Aromatase expression in prepuberal Sertoli cells: effect of thyroid hormone

Aromatase activity has recently been assumed as a Sertoli cell functional m aturation marker since it is maximally expressed in prepuberal age then it dramatically decreases at puberty and is virtually absent in adult age. Neo natal hypothyroidism is associated with a prolonged proliferation of Sertol i cells. This immature stage persists concomitantly with a dramatic enhance ment of aromatase activity reversed by triiodothyronine (T3) either in vivo or in vitro administration. Therefore, in the present study, after immunol ocalisation of aromatase in the cytoplasm of cultured Sertoli cells as well as in testis section, we investigate the regulatory effects of T3 in the s ame cells just at the age when aromatase activity is reported to be maximal ly expressed. In this aim, the effects of thyroid hormone have been evaluat ed in 2-weeks-old rats, in basal condition and upon stimulation with dibuty ryl cyclic AMP[(Bu)(2)cAMP] by simultaneously analysing three functional le vels of aromatase, mRNA expression; protein content; enzymatic activity. We stern-blot analysis of Sertoli cell extracts revealed a protein, which co-m igrated with a 55 kDa protein detected ill human placenta used a positive c ontrol. The presence of a functional P450 aromatase protein in purified Ser toli cells was confirmed by the measurement [H-3]H2O released after incubat ion with [1 beta-H-3]androst-4-3,17-dione. At the dose used, T3 down-regula tes basal aromatase activity, while aromatase mRNA expression was apparentl y not inhibited. It is noteworthy that aromatase content pattern evaluated by Western blot analysis did not tightly parallel the aromatase activity pa ttern which clearly displays the inhibitory effects of T3, in basal conditi on ad upon (Bu)(2)cAMP stimulation, simulating FSH stimulation. The detecti on of mRNA altered transcript coding for putative protein lacking both arom atic and heme-binding regions upon T3 treatment and unable to convert andro gens into estrogens, provides a reasonable explanation for the observed dis crepancies between aromatase protein pattern, P450arom mRNA levels and arom atase activity. The authors conclude that although the altered transcript i nduced by prolonged exposure to T3 is a mechanism by which T3 may down regu late aromatase activity, it cannot be ruled out a direct effect of this hor mone at the transcription levels since a recognisable emisite for potential TR(s) binding is located in the promoter region of aromatase gene. Thus a further investigation on T3 modulator role on aromatase gene promoter shoul d be pursued even utilising higher doses of T3. (C) 2001 Elsevier Science I reland Ltd. All rights reserved.