Tsc2 disruption in mesenchymal progenitors results in tumors with vascular anomalies overexpressing Lgals3.

Increased mTORC1 signaling from TSC1/TSC2 inactivation is found in cancer and causes tuberous sclerosis complex (TSC). The role of mesenchymal-derived cells in TSC tumorigenesis was investigated through disruption of Tsc2 in craniofacial and limb bud mesenchymal progenitors. Tsc2cKOPrrx1-cre mice had shortened lifespans and extensive hamartomas containing abnormal tortuous, dilated vessels prominent in the forelimbs. Abnormalities were blocked by the mTORC1 inhibitor sirolimus. A Tsc2/mTORC1 expression signature identified in Tsc2-deficient fibroblasts was also increased in bladder cancers with TSC1/TSC2 mutations in the TCGA database. Signature component Lgals3 encoding galectin-3 was increased in Tsc2-deficient cells and serum of Tsc2cKOPrrx1-cre mice. Galectin-3 was increased in TSC-related skin tumors, angiomyolipomas, and lymphangioleiomyomatosis with serum levels in patients with lymphangioleiomyomatosis correlating with impaired lung function and angiomyolipoma presence. Our results demonstrate Tsc2-deficient mesenchymal progenitors cause aberrant morphogenic signals, and identify an expression signature including Lgals3 relevant for human disease of TSC1/TSC2 inactivation and mTORC1 hyperactivity.

STAT1 Signaling in Astrocytes Is Essential for Control of Infection in the Central Nervous System.

The local production of gamma interferon (IFN-γ) is important to control Toxoplasma gondii in the brain, but the basis for these protective effects is not fully understood. The studies presented here reveal that the ability of IFN-γ to inhibit parasite replication in astrocytes in vitro is dependent on signal transducer and activator of transcription 1 (STAT1) and that mice that specifically lack STAT1 in astrocytes are unable to limit parasite replication in the central nervous system (CNS). This susceptibility is associated with a loss of antimicrobial pathways and increased cyst formation in astrocytes. These results identify a critical role for astrocytes in limiting the replication of an important opportunistic pathogen. IMPORTANCE: Astrocytes are the most numerous cell type in the brain, and they are activated in response to many types of neuroinflammation, but their function in the control of CNS-specific infection is unclear. The parasite Toxoplasma gondii is one of the few clinically relevant microorganisms that naturally infects astrocytes, and the studies presented here establish that the ability of astrocytes to inhibit parasite replication is essential for the local control of this opportunistic pathogen. Together, these studies establish a key role for astrocytes as effector cells and in the coordination of many aspects of the protective immune response that operates in the brain.

Absence of STAT1 in donor-derived plasmacytoid dendritic cells results in increased STAT3 and attenuates murine GVHD.

Selective targeting of non-T cells, including antigen-presenting cells (APCs), is a potential strategy to prevent graft-versus-host-disease (GVHD) but to maintain graft-versus-tumor (GVT) effects. Because type I and II interferons signal through signal transducer and activator of transcription-1 (STAT1), and contribute to activation of APCs after allogeneic bone marrow transplant (alloBMT), we examined whether the absence of STAT1 in donor APCs could prevent GVHD while preserving immune competence. Transplantation of STAT1(-/-) bone marrow (BM) prevented GVHD induced by STAT1(+/+) T cells, leading to expansion of B220(+) cells and regulatory T cells. STAT1(-/-) BM also preserved GVT activity and enhanced overall survival of tumor-challenged mice in the setting of GVHD. Furthermore, recipients of allogeneic STAT1(-/-) BM demonstrated increased CD9(-)Siglec H(hi) plasmacytoid dendritic cells (pDCs), and depletion of pDCs after STAT1(-/-) BM transplantation prevented GVHD resistance. STAT1(-/-) pDCs were found to produce decreased free radicals, IFNα, and interleukin (IL)-12, and increased IL-10. Additionally, STAT1(-/-) pDCs that were isolated after alloBMT showed increased gene expression of S100A8 and S100A9, and transplantation of S100A9(-/-) BM reduced GVHD-free survival. Finally, elevated STAT3 was found in STAT1(-/-) pDCs isolated after alloBMT. We conclude that interfering with interferon signaling in APCs such as pDCs provides a novel approach to regulate the GVHD/GVT axis.

Loss of STAT1 from mouse mammary epithelium results in an increased Neu-induced tumor burden.

Type I and type II classes of interferons (IFNs) signal through the JAK/STAT1 pathway and are known to be important in adaptive and innate immune responses and in protection against tumors. Although STAT1 is widely considered a tumor suppressor, it remains unclear, however, if this function occurs in tumor cells (cell autonomous) or if STAT1 acts primarily through immune cells. Here, the question of whether STAT1 has a cell autonomous role in mammary tumor formation was addressed in a mouse model of ERBB2/neu-induced breast cancer in the absence and presence of STAT1. For this purpose, mice that carry floxed Stat1 alleles, which permit cell-specific removal of STAT1, were generated. To induce tumors only in mammary cells lacking STAT1, Stat1 floxed mice were crossed with transgenic mice that express cre recombinase and the neu oncogene under the mouse mammary tumor virus LTR (Stat1fl/fl NIC). Stat1 was effectively deleted in mammary epithelium of virgin Stat1fl/fl NIC females. Time-to-tumor onset was significantly shorter in Stat1fl/fl NIC females than in WT NIC (Wilcoxon rank sum test, P = .02). The median time-to-tumor onset in the Stat1fl/fl NIC mice was 49.4 weeks, whereas it was 62.4 weeks in the WT NIC mice. These results suggest that STAT1 in mammary epithelial cells may play a role in suppressing tumorigenesis. The Stat1 floxed allele described in this study is also a unique resource to determine the cellular targets of IFNs and STAT1 action, which should aid our understanding and appreciation of these pathways.

Interleukin-6 depletion selectively improves hepatic insulin action in obesity.

Obesity and insulin resistance are considered chronic inflammatory states, in part because circulating IL-6 is elevated. Exogenous IL-6 can induce hepatic insulin resistance in vitro and in vivo. The importance of endogenous IL-6, however, to insulin resistance of obesity is unresolved. To test the hypothesis that IL-6 contributes to the inflammation and insulin resistance of obesity, IL-6 was depleted in Lep(ob) mice by injection of IL-6-neutralizing antibody. In untreated Lep(ob) mice, signal transducer and activator of transcription-3 (STAT3) activation was increased compared with that in lean controls, consistent with an inflammatory state. With IL-6 depletion, activation of STAT3 in liver and adipose tissue and expression of haptoglobin were reduced. Expression of the IL-6-dependent, hepatic acute phase protein fibrinogen was also decreased. Using the hyperinsulinemic-euglycemic clamp technique, insulin-dependent suppression of endogenous glucose production was 89% in IL-6-depleted Lep(ob) mice, in contrast to only 32% in Lep(ob) controls, indicating a marked increase in hepatic insulin sensitivity. A significant change in glucose uptake in skeletal muscle after IL-6 neutralization was not observed. In a direct comparison of hepatic insulin signaling in Lep(ob) mice treated with anti-IL-6 vs. IgG-treated controls, insulin-dependent insulin receptor autophosphorylation and activation of Akt (pSer473) were increased by nearly 50% with IL-6 depletion. In adipose tissue, insulin receptor signaling showed no significant change despite major reductions in STAT3 phosphorylation and haptoglobin expression. In diet-induced obese mice, depletion of IL-6 improved insulin responsiveness in 2-h insulin tolerance tests. In conclusion, these results indicate that IL-6 plays an important and selective role in hepatic insulin resistance of obesity.

Hepatocytes: critical for glucose homeostasis.

Maintaining blood glucose levels within a narrow range is a critical physiological function requiring multiple metabolic pathways and involving several cell types, including a prominent role for hepatocytes. Under hormonal control, hepatocytes can respond to either feeding or fasting conditions by storing or producing glucose as necessary. In the fasting state, the effects of glucagon avoid hypoglycemia by stimulating glucogenesis and glycogenolysis and initiating hepatic glucose release. Postprandially, insulin prevents hyperglycemia, in part, by suppressing hepatic gluconeogenesis and glycogenolysis and facilitating hepatic glycogen synthesis. Both transcriptional regulation of rate limiting enzymes and modulation of enzyme activity through phosphorylation and allosteric regulation are involved. Type 2 diabetes mellitus is the most common serious metabolic condition in the world, and results from a subnormal response of tissues to insulin (insulin resistance) and a failure of the insulin-secreting beta cells to compensate. In type 2 diabetes, glucose is overproduced by the hepatocyte and is ineffectively metabolized by other organs. Impairments in the insulin signal transduction pathway appear to be critical lesions contributing to insulin resistance and type 2 diabetes.

Chronic exposure to interleukin-6 causes hepatic insulin resistance in mice.

Interleukin (IL)-6 is one of several proinflammatory cytokines associated with the insulin resistance of obesity and type 2 diabetes. There is, however, little direct evidence in vivo for a causative role of IL-6 in insulin resistance. Here, a 5-day constant subcutaneous infusion of hIL-6 before portal vein insulin challenge resulted in impairment of early insulin receptor signaling in the liver of mice. Importantly, the sixfold elevation of IL-6 attained with constant infusion was similar to levels reached in obesity. Consistent with an hepatic response to IL-6, STAT3 phosphorylation was increased in livers of IL-6-treated mice at 5 days. Chronic infusion of IL-6 also reduced hepatic insulin receptor autophosphorylation by 60% and tyrosine phosphorylation of insulin receptor substrates-1 and -2 by 60 and 40%, respectively. IL-6 had no effect on the mass of these proteins. IL-6 also decreased refeeding-dependent glucokinase mRNA induction by approximately 40%. Insulin tolerance tests revealed reduced insulin sensitivity. In contrast to hepatic insulin receptor signal transduction, 5-day IL-6 exposure failed to suppress skeletal muscle insulin receptor signal transduction. These data suggest that chronic IL-6 treatment selectively impairs hepatic insulin signaling in vivo, further supporting a role for IL-6 in hepatic insulin resistance of obesity.

Suppressor of cytokine signaling-3 (SOCS-3), a potential mediator of interleukin-6-dependent insulin resistance in hepatocytes.

Interleukin-6 (IL-6) is one of several pro-inflammatory cytokines implicated in insulin resistance during infection, cachexia, and obesity. We recently demonstrated that IL-6 inhibits insulin signaling in hepatocytes (Senn, J. J., Klover, P. J., Nowak, I. A., and Mooney, R. A. (2002) Diabetes 51, 3391-3399). Members of the suppressors of cytokine signaling (SOCS) family associate with the insulin receptor (IR), and their ectopic expression inhibits IR signaling. Since several SOCS proteins are induced by IL-6, a working hypothesis is that IL-6-dependent insulin resistance is mediated, at least in part, by induction of SOCS protein(s) in insulin target cells. To examine the involvement of SOCS protein(s) in IL-6-dependent inhibition of insulin receptor signaling, HepG2 cells were treated with IL-6 (20 ng/ml) for periods from 1 min to 8 h. IL-6 induced SOCS-3 transcript at 30 min with a maximum effect at 1 h. SOCS-3 protein levels were also markedly elevated at 1 h. Transcript and protein levels returned to near basal levels by 2 h. SOCS-3 induction by IL-6 paralleled IL-6-dependent inhibition of IR signal transduction. Ectopically expressed SOCS-3 associated with the IR and suppressed insulin-dependent receptor autophosphorylation, insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, association of IRS-1 with the p85 subunit of phosphatidylinositol 3-kinase, and activation of Akt. SOCS-3 was also a direct inhibitor of insulin receptor autophosphorylation in vitro. In mice exposed to IL-6 for 60-90 min, hepatic SOCS-3 expression was increased. This was associated with inhibition of hepatic insulin-dependent receptor autophosphorylation and IRS-1 tyrosine phosphorylation. These data suggest that induction of SOCS-3 in liver may be an important mechanism of IL-6-mediated insulin resistance.

Interleukin-6 induces cellular insulin resistance in hepatocytes.

Interleukin (IL)-6 is one of several proinflammatory cytokines that have been associated with insulin resistance and type 2 diabetes. A two- to threefold elevation of circulating IL-6 has been observed in these conditions. Nonetheless, little evidence supports a direct role for IL-6 in mediating insulin resistance. Here, we present data that IL-6 can inhibit insulin receptor (IR) signal transduction and insulin action in both primary mouse hepatocytes and the human hepatocarcinoma cell line, HepG2. This inhibition depends on duration of IL-6 exposure, with a maximum effect at 1-1.5 h of pretreatment with IL-6 in both HepG2 cells and primary hepatocytes. The IL-6 effect is characterized by a decreased tyrosine phosphorylation of IR substrate (IRS)-1 and decreased association of the p85 subunit of phosphatidylinositol 3-kinase with IRS-1 in response to physiologic insulin levels. In addition, insulin-dependent activation of Akt, important in mediating insulin's downstream metabolic actions, is markedly inhibited by IL-6 treatment. Finally, a 1.5-h preincubation of primary hepatocytes with IL-6 inhibits insulin-induced glycogen synthesis by 75%. These data suggest that IL-6 plays a direct role in insulin resistance at the cellular level in both primary hepatocytes and HepG2 cell lines and may contribute to insulin resistance and type 2 diabetes.