Summary Although all-trans retinoic acid (atRA) is a key regulator of intestinal immunity, its role in colorectal cancer (CRC) is unknown. We found that mice with colitis-associated CRC had a marked deficiency in colonic atRA due to alterations in atRA metabolism mediated by microbiota-induced intestinal inflammation. Human ulcerative colitis (UC), UC-associated CRC, and sporadic CRC specimens have similar alterations in atRA metabolic enzymes, consistent with reduced colonic atRA. Inhibition of atRA signaling promoted tumorigenesis whereas atRA supplementation reduced tumor burden. The benefit of atRA treatment was mediated by cytotoxic CD8+ T cells, activated due to MHCI upregulation on tumor cells. Consistent with these findings, increased colonic expression of the atRA-catabolizing enzyme, CYP26A1, correlated with reduced frequencies of tumoral cytotoxic CD8+ T cells and with worse disease prognosis in human CRC. These results reveal a mechanism by which microbiota drive colon carcinogenesis and highlight atRA metabolism as a therapeutic target for CRC.

The all-trans-retinoic acid (atRA) isomer, 9-cis-retinoic acid (9cRA), activates retinoic acid receptors (RARs) and retinoid X receptors (RXRs) in vitro. RARs control multiple genes, whereas RXRs serve as partners for RARs and other nuclear receptors that regulate metabolism. Physiological function has not been determined for 9cRA, because it has not been detected in serum or multiple tissues with analytically validated assays. Here, we identify 9cRA in mouse pancreas by liquid chromatography/tandem mass spectrometry (LC/MS/MS), and show that 9cRA decreases with feeding and after glucose dosing and varies inversely with serum insulin. 9cRA reduces glucose-stimulated insulin secretion (GSIS) in mouse islets and in the rat β-cell line 832/13 within 15 min by reducing glucose transporter type 2 (Glut2) and glucokinase (GK) activities. 9cRA also reduces Pdx-1 and HNF4α mRNA expression, ∼8-and 80-fold, respectively: defects in Pdx-1 or HNF4α cause maturity onset diabetes of the young (MODY4 and 1, respectively), as does a defective GK gene (MODY2). Pancreas β-cells generate 9cRA, and mouse models of reduced β-cell number, heterozygous Akita mice, and streptozotocin-treated mice have reduced 9cRA. 9cRA is abnormally high in glucose-intolerant mice, which have β-cell hypertropy, including mice with diet-induced obesity (DIO) and ob/ob and db/db mice. These data establish 9cRA as a pancreas-specific autacoid with multiple mechanisms of action and provide unique insight into GSIS.retinol | vitamin A | rexinoids I mpaired glucose-stimulated insulin secretion (GSIS) develops through multiple mechanisms, including actions of metabolic hormones and inflammatory cytokines, products of metabolic overload, and endoplasmic reticulum stress; however, mechanisms of GSIS and impaired glucose tolerance remain incompletely understood (1-4). Also uncertain is the contribution of impaired glucose tolerance to diminished pancreatic β-cell function and mass associated with type 2 diabetes (5). GSIS relies on the pancreas, and pancreas development, islet formation, and function require normal vitamin A nutriture (6-8). Vitamin A restriction during development impairs islet development and promotes glucose intolerance in adult rodents. On the other hand, restricting vitamin A in mature diabetes-prone rats reduces diabetes and insulitis, possibly through enhancing glucose sensing and metabolism. Alltrans-retinoic acid (atRA), an activated metabolite of vitamin A, regulates pancreas development, and atRA does not enhance the incidence of diabetes in diabetes-prone rats fed a vitamin Adeficient diet (7, 9, 10). Although the contribution of vitamin A to pancreas development through atRA seems clear, mechanisms whereby vitamin A affects mature pancreas function have not been determined in depth, nor have the specific vitamin A metabolites been identified that contribute to GSIS control.atRA induces differentiation and regulates cell processes by activating the nuclear receptors RAR α, -β, and -γ, which regulate transcription and translation (11...

Rat RoDH performs efficiently (V(m)/K(m)) in a pathway of all-trans-retinoic acid biosynthesis in cells and recognizes the physiological form of vitamin A, i.e., retinol bound with cellular retinol binding-protein, type I. Here we report that mouse embryo (e7.5 to e18.5) and liver (e12.5 to P2M) display inversely related mRNA expression of an Rodh ortholog, rdh1, and a major retinoic acid catabolic enzyme, cyp26a1, suggesting coordinate modulation of retinoic acid homeostasis. Rdh1 inactivation by homologous recombination produces mice with decreased liver cyp26a1 mRNA and protein and increased liver and kidney retinoid stores, when fed vitamin A-restricted diets. Thus, null mice autocompensate by down-regulating cyp26a1 and sparing retinoids, indicating that rdh1 metabolizes retinoids in vivo. Surprisingly, rdh1-null mice grow longer than wild type, with increased weight and adiposity, when restricted in vitamin A. Liver, kidney, and multiple fat pads increase in weight. Some differences reflect the larger sizes of rdh1-null mice, but mesentery, femoral, and inguinal fat pads grow disproportionately larger. These data reveal an unexpected contribution of Rdh1 to size and adiposity and provide the first genetic evidence of a candidate retinol dehydrogenase affecting either vitamin A-related homeostasis physiologically or vitamin A-related gene expression or biological function in vivo.

Cellular retinol-binding protein type I (CrbpI), encoded by Rpb1, serves as a chaperone of retinol homeostasis, but its physiological effects remain incompletely understood. We show here that the Rbp1 ؊/؊ mouse has disrupted retinoid homeostasis in multiple tissues, with abnormally high 9-cis-retinoic acid (9cRA), a pancreas autacoid that attenuates glucose-stimulated insulin secretion. The Rbp1 ؊/؊ pancreas has increased retinol and intense ectopic expression of Rpb2 mRNA, which encodes CrbpII: both would contribute to increased ␤-cell 9cRA biosynthesis. 9cRA in Rbp1 ؊/؊ pancreas resists postprandial and glucose-induced decreases. Rbp1 (27,36). The relative amounts of apo-and holo-CrbpI facilitate cellular retinol uptake, modulate retinol storage as retinyl esters (RE), and affect retinoid homeostasis by regulating enzyme activity differentially (25, 34). Retinoic acid receptor (RAR) activation depends on CrbpI-mediated retinol uptake. CrbpI mutants with low retinol-binding affinity reduce RAR function in human mammary epithelial cells, leading to a loss of differentiation (10). Nevertheless, Rbp1 Ϫ/Ϫ mice are seemingly healthy and show no gross abnormalities characteristic of overt retinoid deficiency (13). All-trans-retinoic acid (atRA) in serum and several target tissues does not differ significantly between Rbp1 Ϫ/Ϫ and wild-type (WT) mice, likely accounting for lack of gross abnormalities (20, 29). Despite these insights, physiological effects of CrbpI remain to be elucidated fully.Retinol functions primarily through its metabolite atRA, which has diverse effects on energy metabolism. atRA induces pancreas development and differentiation into acini (18,24,28,32), but restricting dietary vitamin A in diabetes-prone rats reduces diabetes and insulitis, an effect not reversed by dosing atRA, suggesting contributions of additional retinoids (9). atRA arrests differentiation of preadipocytes into mature white adipocytes, early in the differentiation process (43, 48). Ablation of Rdh1, which encodes one of several short-chain dehydrogenases that catalyze the first and rate-limiting step in atRA biosynthesis from retinol, increases adiposity in mice fed a nonobesogenic diet (50). Dosing atRA ameliorates weight gain and represses insulin resistance in mice fed an obesogenic (high-fat) diet (3). atRA induces Ucp1, which encodes an uncoupling protein expressed in brown adipose tissue that supports thermogenesis (31).Although atRA mediates most vitamin A functions, 9-cis-RA (9cRA) occurs as an endogenous pancreas retinoid with unique actions independent of atRA (22). 9cRA, biosynthesized primarily by ␤-cells and detected only in pancreas, varies rapidly and inversely with serum glucose. 9cRA attenuates glucose-stimulated insulin secretion (GSIS): a decrease in 9cRA allows optimum glucose-stimulated insulin secretion, whereas dosing with 9cRA impairs insulin secretion and glucose tolerance. 9cRA acts, in part, through reduction of Glut2 and GK activities and through repressing expression of genes that function to pro...

Background: Retinoic acid regulates energy balance and induces phosphoenolpyruvate carboxykinase gene expression. Results: Refeeding, glucose, and insulin decrease retinoic acid in vivo. Insulin suppresses retinol dehydrogenase gene expression through suppressing FoxO1. Conclusion: Insulin inhibits retinoic acid biosynthesis through inhibition of FoxO1-induced Rdh10 gene expression. Significance: Insulin and retinoic acid exert counter balancing effects in regulating energy status.