Distinct mTORC1 pathways for transcription and cleavage of SREBP-1c

Transcriptomic and lipid profiling analysis reveals a functional interplay between testosterone and growth hormone in hypothyroid liver

Preclinical and clinical studies suggest that hypothyroidism might cause hepatic endocrine and metabolic disturbances with features that mimic deficiencies of testosterone and/or GH. The absence of physiological interactions between testosterone and GH can be linked to male differentiated liver diseases. Testosterone plays relevant physiological effects on somatotropic-liver axis and liver composition and the liver is a primary organ of interactions between testosterone and GH. However, testosterone exerts many effects on liver through complex and poorly understood mechanisms. Testosterone impacts liver functions by binding to the Androgen Receptor, and, indirectly, through its conversion to estradiol, and cooperation with GH. However, the role of testosterone, and its interaction with GH, in the hypothyroid liver, remains unclear. In the present work, the effects of testosterone, and how they impact on GH-regulated whole transcriptome and lipid composition in the liver, were studied in the context of adult hypothyroid-orchiectomized rats. Testosterone replacement positively modulated somatotropic-liver axis and impacted liver transcriptome involved in lipid and glucose metabolism. In addition, testosterone enhanced the effects of GH on the transcriptome linked to lipid biosynthesis, oxidation-reduction, and metabolism of unsaturated and long-chain fatty acids (FA). However, testosterone decreased the hepatic content of cholesterol esters and triacylglycerols and increased fatty acids whereas GH increased neutral lipids and decreased polar lipids. Biological network analysis of the effects of testosterone on GH-regulated transcriptome confirmed a close connection with crucial proteins involved in steroid and fatty acid metabolism. Taken together, this comprehensive analysis of gene expression and lipid profiling in hypothyroid male liver reveals a functional interplay between testosterone and pulsed GH administration.

Insulin and the sebaceous gland function

Insulin affects metabolic processes in different organs, including the skin. The sebaceous gland (SG) is an important appendage in the skin, which responds to insulin-mediated signals, either directly or through the insulin growth factor 1 (IGF-1) axis. Insulin cues are differently translated into the activation of metabolic processes depending on several factors, including glucose levels, receptor sensitivity, and sebocyte differentiation. The effects of diet on both the physiological function and pathological conditions of the SG have been linked to pathways activated by insulin and IGF-1. Experimental evidence and theoretical speculations support the association of insulin resistance with acne vulgaris, which is a major disorder of the SG. In this review, we examined the effects of insulin on the SG function and their implications in the pathogenesis of acne.

Decoding the role of immune T cells: A new territory for improvement of metabolic-associated fatty liver disease

Metabolic-associated fatty liver disease (MAFLD) is a new emerging concept and is associated with metabolic dysfunction, generally replacing the name of nonalcoholic fatty liver disease (NAFLD) due to heterogeneous liver condition and inaccuracies in definition. The prevalence of MAFLD is rising by year due to dietary changes, metabolic disorders, and no approved therapy, affecting a quarter of the global population and representing a major economic problem that burdens healthcare systems. Currently, in addition to the common causative factors like insulin resistance, oxidative stress, and lipotoxicity, the role of immune cells, especially T cells, played in MAFLD is increasingly being emphasized by global scholars. Based on the diverse classification and pathophysiological effects of immune T cells, we comprehensively analyzed their bidirectional regulatory effects on the hepatic inflammatory microenvironment and MAFLD progression. This interaction between MAFLD and T cells was also associated with hepatic-intestinal immune crosstalk and gut microbiota homeostasis. Moreover, we pointed out several T-cell-based therapeutic approaches including but not limited to adoptive transfer of T cells, fecal microbiota transplantation, and drug therapy, especially for natural products and Chinese herbal prescriptions. Overall, this study contributes to a better understanding of the important role of T cells played in MAFLD progression and corresponding therapeutic options and provides a potential reference for further drug development.

Insulin activates LC-PUFA biosynthesis of hepatocytes by regulating the PI3K/Akt/mTOR/Srebp1 pathway in teleost Siganus canaliculatus

Insulin is well known an important metabolic regulator in glucose and lipid metabolism. It has been proved to activate long-chain (≥ C₂₀) polyunsaturated fatty acids (LC-PUFA) biosynthesis in mammals, but little is known about such a role in fish. To explore the effects and molecular mechanisms of insulin in fish LC-PUFA biosynthesis, we treated the rabbitfish S. canaliculatus hepatocyte line (SCHL) cells with 65 nM insulin for 12 h, and the results showed that the mRNA levels of genes encoding the key enzymes and transcription factor involved in rabbitfish LC-PUFA biosynthesis such as Δ6Δ5 fads2, elovl5 and srebp1, as well as those of PI3K pathway genes including pdk1, akt2 and mtor increased significantly. Moreover, SCHL cells treated with different PI3K/Akt pathway inhibitors (LY294002, Wortmannin, AKTi-1/2) alone or combined with insulin decreased the mRNA levels of PI3K/Akt/mTOR downstream signaling genes, including Δ6Δ5 fads2, Δ4 fads2, elovl5, elovl4 and srebp1. While PI3K/Akt agonists (740 Y-P, IGF-1, SC-79) had the opposite results. The results of fatty acid composition analysis of hepatocytes showed that insulin stimulation increased the Δ6Δ5 Fads2-dependent PUFA desaturation indexes, while Elovl5-dependent PUFA elongation indexes had upward trends, and consequently LC-PUFA contents increased. Taken together, these results indicated that insulin activated LC-PUFA biosynthesis probably through PI3K/Akt/mTOR/Srebp1 pathway in S. canaliculatus hepatocytes.

Discovery of a potent FKBP38 agonist that ameliorates HFD-induced hyperlipidemia via mTOR/P70S6K/SREBPs pathway

The mammalian target of rapamycin (mTOR)-sterol regulatory element-binding proteins (SREBPs) signaling promotes lipogenesis. However, mTOR inhibitors also displayed a significant side effect of hyperlipidemia. Thus, it is essential to develop mTOR-specific inhibitors to inhibit lipogenesis. Here, we screened the endogenous inhibitors of mTOR, and identified that FKBP38 as a vital regulator of lipid metabolism. FKBP38 decreased the lipid content in vitro and in vivo via suppression of the mTOR/P70S6K/SREBPs pathway. 3,5,6,7,8,3ʹ,4ʹ-Heptamethoxyflavone (HMF), a citrus flavonoid, was found to target FKBP38 to suppress the mTOR/P70S6K/SREBPs pathway, reduce lipid level, and potently ameliorate hyperlipidemia and insulin resistance in high fat diet (HFD)-fed mice. Our findings suggest that pharmacological intervention by targeting FKBP38 to suppress mTOR/P70S6K/SREBPs pathway is a potential therapeutic strategy for hyperlipidemia, and HMF could be a leading compound for development of anti-hyperlipidemia drugs.

Phloridzin Ameliorates Lipid Deposition in High-Fat-Diet-Fed Mice with Nonalcoholic Fatty Liver Disease via Inhibiting the mTORC1/SREBP-1c Pathway

We aimed to investigate whether phloridzin could alleviate nonalcoholic fatty liver disease (NAFLD) in mice, which was induced by feeding a high-fat diet (HFD). We initially analyzed the effect of phloridzin on alleviating HFD-induced NAFLD in C57BL/6J mice and oleic acid (OA)-stimulated human normal liver L-02 cells (L02). Then, we investigated the mechanism of phloridzin on the mTORC1/sterol-regulatory element-binding protein-1c (SREBP-1c) signaling pathway by siRNA analysis, qRT-PCR, flow cytometry, and western blot analysis in vivo and in vitro. The results revealed that phloridzin significantly inhibited the increase in body weight, alleviated abnormal lipid metabolism, and decreased lipid biosynthesis and insulin resistance. Moreover, phloridzin augmented the number of CD8⁺CD122⁺PD-1⁺ Tregs and CD4⁺FoxP3⁺ Tregs in HFD-fed C57BL/6J mice and HFD-fed aP2-SREBF1c mice and downregulated the mTORC1/SREBP-1c signaling pathway-related protein expressions in vivo and in vitro. Furthermore, phloridzin reduced the expression of SREBP-1c in SREBP-1c-RNAi-lentivirus-transfected L02 cells and reversed the SREBP-1c expression in HFD-fed aP2-SREBF1c transgenic mice. Phloridzin ameliorates lipid accumulation and insulin resistance via inhibiting the mTORC1/SREBP-1c pathways. These results indicated that phloridzin may actively ameliorate NAFLD.

TNF-Receptor-1 inhibition reduces liver steatosis, hepatocellular injury and fibrosis in NAFLD mice

Non-alcoholic fatty liver disease (NAFLD) shows an increasing prevalence and is associated with the development of liver fibrosis and cirrhosis as the major risk factors of liver-related mortality in this disease. The therapeutic possibilities are limited and restricted to life style intervention, since specific drugs for NAFLD are unavailable so far. TNFα has been implicated as a major pathogenic driver of NAFLD. TNFα-mediated liver injury occurs mainly via TNF-receptor-1 (TNFR1) signaling, whereas TNFR2 mediates protective pathways. In this study, we analyzed the therapeutic effects of a novel antibody, which selectively inhibits TNFR1 while retaining protective TNFR2 signaling in a high-fat diet (HFD) mouse model of NAFLD. Mice were fed with HFD for 32 weeks and treated with anti-TNFR1-antibody or control-antibody for the last 8 weeks. We then investigated the mechanisms of TNFR1 inhibition on liver steatosis, inflammatory liver injury, insulin resistance and fibrosis. Compared to control-antibody treatment, TNFR1 inhibition significantly reduced liver steatosis and triglyceride content, which was accompanied by reduced expression and activation of the transcription factor SREBP1 and downstream target genes of lipogenesis. Furthermore, inhibition of TNFR1 resulted in reduced activation of the MAP kinase MKK7 and its downstream target JNK, which was associated with significant improvement of insulin resistance. Apoptotic liver injury, NAFLD activity and alanine aminotransferase (ALT) levels, as well as liver fibrosis significantly decreased by anti-TNFR1 compared to control-antibody treatment. Thus, our results suggest selective TNFR1 inhibition as a promising approach for NAFLD treatment.

Deciphering the expression dynamics of ANGPTL8 associated regulatory network in insulin resistance using formal modelling approaches

ANGPTL8 is a recently identified novel hormone which regulates both glucose and lipid metabolism. The increase in ANGPTL8 during compensatory insulin resistance has been recently reported to improve glucose tolerance and a part of cytoprotective metabolic circuit. However, the exact signalling entities and dynamics involved in this process have remained elusive. Therefore, the current study was conducted with a specific aim to model the regulation of ANGPTL8 with emphasis on its role in improving glucose tolerance during insulin resistance. The main contribution of this study is the construction of a discrete model (based on kinetic logic of René Thomas) and its equivalent Stochastic Petri Net model of ANGPTL8 associated Biological Regulatory Network (BRN) which can predict its dynamic behaviours. The predicted results of these models are in‐line with the previous experimental observations and provide comprehensive insights into the signalling dynamics of ANGPTL8 associated BRN. The authors’ results support the hypothesis that ANGPTL8 plays an important role in supplementing the insulin signalling pathway during insulin resistance and its loss can aggravate the pathogenic process by quickly leading towards Diabetes Mellitus. The results of this study have potential therapeutic implications for treatment of Diabetes Mellitus and are suggestive of its potential as a glucose‐lowering agent.

Understanding Obesity as a Risk Factor for Uterine Tumors Using Drosophila

Multiple large-scale epidemiological studies have identified obesity as an important risk factor for a variety of human cancers, particularly cancers of the uterus, gallbladder, kidney, liver, colon, and ovary, but there is much uncertainty regarding how obesity increases the cancer risks. Given that obesity has been consistently identified as a major risk factor for uterine tumors, the most common malignancies of the female reproductive system, we use uterine tumors as a pathological context to survey the relevant literature and propose a novel hypothesis: chronic downregulation of the cyclin-dependent kinase 8 (CDK8) module, composed of CDK8 (or its paralog CDK19), Cyclin C, MED12 (or MED12L), and MED13 (or MED13L), by elevated insulin or insulin-like growth factor signaling in obese women may increase the chances to dysregulate the activities of transcription factors regulated by the CDK8 module, thereby increasing the risk of uterine tumors. Although we focus on endometrial cancer and uterine leiomyomas (or fibroids), two major forms of uterine tumors, our model may offer additional insights into how obesity increases the risk of other types of cancers and diseases. To illustrate the power of model organisms for studying human diseases, here we place more emphasis on the findings obtained from Drosophila melanogaster.

mTOR signalling: jack-of-all-trades 1

The mechanistic target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase that senses and integrates environmental information into cellular regulation and homeostasis. Accumulating evidence has suggested a master role of mTOR signalling in many fundamental aspects of cell biology and organismal development. mTOR deregulation is implicated in a broad range of pathological conditions, including diabetes, cancer, neurodegenerative diseases, myopathies, inflammatory, infectious, and autoimmune conditions. Here, we review recent advances in our knowledge of mTOR signalling in mammalian physiology. We also discuss the impact of mTOR alteration in human diseases and how targeting mTOR function can treat human diseases.

Systems Analysis of the Liver Transcriptome in Adult Male Zebrafish Exposed to the Plasticizer (2-Ethylhexyl) Phthalate (DEHP)

The organic compound diethylhexyl phthalate (DEHP) represents a high production volume chemical found in cosmetics, personal care products, laundry detergents, and household items. DEHP, along with other phthalates causes endocrine disruption in males. Exposure to endocrine disrupting chemicals has been linked to the development of several adverse health outcomes with apical end points including Non-Alcoholic Fatty Liver Disease (NAFLD). This study examined the adult male zebrafish (Danio rerio) transcriptome after exposure to environmental levels of DEHP and 17α-ethinylestradiol (EE2) using both DNA microarray and RNA-sequencing technologies. Our results show that exposure to DEHP is associated with differentially expressed (DE) transcripts associated with the disruption of metabolic processes in the liver, including perturbation of five biological pathways: ‘FOXA2 and FOXA3 transcription factor networks’, ‘Metabolic pathways’, ‘metabolism of amino acids and derivatives’, ‘metabolism of lipids and lipoproteins’, and ‘fatty acid, triacylglycerol, and ketone body metabolism’. DE transcripts unique to DEHP exposure, not observed with EE2 (i.e. non-estrogenic effects) exhibited a signature related to the regulation of transcription and translation, and ruffle assembly and organization. Collectively our results indicate that exposure to low DEHP levels modulates the expression of liver genes related to fatty acid metabolism and the development of NAFLD.

Linking diet to acne metabolomics, inflammation, and comedogenesis: an update

Acne vulgaris, an epidemic inflammatory skin disease of adolescence, is closely related to Western diet. Three major food classes that promote acne are: 1) hyperglycemic carbohydrates, 2) milk and dairy products, 3) saturated fats including trans-fats and deficient ω-3 polyunsaturated fatty acids (PUFAs). Diet-induced insulin/insulin-like growth factor (IGF-1)-signaling is superimposed on elevated IGF-1 levels during puberty, thereby unmasking the impact of aberrant nutrigenomics on sebaceous gland homeostasis. Western diet provides abundant branched-chain amino acids (BCAAs), glutamine, and palmitic acid. Insulin and IGF-1 suppress the activity of the metabolic transcription factor forkhead box O1 (FoxO1). Insulin, IGF-1, BCAAs, glutamine, and palmitate activate the nutrient-sensitive kinase mechanistic target of rapamycin complex 1 (mTORC1), the key regulator of anabolism and lipogenesis. FoxO1 is a negative coregulator of androgen receptor, peroxisome proliferator-activated receptor-γ (PPARγ), liver X receptor-α, and sterol response element binding protein-1c (SREBP-1c), crucial transcription factors of sebaceous lipogenesis. mTORC1 stimulates the expression of PPARγ and SREBP-1c, promoting sebum production. SREBP-1c upregulates stearoyl-CoA- and Δ6-desaturase, enhancing the proportion of monounsaturated fatty acids in sebum triglycerides. Diet-mediated aberrations in sebum quantity (hyperseborrhea) and composition (dysseborrhea) promote Propionibacterium acnes overgrowth and biofilm formation with overexpression of the virulence factor triglyceride lipase increasing follicular levels of free palmitate and oleate. Free palmitate functions as a "danger signal," stimulating toll-like receptor-2-mediated inflammasome activation with interleukin-1β release, Th17 differentiation, and interleukin-17-mediated keratinocyte proliferation. Oleate stimulates P. acnes adhesion, keratinocyte proliferation, and comedogenesis via interleukin-1α release. Thus, diet-induced metabolomic alterations promote the visible sebofollicular inflammasomopathy acne vulgaris. Nutrition therapy of acne has to increase FoxO1 and to attenuate mTORC1/SREBP-1c signaling. Patients should balance total calorie uptake and restrict refined carbohydrates, milk, dairy protein supplements, saturated fats, and trans-fats. A paleolithic-like diet enriched in vegetables and fish is recommended. Plant-derived mTORC1 inhibitors and ω-3-PUFAs are promising dietary supplements supporting nutrition therapy of acne vulgaris.

mTORC1 Down-Regulates Cyclin-Dependent Kinase 8 (CDK8) and Cyclin C (CycC)

In non-alcoholic fatty liver disease (NAFLD) and insulin resistance, hepatic de novo lipogenesis is often elevated, but the underlying mechanisms remain poorly understood. Recently, we show that CDK8 functions to suppress de novo lipogenesis. Here, we identify the mammalian target of rapamycin complex 1 (mTORC1) as a critical regulator of CDK8 and its activating partner CycC. Using pharmacologic and genetic approaches, we show that increased mTORC1 activation causes the reduction of the CDK8-CycC complex in vitro and in mouse liver in vivo. In addition, mTORC1 is more active in three mouse models of NAFLD, correlated with the lower abundance of the CDK8-CycC complex. Consistent with the inhibitory role of CDK8 on de novo lipogenesis, nuclear SREBP-1c proteins and lipogenic enzymes are accumulated in NAFLD models. Thus, our results suggest that mTORC1 activation in NAFLD and insulin resistance results in down-regulation of the CDK8-CycC complex and elevation of lipogenic protein expression.

Livers with constitutive mTORC1 activity resist steatosis independent of feedback suppression of Akt

Insulin resistance is an important contributing factor in non-alcoholic fatty liver disease. AKT and mTORC1 are key components of the insulin pathway, and play a role in promoting de novo lipogenesis. However, mTORC1 hyperactivity per se does not induce steatosis in mouse livers, but instead, protects against high-fat diet induced steatosis. Here, we investigate the in vivo mechanism of steatosis-resistance secondary to mTORC1 activation, with emphasis on the role of S6K1-mediated feedback inhibition of AKT. Mice with single or double deletion of Tsc1 and/or S6k1 in a liver-specific or whole-body manner were generated to study glucose and hepatic lipid metabolism between the ages of 6–14 weeks. Following 8 weeks of high-fat diet, the Tsc1-/-;S6k1-/- mice had lower body weights but higher liver TG levels compared to that of the Tsc1-/- mice. However, the loss of S6k1 did not relieve feedback inhibition of Akt activity in the Tsc1-/- livers. To overcome Akt suppression, Pten was deleted in Tsc1-/- livers, and the resultant mice showed improved glucose tolerance compared with the Tsc1-/- mice. However, liver TG levels were significantly reduced in the Tsc1-/-;Pten-/- mice compared to the Pten-/- mice, which was restored with rapamycin. We found no correlation between liver TG and serum NEFA levels. Expression of lipogenic genes (Srebp1c, Fasn) were elevated in the Tsc1-/-;Pten-/- livers, but this was counter-balanced by an up-regulation of Cpt1a involved in fatty acid oxidation and the anti-oxidant protein, Nrf2. In summary, our in vivo models showed that mTORC1-induced resistance to steatosis was dependent on S6K1 activity, but not secondary to AKT suppression. These findings confirm that AKT and mTORC1 have opposing effects on hepatic lipid metabolism in vivo.

Phosphoinositides: Key modulators of energy metabolism

Phosphoinositides are key players in many trafficking and signaling pathways. Recent advances regarding the synthesis, location and functions of these lipids have dramatically improved our understanding of how and when these lipids are generated and what their roles are in animal physiology. In particular, phosphoinositides play a central role in insulin signaling, and manipulation of PtdIns(3,4,5)P₃levels in particular, may be an important potential therapeutic target for the alleviation of insulin resistance associated with obesity and the metabolic syndrome. In this article we review the metabolism, regulation and functional roles of phosphoinositides in insulin signaling and the regulation of energy metabolism. This article is part of a Special Issue entitled Phosphoinositides.

SREBP-1c overexpression induces triglycerides accumulation through increasing lipid synthesis and decreasing lipid oxidation and VLDL assembly in bovine hepatocytes

The natural incidence of fatty liver in ruminants is significantly higher than in monogastric animals. Fatty liver is associated with sterol regulatory element-binding protein 1c (SREBP-1c). The aim of this study was to investigate the regulatory network effects of SREBP-1c on the lipid metabolic genes involved in fatty acid uptake, activation, oxidation, synthesis, and very low-density lipoprotein (VLDL) assembly in bovine hepatocytes. In vitro, bovine hepatocytes were transfected with an adenovirus-mediated SREBP-1c overexpression vector. SREBP-1c overexpression significantly up-regulated the expression and activity of the fatty acid uptake, activation, and synthesis enzymes: liver fatty acid binding protein, fatty acid translocase, acyl-CoA synthetase long-chain 1, acetyl-CoA carboxylase 1, and fatty acid synthase, increasing triglyceride (TG) synthesis and accumulation. SREBP-1c overexpression down-regulated the expression and activity of the lipid oxidation enzymes: carnitine palmitoyltransferase 1 and carnitine palmitoyltransferase 2. Furthermore, the apolipoprotein B100 expression and microsomal triglyceride transfer protein activity were significantly decreased. SREBP-1c overexpression reduced lipid oxidation and VLDL synthesis, thereby decreasing TG disposal and export. Therefore, large amounts of TG accumulated in the bovine hepatocytes. Taken together, these results indicate that SREBP-1c overexpression increases lipid synthesis and decreases lipid oxidation and VLDL export, thereby inducing TG accumulation in bovine hepatocytes.

The pentose phosphate pathway and cancer

The pentose phosphate pathway (PPP), which branches from glycolysis at the first committed step of glucose metabolism, is required for the synthesis of ribonucleotides and is a major source of NADPH. NADPH is required for and consumed during fatty acid synthesis and the scavenging of reactive oxygen species (ROS). Therefore, the PPP plays a pivotal role in helping glycolytic cancer cells to meet their anabolic demands and combat oxidative stress. Recently, several neoplastic lesions were shown to have evolved to facilitate the flux of glucose into the PPP. This review summarizes the fundamental functions of the PPP, its regulation in cancer cells, and its importance in cancer cell metabolism and survival.

Decreased genetic dosage of hepatic Yin Yang 1 causes diabetic-like symptoms

Insulin sensitivity in liver is characterized by the ability of insulin to efficiently inhibit glucose production and fatty acid oxidation as well as promote de novo lipid biosynthesis. Specific dysregulation of glucose and lipid metabolism in liver is sufficient to cause insulin resistance and type 2 diabetes; this is seen by a selective inability of insulin to suppress glucose production while remaining insulin-sensitive to de novo lipid biosynthesis. We have previously shown that the transcription factor Yin Yang 1 (YY1) controls diabetic-linked glucose and lipid metabolism gene sets in skeletal muscle, but whether liver YY1-targeted metabolic genes impact a diabetic phenotype is unknown. Here we show that decreased genetic dosage of YY1 in liver causes insulin resistance, hepatic lipid accumulation, and dyslipidemia. Indeed, YY1 liver-specific heterozygous mice exhibit blunted activation of hepatic insulin signaling in response to insulin. Mechanistically, YY1, through direct recruitment to promoters, functions as a suppressor of genes encoding for metabolic enzymes of the gluconeogenic and lipogenic pathways and as an activator of genes linked to fatty acid oxidation. These counterregulatory transcriptional activities make targeting hepatic YY1 an attractive approach for treating insulin-resistant diabetes.

Insulin signalling mechanisms for triacylglycerol storage

Insulin signalling is uniquely required for storing energy as fat in humans. While de novo synthesis of fatty acids and triacylglycerol occurs mostly in liver, adipose tissue is the primary site for triacylglycerol storage. Insulin signalling mechanisms in adipose tissue that stimulate hydrolysis of circulating triacylglycerol, uptake of the released fatty acids and their conversion to triacylglycerol are poorly understood. New findings include (1) activation of DNA-dependent protein kinase to stimulate upstream stimulatory factor (USF)1/USF2 heterodimers, enhancing the lipogenic transcription factor sterol regulatory element binding protein 1c (SREBP1c); (2) stimulation of fatty acid synthase through AMP kinase modulation; (3) mobilisation of lipid droplet proteins to promote retention of triacylglycerol; and (4) upregulation of a novel carbohydrate response element binding protein β isoform that potently stimulates transcription of lipogenic enzymes. Additionally, insulin signalling through mammalian target of rapamycin to activate transcription and processing of SREBP1c described in liver may apply to adipose tissue. Paradoxically, insulin resistance in obesity and type 2 diabetes is associated with increased triacylglycerol synthesis in liver, while it is decreased in adipose tissue. This and other mysteries about insulin signalling and insulin resistance in adipose tissue make this topic especially fertile for future research.

A pathway approach to investigate the function and regulation of SREBPs

The essential function of sterol regulatory element-binding proteins (SREBPs) in cellular lipid metabolism and homeostasis has been recognized for a long time, and the basic biological pathway involving SREBPs has been well described; however, a rapidly growing number of studies reveal the complex regulation of these SREBP transcription factors at multiple levels. This regulation allows the integration of signals of diverse pathways involving nutrients, contributing to cellular lipid and energy homeostasis. This review attempts to integrate this knowledge. The description of the SREBP pathway is Web-linked as it refers to the online version of the pathway on wikipathways.org , which is interactively linked to genomics databases and literature. This allows a more extensive study of the pathway through reviewing these links.