Hyokjoon Kwon

Loss of autophagy in hypothalamic POMC neurons impairs lipolysis

Autophagy degrades cytoplasmic contents to achieve cellular homeostasis. We show that selective loss of autophagy in hypothalamic proopiomelanocortin (POMC) neurons decreases α‐melanocyte‐stimulating hormone (MSH) levels, promoting adiposity, impairing lipolysis and altering glucose homeostasis. Ageing reduces hypothalamic autophagy and α‐MSH levels, and aged‐mice phenocopy, the adiposity and lipolytic defect observed in POMC neuron autophagy‐null mice. Intraperitoneal isoproterenol restores lipolysis in both models, demonstrating normal adipocyte catecholamine responsiveness. We propose that an unconventional, autophagosome‐mediated form of secretion in POMC neurons controls energy balance by regulating α‐MSH production. Modulating hypothalamic autophagy might have implications for preventing obesity and metabolic syndrome of ageing.

IL-18 Induces Monocyte Chemotactic Protein-1 Production in Macrophages through the Phosphatidylinositol 3-Kinase/Akt and MEK/ERK1/2 Pathways

Macrophages are activated during an inflammatory response and produce multiple inflammatory cytokines. IL-18 is one of the most important innate cytokines produced from macrophages in the early stages of the inflammatory immune response. Monocyte chemoattractant protein (MCP-1) is expressed in many inflammatory diseases such as multiple sclerosis and rheumatoid arthritis, and its expression is correlated with the severity of the disease. Both IL-18 and MCP-1 have been shown to be involved in inflammatory immune responses. However, it has been unclear whether IL-18 is involved in the induction of MCP-1. This investigation was initiated to determine whether IL-18 can induce MCP-1 production, and if so, by which signal transduction pathways. We found that IL-18 induced the production of MCP-1 in macrophages, which was IL-12-independent and was not mediated by autocrine cytokines such as IFN-γ or TNF-α. We then examined signal transduction pathways involved in IL-18-induced MCP-1 production. We found that IL-18 did not activate the IκB kinase/NF-κB pathway, evidenced by no degradation of IκBα and no translocation of NF-κB p65 to the nucleus in IL-18-stimulated macrophages. Instead, IL-18 activated the PI3K/Akt and MEK/ERK1/2 pathways. Inhibition of either of these pathways attenuated MCP-1 production in macrophages, and inhibition of both signaling pathways resulted in the complete inhibition of MCP-1 production. On the basis of these observations, we conclude that IL-18 induces MCP-1 production through the PI3K/Akt and MEK/ERK1/2 pathways in macrophages.

High-Fat Diet–Induced Adipocyte Cell Death Occurs Through a Cyclophilin D Intrinsic Signaling Pathway Independent of Adipose Tissue Inflammation

OBJECTIVE Previous studies have demonstrated that mice fed a high-fat diet (HFD) develop insulin resistance with proinflammatory macrophage infiltration into white adipose tissue. Concomitantly, adipocytes undergo programmed cell death with the loss of the adipocyte-specific lipid droplet protein perilipin, and the dead/dying adipocytes are surrounded by macrophages that are organized into crown-like structures. This study investigated whether adipocyte cell death provides the driving signal for macrophage inflammation or if inflammation induces adipocyte cell death. RESEARCH DESIGN AND METHODS Two knockout mouse models were used: granulocyte/monocyte-colony stimulating factor (GM-CSF)–null mice that are protected against HFD-induced adipose tissue inflammation and cyclophilin D (CyP-D)–null mice that are protected against adipocyte cell death. Mice were fed for 4–14 weeks with a 60% HFD, and different markers of cell death and inflammation were analyzed. RESULTS HFD induced a normal extent of adipocyte cell death in GM-CSF–null mice, despite a marked reduction in adipose tissue inflammation. Similarly, depletion of macrophages by clodronate treatment prevented HFD-induced adipose tissue inflammation without any affect on adipocyte cell death. However, CyP-D deficiency strongly protected adipocytes from HFD-induced cell death, without affecting adipose tissue inflammation. CONCLUSIONS These data demonstrate that HFD-induced adipocyte cell death is an intrinsic cellular response that is CyP-D dependent but is independent of macrophage infiltration/activation.

Adipocyte-Specific IKKβ Signaling Suppresses Adipose Tissue Inflammation through an IL-13-Dependent Paracrine Feedback Pathway

SUMMARY Adipose tissue inflammation is one pathway shown to mediate insulin resistance in obese humans and rodents. Obesity induces dynamic cellular changes in adipose tissue to increase proinflammatory cytokines and diminish anti-inflammatory cytokines. However, we have found that anti-inflammatory interleukin-13 (IL-13) is unexpectedly induced in adipose tissue of obese humans and high-fat diet (HFD)-fed mice, and the source of IL-13 is primarily the adipocyte. Moreover, HFD-induced proinflammatory cytokines such as tumor necrosis factor alpha (TNF-α) and IL-1β mediate IL-13 production in adipocytes in an IKKβ-dependent manner. In contrast, adipocyte-specific IKKβ-deficient mice show diminished IL-13 expression and enhanced inflammation after HFD feeding, resulting in a worsening of the insulin-resistant state. Together these data demonstrate that although IKKβ activates the expression of proinflammatory mediators, in adipocytes, IKKβ signaling also induces the expression of the anti-inflammatory cytokine IL-13, which plays a unique protective role by limiting adipose tissue inflammation and insulin resistance.

Role of Hck in the pathogenesis of encephalomyocarditis virus-induced diabetes in mice.

ABSTRACT Soluble mediators such as interleukin-1β, tumor necrosis factor alpha (TNF-α), and inducible nitric oxide synthase (iNOS) produced from activated macrophages play an important role in the destruction of pancreatic β cells in mice infected with a low dose of the D variant of encephalomyocarditis (EMC-D) virus. The tyrosine kinase signaling pathway was shown to be involved in EMC-D virus-induced activation of macrophages. This investigation was initiated to determine whether the Src family of kinases plays a role in the activation of macrophages, subsequently resulting in the destruction of β cells, in mice infected with a low dose of EMC-D virus. We examined the activation of p59/p56Hck, p55Fgr, and p56/p53Lynin macrophages from DBA/2 mice infected with the virus. We found that p59/p56Hck showed a marked increase in both autophosphorylation and kinase activity at 48 h after infection, whereas p55Fgr and p56/p53Lyn did not. The p59/p56Hck activity was closely correlated with the tyrosine phosphorylation level of Vav. Treatment of EMC-D virus-infected mice with the Src kinase inhibitor, PP2, resulted in the inhibition of p59/p56Hck activity and almost complete inhibition of the production of TNF-α and iNOS in macrophages and the subsequent prevention of diabetes in mice. On the basis of these observations, we conclude that the Src kinase, p59/p56Hck, plays an important role in the activation of macrophages and the subsequent production of TNF-α and nitric oxide, leading to the destruction of pancreatic β cells, which results in the development of diabetes in mice infected with a low dose of EMC-D virus.

Fyn deficiency promotes a preferential increase in subcutaneous adipose tissue mass and decreased visceral adipose tissue inflammation.

Previous studies have demonstrated that Fyn knockout (FynKO) mice on a standard chow diet display increased glucose clearance and whole-body insulin sensitivity associated with decreased adiposity resulting from increased fatty acid use and energy expenditure. Surprisingly, however, despite a similar extent of adipose tissue (AT) mass accumulation on a high-fat diet, the FynKO mice remained fully glucose tolerant and insulin sensitive. Physiologic analyses demonstrated that the FynKO mice had a combination of skewed AT expansion into the subcutaneous compartment rather than to the visceral depot, reduced AT inflammation associated with reduced T-cell and macrophage infiltration, and increased proportion of anti-inflammatory M2 macrophages. These data demonstrate that Fyn is an important regulator of whole-body integrative metabolism that coordinates AT expansion, inflammation, and insulin sensitivity in states of nutrient excess. These data further suggest that inhibition of Fyn function may provide a novel target to prevent AT inflammation, insulin resistance, and the dyslipidemia components of the metabolic syndrome.

Role of CTLA-4 in the Activation of Single- and Double-Positive Thymocytes

CTLA-4, a homologue of CD28, is a negative regulator of T cell activation in the periphery and is transiently expressed on the cell surface after T cell activation. However, the role of CTLA-4 in T cell activation in the thymus is not clear. This investigation was initiated to determine the role of CTLA-4 in the activation of CD4+CD8+ double-positive (DP) and CD4+CD8− and CD4−CD8+ single-positive (SP) thymocytes using fetal thymic organ cultures (FTOC) of MHC class II-restricted, OVA323–339-restricted TCR transgenic mice (DO11.10). We found that treatment of the FTOC with anti-CTLA-4-blocking Ab during activation with OVA323–339 increased the proportion and number of DP thymocytes, but decreased the proportion and number of SP thymocytes compared with OVA323–339-stimulated FTOC without anti-CTLA-4 Ab treatment. In addition, anti-CTLA-4 Ab treatment inhibited OVA323–339-induced expression of the early activation marker, CD69, in DP thymocytes, but increased CD69 in SP thymocytes. Similarly, CTLA-4 blockage decreased phosphorylation of ERK in DP thymocytes by Ag-specific TCR engagement, but increased phosphorylation of ERK in SP thymocytes. CTLA-4 blockage inhibited deletion of DP thymocytes treated with a high dose of OVA323–339, whereas CTLA-4 blockage did not inhibit deletion of DP thymocytes treated with a low dose of OVA323–339. We conclude that CTLA-4 positively regulates the activation of DP thymocytes, resulting in their deletion, whereas blocking CTLA-4 suppresses the activation of DP thymocytes, leading to inhibition of DP thymocyte deletion. In contrast, CTLA-4 negatively regulates the activation of SP thymocytes.

How Does High-Fat Diet Induce Adipose Tissue Fibrosis?

Obesity is one of the most serious pandemic health problems in modern society and the predisposing factor for the type 2 diabetes mellitus. Chronic low-grade inflammation mediates the pathogenesis of insulin resistance in obese humans and rodents, and white adipose tissue is one of major tissues to modulate inflammation. Obese humans and rodents show dynamic changes of immunocellular compositions in white adipose tissue to induce inflammatory responses. Innate and adaptive immune responses mainly mediated by macrophages and T cells contribute insulin resistance. Recently, it has been shown that adipose tissue fibrosis is also enhanced in obese humans and rodents along with inflammatory responses, and suppression of adipose tissue fibrosis shows improved insulin sensitivity in rodent models, suggesting that adipose tissue fibrosis is involved in insulin resistance.

Adipokines, Inflammation, and Insulin Resistance in Obesity

Humans have highly integrated system to regulate energy storage and expenditure. Adipose tissue is a major depot to store triglycerides during energy excess and release fatty acids and glycerol for systemic energy need. However, adipose tissues have also been shown as highly active endocrine and metabolically important organs to modulate energy expenditure and glucose homeostasis. Brown adipose tissue plays an essential role in non-shivering thermogenesis and in energy dissipation that can serve to protect against obesity. White adipose tissue, referred as either subcutaneous or visceral adipose tissue, has been shown to secret an array of molecules, termed adipokines. These adipokines function as circulating hormones to communicate with other organs including the brain, liver, muscle, immune system, and adipose tissue itself, resulting in the regulation of glucose homeostasis. The dysregulation of adipokines has been implicated in obesity, type 2 diabetes, and cardiovascular disease. Recently, inflammatory responses in adipose tissue have also been shown as one of the major mechanisms to induce peripheral tissue glucose intolerance and insulin resistance. Although leptin and adiponectin regulate feeding behavior and energy expenditure, these adipokines are also involved in the regulation of obesity-induced inflammation. Adipose tissue secretes various pro- and anti-inflammatory adipokines to modulate inflammation and insulin resistance. In obese humans and rodent models, the expression of pro-inflammatory adipokines is enhanced and can directly result in insulin resistance. Collectively, these findings have suggested that obesity-induced insulin resistance may result, at least in part, from an imbalance in the production of pro- and anti-inflammatory adipokines at adipose tissues. Thus, we will describe the recent progress regarding the physiological and molecular function of adipokines in the obesity-induced inflammation and insulin resistance.