A novel steroid-binding protein in the testis of the dogfish Squalus acanthias

Abstract

In an earlier study, we identified and characterized a testicular estrogen receptor in the spiny dogfish ( Squalus acanthias); however, in this species estrogen receptors were located exclusively in nuclear subfractions. We describe here a nonreceptor, sex hormone-binding protein (SBP) present in Squalus testicular cytosol and distinguishable in its physicochemical characteristics from both the estrogen receptor and serum SBP of the same species. Cytosol (100,000 g supernatant) was prepared by differential centrifugation of testicular homogenates and incubated with [ 3H]estradiol (E 2) or [ 3H]testosterone (T) (10–15 n M ± 100-fold excess radioinert competitor) to determine total and nonspecific bound radioactivity. The testicular SBP had a broad specificity: E 2 = T > progesterone > 5α-dihydrotestosterone > estrone, but diethylstilbestrol was not an effective competitor. It displayed a high affinity for both E 2 and T ( K d = 2.2–2.5 × 10 −9 M), sedimented at 8–10 S in both low- and high-salt sucrose gradients, and migrated more slowly than BSA during polyacrylamide disc gel electrophoresis. The testicular SBP-E 2 complex was relatively stable ( t 1 2 = 160 min ) compared to the serum SBP-E 2 complex ( t 1 2 = < 30 min ). The testicular SBP was not found in nuclear subfractions nor did it bind to DNA-cellulose affinity columns. Its intratesticular distribution was stage-dependent: Zone III (mature spermatids) > Zone II (spermatocytes) > Zone I (stem cells and spermatogonia). Moreover, increased binding activity corresponded exactly to the hypertrophy and differentiation of Sertoli cells in the same zones, pointing to this cell as the possible site of SBP synthesis. We conclude that this testicular SBP is the Squalus counterpart of mammalian androgen binding protein. Its presence at this phylogenetic level signifies an important, conserved function.