Retinoic acid upregulates preadipocyte genes to block adipogenesis and suppress diet-induced obesity

Metabolic Sparks in the Liver: Metabolic and Epigenetic Reprogramming in Hepatic Stellate Cells Activation and Its Implications for Human Metabolic Diseases

The liver plays a fundamental role in metabolic homeostasis, integrating systemic fuel utilization with the progression of various metabolic diseases. Hepatic stellate cells (HSCs) are a key nonparenchymal cell type in the liver, which is essential for maintaining hepatic architecture in their quiescent state. However, upon chronic liver injury or metabolic stress, HSCs become activated, leading to excessive extracellular matrix deposition and pro-fibrotic signaling, ultimately positioning them as key players in liver pathology. Emerging evidence highlights the critical roles of metabolic reprogramming and epigenetic regulation in HSCs activation. HSCs activation is driven by both intrinsic fuel metabolism reprogramming and extrinsic metabolic cues from the microenvironment, while the metabolic intermediates actively reshape the epigenetic landscape, reinforcing fibrogenic transcriptional programs. In this review, we summarize recent advances in understanding how metabolic and epigenetic alterations drive HSCs activation, thereby shaping transcriptional programs that sustain fibrosis, and discuss potential therapeutic strategies to target these interconnected pathways in human metabolic diseases.

Vitamin A mediates FABP4 to regulate intramuscular fat production: a new target and strategy for optimizing beef quality

Beef quality is critically determined by intramuscular fat (IMF) deposition. Retinoic acid (RA), the active metabolite of vitamin A, plays an essential regulatory role in IMF development. To systematically investigate RA-mediated regulation of IMF formation in cattle, we established a concentration gradient of RA supplementation and employed a systematic screening approach to identify the optimal dosage for modulating bovine intramuscular adipocytes (IMAs) proliferation and differentiation. Subsequently, leveraging integrated multi-omics approaches, we screened the key downstream molecular targets through which RA governs IMF biosynthesis, and clarified the potential regulatory mechanism of this target. Our experimental data establish that RA promotes the proliferation of IMAs through modulation of G1/S phase progression. Concurrently, RA enhances triglyceride biosynthesis in IMAs by activating PPARγ-mediated cell differentiation and LPL-mediated intracellular lipid accumulation. Integrated transcriptomics and metabonomics analyses identified FABP4, CD36, EBF2, LRP1 and CAV1 as key candidate genes involved in RA-mediated IMF production. Functional interrogation revealed that FABP4 knockdown markedly attenuated lipid accumulation in IMAs, a phenotype rescued through RA supplementation, confirming FABP4 as the critical effector mediating vitamin A's regulation of bovine IMF deposition. These results provide a new understanding of how nutritional factors affect beef quality at the molecular level.

Weight loss and metabolic effects of an ALDH1A1-specific inhibitor, FSI-TN42, in a diet induced mouse model of obesity

BACKGROUND

Retinoic acid (RA) participates in weight regulation and energy metabolism. Mice lacking ALDH1A1, one of the major enzymes responsible for RA biosynthesis, are resistant to diet-induced obesity. Previously, we identified FSI-TN42 (N42) as an ALDH1A1-specific inhibitor and reported its pharmacokinetics and pharmacodynamics as well as its efficacy in weight suppression.

METHODS

In the first study, C57BL/6 J male mice were fed a high fat diet for 8 weeks to induce obesity. Mice were then divided into three groups and fed (1) moderate fat diet (MFD), (2) MFD + WIN 18,446 (1 g/kg diet), or (3) MFD + N42 (1 g/kg diet) for 8 weeks. A control group of mice were fed a low-fat diet for the entire period. Mice were weighed weekly and fasting glucose was determined every 4 weeks. Tissues were examined for potential toxicity using histopathology and complete blood counts. In the second study, we examined influences of N42 on energy balance and/or appetite by determining food intake, activity and energy expenditure in mice with obesity treated with MFD or MFD + N42. Lastly, we tested fertility with a mating study.

RESULTS

N42 significantly accelerated weight loss compared to MFD alone in mice with obesity by reducing fat mass without decreasing lean mass. N42 did not alter food intake or activity levels. While mice treated with N42 lost significantly more weight, they maintained a similar level of energy expenditure compared to mice fed MFD only. Mice fed N42 preferentially used fat postprandially, especially under thermoneutral or mild cold challenge. N42 did not affect male fertility.

CONCLUSIONS

N42 promotes weight loss when used with MFD in mice with diet-induced obesity without causing significant organ toxicity or male infertility. Future studies will determine if N42 can be used to promote further weight loss if combined with current weight loss drugs.

The brown fat-specific overexpression of RBP4 improves thermoregulation and systemic metabolism by activating the canonical adrenergic signaling pathway

Retinol-binding protein 4 (RBP4), the sole specific carrier for retinol (vitamin A) in circulation, is highly expressed in liver and adipose tissues. Previous studies have demonstrated that RBP4 plays a role in cold-mediated adipose tissue browning and thermogenesis. However, the role of RBP4 in brown adipose tissue and its metabolic significance remain unclear. Here we generated and studied transgenic mice that express human RBP4 (hRBP4), specifically in brown adipocytes (UCP1-RBP4 mice), to better understand these uncertainties. When fed a chow diet, these mice presented significantly lower body weights and fat mass than their littermate controls. The UCP1-RBP4 mice also showed significant improvements in glucose clearance, enhanced energy expenditure and increased thermogenesis in response to a cold challenge. This was associated with increased lipolysis and fatty acid oxidation in brown adipose tissue, which was attributed to the activation of canonical adrenergic signaling pathways. In addition, high-performance liquid chromatography analysis revealed that plasma RBP4 and retinol levels were elevated in the UCP1-RBP4 mice, whereas their hepatic retinol levels decreased in parallel with a chow diet. Steady-state brown fat levels of total retinol were significantly elevated in the UCP1-RBP4 mice, suggesting that their retinol uptake was increased in RBP4-expressing brown adipocytes when fed a chow diet. These findings reveal a critical role for RBP4 in canonical adrenergic signaling that promotes lipid mobilization and oxidation in brown adipocytes, where the harnessed energy is dissipated as heat by adaptive thermogenesis.

Serum markers for beef meat quality: Potential media supplement for cell-cultured meat production

As the global population continues to grow and food demands increase, the food industry faces mounting pressure to develop innovative solutions. Cell-cultured meat involves cultivating cells from live animals through self-renewal methods or scaffolding and presents a promising alternative to traditional meat production by generating nutritionally rich biomass. However, significant research is still needed to overcome challenges such as developing serum-free media, identifying suitable additives to support cell growth, and ensuring the quality of cell-cultured meat closely resembles that of traditional meat. Meat quality, which is influenced by various sensorial factors (color, texture, and taste), tenderness, and nutritional values, is determined by the level of intramuscular fat deposition, which significantly influences both meat yield and quality. This paper offers a concise overview of serum markers used to assess beef quality and yield and potential additives currently used in culture media for cell-cultured meat production. We also proposed the potential of using serum markers as additives in the culture media to enhance production of cell-cultured meat. Overall, this review highlights the significance of cell-cultured meat production as a viable solution to address the challenges posed by increasing food demands.

Neonatal vitamin A but not retinoic acid administration increases intramuscular adipocyte number in sheep by promoting vascularization

This study investigated whether vitamin A (VA) administration during the neonatal stage could increase the number of intramuscular adipocytes in Hu sheep by promoting vascularity. A total of 56 newborn male Hu sheep were divided into four groups and received intramuscular injections of either 0, 7500 IU retinoic acid (RA), 7500 IU VA, or a combination of 7500 IU VA and 5 mg SU5416 (an angiogenic inhibitor), at 1, 7, 14, and 21 days of age. At 15 days of age, 6 sheep from each group were randomly selected and sacrificed for intramuscular adipogenic capacity analysis. The remaining 8 sheep in each group were raised until they were 8 months old. VA-treated sheep exhibited an increase in preadipocytes, elevated expression of adipogenic genes (CCAAT enhancer binding protein alpha [CEBPA] and CCAAT enhancer binding protein beta [CEBPB]) and angiogenic genes (vascular endothelial growth factor A [VEGFA]), and stromal vascular fraction cells in the longissimus dorsi (LD) muscle with enhanced adipogenic capacity (P ＜ 0.05). These effects were entirely negated by SU5416. Upon slaughter, VA increased final weight, carcass weight, and average daily gain (P ＜ 0.05) but did not affect feed intake at 21 to 32 weeks (P = 0.824). VA increased the number of intramuscular adipocytes in the LD and semitendinosus (ST) muscle (P ＜ 0.05) without changing the adipocyte number of the omentum, perirenal and subcutaneous fats (P ＞ 0.05). VA injections also increased intramuscular triglyceride (TG) content (P = 0.016) without changing the omentum fat weight or subcutaneous fat thickness (P ＞ 0.05), but it did increase the perirenal fat weight (P = 0.011). Consistently, SU5416 mitigated the effects of VA on intramuscular TG content and adipocyte count, correlating with a decrease in vascularity. In contrast, RA injections didn't affect the intramuscular fat (P = 0.744) but reduced the TG content of the omentum and perirenal fat (P ＜ 0.05). In conclusion, intramuscular injections of VA but not RA at the neonatal stage improved the growth performance of Hu sheep, increasing the number of intramuscular adipocytes and marbling by promoting angiogenesis.

Identification of Genetic Polymorphisms Associated with Interindividual Variability of Vitamin A Concentration in Adipose Tissue of Healthy Male Adults

BACKGROUND

Adipose tissue vitamin A (VA), that is, mainly retinol (RET) and its esters, comes from preformed VA and proVA carotenoids present in our food. Adipose tissue VA acts as hormonal cue maintaining essential aspects of adipocyte biology, which includes fat mobilization and catabolism, energy balance, and glucose homeostasis, and it is thus of particular interest to study its determinants, including genetic ones.

OBJECTIVES

This study aimed to identify genetic variations associated with adipose tissue VA concentration.

METHODS

Forty-two healthy male adults received, in a randomized crossover design, 3 test meals. Periumbilical adipose tissue samples were collected on 6 occasions, that is, at fast and 8 h after consumption of each meal. RET concentration was measured in both plasma and the adipose tissue following saponification. Participants were genotyped using whole-genome microarrays. A total of 1305 single nucleotide polymorphism (SNPs) in or near 27 candidate genes were included for univariate analysis. Partial least squares (PLS) regression was carried out to find the best combination of SNPs associated with the interindividual variability in adipose tissue RET concentration.

RESULTS

Adipose tissue RET concentration was not associated with plasma RET concentrations (r = -0.184, P = 0.28). Interindividual variability of adipose tissue RET concentration was high (coefficient of variation = 62%). Twenty-nine SNPs were significantly (P < 0.05) associated with adipose tissue RET concentration and a PLS regression model identified 16 SNPs as explanatory variables of this concentration. The SNPs were in or near peroxisome proliferator activated receptor gamma, retinoid X receptor alpha, signaling receptor and transporter of retinol, cluster of differentiation 36, free fatty acid receptor 4, aldehyde dehydrogenase 1 family member A1, monoglyceride lipase, diacylglycerol O-acyltransferase 2, and polycystic kidney disease 1-like 2.

CONCLUSIONS

A combination of 16 SNPs has been associated with the interindividual of adipose tissue VA concentration in humans. This trial was registered at clinicaltrials.gov as NCT02100774.

Emergence of microgreens as a valuable food, current understanding of their market and consumer perception: A review

Green leafy vegetables, especially microgreens are gaining popularity due to their high nutritional profiles, rich phytochemical content, and intense flavors. This review explores the growing commercial market for microgreens, especially in upscale dining and premium grocery outlets, highlighting consumer perceptions and their effect on market dynamics. Apart from these, the effect of modern agricultural methods that maximize the growth of microgreens is also examined. The value is anticipated to increase significantly, according to market predictions, from $1.7 billion in 2022 to $2.61 billion by 2029. Positive consumer views on microgreens health benefits drive this growth, although challenges such as varying levels of consumer awareness and income disparities affect sales. The review underscores the need for targeted research and strategic initiatives to enhance consumer understanding and improve cultivation methods to support market expansion in upcoming years.

Maternal genetics and diet modulate vitamin A homeostasis of the offspring and affect the susceptibility to obesity in adulthood in mice

Perinatal nutrition exerts a profound influence on adult metabolic health. This study aimed to investigate whether increased maternal vitamin A (VA) supply can lead to beneficial metabolic phenotypes in the offspring. The researchers utilized mice deficient in the intestine-specific homeobox (ISX) transcription factor, which exhibits increased intestinal VA retinoid production from dietary β-carotene (BC). ISX-deficient dams were fed a VA-sufficient or a BC-enriched diet during the last week of gestation and the whole lactation period. Total retinol levels in milk and weanling livers were 2- to 2.5-fold higher in the offspring of BC-fed dams (BC offspring), indicating increased VA supplies during late gestation and lactation. The corresponding VA-sufficient and BC offspring (males and females) were compared at weaning and adulthood after being fed either a standard or high-fat diet (HFD) with regular VA content for 13 weeks from weaning. HFD-induced increases in adiposity metrics, such as fat depot mass and adipocyte diameter, were more pronounced in males than females and were attenuated or suppressed in the BC offspring. Notably, the BC offspring were protected from HFD-induced increases in circulating triacylglycerol levels and hepatic steatosis. These protective effects were associated with reduced food efficiency, enhanced capacity for thermogenesis and mitochondrial oxidative metabolism in adipose tissues, and increased adipocyte hyperplasia rather than hypertrophy in the BC offspring. In conclusion, maternal VA nutrition influenced by genetics may confer metabolic benefits to the offspring, with mild increases in late gestation and lactation protecting against obesity and metabolic dysregulation in adulthood.NEW & NOTEWORTHY A genetic mouse model, deficient in intestine-specific homeobox (ISX) transcription factor, is used to show that a mildly increased maternal vitamin A supply from β-carotene feeding during late gestation and lactation programs energy and lipid metabolism in tissues and protects the offspring from diet-induced hypertrophic obesity and hepatic steatosis. This knowledge may have implications for human populations where polymorphisms in ISX and ISX target genes involved in vitamin A homeostasis are prevalent.

Retinoic acid generates a beneficial microenvironment for liver progenitor cell activation in acute liver failure

BACKGROUND

When massive necrosis occurs in acute liver failure (ALF), rapid expansion of HSCs called liver progenitor cells (LPCs) in a process called ductular reaction is required for survival. The underlying mechanisms governing this process are not entirely known to date. In ALF, high levels of retinoic acid (RA), a molecule known for its pleiotropic roles in embryonic development, are secreted by activated HSCs. We hypothesized that RA plays a key role in ductular reaction during ALF.

METHODS

RNAseq was performed to identify molecular signaling pathways affected by all-trans retinoid acid (atRA) treatment in HepaRG LPCs. Functional assays were performed in HepaRG cells treated with atRA or cocultured with LX-2 cells and in the liver tissue of patients suffering from ALF.

RESULTS

Under ALF conditions, activated HSCs secreted RA, inducing RARα nuclear translocation in LPCs. RNAseq data and investigations in HepaRG cells revealed that atRA treatment activated the WNT-β-Catenin pathway, enhanced stemness genes (SOX9, AFP, and others), increased energy storage, and elevated the expression of ATP-binding cassette transporters in a RARα nuclear translocation-dependent manner. Further, atRA treatment-induced pathways were confirmed in a coculture system of HepaRG with LX-2 cells. Patients suffering from ALF who displayed RARα nuclear translocation in the LPCs had significantly better MELD scores than those without.

CONCLUSIONS

During ALF, RA secreted by activated HSCs promotes LPC activation, a prerequisite for subsequent LPC-mediated liver regeneration.

Randomized Trial to Improve Body Composition and Micronutrient Status Among South African Children

INTRODUCTION

Physical activity (PA) promotion combined with multimicronutrient supplementation (MMNS) among school-age children may reduce fat mass accrual and increase muscle mass through different mechanisms and so benefit child health. This study determined the efficacy of combined interventions on body composition among South African schoolchildren and determined if micronutrients mediate these effects.

STUDY DESIGN

Longitudinal cluster randomized controlled trial of children followed from 2019 to 2021. Statistical analyses carried from 2022 to 2023.

SETTING/PARTICIPANTS

A total of 1,304 children 6-12 years of age recruited from public schools in Gqeberha, South Africa.

INTERVENTION

Children were randomized by classes to either: (a) a physical activity group (PA); (b) a MMNS group; (c) a physical activity + multimicronutrient supplementation group (PA + MMNS); and (d) a placebo control group.

MAIN OUTCOME MEASURES

Trajectories of overall and truncal fat free mass (FFM) and fat mass (FM) estimates in modeled at 9 and 21 months using latent growth curve models (LGCM). Changes in micronutrient concentrations at 9 months from baseline.

RESULTS

An increased FFM trajectory was found among children in the MMNS arm at 9 months (Beta 0.16, 95% CI = 0.12, 0.31). The PA and MMNS arms both had positive indirect effects on this trajectory at 9 months (Beta 0.66, 95% CI = 0.44, 0.88 and Beta 0.32 95% CI = 0.1 0.5, respectively) and similarly at 21 months when mediated by zinc concentration changes. A reduced FM trajectory was found among children in the PA promotion arm at 9 months when using this collection point as the referent intercept. This arm was inversely associated with the FM trajectory at 9 months when mediated by zinc changes.

CONCLUSIONS

PA and MMNS promotion in school-based interventions directly contributed to reductions in FM and increased FFM among South African children and indirectly through changes in micronutrient status.

TRIAL REGISTRATION

ISRCTN, ISRCTN29534081. Registered on August 9, 2018 Institutional review board: Ethikkommission Nordwest- und Zentralschweiz" (EKNZ, project number: Req-2018-00608). Date of approval: 2018.

Early Life Programming of Adipose Tissue Remodeling and Browning Capacity by Micronutrients and Bioactive Compounds as a Potential Anti-Obesity Strategy

The early stages of life, especially the period from conception to two years, are crucial for shaping metabolic health and the risk of obesity in adulthood. Adipose tissue (AT) plays a crucial role in regulating energy homeostasis and metabolism, and brown AT (BAT) and the browning of white AT (WAT) are promising targets for combating weight gain. Nutritional factors during prenatal and early postnatal stages can influence the development of AT, affecting the likelihood of obesity later on. This narrative review focuses on the nutritional programming of AT features. Research conducted across various animal models with diverse interventions has provided insights into the effects of specific compounds on AT development and function, influencing the development of crucial structures and neuroendocrine circuits responsible for energy balance. The hormone leptin has been identified as an essential nutrient during lactation for healthy metabolic programming against obesity development in adults. Studies have also highlighted that maternal supplementation with polyunsaturated fatty acids (PUFAs), vitamin A, nicotinamide riboside, and polyphenols during pregnancy and lactation, as well as offspring supplementation with myo-inositol, vitamin A, nicotinamide riboside, and resveratrol during the suckling period, can impact AT features and long-term health outcomes and help understand predisposition to obesity later in life.

Intermuscular adipose tissue in obesity and related disorders: cellular origins, biological characteristics and regulatory mechanisms

Intermuscular adipose tissue (IMAT) is a unique adipose depot interspersed between muscle fibers (myofibers) or muscle groups. Numerous studies have shown that IMAT is strongly associated with insulin resistance and muscular dysfunction in people with metabolic disease, such as obesity and type 2 diabetes. Moreover, IMAT aggravates obesity-related muscle metabolism disorders via secretory factors. Interestingly, researchers have discovered that intermuscular brown adipocytes in rodent models provide new hope for obesity treatment by acting on energy dissipation, which inspired researchers to explore the underlying regulation of IMAT formation. However, the molecular and cellular properties and regulatory processes of IMAT remain debated. Previous studies have suggested that muscle-derived stem/progenitor cells and other adipose tissue progenitors contribute to the development of IMAT. Adipocytes within IMAT exhibit features that are similar to either white adipocytes or uncoupling protein 1 (UCP1)-positive brown adipocytes. Additionally, given the heterogeneity of skeletal muscle, which comprises myofibers, satellite cells, and resident mesenchymal progenitors, it is plausible that interplay between these cellular components actively participate in the regulation of intermuscular adipogenesis. In this context, we review recent studies associated with IMAT to offer insights into the cellular origins, biological properties, and regulatory mechanisms of IMAT. Our aim is to provide novel ideas for the therapeutic strategy of IMAT and the development of new drugs targeting IMAT-related metabolic diseases.

Apple-derived extracellular vesicles modulate the expression of human intestinal bile acid transporter ASBT/SLC10A2 via downregulation of transcription factor RARα

PURPOSE

Plant-derived extracellular vesicles (EVs) have been reported to exert biological activity on intestinal tissues by delivering their contents into intestinal cells. We previously reported that ASBT/SLC10A2 mRNA was downregulated by apple-derived extracellular vesicles (APEVs). ASBT downregulation is effective in the treatment of cholestasis and chronic constipation, similar to the beneficial effects of apples. Therefore, this study aimed to establish the mechanism of ASBT downregulation by APEVs, focusing on microRNAs present in APEVs.

RESULTS

APEVs downregulated the expression of ASBT, but no significant effect on SLC10A2-3'UTR was observed. Proteomics revealed that APEVs decreased the expression of RARα/NR1B1. The binding of RARα to SLC10A2 promoter was also decreased by APEVs. The stability of NR1B1 mRNA was attenuated by APEVs and its 3'UTR was found to be a target for APEVs. Apple microRNAs that were predicted to interact with NR1B1-3'UTR were present in APEVs, and their mimics suppressed NR1B1 mRNA expression.

CONCLUSIONS

Suppression of ASBT by APEVs was indirectly mediated by the downregulation of RARα, and its stability was lowered by microRNAs present in APEVs. This study suggested that macromolecules in food directly affect intestinal function by means of EVs that stabilize them and facilitate their cellular uptake.

The Effects of Oral Isotretinoin on Atrophic Acne Scars Measured by Shear-wave Elastography: An Observational, Single-center Study

Background

Although the effects of oral isotretinoin (OI) on acne vulgaris and preventing further acne scars have been well-documented, the specific impact of OI alone on pre-existing atrophic acne scars (AAS) remains unclear. No clinical study has objectively evaluated the effect of OI on AAS yet.

Objective

We sought to investigate the OI effect on AAS quantitatively and reliably by shear-wave elastography (SWE).

Methods

This work is a single-center, prospective and observational study. Thirty patients with moderate and severe acne vulgaris accompanied by AAS were included. We started the OI with a standard dose regime. On Days 0 and 90 of treatment, patients' global acne grading system (GAGS) and the Goodman and Baron's Qualitative Global Scar Rating System (GSRS) were evaluated. The dermal thickness, subcutaneous tissue thickness, scar size, and scar and subcutaneous tissue's elastic modules were measured on both cheeks of each patient by SWE.

Results

The improvement in GSRS stages and GAGS scores in 90 days were statistically significant (respectively; p=0.029, <0.001). Scar size and dermal thickness decreased, while the subcutaneous tissue thickness and the elastic modulus of scar and subcutaneous tissue increased in bilateral cheeks. The thickness changes in the right side dermis, and subcutaneous tissue on both sides were noteworthy (p<0.05).

Conclusion

Besides its well-known effect on acne vulgaris, OI also could be an effective treatment option for reducing scar size and severity while improving skin elasticity. SWE may help follow skin and scar properties.

Retinoic Acid Signaling in Fatty Liver Disease

Retinoic acid (RA) is a metabolite of vitamin A and is essential for development and growth as well as cellular metabolism. Through genomic and nongenomic actions, RA regulates a variety of physiological functions. Dysregulation of RA signaling is associated with many diseases. Targeting RA signaling has been proven valuable to human health. All-trans retinoic acid (AtRA) and anthracycline-based chemotherapy are the standard treatment of acute promyelocytic leukemia (APL). Both human and animal studies have shown a significant relationship between RA signaling and the development and progression of nonalcoholic fatty liver disease (NAFLD). In this review article, we will first summarize vitamin A metabolism and then focus on the role of RA signaling in NAFLD. AtRA inhibits the development and progression of NAFLD via regulating lipid metabolism, inflammation, thermogenesis, etc.

Vitamin A Promotes the Repair of Mice Skeletal Muscle Injury through RARα

Vitamin A (VitA) is an important fat-soluble vitamin which plays an important role in cell growth and individual development. However, the effect of VitA on the repair process of muscle injury and its molecular mechanism are still unclear. In this study, VitA and RA were first added to the culture medium of differentiated cells. We then detected cell differentiation marker proteins and myotube fusion. Moreover, the effects of VitA on RARα expression and nuclear translocation were further examined. The results showed that VitA significantly promoted the differentiation of C2C12, and the expression of RARα was significantly increased. Furthermore, VitA was injected into skeletal muscle injury in mice. HE staining and Western Blot results showed that VitA could significantly accelerate the repair of skeletal muscle injury and VitA increase the expression of RARα in mice. This study provides a theoretical basis for elucidating the regulation mechanism of VitA-mediated muscle development and the development of therapeutic drugs for muscle diseases in animals.

Increasing adipocyte number and reducing adipocyte size: the role of retinoids in adipose tissue development and metabolism

The rising prevalence of obesity is a grave public health threat. In response to excessive energy intake, adipocyte hypertrophy impairs cellular function and leads to metabolic dysfunctions while de novo adipogenesis leads to healthy adipose tissue expansion. Through burning fatty acids and glucose, the thermogenic activity of brown/beige adipocytes can effectively reduce the size of adipocytes. Recent studies show that retinoids, especially retinoic acid (RA), promote adipose vascular development which in turn increases the number of adipose progenitors surrounding the vascular vessels. RA also promotes preadipocyte commitment. In addition, RA promotes white adipocyte browning and stimulates the thermogenic activity of brown/beige adipocytes. Thus, vitamin A is a promising anti-obesity micronutrient.

Thermogenic adipose tissue in energy regulation and metabolic health

The ability to generate thermogenic fat could be a targeted therapy to thwart obesity and improve metabolic health. Brown and beige adipocytes are two types of thermogenic fat cells that regulate energy balance. Both adipocytes share common morphological, biochemical, and thermogenic properties. Yet, recent evidence suggests unique features exist between brown and beige adipocytes, such as their cellular origin and thermogenic regulatory processes. Beige adipocytes also appear highly plastic, responding to environmental stimuli and interconverting between beige and white adipocyte states. Additionally, beige adipocytes appear to be metabolically heterogenic and have substrate specificity. Nevertheless, obese and aged individuals cannot develop beige adipocytes in response to thermogenic fat-inducers, creating a key clinical hurdle to their therapeutic promise. Thus, elucidating the underlying developmental, molecular, and functional mechanisms that govern thermogenic fat cells will improve our understanding of systemic energy regulation and strive for new targeted therapies to generate thermogenic fat. This review will examine the recent advances in thermogenic fat biogenesis, molecular regulation, and the potential mechanisms for their failure.

Lipid-induced alteration in retinoic acid signaling leads to mitochondrial dysfunction in HepG2 and Huh7 cells

A surfeit of mitochondrial reactive oxygen species (ROS) and inflammation serve as obligatory mediators of lipid-associated hepatocellular maladies. While retinoid homeostasis is essential in restoring systemic energy balance, its role in hepatic mitochondrial function remains elusive. The role of lecithin-retinol acyltransferase (LRAT) in maintenance of retinoid homeostasis is appreciated earlier; however, its role in modulating retinoic acid (RA) bioavailability upon lipid-imposition is unexplored. We identified LRAT overexpression in high-fat diet (HFD)-fed rats and palmitate-treated hepatoma cells. Elevation in LRAT expression depletes RA production and deregulates RA signaling. This altered RA metabolism enhances fat accumulation, accompanied by inflammation that leads to impaired mitochondrial function through enhanced ROS generation. Hence, LRAT inhibition could be a novel approach preventing lipid-induced mitochondrial dysfunction in hepatoma cells.

Vitamin A: A Key Inhibitor of Adipocyte Differentiation

Inhibiting adipocyte differentiation, the conversion of preadipocytes to mature functional adipocytes, might represent a new approach to treating obesity and related metabolic disorders. Peroxisome proliferator-activated receptor γ and CCAAT-enhancer-binding protein α are two master coregulators controlling adipogenesis both in culture and in vivo. Many recent studies have confirmed the relationship between retinoic acid (RA) and the conversion of embryonic stem cells into adipocytes; however, these studies have shown that RA potently blocks the differentiation of preadipocytes into mature adipocytes. Nevertheless, the functional role of RA in early tissue development and stem cell differentiation, including in adipose tissue, remains unclear. This study highlights transcription factors that block adipocyte differentiation and maintain preadipocyte status, focusing on those controlled by RA. However, some of these novel adipogenesis inhibitors have not been validated in vivo, and their mechanisms of action require further clarification.

Factors and Molecular Mechanisms of Vitamin A and Childhood Obesity Relationship: A Review

Childhood obesity has become a public health concern. As the importance of vitamin A (VA) in the body has become increasingly acknowledged, there is limited clinical trial evidence to substantiate the association between VA and childhood obesity. Vitamin A deficiency (VAD) increases the risk of childhood obesity, a finding consistently reported in pregnant women. VA could regulate the adipogenic process, inflammation, oxidative stress and metabolism-related gene expression in mature adipocytes. VAD disrupts the balance of obesity-related metabolism, thus affecting lipid metabolism and insulin regulation. Conversely, VA supplementation has a major impact on efficacy in obesity, and obese individuals typically have a lower VA status than normal-weight individuals. Several studies have attempted to identify the genetic and molecular mechanisms underlying the association between VA and obesity. In this review, we summarize and discuss recent new developments focusing on retinol, retinoic acid, and RBP4 and elucidate and provide an overview of the complex interrelationships between these critical components of VA and childhood obesity. However, the causal relationship between VA status and childhood obesity remains unclear. It is also unknown whether VA supplementation improves the overall obesogenic metabolic profile.

The conversion of β-carotene to vitamin A in adipocytes drives the anti-obesogenic effects of β-carotene in mice

OBJECTIVE

The β-carotene oxygenase 1 (BCO1) is the enzyme responsible for the cleavage of β-carotene to retinal, the first intermediate in vitamin A formation. Preclinical studies suggest that BCO1 expression is required for dietary β-carotene to affect lipid metabolism. The goal of this study was to generate a gene therapy strategy that over-expresses BCO1 in the adipose tissue and utilizes the β-carotene stored in adipocytes to produce vitamin A and reduce obesity.

METHODS

We generated a novel adipose-tissue-specific, adeno-associated vector to over-express BCO1 (AT-AAV-BCO1) in murine adipocytes. We tested this vector using a unique model to achieve β-carotene accumulation in the adipose tissue, in which Bco1^-/- mice were fed β-carotene. An AT-AAV over-expressing green fluorescent protein was utilized as control. We evaluated the adequate delivery route and optimized cellular and organ specificity, dosage, and exposure of our vectors. We also employed morphometric analyses to evaluate the effect of BCO1 expression in adiposity, as well as HPLC and mass spectrometry to quantify β-carotene and retinoids in tissues, including retinoic acid.

RESULTS

AT-AAV-BCO1 infusions in the adipose tissue of the mice resulted in the production of retinoic acid, a vitamin A metabolite with strong effects on gene regulation. AT-AAV-BCO1 treatment also reduced adipose tissue size and adipocyte area by 35% and 30%, respectively. These effects were sex-specific, highlighting the complexity of vitamin A metabolism in mammals.

CONCLUSIONS

The over-expression of BCO1 through delivery of an AT-AAV-BCO1 leads to the conversion of β-carotene to vitamin A in adipocytes, which subsequently results in reduction of adiposity. These studies highlight for the first time the potential of adipose tissue β-carotene as a target for BCO1 over-expression in the reduction of obesity.

Vitamin A homeostasis and cardiometabolic disease in humans: lost in translation?

Vitamin A (retinol) is an essential, fat-soluble vitamin that plays critical roles in embryonic development, vision, immunity, and reproduction. Severe vitamin A deficiency results in profound embryonic dysgenesis, blindness, and infertility. The roles of bioactive vitamin A metabolites in regulating cell proliferation, cellular differentiation, and immune cell function form the basis of their clinical use in the treatment of dermatologic conditions and hematologic malignancies. Increasingly, vitamin A also has been recognized to play important roles in cardiometabolic health, including the regulation of adipogenesis, energy partitioning, and lipoprotein metabolism. While these roles are strongly supported by animal and in vitro studies, they remain poorly understood in human physiology and disease. This review briefly introduces vitamin A biology and presents the key preclinical data that have generated interest in vitamin A as a mediator of cardiometabolic health. The review also summarizes clinical studies performed to date, highlighting the limitations of many of these studies and the ongoing controversies in the field. Finally, additional perspectives are suggested that may help position vitamin A metabolism within a broader biological context and thereby contribute to enhanced understanding of vitamin A's complex roles in clinical cardiometabolic disease.

The glucocorticoid receptor represses, whereas C/EBPβ can enhance or repress CYP26A1 transcription

Retinoic acid (RA) counters insulin's metabolic actions. Insulin reduces liver RA biosynthesis by exporting FoxO1 from nuclei. RA induces its catabolism, catalyzed by CYP26A1. A CYP26A1 contribution to RA homeostasis with changes in energy status had not been investigated. We found that glucagon, cortisol, and dexamethasone decrease RA-induced CYP26A1 transcription, thereby reducing RA oxidation during fasting. Interaction between the glucocorticoid receptor and the RAR/RXR coactivation complex suppresses CYP26A1 expression, increasing RA's elimination half-life. Interaction between CCAAT-enhancer-binding protein beta (C/EBPβ) and the major allele of SNP rs2068888 enhances CYP26A1 expression; the minor allele restricts the C/EBPβ effect on CYP26A1. The major and minor alleles associate with impaired human health or reduction in blood triglycerides, respectively. Thus, regulating CYP26A1 transcription contributes to adapting RA to coordinate energy availability with metabolism. These results enhance insight into CYP26A1 effects on RA during changes in energy status and glucocorticoid receptor modification of RAR-regulated gene expression.

Retinoic acid promotes tissue vitamin A status and modulates adipose tissue metabolism of neonatal rats exposed to maternal high-fat diet-induced obesity

Maternal obesity may compromise the micronutrient status of the offspring. Vitamin A (VA) is an essential micronutrient during neonatal development. Its active metabolite, retinoic acid (RA), is a key regulator of VA homeostasis, which also regulates adipose tissue (AT) development in obese adults. However, its role on VA status and AT metabolism in neonates was unknown and it was determined in the present study. Pregnant Sprague-Dawley rats were randomised to a normal fat diet (NFD) or a high fat diet (HFD). From postnatal day 5 (P5) to P20, half of the HFD pups received oral RA every 3 d (HFDRA group). NFD pups and the remaining HFD pups (HFD group) received placebo. Six hours after dosing on P8, P14 and P20, n 4 pups per group were euthanised for different measures. It was found that total retinol concentration in neonatal liver and lung was significantly lower in the HFD group than the NFD group, while the concentrations were significantly increased in the HFDRA group. The HFD group exhibited significantly higher body weight (BW) gain, AT mass, serum leptin and adiponectin, and gene expression of these adipokines in white adipose tissue compared with the NFD group; these measures were significantly reduced in the HFDRA group. BAT UCP2 and UCP3 gene expression were significantly higher in pups receiving RA. In conclusion, repeated RA treatment during the suckling period improved the tissue VA status of neonates exposed to maternal obesity. RA also exerted a regulatory effect on neonatal obesity development by reducing BW gain and adiposity and modulating AT metabolism.

Evidence of depot-specific regulation of all-trans-retinoic acid biosynthesis in human adipose tissue

The prevalence of obesity continues to rise, underscoring the need to better understand the pathways mediating adipose tissue (AT) expansion. All-trans-retinoic acid (atRA), a bioactive vitamin A metabolite, regulates adipogenesis and energy metabolism, and, in rodent studies, aberrant vitamin A metabolism appears a key facet of metabolic dysregulation. The relevance of these findings to human disease is unknown, as are the specific enzymes implicated in vitamin A metabolism within human AT. We hypothesized that in human AT, family 1A aldehyde dehydrogenase (ALDH1A) enzymes contribute to atRA biosynthesis in a depot-specific manner. To test this hypothesis, parallel samples of subcutaneous and omental AT from participants (n = 15) were collected during elective abdominal surgeries to quantify atRA biosynthesis and key atRA synthesizing enzymes. ALDH1A1 was the most abundant ALDH1A isoform in both AT depots with expression approximately twofold higher in omental than subcutaneous AT. ALDH1A2 was detected only in omental AT. Formation velocity of atRA was approximately threefold higher (p = 0.0001) in omental AT (9.8 [7.6, 11.2]) pmol/min/mg) than subcutaneous AT (3.2 [2.1, 4.0] pmol/min/mg) and correlated with ALDH1A2 expression in omental AT (β-coefficient = 3.07, p = 0.0007) and with ALDH1A1 expression in subcutaneous AT (β-coefficient = 0.13, p = 0.003). Despite a positive correlation between body mass index (BMI) and omental ALDH1A1 protein expression (Spearman r = 0.65, p = 0.01), BMI did not correlate with atRA formation. Our findings suggest that ALDH1A2 is the primary mediator of atRA formation in omental AT, whereas ALDH1A1 is the principal atRA-synthesizing enzyme in subcutaneous AT. These data highlight AT depot as a critical variable for defining the roles of retinoids in human AT biology.

Retinol Binding Protein 7 Promotes Adipogenesis in vitro and Regulates Expression of Genes Involved in Retinol Metabolism

Retinol is an essential nutrient in animals. Its metabolites, specifically retinoic acid (RA), are crucial for cell differentiation, including adipogenesis. Retinol binding protein 7 (Rbp7) is under the control of PPARγ, the master regulator of adipogenesis. However, the role of RBP7 in adipogenesis is unclear. Our study showed that Rbp7 was abundantly expressed in white and brown mouse adipose tissues and had a higher expression in adipocytes than in stromal vascular fraction. Rbp7 overexpression promoted 3T3-L1 preadipocyte differentiation with increased triglyceride accumulation and up-regulation of Pparγ, Fabp4, C/ebpα, and AdipoQ. Rbp7 deficient adipocytes had opposite effects of the overexpression, which were rescued by RA supplementation. Indirect assessment of relative nuclear RA levels using RAR response element (RARE)-Luc reporter assay demonstrated that Rbp7 overexpression significantly increased RARE-Luc reporter activity. Rbp7 overexpression significantly increased expression of Raldh1, responsible for RA production, and up-regulation of Lrat and Cyp26a1, involved in retinol storage and RA catabolism, respectively, in 3T3-L1 adipocytes. Rbp7 deficient adipocytes had opposite effects of the overexpression of those genes involved in retinol metabolism. These data suggest that RBP7 increases transcriptional activity of RARE that may induce negative feedback responses via regulation of the gene expression for retinol homeostasis. Our data indicate critical RBP7 functions in adipocytes: regulation of transcriptional activity of RARE and adipocytes differentiation, potentially providing a new target for obesity therapy.

Retinoic Acid: Sexually Dimorphic, Anti-Insulin and Concentration-Dependent Effects on Energy

This review addresses the fasting vs. re-feeding effects of retinoic acid (RA) biosynthesis and functions, and sexually dimorphic RA actions. It also discusses other understudied topics essential for understanding RA activities-especially interactions with energy-balance-regulating hormones, including insulin and glucagon, and sex hormones. This report will introduce RA homeostasis and hormesis to provide context. Essential context also will encompass RA effects on adiposity, muscle function and pancreatic islet development and maintenance. These comments provide background for explaining interactions among insulin, glucagon and cortisol with RA homeostasis and function. One aim would clarify the often apparent RA contradictions related to pancreagenesis vs. pancreas hormone functions. The discussion also will explore the adverse effects of RA on estrogen action, in contrast to the enhancing effects of estrogen on RA action, the adverse effects of androgens on RA receptors, and the RA induction of androgen biosynthesis.

CRABP1 in Non-Canonical Activities of Retinoic Acid in Health and Diseases

In this review, we discuss the emerging role of Cellular Retinoic Acid Binding Protein 1 (CRABP1) as a mediator of non-canonical activities of retinoic acid (RA) and relevance to human diseases. We first discuss the role of CRABP1 in regulating MAPK activities and its implication in stem cell proliferation, cancers, adipocyte health, and neuro-immune regulation. We then discuss an additional role of CRABP1 in regulating CaMKII activities, and its implication in heart and motor neuron diseases. Through molecular and genetic studies of Crabp1 knockout (CKO) mouse and culture models, it is established that CRABP1 forms complexes with specific signaling molecules to function as RA-regulated signalsomes in a cell context-dependent manner. Gene expression data and CRABP1 gene single nucleotide polymorphisms (SNPs) of human cancer, neurodegeneration, and immune disease patients implicate the potential association of abnormality in CRABP1 with human diseases. Finally, therapeutic strategies for managing certain human diseases by targeting CRABP1 are discussed.

Impact of a school-based health intervention program on body composition among South African primary schoolchildren: results from the KaziAfya cluster-randomized controlled trial

BACKGROUND

The prevalence of overweight and obesity is increasing among African children potentially predisposing them to greater obesity and non-communicable diseases (NCDs) in adulthood. This risk may be higher among growth-impaired children who may have greater fat mass. Therefore, we examined the effects of school-based physical activity (PA) promotion and multi-micronutrient supplementation (MMNS) on body composition among South African children enrolled in a longitudinal school-based randomized controlled trial.

METHODS

Children were cluster-randomized by class to one of four groups: (a) a physical activity group (PA), (b) a multi-micronutrient supplementation group (MMNS), (c) a physical activity + multi-micronutrient supplementation group (PA + MMNS), and (d) control group, and were being followed for 3 years. Linear random effects regression models with random intercepts for school classes tested the associations of each intervention arm with overall fat mass (FM), fat-free mass (FFM), truncal fat mass (TrFM), and truncal fat-free mass (TrFFM) at 9 months (T2) for boys and girls. These differences were then explored among children who differed in height velocity (HV).

RESULTS

A total of 1304 children (614 girls, 667 boys) in twelve clusters were assessed at baseline and after 9 months follow-up (T2). At baseline, approximately 15% of children were classified as overweight or obese while approximately 38% of children were classified as mildly stunted or moderately/severely stunted. Among girls, promotion of PA was associated with reduced FM and TrFM at T2 while MMNS was associated with increased FFM. Children with reduced HV in the PA arm had reduced FM while children in the MMNS arm with lower HV had increased FM compared to children in the control arm. Similarly, children with lower HV in the MM and PA groups had reduced TrFM compared to children in the control arm.

CONCLUSIONS

Our study suggests that the promotion of school-based physical activity programs and micronutrient supplementation can reduce childhood adiposity and so reduce the risk of obesity and chronic diseases later in adulthood.

TRIAL REGISTRATION

ISRCTN, ISRCTN29534081 . Registered on August 9, 2018. The trial was designed, analyzed, and interpreted based on the CONSORT protocol (Additional file 1: CONSORT checklist for randomized trial).

Differential Effects of Insulin-Deficient Diabetes Mellitus on Visceral vs. Subcutaneous Adipose Tissue-Multi-omics Insights From the Munich MIDY Pig Model

Adipose tissue (AT) is no longer considered to be responsible for energy storage only but is now recognized as a major endocrine organ that is distributed across different parts of the body and is actively involved in regulatory processes controlling energy homeostasis. Moreover, AT plays a crucial role in the development of metabolic disease such as diabetes. Recent evidence has shown that adipokines have the ability to regulate blood glucose levels and improve metabolic homeostasis. While AT has been studied extensively in the context of type 2 diabetes, less is known about how different AT types are affected by absolute insulin deficiency in type 1 or permanent neonatal diabetes mellitus. Here, we analyzed visceral and subcutaneous AT in a diabetic, insulin-deficient pig model (MIDY) and wild-type (WT) littermate controls by RNA sequencing and quantitative proteomics. Multi-omics analysis indicates a depot-specific dysregulation of crucial metabolic pathways in MIDY AT samples. We identified key proteins involved in glucose uptake and downstream signaling, lipogenesis, lipolysis and β-oxidation to be differentially regulated between visceral and subcutaneous AT in response to insulin deficiency. Proteins related to glycogenolysis, pyruvate metabolism, TCA cycle and lipogenesis were increased in subcutaneous AT, whereas β-oxidation-related proteins were increased in visceral AT from MIDY pigs, pointing at a regionally different metabolic adaptation to master energy stress arising from diminished glucose utilization in MIDY AT. Chronic, absolute insulin deficiency and hyperglycemia revealed fat depot-specific signatures using multi-omics analysis. The generated datasets are a valuable resource for further comparative and translational studies in clinical diabetes research.

Main regulatory factors of marbling level in beef cattle

The content of intramuscular fat (IMF), that determines marbling levels is considered as one of the vital factors influencing beef sensory quality including tenderness, juiciness, flavour and colour. The IMF formation in cattle commences around six months after conception, and continuously grows throughout the life of the animal. The accumulation of marbling is remarkably affected by genetic, sexual, nutritional and management factors. In this review, the adipogenesis and lipogenesis process regulated by various factors and genes during fetal and growing stages is briefly presented. We also discuss the findings of recent studies on the effects of breed, gene, heritability and gender on the marbling accumulation. Various research reported that feeding during pregnancy, concentrate to roughage ratios and the supplementation or restriction of vitamin A, C, and D are crucial nutritional factors affecting the formation and development of IMF. Castration and early weaning combined with high energy feeding are effective management strategies for improving the accumulation of IMF. Furthermore, age and weight at slaughter are also reviewed because they have significant effects on marbling levels. The combination of several factors could positively affect the improvement of the IMF deposition. Therefore, advanced strategies that simultaneously apply genetic, sexual, nutritional and management factors to achieve desired IMF content without detrimental impacts on feed efficiency in high-marbling beef production are essential.

The Interactions of Insulin and Vitamin A Signaling Systems for the Regulation of Hepatic Glucose and Lipid Metabolism

The pandemics of obesity and type 2 diabetes have become a concern of public health. Nutrition plays a key role in these concerns. Insulin as an anabolic hormonal was discovered exactly 100 years ago due to its activity in controlling blood glucose level. Vitamin A (VA), a lipophilic micronutrient, has been shown to regulate glucose and fat metabolism. VA's physiological roles are mainly mediated by its metabolite, retinoic acid (RA), which activates retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which are two transcription factors. The VA status and activations of RARs and RXRs by RA and synthetic agonists have shown to affect the glucose and lipid metabolism in animal models. Both insulin and RA signaling systems regulate the expression levels of genes involved in the regulation of hepatic glucose and lipid metabolism. Interactions of insulin and RA signaling systems have been observed. This review is aimed at summarizing the history of diabetes, insulin and VA signaling systems; the effects of VA status and activation of RARs and RXRs on metabolism and RAR and RXR phosphorylation; and possible interactions of insulin and RA in the regulation of hepatic genes for glucose and lipid metabolism. In addition, some future research perspectives for understanding of nutrient and hormone interactions are provided.

Administration of a selective retinoic acid receptor-γ agonist improves neuromuscular strength in a rodent model of volumetric muscle loss

PURPOSE

Volumetric muscle loss is a uniquely challenging pathology that results in irrecoverable functional deficits. Furthermore, a breakthrough drug or bioactive factor has yet to be established that adequately improves repair of these severe skeletal muscle injuries. This study sought to assess the ability of an orally administered selective retinoic acid receptor-γ agonist, palovarotene, to improve recovery of neuromuscular strength in a rat model of volumetric muscle loss.

METHODS

An irrecoverable, full thickness defect was created in the tibialis anterior muscle of Lewis rats and animals were survived for 4 weeks. Functional recovery of the tibialis anterior muscle was assessed in vivo via neural stimulation and determination of peak isometric torque. Histological staining was performed to qualitatively assess fibrous scarring of the defect site.

RESULTS

Treatment with the selective retinoic acid receptor-γ agonist, palovarotene, resulted in a 38% improvement of peak isometric torque in volumetric muscle loss affected limbs after 4 weeks of healing compared to untreated controls. Additionally, preliminary histological assessment suggests that oral administration of palovarotene reduced fibrous scarring at the defect site.

CONCLUSIONS

These results highlight the potential role of selective retinoic acid receptor-γ agonists in the design of regenerative medicine platforms to maximize skeletal muscle healing. Additional studies are needed to further elucidate cellular responses, optimize therapeutic delivery, and characterize synergistic potential with adjunct therapies.

Adipogenesis and therapeutic potentials of antiobesogenic phytochemicals: Insights from preclinical studies

Obesity is one of the most serious public health problems in both developed and developing countries in recent years. While lifestyle and diet modifications are the most important management strategies of obesity, these may be insufficient to ensure long-term weight reduction in certain individuals and alternative strategies including pharmacotherapy need to be considered. However, drugs option remains limited due to low efficacy and adverse effects associated with their use. Hence, identification of safe and effective alternative therapeutic agents remains warranted to combat obesity. In recent years, bioactive phytochemicals are considered as valuable sources for the discovery of new pharmacological agents for the treatment of obesity. Adipocyte hypertrophy and hyperplasia increases with obesity and undergo molecular and cellular alterations that can affect systemic metabolism giving rise to metabolic syndrome and comorbidities such as type 2 diabetes and cardiovascular diseases. Many phytochemicals have been reported to target adipocytes by inhibiting adipogenesis, inducing lipolysis, suppressing the differentiation of preadipocytes to mature adipocytes, reducing energy intake, and boosting energy expenditure mainly in vitro and in animal studies. Nevertheless, further high-quality studies are needed to firmly establish the clinical efficacy of these phytochemicals. This review outlines common pathways involved in adipogenesis and phytochemicals targeting effector molecules of these pathways, the challenges faced and the way forward for the development of phytochemicals as antiobesity agents.

The role of the retinoid receptor, RAR/RXR heterodimer, in liver physiology

Activated by retinoids, metabolites of vitamin A, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs) play important roles in a wide variety of biological processes, including embryo development, homeostasis, cell proliferation, differentiation and death. In this review, we summarized the functional roles of nuclear receptor RAR/RXR heterodimers in liver physiology. Specifically, RAR/RXR modulate the synthesis and metabolism of lipids and bile acids in hepatocytes, regulate cholesterol transport in macrophages, and repress fibrogenesis in hepatic stellate cells. We have also listed the specific genes that carry these functions and how RAR/RXR regulate their expression in liver cells, providing a mechanistic view of their roles in liver physiology. Meanwhile, we pointed out many questions regarding the detailed signaling of RAR/RXR in regulating the expression of liver genes, and hope future studies will address these issues.

Vitamin A regulates intramuscular adipose tissue and muscle development: promoting high-quality beef production

During growth in cattle, the development of intramuscular adipose tissue and muscle is dependent upon cell hyperplasia (increased number of adipocytes) and hypertrophy (increased size of adipocytes). Based on the results of previous studies, other adipose tissue depots (e.g., perirenal and subcutaneous) develop from the fetal stage primarily as brown adipose tissue. The hyperplastic stage of intramuscular adipose is considered to develop from late pregnancy, but there is no evidence indicating that intramuscular adipose tissue develops initially as brown adipose tissue. Hyperplastic growth of intramuscular adipose continues well into postweaning and is dependent on the timing of the transition to grain-based diets; thereafter, the late-stage development of intramuscular adipose tissue is dominated by hypertrophy. For muscle development, hyperplasia of myoblasts lasts from early (following development of somites in the embryo) to middle pregnancy, after which growth of muscle is the result of hypertrophy of myofibers. Vitamin A is a fat-soluble compound that is required for the normal immunologic function, vision, cellular proliferation, and differentiation. Here we review the roles of vitamin A in intramuscular adipose tissue and muscle development in cattle. Vitamin A regulates both hyperplasia and hypertrophy in in vitro experiments. Vitamin A supplementation at the early stage and restriction at fattening stage generate opposite effects in the beef cattle. Appropriate vitamin A supplementation and restriction strategy increase intramuscular adipose tissue development (i.e., marbling or intramuscular fat) in some in vivo trials. Besides, hyperplasia and hypertrophy of myoblasts/myotubes were affected by vitamin A treatment in in vitro trials. Additionally, some studies reported an interaction between the alcohol dehydrogenase-1C (ADH1C) genotype and vitamin A feed restriction for the development of marbling and/or intramuscular adipose tissue, which was dependent on the timing and level of vitamin A restriction. Therefore, the feed strategy of vitamin A has the visible impact on the marbling and muscle development in the cattle, which will be helpful to promote the quality of the beef.

All-trans retinoic acid controls differentiation, proliferation, and lipolysis in isolated subcutaneous adipocytes from peripartal Holstein cows

Preadipocyte proliferation and differentiation are critical for normal adipose tissue development, including achieving a mature phenotype, characterized by its ability to accumulate triacylglycerol and release fatty acids. In nonruminants, it is well known that all-trans retinoic acid (ATRA), the most-active form of vitamin A, helps regulate proliferation, differentiation, and apoptosis in several types of cells including adipocytes. The purpose of this study was to evaluate the effect of ATRA on proliferation, apoptosis, differentiation, and lipolysis of primary bovine adipocytes isolated from subcutaneous adipose tissue of 5 healthy Holstein cows at 17 (±4 standard deviations) d postpartum. Cells were stimulated with increasing concentrations of ATRA (0.2, 2, and 20 nM) at the preconfluent (2 d) and postconfluent (8 d) preadipocyte stage or at the mature adipocyte stage (2 d). All concentrations of ATRA inhibited preconfluent preadipocyte proliferation with decreased proportion of S-phase cells and reduced protein abundance of cyclins (CCND1, CCND2, CCND3, CCNE1) and cyclin-dependent kinases (CDK2, CDK4, CDK6). Compared with vehicle, ATRA treatment induced apoptosis in preconfluent preadipocytes. Additionally, ATRA (0.2, 2, and 20 nM) supplementation also inhibited differentiation of postconfluent preadipocytes through downregulation of protein abundance of PPARγ and C/EBPα. After induction of differentiation, basal lipolysis in mature adipocytes increased upon treatment with all concentrations of ATRA. However, data on phosphorylated hormone-sensitive lipase or PLIN1 indicated that ATRA had no effect on epinephrine-stimulated lipolysis in mature adipocytes. Overall, these results demonstrate that ATRA might inhibit lipid accumulation by suppressing preadipocyte proliferation and differentiation, subsequently leading to apoptosis in postconfluent preadipocytes and promoting basal lipolysis in mature adipocytes. Overall, these in vitro responses provide some insights into the potential for nutritional management to modulate adipose tissue lipolysis, particularly in overconditioned cows during the dry period, which are more susceptible to suffer metabolic disorders due to excessive fat mobilization postpartum.

Retinoic Acid Receptor Agonists Suppress Muscle Fatty Infiltration in Mice

BACKGROUND

The infiltration of fat tissue into skeletal muscle, a condition referred to as muscle fatty infiltration or fatty degeneration, is regarded as an irreversible event that significantly compromises the motor function of skeletal muscle.

PURPOSE

To investigate the effect of retinoic acid receptor (RAR) agonists in suppressing the adipogenic differentiation of fibroadipogenic progenitors (FAPs) in vitro and fatty infiltration after rotator cuff tear in mice.

STUDY DESIGN

Controlled laboratory study.

METHODS

FAPs isolated from mouse skeletal muscle were cultured in adipogenic differentiation medium in the presence or absence of an RAR agonist. At the end of cell culture, adipogenic differentiation was evaluated by gene expression analysis and oil red O staining. A mouse model of fatty infiltration-which includes the resection of the rotator cuff, removal of the humeral head, and denervation the supraspinatus muscle-was used to induce fatty infiltration in the supraspinatus muscle. The mice were orally or intramuscularly administered with an RAR agonist after the surgery. Muscle fatty infiltration was evaluated by histology and gene expression analysis.

RESULTS

RAR agonists effectively inhibited the adipogenic differentiation of FAPs in vitro. Oral and intramuscular administration of RAR agonists suppressed the development of muscle fatty infiltration in the mice after rotator cuff tear. In accordance, we found a significant decrease in the number of intramuscular fat cells and suppressed expression in adipogenic markers. RAR agonists also increased the expression of the transcripts for collagens; however, an accumulation of collagenous tissues was not histologically evident in the present model.

CONCLUSION

Muscle fatty infiltration can be alleviated by RAR agonists through suppressing the adipogenic differentiation of FAPs. The results also suggest that RAR agonists are potential therapeutic agents for treating patients who are at risk of developing muscle fatty infiltration. The consequence of the increased expression of collagen transcripts by RAR agonists needs to be clarified.

CLINICAL RELEVANCE

RAR agonists can be used to prevent the development of muscle fatty infiltration after rotator cuff tear. Nevertheless, further studies are mandatory in a large animal model to examine the safety and efficacy of intramuscular injection of RAR agonists.

The association between carotenoids and subjects with overweight or obesity: a systematic review and meta-analysis

BACKGROUND

Excess body weight, including overweight and obesity, is one of the major factors influencing human health, and plays an important role in the global burden of disease. Carotenoids serve as precursors of vitamin A-related retinoids, and are considered to have potential effects on many diseases. However, the influence of carotenoids on people with excess body weight is unclear.

METHODS

This meta-analysis was conducted to assess the effects of carotenoids on overweight or obese subjects utilizing the available evidence. We searched PubMed, Medline, Cochrane Library, Web of Science and EMBASE databases up to September 2020. Random effects models were used to calculate the standard mean differences (SMDs) and odds ratios (ORs) with their 95% confidence intervals (95% CIs).

RESULTS

A total of seven randomized controlled trials and eight observational studies met the inclusion criteria and contained 28 944 subjects and data on multiple carotenoid subgroups, including lycopene, astaxanthin, cryptoxanthin, α-carotene, and β-carotene. In all included Randomized Controlled Trial (RCT), the intervention duration was 20 days at the shortest and 16 weeks at the longest, and the range of intervention doses was 1.2-60 mg d-1. Our study found that the insufficiency of serum carotenoids was a risk factor for overweight and obesity (OR = 1.73, 95% CI [1.57, 1.91], p < 0.001). Moreover, carotenoid supplementation was significantly associated with body weight reductions (SMD = -2.34 kg, 95% CI [-3.80, -0.87] kg, p < 0.001), body mass index decrease (BMI, SMD = -0.95 kg cm-2, 95% CI [-1.88, -0.01] kg cm-2, p < 0.001) and waist circumference losses (WC, SMD = -1.84 cm, 95% CI [-3.14, -0.54]cm, p < 0.001).

CONCLUSION

In summary, the carotenoids show promising effects in overweight or obese subjects. Additional data from large clinical trials are needed.

Plasma Carboxyl-Metabolome Is Associated with Average Daily Gain Divergence in Beef Steers

We applied an untargeted metabolomics technique to analyze the plasma carboxyl-metabolome of beef steers with divergent average daily gain (ADG). Forty-eight newly weaned Angus crossbred beef steers were fed the same total mixed ration ad libitum for 42 days. On day 42, the steers were divided into two groups of lowest (LF: n = 8) and highest ADG (HF: n = 8), and blood samples were obtained from the two groups for plasma preparation. Relative quantification of carboxylic-acid-containing metabolites in the plasma samples was determined using a metabolomics technique based on chemical isotope labeling liquid chromatography mass spectrometry. Metabolites that differed (fold change (FC) ≥ 1.2 or ≤ 0.83 and FDR ≤ 0.05) between LF and HF were identified using a volcano plot. Metabolite set enrichment analysis (MSEA) of the differential metabolites was done to determine the metabolic pathways or enzymes that were potentially altered. In total, 328 metabolites were identified. Volcano plot analysis revealed 43 differentially abundant metabolites; several short chain fatty acids and ketone bodies had greater abundance in HF steers. Conversely, several long chain fatty acids were greater in LF steers. Five enzymatic pathways, such as fatty acyl CoA elongation and fatty-acid CoA ligase were altered based on MSEA. This study demonstrated that beef steers with divergent ADG had altered plasma carboxyl-metabolome, which is possibly caused by altered abundances and/or activities of enzymes involved in fatty acid oxidation and biosynthesis in the liver.

Retinoic acid receptor γ is a therapeutically targetable driver of growth and survival in prostate cancer

BACKGROUND

Prostate cancer (PC) tissue contains all-trans retinoic acid (ATRA) at a very low level (10-9 M), at least an order of magnitude lower than in adjacent normal healthy prostate cells or benign prostate hyperplasia. When this is coupled with deregulated expression of the intracellular lipid-binding proteins FABP5 and CRABP2 that is frequently found in PC, this is likely to result in the preferential delivery of ATRA to oncogenic PPARβ/δ rather than retinoic acid receptors (RARs). There are three isotypes of RARs (RARα, RARβ, and RARγ) and recent studies have revealed discrete physiological roles. For example, RARα and RARγ promote differentiation and self-renewal, respectively, which are critical for proper hematopoiesis.

AIMS

We have previously shown that ATRA stimulates transactivation of RARγ at sub-nanomolar concentrations (EC50 0.24 nM), whereas an 80-fold higher concentration was required for RARα-mediated transactivation (EC50 19.3 nM). Additionally, we have shown that RAR pan-antagonists inhibit the growth of PC cells (at 16-34 nM). These findings, together with the low level of ATRA in PC, led us to hypothesize that RARγ plays a role in PC pathogenesis and that RARγ-selective antagonism may be an effective treatment.

METHODS AND RESULTS

We found that concentrations of 10-9 M and below of ATRA promoted survival/proliferation and opposed adipogenic differentiation of human PC cell lines by a mechanism that involves RARγ. We also found that a RARγ-selective antagonist (AGN205728) potently induced mitochondria-dependent, but caspase-independent, cell death in PC cell lines. Furthermore, AGN205728 demonstrated synergism in killing PC cells in combination with cytotoxic chemotherapeutic agents.

CONCLUSION

We suggest that the use of RARγ-selective antagonists may be effective in PC (and potentially other cancers), either as a single agent or in combination with cytotoxic chemotherapy.

Retinoic acid signalling in fibro/adipogenic progenitors robustly enhances muscle regeneration

BACKGROUND

During muscle regeneration, excessive formation of adipogenic and fibrogenic tissues, from their respective fibro/adipogenic progenitors (FAPs), impairs functional recovery. Intrinsic mechanisms controlling the proliferation and differentiation of FAPs remain largely unexplored.

METHODS

Here, we investigated the role of retinoic acid (RA) signalling in regulating FAPs and the subsequent effects on muscle restoration from a cardiotoxin-induced injury. Blockage of retinoic acid receptor (RAR) signalling was achieved through dominant negative retinoic acid receptor α (RARα403) expression specific in PDGFRα+ FAPs in vivo and by BMS493 treatment in vitro. Effects of RAR-signalling on FAP cellularity and muscle regeneration were also investigated in a high-fat diet-induced obese mice model.

FINDINGS

Supplementation of RA increased the proliferation of FAPs during the early stages of regeneration while suppressing FAP differentiation and promoting apoptosis during the remodelling stage. Loss of RAR-signalling caused ectopic adipogenic differentiation of FAPs and impaired muscle regeneration. Furthermore, obesity disrupted the cellular transition of FAPs and attenuated muscle regeneration. Supplementation of RA to obese mice not only rescued impaired muscle fibre regeneration, but also inhibited infiltration of fat and fibrotic tissues during muscle repair. These beneficial effects were abolished after blocking RAR-signalling in FAPs of obese mice.

INTERPRETATION

These data suggest that RAR-signalling in FAPs is a critical therapeutic target for suppressing differentiation of FAPs and facilitating the regeneration of muscle and other tissues.

FUNDING

This study was supported by grants from the National Institutes of Health (R01-HD067449 and R21-AG049976) to M.D.

Intact vitamin A transport is critical for cold-mediated adipose tissue browning and thermogenesis

Objective

Transformation of white into brown fat (“browning”) reduces obesity in many preclinical models and holds great promise as a therapeutic concept in metabolic disease. Vitamin A metabolites (retinoids) have been linked to thermogenic programming of adipose tissue; however, the physiologic importance of systemic retinoid transport for adipose tissue browning and adaptive thermogenesis is unknown.

Methods

We performed cold exposure studies in mice and humans and used a genetic model of defective vitamin A transport, the retinol binding protein deficient (Rbp^−/-) mouse, to study the effects of cooling on systemic vitamin A and the relevance of intact retinoid transport on cold-induced adipose tissue browning.

Results

We show that cold stimulation in mice and humans leads to an increase in circulating retinol and its plasma transporter, Rbp. In Rbp^−/- mice, thermogenic programming of adipocytes and oxidative mitochondrial function are dramatically impaired in subcutaneous white fat, which renders Rbp^−/- mice more cold-sensitive. In contrast, retinol stimulation in primary human adipocytes promotes thermogenic gene expression and mitochondrial respiration. In humans, cold-mediated retinol increase is associated with a shift in oxidative substrate metabolism suggestive of higher lipid utilisation.

Conclusions

Systemic vitamin A levels are regulated by cold exposure in mice and humans, and intact retinoid transport is essential for cold-induced adipose tissue browning and adaptive thermogenesis.

Therapeutic Efficacy of Antioxidants in Ameliorating Obesity Phenotype and Associated Comorbidities

Obesity has been a worldwide epidemic for decades. Despite the abundant increase in knowledge regarding the etiology and pathogenesis of obesity, the prevalence continues to rise with estimates predicting considerably higher numbers by the year 2030. Obesity is characterized by an abnormal lipid accumulation, however, the physiological consequences of obesity are far more concerning. The development of the obesity phenotype constitutes dramatic alterations in adipocytes, along with several other cellular mechanisms which causes substantial increase in systemic oxidative stress mediated by reactive oxygen species (ROS). These alterations promote a chronic state of inflammation in the body caused by the redox imbalance. Together, the systemic oxidative stress and chronic inflammation plays a vital role in maintaining the obese state and exacerbating onset of cardiovascular complications, Type II diabetes mellitus, dyslipidemia, non-alcoholic steatohepatitis, and other conditions where obesity has been linked as a significant risk factor. Because of the apparent role of oxidative stress in the pathogenesis of obesity, there has been a growing interest in attenuating the pro-oxidant state in obesity. Hence, this review aims to highlight the therapeutic role of antioxidants, agents that negate pro-oxidant state of cells, in ameliorating obesity and associated comorbidities. More specifically, this review will explore how various antioxidants target unique and diverse pathways to exhibit an antioxidant defense mechanism.

Guide to preclinical models used to study the pathophysiology of idiopathic intracranial hypertension

Idiopathic intracranial hypertension (IIH) is characterised by raised intracranial pressure (ICP) and papilloedema in the absence of an identifiable secondary cause typically occurring in young women with obesity. The impact is considerable with the potential for blindness, chronic disabling headaches, future risk of cardiovascular disease and marked healthcare utilisation. There have been marked advances in our understanding the pathophysiology of IIH including the role of androgen excess. Insight into pathophysiological underpinnings has arisen from astute clinical observations, studies, and an array of preclinical models. This article summarises the current literature pertaining to the pathophysiology of IIH. The current preclinical models relevant to gaining mechanistic insights into IIH are then discussed. In vitro and in vivo models which study CSF secretion and the effect of potentially pathogenic molecules have started to glean important mechanistic insights. These models are also useful to evaluate novel therapeutic targets to abrogate CSF secretion. Importantly, in vitro CSF secretion assays translate into relevant changes in ICP in vivo. Models of CSF absorption pertinent to IIH, are less well established but highly relevant and of future interest. There is no fully developed in vivo model of IIH but this remains an area of importance. Progress is being made to improve our understanding of the underlying aetiology in IIH including the characterisation of disease biomarkers and their mechanistic role in driving disease pathology. Preclinical models, used to evaluate IIH mechanisms are yielding important mechanistic insights. Further work to refine these techniques will provide translatable insights into disease aetiology.