Insulin action is severely impaired in adipocytes of apparently healthy overweight and obese subjects

Sex differences in adipose insulin resistance are linked to obesity, lipolysis and insulin receptor substrate 1

BACKGROUND/OBJECTIVE

Insulin resistance is more prominent in men than women. If this involves adipose tissue is unknown and was presently examined.

SUBJECTS/METHODS

AdipoIR (in vivo adipose insulin resistance index) was measured in 2344 women and 787 men. In 259 of the women and 54 of the men, insulin induced inhibition of lipolysis (acylglycerol breakdown) and stimulation of lipogenesis (glucose conversion to acylglycerols) were determined in subcutaneous adipocytes; in addition, basal (spontaneous) lipolysis was also determined in the fat cells. In 234 women and 115 men, RNAseq expression of canonical insulin signal genes were measured in subcutaneous adipose tissue. Messenger RNA transcripts of the most discriminant genes were quantified in 175 women and 109 men.

RESULTS

Men had higher AdipoIR values than women but only when obesity (body mass index 30 kg/m2 or more) was present (p < 0.0001). The latter sex dimorphism was found among physically active and sedentary people, in those with and without cardiometabolic disease and in people using nicotine or not (p = 0.0003 or less). In obesity, adipocyte insulin sensitivity (half maximum effective hormone concentration) and maximal antilipolytic effect were tenfold and 10% lower, respectively, in men than women (p = 0.005 or less). Basal rate of lipolysis was two times higher in men than women (p > 0.0001). Sensitivity and maximum effect of insulin on lipogenesis were similar in both sexes (p = 0.26 and p = 0.18, respectively). When corrected for multiple comparison only RNAseq expression of insulin receptor substrate 1 (IRS1) was lower in men than women (p < 0.0001). The mRNA transcript for IRS1 was 60% higher in women than men (p < 0.0001).

CONCLUSIONS

In obesity, adipose tissue insulin resistance is more pronounced in men than in women. The mechanism involves less efficient insulin-mediated inhibition of adipocyte lipolysis, increased basal rate of lipolysis and decreased adipose expression of a key element of insulin signaling, IRS1.

Phenotyping obesity: A focus on metabolically healthy obesity and metabolically unhealthy normal weight

Over the past 4 decades, research has shown that having a normal body weight does not automatically imply preserved metabolic health and a considerable number of lean individuals harbour metabolic abnormalities typically associated with obesity. Conversely, excess adiposity does not always equate with an abnormal metabolic profile. In fact, evidence exists for the presence of a metabolically unhealthy normal weight (MUHNW) and a metabolically healthy obese (MHO) phenotype. It has become increasingly recognised that different fat depots exert different effects on the metabolic profile of each individual by virtue of their location, structure and function, giving rise to these different body composition phenotypes. Furthermore, other factors have been implicated in the aetiopathogenesis of the body composition phenotypes, including genetics, ethnicity, age and lifestyle/behavioural factors. Even though to date both MHO and MUHNW have been widely investigated and documented in the literature, studies report different outcomes on long-term cardiometabolic morbidity and mortality. Future large-scale, observational and population-based studies are required for better profiling of these phenotypes as well as to further elucidate the pathophysiological role of the adipocyte in the onset of metabolic disorders to allow for better risk stratification and a personalised treatment paradigm.

Insulin resistance in adipocytes: Novel insights into the pathophysiology of metabolic syndrome

BACKGROUND

Insulin resistance in all major target tissues is present in metabolic syndrome (MetS). The resistance in adipocytes is not well described and was presently examined.

METHODS

In this observational study on isolated abdominal white subcutaneous adipocytes from 419 adults, concentration-response effects of insulin on lipolysis inhibition (glycerol release) and lipogenesis stimulation (glucose conversion to total lipids) were determined. Insights into early and late insulin signaling events were obtained through the determination of insulin sensitivity (half maximum effective concentration) and responsiveness (maximum effect), respectively. In a subgroup of 132 subjects, we analyzed the subcutaneous adipose mRNA expression of genes in the canonical insulin signaling pathway using microarray. These results were validated using quantitative real-time polymerase chain reaction in 74 individuals.

RESULTS

While the insulin responsiveness was similar in subjects with or without Mets, the sensitivity to insulin-mediated inhibition of lipolysis and stimulation of lipogenesis was ∼tenfold lower in subjects with MetS (p < 0.0001). When age, sex, adipocyte volume, body mass index and body shape were considered, only the antilipolytic resistance was independently associated with MetS. The mRNA expression of several genes in the canonical insulin signaling pathway were altered in MetS (p < 0.0006 or better) where the mRNA levels of insulin receptor substrate 2 associated with the antilipolytic effect (Rho = 0.34; p = 0.0016).

CONCLUSION

The sensitivities of the antilipolytic and lipogenic effects of insulin are decreased in the MetS but only antilipolysis remains significant after multiple regression analysis. This resistance is localized at initial and receptor-near events in hormone signaling involving insulin receptor substrate 2.

Increased Adipocyte Hypertrophy in Patients with Nascent Metabolic Syndrome

Background and Aims: Metabolic Syndrome (MetS), a global problem, predisposes to an increased risk for type 2 diabetes and premature cardiovascular disease. While MetS is associated with central obesity, there is scanty data on adipocyte hypertrophy, increased fat cell size (FCS), in MetS. The aim of this study was to investigate FCS status in adipose tissue (AT) biopsy of patients with nascent MetS without the confounding of diabetes, cardiovascular disease, smoking, or lipid therapy. Methods and Results: Fasting blood and subcutaneous gluteal AT biopsies were obtained in MetS (n = 20) and controls (n = 19). Cardio-metabolic features, FFA levels, hsCRP, and HOMA-IR were significantly increased in patients with MetS. Waist-circumference (WC) adjusted-FCS was significantly increased in patients with MetS and increased with increasing severity of MetS. Furthermore, there were significant correlations between FCS with glucose, HDL-C, and the ratio of TG: HDL-C. There were significant correlations between FCS and FFA, as well as endotoxin and monocyte TLR4 abundance. Additionally, FCS correlated with readouts of NLRP3 Inflammasome activity. Most importantly, FCS correlated with markers of fibrosis and angiogenesis. Conclusions: In conclusion, in patients with nascent MetS, we demonstrate WC-adjusted increase in FCS from gluteal adipose tissue which correlated with cellular inflammation, fibrosis, and angiogenesis. While these preliminary observations were in gluteal fat, future studies are warranted to confirm these findings in visceral and other fat depots.

Adipose tissue specific CCL18 associates with cardiometabolic diseases in non-obese individuals implicating CD4+ T cells

AIM

Obesity is linked to cardiometabolic diseases, however non-obese individuals are also at risk for type 2 diabetes (T2D) and cardiovascular disease (CVD). White adipose tissue (WAT) is known to play a role in both T2D and CVD, but the contribution of WAT inflammatory status especially in non-obese patients with cardiometabolic diseases is less understood. Therefore, we aimed to find associations between WAT inflammatory status and cardiometabolic diseases in non-obese individuals.

METHODS

In a population-based cohort containing non-obese healthy (n = 17), T2D (n = 16), CVD (n = 18), T2D + CVD (n = 19) individuals, seventeen different cytokines were measured in WAT and in circulation. In addition, 13-color flow cytometry profiling was employed to phenotype the immune cells. Human T cell line (Jurkat T cells) was stimulated by rCCL18, and conditioned media (CM) was added to the in vitro cultures of human adipocytes. Lipolysis was measured by glycerol release. Blocking antibodies against IFN-γ and TGF-β were used in vitro to prove a role for these cytokines in CCL18-T-cell-adipocyte lipolysis regulation axis.

RESULTS

In CVD, T2D and CVD + T2D groups, CCL18 and CD4+ T cells were upregulated significantly compared to healthy controls. WAT CCL18 secretion correlated with the amounts of WAT CD4+ T cells, which also highly expressed CCL18 receptors suggesting that WAT CD4+ T cells are responders to this chemokine. While direct addition of rCCL18 to mature adipocytes did not alter the adipocyte lipolysis, CM from CCL18-treated T cells increased glycerol release in in vitro cultures of adipocytes. IFN-γ and TGF-β secretion was significantly induced in CM obtained from T cells treated with CCL18. Blocking these cytokines in CM, prevented CM-induced upregulation of adipocyte lipolysis.

CONCLUSION

We suggest that in T2D and CVD, increased production of CCL18 recruits and activates CD4+ T cells to secrete IFN-γ and TGF-β. This, in turn, promotes adipocyte lipolysis - a possible risk factor for cardiometabolic diseases.

Relationship Between a Sedentary Lifestyle and Adipose Insulin Resistance

Sedentary people have insulin resistance in their skeletal muscle, but whether this also occurs in fat cells was unknown. Insulin inhibition of hydrolysis of triglycerides (antilipolysis) and stimulation of triglyceride formation (lipogenesis) were investigated in subcutaneous fat cells from 204 sedentary and 336 physically active subjects. Insulin responsiveness (maximum hormone effect) and sensitivity (half-maximal effective concentration) were determined. In 69 women, hyperinsulinemia-induced circulating fatty acid levels were measured. In 128 women, adipose gene expression was analyzed. Responsiveness of insulin for antilipolysis (60% inhibition) and lipogenesis (twofold stimulation) were similar between sedentary and active subjects. Sensitivity for both measures decreased ˜10-fold in sedentary subjects (P < 0.01). However, upon multiple regression analysis, only the association between antilipolysis sensitivity and physical activity remained significant when adjusting for BMI, age, sex, waist-to-hip ratio, fat-cell size, and cardiometabolic disorders. Fatty acid levels decreased following hyperinsulinemia but remained higher in sedentary compared with active women (P = 0.01). mRNA expression of insulin receptor and its substrates 1 and 2 was decreased in sedentary subjects. In conclusion, while the maximum effect is preserved, sensitivity to insulin's antilipolytic effect in subcutaneous fat cells is selectively lower in sedentary subjects.

Is insulin resistance tissue-dependent and substrate-specific? The role of white adipose tissue and skeletal muscle

Insulin resistance (IR) refers to a reduction in the ability of insulin to exert its metabolic effects in organs such as adipose tissue (AT) and skeletal muscle (SM), leading to chronic diseases such as type 2 diabetes, hepatic steatosis, and cardiovascular diseases. Obesity is the main cause of IR, however not all subjects with obesity develop clinical insulin resistance, and not all clinically insulin-resistant people have obesity. Recent evidence implies that IR onset is tissue-dependent (AT or SM) and/or substrate-specific (glucometabolic or lipometabolic). Therefore, the aims of the present review are 1) to describe the glucometabolic and lipometabolic activities of insulin in AT and SM in the maintenance of whole-body metabolic homeostasis, 2) to discuss the pathophysiology of substrate-specific IR in AT and SM, and 3) to highlight novel validated tests to assess tissue and substrate-specific IR that are easy to perform in clinical practice. In AT, glucometabolic IR reduces glucose availability for glycerol and fatty acid synthesis, thus decreasing the esterification and synthesis of signaling bioactive lipids. Lipometabolic IR in AT impairs the antilipolytic effect of insulin and lipogenesis, leading to an increase in circulating FFAs and generating lipotoxicity in peripheral tissues. In SM, glucometabolic IR reduces glucose uptake, whereas lipometabolic IR impairs mitochondrial lipid oxidation, increasing oxidative stress and inflammation, all of which lead to metabolic inflexibility. Understanding tissue-dependent and substrate-specific IR is of paramount importance for early detection before clinical manifestations and for the development of more specific treatments or direct interventions to prevent chronic life-threatening diseases.

Type 2 Diabetes, Independent of Obesity and Age, Is Characterized by Senescent and Dysfunctional Mature Human Adipose Cells

Obesity with dysfunctional adipose cells is the major cause of the current epidemic of type 2 diabetes (T2D). We examined senescence in human adipose tissue cells from age- and BMI-matched individuals who were lean, obese, and obese with T2D. In obese individuals and, more pronounced, those with T2D, we found mature and fully differentiated adipose cells to exhibit increased senescence similar to what we previously have shown in the progenitor cells. The degree of adipose cell senescence was positively correlated with whole-body insulin resistance and adipose cell size. Adipose cell protein analysis revealed dysfunctional cells in T2D with increased senescence markers reduced PPAR-γ, GLUT4, and pS473AKT. Consistent with a recent study, we found the cell cycle regulator cyclin D1 to be increased in obese cells and further elevated in T2D cells, closely correlating with senescence markers, ambient donor glucose, and, more inconsistently, plasma insulin levels. Furthermore, fully differentiated adipose cells were susceptible to experimentally induced senescence and to conditioned medium increasing cyclin D1 and responsive to senolytic agents. Thus, fully mature human adipose cells from obese individuals, particularly those with T2D become senescent, and SASP secretion by senescent progenitor cells can play an important role in addition to donor hyperinsulinemia.

Biological Role of the Intercellular Transfer of Glycosylphosphatidylinositol-Anchored Proteins: Stimulation of Lipid and Glycogen Synthesis

Glycosylphosphatidylinositol-anchored proteins (GPI-APs), which are anchored at the outer leaflet of plasma membranes (PM) only by a carboxy-terminal GPI glycolipid, are known to fulfill multiple enzymic and receptor functions at the cell surface. Previous studies revealed that full-length GPI-APs with the complete GPI anchor attached can be released from and inserted into PMs in vitro. Moreover, full-length GPI-APs were recovered from serum, dependent on the age and metabolic state of rats and humans. Here, the possibility of intercellular control of metabolism by the intercellular transfer of GPI-APs was studied. Mutant K562 erythroleukemia (EL) cells, mannosamine-treated human adipocytes and methyl-ß-cyclodextrin-treated rat adipocytes as acceptor cells for GPI-APs, based on their impaired PM expression of GPI-APs, were incubated with full-length GPI-APs, prepared from rat adipocytes and embedded in micelle-like complexes, or with EL cells and human adipocytes with normal expression of GPI-APs as donor cells in transwell co-cultures. Increases in the amounts of full-length GPI-APs at the PM of acceptor cells as a measure of their transfer was assayed by chip-based sensing. Both experimental setups supported both the transfer and upregulation of glycogen (EL cells) and lipid (adipocytes) synthesis. These were all diminished by serum, serum GPI-specific phospholipase D, albumin, active bacterial PI-specific phospholipase C or depletion of total GPI-APs from the culture medium. Serum inhibition of both transfer and glycogen/lipid synthesis was counteracted by synthetic phosphoinositolglycans (PIGs), which closely resemble the structure of the GPI glycan core and caused dissociation of GPI-APs from serum proteins. Finally, large, heavily lipid-loaded donor and small, slightly lipid-loaded acceptor adipocytes were most effective in stimulating transfer and lipid synthesis. In conclusion, full-length GPI-APs can be transferred between adipocytes or between blood cells as well as between these cell types. Transfer and the resulting stimulation of lipid and glycogen synthesis, respectively, are downregulated by serum proteins and upregulated by PIGs. These findings argue for the (patho)physiological relevance of the intercellular transfer of GPI-APs in general and its role in the paracrine vs. endocrine (dys)regulation of metabolism, in particular. Moreover, they raise the possibility of the use of full-length GPI-APs as therapeutics for metabolic diseases.

The multifaceted roles of the adipose tissue vasculature

The prevalence of obesity and its associated pathologies continue to increase, which has led to a renewed interest in our major weight-regulating organ, the white adipose tissue. It has become clear that its development, expansion, and physiological function depend on proper crosstalk between each of its cellular constituents, with a central role for the vascular endothelium lining the blood vessels. Although first considered a mere barrier, the endothelium has emerged as a dynamic unit modulating many critical adipose tissue functions. It not only oversees the uptake of all nutrients to be stored in the adipocytes but also provides an important growth niche for adipocyte progenitors and regulates the expandability of the tissue during overfeeding and obesity. In this review, we describe the reciprocal relationship between endothelial cells, adipocytes, and obesity. We present recent studies that support an important role for endothelial cells as central mediators of many of the physiological and pathological functions of the adipose tissue and highlight several unknown aspects of adipose tissue vascular biology. This new perspective could present exciting opportunities to develop new therapeutic approaches against obesity-related pathologies and is thus of great interest in our increasingly obese society.

Metabolically healthy obesity: Is it really healthy for type 2 diabetes mellitus?

Metabolically healthy obese (MHO) individuals are reported to have a lower risk of developing cardiovascular diseases in comparison with individuals with metabolic syndrome. However, the association between MHO and type 2 diabetes (T2DM) is still controversial. Some studies indicated that MHO is a favorable phenotype for T2DM, but more studies showed that MHO individuals have an increased risk of developing T2DM compared with metabolically healthy normal-weight individuals, especially among those who would acquire metabolically unhealthy obesity. This has been supported by finding insulin resistance and low-grade inflammatory responses in MHO individuals with a tendency for impaired beta-cell dysfunction. Studies also showed that liver fat accumulation increased the risk of incidence of T2DM in MHO. Here, we reviewed current literature on the relationship between MHO and T2DM, discussed the determinants for the development of diabetes in MHO, and summarized the measures for the prevention of T2DM in MHO.

Adipose tissue macrophages and atherogenesis – a synergy with cholesterolaemia

Excessive LDL cholesterol concentration together with subclinical inflammation, in which macrophages play a central role, are linked pathologies. The process starts with the accumulation of macrophages in white adipose tissue and the switch of their polarization toward a pro-inflammatory phenotype. The proportion of pro-inflammatory macrophages in adipose tissue is related to the main risk predictors of cardiovascular disease. The cholesterol content of phospholipids of cell membranes seems to possess a crucial role in the regulation of membrane signal transduction and macrophage polarization. Also, different fatty acids of membrane phospholipids influence phenotypes of adipose tissue macrophages with saturated fatty acids stimulating pro-inflammatory whereas ω3 fatty acids anti-inflammatory changes. The inflammatory status of white adipose tissue, therefore, reflects not only adipose tissue volume but also adipose tissue macrophages feature. The beneficial dietary change leading to an atherogenic lipoprotein decrease may therefore synergically reduce adipose tissue driven inflammation.

A longitudinal study of the antilipolytic effect of insulin in women following bariatric surgery

Insulin resistance of glucose utilization is fully restored following BMI normalization after bariatric surgery. We investigated if this also pertains to insulin-induced effects on fatty acid handling. Forty-three women with obesity (OB) were investigated before and 2 years after Roux-en-Y gastric by-pass when BMI was <30 kg/m² (PO) and compared with 26 never obese women (NO). The Adipo-IR index was used as measure of insulin antilipolytic sensitivity. Changes (delta) in circulating glycerol and fatty acid levels during hyperinsulinemic euglycemic clamp represented the insulin maximum antilipolytic effect. Overall fatty acid utilization was reflected by delta fatty acids minus 3 × delta glycerol. Adipo-IR was higher in OB than in NO and PO (p < 0.0001), the latter two groups having similar values. Insulin lowered glycerol levels by about 70% in all groups, but delta glycerol was 30% larger in PO than in NO (p = 0.04). Delta adds and adds utilization were similar in all groups. We conclude that women with obesity, whose BMI is normalized after bariatric surgery, have improved maximum in vivo antilipolytic effect of insulin above expected in absolute but not relative terms as regards glycerol changes, while the handling of circulating fatty acids is changed to the normal state.

Critical Reappraisal of the Role and Importance of Exercise Intervention in the Treatment of Obesity in Adults

In the treatment of obesity in adults, exercise intervention is recommended and some people with obesity even prefer exercise above dietary intervention as a single weight-loss strategy. However, evidence is accumulating that the long-term body weight and adipose tissue mass loss ​as a result of exercise intervention in these individuals is disappointingly small. Although this could be related to various clinical reasons, more recent evidence reveals that also (patho)physiological abnormalities are involved which cannot be remediated by exercise intervention, especially in metabolically compromised patients. As a result, the role and importance of exercise intervention in the treatment of obesity deserve significant reconsideration to avoid confusion and disappointment amongst clinicians, patients and society. Hence, to reduce adipose tissue mass and body weight, dietary intervention is much more effective than exercise intervention, and is, therefore, of key importance in this endeavour. However, dietary interventions must be supplemented by exercise training to induce clinically relevant changes in specific cardiovascular or metabolic risk factors like blood pressure, blood triglycerides and high-density lipoprotein cholesterol concentrations, as well as visceral adipose tissue mass, physical fitness, muscle mass and strength, quality of life and life expectancy. This allows individuals with obesity to preserve their cardiometabolic health or to shift from a metabolically unhealthy phenotype to a metabolically healthy phenotype. Signifying the true clinical value of exercise interventions might lead to a better understanding and appreciation of the goals and associated effects when implemented in the multidisciplinary treatment of obesity, for which a proper tailoring of exercise prescription is required.

Omics Approaches in Adipose Tissue and Skeletal Muscle Addressing the Role of Extracellular Matrix in Obesity and Metabolic Dysfunction

Extracellular matrix (ECM) remodeling plays important roles in both white adipose tissue (WAT) and the skeletal muscle (SM) metabolism. Excessive adipocyte hypertrophy causes fibrosis, inflammation, and metabolic dysfunction in adipose tissue, as well as impaired adipogenesis. Similarly, disturbed ECM remodeling in SM has metabolic consequences such as decreased insulin sensitivity. Most of described ECM molecular alterations have been associated with DNA sequence variation, alterations in gene expression patterns, and epigenetic modifications. Among others, the most important epigenetic mechanism by which cells are able to modulate their gene expression is DNA methylation. Epigenome-Wide Association Studies (EWAS) have become a powerful approach to identify DNA methylation variation associated with biological traits in humans. Likewise, Genome-Wide Association Studies (GWAS) and gene expression microarrays have allowed the study of whole-genome genetics and transcriptomics patterns in obesity and metabolic diseases. The aim of this review is to explore the molecular basis of ECM in WAT and SM remodeling in obesity and the consequences of metabolic complications. For that purpose, we reviewed scientific literature including all omics approaches reporting genetic, epigenetic, and transcriptomic (GWAS, EWAS, and RNA-seq or cDNA arrays) ECM-related alterations in WAT and SM as associated with metabolic dysfunction and obesity.

Hyperinsulinemia and insulin resistance in the obese may develop as part of a homeostatic response to elevated free fatty acids: A mechanistic case-control and a population-based cohort study

Background

It is commonly accepted that in obesity free fatty acids (FFA) cause insulin resistance and hyperglycemia, which drives hyperinsulinemia. However, hyperinsulinemia is observed in subjects with normoglycaemia and thus the paradigm above should be reevaluated.

Methods

We describe two studies: MD-Lipolysis, a case control study investigating the mechanisms of obesity-driven insulin resistance by a systemic metabolic analysis, measurements of adipose tissue lipolysis by microdialysis, and adipose tissue genomics; and POEM, a cohort study used for validating differences in circulating metabolites in relation to adiposity and insulin resistance observed in the MD-Lipolysis study.

Findings

In insulin-resistant obese with normal glycaemia from the MD-Lipolysis study, hyperinsulinemia was associated with elevated FFA. Lipolysis, assessed by glycerol release per adipose tissue mass or adipocyte surface, was similar between obese and lean individuals. Adipose tissue from obese subjects showed reduced expression of genes mediating catecholamine-driven lipolysis, lipid storage, and increased expression of genes driving hyperplastic growth. In the POEM study, FFA levels were specifically elevated in obese-overweight subjects with normal fasting glucose and high fasting levels of insulin and C-peptide.

Interpretation

In obese subjects with normal glycaemia elevated circulating levels of FFA at fasting are the major metabolic derangement candidate driving fasting hyperinsulinemia. Elevated FFA in obese with normal glycaemia were better explained by increased fat mass rather than by adipose tissue insulin resistance. These results support the idea that hyperinsulinemia and insulin resistance may develop as part of a homeostatic adaptive response to increased adiposity and FFA.

Funding

Swedish-Research-Council (2016-02660); Diabetesfonden (DIA2017-250; DIA2018-384; DIA2020-564); Novo-Nordisk-Foundation (NNF17OC0027458; NNF19OC0057174); Cancerfonden (CAN2017/472; 200840PjF); Swedish-ALF-agreement (2018-74560).

Usefulness of surrogate markers to determine insulin action in fat cells

BACKGROUND

Obesity is a major factor behind insulin resistance. The validity of simple biochemical surrogate measures to estimate insulin resistance at the fat cell level is unclear.

OBJECTIVE

To investigate if the surrogate measures HOMA-IR (glucose/insulin product) and Adipo-IR (fatty acids/insulin product) reflect insulin action on glucose/lipid metabolism in fat cells.

DESIGN

Insulin-induced lipogenesis and lipolysis inhibition (antilipolysis) in subcutaneous fat cells were investigated for sensitivity (reflecting receptor-near events) and responsiveness (i.e., maximum action reflecting distal post-receptor events) in 363 subjects. Results were compared with log₁₀ transformed values for HOMA-IR and Adipo-IR.

RESULTS

Individually, the four measures of in vitro insulin action on fat cells correlated significantly (p < 0.0001) but weakly with each other (adjusted r² 0.05-0.23). HOMA-IR and Adipo-IR correlated strongly with each other (adjusted r² = 0.81). Using Spearman or simple linear regression all in vitro measures except antilipolytic responsiveness expressed per lipid weight, correlated significantly with Adipo-IR or HOMA-IR (p values <0.0001). Similar relationships remained after combined correction for age, body mass index and sex. Together, the four in vitro measures explained 50% of the variability in HOMA-IR and ADIPO-IR (p < 0.0001). Receiver-operating characteristic analysis showed good sensitivity and specificity for Adipo-IR and HOMA-IR to detect combined insulin resistance of antilipolysis and lipogenesis in fat cells (area under the curve = 0.8).

CONCLUSIONS

Insulin action at the receptor and post-receptor levels on lipolysis and lipogenesis in fat cells correlates significantly with Adipo-IR and HOMA-IR. Both surrogate measures give similar information about insulin resistance of glucose and lipid metabolism in fat cells.

Anti-Lipolysis Induced by Insulin in Diverse Pathophysiologic Conditions of Adipose Tissue

As an important energy reservoir, adipose tissue maintains lipid balance and regulates energy metabolism. When the body requires energy, adipocytes provide fatty acids to peripheral tissues through lipolysis. Insulin plays an important role in regulating normal fatty acid levels by inhibiting lipolysis. When the morphology of adipose tissue is abnormal, its microenvironment changes and the lipid metabolic balance is disrupted, which seriously impairs insulin sensitivity. As the most sensitive organ to respond to insulin, lipolysis levels in adipose tissue are affected by impaired insulin function, which results in serious metabolic diseases. However, the specific underlying mechanisms of this process have not yet been fully elucidated, and further study is required. The purpose of this review is to discuss the effects of adipose tissue on the anti-lipolysis process triggered by insulin under different conditions. In particular, the functional changes of this process respond to inconsonantly morphological changes of adipose tissue.