9-cis retinoic acid signaling: changing partners causes some excitement

Retinoic acid and methylation cis-regulatory elements control the mouse tissue non-specific alkaline phosphatase gene expression

To understand the mechanisms regulating the tissue non-specific alkaline phosphatase (TNAP) activity during development, we characterized cis-transcriptional regulatory elements. In embryonic cells and tissues, TNAP expression was driven preferentially by the exon 1A (E1A) promoter, one of the two promoters previously defined. Transcriptional activity of E1A promoter was up-regulated by retinoic acid (RA) through a putative RA-responsive element. Transgenic mice analysis with lacZ reporter constructs revealed negative regulatory elements within 8.5 kb of E1A promoter. Promoter sequences of endogenous TNAP in non-expressing tissues and those carried by the 8.5 kb-lacZ transgene were found to be highly methylated. A 1 kb fragment of E1A promoter increased the methylation level of lacZ and promoter sequences. The role of RA and DNA methylation in defining the embryonic expression pattern of TNAP is discussed.

Direct repeats bind the EcR/USP receptor and mediate ecdysteroid responses in Drosophila melanogaster

The steroid hormone 20-hydroxyecdysone plays a key role in the induction and modulation of morphogenetic events throughout Drosophila development. Previous studies have shown that a heterodimeric nuclear receptor composed of the EcR and USP proteins mediates the action of the hormone at the transcriptional through binding to palindromic ecdysteroid mediates the action of the hormone at the transcriptional level through binding to palindromic ecdysteroid response elements (EcREs) such as those present in the promoter of the hsp27 gene or the fat body-specific enhancer of the Fbp1 gene. We show that in addition to palindromic EcREs, the EcR/USP heterodimer can bind in vitro with various affinities to direct repetitions of the motif AGGTCA separated by 1 to 5 nucleotides (DR1 to DR5), which are known to be target sites for vertebrate nuclear receptors. At variance with the receptors, EcR/USP was also found to bind to a DR0 direct repeat with no intervening nucleotide. In cell transformation assays, direct repeats DR0 to DR5 alone can render the minimum viral tk or Drosophila Fbp1 promoter responsive to 20-hydroxyecdysone, as does the palindromic hsp27 EcRE. In a transgenic assay, however, neither the palindromic hsp27 element nor direct repeat DR3 alone can make the Fbp1 minimal promoter responsive to premetamorphic ecdysteroid peaks. In contrast, DR0 and DR3 elements, when substituted for the natural palindromic EcRE in the context of the Fbp1 enhancer, can drive a strong fat body-specific ecdysteroid response in transgenic animals. These results demonstrate that directly repeated EcR/USP binding sites are as effective as palindromic EcREs in vivo. They also provide evidence that additional flanking regulatory sequences are crucially required to potentiate the hormonal response mediated by both types of elements and specify its spatial and temporal pattern.

Specific teratogenic effects of different retinoic acid isomers and analogs in the developing anterior central nervous system of zebrafish

Vertebrate embryos are sensitive to retinoic acid, either in deficiency or in excess. Although all-trans retinoic acid (RA) and 9-cis RA are known to have distinct but overlapping activities in higher organisms, only the all-trans isomer has been investigated in detail as a teratogen in zebrafish. We have identified profound and specific effects of 9-cis RA when administered to zebrafish embryos, and have confirmed the results of prior studies on the teratogenic effects of exogenous all-trans RA. Moreover, we have identified a 1-hr period during gastrulation in which embryos are particularly sensitive to the teratogenic effects of RA. In the course of these investigations, we have also studied the effects of two synthetic retinoids-a 9-cis RA analog, SR11217, and an all-trans RA analog, TTAB. An application of all-trans RA to the early zebrafish gastrula leads to defects that are limited to the caudal midbrain and rostral hindbrain. Our experiments show that an application of exogenous 9-cis RA for a period as short as 1 hr and at a concentration as low as 0.1 mu M can block differentiation of the rostral CNS. We have observed abnormal phenotypes using DIC optics, and have demonstrated further abnormalities using whole-mount immunocytochemical staining with antibodies to HNK-1 and acetylated alpha-tubulin. Major axon tract formation in the anterior CNS is unambiguously disrupted by the administration of 9-cis RA but not all-trans RA. Furthermore, exogenous 9-cis RA produces a qualitative alteration in the multiple-site expression pattern of the hlx-1 gene within the rostral CNS, while treatment with all-trans RA leads only to a weakened expression signal. The administration of TTAB and SR11217 result in distinctive inhibitions of hlx-1 expression. Unlike all-trans RA, which causes premature par-2 expression in the posterior midbrains of a majority of embryos, 9-cis RA leads to a complete deletion of this domain throughout development. These results suggest that 9-cis RA is a more active teratogen than all-trans RA in rostral CNS structures of the zebrafish embryo.

The orphan nuclear hormone receptor LXR alpha interacts with the peroxisome proliferator-activated receptor and inhibits peroxisome proliferator signaling

The yeast two-hybrid system was used to isolate novel cellular factors that interact with the mouse peroxisome proliferator-activated receptor alpha (PPARalpha). One of the interacting clones isolated encoded LXRalpha, a recently described human orphan nuclear hormone receptor. LXRalpha bound directly to PPARalpha, as well as to the common heterodimerization partner 9-cis-retinoic acid receptor (RXRalpha). LXRalpha did not form a DNA binding complex with PPARalpha on synthetic hormone response elements composed of direct repeats of the TGACCT consensus half-site or on naturally occurring peroxisome proliferator response elements (PPREs) or LXRalpha response elements. However, LXRalpha inhibited binding of PPARalpha/RXRalpha heterodimers to PPREs, and coexpression of LXRalpha in mammalian cells antagonized peroxisome proliferator signaling mediated by PPARalpha/RXRalpha in vivo. These findings identify a novel partner for PPARalpha and suggest that LXRalpha plays a role in modulating PPAR-signaling pathways in the cell.

Hepatocyte nuclear factor-4 is responsible for the liver-specific expression of the gene coding for hepatocyte growth factor-like protein

In an attempt to understand the molecular mechanism regulating the expression of the gene coding for human hepatocyte growth factor-like protein/macrophage stimulating protein (HGFL), our laboratory has isolated and characterized approximately 4200 bp of the 5'-flanking region of the HGFL gene. To determine the location of sites which may be critical for the function of the HGFL gene promoter, we constructed a series of hybrid genes containing serial deletions of this region attached to the coding sequences for chloramphenicol acetyltransferase. Expression of these chimeric plasmids was examined by transient transfection of HepG2 and 293 cells. Our results suggest that the transcriptional activity of the HGFL promoter is modulated in HepG2 cells by one positive element at position -135 to -105 (-135/-105). In contrast, only background levels of chloramphenicol acetyltransferase expression have been detected in 293 cells. The -135/-105 region appears to bind a liver-specific transcription factor essential for expression of this gene. Gel mobility shift experiments with antibodies against hepatocyte nuclear factor-4 (HNF-4) and transactivation of the HGFL promoter by a HNF-4 cDNA expression vector suggest that HNF-4 binds to the -135/-105 region and is responsible for the liver-specific expression of HGFL.