Forkhead box-O1 (FOXO1) is a downstream target of AKT and plays crucial roles in cell cycle control, apoptosis, metabolism and adipocyte differentiation. It is thought that FOXO1 affects adipocyte differentiation by regulating lipogenesis and cell cycle. With the deepening in the understanding of this field, it is currently believed that FOXO1 translocation between nuclei and cytoplasm is involved in the regulation of FOXO1 activity, thus affecting adipocyte differentiation. Translocation of FOXO1 depends on its post-translational modifications and interactions with 14-3-3. Based on these modifications and interactions, FOXO1 could regulate lipogenesis through PPARγ and the adipocyte cell cycle through p21 and p27. In this review, we aim to provide a comprehensive FOXO1 regulation network in adipocyte differentiation by linking together distinct functions mentioned above to explain their effects on adipocyte differentiation and to emphasize the regulatory role of FOXO1. In addition, we also focus on the novel findings such as the use of miRNAs in FOXO1 regulation and highlight the improvable issues, such as RNA modifications, for future research in the field.
Junye Chen, Yi Lu, Mengyuan Tian and Qiren Huang
Jiazhong Sun, Yancheng Xu, Haohua Deng, Suxin Sun, Zhe Dai and Yanlei Sun
Hypoadiponectinemia and hyperresistinemia may be important in mediating signals from adipocytes to insulin-sensitive tissue and vasculature. However, the mechanism that mediates the aberrant production of adipokines remains poorly understood. In this study, we have investigated the effect of intermittent high glucose on the expression of adiponectin and resistin, and the production of 8-hydroxydeoxyguanosine (8-OHdG) and nitrotyrosine in the adipocytes, either in the presence or in the absence of Mn(III) tetrakis(4-benzoic acid) porphyrin chloride (MnTBAP) or thenoyltrifluoroacetone (TTFA). 3T3-L1 adipocytes were incubated for 72 h in media containing different glucose concentrations: 5 mmol/l, 20 mmol/l, 5 mmol/l alternating with 20 mmol/l glucose, with or without MnTBAP and TTFA. We measured the expression of resistin and adiponectin. The production of nitrotyrosine and 8-OHdG as oxidative stress parameter was measured. Both constant and intermittent high glucose significantly suppressed the expression and secretion of adiponectin, and increased expression and secretion of resistin in mature adipocytes compared to normal glucose conditions. However, these effects were significantly greater under intermittent high glucose conditions compared to constant high glucose. The levels of nitrotyrosine and 8-OHdG were significantly elevated under both intermittent and constant high glucose conditions, the effect being greater under intermittent high glucose. In addition, the antioxidants MnTBAP or TTFA reversed the aberrant production of adiponectin and resistin, as well as overproduction of nitrotyrosine and 8-OHdG in adipocytes induced by constant or intermittent high glucose. Intermittent high glucose exacerbates the aberrant production of adiponectin and resistin through reactive oxygen species overproduction at the mitochondrial transport chain level in adipocytes, indicating that glycemic variability has important pathological effects on the secretion of adipokines.
Rose Kohlie, Nina Perwitz, Julia Resch, Sebastian M Schmid, Hendrik Lehnert, Johannes Klein and K Alexander Iwen
Brown adipose tissue (BAT) is key to energy homeostasis. By virtue of its thermogenic potential, it may dissipate excessive energy, regulate body weight and increase insulin sensitivity. Catecholamines are critically involved in the regulation of BAT thermogenesis, yet research has focussed on the effects of noradrenaline and adrenaline. Some evidence suggests a role of dopamine (DA) in BAT thermogenesis, but the cellular mechanisms involved have not been addressed. We employed our extensively characterised murine brown adipocyte cells. D1-like and D2-like receptors were detectable at the protein level. Stimulation with DA caused an increase in cAMP concentrations. Oxygen consumption rates (OCR), mitochondrial membrane potential (Δψm) and uncoupling protein 1 (UCP1) levels increased after 24 h of treatment with either DA or a D1-like specific receptor agonist. A D1-like receptor antagonist abolished the DA-mediated effect on OCR, Δψm and UCP1. DA induced the release of fatty acids, which did not additionally alter DA-mediated increases of OCR. Mitochondrial mass (as determined by (i) CCCP- and oligomycin-mediated effects on OCR and (ii) immunoblot analysis of mitochondrial proteins) also increased within 24 h. This was accompanied by an increase in peroxisome proliferator-activated receptor gamma co-activator 1 alpha protein levels. Also, DA caused an increase in p38 MAPK phosphorylation and pharmacological inhibition of p38 MAPK abolished the DA-mediated effect on Δψm. In summary, our study is the first to reveal direct D1-like receptor and p38 MAPK-mediated increases of thermogenesis and mitochondrial mass in brown adipocytes. These results expand our understanding of catecholaminergic effects on BAT thermogenesis.
Andrea Armani, Vincenzo Marzolla, Andrea Fabbri and Massimiliano Caprio
In addition to the well-documented expression and activity of the mineralocorticoid receptor (MR) in the kidney, in the last decade research on MR has also revealed its important role in regulating functions of extrarenal tissues, including adipose tissue, where MR is involved in adipocyte fundamental processes such as differentiation, autophagy and adipokine secretion. MR expression is increased in adipose tissue of murine models of obesity and in obese human subjects, suggesting that over-activation of the mineralocorticoid signaling leads to dysfunctional adipocyte and associated metabolic disorders. Notably, pharmacological blockade of MR prevents metabolic dysfunctions observed in obese mice and suggests a potential therapeutic use of MR antagonists in the treatment of obesity and metabolic syndrome. However, the molecular pathways affected by MR blockade have been poorly investigated. This review summarizes the functions of MR in the adipocyte, discusses potential signaling pathways mediating MR action, and describes post-translational modifications regulating its activity.
Changxue Lu and Sheue-Yann Cheng
Peroxisome proliferator-activated receptors (PPARs) and thyroid hormone receptors (TRs) are members of the nuclear receptor superfamily. They are ligand-dependent transcription factors that interact with their cognate hormone response elements in the promoters to regulate respective target gene expression to modulate cellular functions. While the transcription activity of each is regulated by their respective ligands, recent studies indicate that via multiple mechanisms PPARs and TRs crosstalk to affect diverse biological functions. Here, we review recent advances in the understanding of the molecular mechanisms and biological impact of crosstalk between these two important nuclear receptors, focusing on their roles in adipogenesis and carcinogenesis.
J Boucher, I Castan-Laurell, S Le Lay, D Grujic, D Sibrac, S Krief, M Lafontan, BB Lowell, I Dugail, JS Saulnier-Blache and P Valet
Catecholamines regulate white adipose tissue function and development by acting through beta- and alpha2-adrenergic receptors (ARs). Human adipocytes express mainly alpha 2A- but few or no beta 3-ARs while the reverse is true for rodent adipocytes. Our aim was to generate a mouse model with a human-like alpha2/beta-adrenergic balance in adipose tissue by creating transgenic mice harbouring the human alpha 2A-AR gene under the control of its own regulatory elements in a combined mouse beta 3-AR-/- and human beta 3-AR+/+ background. Transgenic mice exhibit functional human alpha 2A-ARs only in white fat cells. Interestingly, as in humans, subcutaneous adipocytes expressed higher levels of alpha2-AR than perigonadal fat cells, which are associated with a better antilipolytic response to epinephrine. High-fat-diet-induced obesity was observed in transgenic mice in the absence of fat cell size modifications. In addition, analysis of gene expression related to lipid metabolism in isolated adipocytes suggested reduced lipid mobilization and no changes in lipid storage capacity of transgenic mice fed a high-fat diet. Finally, the development of adipose tissue in these mice was not associated with significant modifications of glucose and insulin blood levels. Thus, these transgenic mice constitute an original model of diet-induced obesity for in vivo physiological and pharmacological studies with respect to the alpha2/beta-AR balance in adipose tissue.
Tae Woo Jung, Hyoung-Chun Kim, Yong Kyoo Shin, Hyeyoung Min, Seong-Wan Cho, Zi Soo Kim, Su Mi Han, A M Abd El-Aty, Ahmet Hacımüftüoğlu and Ji Hoon Jeong
An aqueous extract of Humulus japonicus (AH) has been documented to ameliorate hypertension and non-alcoholic fatty liver disease (NAFLD). Here, we investigated the effects of an aqueous extract of AH on thermogenesis and palmitate-induced oxidative stress in adipocytes. To verify the effect of AH on browning, we measured the expression levels of specific markers in 3T3-L1 adipocytes using qPCR and Western blotting, respectively. To assess the role of oxidative stress, cells were stained with DCFDA and observed by fluorescence microscopy. AH increased the expression of brown adipose tissue-specific markers. Additionally, it induced fatty acid oxidation and lipolysis and suppressed both lipogenic markers and lipid accumulation. Furthermore, AH ameliorated hydrogen peroxide-induced oxidative stress. Enhanced expression of these markers contributed to fat browning, fatty acid oxidation and lipolysis of 3T3-L1 adipocytes via the AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor delta (PPARδ) signaling pathways. Moreover, AMPK and PPARδ resulting in protective effects of AH against oxidative stress. In sum, AH could promote the browning, lipolysis and thermogenesis in 3T3-L1 adipocytes and would suppress the hydrogen peroxide-induced oxidative stress and lipogenesis during differentiation. We therefore suggest that AH could be used as a potential candidate for treating obesity and related metabolic disorders.
C Attané, D Daviaud, C Dray, R Dusaulcy, M Masseboeuf, D Prévot, C Carpéné, I Castan-Laurell and P Valet
Apelin is a peptide present in different cell types and secreted by adipocytes in humans and rodents. Apelin exerts its effects through a G-protein-coupled receptor called APJ. During the past years, a role of apelin/APJ in energy metabolism has emerged. Apelin was shown to stimulate glucose uptake in skeletal muscle through an AMP-activated protein kinase (AMPK)-dependent pathway in mice. So far, no metabolic effects of apelin have been reported on human adipose tissue (AT). Thus, the effect of apelin on AMPK in AT was measured as well as AMPK-mediated effects such as inhibition of lipolysis and stimulation of glucose uptake. AMPK and acetyl-CoA carboxylase phosphorylation were measured by western blot to reflect the AMPK activity. Lipolysis and glucose uptake were measured, ex vivo, in response to apelin on isolated adipocytes and explants from AT of the subcutaneous region of healthy subjects (body mass index: 25.6±0.8 kg/m2, n=30 in total). APJ mRNA and protein are present in human AT and isolated adipocytes. Apelin stimulated AMPK phosphorylation at Thr-172 in a dose-dependent manner in human AT, which was associated with increased glucose uptake since C compound (20 μM), an AMPK inhibitor, completely prevented apelin-induced glucose uptake. However, in isolated adipocytes or AT explants, apelin had no significant effect on basal and isoprenaline-stimulated lipolysis. Thus, these results reveal, for the first time, that apelin is able to act on human AT in order to stimulate AMPK and glucose uptake.
Rafaela Fadoni Alponti, Luciana Godoy Viana, Norma Yamanouye and Paulo Flavio Silveira
Insulin-regulated aminopeptidase (IRAP, EC 188.8.131.52) in adipocytes is well known to traffic between high (HDM) and low (LDM) density microsomal fractions toward the plasma membrane (MF) under stimulation by insulin. However, its catalytic preference for aminoacyl substrates with N-terminal Leu or Cys is controversial. Furthermore, possible changes in its traffic under metabolic challenges are unknown. The present study investigated the catalytic activity attributable to EC 184.108.40.206 in HDM, LDM and MF from isolated adipocytes of healthy (C), food deprived (FD) and monosodium glutamate (MSG) obese rats on aminoacyl substrates with N-terminal Cys or Leu, in absence or presence of insulin. Efficacy and reproducibility of subcellular adipocyte fractionation procedure were demonstrated. Comparison among HDM vs LDM vs MF intragroup revealed that hydrolytic activity trafficking from LDM to MF under influence of insulin in C, MSG and FD is only on N-terminal Cys. In MSG the same pattern of anterograde traffic and aminoacyl preference occurred independently of insulin stimulation. The pathophysiological significance of IRAP in adipocytes seems to be linked to comprehensive energy metabolism related roles of endogenous substrates with N-terminal cysteine pair such as vasopressin and oxytocin.
Yanyan Cao, Yunsheng Li, Jaekyung Kim, Yulin Ren, Klaus Himmeldirk, Yi Liu, Yanrong Qian, Fengming Liu and Xiaozhuo Chen
Type 2 diabetes (T2D) has become an epidemic worldwide while T1D remains a great medical challenge. Insulin receptor (IR) signaling activators could alleviate hyperglycemia, reduce the burden on the pancreas, and contribute to prevention and treatment of both types of diabetes. Previously, we reported the synthesis and identification of a natural antidiabetic compound α-penta-galloyl-glucose (α-PGG). Subsequent studies led to the identification of an α-P6GG derivative, 6-chloro-6-deoxy-1,2,3,4-tetra-O-galloyl-α-d-glucopyranose (6Cl-TGQ). Here, we report that 6Cl-TGQ not only induced rapid and long-lasting glucose uptake comparable to insulin in adipocytes but also reduced high blood glucose levels to near normal and significantly decreased plasma insulin levels and improved glucose tolerance performance in high-fat diet-induced T2D mice when administered orally at 5 mg/kg once every other day. Moreover, a single gavage of 6Cl-TGQ at 10 mg/kg induced rapid and sharp decline of blood glucose in streptozotocin-induced T1D mice. Our studies further indicated that 6Cl-TGQ activated IR signaling in cell models and insulin-responsive tissues of mice. 6Cl-TGQ-induced Akt phosphorylation was completely blocked by IR and PI3K inhibitors, while the induced glucose uptake was blocked by the same compounds and a Glut4 inhibitor. Receptor binding studies indicated that 6Cl-TGQ bound to IR with a higher affinity than α-PGG. Importantly, 6Cl-TGQ, unlike insulin, selectively induced phosphorylation of IR without activating IGF1R or its signaling and did not increase cancer cell proliferation. These results indicate that 6Cl-TGQ is a potent orally efficacious compound with low carcinogenic potential and may contribute to the prevention and treatment of T1D and T2D.