Basal lipolysis, not the degree of insulin resistance, differentiates large from small isolated adipocytes in high-fat fed mice

Aims/hypothesis: Adipocytes in obesity are characterised by increased cell size and insulin resistance compared with adipocytes isolated from lean patients. However, it is not clear at present whether hypertrophy actually does drive adipocyte insulin resistance. Thus, the aim of the present study was to metabolically characterise small and large adipocytes isolated from epididymal fat pads of mice fed a high-fat diet (HFD). Methods: C57BL/6J mice were fed normal chow or HFD for 8weeks. Adipocytes from epididymal fat pads were isolated by collagenase digestion and, in HFD-fed mice, separated into two fractions according to their size by filtration through a nylon mesh. Viability was assessed by lactate dehydrogenase and 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium assays. Basal and insulin-stimulated d-[U-14C]glucose incorporation and lipolysis were measured. Protein levels and mRNA expression were determined by western blot and real-time RT-PCR, respectively. Results: Insulin-stimulated D-[U-14C]glucose incorporation into adipocytes isolated from HFD-fed mice was reduced by 50% compared with adipocytes from chow-fed mice. However, it was similar between small (average diameter 60.9 ± 3.1μm) and large (average diameter 83.0 ± 6.6μm) adipocytes. Similarly, insulin-stimulated phosphorylation of protein kinase B and AS160 were reduced to the same extent in small and large adipocytes isolated from HFD-mice. In addition, insulin failed to inhibit lipolysis in both adipocyte fractions, whereas it decreased lipolysis by 30% in adipocytes of chow-fed mice. In contrast, large and small adipocytes differed in basal lipolysis rate, which was twofold higher in the larger cells. The latter finding was associated with higher mRNA expression levels of Atgl (also known as Pnpla2) and Hsl (also known as Lipe) in larger adipocytes. Viability was not different between small and large adipocytes. Conclusions/interpretation: Rate of basal lipolysis but not insulin responsiveness is different between small and large adipocytes isolated from epididymal fat pads of HFD-fed mic

The Portal Theory Supported by Venous Drainage–Selective Fat Transplantation

OBJECTIVE The "portal hypothesis" proposes that the liver is directly exposed to free fatty acids and cytokines increasingly released from visceral fat tissue into the portal vein of obese subjects, thus rendering visceral fat accumulation particularly hazardous for the development of hepatic insulin resistance and type 2 diabetes. In the present study, we used a fat transplantation paradigm to (artificially) increase intra-abdominal fat mass to test the hypothesis that venous drainage of fat tissue determines its impact on glucose homeostasis. RESEARCH DESIGN AND METHODS Epididymal fat pads of C57Bl6/J donor mice were transplanted into littermates, either to the parietal peritoneum (caval/systemic venous drainage) or, by using a novel approach, to the mesenterium, which confers portal venous drainage. RESULTS Only mice receiving the portal drained fat transplant developed impaired glucose tolerance and hepatic insulin resistance. mRNA expression of proinflammatory cytokines was increased in both portally and systemically transplanted fat pads. However, portal vein (but not systemic) plasma levels of interleukin (IL)-6 were elevated only in mice receiving a portal fat transplant. Intriguingly, mice receiving portal drained transplants from IL-6 knockout mice showed normal glucose tolerance. CONCLUSIONS These results demonstrate that the metabolic fate of intra-abdominal fat tissue transplantation is determined by the delivery of inflammatory cytokines to the liver specifically via the portal system, providing direct evidence in support of the portal hypothesis