Reduced Socs3 expression in adipose tissue protects female mice against obesity-induced insulin resistance

AIMS/HYPOTHESIS

Inflammation in obesity increases the levels of the suppressor of cytokine signalling-3 (SOCS3) protein in adipose tissue, but the physiological importance of this protein in regulating whole-body insulin sensitivity in obesity is not known.

METHODS

We generated Socs3 floxed (wild-type, WT) and Socs3 aP2 (also known as Fabp4)-Cre null (Socs3 AKO) mice. Mice were maintained on either a regular chow or a high-fat diet (HFD) for 16 weeks during which time body mass, adiposity, glucose homeostasis and insulin sensitivity were assessed.

RESULTS

The HFD increased SOCS3 levels in adipose tissue of WT but not Socs3 AKO mice. WT and Socs3 AKO mice had similar body mass and adiposity, assessed using computed tomography (CT) imaging, irrespective of diet or sex. On a control chow diet there were no differences in insulin sensitivity or glucose tolerance. When fed a HFD, female but not male Socs3 AKO mice had improved glucose tolerance as well as lower fasting glucose and insulin levels compared with WT littermates. Hyperinsulinaemic-euglycaemic clamps and positron emission tomography (PET) imaging demonstrated that improved insulin sensitivity was due to elevated adipose tissue glucose uptake. Increased insulin-stimulated glucose uptake in adipose tissue was associated with enhanced levels and activating phosphorylation of insulin receptor substrate-1 (IRS1).

CONCLUSIONS/INTERPRETATION

These data demonstrate that inhibiting SOCS3 production in adipose tissue of female mice is effective for improving whole-body insulin sensitivity in obesity.

Enhancement by galactosamine of lipopolysaccharide(LPS)-induced tumour necrosis factor production and lethality: its suppression by LPS pretreatment

1. D-Galactosamine (GalN) depletes UTP primarily in the liver, resulting in decreased RNA synthesis in hepatocytes. Co-injection of GalN and lipopolysaccharide (LPS) into mice produces fulminant hepatitis with severe hepatic congestion, resulting in rapid death. Although the underlying mechanism is uncertain, GalN enhances the sensitivity to tumour necrosis factor (TNF). Administration of uridine (a precursor of UTP) prior injection of either LPS itself or interleukin-1 (IL-1) reduces the lethality of GalN+LPS. The present study focused on the effects of these agents on TNF production. 2. Intraperitoneal injection of GalN+LPS into mice greatly elevated serum TNF. Although large doses of LPS alone also greatly elevated serum TNF, LPS itself induced neither hepatic congestion nor rapid death. Administration of a macrophage depletor, liposomes encapsulated with dichloromethylene bisphosphonate, reduced both the TNF production and mortality induced by GalN+LPS. 3. Uridine, when injected 0.5 h after the injection of GalN+LPS, reduced the production of TNF. Prior injection of LPS, but not of IL-1, also reduced this TNF production. 4. Serum from LPS-injected mice reduced the TNF production induced by GalN+LPS, but it was less effective at reducing the lethality. Its ability to reduce TNF production was abolished by heat-treatment. 5. We hypothesize that a factor inhibiting TNF production by macrophages is produced by hepatocytes in response to LPS. Possibly, production of this hepatocyte-derived TNF-down-regulator (TNF-DRh) may be: (i) inhibited by GalN, causing over-production of TNF by macrophages and (ii) stimulated by LPS-pretreatment (and restored by uridine), causing reduced TNF production.

Therapy with interleukin-2 induces the systemic release of phospholipase-A2

Therapy with interleukin-2 (IL-2) induces remissions in some forms of cancer. This treatment however, is accompanied by side-effects which, in part, may be mediated by the formation of eicosanoids and platelet-activating factor. We investigated the systemic release of phospholipase A2 (PLA2), a rate-limiting enzyme in the formation of these lipid mediators, in patients receiving IL-2. In a pilot study of 4 patients we observed an increase in PLA2 activity in serial plasma samples obtained during the first day after a bolus infusion of IL-2, which increase closely correlated with that of antigen levels of secretory phospholipase A2 (sPLA2) as measured by enzyme-linked immunosorbent assay (r = 0.92; P < 0.001). In 20 patients, receiving 12 x 10(6)-18 x 10(6) IU IL-2/m2, we then investigated the course of antigenic levels of sPLA2 in relation to those of the cytokines tumour necrosis factor alpha (TNF) and interleukin-6 (IL-6) (both cytokines may induce sPLA2 in vivo). From 4 h on, sPLA2 levels significantly increased, reaching a peak 24 h after the IL-2 infusion. Subsequent IL-2 infusions even induced a further increase of sPLA2. This increase of sPLA2 was presumably not due to a direct effect of IL-2 on, for example, hepatocytes, since this cytokine, in contrast to IL-1, IL-6, TNF and interferon gamma, was not able to induce the synthesis of sPLA2 by Hep G2 cells in vitro. Consistent with this, plasma levels of TNF and IL-6 in the patients rose, reaching peak levels before a zenith of sPLA2 occurred, i.e. at 2 h and 4 h after the start of the IL-2 infusion respectively. sPLA2 levels significantly correlated with the development of the side-effects increase in body weight (r = 0.49; P < 0.0001) and decrease in mean arterial blood pressure (r = 0.40; P < 0.0001). Moreover, maximum sPLA2 levels induced by IL-2 were higher in patients who had progressive disease after therapy than in patients who had stable disease or a partial response.