George D. Mcilroy

Adipocyte-Specific Protein Tyrosine Phosphatase 1B Deletion Increases Lipogenesis, Adipocyte Cell Size and Is a Minor Regulator of Glucose Homeostasis

Protein tyrosine phosphatase 1B (PTP1B), a key negative regulator of leptin and insulin signaling, is positively correlated with adiposity and contributes to insulin resistance. Global PTP1B deletion improves diet-induced obesity and glucose homeostasis via enhanced leptin signaling in the brain and increased insulin signaling in liver and muscle. However, the role of PTP1B in adipocytes is unclear, with studies demonstrating beneficial, detrimental or no effect(s) of adipose-PTP1B-deficiency on body mass and insulin resistance. To definitively establish the role of adipocyte-PTP1B in body mass regulation and glucose homeostasis, adipocyte-specific-PTP1B knockout mice (adip-crePTP1B−/−) were generated using the adiponectin-promoter to drive Cre-recombinase expression. Chow-fed adip-crePTP1B−/− mice display enlarged adipocytes, despite having similar body weight/adiposity and glucose homeostasis compared to controls. High-fat diet (HFD)-fed adip-crePTP1B−/− mice display no differences in body weight/adiposity but exhibit larger adipocytes, increased circulating glucose and leptin levels, reduced leptin sensitivity and increased basal lipogenesis compared to controls. This is associated with decreased insulin receptor (IR) and Akt/PKB phosphorylation, increased lipogenic gene expression and increased hypoxia-induced factor-1-alpha (Hif-1α) expression. Adipocyte-specific PTP1B deletion does not beneficially manipulate signaling pathways regulating glucose homeostasis, lipid metabolism or adipokine secretion in adipocytes. Moreover, PTP1B does not appear to be the major negative regulator of the IR in adipocytes.

Fenretinide Treatment Prevents Diet-Induced Obesity in Association With Major Alterations in Retinoid Homeostatic Gene Expression in Adipose, Liver, and Hypothalamus

The synthetic retinoid, Fenretinide (FEN), inhibits obesity and insulin resistance in mice and is in early clinical trials for treatment of insulin resistance in obese humans. We aimed to determine whether alterations in retinoic acid (RA)-responsive genes contribute to the beneficial effects of FEN. We examined the effect of FEN on 3T3-L1 adipocyte differentiation and alterations in gene expression in C57Bl/6 and retinaldehyde dehydrogenase (RALDH) 1 knockout (KO) mice fed a high-fat (HF) diet. FEN completely inhibited adipocyte differentiation by blocking CCAAT/enhancer-binding protein (C/EBP) α/peroxisome proliferator–activated receptor (PPAR) γ−mediated induction of downstream genes and upregulating RA-responsive genes like cellular retinol-binding protein-1. In mice fed an HF diet, RA-responsive genes were markedly increased in adipose, liver, and hypothalamus, with short-term and long-term FEN treatment. In adipose, FEN inhibited the downregulation of PPARγ and improved insulin sensitivity and the levels of adiponectin, resistin, and serum RBP (RBP4). FEN inhibited hyperleptinemia in vivo and leptin expression in adipocytes. Surprisingly, hypothalamic neuropeptide Y expression was completely suppressed, suggesting a central effect of FEN to normalize hyperglycemia. Moreover, FEN induced RA-responsive genes in RALDH1 KO mice, demonstrating that FEN can augment RA signaling when RA synthesis is impaired. We show that FEN-mediated beneficial effects are through alterations in retinoid homeostasis genes, and these are strong candidates as therapeutic targets for the treatment of obesity and insulin resistance.

The mechanisms of Fenretinide-mediated anti-cancer activity and prevention of obesity and type-2 diabetes.

Fenretinide remains the most investigated retinoid compound for the prevention of cancer. Its clinical use remains a genuine possibility due to a favourable toxicological profile and accumulation in fatty tissues. Like other well-characterised pharmacological therapies, Fenretinide has been shown to affect multiple signalling pathways. Recent findings have discovered additional beneficial properties the synthetic retinoid was not intentionally designed for, including the prevention of high-fat diet-induced obesity and insulin resistance. These preclinical findings in rodents are timely since obesity has reached pandemic proportions and safe effective therapeutics are severely lacking. Recent investigations have proposed various mechanisms of action for the beneficial effects of Fenretinide. This review covers the current knowledge about Fenretinides use as a therapy for cancer and potential to treat obesity, insulin resistance and glucose intolerance. An overview of the signalling pathways manipulated by Fenretinide including retinoid homeostasis, reactive oxygen species generation and inhibition of ceramide synthesis will be presented and insights into apoptosis and/or autophagy induction by Fenretinide will also be discussed. The largely unexplored area of Fenretinide metabolites as alternative therapeutic options and how these may be relevant will also be presented. Fenretinide shows great promise, but unfortunately evidence is lacking from clinical trials on Fenretinides effectiveness in humans. Finally we identify what action can be taken to further progress the investigation of this extremely important retinoid.

Fenretinide mediated retinoic acid receptor signalling and inhibition of ceramide biosynthesis regulates adipogenesis, lipid accumulation, mitochondrial function and nutrient stress signalling in adipocytes and adipose tissue

Graphical abstract Proposed mechanistic action of FEN in adipocytes. RA inhibits adipogenesis through early RAR activation, which PPARγ agonist ROSI cannot prevent. FEN inhibits adipogenesis with a delayed response in RAR signalling, however, inhibition is lost when combined with ROSI. The 4-OXO FEN catabolite cannot inhibit adipogenesis through RAR activation, but like FEN’s RAR-independent effects, displays increased phosphorylation of Akt, mild cellular stress/autophagy induction and decreased lipid accumulation. Moreover, FEN-mediated inhibition of DES-1 increases dihydroceramide levels in a RAR-independent manner, and is linked to a complete prevention of mitochondrial dysfunction and decreased adiposity in high-fat fed obese mice. Thus an alternative to the additive beneficial effects of FEN-mediated RAR-dependant and -independent signalling, 4-OXO FEN may be a novel therapeutic candidate to improve adipocyte hypertrophy via inhibition of ceramide biosynthesis and modulation of nutrient stress pathways.

Elevated Fibroblast growth factor 21 (FGF21) in obese, insulin resistant states is normalised by the synthetic retinoid Fenretinide in mice.

Fibroblast growth factor 21 (FGF21) has emerged as an important beneficial regulator of glucose and lipid homeostasis but its levels are also abnormally increased in insulin-resistant states in rodents and humans. The synthetic retinoid Fenretinide inhibits obesity and improves glucose homeostasis in mice and has pleotropic effects on cellular pathways. To identify Fenretinide target genes, we performed unbiased RNA-seq analysis in liver from mice fed high-fat diet ± Fenretinide. Strikingly, Fgf21 was the most downregulated hepatic gene. Fenretinide normalised elevated levels of FGF21 in both high-fat diet-induced obese mice and in genetically obese-diabetic Leprdbmice. Moreover, Fenretinide-mediated suppression of FGF21 was independent of body weight loss or improved hepatic insulin sensitivity and importantly does not induce unhealthy metabolic complications. In mice which have substantially decreased endogenous retinoic acid biosynthesis, Fgf21 expression was increased, whereas acute pharmacological retinoid treatment decreased FGF21 levels. The repression of FGF21 levels by Fenretinide occurs by reduced binding of RARα and Pol-II at the Fgf21 promoter. We therefore establish Fgf21 as a novel gene target of Fenretinide signalling via a retinoid-dependent mechanism. These results may be of nutritional and therapeutic importance for the treatment of obesity and type-2 diabetes.

Fenretinide prevents obesity in aged female mice in association with increased retinoid and estrogen signaling.

The synthetic retinoid fenretinide (FEN) inhibits adiposity in male mice fed a high‐fat diet (HFD) in association with alterations in retinoic acid (RA) signaling. Young female mice are protected from obesity via estrogen signaling. We, therefore, investigated whether FEN also influences adiposity in aged female mice differing in parity and whether such effects are mediated by retinoid and estrogen signaling.

Adipose specific disruption of seipin causes early-onset generalised lipodystrophy and altered fuel utilisation without severe metabolic disease

Objective Mutations to the BSCL2 gene disrupt the protein seipin and cause the most severe form of congenital generalised lipodystrophy (CGL). Affected individuals exhibit a near complete loss of white adipose tissue (WAT) and suffer from metabolic disease. Seipin is critical for adipocyte development in culture and mice with germline disruption to Bscl2 recapitulate the effects of BSCL2 disruption in humans. Here we examined whether loss of Bscl2 specifically in developing adipocytes in vivo is sufficient to prevent adipose tissue development and cause all features observed with congenital BSCL2 disruption. Methods We generated and characterised a novel mouse model of Bscl2 deficiency in developing adipocytes (Ad-B2(−/−)) using the adipose-specific Adiponectin-Cre line. Results We demonstrate that Ad-B2(−/−) mice display early onset lipodystrophy, in common with congenital Bscl2 null mice and CGL2 patients. However, glucose intolerance, insulin resistance, and severe hepatic steatosis are not apparent. Food intake and energy expenditure are unchanged, but Ad-B2(−/−) mice exhibit significantly altered substrate utilisation. We also find differential effects of seipin loss between specific adipose depots revealing new insights regarding their varied characteristics. When fed a high-fat diet, Ad-B2(−/−) mice entirely fail to expand adipose mass but remain glucose tolerant. Conclusions Our findings demonstrate that disruption of Bscl2 specifically in developing adipocytes is sufficient to cause the early-onset generalised lipodystrophy observed in patients with mutations in BSCL2. However, this significant reduction in adipose mass does not cause the overt metabolic dysfunction seen in Bscl2 knockout mice, even following a high-fat diet challenge.