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Thyroid hormone-induced protein (TIP) gene expression by 3,5,3(')-triiodothyronine in the ovarian follicle of perch (Anabas testudineus, Bloch): modulation of 3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4)-isomerase enzyme by TIP.

Our previous reports had shown that 3,5,3'-triiodothyronine (T(3)) induced the generation of a 52-kDa monomer protein, i.e., TIP (thyroid hormone-induced protein) in the perch ovarian follicle. TIP, in turn, increased progesterone formation by stimulating Delta(5)-3beta-HSD activity (3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4) isomerase) [Eur. J. Endocrinol. 134 (1996) 128-135; Gen. Comp. Endocrinol. 113 (1999) 212-220]. In the present investigation, perch ovarian follicles were incubated in the absence (control) or the presence of T(3) or gonadotropin (GTH) or human chorionic gonadotropin (hCG). RNAs were isolated and allowed to hybridize with a radiolabeled TIP oligonucleotide probe prepared on the basis of the N-terminal 17-amino-acid sequence of TIP. Only RNA from T(3)-incubated follicles hybridized with the probe, while RNA from control or GTH- or hCG-incubated follicles did not hybridize with the probe. The transcript size of TIP mRNA was approximately 1.8 kb. mRNA isolated from T(3)-incubated ovarian follicles subjected to in vitro translation and Western blot analysis clearly identified a 52-kDa protein which was not found with the mRNA from the control follicles. However, both TIP and GTH stimulated progesterone secretion from perch ovarian follicles in vitro. GTH stimulation of Delta(5)-3beta-HSD was due to the stimulation of enzyme protein synthesis as a more than twofold increase in Delta(5)-3beta-HSD occurred in response to GTH. But TIP did not stimulate synthesis of Delta(5)-3beta-HSD protein. However, in vitro incubation of Delta(5)-3beta-HSD enzyme with TIP in the presence of NAD and substrate (pregnenolone) greatly stimulated enzyme activity, while incubation with GTH had no effect, indicating a modulation of Delta(5)-3beta-HSD protein from a less active to a more active state by TIP. This has been supported by another observation, in which TIP (52 kDa) and Delta(5)-3beta-HSD (45 kDa) incubation resulted in a complex of 99 kDa. This suggests a protein-protein interaction in the process of Delta(5)-3beta-HSD activation by TIP. The present work, therefore, shows some new and interesting aspects of thyroid hormone regulation of the reproductive control mechanism.