Interaction of thyroid hormone and steroidogenic acute regulatory (StAR) protein in the regulation of murine Leydig cell steroidogenesis

The steroidogenic acute regulatory (StAR) protein, a novel phosphoprotein, is a crucial factor involved in intramitochondrial cholesterol transportati on, the rate-limiting step in steroidogenesis. The present investigations w ere undertaken to elucidate involvement of thyroid hormone and StAR protein in the regulation of steroidogenesis in mouse Leydig cells. Treatment of c ells with triiodothyronine (T-3) coordinately augmented the levels of StAR protein, StAR mRNA, and steroid production, and these responses were progre ssively dependent on expression of steroidogenic factor 1 (SF-1). With rega rd to steroidogenesis and StAR expression, the T-3 response requires both o n-going mRNA and protein synthesis. In addition, the effects of T-3 were ac utely modulated at the steroidogenic machinery and luteinizing hormone rece ptor (LHR) function, while these levels were suppressed following longer pe riods of exposure to T-3. Furthermore, the inhibition of SF-1 expression by DAX-1 markedly abolished T-3-mediated StAR expression in a time frame, whi ch was consistent with decreased steroid biosynthesis. Specific involvement of SF-1 was further confirmed by assessing the 5'-flanking region of the m ouse StAR gene, which identified a region between -254 and -110 bp that was essential for T-3 function. Importantly, it was found that the SF-1 bindin g site at position -135 bp of the 5'-flanking region was greatly involved i n T-3-mediated reporter activity. Electrophoretic mobility shift assays (EM SA) also demonstrated involvement of SF-1 in T-3 function. The relevance of T-3-mediated LHR function was investigated in mice rendered hypo-and hyper thyroid, which accounted for up-regulation in the former and down-regulatio n in the latter group, respectively. These findings demonstrate a key role of thyroid hormone in maintaining mouse Leydig cell function, where thyroid hormone and StAR protein coordinately regulate steroid hormone biosynthesi s. (C) 2001 Elsevier Science. Ltd. All rights reserved.