Stromal progenitors are found in many different tissues, where they play an important role in the maintenance of tissue homeostasis owing to their ability to differentiate into parenchymal cells. ...These progenitor cells are differentially pre-programmed by their tissue microenvironment but, when cultured and stimulated in vitro, these cells - commonly referred to as mesenchymal stromal cells (MSCs) - exhibit a marked plasticity to differentiate into many different cell lineages. Loss-of-function studies in vitro and in vivo have uncovered the involvement of specific signalling pathways and key transcriptional regulators that work in a sequential and coordinated fashion to activate lineage-selective gene programmes. Recent advances in omics and single-cell technologies have made it possible to obtain system-wide insights into the gene regulatory networks that drive lineage determination and cell differentiation. These insights have important implications for the understanding of cell differentiation, the contribution of stromal cells to human disease and for the development of cell-based therapeutic applications.
PPARs: Fatty acid sensors controlling metabolism Poulsen, Lars la Cour; Siersbæk, Majken; Mandrup, Susanne
Seminars in cell & developmental biology,
08/2012, Letnik:
23, Številka:
6
Journal Article
Recenzirano
► PPARs are molecular sensors of FAs and FA derivatives and control metabolism. ► The three subtypes display distinct but overlapping expression and functions. ► PPARα is a major activator of FAO, ...e.g. in liver and BAT. ► PPARδ is a major activator of oxidative metabolism and is ubiquitously expressed. ► PPARγ is a major activator of adipogenesis and FA storage.
The peroxisome proliferator activated receptors (PPARs) are nuclear receptors that play key roles in the regulation of lipid metabolism, inflammation, cellular growth, and differentiation. The receptors bind and are activated by a broad range of fatty acids and fatty acid derivatives and they thereby serve as major transcriptional sensors of fatty acids. Here we review the function, regulation, and mechanism of the different PPAR subtypes with special emphasis on their role in the regulation of lipid metabolism.
Adipocyte differentiation is tightly controlled by a transcriptional cascade, which directs the extensive reprogramming of gene expression required to convert fibroblast-like precursor cells into ...mature lipid-laden adipocytes. Recent global analyses of transcription factor binding and chromatin remodeling have revealed ‘snapshots’ of this cascade and the chromatin landscape at specific time-points of differentiation. These studies demonstrate that multiple adipogenic transcription factors co-occupy hotspots characterized by an open chromatin structure and specific epigenetic modifications. Such transcription factor hotspots are likely to represent key signaling nodes which integrate multiple adipogenic signals at specific chromatin sites, thereby facilitating coordinated action on gene expression.
Interactions between transcriptional promoters and their distal regulatory elements play an important role in transcriptional regulation; however, the extent to which these interactions are subject ...to rapid modulations in response to signals is unknown. Here, we use promoter capture Hi-C to demonstrate a rapid reorganization of promoter-anchored chromatin loops within 4 hr after inducing differentiation of 3T3-L1 preadipocytes. The establishment of new promoter-enhancer loops is tightly coupled to activation of poised (histone H3 lysine 4 mono- and dimethylated) enhancers, as evidenced by the acquisition of histone H3 lysine 27 acetylation and the binding of MED1, SMC1, and P300 proteins to these regions, as well as to activation of target genes. Intriguingly, formation of loops connecting activated enhancers and promoters is also associated with extensive recruitment of corepressors such as NCoR and HDACs, indicating that this class of coregulators may play a previously unrecognized role during enhancer activation.
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•Dynamic promoters and enhancers are connected through dynamic chromatin loops•Chromatin loop formation is coupled to activation of poised enhancers•Enhancer activation is associated with corepressor recruitment
Siersbæk et al. demonstrate a rapid and dynamic rewiring of promoter-anchored chromatin loops during adipocyte differentiation, which is linked to changes in the activity of promoters and enhancers. Loop formation involves activation of poised enhancers and is associated with extensive recruitment of both coactivators and corepressors.
Non-alcoholic steatohepatitis (NASH) signified by hepatic steatosis, inflammation, hepatocellular injury, and fibrosis is a growing cause of chronic liver disease, cirrhosis, and hepatocellular ...carcinoma. Hepatic fibrosis resulting from accumulation of extracellular matrix proteins secreted by hepatic myofibroblasts plays an important role in disease progression. Activated hepatic stellate cells (HSCs) have been identified as the primary source of myofibroblasts in animal models of hepatotoxic liver injury; however, so far HSC activation and plasticity have not been thoroughly investigated in the context of NASH-related fibrogenesis. Here we have determined the time-resolved changes in the HSC transcriptome during development of Western diet- and fructose-induced NASH in mice, a NASH model recapitulating human disease. Intriguingly, HSC transcriptional dynamics are highly similar across disease models pointing to HSC activation as a point of convergence in the development of fibrotic liver disease. Bioinformatic interrogation of the promoter sequences of activated genes combined with loss-of-function experiments indicates that the transcriptional regulators ETS1 and RUNX1 act as drivers of NASH-associated HSC plasticity. Taken together, our results implicate HSC activation and transcriptional plasticity as key aspects of NASH pathophysiology.
C57BL/6J-related mouse strains are widely used animal models for diet-induced obesity (DIO). Multiple vendors breed C57BL/6J-related substrains which may introduce genetic drift and environmental ...confounders such as microbiome differences. To address potential vendor/substrain specific effects, we compared DIO of C57BL/6J-related substrains from three different vendors: C57BL/6J (Charles Rivers), C57BL/6JBomTac (Taconic Bioscience) and C57BL/6JRj (Janvier). After local acclimatization, DIO was induced by either a high-fat diet (HFD, 60% energy from fat) or western diet (WD, 42% energy from fat supplemented with fructose in the drinking water). All three groups on HFD gained a similar amount of total body weight, yet the relative amount of fat percentage and mass of inguinal- and epididymal white adipose tissue (iWAT and eWAT) was lower in C57BL/6JBomTac compared to the two other C57BL/6J-releated substrains. In contrast to HFD, the three groups on WD responded differently in terms of body weight gain, where C57BL/6J was particularly prone to WD. This was associated with a relative higher amount of eWAT, iWAT, and liver triglycerides. Although the HFD and WD had significant impact on the microbiota, we did not observe any major differences between the three groups of mice. Together, these data demonstrate significant differences in HFD- and WD-induced adiposity in C57BL/6J-related substrains, which should be considered in the design of animal DIO studies.
Recent studies suggest that exposure to endocrine-disrupting compounds (EDCs) may play a role in the development of obesity. EDCs such as the flame retardant 2,2′,4,4′-tetrabrominated diphenyl ether ...(BDE-47) have been shown to enhance adipocyte differentiation in the murine 3T3-L1 model. The mechanisms by which EDCs direct preadipocytes to form adipocytes are poorly understood. Here, we examined transcriptional and epigenetic mechanisms underlying the induction of in vitro adipocyte differentiation by BDE-47. Quantitative high content microscopy revealed concentration-dependent enhanced adipocyte differentiation following exposure to BDE-47 or the antidiabetic drug troglitazone (TROG). BDE-47 modestly activated the key adipogenic transcription factor peroxisome proliferator-activated receptor gamma (PPARγ) in COS7 cells, transiently transfected with a GAL4 reporter construct. Increased gene expression was observed for Pparγ2, leptin (Lep), and glucose-6-phophatase catalytic subunit (G6pc) in differentiated 3T3-L1 cells after BDE-47 exposure compared to TROG. Methylation-sensitive high resolution melting (MS-HRM) revealed significant demethylation of three CpG sites in the Pparγ2 promoter after exposure to both BDE-47 and TROG in differentiated 3T3-L1 cells. This study shows the potential of BDE-47 to induce adipocyte differentiation through various mechanisms that include Pparγ2 gene induction and promoter demethylation accompanied by activation of PPARγ, and possible disruption of glucose homeostasis and IGF1 signaling.
Aims/hypothesis
Chronic exposure of pancreatic beta cells to high glucose and fatty acids has been proposed to induce glucolipotoxicity. However, contradictory results suggest adaptations of the beta ...cells, which might be instrumental for partial preservation of the secretory response. In this context, we delineated the expression pattern of genes related to lipid pathways along with fat storage/mobilisation during glucose-stimulated insulin secretion.
Methods
Insulin-secreting cells were cultured for 3 days at different glucose concentrations (5.5, 11.1, 25 mmol/l) without or with BSA-complexed 0.4 mmol/l palmitate and oleate. Then, transcriptomic analyses of lipid pathways were performed in human islets by RNA-Seq and in INS-1E cells and rat islets by quantitative RT-PCR. Storage of fat was assessed in INS-1E cells by electron microscopy and Bodipy staining, which was also used for measuring lipid mobilisation rate. The secretory response was monitored during acute 15 mmol/l glucose stimulation using online luminescence assay for INS-1E cells and by radioimmunoassay for rat islets.
Results
In human islets, chronic exposure to palmitate and oleate modified expression of a panel of genes involved in lipid handling. Culture at 25 mmol/l glucose upregulated genes encoding for enzymes of the glycerolipid/NEFA cycle and downregulated receptors implicated in fatty acid signalling. Similar results were obtained in INS-1E cells, indicating enhanced capacity of the glycerolipid/NEFA cycle under glucotoxic conditions. Exposure to unsaturated C18:1 fatty acid favoured intracellular lipid accumulation in a glucose-dependent way, an effect also observed with saturated C16:0 fatty acid when combined with the panlipase inhibitor Orlistat. After the glucolipotoxic culture, intracellular fat mobilisation was required for acute glucose-stimulated secretion, particularly in oleate-treated cells under glucotoxic culture conditions. The lipid mobilisation rate was governed chiefly by the levels of stored fat as a direct consequence of the culture conditions rather than energetic demands, except in palmitate-loaded cells.
Conclusions/interpretation
Glucolipotoxic conditions promote the capacity of the glycerolipid/NEFA cycle thereby preserving part of the secretory response. The cycle of fat storage/mobilisation emerges as a mechanism helping the beta cell to cope with glucotoxic conditions.
Graphical abstract
Background and Aims
Hepatic sinusoidal cells are known actors in the fibrogenic response to injury. Activated hepatic stellate cells (HSCs), liver sinusoidal endothelial cells, and Kupffer cells are ...responsible for sinusoidal capillarization and perisinusoidal matrix deposition, impairing vascular exchange and heightening the risk of advanced fibrosis. While the overall pathogenesis is well understood, functional relations between cellular transitions during fibrogenesis are only beginning to be resolved. At single‐cell resolution, we here explored the heterogeneity of individual cell types and dissected their transitions and crosstalk during fibrogenesis.
Approach and Results
We applied single‐cell transcriptomics to map the heterogeneity of sinusoid‐associated cells in healthy and injured livers and reconstructed the single‐lineage HSC trajectory from pericyte to myofibroblast. Stratifying each sinusoidal cell population by activation state, we projected shifts in sinusoidal communication upon injury. Weighted gene correlation network analysis of the HSC trajectory led to the identification of core genes whose expression proved highly predictive of advanced fibrosis in patients with nonalcoholic steatohepatitis (NASH). Among the core members of the injury‐repressed gene module, we identified plasmalemma vesicle–associated protein (PLVAP) as a protein amply expressed by mouse and human HSCs. PLVAP expression was suppressed in activated HSCs upon injury and may hence define hitherto unknown roles for HSCs in the regulation of microcirculatory exchange and its breakdown in chronic liver disease.
Conclusions
Our study offers a single‐cell resolved account of drug‐induced injury of the mammalian liver and identifies key genes that may serve important roles in sinusoidal integrity and as markers of advanced fibrosis in human NASH.
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