Adipose Tissue and Metabolic Health.

In this review, we provide a brief synopsis of the connections between adipose tissue and metabolic health and highlight some recent developments in understanding and exploiting adipocyte biology. Adipose tissue plays critical roles in the regulation of systemic glucose and lipid metabolism and secretes bioactive molecules possessing endocrine, paracrine, and autocrine functions. Dysfunctional adipose tissue has a detrimental impact on metabolic health and is intimately involved in key aspects of metabolic diseases such as insulin resistance, lipid overload, inflammation, and organelle stress. Differences in the distribution of fat depots and adipose characteristics relate to divergent degrees of metabolic dysfunction found in metabolically healthy and unhealthy obese individuals. Thermogenic adipocytes increase energy expenditure via mitochondrial uncoupling or adenosine triphosphate-consuming futile substrate cycles, while functioning as a metabolic sink and participating in crosstalk with other metabolic organs. Manipulation of adipose tissue provides a wealth of opportunities to intervene and combat the progression of associated metabolic diseases. We discuss current treatment modalities for obesity including incretin hormone analogs and touch upon emerging strategies with therapeutic potential including exosome-based therapy, pharmacological activation of brown and beige adipocyte thermogenesis, and administration or inhibition of adipocyte-derived factors.

Regulation of beige adipocyte thermogenesis by the cold-repressed ER protein NNAT.

OBJECTIVE: Cold stimuli trigger the conversion of white adipose tissue into beige adipose tissue, which is capable of non-shivering thermogenesis. However, what process drives this activation of thermogenesis in beige fat is not well understood. Here, we examine the ER protein NNAT as a regulator of thermogenesis in adipose tissue. METHODS: We investigated the regulation of adipose tissue NNAT expression in response to changes in ambient temperature. We also evaluated the functional role of NNAT in thermogenic regulation using Nnat null mice and primary adipocytes that lack or overexpress NNAT. RESULTS: Cold exposure or treatment with a ß3-adrenergic agonist reduces the expression of adipose tissue NNAT in mice. Genetic disruption of Nnat in mice enhances inguinal adipose tissue thermogenesis. Nnat null mice exhibit improved cold tolerance both in the presence and absence of UCP1. Gain-of-function studies indicate that ectopic expression of Nnat abolishes adrenergic receptor-mediated respiration in beige adipocytes. NNAT physically interacts with the ER Ca2+-ATPase (SERCA) in adipocytes and inhibits its activity, impairing Ca2+ transport and heat dissipation. We further demonstrate that NHLRC1, an E3 ubiquitin protein ligase implicated in proteasomal degradation of NNAT, is induced by cold exposure or ß3-adrenergic stimulation, thus providing regulatory control at the protein level. This serves to link cold stimuli to NNAT degradation in adipose tissue, which in turn leads to enhanced SERCA activity. CONCLUSIONS: Our study implicates NNAT in the regulation of adipocyte thermogenesis.

CFTR regulates brown adipocyte thermogenesis via the cAMP/PKA signaling pathway.

BACKGROUND: Cystic fibrosis (CF) is characterized by reduced growth and lower body weight, which are multifactorial. CF mouse models lack key disease characteristics that predispose to a negative energy balance, such as pulmonary infections or exocrine pancreatic insufficiency, and yet they still exhibit a growth defect and an abnormally increased energy expenditure. Whether adipocyte thermogenesis contributes to the elevated resting energy expenditure in CF mice is unknown. METHODS: We examined the expression of CFTR in thermogenic brown adipose tissue (BAT) and investigated a functional role for CFTR using BAT-specific CFTR null mice (CFTRBATKO). RESULTS: The CFTR protein is expressed in mouse BAT at levels comparable to those in the lungs. BAT-specific inactivation of CFTR in mice increases whole-body energy expenditure associated with sympathetic stimulation by cold exposure. Weight gain on a high-fat diet is attenuated in these mice. However, CFTR-deficient brown adipocytes themselves have impaired, rather than enhanced, thermogenic responses. These cells feature decreased lipolysis and blunted activation of the cAMP/PKA signaling pathway in response to adrenergic stimulation. This suggests that compensatory heat production in other tissues likely accounts for the increased systemic energy expenditure seen in CFTRBATKO mice. CONCLUSIONS: Our data reveal a new role for CFTR in the regulation of adipocyte thermogenesis.

Adipose Mitochondrial Complex I Deficiency Modulates Inflammation and Glucose Homeostasis in a Sex-Dependent Manner.

Mitochondrial dysfunction in adipose tissue has been associated with type 2 diabetes, but it is unclear whether it is a cause or the consequence. Mitochondrial complex I is a major site of reactive oxygen species generation and a therapeutic target. Here we report that genetic deletion of the complex I subunit Ndufs4 specifically in adipose tissue results in an increased propensity to develop diet-induced weight gain, glucose intolerance, and elevated levels of fat inflammatory genes. This outcome is apparent in young males but not in young females, suggesting that females are relatively protected from the adverse consequences of adipose mitochondrial dysfunction for metabolic health. Mutant mice of both sexes exhibit defects in brown adipose tissue thermogenesis. Fibroblast growth factor 21 (FGF21) signaling in adipose tissue is selectively blunted in male mutant mice relative to wild-type littermates, consistent with sex-dependent regulation of its autocrine/paracrine action in adipocytes. Together, these findings support that adipocyte-specific mitochondrial dysfunction is sufficient to induce tissue inflammation and can cause systemic glucose abnormalities in male mice.

Defective brown adipose tissue thermogenesis and impaired glucose metabolism in mice lacking Letmd1.

Manipulation of energy-dissipating adipocytes has the potential to produce metabolic benefits. To this end, it is valuable to understand the mechanisms controlling the generation and function of thermogenic fat. Here, we identify Letm1 domain containing 1 (Letmd1) as a regulator of brown fat formation and function. The expression of Letmd1 is induced in brown fat by cold exposure and by ß-adrenergic activation. Letmd1-deficient mice exhibit severe cold intolerance concomitant with abnormal brown fat morphology, reduced thermogenic gene expression, and low mitochondrial content. The null mice exhibit impaired ß3-adrenoreceptor-dependent thermogenesis and are prone to diet-induced obesity and defective glucose disposal. Letmd1 was previously described as a mitochondrial protein, and we find that it also localizes to the nucleus and interacts with the transcriptional coregulator and chromatin remodeler Brg1/Smarca4, thus providing a way to impact thermogenic gene expression. Our study uncovers a role for Letmd1 as a key regulatory component of adaptive thermogenesis.

Male reproductive health in cystic fibrosis.

The impact of cystic fibrosis (CF) on male reproductive health is profound. The vast majority of men with CF are infertile due to obstructive azoospermia. Multiple factors associated with CF contribute to an increased prevalence of testosterone deficiency, which adversely affects muscle mass, bone density, and quality of life. This article reviews the pathophysiology, diagnosis, and management of infertility and testosterone deficiency that occur in men with CF. With improving survival of CF patients, these topics are becoming more significant in their clinical care.

Energy balance and obesity in individuals with cystic fibrosis.

This article reviews the significance of nutritional status in patients with cystic fibrosis (CF), and sheds light on the reasons behind the intense focus placed on perpetual weight gain and increased caloric intake by CF patients and their providers. The manuscript explores the potential mechanisms by which aberrant CFTR may contribute to increased resting energy expenditure (REE), and how correcting and potentiating its activity, possibly by reducing REE, among other intended and off-target effects, can contribute to weight gain in this patient population. The commentary also examines what is currently known about metabolic and vascular complications of obesity in patients with CF, and presents dietary, nutritional, pharmacologic and surgical approaches that may help obese patients with CF lose weight and build more lean body mass.

Evolving Mechanistic Views and Emerging Therapeutic Strategies for Cystic Fibrosis-Related Diabetes.

Diabetes is a common and important complication of cystic fibrosis, an autosomal recessive genetic disease due to mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Cystic fibrosis-related diabetes (CFRD) is associated with profound detrimental effects on the disease course and mortality and is expected to increase in prevalence as the survival of patients with cystic fibrosis continues to improve. Despite progress in the functional characterization of CFTR molecular defects, the mechanistic basis of CFRD is not well understood, in part because of the relative inaccessibility of the pancreatic tissue and the limited availability of representative animal models. This review presents a concise overview of the current understanding of CFRD pathogenesis and provides a cutting-edge update on novel findings from human and animal studies. Potential contributions from paracrine mechanisms and ß-cell compensatory mechanisms are highlighted, as well as functional ß-cell and α-cell defects, incretin defects, exocrine pancreatic insufficiency, and loss of islet cell mass. State-of-the-art and emerging treatment options are explored, including advances in insulin administration, CFTR modulators, cell replacement, gene replacement, and gene editing therapies.

GLTSCR2/PICT1 links mitochondrial stress and Myc signaling.

Mitochondrial defects underlie a multitude of human diseases. Genetic manipulation of mitochondrial regulatory pathways represents a potential therapeutic approach. We have carried out a high-throughput overexpression screen for genes that affect mitochondrial abundance or activity using flow-cytometry-based enrichment of a cell population expressing a high-complexity, concentration-normalized pool of human ORFs. The screen identified 94 candidate mitochondrial regulators including the nuclear protein GLTSCR2, also known as PICT1. GLTSCR2 enhances mitochondrial function and is required for the maintenance of oxygen consumption, consistent with a pivotal role in the control of cellular respiration. RNAi inactivation of the Caenorhabditis elegans ortholog of GLTSCR2 reduces respiration in worms, indicating functional conservation across species. GLTSCR2 controls cellular proliferation and metabolism via the transcription factor Myc, and is induced by mitochondrial stress, suggesting it may constitute a significant component of the mitochondrial signaling pathway.

Cumulative haploinsufficiency and triplosensitivity drive aneuploidy patterns and shape the cancer genome.

Aneuploidy has been recognized as a hallmark of cancer for more than 100 years, yet no general theory to explain the recurring patterns of aneuploidy in cancer has emerged. Here, we develop Tumor Suppressor and Oncogene (TUSON) Explorer, a computational method that analyzes the patterns of mutational signatures in tumors and predicts the likelihood that any individual gene functions as a tumor suppressor (TSG) or oncogene (OG). By analyzing >8,200 tumor-normal pairs, we provide statistical evidence suggesting that many more genes possess cancer driver properties than anticipated, forming a continuum of oncogenic potential. Integrating our driver predictions with information on somatic copy number alterations, we find that the distribution and potency of TSGs (STOP genes), OGs, and essential genes (GO genes) on chromosomes can predict the complex patterns of aneuploidy and copy number variation characteristic of cancer genomes. We propose that the cancer genome is shaped through a process of cumulative haploinsufficiency and triplosensitivity.

Polyubiquitinated PCNA recruits the ZRANB3 translocase to maintain genomic integrity after replication stress.

Completion of DNA replication after replication stress depends on PCNA, which undergoes monoubiquitination to stimulate direct bypass of DNA lesions by specialized DNA polymerases or is polyubiquitinated to promote recombination-dependent DNA synthesis across DNA lesions by template switching mechanisms. Here we report that the ZRANB3 translocase, a SNF2 family member related to the SIOD disorder SMARCAL1 protein, is recruited by polyubiquitinated PCNA to promote fork restart following replication arrest. ZRANB3 depletion in mammalian cells results in an increased frequency of sister chromatid exchange and DNA damage sensitivity after treatment with agents that cause replication stress. Using in vitro biochemical assays, we show that recombinant ZRANB3 remodels DNA structures mimicking stalled replication forks and disassembles recombination intermediates. We therefore propose that ZRANB3 maintains genomic stability at stalled or collapsed replication forks by facilitating fork restart and limiting inappropriate recombination that could occur during template switching events.

Wnt signaling regulates mitochondrial physiology and insulin sensitivity.

Mitochondria serve a critical role in physiology and disease. The genetic basis of mitochondrial regulation in mammalian cells has not yet been detailed. We performed a large-scale RNAi screen to systematically identify genes that affect mitochondrial abundance and function. This screen revealed previously unrecognized roles for >150 proteins in mitochondrial regulation. We report that increased Wnt signals are a potent activator of mitochondrial biogenesis and reactive oxygen species (ROS) generation, leading to DNA damage and acceleration of cellular senescence in primary cells. The signaling protein insulin receptor substrate-1 (IRS-1), shown here to be a transcriptional target of Wnt, is induced in this setting. The increased level of IRS-1 drives activation of mitochondrial biogenesis; furthermore, in insulin-responsive cell types, it enhances insulin signaling, raising the possibility that Wnt proteins may be used to modulate glucose homeostasis. Our results identify a key component of the mitochondrial regulatory apparatus with a potentially important link to metabolic and degenerative disorders.