The circadian rhythms in mammals are regulated by a pacemaker located in the suprachiasmatic nucleus of the hypothalamus. Four clock-gene families have been found to be involved in a transcription-translation feedback loop that generates the circadian rhythm at the intracellular level. The proteins Clock and Bmal1 form a heterodimer which activates the transcription of the Per gene from the E-box elements in its promoter region. Protein products of Per act together with Cry proteins to inhibit Per transcription, thus closing the autoregulatory feedback loop. We found that Dec1 and Dec2, basic helix-loop-helix transcription factors, repressed Clock/Bmal1-induced transactivation of the mouse Per1 promoter through direct protein-protein interactions with Bmal1 and/or competition for E-box elements. Dec1 and Dec2 are expressed in the suprachiasmic nucleus in a circadian fashion, with a peak in the subjective day. A brief light pulse induced Dec1 but not Dec2 expression in the suprachiasmic nucleus in a phase-dependent manner. Dec1 and Dec2 are regulators of the mammalian molecular clock, and form a fifth clock-gene family.

The endogenous CYP2B6 gene becomes phenobarbital (PB) inducible in androstenol-treated HepG2 cells either transiently or stably transfected with a nuclear receptor CAR expression vector. The PB induction mediated by CAR is regulated by a conserved 51-base pair element called PB-responsive enhancer module (PBREM) that has now been located between ؊1733 and ؊1683 bp in the gene's 5-flanking region. An in vitro translated CAR acting as a retinoid X receptor ␣ heterodimer binds directly to the two nuclear receptor sites NR1 and NR2 within PBREM. In a stably transfected HepG2 cell line, both PBREM and NR1 are activated by PB and PB-type compounds such as chlorinated pesticides, polychlorinated biphenyls and chlorpromazine. In addition to PBREM, CAR also transactivates the steroid/rifampicin-response element of the human CYP3A4 gene in HepG2 cells. Thus, activation of the repressed nuclear receptor CAR appears to be a versatile mediator that regulates PB induction of the CYP2B and other genes.

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In the present study, we found that hypoxia or CoCl 2 enhanced the mRNA expression of DEC2, as well as DEC1, within 24 h in chondrogenic ATDC5, 293T, and HeLa cells. In luciferase assays, the regions between ؊524 and ؊401 in the DEC1 promoter, and between ؊863 and ؊258 in the DEC2 promoter, were responsible for the hypoxia-or hypoxiainducible factor-1␣ (HIF-1␣)-induced transcription. In these regions, we identified functional hypoxia response elements (HREs) that bound to HIF-1␣ and HIF-1␤. In addition to an HIF-1 binding site consensus sequence, the DEC1 HRE had cAMP response element-like and CACAG sequences, which were also involved in the transcription activation in response to HIF-1␣. Although the DEC2 HRE did not have a cAMP response element-like or CACAG sequence, it showed a higher affinity for HIF-1 than did the DEC1 HRE. Because DEC1 and DEC2 are directly inducible by HIF-1, these transcription factors may be crucial for the adaptation to hypoxia.Recently we cloned the cDNA for the novel basic helix-loophelix (bHLH) 1 transcription factor DEC1 (BHLHB2), which was expressed at a higher level in human primary chondrocytes than in fibroblastic cells (1). A mouse ortholog (Stra13) and a rat ortholog (SHARP-2) of DEC1 were identified independently by others in the P19 embryonal carcinoma cells and rat brain, respectively (2, 3), and the mRNA of DEC1 was found in a variety of embryonic and adult tissues (1-3). In addition, we cloned a member of the DEC subfamily of bHLH proteins, DEC2 (BHLHB3), by searching the expressed sequence tags data bank (4). DEC2 is similar to SHARP-1 (3), which is a truncated molecule of DEC2 produced by a sequencing error or a minor frameshift mutant (4). The bHLH regions of DEC1 and DEC2 exhibit the highest similarities to those of Drosophila Hairy, Enhancer of split and mammal HES, which are known as transcriptional repressors with the WRPW motif, which interacts with the Groucho family members of corepressors. Although the Stra13/DEC1 and DEC2/SHARP-1 lack the WRPW motif, they also act as transcriptional repressors by a discrete mechanism (5, 6). Stra13/DEC1 interacts physically with the components of the basal transcription machineries, such as TATA-binding protein and TFIIB, and can recruit the histone deacetylase 1-Sin3A-NcoR corepressor complex through their carboxyl-terminal repression domain (5).DEC1/Stra13/SHARP-2 may be involved in the control of the proliferation and/or differentiation of chondrocytes, nerve cells, fibroblasts, and T cells (1-3, 7). The overexpression of DEC1/ Stra13 promoted a chondrogenic differentiation of the mesenchymal stem cells, 2 and a neural differentiation of the P19 cells (2). Recently, Stra13-deficient mice were generated, and it was shown that Stra13/DEC1 is a key regulator of lymphocyte activation that is vital for the maintenance of self-tolerance and constraint of autoimmunity. Other phenotypic differences between Stra13 Ϫ/Ϫ mutants and the wild-type littermates have not been reported (7). On the other hand, DEC2/SHARP-1 worked as a tran...

BiochemistryRole of steroid 11,6-hydroxylase and steroid 18-hydroxylase in the biosynthesis of glucocorticoids and mineralocorticoids in humans (cortisol (9) recently isolated aldosterone synthase cytochrome P-450 (P-450ado), which is induced in Na+-depleted K+-replete rat adrenal cortex (10,11), and demonstrated that rat P-450ado catalyzes three successive monooxygenation reactions of DOC to form aldosterone as a final product, whereas rat P-45011p does not substantially catalyze the reaction to form aldosterone. In regard to aldosterone biosynthesis in the human adrenal cortex, it has been postulated for a long time (12)(13)(14)

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