PAQR proteins and the evolution of a superpower: Eating all kinds of fats: Animals rely on evolutionarily conserved membrane homeostasis proteins to compensate for dietary variation

Recently published work showed that members of the PAQR protein family are activated by cell membrane rigidity and contribute to our ability to eat a wide variety of diets. Cell membranes are primarily composed of phospholipids containing dietarily obtained fatty acids, which poses a challenge to membrane properties because diets can vary greatly in their fatty acid composition and could impart opposite properties to the cellular membranes. In particular, saturated fatty acids (SFAs) can pack tightly and form rigid membranes (like butter at room temperature) while unsaturated fatty acids (UFAs) form more fluid membranes (like vegetable oils). Proteins of the PAQR protein family, characterized by the presence of seven transmembrane domains and a cytosolic N-terminus, contribute to membrane homeostasis in bacteria, yeasts, and animals. These proteins respond to membrane rigidity by stimulating fatty acid desaturation and incorporation of UFAs into phospholipids and explain the ability of animals to thrive on diets with widely varied fat composition. Also see the video abstract here: https://youtu.be/6ckcvaDdbQg.

APN Expression in Serum and Corpus Luteum: Regulation of Luteal Steroidogenesis Is Possibly Dependent on the AdipoR2/AMPK Pathway in Goats

Adiponectin (APN) is an essential adipokine for a variety of reproductive processes. To investigate the role of APN in goat corpora lutea (CLs), CLs and sera from different luteal phases were collected for analysis. The results showed that the APN structure and content had no significant divergence in different luteal phases both in CLs and sera; however, high molecular weight APN was dominant in serum, while low molecular weight APN was more present in CLs. The luteal expression of both AdipoR1/2 and T-cadherin (T-Ca) increased on D11 and 17. APN and its receptors (AdipoR1/2 and T-Ca) were mainly expressed in goat luteal steroidogenic cells. The steroidogenesis and APN structure in pregnant CLs had a similar model as in the mid-cycle CLs. To further explore the effects and mechanisms of APN in CLs, steroidogenic cells from pregnant CLs were isolated to detect the AMPK-mediated pathway by the activation of APN (AdipoRon) and knockdown of APN receptors. The results revealed that P-AMPK in goat luteal cells increased after incubation with APN (1 μg/mL) or AdipoRon (25 μM) for 1 h, and progesterone (P4) and steroidogenic proteins levels (STAR/CYP11A1/HSD3B) decreased after 24 h. APN did not affect the steroidogenic protein expression when cells were pretreated with Compound C or SiAMPK. APN increased P-AMPK and reduced the CYP11A1 expression and P4 levels when cells were pretreated with SiAdipoR1 or SiT-Ca, while APN failed to affect P-AMPK, the CYP11A1 expression or the P4 levels when pretreated with SiAdipoR2. Therefore, the different structural forms of APN in CLs and sera may possess distinct functions; APN might regulate luteal steroidogenesis through AdipoR2 which is most likely dependent on AMPK.

Distribution of Adiponectin Receptors in the Brain of Adult Mouse: Effect of a Single Dose of the Adiponectin Receptor Agonist, AdipoRON, on Ischemic Stroke

Adiponectin exhibits pleiotropic effects, including anti-inflammatory, anti-apoptotic, anti-oxidant, and neuroprotective ones. Although some studies have documented brain expression in different rodent models of its receptors, AdipoR1 and AdipoR2, their global distribution remains incomplete. Here, we demonstrated that both AdipoR are widely distributed in the brains of adult mice. Furthermore, by double immunostaining studies, we showed that AdipoR1 and AdipoR2 are mainly expressed in neurons and blood vessels. Then, considering the wide distribution of both receptors and the neuroprotective effects of adiponectin, we tested the therapeutic effect of a single injection of the adiponectin receptor agonist, AdipoRON (5 mg.kg⁻¹), 24 h after stroke in a model of middle cerebral artery occlusion technique (MCAO). Under our experimental conditions, we demonstrated that AdipoRON did not modulate the infarct volume, cell death, neuroinflammatory parameters including microglia activation and oxidative stress. This study suggests that a protocol based on multiple injections of AdipoRON at a higher dose after MCAO could be considered to promote the therapeutic properties of AdipoRON on the brain repair mechanism and recovery.

Elucidating exercise-induced skeletal muscle signaling pathways and applying relevant findings to preemptive therapy for lifestyle-related diseases

While it is well recognized that exercise represents a radical preventive and therapeutic measure for lifestyle-related diseases, it is clear that contemporary lifestyles abound with situations where exercise may be found difficult to implement on a continuous basis. Indeed, this has led to global expectations for elucidation of the exercise-activated skeletal muscle signaling pathways as well as for development of exercise mimics that effectively activate such pathways. It is shown that exercise activates the transcriptional coactivator PGC-1α via AMPK/SIRT1 in muscle, thereby not only enhancing mitochondrial function and muscle endurance but upregulating energy metabolism. Further, adipocyte-derived adiponectin is also shown to activate AMPK/SIRT1/PGC-1α via its receptor AdipoR1 in skeletal muscles. Thus, adiponectin/AdipoR1 signaling is thought to constitute exercise-mimicking signaling. Indeed, it has become clear that AMPK, SIRT1 and AdipoR activators act as exercise mimetics. With the crystal structures of AdipoR elucidated and humanized AdipoR mice generated toward optimization of candidate AdipoR-activators for human use, expectations are mounting for the clinical application in the near future of AdipoR activators as exercise mimetics in humans. This review provides an overview of molecules activated by exercise and compounds activating these molecules, with a focus on the therapeutic potential of AdipoR activators as exercise mimetics.

Estradiol overcomes adiponectin-resistance in diabetic mice by regulating skeletal muscle adiponectin receptor 1 expression

Adiponectin and insulin resistance creates a vicious cycle that exacerbates type 2 diabetes. Earlier, we observed that female leptin receptor-deficient BLKS mice (BKS-db/db) were more sensitive to an adiponectin mimetic GTDF than males, which led us to explore if E2 plays a crucial role in modulation of adiponectin-sensitivity. Male but not female BKS-db/db mice were resistant to metabolic effects of globular adiponectin treatment. Male BKS-db/db displayed reduced skeletal muscle AdipoR1 protein expression, which was consequent to elevated polypyrimidine tract binding protein 1 (PTB) and miR-221. E2 treatment in male BKS-db/db, and ovariectomized BALB/c mice rescued AdipoR1 protein expression via downregulation of PTB and miR-221, and also directly increased AdipoR1 mRNA by its classical nuclear receptors. Estrogen receptor regulation via dietary or pharmacological interventions may improve adiponectin resistance and consequently ameliorate insulin resistance in type 2 diabetes.

Adiponectin regulates electroacupuncture-produced analgesic effects in association with a crosstalk between the peripheral circulation and the spinal cord

Neurotransmitter-mediated acupuncture analgesia has been widely studied in nervous systems. It remains largely unclear if peripheral substances are involved the acupuncture analgesia. Adiponectin (APN), a circulating adipokine, shows analgesic effects. The study aimed to examine whether APN regulates analgesic effects of electroacupuncture (EA) in the complete Freund's adjuvant (CFA)-induced mouse model. APN wild type (WT) and knockout (KO) mouse were employed in the study. We found that EA attenuates the CFA-induced pain as demonstrated by the Hargreaves thermal test and the von Frey filament test. The deletion of APN significantly reduced the acupuncture analgesia in the CFA-treated APN KO mice while the intrathecal administration of APN mimicked the analgesic effects of EA. We further revealed that EA produced analgesic effects mainly via APN/AdipoR2-mediated AMPK pathway by the siRNA inhibitions of APN receptors (adipoR1/2) in the spinal cord. The immunofluorescence staining analysis showed that EA increased the APN accumulation in spinal cord through the blood circulation. In conclusion, the study indicates a novel mechanism that acupuncture produces analgesic effects at least partially via APN/AdipoR2-AMPK pathway in the spinal cord.

Tetrahydrocurcumin Upregulates the Adiponectin-AdipoR Pathway and Improves Insulin Signaling and Pancreatic β-Cell Function in High-Fat Diet/Streptozotocin-Induced Diabetic Obese Mice

Impairment of adiponectin production and function is closely associated with insulin resistance and type 2 diabetes, which are linked to obesity. Studies in animal models have documented the anti-diabetic effects of tetrahydrocurcumin (THC). Although several possible mechanisms have been proposed, the contribution of adiponectin signaling on THC-mediated antihyperglycemic effects remains unknown. Here, we report that adiposity, steatosis, and hyperglycemia were potently attenuated in high-fat diet/streptozotocin-induced diabetic obese mice after they received 20 and 100 mg/kg THC for 14 weeks. THC upregulated UCP-1 in adipose tissue and elevated adiponectin levels in the circulation. THC upregulated the AdipoR1/R2-APPL1-mediated pathway in the liver and skeletal muscle, which contributes to improved insulin signaling, glucose utilization, and lipid metabolism. Furthermore, THC treatment significantly (p < 0.05) preserved islet mass, reduced apoptosis, and restored defective insulin expression in the pancreatic β-cells of diabetic obese mice, which was accompanied by an elevation of AdipoR1 and APPL1. These results demonstrated a potential mechanism underlying the beneficial effects of THC against hyperglycemia via the adiponectin-AdipoR pathway, and thus, may lead to a novel therapeutic use for type 2 diabetes.

A Genetic Titration of Membrane Composition in C. elegans Reveals its Importance for Multiple Cellular and Physiological Traits

Communicating editor: B. Grant

The composition and biophysical properties of cellular membranes must be tightly regulated to maintain the proper functions of myriad processes within cells. To better understand the importance of membrane homeostasis, we assembled a panel of five Caenorhabditis elegans strains that show a wide span of membrane composition and properties, ranging from excessively rich in saturated fatty acids (SFAs) and rigid to excessively rich in polyunsaturated fatty acids (PUFAs) and fluid. The genotypes of the five strain are, from most rigid to most fluid: paqr-1(tm3262); paqr-2(tm3410), paqr-2(tm3410), N2 (wild-type), mdt-15(et14); nhr-49(et8), and mdt-15(et14); nhr-49(et8); acs-13(et54). We confirmed the excess SFA/rigidity-to-excess PUFA/fluidity gradient using the methods of fluorescence recovery after photobleaching (FRAP) and lipidomics analysis. The five strains were then studied for a variety of cellular and physiological traits and found to exhibit defects in: permeability, lipid peroxidation, growth at different temperatures, tolerance to SFA-rich diets, lifespan, brood size, vitellogenin trafficking, oogenesis, and autophagy during starvation. The excessively rigid strains often exhibited defects in opposite directions compared to the excessively fluid strains. We conclude that deviation from wild-type membrane homeostasis is pleiotropically deleterious for numerous cellular/physiological traits. The strains introduced here should prove useful to further study the cellular and physiological consequences of impaired membrane homeostasis.

Comparative Analysis of CTRP-Mediated Effects on Cardiomyocyte Glucose Metabolism: Cross Talk between AMPK and Akt Signaling Pathway

C1q/tumor necrosis factor -alpha-related proteins (CTRPs) have been shown to mediate protective cardiovascular effects, but no data exists on their effects on glucose and fatty acid (FA) metabolism in cardiomyocytes. In the present study, adult rat cardiomyocytes and H9C2 cardiomyoblasts were stimulated with various recombinant CTRPs. Glucose or FA uptake, expression of genes involved in glucose or FA metabolism and the role of the AMP-activated protein kinase (AMPK) and Akt were investigated. Although most CTRPs induced an increase in phosphorylation of AMPK and Akt in cardiomyocytes, mainly CTRP2, 7, 9 and 13 induced GLUT1 and GLUT4 translocation and glucose uptake in cardiomyocytes, despite high structural similarities among CTRPs. AMPK inhibition reduced the CTRPs-mediated activation of Akt, while Akt inhibition did not impair AMPK activation. In addition, CTRP2, 7, 9 and 13 mediated strong effects on the expression of enzymes involved in glucose or FA metabolism. Loss of adiponectin receptor 1, which has been suggested to be involved in CTRP-induced signal transduction, abolished the effects of some but not all CTRPs on glucose metabolism. Targeting the AMPK signaling pathway via CTRPs may offer a therapeutic principle to restore glucose homeostasis by acting on glucose uptake independent of the Akt pathway.

Adiponectin: Role in Physiology and Pathophysiology

Adiponectin, an adipokine secreted by adipocytes, is a well-known homeostatic factor for regulating glucose levels, lipid metabolism, and insulin sensitivity through its anti-inflammatory, anti-fibrotic, and antioxidant effects. All these metabolic processes are mediated via two adiponectin receptors, AdipoR1 and AdipoR2. In addition, adiponectin is one of the hormones with the highest plasma concentrations. Weight loss or caloric restriction leads to increasing adiponectin levels, and this increase is associated with increased insulin sensitivity. Therefore, the adiponectin pathway can play a crucial role in the development of drugs to treat type 2 diabetes mellitus and other obesity-related diseases affected by insulin resistance like cancers or cardiovascular diseases. Adiponectin appears to increase insulin sensitivity by improving glucose and lipid metabolisms. The objective of this review is to analyze current knowledge concerning adiponectin and, in particular, its role in physiology and pathophysiology.

Adipokines underlie the early origins of obesity and associated metabolic comorbidities in the offspring of women with pregestational obesity

Maternal pregestational obesity is a well-known risk factor for offspring obesity, metabolic syndrome, cardiovascular disease and type 2 diabetes. The mechanisms by which maternal obesity can induce alterations in fetal and later neonatal metabolism are not fully elucidated due to its complexity and multifactorial causes. Two adipokines, leptin and adiponectin, are involved in fetal and postnatal growth trajectories, and both are altered in women with pregestational obesity. The placenta synthesizes leptin, which goes mainly to the maternal circulation and in lesser amount to the developing fetus. Maternal pregestational obesity and hyperleptinemia are associated with placental dysfunction and changes in nutrient transporters which directly affect fetal growth and development. By the other side, the embryo can produce its own leptin from early in development, which is associated to fetal weight and adiposity. Adiponectin, an insulin-sensitizing adipokine, is downregulated in maternal obesity. High molecular weight (HMW) adiponectin is the most abundant form and with most biological actions. In maternal obesity lower total and HMW adiponectin levels have been described in the mother, paralleled with high levels in the umbilical cord. Several studies have found that cord blood adiponectin levels are related with postnatal growth trajectories, and it has been suggested that low adiponectin levels in women with pregestational obesity enhance placental insulin sensitivity and activation of placental amino acid transport systems, supporting fetal overgrowth. The possible mechanisms by which maternal pregestational obesity, focusing in the actions of leptin and adiponectin, affects the fetal development and postnatal growth trajectories in their offspring are discussed.

Drug development research for novel adiponectin receptor-targeted antidiabetic drugs contributing to healthy longevity

It is well recognized that the decrease of adiponectin associated with high-fat diet and lack of exercise accounts for the onset of insulin resistance, type 2 diabetes, the metabolic syndrome, and cardiovascular disease. Our research efforts have led to the identification of adiponectin receptors, AdipoR1 and AdipoR2, with the former shown to activate AMP kinase in the liver and the latter shown to activate peroxisome proliferator-activated receptor-α signaling thereby increasing fatty acid oxidation. Again, adiponectin upregulates mitochondrial function in the skeletal muscle thereby improving glucose/lipid metabolism and insulin resistance. These findings suggested that activation of adiponectin/AdipoR signaling could represent a viable therapeutic approach to lifestyle-linked diseases associated with prevalent obesity thus contributing to healthy longevity in humans. Indeed, they have led to the successful discovery of AdipoRon, a small-molecule AdipoR-activating compound. Thus far, AdipoRon has been found not only to improve insulin resistance in mice but to prolong their lifespan shortened by high-fat diet. Additionally, our structure-based drug discovery research has led to AdipoR being identified as an entirely novel structure having a zinc iron bound within its seven-transmembrane domain as well as an opposite orientation to that of G protein-coupled receptors. It is expected that increasing insight into AdipoR signaling will facilitate the structure-based optimization of candidate small-molecule AdipoR-activating compounds for human use as well as the development of molecularly targeted and calorie-limiting/exercise-mimicking agents for lifestyle-linked diseases.

Adiponectin/AdipoR Research and Its Implications for Lifestyle-Related Diseases

The number of patients with obesity continues to increase seriously worldwide. It has become clear that, against a background of insulin resistance, obesity induces the so-called metabolic syndrome consisting of diabetes, hypertension, and dyslipidemia, leading, consequently, to an increased incidence of cardiovascular disease in affected individuals. It is shown that environmental factors, e.g., high-fat diet and lack of physical activity, not only promote the onset of obesity but lead to impairment of the action of adiponectin and its receptors, thus accounting in part for the onset of insulin resistance, type 2 diabetes/metabolic syndrome, and atherosclerosis in modern society. This review is intended to highlight some milestones in adipocyte research from the discovery of the insulin-sensitizing properties of adiponectin to the elucidation of the structures of its receptors, as well as to clarify their therapeutic implications and prospects for lifestyle-related diseases.

AdipoRs- a potential therapeutic target for fibrotic disorders

Introduction: Fibrotic disorders are a leading cause of morbidity and mortality; hence effective treatments are still vigorously sought. AdipoRs (AdipoR1 and Adipo2) are responsible for the antifibrotic effects of adiponectin (APN). APN exerts antifibrotic effects by binding to its receptors. APN concentration and AdipoR expression are closely associated with fibrotic disorders. Decreased AdipoR expression may reduce APN-AdipoR signaling, while the upregulation of AdipoR expression may restore the anti-fibrotic effects of APN. Loss of APN signaling exacerbates fibrosis in vivo and in vitro. Areas covered: We assess the relationship between APN and fibrotic disorders, the structure of receptors for APN and the pathways accounting for APN or its analogs blocking fibrotic disorders. This article also discusses designed APN products and their therapeutic prospects for fibrotic disorders. Expert opinion: AdipoRs have a critical role in blocking fibrosis. The development of small-molecule agonists toward this target represents a valid drug development pathway.

Structure and function analysis of adiponectin receptors toward development of novel antidiabetic agents promoting healthy longevity

Plasma adiponectin levels and expression of its receptors, AdipoRs are decreased in obesity, which cause insulin resistance and diabetes. AdipoR-deficient mice exhibit insulin resistance and impaired glucose tolerance. Moreover, newly identified AdipoR agonists not only improve insulin resistance but prolong lifespan shortened by obesity via AdipoR. Furthermore, efforts to promote structure-based drug discovery research at our laboratory have led to the first ever successful crystallization of AdipoR as well as to clarification of their structures. Structural analysis of AdipoRs as key molecules in lifestyle-related diseases is thus expected to lead not only to the acceleration of structure-based drug discovery but to the elucidation of novel aspects of the AdipoR structures and functions in the years to come. Finally, with the development of novel AdipoR-targeted antidiabetic agents also capable of prolonging lifespan, the attainment of healthy longevity may finally be brought within reach.

AdipoR-increased intracellular ROS promotes cPLA2 and COX-2 expressions via activation of PKC and p300 in adiponectin-stimulated human alveolar type II cells

Adiponectin, an adipokine, accumulated in lung system via T-cadherin after allergens/ozone challenge. However, the roles of adiponectin on lung pathologies were controversial. Here we reported that adiponectin stimulated expression of inflammatory proteins, cytosolic phospholipase A2 (cPLA2), cyclooxygenase-2 (COX-2), and production of reactive oxygen species (ROS) in human alveolar type II A549 cells. AdipoR1/2 involved in adiponectin-activated NADPH oxidase and mitochondria, which further promoted intracellular ROS accumulation. Protein kinase C (PKC) may involve an adiponectin-activated NADPH oxidase. Similarly, p300 phosphorylation and histone H4 acetylation occurred in adiponectin-challenged A549 cells. Moreover, adiponectin-upregulated cPLA2 and COX-2 expression was significantly abrogated by ROS scavenger (N-acetylcysteine) or the inhibitors of NADPH oxidase (apocynin), mitochondrial complex I (rotenone), PKC (Ro31-8220, Gö-6976, and rottlerin), and p300 (garcinol). Briefly, we reported that adiponectin stimulated cPLA2 and COX-2 expression via AdipoR1/2-dependent activation of PKC/NADPH oxidase/mitochondria resulting in ROS accumulation, p300 phosphorylation, and histone H4 acetylation. These results suggested that adiponectin promoted lung inflammation, resulting in exacerbation of pulmonary diseases via upregulating cPLA2 and COX-2 expression together with intracellular ROS production. Understanding the adiponectin signaling pathways on regulating cPLA2 and COX-2 may help develop therapeutic strategies on pulmonary diseases.

Perspective of Small-Molecule AdipoR Agonist for Type 2 Diabetes and Short Life in Obesity

Obesity associated with unhealthy diet and lack of exercise is shown to contribute to the onset and/or aggravation of the metabolic syndrome and diabetes, thus placing affected individuals at increased risk of cardiovascular disease and cancer. Plasma adiponectin levels are decreased in obesity, which causes insulin resistance and diabetes. Therefore, we identified adiponectin receptors (AdipoRs) as the therapeutic target. It was suggested that, similarly to caloric restriction and exercise, activation of the AdipoRs may have the potential not only to improve lifestyle-related diseases but to contribute to prolonged the shortened lifespan on a high caloric unhealthy diet. To this end, we have identified "AdipoRon" as an adiponectin receptor agonist. Indeed, AdipoRon ameliorated diabetes associated with obesity as well as to increase exercise endurance, thus prolonging shortened lifespan of obese mice fed on a high fat diet. Additionally, we have recently determined the crystal structures of the human AdipoRs. The seven-transmembrane helices of AdipoRs are structurally distinct from those of G-protein coupled receptors. It is expected that these findings will contribute not only to the elucidation of the AdipoR-related signal transduction but to the development and optimization of AdipoR-targeted therapeutics for obesity-related diseases such as diabetes.

Insulin-independent role of adiponectin receptor signaling in Drosophila germline stem cell maintenance

Adipocytes have key endocrine roles, mediated in large part by secreted protein hormones termed adipokines. The adipokine adiponectin is well known for its role in sensitizing peripheral tissues to insulin, and several lines of evidence suggest that adiponectin might also modulate stem cells/precursors. It remains unclear, however, how adiponectin signaling controls stem cells and whether this role is secondary to its insulin-sensitizing effects or distinct. Drosophila adipocytes also function as an endocrine organ and, although no obvious adiponectin homolog has been identified, Drosophila AdipoR encodes a well-conserved homolog of mammalian adiponectin receptors. Here, we generate a null AdipoR allele and use clonal analysis to demonstrate an intrinsic requirement for AdipoR in germline stem cell (GSC) maintenance in the Drosophila ovary. AdipoR null GSCs are not fully responsive to bone morphogenetic protein ligands from the niche and have a slight reduction in E-cadherin levels at the GSC-niche junction. Conversely, germline-specific overexpression of AdipoR inhibits natural GSC loss, suggesting that reduction in adiponectin signaling might contribute to the normal decline in GSC numbers observed over time in wild-type females. Surprisingly, AdipoR is not required for insulin sensitization of the germline, leading us to speculate that insulin sensitization is a more recently acquired function than stem cell regulation in the evolutionary history of adiponectin signaling. Our findings establish Drosophila female GSCs as a new system for future studies addressing the molecular mechanisms whereby adiponectin receptor signaling modulates stem cell fate.

Globular adiponectin and its downstream target genes are up-regulated locally in human colorectal tumors: ex vivo and in vitro studies

OBJECTIVE

Low plasma adiponectin levels are linked to obesity, insulin resistance, and the risk of several types of malignancy. Despite the decline in circulating adiponectin concentrations, the increase in the expression of adiponectin receptors AdipoR1 and AdipoR2 is greater in cancerous than in normal colonic tissue. The purpose of this study was to obtain new information regarding local adiponectin signaling in the pathogenesis of colorectal cancer (CRC).

METHODS

We characterized the expressions of adiponectin and several of its downstream targets in paired samples of tumor tissue and adjacent noncancerous mucosa in 60 surgical patients with colorectal adenocarcinomas.

RESULTS

Adiponectin was expressed in both colorectal tumors and the adjacent mucosa. The expressions of adiponectin mRNA and its globular protein variant (gAd), adiponectin receptor type 1 and 5' AMP-activated protein kinase (AMPK) mRNA were significantly higher in colorectal tumors than in the adjacent mucosa. This finding was accompanied by increased mRNA expression of genes encoding proteins involved in fatty-acid trafficking and oxidation. The potential interference between adiponectin stimulation and AMPK activation through AMPK1 was examined in an in vitro model with the aid of silencing-RNA experiments. Furthermore, AMPK mRNA expression on tumors was positively correlated with a more advanced tumor stage in the patients.

CONCLUSION

We propose that the globular adiponectin-AMPK pathway functions in an autocrine manner in colorectal tumors, explaining some of the beneficial changes in cellular oxidative capacity in tumors in favor of tumorigenesis.

Adiponectin, driver or passenger on the road to insulin sensitivity?

Almost 20 years have passed since the first laboratory evidence emerged that an abundant message encoding a protein with homology to the C1q superfamily is highly specifically expressed in adipocytes. At this stage, we refer to this protein as adiponectin. Despite more than 10,000 reports in the literature since its initial description, we seem to have written only the first chapter in the textbook on adiponectin physiology. With every new aspect we learn about adiponectin, a host of new questions arise with respect to the underlying molecular mechanisms. Here, we aim to summarize recent findings in the field and bring the rodent studies that suggest a causal relationship between adiponectin levels in plasma and systemic insulin sensitivity in perspective with the currently available data on the clinical side.