Steroidogenic factor 1 (SF-1), an orphan member of the intracellular receptor superfamily, plays an essential role in the development and function of multiple endocrine organs. It is expressed in all steroidogenic tissues where it regulates the P450 steroidogenic genes to generate physiologically active steroids. Although many of the functions of SF-1 in vivo have been defined, an unresolved question is whether a ligand modulates its transcriptional activity. Here, we show that 25-, 26-, or 27-hydroxycholesterol, known suppressors of cholesterol biosynthesis, enhance SF-1-dependent transcriptional activity. This activation is dependent upon the SF-1 activation function domain, and, is specific for SF-1 as several other receptors do not respond to these molecules. The oxysterols activate at concentrations comparable to those previously shown to inhibit cholesterol biosynthesis, and, can be derived from cholesterol by P450c27, an enzyme expressed within steroidogenic tissues. Recent studies have shown that the nuclear receptor LXR also is activated by oxysterols. We demonstrate that different oxysterols differ in their rank order potency for these two receptors, with 25-hydroxycholesterol preferentially activating SF-1 and 22(R)-hydroxycholesterol preferentially activating LXR. These results suggest that specific oxysterols may mediate transcriptional activation via different intracellular receptors. Finally, ligand-dependent transactivation of SF-1 by oxysterols may play an important role in enhancing steroidogenesis in vivo.Steroidogenic factor 1 (SF-1), a monomer member of the orphan nuclear receptor family, is expressed from the inception of adrenal and gonadal development, and mice deficient in SF-1 lack these organs (1-4). SF-1 also plays an important role in regulating the expression of multiple components of steroidogenesis, including the cytochrome P450 steroid hydroxylases and the steroidogenic acute regulatory protein (5, 6). Finally, SF-1 is important for the expression of several key products of pituitary gonadotropes, including folliclestimulating hormone, leutinizing hormone, and the receptor for gonadotropin-releasing hormone (7). These results have established a critical role for SF-1 at multiple levels of endocrine development and function. An unresolved question, however, has been whether SF-1-dependent transcriptional activity is modulated by a ligand-either endogenous to steroidogenic cells or supplied in an endocrine fashion. In the present study, we have begun to answer this question by examining several different steroidogenic intermediates or their derivatives for their ability to activate SF-1. Our results demonstrate that certain endogenous oxysterols such as 25-, 26-, or 27-hydroxycholesterol (OHC), known inhibitors of cholesterol biosynthesis (8, 9), selectively enhance SF-1 mediated transcriptional activity. These compounds may be generated in vivo through the actions of the enzyme P450c27 (10-12). Furthermore, activation by these molecules is observed at concentrations compar...

Tissue transglutaminase (transglutaminase type II) is an intracellular protein cross-linking enzyme that accumulates in connective tissue and in cells undergoing apoptosis. Retinoids regulate the transcription of the mouse tissue transglutaminase gene via activation of regulatory elements contained within 4 kilobases of the 5'-end of the gene. Co-transfection studies with retinoid receptor expression vectors in CV-1 cells demonstrated that the mouse tissue transglutaminase promoter is activated by ligand activation of either retinoic acid receptor-retinoid X receptor (RAR.RXR) heterodimers or RXR homodimers. Optimal induction is achieved with retinoid receptor panagonists; partial activation can also be achieved with either RAR-specific or RXR-specific retinoids. Retinoid-dependent activation of the tissue transglutaminase promoter depends on both a proximal regulatory region containing sequences highly conserved between the human and the mouse tissue transglutaminase promoters and a distal region that includes a 30-base pair retinoid response element (mTGRRE1). mTGRRE1 contains three hexanucleotide half-sites (two canonical and one non-canonical) in a DR7/DR5 motif that bind both RAR*RXR heterodimers and RXR homodimers. These studies suggest that retinoid-dependent expression of the mouse tissue transglutaminase gene is mediated by a versatile tripartite retinoid response element located 1.7 kilobases upstream of the transcription start site.