Diet-induced obesity prevents interstitial dispersion of insulin in skeletal muscle

Fibro-adipogenic progenitors in physiological adipogenesis and intermuscular adipose tissue remodeling

Excessive accumulation of intermuscular adipose tissue (IMAT) is a common pathological feature in various metabolic and health conditions and can cause muscle atrophy, reduced function, inflammation, insulin resistance, cardiovascular issues, and unhealthy aging. Although IMAT results from fat accumulation in muscle, the mechanisms underlying its onset, development, cellular components, and functions remain unclear. IMAT levels are influenced by several factors, such as changes in the tissue environment, muscle type and origin, extent and duration of trauma, and persistent activation of fibro-adipogenic progenitors (FAPs). FAPs are a diverse and transcriptionally heterogeneous population of stromal cells essential for tissue maintenance, neuromuscular stability, and tissue regeneration. However, in cases of chronic inflammation and pathological conditions, FAPs expand and differentiate into adipocytes, resulting in the development of abnormal and ectopic IMAT. This review discusses the role of FAPs in adipogenesis and how they remodel IMAT. It highlights evidence supporting FAPs and FAP-derived adipocytes as constituents of IMAT, emphasizing their significance in adipose tissue maintenance and development, as well as their involvement in metabolic disorders, chronic pathologies and diseases. We also investigated the intricate molecular pathways and cell interactions governing FAP behavior, adipogenesis, and IMAT accumulation in chronic diseases and muscle deconditioning. Finally, we hypothesize that impaired cellular metabolic flexibility in dysfunctional muscles impacts FAPs, leading to IMAT. A deeper understanding of the biology of IMAT accumulation and the mechanisms regulating FAP behavior and fate are essential for the development of new therapeutic strategies for several debilitating conditions.

Capillary Endothelial Insulin Transport: The Rate-limiting Step for Insulin-stimulated Glucose Uptake

The rate-limiting step for skeletal muscle glucose uptake is transport from microcirculation to muscle interstitium. Capillary endothelium poses a barrier that delays the onset of muscle insulin action. Defining physiological barriers that control insulin access to interstitial space is difficult because of technical challenges that confront study of microscopic events in an integrated physiological system. Two physiological variables determine muscle insulin access. These are the number of perfused capillaries and the permeability of capillary walls to insulin. Disease states associated with capillary rarefaction are closely linked to insulin resistance. Insulin permeability through highly resistant capillary walls of muscle poses a significant barrier to insulin access. Insulin may traverse the endothelium through narrow intercellular junctions or vesicular trafficking across the endothelial cell. Insulin is large compared with intercellular junctions, making this an unlikely route. Transport by endothelial vesicular trafficking is likely the primary route of transit. Studies in vivo show movement of insulin is not insulin receptor dependent. This aligns with single-cell transcriptomics that show the insulin receptor is not expressed in muscle capillaries. Work in cultured endothelial cell lines suggest that insulin receptor activation is necessary for endothelial insulin transit. Controversies remain in the understanding of transendothelial insulin transit to muscle. These controversies closely align with experimental approaches. Control of circulating insulin accessibility to skeletal muscle is an area that remains ripe for discovery. Factors that impede insulin access to muscle may contribute to disease and factors that accelerate access may be of therapeutic value for insulin resistance.

Adipose tissue spexin in physical exercise and age-associated diseases

It is known that a strong association exists between a suboptimal lifestyle (physical inactivity and sedentary behavior and/or high calorie diet) and increased propensity of developing age-associated diseases, such as obesity and T2DM. Physical exercise can alleviate obesity-induced insulin resistance and T2DM, however, the precise mechanism for this outcome is not fully understood. The endocrine disorder of adipose tissue in obesity plays a critical role in the development of insulin resistance. In this regard, spexin has been recently described as an adipokine that plays an important role in the pathophysiology of obesity-induced insulin resistance and T2DM. In obese states, expression of adipose tissue spexin is reduced, inducing the adipose tissue and skeletal muscle more susceptible to insulin resistance. Emerging evidences point out that exercise can increase spexin expression. In return, spexin could exert the exercise-protective roles to ameliorate insulin resistance, suggesting that spexin is a potential mediator for exercise to ameliorate obesity-induced insulin resistance and T2DM, namely, the beneficial effect of exercise on insulin sensitivity is at least partly mediated by spexin. This review summarizes our and others' recent studies regarding the effects of obesity on adipose tissue spexin induction, along with the potential effect of exercise on this response in obese context, and provides a new insight into the multivariate relationship among exercise, spexin and T2DM. It should be therefore taken into account that a combination of spexin and exercise training is an effective therapeutic strategy for age-associated diseases.

Anti-diabetic effects and mechanisms of action of a Chinese herbal medicine preparation JQ-R in vitro and in diabetic KKAy mice

Refined-JQ (JQ-R) is a mixture of refined extracts from Coptis chinensis (Ranunculaceae), Astragalus membranaceus (Leguminosae) and Lonicera japonica (Caprifoliaceae), the three major herbs of JinQi-JiangTang tablet, a traditional Chinese medicine (TCM) formula. The mechanisms by which JQ-R regulates glucose metabolism and improves insulin sensitivity were studied in type 2 diabetic KK^Ay mice and insulin-resistant L6 myotubes. To investigate the mechanisms by which JQ-R improves insulin sensitivity, a model of insulin-resistant cells induced with palmitic acid (PA) was established in L6 myotubes. Glucose uptake and expression of factors involved in insulin signaling, stress, and inflammatory pathways were detected by immunoblotting. JQ-R showed beneficial effects on glucose homeostasis and insulin resistance in a euglycemic clamp experiment and decreased fasting insulin levels in diabetic KK^Ay mice. JQ-R also improved the plasma lipid profiles. JQ-R directly increased the activity of superoxide dismutase (SOD) and decreased malondialdehyde (MDA) as well as inducible nitric oxide synthase (iNOS) levels in insulin-resistant L6 cells, and elevated the insulin-stimulated glucose uptake with upregulated phosphorylation of AKT. The phosphorylation levels of nuclear factor kappa B (NF-κB p65), inhibitor of NF-κB (IκB α), c-Jun N-terminal kinase (JNK1/2) and extracellular-signal-regulated kinases (ERK1/2) were also changed after JQ-R treatment compared with the control group. Together these findings suggest that JQ-R improved glucose and lipid metabolism in diabetic KK^Ay mice. JQ-R directly enhanced insulin-stimulated glucose uptake in insulin-resistant myotubes with improved insulin signalling and inflammatory response and oxidative stress. JQ-R could be a candidate to achieve improved glucose metabolism and insulin sensitivity in type 2 diabetes mellitus.

Insulin access to skeletal muscle is impaired during the early stages of diet-induced obesity

OBJECTIVE

Insulin must move from the blood to the interstitium to initiate signaling, yet access to the interstitium may be impaired in cases of insulin resistance, such as obesity. This study investigated whether consuming a short- and long-term high-fat diet (HFD) impairs insulin access to skeletal muscle, the major site of insulin-mediated glucose uptake.

METHODS

Male mongrel dogs were divided into three groups consisting of control diet (n = 16), short-term (n = 8), and long-term HFD (n = 8). Insulin sensitivity was measured with intravenous glucose tolerance tests. A hyperinsulinemic euglycemic clamp was performed in each animal at the conclusion of the study. During the clamp, lymph fluid was measured as a representation of the interstitial space to assess insulin access to muscle.

RESULTS

Short- and long-term HFD induced obesity and reduced insulin sensitivity. Lymph insulin concentrations were approximately 50% of plasma insulin concentrations under control conditions. Long-term HFD caused fasting plasma hyperinsulinemia; however, interstitial insulin concentrations were not increased, suggesting impaired insulin access to muscle.

CONCLUSIONS

A HFD rapidly induces insulin resistance at the muscle and impairs insulin access under basal insulin concentrations. Hyperinsulinemia induced by a long-term HFD may be a compensatory mechanism necessary to maintain healthy insulin levels in muscle interstitium.

Rapid development of cardiac dysfunction in a canine model of insulin resistance and moderate obesity

AIMS/HYPOTHESIS

The worldwide incidence of obesity and diabetes continues to rise at an alarming rate. A major cause of the morbidity and mortality associated with obesity and diabetes is heart disease, yet the mechanisms that lead to cardiovascular complications remain unclear.

METHODS

We performed cardiac MRI to assess left ventricular morphology and function during the development of moderate obesity and insulin resistance in a well-established canine model (n = 26). To assess the influence of dietary fat composition, we randomised animals to a traditional lard diet (rich in saturated and monounsaturated fat; n = 12), a salmon oil diet (rich in polyunsaturated fat; n = 8) or a control diet (n = 6).

RESULTS

High-fat feeding with lard increased body weight and fasting insulin and markedly reduced insulin sensitivity. Lard feeding also significantly reduced left ventricular function, evidenced by a worsening of circumferential strain and impairment in left ventricular torsion. High-fat feeding with salmon oil increased body weight; however, salmon oil feeding did not impair insulin sensitivity or cardiac function.

CONCLUSIONS/INTERPRETATION

These data emphasise the importance of dietary fat composition on both metabolic and cardiac function, and have important implications for the relationship between diet and health.

A Review of the Toxicologic Implications of Obesity

The increasing prevalence of obesity in developed nations has far-reaching implications for medical toxicology. The management of obese patients is complicated by comorbid illnesses, changes in cardiovascular and respiratory physiology, alterations in pharmacokinetics, and a lack of studies to identify appropriate dosing for current therapeutics and antidotes. In this review article, we examine obesity-associated physiologic and pharmacokinetic changes that may increase the vulnerability of obese patients to overdose. Further research is needed to characterize the relationship between drug toxicity and obesity.

Palmitate-induced inflammatory pathways in human adipose microvascular endothelial cells promote monocyte adhesion and impair insulin transcytosis

Obesity is associated with inflammation and immune cell recruitment to adipose tissue, muscle and intima of atherosclerotic blood vessels. Obesity and hyperlipidemia are also associated with tissue insulin resistance and can compromise insulin delivery to muscle. The muscle/fat microvascular endothelium mediates insulin delivery and facilitates monocyte transmigration, yet its contribution to the consequences of hyperlipidemia is poorly understood. Using primary endothelial cells from human adipose tissue microvasculature (HAMEC), we investigated the effects of physiological levels of fatty acids on endothelial inflammation and function. Expression of cytokines and adhesion molecules was measured by RT-qPCR. Signaling pathways were evaluated by pharmacological manipulation and immunoblotting. Surface expression of adhesion molecules was determined by immunohistochemistry. THP1 monocyte interaction with HAMEC was measured by cell adhesion and migration across transwells. Insulin transcytosis was measured by total internal reflection fluorescence microscopy. Palmitate, but not palmitoleate, elevated the expression of IL-6, IL-8, TLR2 (Toll-like receptor 2), and intercellular adhesion molecule 1 (ICAM-1). HAMEC had markedly low fatty acid uptake and oxidation, and CD36 inhibition did not reverse the palmitate-induced expression of adhesion molecules, suggesting that inflammation did not arise from palmitate uptake/metabolism. Instead, inhibition of TLR4 to NF-κB signaling blunted palmitate-induced ICAM-1 expression. Importantly, palmitate-induced surface expression of ICAM-1 promoted monocyte binding and transmigration. Conversely, palmitate reduced insulin transcytosis, an effect reversed by TLR4 inhibition. In summary, palmitate activates inflammatory pathways in primary microvascular endothelial cells, impairing insulin transport and increasing monocyte transmigration. This behavior may contribute in vivo to reduced tissue insulin action and enhanced tissue infiltration by immune cells.

Lipid-induced insulin resistance does not impair insulin access to skeletal muscle

Elevated plasma free fatty acids (FFA) induce insulin resistance in skeletal muscle. Previously, we have shown that experimental insulin resistance induced by lipid infusion prevents the dispersion of insulin through the muscle, and we hypothesized that this would lead to an impairment of insulin moving from the plasma to the muscle interstitium. Thus, we infused lipid into our anesthetized canine model and measured the appearance of insulin in the lymph as a means to sample muscle interstitium under hyperinsulinemic euglycemic clamp conditions. Although lipid infusion lowered the glucose infusion rate and induced both peripheral and hepatic insulin resistance, we were unable to detect an impairment of insulin access to the lymph. Interestingly, despite a significant, 10-fold increase in plasma FFA, we detected little to no increase in free fatty acids or triglycerides in the lymph after lipid infusion. Thus, we conclude that experimental insulin resistance induced by lipid infusion does not reduce insulin access to skeletal muscle under clamp conditions. This would suggest that the peripheral insulin resistance is likely due to reduced cellular sensitivity to insulin in this model, and yet we did not detect a change in the tissue microenvironment that could contribute to cellular insulin resistance.

Modest hyperglycemia prevents interstitial dispersion of insulin in skeletal muscle

Insulin injected directly into skeletal muscle diffuses rapidly through the interstitial space to cause glucose uptake, but this is blocked in insulin resistance. As glucotoxicity is associated with endothelial dysfunction, the observed hyperglycemia in diet-induced obese dogs may inhibit insulin access to muscle cells, and exacerbate insulin resistance. Here we asked whether interstitial insulin diffusion is reduced in modest hyperglycemia, similar to that induced by a high fat diet.

METHODS

During normoglycemic (100 mg/dl) and moderately hyperglycemic (120 mg/dl) clamps in anesthetized canines, sequential doses of insulin were injected into the vastus medialis of one hindlimb; the contra-lateral limb served as a control. Plasma samples were collected and analyzed for insulin content. Lymph vessels of the hind leg were also catheterized, and lymph samples were analyzed as an indicator of interstitial insulin concentration.

RESULTS

Insulin injection increased lymph insulin in normoglycemic animals, but not in hyperglycemic animals. Muscle glucose uptake was elevated in response to hyperglycemia, however the insulin-mediated glucose uptake in normoglycemic controls was not observed in hyperglycemia. Modest hyperglycemia prevented intra-muscularly injected insulin from diffusing through the interstitial space reduced insulin-mediated glucose uptake.

CONCLUSION

Hyperglycemia prevents the appearance of injected insulin in the interstitial space, thus reducing insulin action on skeletal muscle cells.

Altered transendothelial transport of hormones as a contributor to diabetes

The vascular endothelium is a dynamic structure responsible for the separation and regulated movement of biological material between circulation and interstitial fluid. Hormones and nutrients can move across the endothelium either via a transcellular or paracellular route. Transcellular endothelial transport is well understood and broadly acknowledged to play an important role in the normal and abnormal physiology of endothelial function. However, less is known about the role of the paracellular route. Although the concept of endothelial dysfunction in diabetes is now widely accepted, we suggest that alterations in paracellular transport should be studied in greater detail and incorporated into this model. In this review we provide an overview of endothelial paracellular permeability and discuss its potential importance in contributing to the development of diabetes and associated complications. Accordingly, we also contend that if better understood, altered endothelial paracellular permeability could be considered as a potential therapeutic target for diabetes.

Treating Diabetes with Exercise - Focus on the Microvasculature

The rising incidence of diabetes and the associated metabolic diseases including obesity, cardiovascular disease and hypertension have led to investigation of a number of drugs to treat these diseases. However, lifestyle interventions including diet and exercise remain the first line of defense. The benefits of exercise are typically presented in terms of weight loss, improved body composition and reduced fat mass, but exercise can have many other beneficial effects. Acute effects of exercise include major changes in blood flow through active muscle, an active hyperemia that increases the delivery of oxygen to the working muscle fibers. Longer term exercise training can affect the vasculature, improving endothelial health and possibly basal metabolic rates. Further, insulin sensitivity is improved both acutely after a single bout of exercise and shows chronic effects with exercise training, effectively reducing diabetes risk. Exercise-mediated improvements in endothelial function may also reduce complications associated with both diabetes and other metabolic disease. Thus, while drugs to improve microvascular function in diabetes continue to be investigated, exercise can also provide many similar benefits on endothelial function and should remain the first prescription when treating insulin resistance and diabetes. This review will investigate the effects of exercise on the blood vessel and the potential benefits of exercise on cardiovascular disease and diabetes.

Insulin resistance protects the heart from fuel overload in dysregulated metabolic states

Reversing impaired insulin sensitivity has been suggested as treatment for heart failure. However, recent clinical evidence suggests the opposite. Here we present a line of reasoning in support of the hypothesis that insulin resistance protects the heart from the consequences of fuel overload in the dysregulated metabolic state of obesity and diabetes. We discuss pathways of myocardial fuel toxicity, as well as several layers of defense against fuel overload. Our reassessment of the literature suggests that in the heart, insulin-sensitizing agents result in an elimination of some of the defenses, leading to cytotoxic damage. In contrast, a normalization of fuel supply should either prevent or reverse the process. Taken together, we offer a new perspective on insulin resistance of the heart.

Skeletal muscle tissue transcriptome differences in lean and obese female beagle dogs

Skeletal muscle is a large and insulin-sensitive tissue that is an important contributor to metabolic homeostasis and energy expenditure. Many metabolic processes are altered with obesity, but the contribution of muscle tissue in this regard is unclear. A limited number of studies have compared skeletal muscle gene expression of lean and obese dogs. Using microarray technology, our objective was to identify genes and functional classes differentially expressed in skeletal muscle of obese (14.6 kg; 8.2 body condition score; 44.5% body fat) vs. lean (8.6 kg; 4.1 body condition score; 22.9% body fat) female beagle adult dogs. Alterations in 77 transcripts was observed in genes pertaining to the functional classes of signaling, transport, protein catabolism and proteolysis, protein modification, development, transcription and apoptosis, cell cycle and differentiation. Genes differentially expressed in obese vs. lean dog skeletal muscle indicate oxidative stress and altered skeletal muscle cell differentiation. Many genes traditionally associated with lipid, protein and carbohydrate metabolism were not altered in obese vs. lean dogs, but genes pertaining to endocannabinoid metabolism, insulin signaling, type II diabetes mellitus and carnitine transport were differentially expressed. The relatively small response of skeletal muscle could indicate that changes are occurring at a post-transcriptional level, that other tissues (e.g., adipose tissue) were buffering skeletal muscle from metabolic dysfunction or that obesity-induced changes in skeletal muscle require a longer period of time and that the length of our study was not sufficient to detect them. Although only a limited number of differentially expressed genes were detected, these results highlight genes and functional classes that may be important in determining the etiology of obesity-induced derangement of skeletal muscle function.

The endothelium in diabetes: its role in insulin access and diabetic complications

The vascular endothelium has been identified as an important component in diabetes-associated complications, which include many cardiovascular disorders such as atherosclerosis, hypertension and peripheral neuropathy. Additionally, insulin's actions on the endothelium are now seen as a major factor in the metabolic effects of the hormone by increasing access to insulin sensitive tissues. Endothelial function is impaired in diabetes, obesity, and the metabolic syndrome, which could reduce insulin access to the tissue, and thus reduce insulin sensitivity independently of direct effects at the muscle cell. As such, the endothelium is a valid target for treatment of both the impaired glucose metabolism in diabetes, as well as the vascular based complications of diabetes. Here we review the basics of the endothelium in insulin action, with a focus on the skeletal muscle as insulin's major metabolic organ, and how this is affected by diabetes. We will focus on the most recent developments in the field, including current treatment possibilities.

Endothelial dysfunction in (pre)diabetes: characteristics, causative mechanisms and pathogenic role in type 2 diabetes

Endothelial dysfunction associated with diabetes and cardiovascular disease is characterized by changes in vasoregulation, enhanced generation of reactive oxygen intermediates, inflammatory activation, and altered barrier function. These endothelial alterations contribute to excess cardiovascular disease in diabetes, but may also play a role in the pathogenesis of diabetes, especially type 2. The mechanisms underlying endothelial dysfunction in diabetes differ between type 1 (T1D) and type 2 diabetes (T2D): hyperglycemia contributes to endothelial dysfunction in all individuals with diabetes, whereas the causative mechanisms in T2D also include impaired insulin signaling in endothelial cells, dyslipidemia and altered secretion of bioactive substances (adipokines) by adipose tissue. The close association of so-called perivascular adipose tissue with arteries and arterioles facilitates the exposure of vascular endothelium to adipokines, particularly if inflammation activates the adipose tissue. Glucose and adipokines activate specific intracellular signaling pathways in endothelium, which in concert result in endothelial dysfunction in diabetes. Here, we review the characteristics of endothelial dysfunction in diabetes, the causative mechanisms involved and the role of endothelial dysfunction(s) in the pathogenesis of T2D. Finally, we will discuss the therapeutic potential of endothelial dysfunction in T2D.

The barrier within: endothelial transport of hormones

Hormones are involved in a plethora of processes including development and growth, metabolism, mood, and immune responses. These essential functions are dependent on the ability of the hormone to access its target tissue. In the case of endocrine hormones that are transported through the blood, this often means that the endothelium must be crossed. Many studies have shown that the concentrations of hormones and nutrients in blood can be very different from those surrounding the cells on the tissue side of the blood vessel endothelium, suggesting that transport across this barrier can be rate limiting for hormone action. This transport can be regulated by altering the surface area of the blood vessel available for diffusion through to the underlying tissue or by the permeability of the endothelium. Many hormones are known to directly or indirectly affect the endothelial barrier, thus affecting their own distribution to their target tissues. Dysfunction of the endothelial barrier is found in many diseases, particularly those associated with the metabolic syndrome. The interrelatedness of hormones may help to explain why the cluster of diseases in the metabolic syndrome occur together so frequently and suggests that treating the endothelium may ameliorate defects in more than one disease. Here, we review the structure and function of the endothelium, its contribution to the function of hormones, and its involvement in disease.

Lipid-induced insulin resistance is not mediated by impaired transcapillary transport of insulin and glucose in humans

Increased lipid availability reduces insulin-stimulated glucose disposal in skeletal muscle, which is generally explained by fatty acid-mediated inhibition of insulin signaling. It remains unclear whether lipids also impair transcapillary transport of insulin and glucose, which could become rate controlling for glucose disposal. We hypothesized that lipid-induced insulin resistance is induced by inhibiting myocellular glucose uptake and not by interfering with the delivery of insulin or glucose. We measured changes in interstitial glucose and insulin in skeletal muscle of healthy volunteers during intravenous administration of triglycerides plus heparin or glycerol during physiologic and supraphysiologic hyperinsulinemia, by combining microdialysis with oral glucose tolerance tests and euglycemic-hyperinsulinemic clamps. Lipid infusion reduced insulin-stimulated glucose disposal by ~70% (P < 0.05) during clamps and dynamic insulin sensitivity by ~12% (P < 0.05) during oral glucose loading. Dialysate insulin and glucose levels were unchanged or even transiently higher (P < 0.05) during lipid than during glycerol infusion, whereas regional blood flow remained unchanged. These results demonstrate that short-term elevation of free fatty acids (FFAs) induces insulin resistance, which in skeletal muscle occurs primarily at the cellular level, without impairment of local perfusion or transcapillary transport of insulin and glucose. Thus, vascular effects of FFAs are not rate controlling for muscle insulin-stimulated glucose disposal.