Vitamin A and retinoid signaling: genomic and nongenomic effects

Adapalene induces adipose browning through the RARβ-p38 MAPK-ATF2 pathway

Adipose browning has recently been reported to be a novel therapeutic strategy for obesity. Because the retinoic acid receptor (RAR) is a potential target involved in browning, adapalene (AD), an anti-acne agent with RAR agonism, was examined in detail for its effects on adipose browning and the underlying mechanisms in vitro and in vivo. AD upregulated the expression of adipose browning-related markers in a concentration-dependent manner, promoted mitochondrial biogenesis, increased oxygen consumption rates, and lowered lipid droplet sizes in differentiated 3T3/L1 white adipocytes. Among the three retinoic acid receptors (RARα, RARβ, and RARγ), knockdown of the gene encoding RARβ mitigated AD-induced adipose browning. Similarly, LE135 (a selective RARβ antagonist) attenuated AD action, suggesting that AD promotes adipose browning through RARβ. Sequential phosphorylation of p38 mitogen-activated protein kinase (MAPK) and activating transcription factor 2 (ATF2) was critical for AD-induced adipose browning, based on the observations that either SB203580 (a p38 MAPK inhibitor) or ATF2 siRNA reduced the effects of AD. In vivo browning effects of AD were confirmed in C57BL/6J mice and high-fat diet-induced obese (DIO) mice after oral administration of AD either acutely or chronically. This study identifies new actions of AD as an adipose browning agent and demonstrates that RARβ activation followed by increased phosphorylation of p38 MAPK and ATF2 appears to be a key mechanism of AD action.

Antiretroviral Therapy-Induced Dysregulation of Gene Expression and Lipid Metabolism in HIV+ Patients: Beneficial Role of Antioxidant Phytochemicals

Human immunodeficiency virus (HIV) infection has continued to be the subject of study since its discovery nearly 40 years ago. Significant advances in research and intake of antiretroviral therapy (ART) have slowed the progression and appearance of the disease symptoms and the incidence of concomitant diseases, which are the leading cause of death in HIV+ persons. However, the prolongation of ART is closely related to chronic degenerative diseases and pathologies caused by oxidative stress (OS) and alterations in lipid metabolism (increased cholesterol levels), both of which are conditions of ART. Therefore, recent research focuses on using natural therapies to diminish the effects of ART and HIV infection: regulating lipid metabolism and reducing OS status. The present review summarizes current information on OS and cholesterol metabolism in HIV+ persons and how the consumption of certain phytochemicals can modulate these. For this purpose, MEDLINE and SCOPUS databases were consulted to identify publications investigating HIV disease and natural therapies and their associated effects.

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.

All trans retinoic acid as a host-directed immunotherapy for tuberculosis

Tuberculosis (TB) is the top bacterial infectious disease killer and one of the top ten causes of death worldwide. The emergence of strains of multiple drug-resistant tuberculosis (MDR-TB) has pushed our available stock of anti-TB agents to the limit of effectiveness. This has increased the urgent need to develop novel treatment strategies using currently available resources. An adjunctive, host-directed therapy (HDT) designed to act on the host, instead of the bacteria, by boosting the host immune response through activation of intracellular pathways could be the answer. The integration of multidisciplinary approaches of repurposing currently FDA-approved drugs, with a targeted drug-delivery platform is a very promising option to reduce the long timeline associated with the approval of new drugs - time that cannot be afforded given the current levels of morbidity and mortality associated with TB infection. The deficiency of vitamin A has been reported to be highly associated with the increased susceptibility of TB. All trans retinoic acid (ATRA), the active metabolite of vitamin A, has proven to be very efficacious against TB both in vitro and in vivo. In this review, we discuss and summarise the importance of vitamin A metabolites in the fight against TB and what is known regarding the molecular mechanisms of ATRA as a host-directed therapy for TB including its effect on macrophages cytokine profile and cellular pathways. Furthermore, we focus on the issues behind why previous clinical trials with vitamin A supplementation have failed, and how these issues might be overcome.

All-trans retinoic acid changes muscle fiber type via increasing GADD34 dependent on MAPK signal

ATRA increases GADD34 expression by decreasing the expression of Six1, which down-regulates the transcriptional activity with TLE3 and increasing mRNA stability through blocking the interaction between TTP and ARE on GADD34 mRNA, resulting in muscle fiber type change.

All-trans retinoic acid (ATRA) increases the sensitivity to unfolded protein response in differentiating leukemic blasts. The downstream transcriptional factor of PERK, a major arm of unfolded protein response, regulates muscle differentiation. However, the role of growth arrest and DNA damage-inducible protein 34 (GADD34), one of the downstream factors of PERK, and the effects of ATRA on GADD34 expression in muscle remain unclear. In this study, we identified ATRA increased the GADD34 expression independent of the PERK signal in the gastrocnemius muscle of mice. ATRA up-regulated GADD34 expression through the transcriptional activation of GADD34 gene via inhibiting the interaction of homeobox Six1 and transcription co-repressor TLE3 with the MEF3-binding site on the GADD34 gene promoter in skeletal muscle. ATRA also inhibited the interaction of TTP, which induces mRNA degradation, with AU-rich element on GADD34 mRNA via p-38 MAPK, resulting in the instability of GADD34 mRNA. Overexpressed GADD34 in C2C12 cells changes the type of myosin heavy chain in myotubes. These results suggest ATRA increases GADD34 expression via transcriptional and post-transcriptional regulation, which changes muscle fiber type.

Mechanisms of Feedback Regulation of Vitamin A Metabolism

Vitamin A is an essential nutrient required throughout life. Through its various metabolites, vitamin A sustains fetal development, immunity, vision, and the maintenance, regulation, and repair of adult tissues. Abnormal tissue levels of the vitamin A metabolite, retinoic acid, can result in detrimental effects which can include congenital defects, immune deficiencies, proliferative defects, and toxicity. For this reason, intricate feedback mechanisms have evolved to allow tissues to generate appropriate levels of active retinoid metabolites despite variations in the level and format, or in the absorption and conversion efficiency of dietary vitamin A precursors. Here, we review basic mechanisms that govern vitamin A signaling and metabolism, and we focus on retinoic acid-controlled feedback mechanisms that contribute to vitamin A homeostasis. Several approaches to investigate mechanistic details of the vitamin A homeostatic regulation using genomic, gene editing, and chromatin capture technologies are also discussed.

Role of Nuclear Receptors in Controlling Erythropoiesis

Nuclear receptors (NRs), are a wide family of ligand-regulated transcription factors sharing a common modular structure composed by an N-terminal domain and a ligand-binding domain connected by a short hinge linker to a DNA-binding domain. NRs are involved in many physiological processes, including metabolism, reproduction and development. Most of them respond to small lipophilic ligands, such as steroids, retinoids, and phospholipids, which act as conformational switches. Some NRs are still "orphan" and the search for their ligands is still ongoing. Upon DNA binding, NRs can act both as transcriptional activators or repressors of their target genes. Theoretically, the possibility to modulate NRs activity with small molecules makes them ideal therapeutic targets, although the complexity of their signaling makes drug design challenging. In this review, we discuss the role of NRs in erythropoiesis, in both homeostatic and stress conditions. This knowledge is important in view of modulating red blood cells production in disease conditions, such as anemias, and for the expansion of erythroid cells in culture for research purposes and for reaching the long-term goal of cultured blood for transfusion.

Retinoic Acid Grafted to Hyaluronic Acid Activates Retinoid Gene Expression and Removes Cholesterol from Cellular Membranes

All-trans-retinoic acid (atRA) is a potent ligand that regulates gene expression and is used to treat several skin disorders. Hyaluronic acid (HA) was previously conjugated with atRA (HA-atRA) to obtain a novel amphiphilic compound. HA-atRA forms micelles that incorporate hydrophobic molecules and facilitate their transport through the skin. The aim of this study was to determine the influence of HA-atRA on gene expression in skin cells and to compare it with that of unbound atRA. Gene expression was investigated using microarrays and a luciferase system with a canonical atRA promoter. HA-atRA upregulated gene expression similarly to atRA. However, HA-atRA activated the expression of cholesterol metabolism genes, unlike atRA. Further investigation using HPLC and filipin III staining suggested that the treated cells induced cholesterol synthesis to replenish the cholesterol removed from the cells by HA-atRA. HA modified with oleate (HA-C18:1) removed cholesterol from the cells similarly to HA-atRA, suggesting that the cholesterol removal stemmed from the amphiphilic nature of the two derivatives. HA-atRA induces retinoid signaling. Thus, HA-atRA could be used to treat skin diseases, such as acne and psoriasis, where the combined action of atRA signaling and anti-inflammatory cholesterol removal may be potentially beneficial.

Evolution of non-genomic nuclear receptor function

Nuclear receptors (NRs) are responsible for the regulation of diverse developmental and physiological systems in metazoans. NR actions can be the result of genomic and non-genomic mechanisms depending on whether they act inside or outside of the nucleus respectively. While the actions of both mechanisms have been shown to be crucial to NR functions, non-genomic actions are considered less frequently than genomic actions. Furthermore, hypotheses on the origin and evolution of non-genomic NR signaling pathways are rarely discussed in the literature. Here we summarize non-genomic NR signaling mechanisms in the context of NR protein family evolution and animal phyla. We find that NRs across groups and phyla act via calcium flux as well as protein phosphorylation cascades (MAPK/PI3K/PKC). We hypothesize and discuss a possible synapomorphy of NRs in the NR1 and NR3 families, including the thyroid hormone receptor, vitamin D receptor, ecdysone receptor, retinoic acid receptor, steroid receptors, and others. In conclusion, we propose that the advent of non-genomic NR signaling may have been a driving force behind the expansion of NR diversity in Cnidarians, Placozoans, and Bilaterians.

Septoclasts expressing epidermal fatty acid-binding protein (E-FABP, FABP5) in endochondral ossification

BACKGROUND

Long-chain fatty acids (LCFAs) and retinoic acid (RA) are abundant in the growth plates (GPs) of long bones; however, their roles have not been elucidated. We observed that epidermal fatty acid-binding protein (E-FABP/FABP5) with a high affinity for both LCFAs and RA is exclusively expressed in the septoclasts located at the chondro-osseous junction (COJ) of the GP.

HIGHLIGHTS

E-FABP expressed in septoclasts is involved in both LCFA metabolism and RA signaling as an intracellular transporter of both LCFAs and RA. Septoclasts with shortened cytoplasmic processes are associated with cartilage resorptive activity downregulation because of E-FABP deficiency or excess or deficiency of RA. In ontogeny, the septoclasts are differentiated from the pericytes and involved in the resorption of the uncalcified matrix of the cartilage templates in endochondral ossification.

CONCLUSION

Septoclasts originate from pericytes and express E-FABP to play crucial roles in uncalcified matrix resorption by LCFA metabolism and RA signaling during endochondral ossification.

Physiologically Relevant Free Ca2+ Ion Concentrations Regulate STRA6-Calmodulin Complex Formation via the BP2 Region of STRA6

The interaction of calmodulin (CaM) with the receptor for retinol uptake, STRA6, involves an α-helix termed BP2 that is located on the intracellular side of this homodimeric transporter (Chen et al., 2016 [1]). In the absence of Ca2+, NMR data showed that a peptide derived from BP2 bound to the C-terminal lobe (C-lobe) of Mg2+-bound CaM (MgCaM). Upon titration of Ca2+ into MgCaM-BP2, NMR chemical shift perturbations (CSPs) were observed for residues in the C-lobe, including those in the EF-hand Ca2+-binding domains, EF3 and EF4 (CaKD = 60 ± 7 nM). As higher concentrations of free Ca2+ were achieved, CSPs occurred for residues in the N-terminal lobe (N-lobe) including those in EF1 and EF2 (CaKD = 1000 ± 160 nM). Thermodynamic and kinetic Ca2+ binding studies showed that BP2 addition increased the Ca2+-binding affinity of CaM and slowed its Ca2+ dissociation rates (koff) in both the C- and N-lobe EF-hand domains, respectively. These data are consistent with BP2 binding to the C-lobe of CaM at low free Ca2+ concentrations (<100 nM) like those found at resting intracellular levels. As free Ca2+ levels approach 1000 nM, which is typical inside a cell upon an intracellular Ca2+-signaling event, BP2 is shown here to interact with both the N- and C-lobes of Ca2+-loaded CaM (CaCaM-BP2). Because this structural rearrangement observed for the CaCaM-BP2 complex occurs as intracellular free Ca2+ concentrations approach those typical of a Ca2+-signaling event (CaKD = 1000 ± 160 nM), this conformational change could be relevant to vitamin A transport by full-length CaCaM-STRA6.

A novel hypothesis for COVID-19 pathogenesis: Retinol depletion and retinoid signaling disorder

The SARS-CoV-2 virus has caused a worldwide COVID-19 pandemic. In less than a year and a half, more than 200 million people have been infected and more than four million have died. Despite some improvement in the treatment strategies, no definitive treatment protocol has been developed. The pathogenesis of the disease has not been clearly elucidated yet. A clear understanding of its pathogenesis will help develop effective vaccines and drugs. The immunopathogenesis of COVID-19 is characteristic with acute respiratory distress syndrome and multiorgan involvement with impaired Type I interferon response and hyperinflammation. The destructive systemic effects of COVID-19 cannot be explained simply by the viral tropism through the ACE2 and TMPRSS2 receptors. In addition, the recently identified mutations cannot fully explain the defect in all cases of Type I interferon synthesis. We hypothesize that retinol depletion and resulting impaired retinoid signaling play a central role in the COVID-19 pathogenesis that is characteristic for dysregulated immune system, defect in Type I interferon synthesis, severe inflammatory process, and destructive systemic multiorgan involvement. Viral RNA recognition mechanism through RIG-I receptors can quickly consume a large amount of the body's retinoid reserve, which causes the retinol levels to fall below the normal serum levels. This causes retinoid insufficiency and impaired retinoid signaling, which leads to interruption in Type I interferon synthesis and an excessive inflammation. Therefore, reconstitution of the retinoid signaling may prove to be a valid strategy for management of COVID-19 as well for some other chronic, degenerative, inflammatory, and autoimmune diseases.

A Metabolomics Analysis of Circulating Carotenoids and Breast Cancer Risk

BACKGROUND

Higher circulating carotenoids are associated with lower breast cancer risk. The underlying biology remains under-explored.

METHODS

We profiled 293 prediagnostic plasma metabolites in a nested case-control study (n = 887 cases) within the Nurses' Health Studies. Associations between circulating carotenoids and metabolites were identified using linear-mixed models (FDR ≤ 0.05), and we further selected metabolites most predictive of carotenoids with LASSO. Metabolic signatures for carotenoids were calculated as weighted sums of LASSO selected metabolites. We further evaluated the metabolic signatures in relation to breast cancer risk using conditional logistic-regression.

RESULTS

We identified 48 to 110 metabolites associated with plasma levels of α-carotene, β-carotene, β-cryptoxanthin, estimated-vitamin-A-potential, lutein/zeaxanthin, and lycopene, which included primarily positively associated metabolites implicated in immune regulation (tryptophan), redox balance (plasmalogens, glutamine), epigenetic regulations (acetylated-/methylated-metabolites), and primarily inversely associated metabolites involved in β-oxidation (carnitines; FDR ≤ 0.05). The metabolomic signatures derived for β-carotene (Q4 vs. Q1 relative risk RR = 0.74, P _trend = 0.02), and estimated-vitamin-A-potential (Q4 vs. Q1 RR = 0.74, P _trend = 0.02)-measured ≥10 years before diagnosis-were associated with lower breast cancer risk. Modest attenuations of RR for measured levels of β-carotene and estimated-vitamin-A-potential were seen when we adjusted for their corresponding metabolic signatures.

CONCLUSIONS

Metabolites involved in immune regulation, redox balance, membrane signaling, and β-oxidation were associated with plasma carotenoids. Although some metabolites may reflect shared common food sources or compartmental colocalization with carotenoids, others may signal the underlying pathways of carotenoids-associated lowered breast cancer risk.

IMPACT

Consumption of carotenoid-rich diet is associated with a wide-range of metabolic changes which may help to reduce breast cancer risk.

Characterization of subunit interactions in the hetero-oligomeric retinoid oxidoreductase complex

The hetero-oligomeric retinoid oxidoreductase complex (ROC) catalyzes the interconversion of all-trans-retinol and all-trans-retinaldehyde to maintain the steady-state output of retinaldehyde, the precursor of all-trans-retinoic acid that regulates the transcription of numerous genes. The interconversion is catalyzed by two distinct components of the ROC: the NAD(H)-dependent retinol dehydrogenase 10 (RDH10) and the NADP(H)-dependent dehydrogenase reductase 3 (DHRS3). The binding between RDH10 and DHRS3 subunits in the ROC results in mutual activation of the subunits. The molecular basis for their activation is currently unknown. Here, we applied site-directed mutagenesis to investigate the roles of amino acid residues previously implied in subunit interactions in other SDRs to obtain the first insight into the subunit interactions in the ROC. The results of these studies suggest that the cofactor binding to RDH10 subunit is critical for the activation of DHRS3 subunit and vice versa. The C-terminal residues 317-331 of RDH10 are critical for the activity of RDH10 homo-oligomers but not for the binding to DHRS3. The C-terminal residues 291-295 are required for DHRS3 subunit activity of the ROC. The highly conserved C-terminal cysteines appear to be involved in inter-subunit communications, affecting the affinity of the cofactor binding site in RDH10 homo-oligomers as well as in the ROC. Modeling of the ROC quaternary structure based on other known structures of SDRs suggests that its integral membrane-associated subunits may be inserted in adjacent membranes of the endoplasmic reticulum (ER), making the formation and function of the ROC dependent on the dynamic nature of the tubular ER network.

Vitamin A and Vitamin E: Will the Real Antioxidant Please Stand Up?

Vitamin A, acting through its metabolite, all-trans-retinoic acid, is a potent transcriptional regulator affecting expression levels of hundreds of genes through retinoic acid response elements present within these genes. However, the literature is replete with claims that consider vitamin A to be an antioxidant vitamin, like vitamins C and E. This apparent contradiction in the understanding of how vitamin A acts mechanistically within the body is a major focus of this review. Vitamin E, which is generally understood to act as a lipophilic antioxidant protecting polyunsaturated fatty acids present in membranes, is often proposed to be a transcriptional regulator. The evaluation of this claim is another focus of the review. We conclude that vitamin A is an indirect antioxidant, whose indirect function is to transcriptionally regulate a number of genes involved in mediating the body's canonical antioxidant responses. Vitamin E, in addition to being a direct antioxidant, prevents the increase of peroxidized lipids that alter both metabolic pathways and gene expression profiles within tissues and cells. However, there is little compelling evidence that vitamin E has a direct transcriptional mechanism like that of vitamin A. Thus, we propose that the term antioxidant not be applied to vitamin A, and we discourage the use of the term transcriptional mediator when discussing vitamin E.

Dermal EZH2 orchestrates dermal differentiation and epidermal proliferation during murine skin development

Skin development and patterning is dependent on factors that regulate the stepwise differentiation of dermal fibroblasts concomitant with dermal-epidermal reciprocal signaling, two processes that are poorly understood. Here we show that dermal EZH2, the methyltransferase enzyme of the epigenetic Polycomb Repressive Complex 2 (PRC2), is a new coordinator of both these processes. Dermal EZH2 activity is present during dermal fibroblast differentiation and is required for spatially restricting Wnt/β-catenin signaling to reinforce dermal fibroblast cell fate. Later in development, dermal EZH2 regulates the expression of reticular dermal markers and initiation of secondary hair follicles. Embryos lacking dermal Ezh2 have elevated epidermal proliferation and differentiation that can be rescued by small molecule inhibition of retinoic acid (RA) signaling. Together, our study reveals that dermal EZH2 is acting like a rheostat to control the levels of Wnt/β-catenin and RA signaling to impact fibroblast differentiation cell autonomously and epidermal keratinocyte development non-cell autonomously, respectively.

The Role of Vitamins in Neurodegenerative Disease: An Update

Acquiring the recommended daily allowance of vitamins is crucial for maintaining homeostatic balance in humans and other animals. A deficiency in or dysregulation of vitamins adversely affects the neuronal metabolism, which may lead to neurodegenerative diseases. In this article, we discuss how novel vitamin-based approaches aid in attenuating abnormal neuronal functioning in neurodegeneration-based brain diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, Amyotrophic lateral sclerosis, and Prion disease. Vitamins show their therapeutic activity in Parkinson's disease by antioxidative and anti-inflammatory activity. In addition, different water- and lipid-soluble vitamins have also prevented amyloid beta and tau pathology. On the other hand, some results also show no correlation between vitamin action and the prevention of neurodegenerative diseases. Some vitamins also exhibit toxic activity too. This review discusses both the beneficial and null effects of vitamin supplementation for neurological disorders. The detailed mechanism of action of both water- and lipid-soluble vitamins is addressed in the manuscript. Hormesis is also an essential factor that is very helpful to determine the effective dose of vitamins. PubMed, Google Scholar, Web of Science, and Scopus were employed to conduct the literature search of original articles, review articles, and meta-analyses.

Micronutrient Food Supplements in Patients with Gastro-Intestinal and Hepatic Cancers

Colorectal carcinogenesis is the second most common cause of mortality across all types of malignancies, followed by hepatic and stomach cancers. Chemotherapy and radiotherapy are key approaches to treating cancer patients, but these carry major concerns, such as a high risk of side effects, poor accessibility, and the non-selective nature of chemotherapeutics. A number of natural products have been identified as countering various forms of cancer with fewer side effects. The potential impact of vitamins and minerals on long-term health, cognition, healthy development, bone formation, and aging has been supported by experimental and epidemiological studies. Successful treatment may thus be highly influenced by the nutritional status of patients. An insufficient diet could lead to detrimental effects on immune status and tolerance to treatment, affecting the ability of chemotherapy to destroy cancerous cells. In recent decades, most cancer patients have been taking vitamins and minerals to improve standard therapy and/or to decrease the undesirable side effects of the treatment together with the underlying disease. On the other hand, taking dietary supplements during cancer therapy may affect the effectiveness of chemotherapy. Thus, micronutrients in complementary oncology must be selected appropriately and should be taken at the right time. Here, the potential impact of micronutrients on gastro-intestinal and hepatic cancers is explored and their molecular targets are laid down.

Signaling through retinoic acid receptors is essential for mammalian uterine receptivity and decidualization

Retinoic acid (RA) signaling has long been speculated to regulate embryo implantation, because many enzymes and proteins responsible for maintaining RA homeostasis and transducing RA signals are tightly regulated in the endometrium during this critical period. However, due to a lack of genetic data, it was unclear whether RA signaling is truly required for implantation and which specific RA signaling cascades are at play. Herein we utilize a genetic murine model that expresses a dominant-negative form of RA receptor (RAR) specifically in female reproductive organs to show that functional RA signaling is fundamental to female fertility, particularly implantation and decidualization. Reduction in RA signaling activity severely affects the ability of the uterus to achieve receptive status and decidualize, partially through dampening follistatin expression and downstream activin B/bone morphogenetic protein 2 signaling. To confirm translational relevance of these findings to humans, human endometrial stromal cells (hESCs) were treated with a pan-RAR antagonist to show that in vitro decidualization is impaired. RNA interference perturbation of individual RAR transcripts in hESCs revealed that RARα in particular was essential for proper decidualization. These data provide direct functional evidence that uterine RAR-mediated RA signaling was crucial for mammalian embryo implantation, and its disruption led to failure of uterine receptivity and decidualization, resulting in severely compromised fertility.

Acitretin and Retinoic Acid Derivatives Inhibit BK Polyomavirus Replication in Primary Human Proximal Renal Tubular Epithelial and Urothelial Cells

Small-molecule drugs inhibiting BK polyomavirus (BKPyV) represent a significant unmet clinical need in view of polyomavirus-associated nephropathy or hemorrhagic cystitis, which complicate 5% to 25% of kidney and hematopoietic cell transplantations. We characterized the inhibitory activity of acitretin on BKPyV replication in primary human renal proximal tubular epithelial cells (RPTECs). Effective inhibitory concentrations of 50% (EC₅₀) and 90% (EC₉₀) were determined in dilution series measuring BKPyV loads, transcripts, and protein expression, using cell proliferation, metabolic activity, and viability to estimate cytotoxic concentrations and selectivity indices (SI). The acitretin EC₅₀ and EC₉₀ in RPTECs were 0.64 (SI₅₀, 250) and 3.25 μM (SI₉₀, 49.2), respectively. Acitretin effectively inhibited BKPyV replication until 72 h postinfection when added 24 h before infection until 12 h after infection, but decreased to <50% at later time points. Acitretin did not interfere with nuclear delivery of BKPyV genomes, but it decreased large T-antigen transcription and protein expression. Acitretin did not inhibit the initial round of BKPyV replication following transfection of full-length viral genomes, but it affected subsequent rounds of reinfection. Acitretin also inhibited BKPyV replication in human urothelial cells and in Vero cells, but not in COS-7 cells constitutively expressing Simian virus 40 (SV40) large T antigen. Retinoic acid agonists (all-trans retinoic acid, 9-cis retinoic acid [9-cis-RA], 13-cis-RA, bexarotene, and tamibarotene) and the RAR/RXR antagonist RO41-5253 also inhibited BKPyV replication, pointing to an as-yet-undefined mechanism. IMPORTANCE Acitretin selectively inhibits BKPyV replication in primary human cell culture models of nephropathy and hemorrhagic cystitis. Since acitretin is an approved drug in clinical use reaching BKPyV-inhibiting concentrations in systemically treated patients, further studies are warranted to provide data for clinical repurposing of retinoids for treatment and prevention of replicative BKPyV-diseases.

Topical and nutricosmetic products for healthy hair and dermal antiaging using "dual-acting" (2 for 1) plant-based peptides, hormones, and cannabinoids

One of the side effects of oral antiaging retinoids is increased hair shedding. Retinoids promote the expression of TGF-β2 from fibroblasts, which stimulate collagen expression but silences keratinocytes. Since keratinocytes normally influence differentiation of dermal papilla cells at the base of the hair follicle, retinoids feasibly inhibit hair growth via the increased expression of TGF-β2, which inhibits Wnt/β-catenin signaling. Fortunately, the plant kingdom provides an array of alternatives as dual-acting nutricosmetics and topicals that work independently of TGF-β2 to confer dermal antiaging and hair health effects. These alternatives include "plant hormones" such as cytokinins and phytoestrogens. Many cytokinins are agonists of the G-coupled adenosine receptors. Partial agonism of adenosine receptors promotes collagen synthesis independently of TGF-β2 signaling. Adenosine expression is potentially also the mechanism of minoxidil in promotion of scalp hair growth. Because of crosstalk between adenosine and cannabinoid receptors it makes sense to try combinations of specific CB2 agonists and cytokinins (or phytoestrogens). However, dual-acting cosmetics including peptides with high numbers of positively charged amino acids, such as lysine or arginine, offer real potential as they can be processed from multiple botanical candidates, including almond, fenugreek, pea sprouts, soy, and seaweeds. The current review summarizes much of what is known about retinoid alternatives in the plant kingdom and identifies potentially fruitful new areas of research.

The Influence of Nutritional Factors on Immunological Outcomes

Through food intake, humans obtain a variety of nutrients that are essential for growth, cellular function, tissue development, energy, and immune defense. A special interaction between nutrients and gut-associated lymphoid tissue occurs in the intestinal tract. Enterocytes of the intestinal barrier act as sensors for antigens from nutrients and the intestinal microbiota, which they deliver to the underlying immune system of the lamina propria, triggering an immune response. Studies investigating the mechanism of influence of nutrition on immunological outcomes have highlighted an important role of macronutrients (proteins, carbohydrates, fatty acids) and micronutrients (vitamins, minerals, phytochemicals, antioxidants, probiotics) in modulating immune homeostasis. Nutrients exert their role in innate immunity and inflammation by regulating the expression of TLRs, pro- and anti-inflammatory cytokines, thus interfering with immune cell crosstalk and signaling. Chemical substrates derived from nutrient metabolism may act as cofactors or blockers of enzymatic activity, influencing molecular pathways and chemical reactions associated with microbial killing, inflammation, and oxidative stress. Immune cell function appears to be influenced by certain nutrients that form parts of the cell membrane structure and are involved in energy production and prevention of cytotoxicity. Nutrients also contribute to the initiation and regulation of adaptive immune responses by modulating B and T lymphocyte differentiation, proliferation and activation, and antibody production. The purpose of this review is to present the available data from the field of nutritional immunology to elucidate the complex and dynamic relationship between nutrients and the immune system, the delineation of which will lead to optimized nutritional regimens for disease prevention and patient care.

Retinoic acid stimulates transcription of the rat SHARP-2 gene via multiple pathways

Members of the enhancer of split- and hairy-related protein (SHARP) family, SHARP-1 and SHARP-2, are basic helix-loop-helix transcriptional repressors and belong to the clock genes. In this study, an effect of retinoic acid (RA) on the SHARP family gene expression in the differentiated cells was examined. RA rapidly and temporarily induced the SHARP-2 mRNA expression in hepatic H4IIE cells. Then, whether the SHARP-2 mRNA expression is altered by dexamethasone (Dex), insulin, and the combination of RA and Dex or RA and insulin was examined. Dex had different effects on the expression of SHARP-2 mRNA in the presence or absence of RA. Then, the molecular mechanisms were investigated using inhibitors of various signaling molecules. The RA-induction of SHARP-2 mRNA level was mainly inhibited by LY294002, staurosporine, and actinomycin D, respectively. Finally, whether RA acts on the transcriptional regulatory region of the SHARP-2 gene was analyzed using luciferase reporter gene assay. At least two RA-responsive regions were mapped at the nucleotide sequences between -3,700 and -1,600 of the SHARP-2 gene. In addition, this effect was dependent on the retinoic acid receptor and retinoid X receptor. Thus, we conclude that RA stimulated transcription of the SHARP-2 gene via multiple pathways.

Overview of all-trans-retinoic acid (ATRA) and its analogues: Structures, activities, and mechanisms in acute promyelocytic leukaemia

All-trans-retinoic acid (ATRA) is effective for preventing cancer and treating skin diseases and acute promyelocytic leukaemia (APL). These pharmacological effects of ATRA are mainly mediated by retinoid X receptors (RXRs) and retinoic acid receptors (RARs). This article provides a comprehensive overview of the clinical progress on and the molecular mechanisms of ATRA in the treatment of APL. ATRA can promote the transcriptional activation of differentiation-related genes and regulate autophagy by inhibiting mTOR, which results in anti-APL effects. In detail, the structures, pharmacological effects, and clinical studies of 68 types of ATRA analogues are described. These compounds have excellent antitumour therapeutic potential and could be used as lead compounds for further development and research.

Nutrigenomics and Life Style Facet- A Modulatory Molecular Evidence in Progression of Breast and Colon Cancer with Emerging Importance

Legitimate nutrition assumes a significant role in preventing diseases and, in this way, nutritional interventions establish vital strategies in the area of public health. Nutrigenomics centres on the different genes and diet in an individual and how an individual's genes influence the reaction to bioactive foodstuff. It targets considering the genetic and epigenetic interactions with nutrients to lead to a phenotypic alteration and consequently to metabolism, differentiation, or even apoptosis. Nutrigenomics and lifestyle factors play a vital role in health management and represent an exceptional prospect for the improvement of personalized diets to the individual at risk of developing diseases like cancer. Concerning cancer as a multifactorial genetic ailment, several aspects need to be investigated and analysed. Various perspectives should be researched and examined regarding the development and prognosis of breast and colon cancer. Malignant growth occurrence is anticipated to upsurge in the impending days, and an effective anticipatory strategy is required. The effect of dietary components, basically studied by nutrigenomics, looks at gene expression and molecular mechanisms. It also interrelates bioactive compounds and nutrients because of different 'omics' innovations. Several preclinical investigations demonstrate the pertinent role of nutrigenomics in breast and colon cancer, and change of dietary propensities is conceivably a successful methodology for reducing cancer risk. The connection between the genomic profile of patients with breast or colon cancer and their supplement intake, it is conceivable to imagine an idea of personalized medicine, including nutrition and medicinal services.

All-trans retinoic acid induces synaptic plasticity in human cortical neurons

A defining feature of the brain is the ability of its synaptic contacts to adapt structurally and functionally in an experience-dependent manner. In the human cortex, however, direct experimental evidence for coordinated structural and functional synaptic adaptation is currently lacking. Here, we probed synaptic plasticity in human cortical slices using the vitamin A derivative all-trans retinoic acid (atRA), a putative treatment for neuropsychiatric disorders such as Alzheimer's disease. Our experiments demonstrated that the excitatory synapses of superficial (layer 2/3) pyramidal neurons underwent coordinated structural and functional changes in the presence of atRA. These synaptic adaptations were accompanied by ultrastructural remodeling of the calcium-storing spine apparatus organelle and required mRNA translation. It was not observed in synaptopodin-deficient mice, which lack spine apparatus organelles. We conclude that atRA is a potent mediator of synaptic plasticity in the adult human cortex.

Mitochondria and metabolic transitions in cardiomyocytes: lessons from development for stem cell-derived cardiomyocytes

Current methods to differentiate cardiomyocytes from human pluripotent stem cells (PSCs) inadequately recapitulate complete development and result in PSC-derived cardiomyocytes (PSC-CMs) with an immature or fetal-like phenotype. Embryonic and fetal development are highly dynamic periods during which the developing embryo or fetus is exposed to changing nutrient, oxygen, and hormone levels until birth. It is becoming increasingly apparent that these metabolic changes initiate developmental processes to mature cardiomyocytes. Mitochondria are central to these changes, responding to these metabolic changes and transitioning from small, fragmented mitochondria to large organelles capable of producing enough ATP to support the contractile function of the heart. These changes in mitochondria may not simply be a response to cardiomyocyte maturation; the metabolic signals that occur throughout development may actually be central to the maturation process in cardiomyocytes. Here, we review methods to enhance maturation of PSC-CMs and highlight evidence from development indicating the key roles that mitochondria play during cardiomyocyte maturation. We evaluate metabolic transitions that occur during development and how these affect molecular nutrient sensors, discuss how regulation of nutrient sensing pathways affect mitochondrial dynamics and function, and explore how changes in mitochondrial function can affect metabolite production, the cell cycle, and epigenetics to influence maturation of cardiomyocytes.

Circulating carotenoids and breast cancer among high-risk individuals

BACKGROUND

Carotenoids represent 1 of few modifiable factors to reduce breast cancer risk. Elucidation of interactions between circulating carotenoids and genetic predispositions or mammographic density (MD) may help inform more effective primary preventive strategies in high-risk populations.

OBJECTIVES

We tested whether women at high risk for breast cancer due to genetic predispositions or high MD would experience meaningful and greater risk reduction from higher circulating levels of carotenoids in a nested case-control study in the Nurses' Health Studies (NHS and NHSII).

METHODS

This study included 1919 cases and 1695 controls in a nested case-control study in the NHS and NHSII. We assessed both multiplicative and additive interactions. RR reductions and 95% CIs were calculated using unconditional logistic regressions, adjusting for matching factors and breast cancer risk factors. Absolute risk reductions (ARR) were calculated based on Surveillance, Epidemiology, and End Results incidence rates.

RESULTS

We showed that compared with women at low genetic risk or low MD, those with higher genetic risk scores or high MD had greater ARRs for breast cancer as circulating carotenoid levels increase (additive P-interaction = 0.05). Among women with a high polygenic risk score, those in the highest quartile of circulating carotenoids had a significant ARR (28.6%; 95% CI, 14.8-42.1%) compared to those in the lowest quartile of carotenoids. For women with a high percentage MD (≥50%), circulating carotenoids were associated with a 37.1% ARR (95% CI, 21.7-52.1%) when comparing the highest to the lowest quartiles of circulating carotenoids.

CONCLUSIONS

The inverse associations between circulating carotenoids and breast cancer risk appeared to be more pronounced in high-risk women, as defined by germline genetic makeup or MD.

Nuclear Receptors as Multiple Regulators of NLRP3 Inflammasome Function

Nuclear receptors are important bridges between lipid signaling molecules and transcription responses. Beside their role in several developmental and physiological processes, many of these receptors have been shown to regulate and determine the fate of immune cells, and the outcome of immune responses under physiological and pathological conditions. While NLRP3 inflammasome is assumed as key regulator for innate and adaptive immune responses, and has been associated with various pathological events, the precise impact of the nuclear receptors on the function of inflammasome is hardly investigated. A wide variety of factors and conditions have been identified as modulators of NLRP3 inflammasome activation, and at the same time, many of the nuclear receptors are known to regulate, and interact with these factors, including cellular metabolism and various signaling pathways. Nuclear receptors are in the focus of many researches, as these receptors are easy to manipulate by lipid soluble molecules. Importantly, nuclear receptors mediate regulatory mechanisms at multiple levels: not only at transcription level, but also in the cytosol via non-genomic effects. Their importance is also reflected by the numerous approved drugs that have been developed in the past decade to specifically target nuclear receptors subtypes. Researches aiming to delineate mechanisms that regulate NLRP3 inflammasome activation draw a wide range of attention due to their unquestionable importance in infectious and sterile inflammatory conditions. In this review, we provide an overview of current reports and knowledge about NLRP3 inflammasome regulation from the perspective of nuclear receptors, in order to bring new insight to the potentially therapeutic aspect in targeting NLRP3 inflammasome and NLRP3 inflammasome-associated diseases.

Non-Coding RNAs in Retinoic Acid as Differentiation and Disease Drivers

All-trans retinoic acid (RA) is the most active metabolite of vitamin A. Several studies have described a pivotal role for RA signalling in different biological processes such as cell growth and differentiation, embryonic development and organogenesis. Since RA signalling is highly dose-dependent, a fine-tuning regulatory mechanism is required. Thus, RA signalling deregulation has a major impact, both in development and disease, related in many cases to oncogenic processes. In this review, we focus on the impact of ncRNA post-transcriptional regulatory mechanisms, especially those of microRNAs and lncRNAs, in RA signalling pathways during differentiation and disease.

Changes in retinoid metabolism and signaling associated with metabolic remodeling during fasting and in type I diabetes

Liver is the central metabolic hub that coordinates carbohydrate and lipid metabolism. The bioactive derivative of vitamin A, retinoic acid (RA), was shown to regulate major metabolic genes including phosphoenolpyruvate carboxykinase, fatty acid synthase, carnitine palmitoyltransferase 1, and glucokinase among others. Expression levels of these genes undergo profound changes during adaptation to fasting or in metabolic diseases such as type 1 diabetes (T1D). However, it is unknown whether the levels of hepatic RA change during metabolic remodeling. This study investigated the dynamics of hepatic retinoid metabolism and signaling in the fed state, in fasting, and in T1D. Our results show that fed-to-fasted transition is associated with significant decrease in hepatic retinol dehydrogenase (RDH) activity, the rate-limiting step in RA biosynthesis, and downregulation of RA signaling. The decrease in RDH activity correlates with the decreased abundance and altered subcellular distribution of RDH10 while Rdh10 transcript levels remain unchanged. In contrast to fasting, untreated T1D is associated with upregulation of RA signaling and an increase in hepatic RDH activity, which correlates with the increased abundance of RDH10 in microsomal membranes. The dynamic changes in RDH10 protein levels in the absence of changes in its transcript levels imply the existence of posttranscriptional regulation of RDH10 protein. Together, these data suggest that the downregulation of hepatic RA biosynthesis, in part via the decrease in RDH10, is an integral component of adaptation to fasting. In contrast, the upregulation of hepatic RA biosynthesis and signaling in T1D might contribute to metabolic inflexibility associated with this disease.

Antiepileptogenic Effect of Retinoic Acid

Retinoic acid, a metabolite of vitamin A, acts through either genomic or nongenomic actions. The genomic action of retinoids exerts effects on gene transcription through interaction with retinoid receptors such as retinoic acid receptors (RARα, β, and γ) and retinoid X receptors (RXRα, β, and γ) that are primarily concentrated in the amygdala, pre-frontal cortex, and hippocampal areas in the brain. In response to retinoid binding, RAR/RXR heterodimers undergo major conformational changes and orchestrate the transcription of specific gene networks. Previous experimental studies have reported that retinoic acid exerts an antiepileptogenic effect through diverse mechanisms, including the modulation of gap junctions, neurotransmitters, long-term potentiation, calcium channels and some genes. To our knowledge, there are no previous or current clinical trials evaluating the use of retinoic acid for seizure control.

Vitamin A and Its Multi-Effects on Pancreas: Recent Advances and Prospects

Vitamin A (VA), which is stored in several forms in most tissues, is required to maintain metabolite homeostasis and other processes, including the visual cycle, energy balance, epithelial cell integrity, and infection resistance. In recent years, VA molecules, also known as retinoids, have been extensively explored and used in the treatment of skin disorders and immune-related tumors. To date, several observational and interventional studies have explored the relationship between VA status and the pathogenesis of diabetes. In particular, VA micronutrients have been shown to regulate pancreatic development, β-cell function, pancreatic innate immune responses, and pancreatic stellate cells phenotypes through multiple mechanisms. However, there are still many problems to be proven or resolved. In this review, we summarize and discuss recent and available evidence on VA biological metabolism in the pancreas. Analysis of the effects of VA on metabolism in the pancreas will contribute to our understanding of the supportive physiological roles of VA in pancreas protection.

Retinoid signaling in skeletal development: Scoping the system for predictive toxicology

All-trans retinoic acid (ATRA), the biologically active form of vitamin A, is instrumental in regulating the patterning and specification of the vertebrate embryo. Various animal models demonstrate adverse developmental phenotypes following experimental retinoid depletion or excess during pregnancy. Windows of vulnerability for altered skeletal patterning coincide with early specification of the body plan (gastrulation) and regional specification of precursor cell populations forming the facial skeleton (cranial neural crest), vertebral column (somites), and limbs (lateral plate mesoderm) during organogenesis. A common theme in physiological roles of ATRA signaling is mutual antagonism with FGF signaling. Consequences of genetic errors or environmental disruption of retinoid signaling include stage- and region-specific homeotic transformations to severe deficiencies for various skeletal elements. This review derives from an annex in Detailed Review Paper (DRP) of the OECD Test Guidelines Programme (Project 4.97) to support recommendations regarding assay development for the retinoid system and the use of resulting data in a regulatory context for developmental and reproductive toxicity (DART) testing.

Structural basis for DNA recognition and allosteric control of the retinoic acid receptors RAR-RXR

Retinoic acid receptors (RARs) as a functional heterodimer with retinoid X receptors (RXRs), bind a diverse series of RA-response elements (RAREs) in regulated genes. Among them, the non-canonical DR0 elements are bound by RXR–RAR with comparable affinities to DR5 elements but DR0 elements do not act transcriptionally as independent RAREs. In this work, we present structural insights for the recognition of DR5 and DR0 elements by RXR–RAR heterodimer using x-ray crystallography, small angle x-ray scattering, and hydrogen/deuterium exchange coupled to mass spectrometry. We solved the crystal structures of RXR–RAR DNA-binding domain in complex with the Rarb2 DR5 and RXR–RXR DNA-binding domain in complex with Hoxb13 DR0. While cooperative binding was observed on DR5, the two molecules bound non-cooperatively on DR0 on opposite sides of the DNA. In addition, our data unveil the structural organization and dynamics of the multi-domain RXR–RAR DNA complexes providing evidence for DNA-dependent allosteric communication between domains. Differential binding modes between DR0 and DR5 were observed leading to differences in conformation and structural dynamics of the multi-domain RXR–RAR DNA complexes. These results reveal that the topological organization of the RAR binding element confer regulatory information by modulating the overall topology and structural dynamics of the RXR–RAR heterodimers.

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.

The role of essential fatty acids in cystic fibrosis and normalizing effect of fenretinide

Cystic fibrosis (CF) is the most common autosomal-recessive disease in Caucasians caused by mutations in the CF transmembrane regulator (CFTR) gene. Patients are usually diagnosed in infancy and are burdened with extensive medical treatments throughout their lives. One of the first documented biochemical defects in CF, which predates the cloning of CFTR gene for almost three decades, is an imbalance in the levels of polyunsaturated fatty acids (PUFAs). The principal hallmarks of this imbalance are increased levels of arachidonic acid and decreased levels of docosahexaenoic acids (DHA) in CF. This pro-inflammatory profile of PUFAs is an important component of sterile inflammation in CF, which is known to be detrimental, rather than protective for the patients. Despite decades of intensive research, the mechanistic basis of this phenomenon remains unclear. In this review we summarized the current knowledge on the biochemistry of PUFAs, with a focus on the metabolism of AA and DHA in CF. Finally, a synthetic retinoid called fenretinide (N-(4-hydroxy-phenyl) retinamide) was shown to be able to correct the pro-inflammatory imbalance of PUFAs in CF. Therefore, its pharmacological actions and clinical potential are briefly discussed as well.

Enhanced Antiproliferative Effect of Combined Treatment with Calcitriol and All-Trans Retinoic Acid in Relation to Vitamin D Receptor and Retinoic Acid Receptor α Expression in Osteosarcoma Cell Lines

The main objective of this study was to analyze changes in the antiproliferative effect of vitamin D3, in the form of calcitriol and calcidiol, via its combined application with all-trans retinoic acid (ATRA) in osteosarcoma cell lines. The response to treatment with calcitriol and calcidiol alone was specific for each cell line. Nevertheless, we observed an enhanced effect of combined treatment with ATRA and calcitriol in the majority of the cell lines. Although the levels of respective nuclear receptors did not correlate with the sensitivity of cells to these drugs, vitamin D receptor (VDR) upregulation induced by ATRA was found in cell lines that were the most sensitive to the combined treatment. In addition, all these cell lines showed high endogenous levels of retinoic acid receptor α (RARα). Our study confirmed that the combination of calcitriol and ATRA can achieve enhanced antiproliferative effects in human osteosarcoma cell lines in vitro. Moreover, we provide the first evidence that ATRA is able to upregulate VDR expression in human osteosarcoma cells. According to our results, the endogenous levels of RARα and VDR could be used as a predictor of possible synergy between ATRA and calcitriol in osteosarcoma cells.

Retinoic Acid and the Gut Microbiota in Alzheimer's Disease: Fighting Back-to-Back?

BACKGROUND

There is growing evidence that the gut microbiota may play an important role in neurodegenerative diseases such as Alzheimer's disease. However, how these commensals influence disease risk and progression still has to be deciphered.

OBJECTIVE

The objective of this review was to summarize current knowledge on the interplay between gut microbiota and retinoic acid. The latter one represents one of the important micronutrients, which have been correlated to Alzheimer's disease and are used in initial therapeutic intervention studies.

METHODS

A selective overview of the literature is given with the focus on the function of retinoic acid in the healthy and diseased brain, its metabolism in the gut, and the potential influence that the bioactive ligand may have on microbiota, gut physiology and, Alzheimer's disease.

RESULTS

Retinoic acid can influence neuronal functionality by means of plasticity but also by neurogenesis and modulating proteostasis. Impaired retinoid-signaling, therefore, might contribute to the development of diseases in the brain. Despite its rather direct impact, retinoic acid also influences other organ systems such as gut by regulating the residing immune cells but also factors such as permeability or commensal microbiota. These in turn can also interfere with retinoid-metabolism and via the gutbrain- axis furthermore with Alzheimer's disease pathology within the brain.

CONCLUSION

Potentially, it is yet too early to conclude from the few reports on changed microbiota in Alzheimer's disease to a dysfunctional role in retinoid-signaling. However, there are several routes how microbial commensals might affect and might be affected by vitamin A and its derivatives.

Protein Kinase C Alpha (PKCα) overexpression leads to a better response to retinoid acid therapy through Retinoic Acid Receptor Beta (RARβ) activation in mammary cancer cells

PURPOSE

Retinoids have proved to be effective for hematologic malignancies treatment but till nowadays, their use as single agent for the solid tumor's management is still controversial. All-trans retinoic acid (ATRA), the main active metabolite of vitamin A, exerts non-genomic interactions with different members of the protein kinase C (PKC) family, recognized modulators of different tumor progression pathways. To determine whether a group of patients could become benefited employing a retinoid therapy, in this study we have evaluated whether PKCα expression (a poor prognosis marker in breast cancer) could sensitizes mammary cells to ATRA treatment.

METHODS

PKCα overexpression was achieved by stable transfection and confirmed by western blot. Transfected PKC functionality was determined by nuclear translocation-induction and confocal microscopy. In vitro proliferation was evaluated by cell counting and cell cycle distribution was analyzed by flow cytometry. In vivo studies were performed to evaluate orthotopic tumor growth and experimental lung colonization. Retinoic acid response elements (RARE) and AP1 sites-dependent activity was studied by gene reporter assays and retinoic acid receptors (RARs) were measured by RT-qPCR.

RESULTS

Our findings suggest that high PKCα levels improve the differentiation response to ATRA in a RAR signaling-dependent manner. Moreover, RARβ expression appears to be critical to induce ATRA sensitization, throughout AP1 trans-repression.

CONCLUSION

Here we propose that retinoids could lead a highly personalized anticancer treatment, bringing benefits to patients with aggressive breast tumors resulting from high PKCα expression but, an adequate expression of the RARβ receptor is required to ensure the effect on this process.

Advances and challenges in retinoid delivery systems in regenerative and therapeutic medicine

Retinoids regulate a wide spectrum of cellular functions from the embryo throughout adulthood, including cell differentiation, metabolic regulation, and inflammation. These traits make retinoids very attractive molecules for medical purposes. In light of some of the physicochemical limitations of retinoids, the development of drug delivery systems offers several advantages for clinical translation of retinoid-based therapies, including improved solubilization, prolonged circulation, reduced toxicity, sustained release, and improved efficacy. In this Review, we discuss advances in preclinical and clinical tests regarding retinoid formulations, specifically the ones based in natural retinoids, evaluated in the context of regenerative medicine, brain, cancer, skin, and immune diseases. Advantages and limitations of retinoid formulations, as well as prospects to push the field forward, will be presented.

Current Trends in ATRA Delivery for Cancer Therapy

All-Trans Retinoic Acid (ATRA) is the most active metabolite of vitamin A. It is critically involved in the regulation of multiple processes, such as cell differentiation and apoptosis, by activating specific genomic pathways or by influencing key signaling proteins. Furthermore, mounting evidence highlights the anti-tumor activity of this compound. Notably, oral administration of ATRA is the first choice treatment in Acute Promyelocytic Leukemia (APL) in adults and NeuroBlastoma (NB) in children. Regrettably, the promising results obtained for these diseases have not been translated yet into the clinics for solid tumors. This is mainly due to ATRA-resistance developed by cancer cells and to ineffective delivery and targeting. This up-to-date review deals with recent studies on different ATRA-loaded Drug Delivery Systems (DDSs) development and application on several tumor models. Moreover, patents, pre-clinical, and clinical studies are also reviewed. To sum up, the main aim of this in-depth review is to provide a detailed overview of the several attempts which have been made in the recent years to ameliorate ATRA delivery and targeting in cancer.

Predictive role of vitamin A serum concentration in psoriatic patients treated with IL-17 inhibitors to prevent skin and systemic fungal infections

The use of biological drugs in psoriasis is replacing traditional therapies due to their specific mechanism and limited side effects. However, the use of Interleukin 17 inhibitors and the modification of its cytokine pathway could favor the risk of fungal infections. All-trans retinoic acid is an active metabolite of vitamin A with anti-inflammatory and immunoregulatory properties through its capacity to stimulate both innate and adaptive immunity and to its effects on proliferation, differentiation and apoptosis in a variety of immune cells. Furthermore, it has been recently discovered that All-trans retinoic acid has a direct fungistatic effect against Candida and Aspergillus Fumigatus. On the basis of these new insights, in the current review, we suggest that the evaluation of serum level of All-trans retinoic acid or vitamin A should be considered as a predictive marker for the development of fungal infections among psoriatic patients treated with Interleukin 17 inhibitors. In clinical practice, vitamin A test could be added in the routine hospital diagnostic management for a better selection of psoriatic patients eligible to Interleukin 17 inhibitors.

Tissue localization of retinoic acid receptor (RAR) active drugs

The retinoic acid (RA) signaling pathway is crucial for the control of embryonic development and also regulates function of several organ systems in the adult, including the central nervous system. The retinoic acid receptors (RARs) that mediate the majority of the functions of RA can promote proliferation, differentiation, morphogenesis and cell survival. Dysregulation of this signaling pathway has been considered in the pathophysiology of various diseases including neurodegenerative disorders such Alzheimer's disease and amyotrophic lateral sclerosis. Thus, drugs targeted to the RARs have been proposed as treatments for such diseases. Understanding how these drugs distribute in the body is essential to determine their potential effectiveness. However measuring tissue levels of what are often lipophilic drugs can be difficult. Here we describe an indirect measurement of RAR ligand tissue distribution after intraperitoneal injection into rodents that uses a sensitive RA reporter cell line.

Retinol, Retinoic Acid, and Retinol-Binding Protein 4 are Differentially Associated with Cardiovascular Disease, Type 2 Diabetes, and Obesity: An Overview of Human Studies

Vitamin A is a fat-soluble essential nutrient obtained from plant- and animal-based sources that has roles in growth, vision, and metabolism. Vitamin A circulates mainly as retinol bound to retinol-binding protein 4 (RBP4), and is delivered to tissues and converted to retinoic acid, which is a ligand for several nuclear receptors. In recent years, aspects of vitamin A metabolism have been under scrutiny with regards to the development of metabolic and lifestyle diseases including cardiovascular disease (CVD), type 2 diabetes mellitus (T2DM), and overweight and obesity in humans. Studies have mainly focused on RBP4 in this context, whereas the major circulating form, retinol, and the major bioactive form, retinoic acid, have been overlooked in this regard until recently. As one of the main roles of RBP4 is to deliver retinol to tissues for biological action, the associations of retinol and retinoic acid with these diseases must also be considered. In this review, we summarize and discuss recent and available evidence from human studies with focus on retinol, retinoic acid, and RBP4 and provide an overview of these crucial components of vitamin A metabolism in CVD, T2DM, and obesity. In summary, retinol was found to be both inversely and positively associated with CVD whereas the associations with T2DM and obesity were less clear. Although only a few studies have been published on retinoic acid, it was inversely associated with CVD. In contrast, serum RBP4 was mostly found to be positively associated with CVD, T2DM, and obesity. At present, it is difficult to ascertain why the reported associations differ depending on the compound under study, but there is a clear imbalance in the literature in disfavor of retinol and retinoic acid, which needs to be considered in future human studies.

Meiosis occurs normally in the fetal ovary of mice lacking all retinoic acid receptors

Gametes are generated through a specialized cell differentiation process, meiosis, which, in ovaries of most mammals, is initiated during fetal life. All-trans retinoic acid (ATRA) is considered as the molecular signal triggering meiosis initiation. In the present study, we analyzed female fetuses ubiquitously lacking all ATRA nuclear receptors (RAR), obtained through a tamoxifen-inducible cre recombinase-mediated gene targeting approach. Unexpectedly, mutant oocytes robustly expressed meiotic genes, including the meiotic gatekeeper STRA8. In addition, ovaries from mutant fetuses grafted into adult recipient females yielded offspring bearing null alleles for all Rar genes. Thus, our results show that RAR are fully dispensable for meiotic initiation, as well as for the production of functional oocytes. Assuming that the effects of ATRA all rely on RAR, our study goes against the current model according to which meiosis is triggered by endogenous ATRA in the developing ovary. It therefore revives the search for the meiosis-inducing substance.

Identifying vitamin A signaling by visualizing gene and protein activity, and by quantification of vitamin A metabolites

Vitamin A (retinol) is an essential nutrient for embryonic development and adult homeostasis. Signaling by vitamin A is carried out by its active metabolite, retinoic acid (RA), following a two-step conversion. RA is a small, lipophilic molecule that can diffuse from its site of synthesis to neighboring RA-responsive cells where it binds retinoic acid receptors within RA response elements of target genes. It is critical that both vitamin A and RA are maintained within a tight physiological range to protect against developmental disorders and disease. Therefore, a series of compensatory mechanisms exist to ensure appropriate levels of each. This strict regulation is provided by a number synthesizing and metabolizing enzymes that facilitate the precise spatiotemporal control of vitamin A metabolism, and RA synthesis and signaling. In this chapter we describe protocols that (1) biochemically isolate and quantify vitamin A and its metabolites and (2) visualize the spatiotemporal activity of genes and proteins involved in the signaling pathway.