The retinol esterifying enzyme LRAT supports cell signaling by retinol-binding protein and its receptor STRA6

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.

Combining multiaspect factors to predict the risk of childhood hypertension incidence

Childhood hypertension has become a global public health issue due to its increasing prevalence and association with cerebral-cardiovascular disease in adults. In this study, we developed a predictive model for childhood hypertension based on environmental and genetic factors to identify at-risk individuals. Eighty children diagnosed with childhood hypertension and 84 children in the control group matched by sex and age from an established cohort were included in a nested case-control study. We constructed a multiple logistic regression model to analyze the factors associated with hypertension and applied the 10-fold cross-validation method to verify the prediction stability of the model. Childhood hypertension was found positively correlated with triglyceride level ≥150 mg/dL; low-density lipoprotein cholesterol level ≥110 mg/dL; body mass index Z score; waist-to-height ratio Z score; and red blood cell count (all P < .01) and negatively correlated with the relative expression level of retinol acyltransferase; relative expression level of vitamin D receptor; and dietary intake of fiber, vitamin C and copper (all P < .05) in this study. BMI Z score, triglyceride ≥150 mg/dL, RBC count, VDR/β-actin and LRAT/β-actin ratios were used to construct the predictive model. The area under the receiver operating characteristic curve was 94.45% (95% CI = 89.35%∼98.65%, P < .001). The accuracy, sensitivity, specificity, positive predictive value, and negative predictive value were all above 80% in both the training and validation sets. In conclusion, this model can predict the risk of childhood hypertension and could provide a theoretical basis for early prevention and intervention to improve the cardiovascular health of children.

Intestinal transcriptomes in Kazakh sheep with different haplotypes after experimental Echinococcus granulosus infection

Cystic echinococcosis (CE) is a chronic zoonosis caused by infection with the larval stage of the cestode Echinococcus granulosus. As the intermediate host, sheep are highly susceptible to this disease. Our previous studies have shown that sheep with haplotype MHC Mva Ibc-Sac IIab-Hin1I ab were resistant to CE infection, while their counterparts without this haplotype were not. In order to reveal the molecular mechanism of resistance in Kazakh sheep, after selecting the differential miRNA in our previous study, herein, transcriptome analyses were conducted to detect the differential expression genes in the intestinal tissue of Kazakh sheep with resistant and non-resistant MHC haplotypes, after peroral infection with E. granulosus eggs. A total of 3835 differentially expressed genes were identified between the two groups, with 2229 upregulated and 1606 downregulated. Further function analysis showed that the most significant genes were related to both innate immune response and adaptive response participating in the defense against E. granulosus infection and the metabolic changes associated with it. The results suggest that genes related to lectin receptors, NK cells activation, chemokines, and tumor necrosis factor, may play important roles in the response of intestinal tissue to E. granulosus.

Vitamin A deficiency indicating as low expression of LRAT may be a novel biomarker of primary hypertension

AIMS

Vitamin A (VA) deficiency triggers many diseases and is a worldwide nutrition problem. The Retinol acyltransferase (LRAT) is an indicator of VA storage function, and the relationship between LRAT and blood pressure level and the regulation mechanism will be elucidated.

METHODS

160 children aged 6-12 years were included, and the serum VA and, the transcription levels of LRAT and RARs, were measured. Spontaneously hypertensive rats (SHRs) and WKY rats were treated with VA deficiency (VAD) or normal (VAN) fodder for 20 weeks. LRAT, retinoic acid, renin angiotensin system (RAS) biomarkers, and the structure and function of the heart for SHRs were measured.

RESULTS

The serum retinol and serum retinol/BMI levels were lower in children in the low LRAT group (LRAT

CONCLUSIONS

LRAT may be an important biomarker of vitamin A deficiency in target organs and may regulate BP by affecting RAS biomarkers.

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Key Words

• Childhood Primary Hypertension
• Renin Angiotensin System
• Retinol Acyltransferase
• Spontaneously Hypertensive Rats
• Vitamin A Deficiency

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MESH Headings

• Acyltransferases/metabolism
• Animals
• Biomarkers/metabolism
• Blood Pressure/physiology
• Child
• Female
• Humans
• Hypertension/diagnosis
• Hypertension/drug therapy
• Hypertension/metabolism
• Male
• Rats
• Rats, Inbred SHR
• Renin-Angiotensin System/drug effects
• Renin-Angiotensin System/physiology
• Tretinoin/metabolism
• Vitamin A/blood
• Vitamin A/pharmacology
• Vitamin A Deficiency/diagnosis
• Vitamin A Deficiency/metabolism
• Vitamin A Deficiency/therapy
• Vitamins/blood
• Vitamins/pharmacology

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Affiliation(s)

Xiaohua Liang Clinical Epidemiology and Biostatistics Department, Children's Hospital of Chongqing Medical University, Ministry of Education Key Laboratory of Child Development and Disorders, National Clinical Research Center for Child Health and Disorders, Key Laboratory of Pediatrics in Chongqing, China International Science and Technology Cooperation Center of Child Development and Critical Disorders , Chongqing, China
Min Chen Clinical Epidemiology and Biostatistics Department, Children's Hospital of Chongqing Medical University, Ministry of Education Key Laboratory of Child Development and Disorders, National Clinical Research Center for Child Health and Disorders, Key Laboratory of Pediatrics in Chongqing, China International Science and Technology Cooperation Center of Child Development and Critical Disorders , Chongqing, China
Dong Wang Department of Ultrasound, First Affiliated Hospital of Chongqing Medical University , Chongqing, China
Jin Wen Kidney Disease Department, Yongchuan Hospital of Chongqing Medical University , Chongqing, China
Jie Chen Clinical Epidemiology and Biostatistics Department, Children's Hospital of Chongqing Medical University, Ministry of Education Key Laboratory of Child Development and Disorders, National Clinical Research Center for Child Health and Disorders, Key Laboratory of Pediatrics in Chongqing, China International Science and Technology Cooperation Center of Child Development and Critical Disorders , Chongqing, China

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STRA6 Polymorphisms Are Associated With EGFR Mutations in Locally-Advanced and Metastatic Non-Small Cell Lung Cancer Patients

Retinol plays a significant role in several physiological processes through their nuclear receptors, whose expression depends on retinol cytoplasmic concentration. Loss of expression of nuclear receptors and low retinol levels have been correlated with lung cancer development. Stimulated by retinoic acid 6 (STRA6) is the only described cell membrane receptor for retinol uptake. Some chronic diseases have been linked with specific polymorphisms in STRA6. This study aimed to evaluate four STRA6 single nucleotide polymorphisms (SNPs) (rs4886578, rs736118, rs351224, and rs97445) among 196 patients with locally-advanced and metastatic non-small cell lung cancer (NSCLC) patients. Genotyping, through a validated SNP assay and determined using real time-PCR, was correlated with clinical features and outcomes. NSCLC patients with a TT SNP rs4886578 and rs736118 genotype were more likely to be >60 years, non-smokers, and harboring EGFR mutations. Patients with a TT genotype compared with a CC/CT SNP rs974456 genotype had a median progression-free survival (PFS) of 3.2 vs. 4.8 months, p = 0.044, under a platinum-based regimen in the first-line. Furthermore, patients with a TT rs351224 genotype showed a prolonged overall survival (OS), 47.5 months vs. 32.0 months, p = 0.156. This study showed a correlation between clinical characteristics, such as age, non-smoking history, and EGFR mutational status and oncological outcomes depending on STRA6 SNPs. The STRA6 TT genotype SNP rs4886578 and rs736118 might be potential biomarkers in locally-advanced and metastatic NSCLC patients.

Electronegative low-density lipoprotein of patients with metabolic syndrome induces pathogenesis of aorta through disruption of the stimulated by retinoic acid 6 cascade

AIMS/INTRODUCTION

Electronegative low-density lipoprotein (L5) is the most atherogenic fraction of low-density lipoprotein and is elevated in people with metabolic syndrome (MetS), whereas the retinol-binding protein 4 receptor (stimulated by retinoic acid 6 [STRA6]) cascade is disrupted in various organs of patients with obesity-related diseases. Our objective was to investigate whether L5 from MetS patients capably induces pathogenesis of aorta through disrupting the STRA6 cascade.

MATERIAL AND METHODS

We examined the in vivo and in vitro effects of L5 on the STRA6 cascade and aortic atherogenic markers. To investigate the role of this cascade on atherosclerotic formation, crbp1 transfection was carried out in vitro.

RESULTS

This study shows that L5 activates atherogenic markers (p38 mitogen-activated protein kinases, pSmad2 and matrix metallopeptidase 9) and simultaneously suppresses STRA6 signals (STRA6, cellular retinol-binding protein 1, lecithin-retinol acyltransferase, retinoic acid receptor-α and retinoid X receptor-α) in aortas of L5-injected mice and L5-treated human aortic endothelial cell lines and human aortic smooth muscle cell lines. These L5-induced changes of the STRA6 cascade and atherogenic markers were reversed in aortas of LOX1-/- mice and in LOX1 ribonucleic acid-silenced human aortic endothelial cell lines and human aortic smooth muscle cell lines. Furthermore, crbp1 gene transfection reversed the disruption of the STRA6 cascade, the phosphorylation of p38 mitogen-activated protein kinases and Smad2, and the elevation of matrix metallopeptidase 9 in L5-treated human aortic endothelial cell lines.

CONCLUSIONS

This study shows that L5 from MetS patients induces atherogenic markers by disrupting STRA6 signaling. Suppression of STRA6 might be one novel pathogenesis of aorta in patients with MetS.

Vitamin A and inflammatory bowel diseases: from cellular studies and animal models to human disease

Vitamin A (VA) and metabolites such as Retinoic Acid (RA) and all-trans-RA (at-RA) are crucial in the modulation of the immune system and may be determinative in the balance of the immune responses. Inflammatory bowel diseases (IBD) consist of chronic relapsing and heterogeneous disorders with not well-known etiology. Due to its role in inflammatory processes, VA may be helpful in the treatment of IBD. Area covered: As VA plays a significant role in the inflammatory processes, this review aims to show the potential role of this vitamin in IBD, searching for cellular studies, animal models, and studies with humans. Expert commentary: Many studies have described the importance of alternative therapeutic approaches for IBD. Due to its role in the immune system, VA may also exert an indispensable role in the IBD. Nevertheless, some authors have shown that these compounds could stimulate the release of pro-inflammatory cytokines. For these reasons, more studies should be performed to establish the precise mechanisms of VA and its metabolites in systemic and intestinal inflammation.

Palmitate-induced C/EBP homologous protein activation leads to NF-κB-mediated increase in BACE1 activity and amyloid beta genesis

The etiology of Alzheimer's disease (AD) is egregiously comprehended, but epidemiological studies have posited that diets rich in the saturated fatty acid palmitic acid (palmitate) are a significant risk factor. The production and accumulation of amyloid beta peptide (Aβ) is considered the core pathological molecular event in the pathogenesis of AD. The rate-limiting step in Aβ genesis from amyloid-β precursor protein (AβPP) is catalyzed by the enzyme β-site amyloid precursor protein cleaving enzyme 1 (BACE1), the expression and enzymatic activity of which is significantly up-regulated in the AD brain. In this study, we determined the molecular mechanisms that potentially underlie the palmitate-induced up-regulation in BACE1 expression and augmented Aβ production. We demonstrate that a palmitate-enriched diet and exogenous palmitate treatment evoke an increase in BACE1 expression and activity leading to enhanced Aβ genesis in the mouse brain and SH-SY5Y-APPSwe cells, respectively, through the activation of the transcription factor NF-κB. Chromatin immunoprecipitation (ChIP) assays and luciferase reporter assays revealed that palmitate enhances BACE1 expression by increasing the binding of NF-κB in the BACE1 promoter followed by an enhancement in the transactivation of the BACE1 promoter. Elucidation and delineation of upstream molecular events unveiled a critical role of the endoplasmic reticulum stress-associated transcription factor, C/EBP homologous protein (CHOP) in the palmitate-induced NF-κB activation, as CHOP knock-down cells and Chop-/- mice do not exhibit the same degree of NF-κB activation in response to the palmitate challenge. Our study delineates a novel CHOP-NF-κB signaling pathway that mediates palmitate-induced up-regulation of BACE1 expression and Aβ genesis.

Functions of Intracellular Retinoid Binding-Proteins

Multiple binding and transport proteins facilitate many aspects of retinoid biology through effects on retinoid transport, cellular uptake, metabolism, and nuclear delivery. These include the serum retinol binding protein sRBP (aka Rbp4), the plasma membrane sRBP receptor Stra6, and the intracellular retinoid binding-proteins such as cellular retinol-binding proteins (CRBP) and cellular retinoic acid binding-proteins (CRABP). sRBP transports the highly lipophilic retinol through an aqueous medium. The major intracellular retinol-binding protein, CRBP1, likely enhances efficient retinoid use by providing a sink to facilitate retinol uptake from sRBP through the plasma membrane or via Stra6, delivering retinol or retinal to select enzymes that generate retinyl esters or retinoic acid, and protecting retinol/retinal from excess catabolism or opportunistic metabolism. Intracellular retinoic acid binding-proteins (CRABP1 and 2, and FABP5) seem to have more diverse functions distinctive to each, such as directing retinoic acid to catabolism, delivering retinoic acid to specific nuclear receptors, and generating non-canonical actions. Gene ablation of intracellular retinoid binding-proteins does not cause embryonic lethality or gross morphological defects. Metabolic and functional defects manifested in knockouts of CRBP1, CRBP2 and CRBP3, however, illustrate their essentiality to health, and in the case of CRBP2, to survival during limited dietary vitamin A. Future studies should continue to address the specific molecular interactions that occur between retinoid binding-proteins and their targets and their precise physiologic contributions to retinoid homeostasis and function.

Palmitate Increases β-site AβPP-Cleavage Enzyme 1 Activity and Amyloid-β Genesis by Evoking Endoplasmic Reticulum Stress and Subsequent C/EBP Homologous Protein Activation

Epidemiological studies implicate diets rich in saturated free fatty acids (sFFA) as a potential risk factor for developing Alzheimer's disease (AD). In particular, high plasma levels of the sFFA palmitic acid (palmitate) were shown to inversely correlate with cognitive function. However, the cellular mechanisms by which sFFA may increase the risk for AD are not well known. Endoplasmic reticulum (ER) stress has emerged as one of the signaling pathways initiating and fostering the neurodegenerative changes in AD by increasing the aspartyl protease β-site AβPP cleaving enzyme 1 (BACE1) and amyloid-β (Aβ) genesis. In this study, we determined the extent to which palmitate increases BACE1 and Aβ levels in vitro and in vivo as well as the potential role of ER stress as cellular mechanism underlying palmitate effects. We demonstrate, in palmitate-treated SH-SY5Y neuroblastoma cells and in the hippocampi of palmitate-enriched diet-fed mice, that palmitate evokes the activation of the C/EBP Homologous Protein (CHOP), a transcription factor that is specifically responsive to ER stress. Induction of CHOP expression is associated with increased BACE1 mRNA, protein and activity levels, and subsequent enhanced amyloidogenic processing of amyloid-β protein precursor (AβPP) that culminates in a substantial increase in Aβ genesis. We further show that CHOP is an indispensable molecular mediator of palmitate-induced upregulation in BACE1 activity and Aβ genesis. Indeed, we show that Chop-/- mice and CHOP knocked-down SH-SY5Y neuroblastoma cells do not exhibit the same commensurate degree of palmitate-induced increase in BACE1 expression levels and Aβ genesis.

The retinol-binding protein receptor STRA6 regulates diurnal insulin responses

It has long been appreciated that insulin action is closely tied to circadian rhythms. However, the mechanisms that dictate diurnal insulin sensitivity in metabolic tissues are not well understood. Retinol-binding protein 4 (RBP4) has been implicated as a driver of insulin resistance in rodents and humans, and it has become an attractive drug target in type II diabetes. RBP4 is synthesized primarily in the liver where it binds retinol and transports it to tissues throughout the body. The retinol-RBP4 complex (holo-RBP) can be recognized by a cell-surface receptor known as stimulated by retinoic acid 6 (STRA6), which transports retinol into cells. Coupled to retinol transport, holo-RBP can activate STRA6-driven Janus kinase (JAK) signaling and downstream induction of signal transducer and activator of transcription (STAT) target genes. STRA6 signaling in white adipose tissue has been shown to inhibit insulin receptor responses. Here, we examined diurnal rhythmicity of the RBP4/STRA6 signaling axis and investigated whether STRA6 is necessary for diurnal variations in insulin sensitivity. We show that adipose tissue STRA6 undergoes circadian patterning driven in part by the nuclear transcription factor REV-ERBα. Furthermore, STRA6 is necessary for diurnal rhythmicity of insulin action and JAK/STAT signaling in adipose tissue. These findings establish that holo-RBP and its receptor STRA6 are potent regulators of diurnal insulin responses and suggest that the holo-RBP/STRA6 signaling axis may represent a novel therapeutic target in type II diabetes.

Maternal low-protein diet decreases brain-derived neurotrophic factor expression in the brains of the neonatal rat offspring

Prenatal exposure to a maternal low-protein (LP) diet has been known to cause cognitive impairment, learning and memory deficits. However, the underlying mechanisms have not been identified. Herein, we demonstrate that a maternal LP diet causes, in the brains of the neonatal rat offspring, an attenuation in the basal expression of the brain-derived neurotrophic factor (BDNF), a neurotrophin indispensable for learning and memory. Female rats were fed either a 20% normal protein (NP) diet or an 8% LP 3 weeks before breeding and during the gestation period. Maternal LP diet caused a significant reduction in the Bdnf expression in the brains of the neonatal rats. We further found that the maternal LP diet reduced the activation of the cAMP/protein kinase A/cAMP response element binding protein (CREB) signaling pathway. This reduction was associated with a significant decrease in CREB binding to the Bdnf promoters. We also show that prenatal exposure to the maternal LP diet results in an inactive or repressed exon I and exon IV promoter of the Bdnf gene in the brain, as evidenced by fluxes in signatory hallmarks in the enrichment of acetylated and trimethylated histones in the nucleosomes that envelop the exon I and exon IV promoters, causing the Bdnf gene to be refractory to transactivation. Our study is the first to determine the impact of a maternal LP diet on the basal expression of BDNF in the brains of the neonatal rats exposed prenatally to an LP diet.

Is retinol binding protein 4 a link between adiposity and cancer?

Retinol binding protein 4 (RBP4) is synthesized in the liver where it binds vitamin A, retinol, and transports it to tissues throughout the body. It has been shown in some studies that the level of circulating RBP4 increases with body mass, and the protein has been implicated as a mediator in the development of insulin resistance and the metabolic disease. Adipose tissue serves as another site of RBP4 synthesis, accounting for its designation as an adipokine. In addition to its function as a transport protein, RBP4 serves as a signaling molecule which, by binding to the membrane receptor STRA6, triggers downstream activation of pro-oncogenic pathways including JAK2/STAT3/5. Taken together, available information suggests the possibility that RBP4 may be a link between obesity and cancer.

Electronegative low density lipoprotein induces renal apoptosis and fibrosis: STRA6 signaling involved

Dyslipidemia has been proven to capably develop and aggravate chronic kidney disease. We also report that electronegative LDL (L5) is the most atherogenic LDL. On the other hand, retinoic acid (RA) and RA receptor (RAR) agonist are reported to be beneficial in some kidney diseases. “Stimulated by retinoic acid 6” (STRA6), one retinol-binding protein 4 receptor, was recently identified to regulate retinoid homeostasis. Here, we observed that L5 suppressed STRA6 cascades [STRA6, cellular retinol-binding protein 1 (CRBP1), RARs, retinoid X receptor α, and retinol, RA], but L5 simultaneously induced apoptosis and fibrosis (TGFβ₁, Smad2, collagen 1, hydroxyproline, and trichrome) in kidneys of L5-injected mice and L5-treated renal tubular cells. These L5-induced changes of STRA6 cascades, renal apoptosis, and fibrosis were reversed in kidneys of LOX1^−/− mice. LOX1 RNA silencing and inhibitor of c-Jun N-terminal kinase and p38MAPK rescued the suppression of STRA6 cascades and apoptosis and fibrosis in L5-treated renal tubular cells. Furthermore, crbp1 gene transfection reversed downregulation of STRA6 cascades, apoptosis, and fibrosis in L5-treated renal tubular cells. For mimicking STRA6 deficiency, efficient silencing of STRA6 RNA was performed and was found to repress STRA6 cascades and caused apoptosis and fibrosis in L1-treated renal tubular cells. In summary, this study reveals that electronegative L5 can cause kidney apoptosis and fibrosis via the suppression of STRA6 cascades, and implicates that STRA6 signaling may be involved in dyslipidemia-mediated kidney disease.

Vitamin A in regulation of insulin responsiveness: mini review

Vitamin A, retinol, circulates in blood bound to retinol-binding protein (RBP4) which, in turn, associates with another serum protein, transthyretin (TTR), to form a ternary retinol-RBP4-TTR complex. At some tissues, retinol-bound (holo-) RBP4 is recognised by a receptor termed stimulated by retinoic acid 6 (STRA6) which transports retinol into cells. This mini-review summarises evidence demonstrating that, in addition to functioning as a retinol transporter, STRA6 is also a signalling receptor which is activated by holo-RBP4. The data show that STRA6-mediated retinol transport induces receptor phosphorylation, in turn activating a Janus kinases2/signal transducers and activators of transcription (STAT)3/5 cascade that culminates in induction of STAT target genes. STRA6-mediated retinol transport and cell signalling are inter-dependent, and both functions critically rely on intracellular retinol trafficking and metabolism. Hence, STRA6 couples 'sensing' of vitamin A homeostasis and metabolism to cell signalling, allowing it to control important biological functions. For example, by inducing the expression of the STAT target gene suppressor of cytokine signalling 3, STRA6 potently suppresses insulin responses. These observations provide a rationale for understanding the reports that elevation in serum levels of RBP4, often observed in obese mice and human subjects, causes insulin resistance. The observations indicate that the holo-RBP4 /STRA6 signalling cascade may comprise an important link through which obesity leads to insulin resistance and suggest that the pathway may be a novel target for treatment of metabolic diseases.

Vitamin A Transport and Cell Signaling by the Retinol-Binding Protein Receptor STRA6

Vitamin A, retinol, circulates in blood bound to retinol binding protein (RBP). In some tissues, the retinol-RBP complex (holo-RBP) is recognized by a membrane receptor, termed STRA6, which mediates uptake of retinol into cells. Recent studies have revealed that, in addition to serving as a retinol transporter, STRA6 is a ligand-activated cell surface signaling receptor that, upon binding of holo-RBP activates JAK/STAT signaling, culminating in the induction of STAT target genes. It has further been shown that retinol transport and cell signaling by STRA6 are critically interdependent and that both are coupled to intracellular vitamin A metabolism. The molecular mechanism of action of STRA6 and its associated machinery is beginning to be revealed, but further work is needed to identify and characterize the complete range of genes and associated signaling cascades that are regulated by STRA6 in different tissues. An understanding of STRA6 is clinically relevant, as for example, it has been shown to be hyper- activated in obese animals, leading to insulin resistance. A potential role for STRA6 in other pathologies, including cancer, awaits further investigation.

Role of Retinoic Acid-Metabolizing Cytochrome P450s, CYP26, in Inflammation and Cancer

Vitamin A (retinol) and its active metabolite, all-trans-retinoic acid (atRA), play critical roles in regulating the differentiation, growth, and migration of immune cells. Similarly, as critical signaling molecules in the regulation of the cell cycle, retinoids are important in cancers. Concentrations of atRA are tightly regulated in tissues, predominantly by the availability of retinol, synthesis of atRA by ALDH1A enzymes and metabolism and clearance of atRA by CYP26 enzymes. The ALDH1A and CYP26 enzymes are expressed in several cell types in the immune system and in cancer cells. In the immune system, the ALDH1A and CYP26 enzymes appear to modulate RA concentrations. Consequently, alterations in the activity of ALDH1A and CYP26 enzymes are expected to change disease outcomes in inflammation. There is increasing evidence from various disease models of intestinal and skin inflammation that treatment with atRA has a positive effect on disease markers. However, whether aberrant atRA concentrations or atRA synthesis and metabolism play a role in inflammatory disease development and progression is not well understood. In cancers, especially in acute promyelocytic leukemia and neuroblastoma, increasing intracellular concentrations of atRA appears to provide clinical benefit. Inhibition of the CYP26 enzymes to increase atRA concentrations and combat therapy resistance has been pursued as a drug target in these cancers. This chapter covers the current knowledge of how atRA and retinol regulate the immune system and inflammation, how retinol and atRA metabolism is altered in inflammation and cancer, and what roles atRA-metabolizing enzymes have in immune responses and cancers.

Signaling by retinol and its serum binding protein

Vitamin A, retinol, circulates in blood bound to retinol-binding protein (RBP) which, in turn, associates with transthyretin (TTR) to form a retinol-RBP-TTR ternary complex. At some tissues, retinol-bound (holo-) RBP is recognized by a membrane protein termed STRA6, which transports retinol from extracellular RBP into cells and, concomitantly, activates a JAK2/STAT3/5 signaling cascade that culminates in induction of STAT target genes. STRA6-mediated retinol transport and cell signaling are critically inter-dependent, and they both require the presence of cellular retinol-binding protein 1 (CRBP1), an intracellular retinol acceptor, as well as a retinol-metabolizing enzyme such as lecithin:retinol acyltransferase (LRAT). STRA6 thus functions as a "cytokine signaling transporter" which couples vitamin A homeostasis and metabolism to cell signaling, thereby regulating gene transcription. Recent studies provided molecular level insights into the mode of action of this unique protein.

PTPRT regulates high-fat diet-induced obesity and insulin resistance

Obesity is a risk factor for many human diseases. However, the underlying molecular causes of obesity are not well understood. Here, we report that protein tyrosine phosphatase receptor T (PTPRT) knockout mice are resistant to high-fat diet-induced obesity. Those mice avoid many deleterious side effects of high-fat diet-induced obesity, displaying improved peripheral insulin sensitivity, lower blood glucose and insulin levels. Compared to wild type littermates, PTPRT knockout mice show reduced food intake. Consistently, STAT3 phosphorylation is up-regulated in the hypothalamus of PTPRT knockout mice. These studies implicate PTPRT-modulated STAT3 signaling in the regulation of high-fat diet-induced obesity.

Retinoid receptor signaling and autophagy in acute promyelocytic leukemia

Retinoids are a family of signaling molecules derived from vitamin A with well established roles in cellular differentiation. Physiologically active retinoids mediate transcriptional effects on cells through interactions with retinoic acid (RARs) and retinoid-X (RXR) receptors. Chromosomal translocations involving the RARα gene, which lead to impaired retinoid signaling, are implicated in acute promyelocytic leukemia (APL). All-trans-retinoic acid (ATRA), alone and in combination with arsenic trioxide (ATO), restores differentiation in APL cells and promotes degradation of the abnormal oncogenic fusion protein through several proteolytic mechanisms. RARα fusion-protein elimination is emerging as critical to obtaining sustained remission and long-term cure in APL. Autophagy is a degradative cellular pathway involved in protein turnover. Both ATRA and ATO also induce autophagy in APL cells. Enhancing autophagy may therefore be of therapeutic benefit in resistant APL and could broaden the application of differentiation therapy to other cancers. Here we discuss retinoid signaling in hematopoiesis, leukemogenesis, and APL treatment. We highlight autophagy as a potential important regulator in anti-leukemic strategies.