Diversity and biological significance of sex hormone-binding globulin in fish, an evolutionary perspective

Abstract

In fish, two different genes, shbga and shbgb, exist that encode for very different proteins. Shbga is the ortholog of mammalian Shbg and was found in all investigated teleosts. In contrast, Shbgb is highly divergent and appears to be a salmonid-specific protein. Here, we review existing data on fish Shbga and Shbgb that have been obtained in chondrichthyes and osteichtyes. Even though other significant expression sites exist, existing data indicate that Shbga is mainly expressed in liver and subsequently secreted into the blood as a homodimer. In contrast, Shbgb is mainly expressed in the ovary, probably secreted as a monomer, and could contribute to the regulation of local steroid action. Binding studies indicate a specialization of circulating Shbg during evolution towards the preferential binding of estradiol and testosterone in teleosts. In contrast, specific fish steroids such as 11-oxo-androgens and oocyte maturation-inducing steroids that are crucial for reproduction are poorly bound by either form of Shbg.

Plasma Nesfatin-1 Is Not Affected by Long-Term Food Restriction and Does Not Predict Rematuration among Iteroparous Female Rainbow Trout (Oncorhynchus mykiss)

“…This difference in circulating E2 levels may be due to differences in the duration of pre-spawn fasting used by the facility from which trout were obtained (as described in Methods section), or to altered clearance of steroids by the liver [56], [57]. Previous work suggests that animals may experience an aging-associated decrease in the rate of steroid clearance via liver conjugation and hydroxylation reactions [58], [59], or an increasing sequestration of steroids by sex hormone-binding globulin [60], or some diffuse age-related effects associated with a diminished basal metabolic rate similar to that described in humans [61], [62] and other animals [63]. Regardless, this difference between age classes was transient: two-year-old and three-year-old fish exhibited similar plasma E2 levels by the second sampling date.…”

Some Aspects of Steroid Regulation in Teleosts

“…В переносе стероидов у рыб участвует ГСПГ [Bobe et al, 2010], а транскор-тин (кортикостероид-связывающий глобулин, КСГ) у рыб практически не изучен.…”

Evolutionary Perspectives on Sex Steroids in the Vertebrates

“…Chondrichthyes (http://tolweb.org/Gnathostomata/14843) show plasma SHBGs with generalized specificities for estradiol, testosterone, progesterone and corticosterone. In the osteichthyes, plasma SHBGs are more specific to estradiol and testosterone compared to progesterone and corticosterone (Bobe et al 2010). In mammals (http://tolweb.org/Therapsida/14973), SHBGs regulate the accessibility of sex-steroids to various organs.…”