Transcription of eukaryotic protein-encoding genes by RNA polymerase II is modulated by two distinct classes of transcription factors. The first class comprises general transcription factors which are necessary for accurate initiation of transcription. These factors include TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIG/J, and TFIIH (11, 82). TFIID is a multiprotein complex consisting of TATA-binding protein (TBP) complexed with a number of TBP-associated factors (73). The binding of TFIID is thought to be the first step in transcriptional initiation (11, 82). The subsequent binding of TFIIB is thought to be involved in bringing RNA polymerase II to the complex through the association of TFIIF (34, 37). This leads to the recruitment of TFIIE and TFIIH and possibly other factors, which ultimately results in the initiation of transcription (11,75,82).The magnitude of transcriptional activity is greatly affected by the second class of transcription factors that generally bind to distal control DNA elements (52). These sequence-specific factors act to either promote or inhibit the formation of an active transcriptional initiation complex. Recent in vitro transcription studies suggest that the entry of TFIIB may be rate limiting for transcriptional initiation and that several transcriptional activators act to recruit or stabilize the interaction of TFIIB with the initiation complex (14, 62). TFIIB contains a potential N-terminal zinc finger structure (amino acids 14 to 36) that may be important for interaction with RNA polymerase II and TFIIF (34), a more C-terminal amphipathic ␣ helix (amino acids 184 to 201), and two imperfect repeats (amino acids 124 to 201 and 218 to 294) (34,37,62) (Fig. 1A). The amphipathic ␣ helix appears to be important for interaction with TBP (34, 37) and several transcriptional activators such as the herpes simplex virus VP16 protein (62).Members of the steroid/thyroid hormone receptor gene family are sequence-specific DNA-binding proteins that play important roles in gene regulation. The steroid hormone receptor subfamily includes the receptors which mediate the effects of glucocorticoids, progestins, mineralocorticoids, androgens, and estrogens (12, 16). The thyroid/retinoid receptor subfamily (16, 21) includes receptors that mediate the effects of thyroid hormone (3,5,3Ј-triiodo-L-thyronine [T3]), all-trans retinoic acid, 9-cis retinoic acid, and 1,25-dihydroxyvitamin D 3 as well as several orphan receptors (e.g., COUP-TF and c-ErbA␣2) whose ligands, if any, are unknown (26,27,44). Steroid/thyroid receptors bind to specific DNA sequences known as hormone response elements (HREs) and mediate ligand-dependent ac-

The receptors for thyroid hormone (T3R), all-trans-retinoic acid (RAR), and 9-cis-retinoic acid (RXR) bind DNA response elements as homo-and heterodimers. The ligand-binding domains of these receptors contain nine conserved heptads proposed to play a role in dimerization. Mutant receptors with changes in the first or last hydrophobic amino acids in the highly conserved ninth heptad of chick T3Ra (cT3Ra) [cT3R(L365R) and cT3R(L372R)] and human RARa (hRARa) [hRAR(M377R) and hRAR(L384R)] reveal that this heptad is essential for certain heterodimeric interactions and for diverse functional activities. Without ligands, wild-type receptors form both homodimers and heterodimers, while these mutants form only homodimers. Surprisingly, the cognate ligand for each mutant enables heterodimer formation between cT3R(L365R) and RAR or RXR and between hRAR(M377R) and T3R or RXR. Both cT3R(L365R) and hRAR(M377R) mediate liganddependent transcriptional regulation. However, unlike the wild-type receptor, non-ligand-associated cT3R(L365R) does not suppress the basal activity of certain promoters containing thyroid hormone response elements, suggesting that this silencing effect of T3R is mediated by unliganded heterodimers of T3R and endogenous RXR or related factors. Heterodimerization is also necessary for the strong ligand-independent inhibition between T3R and RAR on a common response element, since the ninth-heptad mutants function as poor inhibitors. However, with a T3R-specific response element, hRAR(M377R) acts as a retinoic aciddependent inhibitor of cT3R, indicating the importance of heterodimerization for this inhibition. Our studies also suggest that the ninth heptad is necessary for the dominant inhibition of wild-type T3Rs by mutant T3Rs, as has been found for the thyroid hormone-resistant syndrome in humans. Thus, the ninth heptad repeat is required for heterodimerization, suppression of basal promoter activity, and dominant negative effects of T3R and RAR. Lastly, the finding that cT3R(L365R) and hRAR(M377R) require ligands for heterodimer formation also raises the possibility that heterodimeric interactions are mediated by the ninth heptad without ligands but by a second region of these receptors with ligands.

Retinoic acid receptors (RARs) are ligand-dependent transcription factors which are members of the steroid/ thyroid hormone receptor gene family. RAR-agonists inhibit the proliferation of many human breast cancer cell lines, particularly those whose growth is stimulated by estradiol (E2) or growth factors. PCR-amplified subtractive hybridization was used to identify candidate retinoidregulated genes that may be involved in growth inhibition. One candidate gene identified was SOX9, a member of the high mobility group (HMG) box gene family of transcription factors. SOX9 gene expression is rapidly stimulated by RAR-agonists in T-47D cells and other retinoidinhibited breast cancer cell lines. In support of this finding, a database search indicates that SOX9 is expressed as an EST in breast tumor cells. SOX9 is known to be expressed in chondrocytes where it regulates the transcription of type II collagen and in testes where it plays a role in male sexual differentiation. RAR pan-agonists and the RARa-selective agonist Am580, but not RXR agonists, stimulate the expression of SOX9 in a wide variety of retinoid-inhibited breast cancer cell lines. RAR-agonists did not stimulate SOX9 in breast cancer cell lines which were not growth inhibited by retinoids. Expression of SOX9 in T-47D cells leads to cycle changes similar to those found with RARagonists while expression of a dominant negative form of SOX9 blocks RA-mediated cell cycle changes, suggesting a role for SOX9 in retinoid-mediated growth inhibition.

Biochemical analyses of caudate nucleus biopsy samples from patients with Parkinson's disease undergoing autologous adrenal transplantation were performed. Activity of the dopamine biosynthetic enzyme tyrosine hydroxylase, and concentrations of dopamine and its primary metabolite homovanillic acid were significantly greater than anticipated on the basis of previously published postmortem values. These data suggest that postmortem changes in various biochemical parameters of dopamine function are more rapid than has been generally appreciated. Further analysis of striatal biopsy samples may reveal predictive relationships between striatal indices of dopamine function and therapeutic response to adrenal transplantation.