Xinxia Zhu

Systemic inflammation and insulin sensitivity in obese IFN-γ knockout mice

Adipose tissue macrophages are important mediators of inflammation and insulin resistance in obesity. IFN-γ is a central regulator of macrophage function. The role of IFN-γ in regulating systemic inflammation and insulin resistance in obesity is unknown. We studied obese IFN-γ knockout mice to identify the role of IFN-γ in regulating inflammation and insulin sensitivity in obesity. IFN-γ-knockout C57Bl/6 mice and wild-type control litter mates were maintained on normal chow or a high fat diet for 13 weeks and then underwent insulin sensitivity testing then sacrifice and tissue collection. Flow cytometry, intracellular cytokine staining, and QRTPCR were used to define tissue lymphocyte phenotype and cytokine expression profiles. Adipocyte size was determined from whole adipose tissue explants examined under immunofluorescence microscopy. Diet-induced obesity induced systemic inflammation and insulin resistance, along with a pan-leukocyte adipose tissue infiltrate that includes macrophages, T-cells, and NK cells. Obese IFN-γ-knockout animals, compared with obese wild-type control animals, demonstrate modest improvements in insulin sensitivity, decreased adipocyte size, and an M2-shift in ATM phenotype and cytokine expression. These data suggest a role for IFN-γ in the regulation of inflammation and glucose homeostasis in obesity though multiple potential mechanisms, including effects on adipogenesis, cytokine expression, and macrophage phenotype.

Central nervous system inflammation induces muscle atrophy via activation of the hypothalamic–pituitary–adrenal axis

Systemic and CNS-delimited inflammation triggers skeletal muscle catabolism in a manner dependent on glucocorticoid signaling.

Inflammation-induced lethargy is mediated by suppression of orexin neuron activity

In response to illness, animals subvert normal homeostasis and divert their energy utilization to fight infection. An important and unexplored feature of this response is the suppression of physical activity and foraging behavior in the setting of negative energy balance. Inflammatory signaling in the hypothalamus mediates the febrile and anorectic responses to disease, but the mechanism by which locomotor activity (LMA) is suppressed has not been described. Lateral hypothalamic orexin (Ox) neurons link energy status with LMA, and deficiencies in Ox signaling lead to hypoactivity and hypophagia. In the present work, we examine the effect of endotoxin-induced inflammation on Ox neuron biology and LMA in rats. Our results demonstrate a vital role for diminished Ox signaling in mediating inflammation-induced lethargy. This work defines a specific population of inflammation-sensitive, arousal-associated Ox neurons and identifies a proximal neural target for inflammatory signaling to Ox neurons, while eliminating several others.

Regulation of Agouti-Related Protein Messenger Ribonucleic Acid Transcription and Peptide Secretion by Acute and Chronic Inflammation

Agouti-related protein (AgRP) is an orexigenic neuropeptide produced by neurons in the hypothalamic arcuate nucleus (ARC) that is a key component of central neural circuits that control food intake and energy expenditure. Disorders in energy homeostasis, characterized by hypophagia and increased metabolic rate, frequently develop in animals with either acute or chronic diseases. Recently, studies have demonstrated that proopiomelanocortin-expressing neurons in the ARC are activated by the proinflammatory cytokine IL-1beta. In the current study, we sought to determine whether inflammatory processes regulate the expression of AgRP mRNA and to characterize the response of AgRP neurons to IL-1beta. Here, we show by real-time RT-PCR and in situ hybridization analysis that AgRP mRNA expression in rodents is increased in models of acute and chronic inflammation. AgRP neurons were found to express the type I IL-1 receptor, and the percentage of expression was significantly increased after peripheral administration of lipopolysaccharide. Furthermore, we demonstrate that IL-1beta inhibits the release of AgRP from hypothalamic explants. Collectively, these data indicate that proinflammatory signals decrease the secretion of AgRP while increasing the transcription of the AgRP gene. These observations suggest that AgRP neurons may participate with ARC proopiomelanocortin neurons in mediating the anorexic and metabolic responses to acute and chronic disease processes.

Combined effects of ghrelin and higher food intake enhance skeletal muscle mitochondrial oxidative capacity and AKT phosphorylation in rats with chronic kidney disease

Skeletal muscle mitochondrial dysfunction and insulin resistance occur in chronic kidney disease. Ghrelin is a gastric hormone previously shown to enhance muscle mitochondrial enzyme activities and AKT-mediated insulin signaling independent of food intake in healthy rats. Here we determined the impact of ghrelin treatment on anorexia, skeletal muscle mitochondrial oxidative capacity, AKT phosphorylation as a measure of insulin signaling, and lean body mass in a rat model of chronic kidney disease. Ghrelin infusion promoted higher food intake and lean body mass. Further, although muscle mitochondrial enzyme activities were low in the rats with CKD (chronic kidney disease), they normalized with ghrelin treatment, a change that was consistent with the increase in the transcript levels of regulators of mitochondrial biogenesis and lipid metabolism. This was associated with a lower muscle triglyceride content and higher AKT phosphorylation. Pair-feeding showed that mitochondrial effects of ghrelin are independent of changes in food intake, whereas combined ghrelin treatment and higher food intake were needed to enhance AKT phosphorylation. Thus, ghrelin-induced muscle mitochondrial changes and lower tissue triglycerides could favor insulin action and muscle anabolism in the presence of improvement in food intake. Our study shows that combined effects of ghrelin on appetite and muscle mitochondria improve muscle metabolic and nutritional alterations in chronic kidney disease. This could have potential beneficial impact on patient morbidity and survival.

Genetic and Pharmacologic Blockade of Central Melanocortin Signaling Attenuates Cardiac Cachexia in Rodent Models of Heart Failure

The central melanocortin system plays a key role in the regulation of food intake and energy homeostasis. We investigated whether genetic or pharmacologic blockade of central melanocortin signaling attenuates cardiac cachexia in mice and rats with heart failure. Permanent ligation of the left coronary artery (myocardial infarction (MI)) or sham operation was performed in wild-type (WT) or melanocortin-4 receptor (MC4R) knockout mice. Eight weeks after surgery, WT-Sham mice had significant increases in lean body mass (LBM; P<0·05) and fat mass (P<0·05), whereas WT-MI did not gain significant amounts of LBM or fat mass. Resting basal metabolic rate (BMR) was significantly lower in WT-Sham mice compared to WT-MI mice (P<0·001). In contrast, both MC4-Sham and MC4-MI mice gained significant amounts of LBM (P<0·05) and fat mass (P<0·05) over the study period. There was no significant difference in the BMR between MC4-Sham and MC4-MI mice. In the second experiment, rats received aortic bands or sham operations, and after recovery received i.c.v. injections of either artificial cerebrospinal fluid (aCSF) or the melanocortin antagonist agouti-related protein (AGRP) for 2 weeks. Banded rats receiving AGRP gained significant amount of LBM (P<0·05) and fat mass (P<0·05) over the treatment period, whereas banded rats receiving aCSF did not gain significant amounts of LBM or fat mass. These results demonstrated that genetic and pharmacologic blockade of melanocortin signaling attenuated the metabolic manifestations of cardiac cachexia in murine and rat models of heart failure.

Arcuate nucleus proopiomelanocortin neurons mediate the acute anorectic actions of leukemia inhibitory factor via gp130

The proinflammatory cytokine leukemia inhibitory factor (LIF) is induced in disease states and is known to inhibit food intake when administered centrally. However, the neural pathways underlying this effect are not well understood. We demonstrate that LIF acutely inhibits food intake by directly activating pro-opiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus. We show that arcuate POMC neurons express the LIF-R, and that LIF stimulates the release of the anorexigenic peptide, alpha-MSH from ex vivo hypothalami. Transgenic mice lacking gp130, the signal transducing subunit of the LIF-R complex, specifically in POMC neurons fail to respond to LIF. Furthermore, LIF does not stimulate the release of alpha-MSH from the transgenic hypothalamic explants. These findings indicate that POMC neurons mediate the acute anorectic actions of central LIF administration and provide a mechanistic link between inflammation and food intake.

Biodegradable polymers based on bile acids and potential biomedical applications

Bile acids are natural compounds that play an important biological role in the body. They are candidates of choice as building blocks of biocompatible and degradable polymers. Bile acids have been incorporated into biodegradable polymeric structures as pendant moieties on natural polymers, such as dextran and chitosan, and as the core component of the backbone of synthetic polymers and dendrimers. In the two latter cases, linkage of the repeating units relies on anhydride, ester or amide bonds. Owing to the rigidity of the steroidal polycyclic backbone and the amphiphilic properties of bile acids, materials based on these compounds can display interesting tunable properties. Therefore, biodegradable polymers based on bile acids are of particular interest to the fields of drug delivery, where their amphiphilic properties can be used to encapsulate drugs, and tissue engineering, where their rigid steroidal backbone make them good candidates for controlling the mechanical properties of the materials.

Muscle atrophy in response to cytotoxic chemotherapy is dependent on intact glucocorticoid signaling in skeletal muscle

Cancer cachexia is a syndrome of weight loss that results from the selective depletion of skeletal muscle mass and contributes significantly to cancer morbidity and mortality. The driver of skeletal muscle atrophy in cancer cachexia is systemic inflammation arising from both the cancer and cancer treatment. While the importance of tumor derived inflammation is well described, the mechanism by which cytotoxic chemotherapy contributes to cancer cachexia is relatively unexplored. We found that the administration of chemotherapy to mice produces a rapid inflammatory response. This drives activation of the hypothalamic-pituitary-adrenal axis, which increases the circulating level of corticosterone, the predominant endogenous glucocorticoid in rodents. Additionally, chemotherapy administration results in a significant loss of skeletal muscle mass 18 hours after administration with a concurrent induction of genes involved with the ubiquitin proteasome and autophagy lysosome systems. However, in mice lacking glucocorticoid receptor expression in skeletal muscle, chemotherapy-induced muscle atrophy is completely blocked. This demonstrates that cytotoxic chemotherapy elicits significant muscle atrophy driven by the production of endogenous glucocorticoids. Further, it argues that pharmacotherapy targeting the glucocorticoid receptor, given in concert with chemotherapy, is a viable therapeutic strategy in the treatment of cancer cachexia.

Prostacyclin signaling regulates circulating ghrelin during acute inflammation

Ghrelin is an octanoylated 28 amino acid peptide predominantly secreted by the stomach, and has potent stimulatory effects on appetite. Several laboratories, including our own, have demonstrated that ghrelin levels fall in states of acute inflammation brought about by injection of bacterial lipopolysaccharide (LPS). We now demonstrate that the decrease in circulating ghrelin is not due to a decrease in ghrelin gene expression, but is instead likely to be due to an acute decrease in ghrelin secretion. Furthermore, we have found that the change in circulating ghrelin during acute inflammation required a prostaglandin second messenger, but did not require the synthesis of nitric oxide. Interestingly, i.v. injection of prostaglandin E(2) failed to decrease circulating ghrelin levels, whereas prostacyclin decreased circulating ghrelin to a similar extent as did LPS. We also provide anatomical evidence for the mechanism of the regulation of ghrelin by inflammation. We demonstrate that the type 1 interleukin-1beta (IL-1beta) receptor is expressed within the gastric mucosa, but is not expressed by ghrelin cells. The prostacyclin receptor was also expressed in the gastric mucosa, and the majority of ghrelin-producing cells were found to co-express this receptor. Mice with genetic deletion of the type 1 IL-1 receptor do not suppress circulating ghrelin levels with LPS administration. Collectively, these data support a model in which the mechanism of inflammation induced decreases in ghrelin are due to the action of IL-1beta on cells within the gastric mucosa that in turn produce prostacyclin as a second messenger. These data provide further support for the potential role of ghrelin as a therapeutic agent in acute and chronic inflammatory diseases.