Retinoid binding-proteins redirect retinoid metabolism: biosynthesis and metabolism of retinoic acid

Many tissues and cell types, starting early in embryogenesis, convert retinol (vitamin A) into an active form, all-trans-retinoic acid. This article will discuss a current model of retinol and retinoic acid metabolism that integrates the various reactions which maintain retinoic acid homeostasis, and will also integrate the enzymology with the functions of cellular retinoid binding proteins. These conserved, high-affinity binding proteins enjoy widespread expression throughout all vertebrates and throughout most vertebrate tissues. The binding proteins limit access to retinol and retinoic acid to select enzymes and serve as substrates and affecters of retinoid metabolism.     

Structure and function of cytoplasmic retinoid binding proteins

We examined the ligand protein interactions of two highly homologous cellular retinol binding proteins, CRBP and CRBP-II, and two highly homologous cellular retinoic acid binding proteins, CRABP-I and CRABP-II. While the crystal structures of all four have been determined, nuclear magnetic resonance studies provide a means for observing dynamic aspects of ligand protein interactions of these proteins in solution. The cellular functions of these proteins are less well understood. We have modeled retinoid flux between cytoplasmic retinoid proteins and model membranes and with nuclear receptors. Based on our in vitro studies, we propose that certain retinoids may indirectly influence retinoid signaling by displacing endogenous retinoids from the cytoplasmic proteins to the nuclear receptors.     

Carotenoid and retinoid metabolism: insights from isotope studies

Use of isotopes as tracers has had an important role in elucidating key features of vitamin A and retinoid metabolism in animal models and humans. Their use has shown that beta-carotene absorption is variable, and that the appearance of beta-carotene and its metabolites in the blood by time since dosing follows characteristic patterns. Retinol formed from beta-carotene shows a different pattern, as does lutein. In this article, we summarize and discuss insights and some surprises into the absorption and metabolism of vitamin A, beta-carotene, and lutein that were gained with the use of isotope tracers in humans, rats, and cells as models.     

Involvement of retinoid X receptor alpha in coenzyme Q metabolism

The nuclear retinoid X receptor alpha (RXRalpha) is the heterodimer partner in several nuclear receptors, some of them regulating lipid biosynthesis. Since coenzyme Q (CoQ) levels are greatly modified in aging and a number of diseases, we have investigated the involvement of RXRalpha in the biosynthetic regulation of this lipid by using a hepatocyte-specific RXRalpha-deficient mouse strain (RXRalpha-def). In the receptor-deficient liver, the amount of CoQ decreased to half of the control, and it was demonstrated that this decrease was caused by a significantly lowered rate of biosynthesis. On the other hand, induction of CoQ was extensive in both control and RXRalpha-def liver using the peroxisomal inducer di(2-ethylhexyl)phthalate (DEHP). Since the RXRalpha deficiency was specific to liver, no change in CoQ content or biosynthesis was observed in kidney. The other mevalonate pathway lipids, cholesterol and dolichol, were unchanged in the RXRalpha-def liver. Upon treatment with DEHP, cholesterol decreased in the control but remained unchanged in the receptor-deficient mice. In control mice, cold exposure elevated CoQ levels by 60%, but this induction did not occur in the liver of RXRalpha-def mice. In contrast, PPARalpha-null mice, which lack induction upon treatment with peroxisomal inducers, respond to cold exposure and CoQ content is increased. The amount of cholesterol decreased in both control and RXRalpha-def liver upon cold treatment. The results demonstrate that RXRalpha is required for CoQ biosynthesis and for its induction upon cold treatment, but does not appear to be involved in the basic synthesis of cholesterol and dolichol. The receptor is not involved in the elevated CoQ biosynthesis during peroxisomal induction.     

Selenium metabolism and function

Selenium like many other elements has a bimodal effect. At lower concentrations selenium is an essential trace element necessary for the growth of man and animals. However, at higher concentrations, selenium has toxic properties for man and animals. The biochemistry of selenium as a component of glutathione peroxidase and the analytical properties of selenium are also outlined.     

Mathematical modeling in metal metabolism: Overview and perspectives

A review of mathematical modeling in metal metabolism is presented. Both endogenous and exogenous metals are considered. Four classes of methods are considered: Petri nets, multi-agent systems, determinist models based on differential equations and stochastic models. For each, a basic theoretical background is given, then examples of applications are given, detailed and commented. Advantages and disadvantages of each class of model are presented. A special attention is given to determinist differential equation models, since almost all models belong to this class.     

Development of biotin-retinoid conjugates as chemical probes for analysis of retinoid function

Herein, we report the rational design, synthesis and biological evaluation of conjugates consisting of the synthetic retinoid Am580 and biotin connected via a linker moiety. We found that the linking substructure between the retinoid part and the biotin part is critical for retaining the biological activity. Conjugate 4 with a shorter linker showed similar potency to endogenous retinoid ATRA (1) and the parent compound Am580 (2) for neural differentiation of mouse embryotic carcinoma P19 cells, and showed the same pattern of induction of gene expression. It is expected to be useful as a probe for investigations of retinoid function. The design rationale and structure-activity relationship of the linker moiety are expected to be helpful for developing biotin conjugates of other nuclear receptor ligands.     

Diphenylamine-based retinoid antagonists: regulation of RAR and RXR function depending on the N-substituent

Based upon the structure-activity relationships of diphenylamine derivatives with retinoid synergistic activity (RXR agonists), novel diphenylamine derivatives with a long alkyl chain (9a and 9b) or a benzyl group (10a-f) as the N-substituent were designed and synthesized. All the synthesized compounds dose-dependently inhibited HL-60 cell differentiation induced by 3.3×10(-10)M Am80. Among them, compound 10f showed the most potent inhibitory activity, and the mechanism was shown, by means of transactivation assay for RARs and RXRs, to involve antagonism against RARs. The N-substituent of the diphenylamine skeleton plays an important role in determining the receptor selectivity for RARs or RXRs, as well as the agonist or antagonist nature of the activity.     

Overview of coenzyme A metabolism and its role in cellular toxicity

Coenzyme A (CoASH) has a clearly defined role as a cofactor for a number of oxidative and biosynthetic reactions in intermediary metabolism. Formation of acyl-CoA thioesters from organic carboxylic acids activates the acid for further biotransformation reactions and facilitates enzyme recognition. Xenobiotic carboxylic acids can also form CoA-thioesters, and the resulting acyl-CoA may contribute to the compound's toxicity. Generation of an unusual or poorly-metabolized acyl-CoA from a xenobiotic may lead to cellular metabolic dysfunction through several types of mechanisms including: (1) inhibition of key metabolic enzymes by the acyl-CoA; (2) sequestration of the total cellular CoA pool as the unusual acyl-CoA; (3) physical-chemical effects of the acyl-CoA; and (4) sequestration and depletion of carnitine as the acyl group is transformed from the acyl-CoA to form the corresponding acylcarnitine. Many of these toxicities are similar to sequelae observed in the inherited organic acidurias in which endogenously-generated acyl-CoAs accumulate secondary to an enzymopathy. Insights into the cellular mechanisms of xenobiotic acyl-CoA accumulation have been derived from model systems developed to understand organic acidemias, such as the methylmalonyl-CoA accumulation of the methylmalonic acidurias. The relevance of acyl-CoA accretion to human pathophysiology has now been well established, and identification of the relevant mechanism of toxicity can allow implementation of strategies to minimize the metabolic injury. Additionally, recognition of the potential for acyl-CoA mediated xenobiotic injury should result in improved rational drug design and earlier recognition of such toxicity when it develops.     

Molecular basis for differential substrate specificity in class IV alcohol dehydrogenases: a conserved function in retinoid metabolism but not in ethanol oxidation

Mammalian class IV alcohol dehydrogenase enzymes are characteristic of epithelial tissues, exhibit moderate to high K(m) values for ethanol, and are very active in retinol oxidation. The human enzyme shows a K(m) value for ethanol which is 2 orders of magnitude lower than that of rat class IV. The uniquely significant difference in the substrate-binding pocket between the two enzymes appears to be at position 294, Val in the human enzyme and Ala in the rat enzyme. Moreover, a deletion at position 117 (Gly in class I) has been pointed out as probably responsible for class IV specificity toward retinoids. With the aim of establishing the role of these residues, we have studied the kinetics of the recombinant human and rat wild-type enzymes, the human G117ins and V294A mutants, and the rat A294V mutant toward aliphatic alcohols and retinoids. 9-cis-Retinol was the best retinoid substrate for both human and rat class IV, strongly supporting a role of class IV in the generation of 9-cis-retinoic acid. In contrast, 13-cis retinoids were not substrates. The G117ins mutant showed a decreased catalytic efficiency toward retinoids and toward three-carbon and longer primary aliphatic alcohols, a behavior that resembles that of the human class I enzyme, which has Gly(117). The K(m) values for ethanol dramatically changed in the 294 mutants, where the human V294A mutant showed a 280-fold increase, and the rat A294V mutant a 50-fold decrease, compared with those of the respective wild-type enzymes. This demonstrates that the Val/Ala exchange at position 294 is mostly responsible for the kinetic differences with ethanol between the human and rat class IV. In contrast, the kinetics toward retinoids was only slightly affected by the mutations at position 294, compatible with a more conserved function of mammalian class IV alcohol dehydrogenase in retinoid metabolism.     

Overview of interleukin-2 function, production and clinical applications

The existence of interleukin (IL)-2 has been recognized for over 25 years, and it remains one of the most extensively studied cytokines. Here we present a broad overview of IL-2 history, functional activities, biological sources, regulation and applications to disease treatment. IL-2 exerts a wide spectrum of effects on the immune system, and it plays crucial roles in regulating both immune activation and homeostasis. Both IL-2 and its multipartite receptor are prototypical of the Type I receptor superfamily, and both have been exploited in numerous ways in the clinic. Despite the wealth of information generated about IL-2 from in vitro culture systems, in vivo mouse knockout models, and clinical trials in humans, fascinating new aspects of its functions in the immune system continue to emerge.     

Impaired renal function and aluminum metabolism

The consequences of renal functional impairment on aluminum (Al) excretion are not clear inasmuch as little is known about its glomerular filtration, tubular reabsorption, or secretion. The association of Al and the etiology of the dialysis encephalopathy syndrome and osteomalacia among patients with uremia suggests that renal functional impairment is a prerequisite for increased body Al stores. However, considerable evidence argues against the concept that tissue Al accumulation occurs as a simple consequence of renal failure. Many dialysis patients have high parathyroid hormone (PTH) concentrations that have been associated with neurologic abnormalities, bone disease, and anemia. The toxicity of PTH could be either direct or indirect by influencing the metabolism of potentially toxic substances such as Al. Our studies in normal rats suggest that gastrointestinal Al absorption and specific tissue burdens are enhanced by PTH, but not irreversibly, because the withdrawal of PTH resulted in Al egress. Dialysis patients are often treated with vitamin D analogs to prevent or control consequences of hyperparathyroidism and impaired 1,25-dihydroxycholecalciferol synthesis. Although some reports suggest that high bone Al in osteomalacia may be responsible for vitamin D resistance, our studies with normal rats suggest that its metabolites may also increase tissue Al burdens independent of PTH action. Thus, several factors operative in uremia other than impaired renal function may contribute to altered Al metabolism and, consequently, to its toxicity.