A Novel, Functional, and Highly Divergent Sex Hormone-Binding Globulin that May Participate in the Local Control of Ovarian Functions in Salmonids

Abstract

A cDNA encoding for a novel rainbow trout SHBG was identified and characterized. Phylogenetic analysis showed that this novel SHBG, named SHBGb, was a highly divergent paralog of the classical SHBG (SHBGa) form previously known in vertebrates including zebrafish, seabass, and rainbow trout. Using all available sequences, no SHBGb-like sequence could be identified in any fish species besides Atlantic salmon. Rainbow trout SHBGa and SHBGb share only 26% sequence identity at the amino acid level and exhibit totally distinct tissue distribution, thus demonstrating a functional shift of SHBGb. Indeed, shbga mRNA was predominantly expressed in liver and spleen but could not be detected in the ovary, whereas shbgb had a predominant ovarian expression but could not be detected in liver. Despite its high divergence, rainbow trout SHBGb expressed in COS-7 cells could bind estradiol and testosterone with high affinity and specificity. Both rainbow trout shbgb mRNA and proteins were localized to the granulosa cells of vitellogenic ovarian follicles, whereas SHBGb immunoreactivity was also found in theca cells. Finally, shbgb ovarian mRNA expression exhibited a significant drop between late vitellogenesis and oocyte maturation at a time when ovarian aromatase (cyp19a) gene expression and estradiol circulating levels exhibited a dramatic decrease. Together, these observations show that SHBGb is a functional and highly divergent SHBG paralog probably arising from a salmonid-specific duplication of the shbg gene.

Characterization and Measurement of the Plasma α- and β-Sex Hormone-Binding Globulin Paralogs in Salmon

“…This is consistent with a report that a salmonid SHBG with a preference for estradiol, which we now know is SHBG-␤, is characterized by a smaller apparent molecular size when compared with SHBG in other species (3). As we expected, the sequences of csSHBG-␣ and csSHBG-␤ are almost identical (99 and 97%, respectively) to those of the corresponding rainbow trout proteins, consistent with their close phylogenetic relationship as members of the trout genus Oncorhynchus (16). However, as also noted previously (16), the sequence identity between these SHBG-␣ and SHBG-␤ paralogs is very poor (ϳ24%) and is much lower than other paralogs in salmonids that arose as a result of more recent genome duplications in these species.…”

“…As we expected, the sequences of csSHBG-␣ and csSHBG-␤ are almost identical (99 and 97%, respectively) to those of the corresponding rainbow trout proteins, consistent with their close phylogenetic relationship as members of the trout genus Oncorhynchus (16). However, as also noted previously (16), the sequence identity between these SHBG-␣ and SHBG-␤ paralogs is very poor (ϳ24%) and is much lower than other paralogs in salmonids that arose as a result of more recent genome duplications in these species. This suggests that SHBG-␤ is a more ancient paralog that has been retained in salmonids to serve a specialized function in these fish.…”

“…It was therefore not entirely unexpected when a SHBG paralog (SHBG-␤) was recently identified from a novel transcript in the rainbow trout gonad (16), but it was surprising that rainbow trout SHBG-␤ is expressed at high levels in the ovary and several other tissues but not in the liver (16). Moreover, the SHBG-␤ gene appears to be present only in salmonids, and the question of whether it contributes to their plasma SHBG levels remains unanswered.…”

“…Again, however, amino acid residues that are predicted to line the steroidbinding site are highly conserved. The most obvious difference between the coho salmon and rainbow trout (16) or chinook salmon SHBG-␤ sequences is the presence of an Asn residue in the amino-terminal region instead of a Lys residue. This introduces an additional consensus sequence for N-glycosylation in the csSHBG-␤ sequence in a position that is conserved in the csSHBG-␣ sequence.…”

Is Immunohistochemical Sex Hormone Binding Globulin Expression Important in the Differential Diagnosis of Adenocarcinomas?

“…It is a major plasma protein, which provides transport for the biologically active sex steroids and plasma concentration of SHBG, and affects the quantity, half-life and transport of the free sex steroids in the circulation to target tissues (Westphal, 1986;Hammond, 1995). SHBG, a 373-aminoacid glycoprotein, previously known as a carrier protein, is 1 classically understood to be synthesized mainly in the liver and then secreted into the circulating system (Maliqueo et al, 2007;Bobe et al, 2008;Fortunati et al, 2010). It binds to sex steroids in the circulation with a high affinity, modulating the bioavailability of the hormon es such as estrogen, androgen and testosterone (Mahlck et al, 1986;Selby et al, 1990;Garcia-Closas et al, 2007).…”