Thyroid hormone-induced protein (TIP) gene expression by 3,5,3'-triiodothyronine in the ovarian follicle of perch (Anabas testudineus, Bloch): modulation of 3β-hydroxysteroid dehydrogenase δ⁵-δ⁴-isomerase enzyme by TIP

Abstract

Our previous reports had shown that 3,5,3'-triiodothyronine (T₃) 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 δ⁵-3β-HSD activity (3β-hydroxysteroid dehydrogenase/δ⁵-δ⁴ 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 T₃ 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 T₃-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 1.8 kb. mRNA isolated from T₃-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 δ⁵-3β-HSD was due to the stimulation of enzyme protein synthesis as a more than twofold increase in δ⁵-3β-HSD occurred in response to GTH. But TIP did not stimulate synthesis of δ⁵-3β-HSD protein. However, in vitro incubation of δ⁵-3β-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 δ⁵-3β-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 δ⁵-3β-HSD (45 kDa) incubation resulted in a complex of 99 kDa. This suggests a protein-protein interaction in the process of δ⁵-3β-HSD activation by TIP. The present work, therefore, shows some new and interesting aspects of thyroid hormone regulation of the reproductive control mechanism.