In the present study we compared the acute in vivo effects of insulin on adipose tissue transcriptional profiles of obese insulin-resistant and lean insulin-sensitive women, to gain insight into the molecular mechanisms underlying insulin resistance. The most striking difference in gene expression of the insulin-resistant group during hyperinsulinemia was reduced transcription of genes involved in mitochondrial respiration (mitochondrial respiratory chain, GO:0001934). Inflammatory pathways with complement components (inflammatory response, GO:0006954) and cytokines (chemotaxis, GO:0042330) were strongly up-regulated in insulin-resistant as compared to insulin-sensitive subjects both before and during hyperinsulinemia. Furthermore, differences were observed in genes contributing to fatty acid, cholesterol and triglyceride metabolism and in genes involved in regulating lipolysis, between the insulin-resistant and -sensitive subjects especially during hyperinsulinemia.
Low-grade adipose tissue inflammation has been postulated as one of the central factors in the development of insulin resistance. Increased inflammation was a predominant phenotype in the insulin-resistant vs. insulin-sensitive subjects in the present study. Many findings of the up-regulated genes in the insulin-resistant group, such as C1Q peptides, MMP9 and SPP1, are consistent with previous reports on adipose tissue gene expression in the transcriptomics analysis in the fasting state in obese mice [24, 25], and in monozygotic twins discordant for BMI . While these data characterize changes in adipose tissue transcriptome in humans and are unique as such, the role of complement components and inflammatory mediators in the development of insulin resistance cannot be determined in a cross-sectional study in humans. Since many of the inflammatory genes observed in this study are active in cells of the monocyte/macrophage lineage, our observation may reflect increased inflammatory cell content in adipose tissue of the insulin-resistant women. This is in line with the observation that the adipose tissue of lean subjects usually consists of approximately 5-10% of macrophages, whereas in obese patients, adipose tissue macrophage content can be as high as 50% of the total cell number . However, there is increasing evidence that some of the genes we found up-regulated are expressed also by adipocytes, such as complement components, PLA2G7 [27, 28], SYK  and ITGB2. Adipocyte and macrophage trancriptomes are similar and they can become even more similar after macrophages engulf lipids  from the dying adipocytes observed in obesity .
Alterations in fatty acid handling and release characterize adipose tissue in obesity. In the present study one of the most notable changes in adipose tissue transcriptome of the insulin-resistant women during fasting was down-regulation of the monocarboxylic acid metabolism pathway, FATP2 being the most down-regulated gene in this pathway. There are no previous reports describing FATP2 expression in human adipose tissue. Moreover, FATP2 is not expressed in mouse 3T3-L1 adipocytes  and in murine tissues it is expressed most strongly in liver and kidney cortex (data on adipose tissue not available in this study) . It seems unlikely that the FATP2 signal in our study would originate from macrophages, since its transcription is strongly down-regulated in the obese insulin-resistant group where macrophage numbers are elevated . Interestingly, a recent report in rat peripheral blood mononuclear cells suggested FATP2 to be an early marker of obesity .
ELOVL6 is a fatty acid elongation factor specific for long chain fatty acids . It which was recently shown to be regulated by SREBP1c . In the present study we made a novel observation of ELOVL6 down-regulation in the obese insulin resistant as compared to the lean insulin-sensitive group in hyperinsulinemia. Interestingly, liver deficiency of ELOVL6 significantly ameliorates insulin resistance in mice by modifying hepatic fatty acid composition . PNPLA3 is expressed mainly in liver and adipose tissue and its genetic variants associate in multiple studies with increased hepatic fat. PNPLA3 has been shown to have a lipolytic and weak lipogenic effect in vivo but its precise role in vivo is unclear . Its expression in adipose tissue has been reported to be similar [38, 39] or increased  in obese compared to lean patients at fasting state. Both insulin and glucose stimulate adipose tissue PNPLA3 expression . The dramatically lower transcript levels of ELOVL6 as well as PNPLA3 in hyperinsulinemic insulin-resistant subjects might have an impact to adipocyte lipid composition and could further decline adipose tissue function in insulin resistance.
Mitochondrial pathways, especially genes involved in mitochondrial respiration, have been shown to be down-regulated in muscle and adipose tissues of insulin-resistant and type 2 diabetic subjects in fasting state [41, 42]. Acquired obesity and poor physical fitness are known to impair the expression of genes of oxidative phosphorylation (41). However, their response to insulin in insulin resistant subjects has not been reported before. Mootha et al. (41) presented evidence that a number of genes involved in oxidative phosphorylation in skeletal muscle are subject to regulation by PGC-1α encoded by PPARGC1, and are down-regulated in type 2 diabetes. Therefore, the down-regulation of PPARGC1 we observed in the subcutaneous fat of insulin resistant subjects may provide one mechanistic explanation to the reduced expression of mitochondrial respiratory chain genes. Interestingly, decreased expression of mitochondrial pathways is the most prominent finding during hyperinsulinemia reflecting a regulatory defect that may further aggravate the pathogenesis of insulin resistance. Since regular practice of physical exercise is known to improve insulin sensitivity and reduce body weight [43, 44], it is possible that differences in physical activity could have amplified the observed differences in gene expression between the insulin sensitive and insulin resistant subjects.
Activity of lipoprotein lipase (LPL) is an important determinant in the development of obesity in mouse models. As a general rule, high fat diet-induced adipogenesis is aggravated by stimulated LPL activity (e.g. by adipose tissue-specific over expression of LPL or deficiency for APOCIII), and attenuated by inhibited LPL activity . A physiologically important LPL inhibitor, ANGPTL4, is strongly down-regulated by insulin in mouse 3T3-L1 cells . In the present study we found insulin to significantly decrease ANGPTL4 expression in human adipose tissue. This finding is novel as was the finding of impaired acute insulin regulation of ANGPTL4 in obese insulin resistant as compared to lean insulin sensitive subjects. In mice, ANGPTL4 over expression inhibits LPL, which in turn slows down adipogenesis and increases plasma triglyceride concentrations. When ANGPTL4 is deleted the reverse phenotype arises [23, 45, 47]. Interestingly, serum ANGPTL4 levels are inversely correlated with plasma glucose concentrations and the serum levels are significantly lower in type 2 diabetic patients than healthy subjects . ANGPTL4 expression is also stimulated by insulin sensitizing drugs thiazolidinediones via PPARy mediated mechanism suggesting that changes in ANGPTL level could have a role in the development of insulin-resistance .
The insulin-resistant group displayed defective induction of important insulin-induced lipid metabolism pathways. While several genes in these pathways are important players in cholesterol metabolism and have a well established role in cholesterol synthesis in liver, little is known of their importance in adipose tissue. Adipose tissue contains the body's largest pool of free cholesterol and cholesterol imbalance is recognized as a characteristic of enlarged adipocytes in obesity . Therefore, the differentially expressed genes in the sterol metabolic pathway in our data could play an important part in the insulin-resistant phenotype. The molecular mechanism underlying the decreased expression of lipid biosynthesis pathways in the insulin-resistant group could involve SREBP-1c since several of the genes shown in Figure 4 are targets of SREBP-1c including HMGCR and ELOVL6 [35, 51]. SREBP-1c is regulated mainly at the transcriptional level and is a major mediator of fatty acid and triglyceride synthesis induction by insulin in liver and adipose tissue [51–54]. Down-regulation of SREBP-1c was observed in the adipose tissue of leptin deficient mice (ob/ob) , in insulin-resistant humans during fasting and in response to insulin , and in adipose tissue of patients with type 2 diabetes , similar to our study. In contrast to the latter study, we detected no difference in SREBF1 expression between the groups in the basal state. The small sample size of the study could have contributed to failure to detect such a difference and also other small scale changes in gene expression. The defective response of SREBF1 to insulin in the insulin-resistant group could explain down-regulation of ELOVL6 and HMGCR mRNAs in this group. Reduced SREBF1 transcription could be a homeostatic reaction of enlarged adipocytes to prevent further lipid synthesis. On the other hand, the reduced SREBF1 expression could also decrease lipogenesis in smaller adipocytes and hinder adipocyte differentiation via PPARG down-regulation, leading to increased insulin resistance .