Obesity-associated increases in adipose tissue (AT) CD11c+ cells suggest that dendritic cells (DC), which are involved in the tissue recruitment and activation of macrophages, may play a role in determining AT and liver immunophenotype in obesity. This study addressed this hypothesis. With the use of flow cytometry, electron microscopy, and loss-and-gain of function approaches, the contribution of DC to the pattern of immune cell alterations and recruitment in obesity was assessed. In AT and liver there was a substantial, high-fat diet (HFD)–induced increase in DC. In AT, these increases were associated with crown-like structures, whereas in liver the increase in DC constituted an early and reversible response to diet. Notably, mice lacking DC had reduced AT and liver macrophages, whereas DC replacement in DC-null mice increased liver and AT macrophage populations. Furthermore, delivery of bone marrow–derived DC to lean wild-type mice increased AT and liver macrophage infiltration. Finally, mice lacking DC were resistant to the weight gain and metabolic abnormalities of an HFD. Together, these data demonstrate that DC are elevated in obesity, promote macrophage infiltration of AT and liver, contribute to the determination of tissue immunophenotype, and play a role in systemic metabolic responses to an HFD.

Stefanovic-Racic M, Perdomo G, Mantell BS, Sipula IJ, Brown NF, O'Doherty RM. A moderate increase in carnitine palmitoyltransferase 1a activity is sufficient to substantially reduce hepatic triglyceride levels. Am J Physiol Endocrinol Metab 294: E969-E977, 2008. First published March 18, 2008 doi:10.1152/ajpendo.00497.2007.-Nonalcoholic fatty liver disease (NAFLD), hypertriglyceridemia, and elevated free fatty acids are present in the majority of patients with metabolic syndrome and type 2 diabetes mellitus and are strongly associated with hepatic insulin resistance. In the current study, we tested the hypothesis that an increased rate of fatty acid oxidation in liver would prevent the potentially harmful effects of fatty acid elevation, including hepatic triglyceride (TG) accumulation and elevated TG secretion. Primary rat hepatocytes were transduced with adenovirus encoding carnitine palmitoyltransferase 1a (Adv-CPT-1a) or control adenoviruses encoding either ␤-galactosidase (Adv-␤-gal) or carnitine palmitoyltransferase 2 (Adv-CPT-2). Overexpression of CPT-1a increased the rate of ␤-oxidation and ketogenesis by ϳ70%, whereas esterification of exogenous fatty acids and de novo lipogenesis were unchanged. Importantly, CPT-1a overexpression was accompanied by a 35% reduction in TG accumulation and a 60% decrease in TG secretion by hepatocytes. There were no changes in secretion of apolipoprotein B (apoB), suggesting the synthesis of smaller, less atherogenic VLDL particles. To evaluate the effect of increasing hepatic CPT-1a activity in vivo, we injected lean or obese male rats with Adv-CPT-1a, Adv-␤-gal, or Adv-CPT-2. Hepatic CPT-1a activity was increased by ϳ46%, and the rate of fatty acid oxidation was increased by ϳ44% in lean and ϳ36% in obese CPT-1a-overexpressing animals compared with Adv-CPT-2-or Adv-␤-gal-treated rats. Similar to observations in vitro, liver TG content was reduced by ϳ37% (lean) and ϳ69% (obese) by this in vivo intervention. We conclude that a moderate stimulation of fatty acid oxidation achieved by an increase in CPT-1a activity is sufficient to substantially reduce hepatic TG accumulation both in vitro and in vivo. Therefore, interventions that increase CPT-1a activity could have potential benefits in the treatment of NAFLD. fatty liver NONALCOHOLIC FATTY LIVER DISEASE (NAFLD) represents a spectrum of liver abnormalities with an estimated prevalence between 14 and 24% in the general population (4). Furthermore, NAFLD is present in the majority of patients with metabolic syndrome and type 2 diabetes mellitus (DM) (29) and is strongly associated with insulin resistance (38) characterized by the inability of insulin to control hepatic gluconeogenesis (28). Although most patients with NAFLD have asymptomatic fatty liver, the condition predisposes toward the development of steatohepatitis, cirrhosis, and hepatocellular carcinoma. Furthermore, hepatic lipid accumulation is associated with a more atherogenic lipid profile (25), including hypertriglyceridemia, a higher plasma concentration of...

The pathophysiology underlying mitochondrial dysfunction in insulin-resistant skeletal muscle is incompletely characterized. To further delineate this we investigated the interaction between insulin signaling, mitochondrial regulation, and function in C2C12 myotubes and in skeletal muscle. In myotubes elevated insulin and glucose disrupt insulin signaling, mitochondrial biogenesis, and mitochondrial bioenergetics. The insulin-sensitizing thiazolidinedione pioglitazone restores these perturbations in parallel with induction of the mitochondrial biogenesis regulator PGC-1␣. Overexpression of PGC-1␣ rescues insulin signaling and mitochondrial bioenergetics, and its silencing concordantly disrupts insulin signaling and mitochondrial bioenergetics. In primary skeletal myoblasts pioglitazone also up-regulates PGC-1␣ expression and restores the insulin-resistant mitochondrial bioenergetic profile. In parallel, pioglitazone upregulates PGC-1␣ in db/db mouse skeletal muscle. Interestingly, the small interfering RNA knockdown of the insulin receptor in C2C12 myotubes down-regulates PGC-1␣ and attenuates mitochondrial bioenergetics. Concordantly, mitochondrial bioenergetics are blunted in insulin receptor knock-out mouse-derived skeletal myoblasts. Taken together these data demonstrate that elevated glucose and insulin impairs and pioglitazone restores skeletal myotube insulin signaling, mitochondrial regulation, and bioenergetics. Pioglitazone functions in part via the induction of PGC-1␣. Moreover, PGC-1␣ is identified as a bidirectional regulatory link integrating insulin-signaling and mitochondrial homeostasis in skeletal muscle.Understanding the pathophysiology initiating the development of insulin resistance should augment our capacity to identify novel therapeutic targets for the prevention and treatment of type 2 diabetes. This biology remains incompletely characterized in part due to the complexity of the interaction of multiple organ systems and the multiplicity of intracellular perturbations within these organs governing the development of insulin resistance. The major peripheral organ systems implicated in insulin resistance include skeletal muscle, adipose tissue, liver, and the immune system. The enhancement of our understanding of this biology will require the combination of reductionist and systems biological approaches.The complexity of the effects of insulin resistance in a single organ is exemplified in skeletal muscle where disruption in glucose uptake (1), insulin signaling (2, 3), glycogen synthesis (4) and in mitochondrial biology (5-7) are evident in insulin-resistant subjects up to two decades before their developing diabetes. A fundamental question arising is whether disruption of skeletal muscle mitochondria is a primary component in this disease pathophysiology or whether it is a consequence of reduced aerobic activity in response to alternate metabolic perturbations associated with insulin resistance and diabetes (8). Although this question has not been definitively answered, increasing mitocho...

The contribution of natural killer T (NKT) cells to the pathogenesis of metabolic abnormalities of obesity is controversial. While the combined genetic deletion of NKT and CD8+ T-cells improves glucose tolerance and reduces inflammation, interpretation of these data have been complicated by the recent observation that the deletion of CD8+ T-cells alone reduces obesity-induced inflammation and metabolic dysregulation, leaving the issue of the metabolic effects of NKT cell depletion unresolved. To address this question, CD1d null mice (CD1d−/−), which lack NKT cells but have a full complement of CD8+ T-cells, and littermate wild type controls (WT) on a pure C57BL/6J background were exposed to a high fat diet, and glucose intolerance, insulin resistance, dyslipidemia, inflammation, and obesity were assessed. Food intake (15.5±4.3 vs 15.3±1.8 kcal/mouse/day), weight gain (21.8±1.8 vs 22.8±1.4 g) and fat mass (18.6±1.9 vs 19.5±2.1 g) were similar in CD1d−/− and WT, respectively. As would be expected from these data, metabolic rate (3.0±0.1 vs 2.9±0.2 ml O2/g/h) and activity (21.6±4.3 vs 18.5±2.6 beam breaks/min) were unchanged by NKT cell depletion. Furthermore, the degree of insulin resistance, glucose intolerance, liver steatosis, and adipose and liver inflammatory marker expression (TNFα, IL-6, IL-10, IFN-γ, MCP-1, MIP1α) induced by high fat feeding in CD1d−/− were not different from WT. We conclude that deletion of NKT cells, in the absence of alterations in the CD8+ T-cell population, is insufficient to protect against the development of the metabolic abnormalities of diet-induced obesity.

Shulman GI, Jurczak MJ. Reduced intestinal lipid absorption and body weight-independent improvements in insulin sensitivity in high-fat diet-fed Park2 knockout mice. Am J Physiol Endocrinol Metab 311: E105-E116, 2016. First published May 10, 2016; doi:10.1152/ajpendo.00042.2016.-Mitochondrial dysfunction is associated with many human diseases and results from mismatch of damage and repair over the life of the organelle. PARK2 is a ubiquitin E3 ligase that regulates mitophagy, a repair mechanism that selectively degrades damaged mitochondria. Deletion of PARK2 in multiple in vivo models results in susceptibility to stress-induced mitochondrial and cellular dysfunction. Surprisingly, Park2 knockout (KO) mice are protected from nutritional stress and do not develop obesity, hepatic steatosis or insulin resistance when fed a high-fat diet (HFD). However, these phenomena are casually related and the physiological basis for this phenotype is unknown. We therefore undertook a series of acute HFD studies to more completely understand the physiology of Park2 KO during nutritional stress. We find that intestinal lipid absorption is impaired in Park2 KO mice as evidenced by increased fecal lipids and reduced plasma triglycerides after intragastric fat challenge. Park2 KO mice developed hepatic steatosis in response to intravenous lipid infusion as well as during incubation of primary hepatocytes with fatty acids, suggesting that hepatic protection from nutritional stress was secondary to changes in energy balance due to altered intestinal triglyceride absorption. Park2 KO mice showed reduced adiposity after 1-wk HFD, as well as improved hepatic and peripheral insulin sensitivity. These studies suggest that changes in intestinal lipid absorption may play a primary role in protection from nutritional stress in Park2 KO mice by preventing HFD-induced weight gain and highlight the need for tissue-specific models to address the role of PARK2 during metabolic stress. lipid absorption; mitophagy; PARK2; insulin resistance; liver; small intestine; obesity PARK2 IS A UBIQUITIN E3 LIGASE that was first identified in patients with autosomal recessive juvenile Parkinsonism (ARJP) and has since emerged as one of the most frequently mutated genes in this disorder (25,31). PARK2 is comprised of an NH 2 -terminal ubiquitin-like domain, a novel PARK2-specific domain, and two RING-finger domains separated by an inbetween RING-finger (IBR) domain in the COOH terminus (25, 32). PARK2 mutations resulting in ARJP typically lead to loss of E3 ligase activity and commonly occur within the COOH-terminal RING-finger or IBR domains (9). PARK2 acts as a multifunctional E3 ligase in that it cooperates with several E2 conjugating enzymes, such as UBCH7, UBCH8, and UBCH13/UEV1, and catalyzes mono-and polyubiquitination with Lys 48 and Lys 63 linkages, indicating proteasome-dependent and -independent effects of PARK2-mediated ubiquitination (9,11,17,41,53). Multiple PARK2 substrates have been reported, but it still remains unclear how loss of PARK2 E3 li...