directions.