No abstract

Leptin elicits a metabolic response that cannot be explained by its anorectic effects alone. To examine the mechanism underlying leptin's metabolic actions, we used transcription profiling to identify leptin-regulated genes in ob/ob liver. Leptin was found to specifically repress RNA levels and enzymatic activity of hepatic stearoyl-CoA desaturase-1 (SCD-1), which catalyzes the biosynthesis of monounsaturated fatty acids. Mice lacking SCD-1 were lean and hypermetabolic. ob/ob mice with mutations in SCD-1 were significantly less obese than ob/ob controls and had markedly increased energy expenditure. ob/ob mice with mutations in SCD-1 had histologically normal livers with significantly reduced triglyceride storage and VLDL (very low density lipoprotein) production. These findings suggest that down-regulation of SCD-1 is an important component of leptin's metabolic actions.

Summary The transcriptional co-activator peroxisome proliferator-activated receptor-gamma co-activator-1 α (PGC-1α) regulates metabolic genes in skeletal muscle, and contributes substantially to the response of muscle to exercise. Muscle specific PGC-1α transgenic expression and exercise both increase the expression of thermogenic genes within white adipose. How the PGC-1α mediated response to exercise in muscle conveys signals to other tissues remains incompletely defined. We employed a metabolic profiling approach to examine metabolites secreted from myocytes with forced expression of PGC-1α, and identified β-aminoisobutyric acid (BAIBA) as a novel small molecule myokine. BAIBA increases the expression of brown adipocyte-specific genes in white adipose tissue and fatty acid β-oxidation in hepatocytes both in vitro and in vivo through a PPARα mediated mechanism, induces a brown adipose-like phenotype in human pluripotent stem cells, and improves glucose homeostasis in mice. In humans, plasma BAIBA concentrations are increased with exercise and inversely associated with metabolic risk factors. BAIBA may thus contribute to exercise-induced protection from metabolic diseases.

SUMMARY Genetic conservation allows ancient features of fat storage endocrine pathways to be explored in C. elegans. Multiple studies have used Nile red or BODIPY-labeled fatty acids to identify regulators of fat mass. When mixed with their food, E. coli bacteria, Nile red, and BODIPY-labeled fatty acids stain multiple spherical cellular structures in the C. elegans major fat storage organ, the intestine. However, here we demonstrate that, in the conditions previously reported, the lysosome-related organelles stained by Nile red and BODIPY-labeled fatty acids are not the C. elegans major fat storage compartment. We show that the major fat stores are contained in a distinct cellular compartment that is not stained by Nile red. Using biochemical assays, we validate oil red O staining as a method to assess major fat stores in C. elegans, allowing for efficient and accurate genetic and functional genomic screens for genes that control fat accumulation at the organismal level.

Rictor is a component of the target of rapamycin complex 2 (TORC2). While TORC2 has been implicated in insulin and other growth factor signaling pathways, the key inputs and outputs of this kinase complex remain unknown. We identified mutations in the Caenorhabditis elegans homolog of rictor in a forward genetic screen for increased body fat. Despite high body fat, rictor mutants are developmentally delayed, small in body size, lay an attenuated brood, and are short-lived, indicating that Rictor plays a critical role in appropriately partitioning calories between long-term energy stores and vital organismal processes. Rictor is also necessary to maintain normal feeding on nutrient-rich food sources. In contrast to wild-type animals, which grow more rapidly on nutrient-rich bacterial strains, rictor mutants display even slower growth, a further reduced body size, decreased energy expenditure, and a dramatically extended life span, apparently through inappropriate, decreased consumption of nutrient-rich food. Rictor acts directly in the intestine to regulate fat mass and whole-animal growth. Further, the high-fat phenotype of rictor mutants is genetically dependent on akt-1, akt-2, and serum and glucocorticoid-induced kinase-1 (sgk-1). Alternatively, the life span, growth, and reproductive phenotypes of rictor mutants are mediated predominantly by sgk-1. These data indicate that Rictor/TORC2 is a nutrient-sensitive complex with outputs to AKT and SGK to modulate the assessment of food quality and signal to fat metabolism, growth, feeding behavior, reproduction, and life span.[Keywords: C. elegans; fat metabolism; life span; insulin/IGF; AKT] Supplemental material is available at http://www.genesdev.org.

The nuclear receptor peroxisome proliferator-activated receptor ␥ regulates adipose differentiation and systemic insulin signaling via ligand-dependent transcriptional activation of target genes. However, the identities of the biologically relevant target genes are largely unknown. Here we describe the isolation and characterization of a novel target gene induced by PPAR␥ ligands, termed PGAR (for PPAR␥ angiopoietin related), which encodes a novel member of the angiopoietin family of secreted proteins. The transcriptional induction of PGAR follows a rapid time course typical of immediate-early genes and occurs in the absence of protein synthesis. The expression of PGAR is predominantly localized to adipose tissues and placenta and is consistently elevated in genetic models of obesity. Hormone-dependent adipocyte differentiation coincides with a dramatic early induction of the PGAR transcript. Alterations in nutrition and leptin administration are found to modulate the PGAR expression in vivo. Taken together, these data suggest a possible role for PGAR in the regulation of systemic lipid metabolism or glucose homeostasis.The past several years have witnessed an increasing recognition of the adipocyte as a remarkably dynamic entity that uses diverse signaling pathways to interact with other tissues. A compelling body of evidence now implicates adipocyte-derived proteins such as leptin (25a, 31) and tumor necrosis factor ␣ (10) as endocrine and/or autocrine modulators of distant and local targets. This enables the adipose tissue to exercise feedback regulation over systemic energy homeostasis and metabolism.In parallel with the improved functional understanding of the adipocyte, there has been a burgeoning interest in studying the molecular control of adipose differentiation. Research efforts directed at the transcriptional regulation of adipogenesis have led to the elucidation of several transcription factors that play key roles in this process, including the peroxisome proliferator-activated receptor ␥ (PPAR␥) (26) and the CCAAT/ enhancer binding protein (6) family members. PPAR␥, a member of the PPAR subfamily of nuclear hormone receptors, is a ligand-dependent transcription factor expressed in a tissueselective manner, with the highest levels in the adipose tissue. Much evidence from gain-and loss-of-function studies indicates that PPAR␥ is a central regulator of adipogenesis and systemic insulin action (reviewed in references 15 and 24), providing a crucial link between these two major aspects of adipocyte biology. PPAR␥ has been demonstrated to stimulate adipose conversion in a variety of fibroblastic cell lines ectopically expressing PPAR␥, as well as in preadipocyte and mesenchymal precursor cell lines (26). Committed myoblasts stably infected with PPAR␥ and CCAAT/enhancer binding protein ␣ can be induced to undergo transdifferentiation into adipocytes upon PPAR␥ activation (11). More recently, lossof-function experiments using PPAR␥ Ϫ/Ϫ mice or embryonic stem cells have confirmed the requirement of PPAR␥ f...

Obesity, defined as an increase in adipose tissue mass, is the most prevalent nutritional disorder in industrialized countries and is a growing problem in developing countries. An increase in adipose tissue mass can be the result of the production of new fat cells through the process of adipogenesis and/or the deposition of increased amounts of cytoplasmic triglyceride per cell. Although much has been learned about the differentiation of adipocytes in vitro, less is known about the molecular basis for the mechanisms regulating adipogenesis in vivo. Here oligonucleotide microarrays have been used to compare the patterns of gene expression in preadipocytes and adipocytes in vitro and in vivo. These data indicate that the cellular programs associated with adipocyte differentiation are considerably more complex than previously appreciated and that a greater number of heretofore uncharacterized gene regulatory events are activated during this process in vitro. In addition, the gene expression changes associated with adipocyte development in vivo and in vitro, while overlapping, are in some respects quite different. These data further suggest that one or more transcriptional programs are activated exclusively in vivo to generate the full adipocyte phenotype. This gene expression survey now sets the stage for further studies to dissect the molecular differences between in vivo and in vitro adipocytes.

SUMMARY Metformin has utility in cancer prevention and treatment, though the mechanisms for these effects remain elusive. Through genetic screening in C. elegans, we uncover two metformin response elements: the nuclear pore complex (NPC) and acyl-CoA dehydrogenase family member-10 (ACAD10). We demonstrate that biguanides inhibit growth by inhibiting mitochondrial respiratory capacity, which restrains transit of the RagA-RagC GTPase heterodimer through the NPC. Nuclear exclusion renders RagC incapable of gaining the GDP-bound state necessary to stimulate mTORC1. Biguanide-induced inactivation of mTORC1 subsequently inhibits growth through transcriptional induction of ACAD10. This ancient metformin response pathway is conserved from worms to humans. Both restricted nuclear pore transit and upregulation of ACAD10 are required for biguanides to reduce viability in melanoma and pancreatic cancer cells, and to extend C. elegans lifespan. This pathway provides a unified mechanism by which metformin kills cancer cells and extends lifespan, and illuminates potential cancer targets.