The expression of adipogenic genes is decreased in obesity and diabetes mellitus

Obesity is strongly correlated with type 2 diabetes mellitus, a common disorder of glucose and lipid metabolism. Although adipocytes are critical in obesity, their role in diabetes has only recently been appreciated. We conducted studies by using DNA microarrays to identify differences in gene expression in adipose tissue from lean, obese, and obese-diabetic mice. The expression level of over 11,000 transcripts was analyzed, and 214 transcripts showed significant differences between lean and obese mice. Surprisingly, the expression of genes normally associated with adipocyte differentiation were down-regulated in obesity. Not all obese individuals will become diabetic; many remain normoglycemic despite profound obesity. Understanding the transition to obesity with concomitant diabetes will provide important clues to the pathogenesis of type 2 diabetes. Therefore, we examined the levels of gene expression in adipose tissue from five groups of obese mice with varying degrees of hyperglycemia, and we identified 88 genes whose expression strongly correlated with diabetes severity. This group included many genes that are known to be involved in signal transduction and energy metabolism as well as genes not previously examined in the context of diabetes. Our data show that a decrease in expression of genes normally involved in adipogenesis is associated with obesity, and we further identify genes important for subsequent development of type 2 diabetes mellitus.

Genetic obesity unmasks nonlinear interactions between murine type 2 diabetes susceptibility loci

Nonlinear interactions between obesity and genetic risk factors are thought to determine susceptibility to type 2 diabetes. We used genetic obesity as a tool to uncover latent differences in diabetes susceptibility between two mouse strains, C57BL/6J (B6) and BTBR. Although both BTBR and B6 lean mice are euglycemic and glucose tolerant, lean BTBR x B6 F1 male mice are profoundly insulin resistant. We hypothesized that the genetic determinants of the insulin resistance syndrome might also predispose genetically obese mice to severe diabetes. Introgressing the ob allele into BTBR revealed large differences in diabetes susceptibility between the strain backgrounds. In a population of F2-ob/ob mice segregating for BTBR and B6 alleles, we observed large variation in pancreatic compensation for the underlying insulin resistance. We also detected two loci that substantially modify diabetes severity, and a third locus that strongly links to fasting plasma insulin levels. Amplification of the genetic signal from these latent diabetes susceptibility alleles in F2-ob/ob mice permitted discovery of an interaction between the two loci that substantially increased the risk of severe type 2 diabetes.

Gene expression profiles of nondiabetic and diabetic obese mice suggest a role of hepatic lipogenic capacity in diabetes susceptibility

Obesity is a strong risk factor for the development of type 2 diabetes. We have previously reported that in adipose tissue of obese (ob/ob) mice, the expression of adipogenic genes is decreased. When made genetically obese, the BTBR mouse strain is diabetes susceptible and the C57BL/6J (B6) strain is diabetes resistant. We used DNA microarrays and RT-PCR to compare the gene expression in BTBR-ob/ob versus B6-ob/ob mice in adipose tissue, liver, skeletal muscle, and pancreatic islets. Our results show: 1) there is an increased expression of genes involved in inflammation in adipose tissue of diabetic mice; 2) lipogenic gene expression was lower in adipose tissue of diabetes-susceptible mice, and it continued to decrease with the development of diabetes, compared with diabetes-resistant obese mice; 3) hepatic expression of lipogenic enzymes was increased and the hepatic triglyceride content was greatly elevated in diabetes-resistant obese mice; 4) hepatic expression of gluconeogenic genes was suppressed at the prediabetic stage but not at the onset of diabetes; and 5) genes normally not expressed in skeletal muscle and pancreatic islets were expressed in these tissues in the diabetic mice. We propose that increased hepatic lipogenic capacity protects the B6-ob/ob mice from the development of type 2 diabetes.

Genetic and genomic studies of the BTBR ob/ob mouse model of type 2 diabetes

The BTBR mouse strain harbors alleles promoting insulin resistance. When made genetically obese (ob/ob), these mice develop severe type 2 diabetes (fasting glucose >400 mg/dL). By contrast, C57BL/6 ob/ob mice are able to compensate for the obesity-induced insulin resistance by increasing pancreatic insulin secretion and thus maintain only slightly elevated plasma glucose levels (<250 mg/dL). Islet insulin secretory responses to glucose are undiminished in the remaining islets of BTBR ob/ob mice. A genome-wide linkage analysis identified 3 major loci influencing plasma glucose and/or insulin levels in an F2ob/ob sample derived from the 2 strains. A locus on chromosome 2 affects insulin sensitivity and is independent of obesity. Loci on chromosomes 16 and 19 affect fasting glucose and insulin levels and likely affect beta-cell mass or function. Analysis of mRNA expression patterns revealed a reduction in lipogenic gene expression in adipose tissue associated with obesity. Conversely, hepatic lipogenic gene expression increases in obese mice, but to a much greater extent in the diabetes-resistant C57BL/6 strain. We propose that hepatic lipogenic capacity affects susceptibility to obesity-induced diabetes.

Please pass the chips: genomic insights into obesity and diabetes

Type 2 diabetes mellitus is an increasingly common disorder of carbohydrate and lipid metabolism. Approximately 16 million individuals in the United States have diabetes, and 800,000 new cases are identified each year. Two important characteristics of this disease are insulin resistance, the failure of peripheral tissues, including liver, muscle, and adipose tissue, to respond to physiologic doses of insulin, and failure of pancreatic beta-cells to properly secrete insulin in response to elevated blood glucose levels. Obesity is a significant risk factor for the development of type 2 diabetes mellitus. Recent observations of extremely lean, lipoatrophic models have revealed a similar predisposition to developing diabetes. Although it may seem paradoxical that both increased adiposity and severely reduced fat mass cause diabetes, a common pathophysiologic process in fat may be responsible for the predisposition to develop hyperglycemia in both conditions. This review will focus on the important role of adipose tissue dysfunction in the pathogenesis of diabetes, and on insights gained through the application of microarray technology to analyze adipocyte gene expression.

Shedding of Syndecan-1/CXCL1 Complexes by Matrix Metalloproteinase 7 Functions as an Epithelial Checkpoint of Neutrophil Activation

Although neutrophils play critical roles in innate immunity, in excess these cells cause severe tissue damage. Thus, neutrophil activation must be tightly regulated to prevent indiscriminant damage. Previously, we reported that mice lacking matrix metalloproteinase (MMP) 7 are protected from lung injury owing to markedly impaired neutrophil movement from the interstitium into mucosal lumenal spaces. This phenotype resulted from a lack of MMP7 shedding of syndecan-1, a heparan sulfate proteoglycan that carries the neutrophil chemokine CXCL1 as cargo. Here, we assessed if shedding syndecan-1/CXCL1 complexes affects neutrophil activation. Whereas injured monolayers of wild-type alveolar type II cells potently stimulated neutrophil activation, as gauged by release of myeloperoxidase, cells from Mmp7(-/-) or syndecan-1-null (Sdc1(-/-)) mice or human cells with MMP7 knockdown did not. In vivo, we observed reduced myeloperoxidase release relative to neutrophil numbers in bleomycin-injured Mmp7(-/-) and Sdc1(-/-) mice. Furthermore, we determined that soluble syndecan-1 directly stimulated neutrophil activation in the absence of cellular damage. These data indicate that MMP7 shedding of syndecan-1/CXCL1 complexes functions as a checkpoint that restricts neutrophil activation at sites of epithelial injury.

Dimension reduction for mapping mRNA abundance as quantitative traits

The advent of sophisticated genomic techniques for gene mapping and microarray analysis has provided opportunities to map mRNA abundance to quantitative trait loci (QTL) throughout the genome. Unfortunately, simple mapping of each individual mRNA trait on the scale of a typical microarray experiment is computationally intensive, subject to high sample variance, and therefore underpowered. However, this problem can be addressed by capitalizing on correlation among the large number of mRNA traits. We present a method to reduce the dimensionality for mapping gene expression data as quantitative traits. We used a blind method, principal components, and a sighted method, hierarchical clustering seeded by disease relevant traits, to define new traits composed of a small collection of promising mRNAs. We validated the principle of our approach by mapping the expression levels of metabolism genes in a population of F(2)-ob/ob mice derived from the BTBR and C57BL/6J strains. We found that lipogenic and gluconeogenic mRNAs, which are known targets of insulin action, were closely associated with the insulin trait. Multiple interval mapping and Bayesian interval mapping of this new trait revealed significant linkages to chromosome regions that were contained in loci associated with type 2 diabetes in this same mouse sample. As a further statistical refinement, we show that principal component analysis also effectively reduced dimensions for mapping phenotypes composed of mRNA abundances.

Normal Akt/PKB with reduced PI3K activation in insulin-resistant mice

Insulin stimulates muscle and adipose tissue to absorb glucose through a signaling cascade that is incompletely understood. Insulin resistance, the inability of insulin to appropriately stimulate glucose uptake, is a hallmark of type 2 diabetes mellitus. The development of experimental systems that model human insulin resistance is important in elucidating the defects responsible for the development of type 2 diabetes. When two strains of mice, BTBR and C57BL/6J (B6), are crossed, the resultant male offspring (BtB6) demonstrate insulin resistance in muscle tissue. Here, we report an insulin resistance phenotype in adipose tissue from lean, nondiabetic BtB6 mice similar to that observed in human muscle. Adipocytes isolated from insulin-resistant male mice display 65% less insulin-stimulated glucose uptake compared with insulin-sensitive female mice. Similarly, adipocytes from insulin-resistant mice have diminished insulin-stimulated IRS-1 phosphorylation and phosphatidylinositol 3-kinase (PI3K) activation. However, normal activation of protein kinase B (Akt/PKB) by insulin is observed. Thus BtB6 mice demonstrate the dissociation of insulin-stimulated PI3K activity and Akt/PKB activation and represent a useful model to investigate the causes of insulin resistance in humans.

Abdominal obesity in BTBR male mice is associated with peripheral but not hepatic insulin resistance

Insulin resistance is a common feature of obesity. BTBR mice have more fat mass than most other inbred mouse strains. On a chow diet, BTBR mice have elevated insulin levels relative to the C57BL/6J (B6) strain. Male F1 progeny of a B6 x BTBR cross are insulin resistant. Previously, we reported insulin resistance in isolated muscle and in isolated adipocytes in this strain. Whereas the muscle insulin resistance was observed only in male F1 mice, adipocyte insulin resistance was also present in male BTBR mice. We examined in vivo mechanisms of insulin resistance with the hyperinsulinemic euglycemic clamp technique. At 10 wk of age, BTBR and F1 mice had a >30% reduction in whole body glucose disposal primarily due to insulin resistance in heart, soleus muscle, and adipose tissue. The increased adipose tissue mass and decreased muscle mass in BTBR and F1 mice were negatively and positively correlated with whole body glucose disposal, respectively. Genes involved in focal adhesion, actin cytoskeleton, and inflammation were more highly expressed in BTBR and F1 than in B6 adipose tissue. The BTBR and F1 mice have higher levels of testosterone, which may be related to the pathological changes in adipose tissue that lead to systemic insulin resistance. Despite profound peripheral insulin resistance, BTBR and F1 mice retained hepatic insulin sensitivity. These studies reveal a genetic difference in body composition that correlates with large differences in peripheral insulin sensitivity.

Inhaled treprostinil and pulmonary arterial hypertension

Multiple conditions result in development of pulmonary hypertension. Pulmonary arterial hypertension (PAH) is the subclassification of pulmonary hypertension, in which known or unknown underlying conditions lead to similar intrinsic alterations in the pulmonary vasculature. PAH is a progressive condition characterized by restricted blood flow through the pulmonary circulation leading to poor survival in the absence of effective therapy. Over the last two decades, new therapeutic agents have substantially improved the course and prognosis for PAH patients. Three available classes of drugs, ie, prostacyclins, endothelin receptor antagonists, and phosphodiesterase-5 inhibitors provide multiple options for treatment of PAH. Endothelin receptor antagonists and phosphodiesterase-5 inhibitors are administered orally, whereas prostacyclin therapies are delivered by continuous intravenous or subcutaneous infusion, or as aerosols by nebulization. Because of the risks and inconveniences associated with administration, prostacyclins are typically reserved for patients with more advanced disease or progression despite oral therapy. Inhaled administration may be a safer and easier route for prostacyclin administration. Treprostinil is a prostacyclin analog that has been demonstrated to be effective when administered by continuous subcutaneous or intravenous infusion, and more recently by nebulization.