9-cis Retinoic acid receptor

ASBT has a complex regulatory system reflecting the importance of this transporter to bile-acid pool size and bile-acid synthesis rates. Hepatic nuclear factor la (HNF-la) is necessary for expression of ASBT as knockout mice showed no expression and had defective bile-acid transport.Conversely, FXR-null mice showed no difference in expression of ASBT, showing that FXR plays no part in regulation of ASBT. In man, HNF-la controls baseline promoter activity of the ASBT gene as the minimal construct with full promoter activity was found to have 3 HNF-la binding sites. These authors also showed that the promoter construct bound peroxisome proliferator activated receptor a (PPARa)/9 cis retinoic acid receptor heterodimer, demonstrating a link between bile-acid absorption and hepatic lipid metabolism mediated by PPARa. 

The response elements for glucocorticoids and estrogen receptors contain short palindromic sequences with various three-nucleotide "spacer" sequences in the center as follows.308,314,316-318 Two receptor proteins bind to the palindromic DNA forming a ho-modimeric receptor pair. For the 9-cis retinoic acid receptor RXR-a the response element contains a pair of direct repeats of a 6-base consensus sequence with a two-base pair spacer ... 

The hormone-receptor complexes interact pairwise with DNA. These pairs (dimers) may consist of two identical hormone-receptor complexes (homodimeric form, e.g., with adrenal or gonadal hormones). The thyroid hormone-receptor complex occurs in heterodimeric form and combines with a cis-retinoic acid-receptor complex. 

III 9-cis-retinoic acid receptor (RxR) Homodimer 9-c/s retinoic acid Direct repeat... 

The close fit of the recognition helix of the 9-cis retinoic acid receptor, RXR, against the DNA major groove is responsible for the noticeable stagger (1.2A) of the Ti499-Ai54i pair and... 

Grober, J., Zaghini, I., Fujii, H., Jones, S. A., Kliewer, S. A., Willson, T. M., Ono, T., and Besnard, P. (1999) Identification of a bile acid-responsive element in the human ileal bile acid-binding protein gene. Involvement of the famesoid X receptor/9-cis-retinoic acid receptor heterodimer../. Biol. Chem. 274, 29749-29754. 

The abbreviations used are HMG-CoA, 3-hydroxy-3-melhylglutaryl-CoA CAT, chloramphenicol acetyltransferase PPAR, peroxisome proliferator-activated receptor PPRE, peroxisome proliferator-responsive element NRRE, nuclear receptor responsive element RXR, retinoid X receptor hRXRa, human 9-cis-retinoic acid receptor a mPPARa, mouse peroxisome proliferator-activated receptor a COUP-TP, chicken ovalbumin upstream-promoter transcription factor, HNF-4, hepatocyte nuclear factor 4 EMSA, electrophoretic mobility shift analysis tk, thymidine kinase NEFA, nonesterified fatty acids... 

The retinoid X receptor (RXR) is a nuclear receptor that binds and is activated by certain endogenous retinoids, such as 9-cis-retinoic acid. RXR is the obligatory heterodimerization partner for a large number of nonclassic steroid nuclear receptors, such as thyroid hoimone receptor, vitamin D3 receptor, peroxisome proliferator-activated receptor and pregnane X receptor. 

Most recently, a phase-I-study defined a dose of 13-ris-retinoic acid that was tolerable in patients after myeloablative therapy, and a phase-III-trial showed that postconsolidation therapy with 13-cis-retinoic acid improved EFS for patients with high-risk neuroblastoma [7]. Preclinical studies in neuroblastoma indicate that ATRA or 13-cw-RA can antagonize cytotoxic chemotherapy and radiation, such that use of 13-cis-RA in neuroblastoma is limited to maintenance after completion of cytotoxic chemotherapy and radiation. It is likely that recurrent disease seen during or after 13-cis-RA therapy in neuroblastoma is due to tumor cell resistance to retinoid-mediated differentiation induction. Studies in neuroblastoma cell lines resistant to 13-cw-RA and ATRA have shown that they can be sensitive, and in some cases collaterally hypersensitive, to the cytotoxic retinoid fenretinide. Here, fenretinide induces tumor cell cytotoxicity rather than differentiation, acts independently from RA receptors, and in initial phase-I-trials has been well tolerated. Clinical trials of fenretinide, alone and in combination with ceramide modulators, are in development. 

Retinoids (i.e., tretinoin and tazarotene) mediate cellular responses primarily through activation of nuclear retinoid receptors [rr]. There are two types of nuclear retinoic acid receptors the retinoic acid receptors (RARs) and the retinoid X receptors. Each type of receptor contains three receptor subtypes alpha, beta, and gamma [rr, 11]. Among the commonly prescribed retinoids, tretinoin activates the RARs alpha, beta, and gamma directly, and the retinoid X receptors indirectly (through conversion of tretinoin to 9-cis-retinoic acid) [rr, 13]. Conversely,... 

Levin AA, Sturzenbecker LJ, Kazmer S, et al (1992) 9-cis Retinoic acid stereoisomer binds and activates the nuclear receptor RXR alpha. Nature 355 359-361... 

The DNA binding element of the nuclear receptors for all-trans retinoic acid, for 9-cis retinoic acid, for the T3 hormone and for the vitamin D3 hormone usually exhibit a direct repeat of the recognition sequence, resulting in formation of heterodimers on the DNA (fig. 4.7b). One of the partners in the heterodimer is always the receptor for 9-cis retinoic acid, RXR, and which usually occupies the 5 side of the HRE. 

Ligands of the RXR-heterodimer group and the orphan receptors are chemically more diverse than the ligands of the steroid family. Representative hgands of this group are the retinoids all-trans retinoic acid, 9-cis retinoic acid, the T3 hormone and vitamin D3 (fig. 4.1). 

Initial studies on the receptors of all-trans retinoic acid, vitamin D3 and the T3-hor-mone assumed that these receptors boimd their HRE s in homodimeric form. It became clear with the discovery of the receptors for 9-cis retinoic acid that this simple... 

The receptors for all-trans retinoic acid, vitamin D3 and the Ts-hormone (as well as other receptors of this class, see table 4.1) usually perform their regulatory function as heterodimers. RXR plays a special role in the formation of heterodimers the receptor for 9-cis retinoic acid is usually one of the binding partners in the heterodimer. 

The receptor for 9-cis retinoic acid (RXR) usually occupies the 5 position in the heterodimer. The RXR serves quasi as a vehicle to bring other receptor monomers to the 3 half-site of the HRE. 

In contrast to the steroid receptor family, members of the thyroid receptor family typically do not associate with accessory proteins and are not localized to the extranucleus matrix. Rather, these receptors exist in the basal state associated with chromatin in the cell nucleus. When bound by hormone ligand, thyroid receptor family members dissociate from the chromatin and typically form heterodimeric combinations with the retinoid-X receptor (RXR). RXR also is capable of homodimerization in association with its ligand 9-cis retinoic acid. Thus high 9-cis retinoic acid levels apparently promote homodimerization, and low levels are permissive of heterodimerization of RXR with activation by the partner ligand. 

Fig. 8.2 The ligand-binding domain of the nuclear receptor RXR binds the small molecule 9-cis retinoic acid 9-cis RA). In the apo-structure on the left, helix 12 (H12) extends out into the solution. Upon binding 9-cis RA, HI2 folds back on top of the ligand and LBD creating a binding site for the transcription machinery. The transcription machinery is thereby recruited to the transcription start site, resulting in activation of transcription. Nuclear receptors generally follo A/ this mechanism of ligand-activated transcription. Coordinates from ILBD [9] and 1 FBY [10].

Figure 2.6. Interactions of all-trans- and 9-cis-retinoic acids (and other active retinoids) with retinoid receptors. COUP, chicken ovaibumin upstream promoter.

The vitamin D receptor acts mainly as a heterodimer with the retinoid X receptor (RXR Section 2.3.2.1). Binding of calcitriol induces a conformational change in the receptor protein, permitting dimerization with occupied or unoccupied RXR, followed by phosphorylation to activate binding to the vitamin D response element on DNA (DeLuca and Zierold, 1998). Abnormally high concentrations of 9-cis-retinoic acid result in sequestration of RXR as the homodimers, meaning that it is unavailable to form heterodimers with the vitamin D receptor (or other receptors) excessive vitamin A can therefore antagonize the nuclear actions of vitamin D (Haussler et al., 1995 Rohde et al., 1999). 

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