Louise J. Hutley

Fat as an endocrine organ: Relationship to the metabolic syndrome

Obesity and the metabolic syndrome have both reached pandemic proportions. Together they have the potential to impact on the incidence and severity of cardiovascular pathologies, with grave implications for worldwide health care systems. The metabolic syndrome is characterized by visceral obesity, insulin resistance, hypertension, chronic inflammation, and thrombotic disorders contributing to endothelial dysfunction and, subsequently, to accelerated atherosclerosis. Obesity is a key component in development of the metabolic syndrome and it is becoming increasingly clear that a central factor in this is the production by adipose cells of bioactive substances that directly influence insulin sensitivity and vascular injury. In this paper, we review advances in the understanding of biologically active molecules collectively referred to as “adipokines” and how dysregulated production of these factors in obese states mediates the pathogenesis of obesity associated metabolic syndrome.

Characterization of the transcriptional and functional effects of fibroblast growth factor-1 on human preadipocyte differentiation

We recently established that fibroblast growth factor (FGF)‐1 promotes adipogenesis of primary human preadipocytes (phPA). In the current report, we have characterized the adipogenic effects of FGF‐1 in phPA and also in a human PA strain derived from an individual with Simpson‐Golabi‐Behmel syndrome (SGBS PA), which exhibit an intrinsic capacity to differentiate with high efficiency. In further studies, we compared these models with the well‐characterized murine 3T3‐L1 preadipocyte cell line (3T3‐L1 PA). FGF‐1 up‐regulated the adipogenic program in phPA, with increased expression of peroxisome proliferator‐activated receptor‐γ in confluent PA prior to induction of differentiation and increased expression of adipocyte markers during differentiation. Moreover, phPA differentiated in the presence of FGF‐1 were more insulin responsive and secreted increased levels of adiponectin. FGF‐1 treatment of SGBS PA further enhanced differentiation. For the most part, the adipogenic program in phPA paralleled that observed in 3T3‐L1 PA; however, we found no evidence of mitotic clonal expansion in the phPA. Finally, we investigated a role for extracellular regulated kinase 1/2 (ERK 1/2) in adipogenesis of phPA. FGF‐1 induced robust phosphorylation of ERK1/2 in early differentiation and inhibition of ERK1/2 activity significantly reduced phPA differentiation. These data suggest that FGF‐1 treated phPA represent a valuable in vitro model for the study of adipogenesis and insulin action and indicate that ERK1/2 activation is necessary for human adipogenesis in the absence of mitotic clonal expansion.—Newell, F. S., Su, H., Tornqvist, H., Whitehead, J. P., Prins, J. B., Hutley, L. J. Characterization of the transcriptional and functional effects of fibroblast growth factor‐1 on human preadipocyte differentiation FASEB J. 20, E2133–E2145 (2006)

Glucose uptake and insulin action in human adipose tissue—influence of BMI, anatomical depot and body fat distribution

OBJECTIVE: To examine and compare in vitro basal and insulin-stimulated glucose uptake in human omental and subcutaneous adipose tissue derived from lean, overweight or obese individuals, and in those with central or peripheral obesity.DESIGN: In vitro study of basal and insulin-stimulated 2-deoxyglucose uptake in human omental and subcutaneous adipose tissue explants derived from patients undergoing elective abdominal surgery.SUBJECTS: Fourteen lean (average age 47 y, average body mass index (BMI) 22 kg/m2), 12 overweight (average age 51 y, average BMI 27 kg/m2), and 15 obese subjects (average age 45 y, average BMI 39 kg/m2). Ten peripherally obese (average age 43 y, average WHR 0.76) and 17 centrally obese (average age 50 y, average waist-to-hip ratio (WHR) 0.92).MEASUREMENTS: Fatness and fat distribution parameters (by anthropometry), basal and insulin stimulated [3H]-2-deoxyglucose uptake in omental and subcutaneous adipose tissue explants.RESULTS: In adipose tissue from lean subjects transport of 2-deoxyglucose over basal was stimulated approximately two-fold by insulin. In contrast, 2-deoxyglucose transport in adipose tissue of obese or overweight subjects was not responsive to insulin. Following incubation with 100–nM insulin for 35 min, insulin-stimulated 2-deoxyglucose transport was significantly lower in both omental and subcutaneous adipose tissue of obese and overweight compared to lean subjects. Basal 2-deoxyglucose uptake was also significantly reduced in omental and subcutaneous tissue in obese compared to lean subjects. Depot-specific differences in 2-deoxyglucose uptake were also seen. Overall 2-deoxyglucose uptake was greater in omental than subcutaneous adipose tissue but this was due to increased basal levels rather than increased insulin action. The reduction in insulin-stimulated 2-deoxyglucose uptake seen in overweight and obese subjects was relatively similar in both depots. However, insulin responsive 2-deoxyglucose transport was significantly lower in the omental adipose tissue of subjects with central obesity, as compared to that of subjects with peripheral obesity. No difference in insulin induced 2-deoxyglucose transport was observed in the subcutaneous adipose tissue explants of subjects with either central or peripheral obesity.CONCLUSION: In lean individuals insulin responsiveness of omental and subcutaneous adipose tissue was similar, but basal glucose uptake was significantly higher in omental adipose tissue. Adipose tissue obtained from overweight as well as obese individuals is insulin resistant. This insulin resistance occurs at a lower BMI than previously expected and is not adipose-depot specific. However, in obese subjects with a central distribution of adiposity insulin resistance occurs at the site of omental adipose tissue, in contrast to those with peripheral obesity.

Estrogen receptors in human preadipocytes

Estrogen influences regional adipose tissue distribution and the accompanying cardiovascular disease risk. To elucidate the mechanisms of this link further, we assessed whether human preadipocytes (PAs) expressed estrogen receptors (ERs) and whether there were any regional or gender differences in ER complement. Human PAs expressed the ERα gene but not ERβ by reverse transcriptase-polymerase chain reaction, possessed ERα protein on Western blotting, and displayed specific 17β-estradiol (E2) binding with calculated dissociation constants of 0.78 nM, 0.96 nM, and 1.19 nM and maximal binding capacities of 9.3 fmol/mg, 14.6 fmol/mg, and 18.2 fmol/mg from three whole cell binding assays. There were no regional differences in ERα complement for males or females. There were no gender differences in ERα complement for subcutaneous or visceral samples. We conclude that ERα but not ERβ is present in human PAs. This suggests that the effect of estrogen on adipose tissue deposition has a contribution from the direct effect of estrogen on human PAs via ERα.

TNF and TNF receptor expression and insulin sensitivity in human omental and subcutaneous adipose tissue - influence of BMI and adipose distribution

Tumour necrosis factor (TNF)α is implicated in the relationship between obesity and insulin resistance/type 2 diabetes. In an effort to understand this association better we (i) profiled gene expression patterns of TNF, TNFR1 and TNFR2 and (ii) investigated the effects of TNF on glucose uptake in isolated adipocytes and adipose tissue explants from omental and subcutaneous depots from lean, overweight and obese individuals. TNF expression correlated with expression of TNFR2, but not TNFR1, and TNF and TNFR2 expression increased in obesity. TNFR1 expression was higher in omental than in subcutaneous adipocytes. Expression levels of TNF or either receptor did not differ between adipocytes from individuals with central and peripheral obesity. TNF only suppressed glucose uptake in insulin-stimulated subcutaneous tissue and this suppression was only observed in tissue from lean subjects. These data support a relationship between the TNF system and body mass index (BMI), but not fat distribution, and suggest depot specificity of the TNF effect on glucose uptake. Furthermore, adipose tissue from obese subjects already appears insulin ‘resistant’ and this may be a result of the increased TNF levels.

Effects of rosiglitazone and linoleic acid on human preadipocyte differentiation

Background Peroxisome proliferator activated receptor gamma (PPARγ) is a ligand‐activated transcription factor known to be central to both adipose tissue development and insulin action. Growth of adipose tissue requires differentiation of preadipocytes with acquisition of specific cellular functions including insulin sensitivity, leptin secretion and the capacity to store triglyceride. Dietary fatty acids and members of the thiazolidinedione class of compounds have been reported to influence adipogenesis at the transcriptional level. Here, we compare the effects of a dietary fatty acid, linoleic acid, and a thiazolidinedione, rosiglitazone, on biochemical and functional aspects of human preadipocyte differentiation in vitro.

Identification of BMP and Activin Membrane-Bound Inhibitor (BAMBI) as a Potent Negative Regulator of Adipogenesis and Modulator of Autocrine/Paracrine Adipogenic Factors

Adipose tissue dysfunction underpins the association of obesity with type 2 diabetes. Adipogenesis is required for the maintenance of adipose tissue function. It involves the commitment and subsequent differentiation of preadipocytes and is coordinated by autocrine, paracrine, and endocrine factors. We previously reported that fibroblast growth factor-1 (FGF-1) primes primary human preadipocytes and Simpson Golabi Behmel syndrome (SGBS) preadipocytes and increases adipogenesis through a cascade involving extracellular signal–related kinase 1/2 (ERK1/2). Here, we aimed to use the FGF-1 system to identify novel adipogenic regulators. Expression profiling revealed bone morphogenetic protein (BMP) and activin membrane-bound inhibitor (BAMBI) as a putative FGF-1 effector. BAMBI is a transmembrane protein and modulator of paracrine factors that regulate adipogenesis, including transforming growth factor (TGF) superfamily members (TGF-β and BMP) and Wnt. Functional investigations established BAMBI as a negative regulator of adipogenesis and modulator of the anti- and proadipogenic effects of Wnt3a, TGF-β1, and BMP-4. Further studies showed that BAMBI expression levels are decreased in a mouse model of diet-induced obesity. Collectively, these findings establish BAMBI as a novel, negative regulator of adipogenesis that can act as a nexus to integrate multiple paracrine signals to coordinate adipogenesis. Alterations in BAMBI may play a role in the (patho)physiology of obesity, and manipulation of BAMBI may present a novel therapeutic approach to improve adipose tissue function.

A putative role for endogenous FGF-2 in FGF-1 mediated differentiation of human preadipocytes

The defining characteristic of obesity is increased adipose tissue (AT) mass following chronic positive energy supply. AT mass is determined by adipocyte number and size, which reflect proliferation and differentiation of preadipocytes and hypertrophy of pre-existing adipocytes. The molecular pathways governing AT expansion are incompletely defined. We previously reported that FGF-1 primes proliferating primary human preadipocytes (phPA), thereby increasing adipogenesis. Here we examined whether FGF-1s adipogenic actions were due to modulation of other FGFs. Treatment of phPA with FGF-1 reduced FGF-2 mRNA/protein by 80%. To examine a putative functional role we performed siRNA knockdown studies. Following FGF-2 knockdown preadipocyte proliferation was decreased and expression of adipogenic genes (PPARγ, G3PDH and adiponectin) was increased at day 1 of differentiation. These results suggest that changes in endogenous FGF-2 levels contribute to FGF-1s early adipogenic effects and highlight the complexity of the paracrine interplay between FGFs within human AT.

Inhibition of inosine monophosphate dehydrogenase reduces adipogenesis and diet-induced obesity.

We previously described a putative role for inosine monophosphate dehydrogenase (IMPDH), a rate-limiting enzyme in de novo guanine nucleotide biosynthesis, in lipid accumulation. Here we present data which demonstrate that IMPDH activity is required for differentiation of preadipocytes into mature, lipid-laden adipocytes and maintenance of adipose tissue mass. In 3T3-L1 preadipocytes inhibition of IMPDH with mycophenolic acid (MPA) reduced intracellular GTP levels by 60% (p<0.05) and blocked adipogenesis (p<0.05). Co-treatment with guanosine, a substrate in the salvage pathway of nucleotide biosynthesis, restored GTP levels and adipogenesis demonstrating the specificity of these effects. Treatment of diet-induced obese mice with mycophenolate mofetil (MMF), the prodrug of MPA, for 28 days did not affect food intake or lean body mass but reduced body fat content (by 36%, p=0.002) and adipocyte size (p=0.03) and number. These data suggest that inhibition of IMPDH may represent a novel strategy to reduce adipose tissue mass.