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The structural features which underlie the nucleocytoplasmic transport of FABP4 is explained. FABP4 delivers specific ligands from the cytosol to the nuclesr receptor PPARgamma in the nucleus, thereby facilitating the ligation and enhancing the transcriptional activity of the receptor.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2007.02.050 Byline: Thaddeus T. Schug (1), Daniel C. Berry (1)(2), Natacha S. Shaw (1), Skylar N. Travis (1), Noa Noy (1)(2) Abstract: Transcriptional activation of the nuclear receptor RAR by retinoic acid (RA) often leads to inhibition of cell growth. However, in some tissues, RA promotes cell survival and hyperplasia, activities that are unlikely to be mediated by RAR. Here, we show that, in addition to functioning through RAR, RA activates the 'orphan' nuclear receptor PPAR[beta]/[delta], which, in turn, induces the expression of prosurvival genes. Partitioning of RA between the two receptors is regulated by the intracellular lipid binding proteins CRABP-II and FABP5. These proteins specifically deliver RA from the cytosol to nuclear RAR and PPAR[beta]/[delta], respectively, thereby selectively enhancing the transcriptional activity of their cognate receptors. Consequently, RA functions through RAR and is a proapoptotic agent in cells with high CRABP-II/FABP5 ratio, but it signals through PPAR[beta]/[delta] and promotes survival in cells that highly express FABP5. Opposing effects of RA on cell growth thus emanate from alternate activation of two different nuclear receptors. Author Affiliation: (1) Division of Nutritional Sciences, Cornell University, Ithaca, NY 14850, USA (2) Department of Pharmacology, Case Western Reserve University School of Medicine, Cleveland, OH 44106, USA Article History: Received 8 August 2006; Revised 21 November 2006; Accepted 23 February 2007 Article Note: (miscellaneous) Published: May 17, 2007

Research has been conducted on the RXR, a nuclear hormone receptor activated by the vitamin A metabolite retionic acid. Results indicate that helix 11 phenylalanine residue mutation diminishes the RXR ability to associate with the coactivators.

Roles of PPAR(beta)/(delta) in keratinocyte inflammation response are discussed. Immediately after skin injury comes release of inflammatory signals. Using cultures of primary keratinocytes from wild-type and PPAR(beta)/(delta)(super.-/-) mice has been helpful in finding that such signals induce keratinocyte differentiation through influencing TNF-alpha and IFN-gamma.

Research has been conducted on the retinoid X receptor (RXR). The structural analysis of several nuclear receptors has been carried out.

The interaction of peroxisome proliferator-activated receptor (PPAR) alpha with ligands has been studied using a fluorescence-based method. PPARs are nuclear hormone receptors that play important roles in lipid metabolism. A synthetic fluorescent long-chain fatty acid, parinaric acid is used as the probe. Results show the long-chain fatty acid binds with PPAR alpha with high affinity and can be used in monitoring protein-ligand interactions by the receptor.

A study was conducted to examine the formation of heterodimers by retinoid X receptor (RXR). The study also analyzed the effects of cognate ligands for RXR and retinoic acid receptors (RAR). RXR-alpha and human RAR-alpha were determined by overexpression in Escherichia coli. Results indicated a molecular mechanism by which ligands may obtain a transcriptional pathway that will be invoked under in vivo conditions.

Fluorescence titration was used to study the ligand-binding characteristics of the retinoid X receptor (RXR) and retinoic acid receptors (RAR) alone or within heterodimers. The ligand-binding capacity of RXR and the dissociation of the bound ligand from RXR was not affected by heterodimerization with RAR. The affinity of RAR with RXR activating factor, 9cRA was higher than RXR and the differential affinity of both receptors for 9cRA was maintained within heterodimers. This indicated the binding of ligands to individual subunits within the RAR-RXR heterodimers.

A study using fluorescence anisotropy was conducted to characterize the interactions between the retinoid X receptor (RXR) and its ligand 9-cis retinoic acid (9cRA). The result shows that there is positive cooperativity in the binding of 9cRA with RXR, which also causes positive cooperativity in the dissociation of tightly bound RXR tetramers upon binding with 9cRA. The results support the hypothesis that regulation of RXR's oligomeric state by ligand binding is an important step in the regulation of gene transcription by retinoids.

A rapid dissociation of protein tetramers results by the binding of retinoid X receptor (RXR) to 9-cis-retinoic acid and thereby induces a new physical state where the protein species act as RXR dimers. This indicates that tetramer formation acts as a moderator of gene transcription in which RXR acts as an intermediary. Fluorescence anisotropy studies reveal that the ligand binding disrupts the tetramer, producing a conformational change, and forms a more compact protein. Retinoids are important in cell growth and differentiation and suppress carcinogenesis.

The measurement of the rates of flip-flop of fatty acids (FA) across phospholipid bilayers using the stopped flow technique show that the rates of transbilayer flip-flop of both physiological and anthroyloxy-labeled fatty acids are very fast. The removal of FA from both leaflets of the lipid bilayers proceeds by a single first-order process. These results confirms the non-necessity of proteins for FA translocation across cell membranes.

A study of the oligomeric state of tetramer formation by retinoid X receptors (RXR) and factors mediating the self-association of proteins may reflect an extra level of regulation of cellular signalling by renoids. Electrophoresis and chemical crosslinking shows that in solution RXR-alpha form heterodimers and homotetramers with high affinities. Dissociation constants of the oligomeric species were calculated by fluorescence anisotropy studies.

A study of the reaction of interphotoreceptor retinoid-binding protein (IRBP) and retinoids reveals that the protein exhibits specificity toward certain types of ligands. The IRBP is an important protein in the interphotoreceptor matrix of the retina, which is involved in the transfer of photoreceptor cells and retinal pigment epithelium. The study includes the measurement of the equilibrium dissociation constants of the complexes of 11-cis and all-trans retinols. The binding affinities also follow the order of 11-cis-retinaldehyde being bound first and 11-cis retinols the last.

Visual cycle reactions induce photoisomerization of the chromophore of rhodopsin, II-cis-retinaldehyde. This further causes regeneration of 11 cis configuration in vertebrates. The cycle II cis-retinol is enzymatically stored, esterified and oxidized. The cellular retinoid binding protein controls visual cycle reactions which involve the partitioning of the substrates.

All trans-retinols and fatty acids bind to interphotoreceptor retinoid-binding protein (IRBP) in a reaction which is characterized. Two binding sites for retinol are seen in IRBP whose binding attractions resemble one another. Retinol fluorescence exhibits modifications when it binds, which are controlled to reveal a more powerful site. Reduced dissociation constant for the site is noted. Retinol is transferred from this region when long-chain fatty acids occur. Reactions between different fatty acids and IRBP are examined by the displacement process.

The binding activity between retinol-binding protein (RBP) and transthyretin (TTR) was investigated using a long-lived fluorescence probe. TTR exhibited the presence of one or two binding sites for RBP with dissociation constants of 0.07 micromole. Retinol was required for binding of RBP to TTR in the presence of 0.15M NaCl. The binding affinity of the two proteins was found to increased with higher ionic strength. Results indicated that the orientation of the TTR binding site on RBP was in close proximity with the retinoid binding site. An RBP-bound retinoic acid exhibited a head group that elicits steric hindrance against TTR.

The transcriptional activities of the nuclear receptors can be regulated by the small molecules comprised of hydrophobic compounds. Examples of such promiscuous receptors are discussed and strategies, which can be employed in shedding light on the nature of bona fide ligands for such receptors are outlined.