To fulfill its role as the major energy-storing tissue, adipose has several unique properties that cannot be seen in any other organ, including an almost unlimited capacity to expand in a non-transformed state. As such, the tissue requires potent mechanisms to remodel, acutely and chronically. Adipocytes can rapidly reach the diffusional limit of oxygen during growth; hypoxia is therefore an early determinant that limits healthy expansion. Proper expansion requires a highly coordinated response among many different cell types, including endothelial precursor cells, immune cells, and preadipocytes. There are therefore remarkable similarities between adipose expansion and growth of solid tumors, a phenomenon that presents both an opportunity and a challenge, since pharmacological interventions supporting healthy adipose tissue adaptation can also facilitate tumor growth. IntroductionAdipose tissue (AT) can respond rapidly and dynamically to alterations in nutrient deprivation and excess through adipocyte hypertrophy and hyperplasia, thereby fulfilling its major role in wholebody energy homeostasis. AT remodeling is an ongoing process that is pathologically accelerated in the obese state, and thus, features such as reduced angiogenic remodeling, ECM overproduction, a heightened state of immune cell infiltration and subsequent proinflammatory responses prevail in many obese fat-pads (1). However, not all AT expansion is necessarily associated with pathological changes. The concept of the "metabolically healthy obese" state (2) suggests that some individuals can preserve systemic insulin sensitivity on the basis of "healthy" AT expansion, bypassing all of the aforementioned pathological consequences associated with obesity (3), thereby also avoiding the obesity-associated lipotoxic side effects. Many physiologically relevant processes important for human AT remodeling can be studied in rodent models, with the added advantage that processes related to AT expansion and reduction can occur at an extremely rapid rate. A 24-hour fast in a mouse is associated with a dramatic loss of AT mass and an acute remodeling process that involves rapid infiltration of macrophages; moreover, merely 24 to 48 hours of exposure to a high-fat diet (HFD) can cause a prompt increase in adipocyte size (4). AT is therefore an ideal model system to study rapid alterations in tissue expansion and reduction, as it adapts to a differential nutrient supply. Here, we will focus on key aspects of the intricate dynamics of AT remodeling and subsequent inflammatory consequences that arise from obesity.

Exercise has beneficial effects on human health, including protection against metabolic disorders such as diabetes1. However, the cellular mechanisms underlying these effects are incompletely understood. The lysosomal degradation pathway, autophagy, is an intracellular recycling system that functions during basal conditions in organelle and protein quality control2. During stress, increased levels of autophagy permit cells to adapt to changing nutritional and energy demands through protein catabolism3. Moreover, in animal models, autophagy protects against diseases such as cancer, neuro-degenerative disorders, infections, inflammatory diseases, ageing and insulin resistance4-6. Here we show that acute exercise induces autophagy in skeletal and cardiac muscle of fed mice. To investigate the role of exercise-mediated autophagy in vivo, we generated mutant mice that show normal levels of basal autophagy but are deficient in stimulus (exercise- or starvation)-induced autophagy. These mice (termed BCL2 AAA mice) contain knock-in mutations in BCL2 phosphorylation sites (Thr69Ala, Ser70Ala and Ser84Ala) that prevent stimulus-induced disruption of the BCL2-beclin-1 complex and autophagy activation. BCL2 AAA mice show decreased endurance and altered glucose metabolism during acute exercise, as well as impaired chronic exercise-mediated protection against high-fat-diet-induced glucose intolerance. Thus, exercise induces autophagy, BCL2 is a crucial regulator of exercise- (and starvation)- induced autophagy in vivo, and autophagy induction may contribute to the beneficial metabolic effects of exercise.

The adipocyte-derived secretory factor adiponectin promotes insulin sensitivity, decreases inflammation and promotes cell survival. To date, no unifying mechanism explains how adiponectin can exert such a variety of beneficial systemic effects. Here, we show that adiponectin potently stimulates a ceramidase activity associated with its two receptors, adipoR1 and adipoR2, and enhances ceramide catabolism and formation of its anti-apoptotic metabolite – sphingosine-1-phosphate (S1P), independently of AMPK. Using models of inducible apoptosis in pancreatic β-cells and cardiomyocytes, we show that transgenic overproduction of adiponectin decreases caspase-8 mediated death, while genetic adiponectin ablation enhances apoptosis in vivo through a sphingolipid-mediated pathway. Ceramidase activity is impaired in cells lacking both adiponectin receptor isoforms, leading to elevated ceramide levels and enhanced susceptibility to palmitate-induced cell death. Combined, our observations suggest a novel unifying mechanism of action for the beneficial systemic effects exerted by adiponectin, with sphingolipid metabolism as its core upstream component.

Fibrosis is increasingly appreciated as a major player in adipose tissue dysfunction. In rapidly expanding adipose tissue, pervasive hypoxia leads to an induction of HIF1α that in turn leads to a potent pro-fibrotic transcriptional program. The pathophysiological impact of adipose tissue fibrosis is likely to play an equally important role on systemic metabolic alterations as fibrotic conditions play in the liver, heart and kidney. Here, we discuss recent advances in our understanding of the genesis, modulation and systemic impact of excessive extracellular matrix (ECM) accumulation in adipose tissue of both rodents and humans and the ensuing impact on metabolic dysfunction.

We examined rodent models with altered levels of mitoNEET, a protein residing in the mitochondrial outer membrane. Adipocyte-specific overexpression of mitoNEET enhances lipid-uptake and storage, leading to an expansion of adipose tissue mass. Despite the resulting massive obesity, benign aspects of adipose tissue expansion prevail and insulin sensitivity is preserved. MitoNEET inhibits mitochondrial iron transport into the matrix. Since iron is a rate-limiting component for electron transport, mitoNEET reduces β-oxidation rates. This is associated with reduced mitochondrial membrane potential and reduced reactive oxygen species damage, along with higher levels of adiponectin production. Conversely, the reduction of mitoNEET enhances mitochondrial respiratory capacity through enhanced iron content in the matrix, with reduced weight gain on a high fat diet. However, a reduction of mitoNEET also causes heightened oxidative-stress and glucose-intolerance. MitoNEET is therefore a potent regulator of mitochondrial function that profoundly impacts the dynamics of cellular and whole-body lipid homeostasis.