Therapeutic potential of the inhibition of the retinoic acid hydroxylases CYP26A1 and CYP26B1 by xenobiotics

Development and validation of CYP26A1 inhibition assay for high-throughput screening

All-trans retinoic acid (atRA) is an endogenous ligand of the retinoic acid receptors, which heterodimerize with retinoid X receptors. AtRA is generated in tissues from vitamin A (retinol) metabolism to form a paracrine signal and is locally degraded by cytochrome P450 family 26 (CYP26) enzymes. The CYP26 family consists of three subtypes: A1, B1, and C1, which are differentially expressed during development. This study aims to develop and validate a high throughput screening assay to identify CYP26A1 inhibitors in a cell-free system using a luminescent P450-Glo assay technology. The assay performed well with a signal to background ratio of 25.7, a coefficient of variation of 8.9%, and a Z-factor of 0.7. To validate the assay, we tested a subset of 39 compounds that included known CYP26 inhibitors and retinoids, as well as positive and negative control compounds selected from the literature and/or the ToxCast/Tox21 portfolio. Known CYP26A1 inhibitors were confirmed, and predicted CYP26A1 inhibitors, such as chlorothalonil, prochloraz, and SSR126768, were identified, demonstrating the reliability and robustness of the assay. Given the general importance of atRA as a morphogenetic signal and the localized expression of Cyp26a1 in embryonic tissues, a validated CYP26A1 assay has important implications for evaluating the potential developmental toxicity of chemicals.

Cytochrome P450 Enzymes as Drug Targets in Human Disease

Although the mention of cytochrome P450 (P450) inhibition usually brings to mind unwanted variability in pharmacokinetics, in several cases P450s are good targets for inhibition. These P450s are essential, but in certain disease states, it is desirable to reduce the concentrations of their products. Most of the attention to date has been with human P450s 5A1, 11A1, 11B1, 11B2, 17A1, 19A1, and 51A1. In some of those cases, there are multiple drugs in use, e.g., exemestane, letrozole, and anastrozole with P450 19A1, the steroid aromatase target in breast cancer. There are also several targets that are less developed, e.g., P450s 2A6, 8B1, 4A11, 24A1, 26A1, and 26B1. SIGNIFICANCE STATEMENT: The selective inhibition of certain cytochrome P450s that have major physiological functions has been shown to be very efficacious in certain human diseases. In several cases, the search for better drugs continues.

Blocking endogenous retinoic acid degradation induces oral tooth formation in zebrafish

According to Dollo's Law of irreversibility in evolution, a lost structure is usually considered to be unable to reappear in evolution due to the accumulation over time of mutations in the genes required for its formation. Cypriniform fish are a classic model of evolutionary loss because, while they form fully operational teeth in the ventral posterior pharynx, unlike other teleosts, they do not possess oral teeth. Paleontological data show that Cypriniforms, a clade of teleost fish that includes the zebrafish, lost their oral teeth 50 to 100 Mya. In order to attempt to reverse oral tooth loss in zebrafish, we block the degradation of endogenous levels of retinoic acid (RA) using a specific inhibitor of the Cyp26 RA degrading enzymes. We demonstrate the inhibition of endogenous RA degradation is sufficient to restore oral tooth induction as marked by the re-appearance of expression of early dental mesenchyme and epithelium genes such as dlx2b and sp7 in the oral cavity. Furthermore, we show that these exogenously induced oral tooth germs are able to be at least partly calcified. Taken together, our data show that modifications of signaling pathways can have a significant effect on the reemergence of once-lost structures leading to experimentally induced reversibility of evolutionary tooth loss in cypriniforms.

CYP26A1 Links WNT and Retinoic Acid Signaling: A Target to Differentiate ALDH+ Stem Cells in APC-Mutant CRC

APC mutation is the main driving mechanism of CRC development and leads to constitutively activated WNT signaling, overpopulation of ALDH+ stem cells (SCs), and incomplete differentiation. We previously reported that retinoic acid (RA) receptors are selectively expressed in ALDH+ SCs, which provides a way to target cancer SCs with retinoids to induce differentiation. Hypotheses: A functional link exists between the WNT and RA pathways, and APC mutation generates a WNT:RA imbalance that decreases retinoid-induced differentiation and increases ALDH+ SCs. Accordingly, to restore parity in WNT:RA signaling, we induce wt-APC expression in APC-mutant CRC cells, and we assess the ability of all-trans retinoic acid (ATRA) to induce differentiation. We found that ATRA increased expression of the WNT target gene, CYP26A1, and inducing wt-APC reduced this expression by 50%. Thus, the RA and WNT pathways crosstalk to modulate CYP26A1, which metabolizes retinoids. Moreover, inducing wt-APC augments ATRA-induced cell differentiation by: (i) decreasing cell proliferation; (ii) suppressing ALDH1A1 expression; (iii) decreasing ALDH+ SCs; and (iv) increasing neuroendocrine cell differentiation. A novel CYP26A1-based network that links WNT and RA signaling was also identified by NanoString profiling/bioinformatics analysis. Furthermore, CYP26A1 inhibitors sensitized CRC cells to the anti-proliferative effect of drugs that downregulate WNT signaling. Notably, in wt-APC-CRCs, decreased CYP26A1 improved patient survival. These findings have strong potential for clinical translation.

Retinoic acid signaling in development and differentiation commitment and its regulatory topology

Retinoic acid (RA), the derivative of vitamin A/retinol, is a signaling molecule with important implications in health and disease. It is a well-known developmental morphogen that functions mainly through the transcriptional activity of nuclear RA receptors (RARs) and, uncommonly, through other nuclear receptors, including peroxisome proliferator-activated receptors. Intracellular RA is under spatiotemporally fine-tuned regulation by synthesis and degradation processes catalyzed by retinaldehyde dehydrogenases and P450 family enzymes, respectively. In addition to dictating the transcription architecture, RA also impinges on cell functioning through non-genomic mechanisms independent of RAR transcriptional activity. Although RA-based differentiation therapy has achieved impressive success in the treatment of hematologic malignancies, RA also has pro-tumor activity. Here, we highlight the relevance of RA signaling in cell-fate determination, neurogenesis, visual function, inflammatory responses and gametogenesis commitment. Genetic and post-translational modifications of RAR are also discussed. A better understanding of RA signaling will foster the development of precision medicine to improve the defects caused by deregulated RA signaling.

Neural plate progenitors give rise to both anterior and posterior pituitary cells

The pituitary is the master neuroendocrine gland, which regulates body homeostasis. It consists of the anterior pituitary/adenohypophysis harboring hormones producing cells and the posterior pituitary/neurohypophysis, which relays the passage of hormones from the brain to the periphery. It is accepted that the adenohypophysis originates from the oral ectoderm (Rathke's pouch), whereas the neural ectoderm contributes to the neurohypophysis. Single-cell transcriptomics of the zebrafish pituitary showed that cyp26b1-positive astroglial pituicytes of the neurohypophysis and prop1-positive adenohypophyseal progenitors expressed common markers implying lineage relatedness. Genetic tracing identifies that, in contrast to the prevailing dogma, neural plate precursors of zebrafish (her4.3+) and mouse (Sox1+) contribute to both neurohypophyseal and a subset of adenohypophyseal cells. Pituicyte-derived retinoic-acid-degrading enzyme Cyp26b1 fine-tunes differentiation of prop1+ progenitors into hormone-producing cells. These results challenge the notion that adenohypophyseal cells are exclusively derived from non-neural ectoderm and demonstrate that crosstalk between neuro- and adeno-hypophyseal cells affects differentiation of pituitary cells.

Genome-Wide Association Studies to Drug: Identifying Retinoic Acid Metabolism Blocking Agents to Suppress Mechanoflammation in Osteoarthritis

Osteoarthritis (OA) is a highly prevalent debilitating joint disease for which there are currently no licensed disease-modifying treatments. The pathogenesis of OA is complex, involving genetic, mechanical, biochemical, and environmental factors. Cartilage injury, arguably the most important driving factor in OA development, is able to activate both protective and inflammatory pathways within the tissue. Recently, >100 genetic risk variants for OA have been identified through Genome Wide Association Studies, which provide a powerful tool to validate existing putative disease pathways and discover new ones. Using such an approach, hypomorphic variants within the aldehyde dehydrogenase 1 family member A2 (ALDH1A2) gene were shown to be associated with increased risk of severe hand OA. ALDH1A2 encodes the enzyme that synthesizes all-trans retinoic acid (atRA), an intracellular signaling molecule. This review summarizes the influence of the genetic variants on expression and function of ALDH1A2 in OA cartilage, its role in the mechanical injury response of cartilage, and its potent anti-inflammatory effect after cartilage injury. In doing so it identifies atRA metabolism-blocking agents as potential treatments for suppressing mechanoflammation in OA.

3,4-Desaturation of retinoic acid by cytochrome P450 27C1 prevents P450-mediated catabolism

Cytochrome P450 (P450, CYP) 27C1 is expressed in human skin and catalyzes the 3,4-desaturation of retinoids. The enzyme has a relatively high specificity constant (k_cat/K_m), and ∼¼ of the retinoids in human skin are in the desaturated form but their function is unknown. 3,4-Dehydroretinoic acid (also didehydroretinoic acid, ddRA) has similar affinity as all-trans retinoic acid (atRA) for retinoid X and retinoic acid receptors (RXRs/RAR). The metabolism of ddRA is unknown, and we considered the hypothesis that desaturation might be a protective mechanism in maintaining active retinoid levels in the body. There are limited theoretical products that can result from ddRA oxidation. We optimized conditions for oxidation of atRA by human liver microsomes-a slow loss of atRA was seen due to 4-oxidation but no loss of ddRA was observed under the same conditions. We evaluated the HPLC peaks that were observed in microsomal incubations with ddRA using UV spectroscopy, NaBH₄ and NaBD₄ reduction, and mass spectrometry. None were potential ddRA oxidation products, and none were increased in the presence of the P450 cofactor NADPH. Known P450 inhibitors had no effects on the levels of these compounds. We conclude that ddRA is not readily oxidized by P450s and that one role of desaturation may be the maintenance of levels of functional retinoids.

Cyp26a1 supports postnatal retinoic acid homeostasis and glucoregulatory control

Considerable evidence confirms the importance of Cyp26a1 to all-trans-retinoic acid (RA) homeostasis during embryogenesis. In contrast, despite its presence in postnatal liver as a potential major RA catabolizing enzyme and its acute sensitivity to induction by RA, some data suggested that Cyp26a1 contributes only marginally to endogenous RA homeostasis postnatally. We report reevaluation of a conditional Cyp26a1 knockdown in the postnatal mouse. The current results show that Cyp26a1 mRNA in WT mouse liver increases 16-fold upon refeeding after a fast, accompanied by an increased rate of RA elimination and a 41% decrease in the RA concentration. In contrast, Cyp26a1 mRNA in the refed homozygotic knockdown reached only 2% of its extent in WT during refeeding, accompanied by a slower rate of RA catabolism and no decrease in liver RA, relative to fasting. Refed homozygous knockdown mice also had decreased Akt1 and 2 phosphorylation and pyruvate dehydrogenase kinase 4 (Pdk4) mRNA and increased glucokinase (Gck) mRNA, glycogen phosphorylase (Pygl) phosphorylation, and serum glucose, relative to WT. Fasted homozygous knockdown mice had increased glucagon/insulin relative to WT. These data indicate that Cyp26a1 participates prominently in moderating the postnatal liver concentration of endogenous RA and contributes essentially to glucoregulatory control.

Disruption of retinol-mediated IL-6 expression in colon cancer-associated fibroblasts: new perspectives on the role of vitamin A metabolism

Stromal myo-/fibroblasts (MFs) account for up to 30% of lamina propria cells in the normal human colon and their number is dramatically increased in colon cancer (CRC). Fibroblasts from cancers, also known as cancer-associated fibroblasts (CAFs), differ from normal colonic MF (N-MFs) and support tumor-promoting inflammation, in part due to increased IL-6 secretion. In this editorial, we highlight recent data obtained regarding IL-6 regulation in colorectal cancer CAFs through vitamin A (retinol) metabolism, discuss current limitations in our understanding of the mechanisms leading to the CAF pro-inflammatory phenotype, and discuss potential approaches to target CAF retinoid metabolism during CRC treatment.

Ethanol exposure perturbs sea urchin development and disrupts developmental timing

Ethanol is a known vertebrate teratogen that causes craniofacial defects as a component of fetal alcohol syndrome (FAS). Our results show that sea urchin embryos treated with ethanol similarly show broad skeletal patterning defects, potentially analogous to the defects associated with FAS. The sea urchin larval skeleton is a simple patterning system that involves only two cell types: the primary mesenchymal cells (PMCs) that secrete the calcium carbonate skeleton and the ectodermal cells that provide migratory, positional, and differentiation cues for the PMCs. Perturbations in RA biosynthesis and Hh signaling pathways are thought to be causal for the FAS phenotype in vertebrates. Surprisingly, our results indicate that these pathways are not functionally relevant for the teratogenic effects of ethanol in developing sea urchins. We found that developmental morphology as well as the expression of some ectodermal and PMC genes was delayed by ethanol exposure. Temporal transcriptome analysis revealed significant impacts of ethanol on signaling and metabolic gene expression, and a disruption in the timing of GRN gene expression that includes both delayed and precocious gene expression throughout the specification network. We conclude that the skeletal patterning perturbations in ethanol-treated embryos likely arise from a loss of temporal synchrony within and between the instructive and responsive tissues.

Variants in ALDH1A2 reveal an anti-inflammatory role for retinoic acid and a new class of disease-modifying drugs in osteoarthritis

More than 40% of individuals will develop osteoarthritis (OA) during their lifetime, yet there are currently no licensed disease-modifying treatments for this disabling condition. Common polymorphic variants in ALDH1A2, which encodes the key enzyme for synthesis of all-trans retinoic acid (atRA), are associated with severe hand OA. Here, we sought to elucidate the biological significance of this association. We first confirmed that ALDH1A2 risk variants were associated with hand OA in the U.K. Biobank. Articular cartilage was acquired from 33 individuals with hand OA at the time of routine hand OA surgery. After stratification by genotype, RNA sequencing was performed. A reciprocal relationship between ALDH1A2 mRNA and inflammatory genes was observed. Articular cartilage injury up-regulated similar inflammatory genes by a process that we have previously termed mechanoflammation, which we believe is a primary driver of OA. Cartilage injury was also associated with a concomitant drop in atRA-inducible genes, which were used as a surrogate measure of cellular atRA concentration. Both responses to injury were reversed using talarozole, a retinoic acid metabolism blocking agent (RAMBA). Suppression of mechanoflammation by talarozole was mediated by a peroxisome proliferator-activated receptor gamma (PPARγ)-dependent mechanism. Talarozole was able to suppress mechano-inflammatory genes in articular cartilage in vivo 6 hours after mouse knee joint destabilization and reduced cartilage degradation and osteophyte formation after 26 days. These data show that boosting atRA suppresses mechanoflammation in the articular cartilage in vitro and in vivo and identifies RAMBAs as potential disease-modifying drugs for OA.

Retinol Depletion in COVID-19

Background and aims

COVID-19 has been a devastating pandemic. There are indications that vitamin A is depleted during infections. Vitamin A is important in development and immune homeostasis. It has been used successfully in measles, RSV and AIDS infections. In this study, we aimed to measure the serum retinol levels in severe COVID-19 patients to assess the importance of vitamin A in the COVID-19 pathogenesis.

Methods

The serum retinol level was measured in two groups of patients: the COVID-19 group, which consisted of 27 severe COVID-19 patients hospitalized in the intensive care unit with respiratory failure, and the control group, which consisted of 23 patients without COVID-19 symptoms.

Results

The mean serum retinol levels were 0.37 mg/L in the COVID-19 group and 0.52 mg/L in the control group. The difference between the serum retinol levels in the two groups was statistically significant. There was no significant difference in retinol levels between different ages and genders within the COVID-19 group. Comorbidity did not affect serum retinol levels.

Conclusion

The serum retinol level was significantly lower in patients with severe COVID-19, and this difference was independent of age or underlying comorbidity. Our data show that retinol and retinoic acid signaling might be important in immunopathogenesis of COVID-19.

Multilayer omics analysis reveals a non-classical retinoic acid signaling axis that regulates hematopoietic stem cell identity

Hematopoietic stem cells (HSCs) rely on complex regulatory networks to preserve stemness. Due to the scarcity of HSCs, technical challenges have limited our insights into the interplay between metabolites, transcription, and the epigenome. In this study, we generated low-input metabolomics, transcriptomics, chromatin accessibility, and chromatin immunoprecipitation data, revealing distinct metabolic hubs that are enriched in HSCs and their downstream multipotent progenitors. Mechanistically, we uncover a non-classical retinoic acid (RA) signaling axis that regulates HSC function. We show that HSCs rely on Cyp26b1, an enzyme conventionally considered to limit RA effects in the cell. In contrast to the traditional view, we demonstrate that Cyp26b1 is indispensable for production of the active metabolite 4-oxo-RA. Further, RA receptor beta (Rarb) is required for complete transmission of 4-oxo-RA-mediated signaling to maintain stem cells. Our findings emphasize that a single metabolite controls stem cell fate by instructing epigenetic and transcriptional attributes.

Retinoids as Chemo-Preventive and Molecular-Targeted Anti-Cancer Therapies

Retinoic acid (RA) agents possess anti-tumor activity through their ability to induce cellular differentiation. However, retinoids have not yet been translated into effective systemic treatments for most solid tumors. RA signaling is mediated by the following two nuclear retinoic receptor subtypes: the retinoic acid receptor (RAR) and the retinoic X receptor (RXR), and their isoforms. The identification of mutations in retinoid receptors and other RA signaling pathway genes in human cancers offers opportunities for target discovery, drug design, and personalized medicine for distinct molecular retinoid subtypes. For example, chromosomal translocation involving RARA occurs in acute promyelocytic leukemia (APL), and all-trans retinoic acid (ATRA) is a highly effective and even curative therapeutic for APL patients. Thus, retinoid-based target discovery presents an important line of attack toward designing new, more effective strategies for treating other cancer types. Here, we review retinoid signaling, provide an update on retinoid agents and the current clinical research on retinoids in cancer, and discuss how the retinoid pathway genotype affects the ability of retinoid agents to inhibit the growth of colorectal cancer (CRC) cells. We also deliberate on why retinoid agents have not shown clinical efficacy against solid tumors and discuss alternative strategies that could overcome the lack of efficacy.

An adverse outcome pathway on the disruption of retinoic acid metabolism leading to developmental craniofacial defects

Adverse outcome pathway (AOP) is a conceptual framework that links a molecular initiating event (MIE) via intermediate key events (KEs) with adverse effects (adverse outcomes, AO) relevant for risk assessment, through defined KE relationships (KERs). The aim of the present work is to describe a linear AOP, supported by experimental data, for skeletal craniofacial defects as the AO. This AO was selected in view of its relative high incidence in humans and the suspected relation to chemical exposure. We focused on inhibition of CYP26, a retinoic acid (RA) metabolizing enzyme, as MIE, based on robust previously published data. Conazoles were selected as representative stressors. Intermediate KEs are RA disbalance, aberrant HOX gene expression, disrupted specification, migration, and differentiation of neural crest cells, and branchial arch dysmorphology. We described the biological basis of the postulated events and conducted weight of evidence (WoE) assessments. The biological plausibility and the overall empirical evidence were assessed as high and moderate, respectively, the latter taking into consideration the moderate evidence for concordance of dose-response and temporal relationships. Finally, the essentiality assessment of the KEs, considered as high, supported the robustness of the presented AOP. This AOP, which appears of relevance to humans, thus contributes to mechanistic underpinning of selected test methods, thereby supporting their application in integrated new approach test methodologies and strategies and application in a regulatory context.

Preclinical assessment of dual CYP26[A1/B1] inhibitor, DX308, as an improved treatment for keratinization disorders

Background

Retinoid‐based therapies are commonly used in the treatment of disorders of keratinization and other skin disorders but can result in non‐specific effects and adverse reactions. Use of retinoic acid metabolism blocking agents (RAMBAs) such as DX308 may address these shortcomings.

Objectives

Characterize the therapeutic potential of recently discovered, CYP26‐selective RAMBA, DX308.

Materials and Methods

Preliminary in vitro assessment of potential off‐target activity, metabolic and toxicologic profiling. Studies to assess safety and efficacy of topical treatment in correcting abnormal skin morphology in rhino mice. Extensive gene expression profiling by RNA sequencing and qPCR in 3D epidermis grown with keratinocytes (KCs) from keratinization disorders and healthy controls, to investigate modulation of retinoid biopathways.

Results

In vitro, DX308 does not interact with off‐target nuclear receptors or CYP450s, is not genotoxic, and is stable in skin, despite vigorous hepatic metabolism. In vivo, topical DX308 induces comedolysis and epidermal thickening without apparent adverse effects. Gene expression profiling shows potent modulation of retinoid‐responsive genes by DX308 in both healthy and keratinization disorder KCs. Pathway analysis suggests DX308 may inhibit inflammatory and immune responses in KCs.

Conclusions

These preliminary studies suggest that DX308 is an efficacious topical therapeutic with a favourable metabolic and safety profiles. DX308 may present an improved therapeutic alternative for the treatment of keratinization disorders and other retinoid‐responsive skin ailments.

Differential RA responsiveness among subsets of mouse late progenitor spermatogonia

Initiation of spermatogonial differentiation in the mouse testis begins with the response to retinoic acid (RA) characterized by activation of KIT and STRA8 expression. In the adult, spermatogonial differentiation is spatiotemporally coordinated by a pulse of RA every 8.6 days that is localized to stages VII-VIII of the seminiferous epithelial cycle. Dogmatically, progenitor spermatogonia that express retinoic acid receptor gamma (RARG) at these stages will differentiate in response to RA, but this has yet to be tested functionally. Previous single-cell RNA-seq data identified phenotypically and functionally distinct subsets of spermatogonial stem cells (SSCs) and progenitor spermatogonia, where late progenitor spermatogonia were defined by expression of RARG and Dppa3. Here, we found late progenitor spermatogonia (RARGhigh KIT-) were further divisible into two subpopulations based on Dppa3 reporter expression (Dppa3-ECFP or Dppa3-EGFP) and were observed across all stages of the seminiferous epithelial cycle. However, nearly all Dppa3+ spermatogonia were differentiating (KIT+) late in the seminiferous epithelial cycle (stages X-XII), while Dppa3- late progenitors remained abundant, suggesting that Dppa3+ and Dppa3- late progenitors differentially responded to RA. Following acute RA treatment (2-4 h), significantly more Dppa3+ late progenitors induced KIT, including at the midpoint of the cycle (stages VI-IX), than Dppa3- late progenitors. Subsequently, single-cell analyses indicated a subset of Dppa3+ late progenitors expressed higher levels of Rxra, which we confirmed by RXRA whole-mount immunostaining. Together, these results indicate RARG alone is insufficient to initiate a spermatogonial response to RA in the adult mouse testis and suggest differential RXRA expression may discriminate responding cells.

Cardiac retinoic acid levels decline in heart failure

Although low circulating levels of the vitamin A metabolite, all-trans retinoic acid (ATRA), are associated with increased risk of cardiovascular events and all-cause mortality, few studies have addressed whether cardiac retinoid levels are altered in the failing heart. Here, we showed that proteomic analyses of human and guinea pig heart failure (HF) were consistent with a decline in resident cardiac ATRA. Quantitation of the retinoids in ventricular myocardium by mass spectrometry revealed 32% and 39% ATRA decreases in guinea pig HF and in patients with idiopathic dilated cardiomyopathy (IDCM), respectively, despite ample reserves of cardiac vitamin A. ATRA (2 mg/kg/d) was sufficient to mitigate cardiac remodeling and prevent functional decline in guinea pig HF. Although cardiac ATRA declined in guinea pig HF and human IDCM, levels of certain retinoid metabolic enzymes diverged. Specifically, high expression of the ATRA-catabolizing enzyme, CYP26A1, in human IDCM could dampen prospects for an ATRA-based therapy. Pertinently, a pan-CYP26 inhibitor, talarozole, blunted the impact of phenylephrine on ATRA decline and hypertrophy in neonatal rat ventricular myocytes. Taken together, we submit that low cardiac ATRA attenuates the expression of critical ATRA-dependent gene programs in HF and that strategies to normalize ATRA metabolism, like CYP26 inhibition, may have therapeutic potential.

Multibiomarker-based assessment of toxicity of central European strains of filamentous cyanobacteria Aphanizomenon gracile and Raphidiopsis raciborskii to zebrafish Danio rerio

The global increase in cyanobacterial blooms poses environmental and health threats. Selected cyanobacterial strains reveal toxicities despite a lack of synthesis of known toxic metabolites, and the mechanisms of these toxicities are not well understood. Here we investigated the toxicity of non-cylindrospermopsin and non-microcystin producing Aphanizomenon gracile and Raphidiopsis raciborskii of Central European origin to zebrafish exposed for 14 days to their extracts. Toxicological screening revealed the presence of anabaenopeptins and a lack of anatoxin-a, ß-methylamino-L-alanine or saxitoxins in examined extracts. The responses were compared to 20 μg L^-1 of common cyanobacterial toxins cylindrospermopsin (CYN) and microcystin-LR (MC-LR). The expression of the marker genes involved in apoptosis (caspase 3a and 3b, Bcl-2, BAX, p53, MAPK, Nrf2), DNA damage detection and repair (GADD45, RAD51, JUN, XPC), detoxification (CYP1A, CYP26, EPHX1), lipid metabolism (PPARa, FABP1, PLA2), phosphorylation/dephosphorylation (PPP6C, PPM1) and cytoskeleton (actin, tubulin) were examined using targeted transcriptomics. Cellular stress and toxicity biomarkers (oxidative injury, antioxidant enzymes, thiol pool status, and lactate dehydrogenase activity) were measured in the liver, and acetylcholinesterase activity was determined as an index of neurotoxicity in the brain. The extracts of three cyanobacterial strains that produce no known cyanotoxins caused marked toxicity in D. rerio, and the biomarker profiles indicate different toxic mechanisms between the bioactive compounds extracted from these strains and the purified cyanotoxins. All studied cyanobacterial extracts and purified cyanotoxins induced oxidative stress and neurotoxicity, downregulated Nrf2 and CYP26B1, disrupted phosphorylation/dephosphorylation processes and actin/tubulin cytoskeleton and upregulated apoptotic activity in the liver. The tested strains and purified toxins displayed distinctively different effects on lipid metabolism. Unlike CYN and MC-LR, the Central European strain of A. gracile and R. raciborskii did not reveal a genotoxic potential. These findings help to further understand the ecotoxicological consequences of toxic cyanobacterial blooms in freshwater ecosystems.

Isoflavones Suppress Cyp26b1 Expression in the Murine Colonic Lamina Propria

Isoflavones have many biological activities and are major bioactive components of kakkonto, a traditional Japanese herbal medicine. We previously reported that the combined therapy of oral immune therapy (OIT) and kakkonto downregulates the mRNA expression of Cyp26b1, a major retinoic acid (RA)-degrading enzyme, in the colon of food allergy mice and thereby ameliorates allergic symptoms. In this study, we evaluated the effects of various isoflavones on Cyp26b1 expression in primary cultured lamina propria (LP) cells isolated from the mouse colon. The mRNA expression of Cyp26b1 was extremely downregulated by all isoflavones tested in the LP cells except for puerarin. In particular, genistein and genistin markedly suppressed Cyp26b1 mRNA expression without affecting RA-synthesizing enzyme expression. Moreover, to evaluate the effects of isoflavones on allergic reactions, genistein and genistin were administered to ovalbumin (OVA)-induced food allergy mice. Oral administration of genistin suppressed the development of allergic symptoms. These results raise the possibility that isoflavones elevated the level of RA in the colon by inhibiting RA degradation and then the high concentration of RA in the colon might exert immunosuppressive and antiallergic effects on food allergy mice.

COVID-19: Endogenous Retinoic Acid Theory and Retinoic Acid Depletion Syndrome

This study presents two new concepts and definitions to the medical literature. One of those is "endogenous retinoic acid theory" and the other "retinoic acid depletion syndrome". A new classification will be provided for the immune system: "retinoic acid-dependent component" and "retinoic acid non-dependent component". If this theory is verified, all the diseases where the retinoic acid metabolism is defective and retinoic acid levels are low will be identified and new approaches will be developed fortreating such diseases. When the need for retinoic acids increases, such as acute infection, high fever, severe catabolic process, or chronic antigenic stimulation, cytochrome oxidase enzymes are inhibited by drugs or internal mechanisms. Metabolism and excretion of retinoic acids stored in the liver are prevented. In this way, retinoic acid levels in the blood are raised to therapeutic levels. This is called "Endogenous Retinoic Acid Theory". Retinoic acids also manage their metabolism through feedback mechanisms. Despite compensatory mechanisms, causes such as high fever, serious catabolic process and excessively large viral genome (SARS-CoV-2), excessive use of RIG-I and Type I interferon synthesis pathway using retinoic acid causes emptying of retinoic acid stores. As a result, the RIG-I pathway becomes ineffective, Type I IFN synthesis stops, and the congenital immune system collapses. Then the immune mechanism passes to TLR3, TLR7, TLR8, TLR9, MDA5 and UPS pathways in the monocyte, macrophage, neutrophil and dendritic cells of the adaptive immune defense system that do not require retinoic acid. This leads to excessive TNFα and cytokine discharge from the pathway. With the depletion of retinoic acid stores as a result of this overuse, the immune defense mechanism switches from the congenital immune system to the adaptive immune system, where retinoic acids cannot be used. As a result of this depletion of retinoic acids, the shift of the immune system to the NFκB arm, which causes excessive cytokine release, is called "retinoic acid depletion syndrome". COVID-19 and previously defined sepsis, SIRS and ARDS are each retinoic acid depletion syndrome. We claim that retinoic acid metabolism is defective in most inflammatory diseases, particularly COVID-19 (cytokine storm) sepsis, SIRS and ARDS. Finding a solution to this mechanism will bring a new perspective and treatment approach to such diseases.

Knockout of Cyp26a1 and Cyp26b1 during postnatal life causes reduced lifespan, dermatitis, splenomegaly, and systemic inflammation in mice

All-trans-retinoic acid (atRA), the active metabolite of vitamin A, is an essential signaling molecule in all chordates. Global knockouts of the atRA clearing enzymes Cyp26a1 or Cyp26b1 are embryonic lethal. In adult rodents, inhibition of Cyp26a1 and Cyp26b1 increases atRA concentrations and signaling. However, postnatal knockout of Cyp26a1 does not cause a severe phenotype. We hypothesized that Cyp26b1 is the main atRA clearing Cyp in postnatal mammals. This hypothesis was tested by generating tamoxifen-inducible knockout mouse models of Cyp26b1 alone or with Cyp26a1. Both mouse models showed dermatitis, blepharitis, and splenomegaly. Histology showed infiltration of inflammatory cells including neutrophils and T lymphocytes into the skin and hyperkeratosis/hyperplasia of the nonglandular stomach. The mice lacking both Cyp26a1 and Cyp26b1 also had a reduced lifespan, failed to gain weight, and showed fat atrophy. There were significant changes in vitamin A homeostasis. Postnatal knockout of Cyp26b1 resulted in increased atRA concentrations in the skin while the postnatal knockout of both Cyp26a1 and Cyp26b1 resulted in increased atRA concentrations in the liver, serum, skin, spleen, and intestines. This study demonstrates the paramount role of Cyp26b1 in regulating retinoid homeostasis in postnatal life.

Liposomal delivery of hydrophobic RAMBAs provides good bioavailability and significant enhancement of retinoic acid signalling in neuroblastoma tumour cells

Retinoid treatment is employed during residual disease treatment in neuroblastoma, where the aim is to induce neural differentiation or death in tumour cells. However, although therapeutically effective, retinoids have only modest benefits and suffer from poor pharmacokinetic properties. In vivo, retinoids induce CYP26 enzyme production in the liver, enhancing their own rapid metabolic clearance, while retinoid resistance in tumour cells themselves is considered to be due in part to increased CYP26 production. Retinoic acid metabolism blocking agents (RAMBAs), which inhibit CYP26 enzymes, can improve retinoic acid (RA) pharmacokinetics in pre-clinical neuroblastoma models. Here, we demonstrate that in cultured neuroblastoma tumour cells, RAMBAs enhance RA action as seen by morphological differentiation, AKT signalling and suppression of MYCN protein. Although active as retinoid enhancers, these RAMBAs are highly hydrophobic and their effective delivery in humans will be very challenging. Here, we demonstrate that such RAMBAs can be loaded efficiently into cationic liposomal particles, where the RAMBAs achieve good bioavailability and activity in cultured tumour cells. This demonstrates the efficacy of RAMBAs in enhancing retinoid signalling in neuroblastoma cells and shows for the first time that liposomal delivery of hydrophobic RAMBAs is a viable approach, providing novel opportunities for their delivery and application in humans.

Characterization of CYP26B1-Selective Inhibitor, DX314, as a Potential Therapeutic for Keratinization Disorders

Inhibition of CYP450-mediated retinoic acid (RA) metabolism by RA metabolism blocking agents increases endogenous retinoids and is an alternative to retinoid therapy. Currently available RA metabolism blocking agents (i.e., liarozole and talarozole) tend to have fewer adverse effects than traditional retinoids but lack target specificity. Substrate-based inhibitor DX314 has enhanced selectivity for RA-metabolizing enzyme CYP26B1 and may offer an improved treatment option for keratinization disorders such as congenital ichthyosis and Darier disease. In this study, we used RT-qPCR, RNA sequencing, pathway, upstream regulator, and histological analyses to demonstrate that DX314 can potentiate the effects of all-trans-RA in healthy and diseased reconstructed human epidermis. We unexpectedly discovered that DX314, but not all-trans-RA or previous RA metabolism blocking agents, appears to protect epidermal barrier integrity. In addition, DX314-induced keratinization and epidermal proliferation effects are observed in a rhino mice model. Altogether, the results indicate that DX314 inhibits all-trans-RA metabolism with minimal off-target activity and shows therapeutic similarity to topical retinoids in vitro and in vivo. Findings of a barrier-protecting effect require further mechanistic study but may lead to a unique strategy in barrier-reinforcing therapies. DX314 is a promising candidate compound for further study and development in the context of keratinization disorders.

Cellular retinoic acid binding protein-II expression and its potential role in skin aging

Skin aging is an intricate biological process consisting of intrinsic and extrinsic alterations of epidermal and dermal structures. Retinoids play an important role in epidermal cell growth and differentiation and are beneficial to counteract skin aging. Cellular retinoic acid binding protein-II (CRABP-II) selectively binds all trans-retinoic acid, the most active retinoid metabolite, contributing to regulate intracytoplasmic retinoid trafficking and keratinocyte differentiation. Immunohistochemistry revealed a reduced epidermal and dermal CRABP-II expression in aged human and mouse skin. To better clarify the role of CRABP-II, we investigated age-related skin changes in CRABP-II knock-out mice. We documented an early reduction of keratinocyte layers, proliferation and differentiation rate, dermal and hypodermal thickness, pilosebaceous units and dermal vascularity in CRABP-II knock-out compared with wild-type mice. Ultrastructural investigation documented reduced number and secretion of epidermal lamellar bodies in CRABP-II knock-out compared with wild-type mice. Cultured CRABP-II knock-out-derived dermal fibroblasts proliferated less and showed reduced levels of TGF-β signal-related genes, Col1A1, Col1A2, and increased MMP2 transcripts compared with those from wild-type. Our data strongly support the hypothesis that a reduction of CRABP-II expression accelerates and promotes skin aging, and suggest CRABP-II as a novel target to improve the efficacy of retinoid-mediated anti-aging therapies.

Regulating Retinoic Acid Availability during Development and Regeneration: The Role of the CYP26 Enzymes

This review focuses on the role of the Cytochrome p450 subfamily 26 (CYP26) retinoic acid (RA) degrading enzymes during development and regeneration. Cyp26 enzymes, along with retinoic acid synthesising enzymes, are absolutely required for RA homeostasis in these processes by regulating availability of RA for receptor binding and signalling. Cyp26 enzymes are necessary to generate RA gradients and to protect specific tissues from RA signalling. Disruption of RA homeostasis leads to a wide variety of embryonic defects affecting many tissues. Here, the function of CYP26 enzymes is discussed in the context of the RA signalling pathway, enzymatic structure and biochemistry, human genetic disease, and function in development and regeneration as elucidated from animal model studies.

Interpreting pathways to discover cancer driver genes with Moonlight

Cancer driver gene alterations influence cancer development, occurring in oncogenes, tumor suppressors, and dual role genes. Discovering dual role cancer genes is difficult because of their elusive context-dependent behavior. We define oncogenic mediators as genes controlling biological processes. With them, we classify cancer driver genes, unveiling their roles in cancer mechanisms. To this end, we present Moonlight, a tool that incorporates multiple -omics data to identify critical cancer driver genes. With Moonlight, we analyze 8000+ tumor samples from 18 cancer types, discovering 3310 oncogenic mediators, 151 having dual roles. By incorporating additional data (amplification, mutation, DNA methylation, chromatin accessibility), we reveal 1000+ cancer driver genes, corroborating known molecular mechanisms. Additionally, we confirm critical cancer driver genes by analysing cell-line datasets. We discover inactivation of tumor suppressors in intron regions and that tissue type and subtype indicate dual role status. These findings help explain tumor heterogeneity and could guide therapeutic decisions.

Biochemical and physiological importance of the CYP26 retinoic acid hydroxylases

The Cytochrome P450 (CYP) family 26 enzymes contribute to retinoic acid (RA) metabolism and homeostasis in humans, mammals and other chordates. The three CYP26 family enzymes, CYP26A1, CYP26B1 and CYP26C1 have all been shown to metabolize all-trans-retinoic acid (atRA) it's 9-cisRA and 13-cisRA isomers and primary metabolites 4-OH-RA and 4-oxo-RA with high efficiency. While no crystal structures of CYP26 enzymes are available, the binding of various ligands has been extensively explored via homology modeling. All three CYP26 enzymes are inducible by treatment with atRA in various prenatal and postnatal tissues and cell types. However, current literature shows that in addition to regulation by atRA, CYP26 enzyme expression is also regulated by other endogenous processes and inflammatory cytokines. In humans and in animal models the expression patterns of CYP26 enzymes have been shown to be tissue and cell type specific, and the expression of the CYP26 enzymes is believed to regulate the formation of critical atRA concentration gradients in various tissue types. Yet, very little data exists on direct disease associations of altered CYP26 expression or activity. Nevertheless, data is emerging describing a variety of human genetic variations in the CYP26 enzymes that are associated with different pathologies. Interestingly, some of these genetic variants result in increased activity of the CYP26 enzymes potentially leading to complex gene-environment interactions due to variability in dietary intake of retinoids. This review highlights the current knowledge of structure-function of CYP26 enzymes and focuses on their role in human retinoid metabolism in different tissues.

Genistein Affects Expression of Cytochrome P450 (CYP450) Genes in Hepatocellular Carcinoma (HEPG2/C3A) Cell Line

BACKGROUND

Genistein (5,7-Dihydroxy-3-(4-hydroxyphenyl)-4H-1-benzopyran-4-one) is the most abundant isoflavone in soybean, which has been associated with a lower risk of development of cancer and cardiovascular diseases. Of particular interest regarding cancer preventive properties of flavonoids is their interaction with cytochrome P450 enzymes (CYPs). However, contradictory data report the effect of genistein on expression of СYPs enzymes.

OBJECTIVE

The aim of this study was to investigate the effects of genistein on cytochrome P450 (CYP) gene expression levels in human hepatocellular carcinoma (HepG2/C3A) and colon adenocarcinoma (HT29) cells.

METHODS

Real-time RT-PCR was used to examine the expression of genes families involved in xenobiotic metabolism, such as CYP1 (CYP1A1, CYP1B1), CYP2 (CYP2E1, CYP2D6), CYP3 (CYP3A4); and of a family involved in the catabolism of the all-trans-retinoic acid (ATRA), CYP26 (CYP26A1, CYP26B1).

RESULTS

RT-qPCR data analysis showed that after 12 h of exposure of HepG2/C3A cells to genistein (5 and 50 µM) there was an upregulation of CYP1A1 and CYP1B1 and downregulation of CYP2D6, CYP26A1 and CYP26B1 mRNA levels. There was no change in the mRNA levels of CYP P450 genes in HT29 cells.

CONCLUSION

Our results suggest that treatment with genistein in non-toxic concentrations may impact the expression level of CYPs involved in the biotransformation of xenobiotics and drug metabolizing enzymes. Moreover, the downregulation of ATRA metabolism-related genes opens a new research path for the study of genistein as retinoic acid metabolism blocking agent for treating cancer and other pathologies.

The retinoic acid hydroxylase Cyp26a1 has minor effects on postnatal vitamin A homeostasis, but is required for exogenous atRA clearance

The all-trans-retinoic acid (atRA) hydroxylase Cyp26a1 is essential for embryonic development and may play a key role in regulating atRA clearance also in adults. We hypothesized that loss of Cyp26a1 activity via inducible knockout in juvenile or adult mice would result in decreased atRA clearance and increased tissue atRA concentrations and atRA-related adverse effects. To test these hypotheses, Cyp26a1 was knocked out in juvenile and adult male and female Cyp26a1 floxed mice using standard Cre-Lox technology and tamoxifen injections. Biochemical and histological methods were used to study the effects of global Cyp26a1 knockout. The Cyp26a1 knockout did not result in consistent histopathological changes in any major organs. Cyp26a1 ^-/- mice gained weight normally and exhibited no adverse phenotypes for up to 1 year after loss of Cyp26a1 expression. Similarly, atRA concentrations were not increased in the liver, testes, spleen, or serum of these mice, and the Cyp26a1 knockout did not cause compensatory induction of lecithin:retinol acetyltransferase (Lrat) or retinol dehydrogenase 11 (Rdh11) mRNA or a decrease in aldehyde dehydrogenase 1a1 (Aldh1a1) mRNA in the liver compared with tamoxifen-treated controls. However, the Cyp26a1 ^-/- mice showed increased bone marrow cellularity and decreased frequency of erythroid progenitor cells in the bone marrow consistent with a retinoid-induced myeloid skewing of hematopoiesis. In addition, the Cyp26a1 knockout decreased clearance of exogenous atRA by 70% and increased atRA half-life 6-fold. These findings demonstrate that despite lacking a major impact on endogenous atRA signaling, Cyp26a1 critically contributes as a barrier for exogenous atRA exposure.

RDH1 suppresses adiposity by promoting brown adipose adaptation to fasting and re-feeding

RDH1 is one of the several enzymes that catalyze the first of the two reactions to convert retinol into all-trans-retinoic acid (atRA). Here, we show that Rdh1-null mice fed a low-fat diet gain more weight as adiposity (17% males, 13% females) than wild-type mice by 20 weeks old, despite neither consuming more calories nor decreasing activity. Glucose intolerance and insulin resistance develop following increased adiposity. Despite the increase in white fat pads, epididymal white adipose does not express Rdh1, nor does muscle. Brown adipose tissue (BAT) and liver express Rdh1 at relatively high levels compared to other tissues. Rdh1 ablation lowered body temperatures during ambient conditions. Given the decreased body temperature, we focused on BAT. A lack of differences in BAT adipogenic gene expression between Rdh1-null mice and wild-type mice, including Pparg, Prdm16, Zfp516 and Zfp521, indicated that the phenotype was not driven by brown adipose hyperplasia. Rather, Rdh1 ablation eliminated the increase in BAT atRA that occurs after re-feeding. This disruption of atRA homeostasis increased fatty acid uptake, but attenuated lipolysis in primary brown adipocytes, resulting in increased lipid content and larger lipid droplets. Rdh1 ablation also decreased mitochondrial proteins, including CYCS and UCP1, the mitochondria oxygen consumption rate, and disrupted the mitochondria membrane potential, further reflecting impaired BAT function, resulting in both BAT and white adipose hypertrophy. RNAseq revealed dysregulation of 424 BAT genes in null mice, which segregated predominantly into differences after fasting vs after re-feeding. Exceptions were Rbp4 and Gbp2b, which increased during both dietary conditions. Rbp4 encodes the serum retinol-binding protein-an insulin desensitizer. Gbp2b encodes a GTPase. Because Gbp2b increased several hundred-fold, we overexpressed it in brown adipocytes. This caused a shift to larger lipid droplets, suggesting that GBP2b affects signaling downstream of the β-adrenergic receptor during basal thermogenesis. Thus, Rdh1-generated atRA in BAT regulates multiple genes that promote BAT adaptation to whole-body energy status, such as fasting and re-feeding. These gene expression changes promote optimum mitochondria function and thermogenesis, limiting adiposity. Attenuation of adiposity and insulin resistance suggests that RDH1 mitigates metabolic syndrome.

Human cytochrome P450 enzymes 5-51 as targets of drugs and natural and environmental compounds: mechanisms, induction, and inhibition - toxic effects and benefits

Cytochrome P450 (P450, CYP) enzymes have long been of interest due to their roles in the metabolism of drugs, pesticides, pro-carcinogens, and other xenobiotic chemicals. They have also been of interest due to their very critical roles in the biosynthesis and metabolism of steroids, vitamins, and certain eicosanoids. This review covers the 22 (of the total of 57) human P450s in Families 5-51 and their substrate selectivity. Furthermore, included is information and references regarding inducibility, inhibition, and (in some cases) stimulation by chemicals. We update and discuss important aspects of each of these 22 P450s and questions that remain open.

Differential RA responsiveness directs formation of functionally distinct spermatogonial populations at the initiation of spermatogenesis in the mouse

In the mammalian testis, sustained spermatogenesis relies on spermatogonial stem cells (SSCs); their progeny either remain as stem cells (self-renewal) or proliferate and differentiate to enter meiosis in response to retinoic acid (RA). Here, we sought to uncover elusive mechanisms regulating a key switch fundamental to spermatogonial fate: the capacity of spermatogonia to respond to RA. Using the developing mouse testis as a model, we found that spermatogonia and precursor prospermatogonia exhibit a heterogeneous capacity to respond to RA with at least two underlying causes. First, progenitor spermatogonia are prevented from responding to RA by catabolic activity of cytochrome P450 family 26 enzymes. Second, a smaller subset of undifferentiated spermatogonia enriched for SSCs exhibit catabolism-independent RA insensitivity. Moreover, for the first time, we observed that precursor prospermatogonia are heterogeneous and comprise subpopulations that exhibit the same differential RA responsiveness found in neonatal spermatogonia. We propose a novel model by which mammalian prospermatogonial and spermatogonial fates are regulated by their intrinsic capacity to respond (or not) to the differentiation signal provided by RA before, and concurrent with, the initiation of spermatogenesis.

All-Trans Retinoic Acid Disrupts Development in Ex Vivo Cultured Fetal Rat Testes. I: Altered Seminiferous Cord Maturation and Testicular Cell Fate

Exposure to excess retinoic acid (RA) disrupts the development of the mammalian testicular seminiferous cord. However, the molecular events surrounding RA-driven loss of cord structure have not previously been examined. To investigate the mechanisms associated with this adverse developmental effect, fetal rat testes were isolated on gestational day 15, after testis determination and the initiation of cord development, and cultured in media containing all-trans RA (ATRA; 10-8 to 10-6 M) or vehicle for 3 days. ATRA exposure resulted in a concentration-dependent decrease in the number of seminiferous cords per testis section and number of germ cells, assessed by histopathology and immunohistochemistry. Following 1 day of culture, genome-wide expression profiling by microarray demonstrated that ATRA exposure altered biological processes related to retinoid metabolism and gonadal sex determination. Real-time RT-PCR analysis confirmed that ATRA enhanced the expression of the key ovarian development gene Wnt4 and the antitestis gene Nr0b1 in a concentration-dependent manner. After 3 days of culture, ATRA-treated testes contained both immunohistochemically DMRT1-positive and FOXL2-positive somatic cells, providing evidence of disrupted testicular cell fate maintenance following ATRA exposure. We conclude that exogenous RA disrupts seminiferous cord development in ex vivo cultured fetal rat testes, resulting in a reduction in seminiferous cord number, and interferes with maintenance of somatic cell fate by enhancing expression of factors that promote ovarian development.

Modest Decreases in Endogenous All-trans-Retinoic Acid Produced by a Mouse Rdh10 Heterozygote Provoke Major Abnormalities in Adipogenesis and Lipid Metabolism

Pharmacological dosing of all-trans-retinoic acid (atRA) controls adiposity in rodents by inhibiting adipogenesis and inducing fatty acid oxidation. Retinol dehydrogenases (Rdh) catalyze the first reaction that activates retinol into atRA. This study examined postnatal contributions of Rdh10 to atRA biosynthesis and physiological functions of endogenous atRA. Embryonic fibroblasts from Rdh10 heterozygote hypomorphs or with a total Rdh10 knockout exhibit decreased atRA biosynthesis and escalated adipogenesis. atRA or a retinoic acid receptor (RAR) pan-agonist reversed the phenotype. Eliminating one Rdh10 copy in vivo (Rdh10^+/- ) yielded a modest decrease (≤25%) in the atRA concentration of liver and adipose but increased adiposity in male and female mice fed a high-fat diet (HFD); increased liver steatosis, glucose intolerance, and insulin resistance in males fed an HFD; and activated bone marrow adipocyte formation in females, regardless of dietary fat. Chronic dosing with low-dose atRA corrected the metabolic defects. These data resolve physiological actions of endogenous atRA, reveal sex-specific effects of atRA in vivo, and establish the importance of Rdh10 to metabolic control by atRA. The consequences of a modest decrease in tissue atRA suggest that impaired retinol activation may contribute to diabesity, and low-dose atRA therapy may ameliorate adiposity and its sequelae of glucose intolerance and insulin resistance.

Why Differentiation Therapy Sometimes Fails: Molecular Mechanisms of Resistance to Retinoids

Retinoids represent a popular group of differentiation inducers that are successfully used in oncology for treatment of acute promyelocytic leukemia in adults and of neuroblastoma in children. The therapeutic potential of retinoids is based on their key role in the regulation of cell differentiation, growth, and apoptosis, which provides a basis for their use both in cancer therapy and chemoprevention. Nevertheless, patients treated with retinoids often exhibit or develop resistance to this therapy. Although resistance to retinoids is commonly categorized as either acquired or intrinsic, resistance as a single phenotypic feature is usually based on the same mechanisms that are closely related or combined in both of these types. In this review, we summarize the most common changes in retinoid metabolism and action that may affect the sensitivity of a tumor cell to treatment with retinoids. The availability of retinoids can be regulated by alterations in retinol metabolism or in retinoid intracellular transport, by degradation of retinoids or by their efflux from the cell. Retinoid effects on gene expression can be regulated via retinoid receptors or via other molecules in the transcriptional complex. Finally, the role of small-molecular-weight inhibitors of altered cell signaling pathways in overcoming the resistance to retinoids is also suggested.

Mechanism of action of the atypical retinoid ST1926 in colorectal cancer: DNA damage and DNA polymerase α

Despite advances in therapeutic strategies, colorectal cancer (CRC) remains the third cause of cancer-related deaths with a relatively low survival rate. Resistance to standard chemotherapy represents a major hurdle in disease management; therefore, developing new therapeutic agents demands a thorough understanding of their mechanisms of action. One of these compounds is ST1926, an adamantyl retinoid that has shown potent antitumor activities in several human cancer models. Here, we show that ST1926 selectively suppressed the proliferation of CRC cells while sparing normal counterparts, and significantly reduced tumor volume in a xenograft cancer mouse model. Next, we investigated the effects of ST1926 in CRC cells and observed early DNA damage, S-phase arrest, dissipation of mitochondrial membrane potential, and apoptosis induction, in a p53 and p21-independent manner. To address the underlying mechanism of resistance to ST1926, we generated ST1926-resistant HCT116 cells and sequenced DNA polymerase α (POLA1), which was reported to be a direct target to the drug's parent molecule, CD437. We identified similar mutations in POLA1 that conferred resistance to ST1926 and CD437. These mutations were absent in 5-fluorouracil-resistant HCT116 cells, clearly validating the specificity of these mutations to the lack of DNA damage and acquired resistance to ST1926. ST1926 also inhibited POLA1 activity and reduced its protein expression levels. Further, in silico analysis of normal and malignant tissue expression data demonstrated that POLA1 levels are elevated in CRC cells and tissues compared to normal counterparts as well as to other cancer types. Our findings highlight previously uncharacterized mechanisms of action of ST1926 in CRC and suggest that elevated POLA1 expression is a pertinent molecular feature and an attractive target in CRC.

Expression and regulation of Angiopoietins and their receptor Tie-2 in sika deer antler

The cartilage vascularization and chondrocyte survival are essential for endochondral ossification which occurs in the process of antler growth. Angiopoietins (Ang) is a family of major angiogenic growth factors and involved in regulating the vascularization. However, the expression and regulation of Angs in the antler are still unknown. The aim of this study is to localize the expression of Ang-1, Ang-2 and their receptor Tie-2 in sika deer antler using in situ hybridization and focused on analyzing the regulation of testosterone, estrogen, all-trans-retinoic acid (ATRA) and 9cRA on their expression in antler chondrocytes. The results showed that Ang-1, Ang-2 and Tie-2 were highly expressed in antler chondrocytes. Administration of testosterone to antler chondrocytes led to a notable increase in the expression of Ang-1 and Tie-2, and a reduction in the expression of Ang-2. The similar result was also observed after estrogen treatment. In contrast, ATRA and 9cRA could inhibit the expression of Ang-1 in antler chondrocytes and heighten the expression of Ang-2. Simultaneously, ATRA could downregulate the expression of Tie-2 in antler chondrocytes at 12 and 24 h, while 9cRA upregulate the expression of Tie-2 at 3 and 6 h. Collectively, Ang-1, Ang-2 and Tie-2 are expressed in antler chondrocytes and their expression can be affected by testosterone, estrogen, ATRA and 9cRA.

Inhibition of the all-trans Retinoic Acid (atRA) Hydroxylases CYP26A1 and CYP26B1 Results in Dynamic, Tissue-Specific Changes in Endogenous atRA Signaling

All-trans retinoic acid (atRA), the active metabolite of vitamin A, is a ligand for several nuclear receptors and acts as a critical regulator of many physiologic processes. The cytochrome P450 family 26 (CYP26) enzymes are responsible for atRA clearance, and are potential drug targets to increase concentrations of endogenous atRA in a tissue-specific manner. Talarozole is a potent inhibitor of CYP26A1 and CYP26B1, and has shown some success in clinical trials. However, it is not known what magnitude of change is needed in tissue atRA concentrations to promote atRA signaling changes. The aim of this study was to quantify the increase in endogenous atRA concentrations necessary to alter atRA signaling in target organs, and to establish the relationship between CYP26 inhibition and altered atRA concentrations in tissues. Following a single 2.5-mg/kg dose of talarozole to mice, atRA concentrations increased up to 5.7-, 2.7-, and 2.5-fold in serum, liver, and testis, respectively, resulting in induction of Cyp26a1 in the liver and testis and Rar β and Pgc 1β in liver. The increase in atRA concentrations was well predicted from talarozole pharmacokinetics and in vitro data of CYP26 inhibition. After multiple doses of talarozole, a significant increase in atRA concentrations was observed in serum but not in liver or testis. This lack of increase in atRA concentrations correlated with an increase in CYP26A1 expression in the liver. The increased atRA concentrations in serum without a change in liver suggest that CYP26B1 in extrahepatic sites plays a key role in regulating systemic atRA exposure.

Physiologically Based Pharmacokinetic Model of All-trans-Retinoic Acid with Application to Cancer Populations and Drug Interactions

All-trans retinoic acid (atRA) is a front-line treatment of acute promyelocytic leukemia (APL). Due to its activity in regulating the cell cycle, it has also been evaluated for the treatment of other cancers. However, the efficacy of atRA has been limited by atRA inducing its own metabolism during therapy, resulting in a decrease of atRA exposure during continuous dosing. Frequent relapse occurs in patients receiving atRA monotherapy. In an attempt to combat therapy resistance, inhibitors of atRA metabolism have been developed. Of these, ketoconazole and liarozole have shown some benefits, but their usage is limited by side effects and low potency toward the cytochrome P450 26A1 isoform (CYP26A1), the main atRA hydroxylase. We determined the pharmacokinetic basis of therapy resistance to atRA and tested whether the complex disposition kinetics of atRA could be predicted in healthy subjects and in cancer patients in the presence and absence of inhibitors of atRA metabolism using physiologically based pharmacokinetic (PBPK) modeling. A PBPK model of atRA disposition was developed and verified in healthy individuals and in cancer patients. The population-based PBPK model of atRA disposition incorporated saturable metabolic clearance of atRA, induction of CYP26A1 by atRA, and the absorption and distribution kinetics of atRA. It accurately predicted the changes in atRA exposure after continuous dosing and when coadministered with ketoconazole and liarozole. The developed model will be useful in interpretation of atRA disposition and efficacy, design of novel dosing strategies, and development of next-generation atRA metabolism inhibitors.

Improvement of Therapeutic Efficacy of Oral Immunotherapy in Combination with Regulatory T Cell-Inducer Kakkonto in a Murine Food Allergy Model

Oral immunotherapy (OIT) has been considered a promising approach for food allergies (FAs). However, the current OIT strategy is limited in terms of the long-term efficacy and safety. We have previously demonstrated that kakkonto, a traditional Japanese herbal medicine, suppresses the occurrence of allergic symptoms in a murine model of ovalbumin (OVA)-induced FA, which is attributed to the induction of the Foxp3⁺ CD4⁺ regulatory T cells. In this study, we established an OIT model using the FA mice with already established allergic symptoms and determined whether kakkonto could improve the efficacy of OIT. The OIT method consisted of initially administrating a very small amount of OVA and slowly increasing the amount. Allergic symptoms decreased in the OIT-treated FA mice. OIT significantly downregulated Th2 immune response-related gene expression in the FA mouse colon, and decreased the level of mouse mast cell protease-1, a marker of mast cell degranulation in the FA mouse plasma. Moreover, the concomitant use of kakkonto significantly enhanced the effectiveness of OIT on the allergic symptoms, and the combination therapy further suppressed the Th2 immune responses and the mast cell degranulation. In addition, OIT significantly increased the population of Foxp3⁺ CD4⁺ regulatory T cells in the FA mouse colon, and this population was further increased by OIT in combination with kakkonto. Furthermore, the combined therapy with kakkonto reduced the expression of RA-degrading enzyme CYP26B1 mRNA in the FA mouse colon. These findings indicated that the combination of OIT with kakkonto represents a promising approach for FA treatment.

13cRA regulates the differentiation of antler chondrocytes through targeting Runx3

Although 13cRA is involved in the regulation of cellular proliferation and differentiation, its physiological roles in chondrocyte proliferation and differentiation still remain unknown. Here, we showed that 13cRA could induce the proliferation of sika deer antler chondrocytes and expression of Ccnd3 and Cdk6. Administration of 13cRA to antler chondrocytes resulted in an obvious increase in the expression of chondrocyte marker Col II and hypertrophic chondrocyte marker Col X. Silencing of Crabp2 expression by specific siRNA could prevent the 13cRA-induced up-regulation of Col X, whereas overexpression of Crabp2 showed the opposite effects. Further study found that Crabp2 mediated the regulation of 13cRA on the expression of Runx3 which was highly expressed in the antler cartilage and inhibited the differentiation of antler chondrocytes. Moreover, attenuation of Runx3 expression greatly raised 13cRA-induced chondrocyte differentiation. Simultaneously, 13cRA could stimulate the expression of Cyp26a1 and Cyp26b1 in the antler chondrocytes. Inhibition of Cyp26a1 and/or Cyp26b1 reinforced the effects of 13cRA on the expression of Col X and Runx3, while overexpression of Cyp26b1 rendered the antler chondrocytes hyposensitive to 13cRA. Collectively, 13cRA may play an important role in the differentiation of antler chondrocytes through targeting Runx3. Crabp2 enhances the effects of 13cRA on chondrocyte differentiation, while Cyp26a1 and Cyp26b1 weaken the sensitivity of antler chondrocytes to 13cRA.

Comparison of the ligand binding site of CYP2C8 with CYP26A1 and CYP26B1: a structural basis for the identification of new inhibitors of the retinoic acid hydroxylases

The CYP26s are responsible for metabolizing retinoic acid and play an important role in maintaining homeostatic levels of retinoic acid. Given the ability of CYP2C8 to metabolize retinoic acid, we evaluated the potential for CYP2C8 inhibitors to also inhibit CYP26. In vitro assays were used to evaluate the inhibition potencies of CYP2C8 inhibitors against CYP26A1 and CYP26B1. Using tazarotenic acid as a substrate for CYP26, IC₅₀ values for 17 inhibitors of CYP2C8 were determined for CYP26A1 and CYP26B1, ranging from ∼20 nM to 100 μM, with a positive correlation observed between IC₅₀s for CYP2C8 and CYP26A1. An evaluation of IC₅₀'s versus in vivo C_max values suggests that inhibitors such as clotrimazole or fluconazole may interact with CYP26 at clinically relevant concentrations and may alter levels of retinoic acid. These findings provide insight into drug interactions resulting in elevated retinoic acid concentrations and expand upon the pharmacophore of CYP26 inhibition.

Development and Characterization of Novel and Selective Inhibitors of Cytochrome P450 CYP26A1, the Human Liver Retinoic Acid Hydroxylase

Cytochrome P450 CYP26 enzymes are responsible for all-trans-retinoic acid (atRA) clearance. Inhibition of CYP26 enzymes will increase endogenous atRA concentrations and is an attractive therapeutic target. However, the selectivity and potency of the existing atRA metabolism inhibitors toward CYP26A1 and CYP26B1 is unknown, and no selective CYP26A1 or CYP26B1 inhibitors have been developed. Here the synthesis and potent inhibitory activity of the first CYP26A1 selective inhibitors is reported. A series of nonazole CYP26A1 selective inhibitors was identified with low nM potency. The lead compound 3-{4-[2-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-yl)-1,3-dioxolan-2-yl] phenyl}4-propanoic acid (24) had 43-fold selectivity toward CYP26A1 with an IC50 of 340 nM. Compound 24 and its two structural analogues also inhibited atRA metabolism in HepG2 cells, resulting in increased potency of atRA toward RAR activation. The identified compounds have potential to become novel treatments aiming to elevate endogenous atRA concentrations and may be useful as cotreatment with atRA to combat therapy resistance.

Improved Homology Model of the Human all-trans Retinoic Acid Metabolizing Enzyme CYP26A1

A new CYP26A1 homology model was built based on the crystal structure of cyanobacterial CYP120A1. The model quality was examined for stereochemical accuracy, folding reliability, and absolute quality using a variety of different bioinformatics tools. Furthermore, the docking capabilities of the model were assessed by docking of the natural substrate all-trans-retinoic acid (atRA), and a group of known azole- and tetralone-based CYP26A1 inhibitors. The preferred binding pose of atRA suggests the (4S)-OH-atRA metabolite production, in agreement with recently available experimental data. The distances between the ligands and the heme group iron of the enzyme are in agreement with corresponding distances obtained for substrates and azole inhibitors for other cytochrome systems. The calculated theoretical binding energies agree with recently reported experimental data and show that the model is capable of discriminating between natural substrate, strong inhibitors (R116010 and R115866), and weak inhibitors (liarozole, fluconazole, tetralone derivatives).

Identification of Tazarotenic Acid as the First Xenobiotic Substrate of Human Retinoic Acid Hydroxylase CYP26A1 and CYP26B1

Cytochrome P450 (CYP) 26A1 and 26B1 are heme-containing enzymes responsible for metabolizing all-trans retinoic acid (at-RA). No crystal structures have been solved, and therefore homology models that provide structural information are extremely valuable for the development of inhibitors of cytochrome P450 family 26 (CYP26). The objectives of this study were to use homology models of CYP26A1 and CYP26B1 to characterize substrate binding characteristics, to compare structural aspects of their active sites, and to support the role of CYP26 in the metabolism of xenobiotics. Each model was verified by dockingat-RA in the active site and comparing the results to known metabolic profiles ofat-RA. The models were then used to predict the metabolic sites of tazarotenic acid with results verified by in vitro metabolite identification experiments. The CYP26A1 and CYP26B1 homology models predicted that the benzothiopyranyl moiety of tazarotenic acid would be oriented toward the heme of each enzyme and suggested that tazarotenic acid would be a substrate of CYP26A1 and CYP26B1. Metabolite identification experiments indicated that CYP26A1 and CYP26B1 oxidatively metabolized tazarotenic acid on the predicted moiety, with in vitro rates of metabolite formation by CYP26A1 and CYP26B1 being the highest across a panel of enzymes. Molecular analysis of the active sites estimated the active-site volumes of CYP26A1 and CYP26B1 to be 918 Å(3)and 977 Å(3), respectively. Overall, the homology models presented herein describe the enzyme characteristics leading to the metabolism of tazarotenic acid by CYP26A1 and CYP26B1 and support a potential role for the CYP26 enzymes in the metabolism of xenobiotics.