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Open Access Publications from the University of California

Scholarly Works (9 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2010)

Adipose tissue is the major energy storage depot in mammals, storing energy in the form triacylglycerol (TAG) during times of energy excess, and mobilizing TAG, via lipolysis, during times of energy need to generate fatty acids. Desnutrin (also known as ATGL) is a novel TAG hydrolase expressed highly only in white and brown adipose tissue (WAT and BAT). To investigate the role of desnutrin in vivo, transgenic mice that overexpress desnutrin predominantly in adipose tissue (aP2-desnutrin mice) were generated. aP2-desnutrin mice had increased lipolysis in adipose tissue and were resistant to diet-induced obesity. Surprisingly, this increased lipolysis in adipose tissue did not result in higher levels of serum non-esterified fatty acids (NEFA), but instead promoted fatty acid oxidation directly within adipocytes resulting in higher energy expenditure and a leaner phenotype. To further investigate the adipose-specific role of desnutrin, gene targeting by homologous recombination in embryonic stem cells was performed to generate adipose-specific conditional desnutrin knockout mice (desnutrin ASKO mice). In contrast to aP2-desnutrin mice, these mice had markedly decreased lipolysis in both WAT and BAT with massive accumulation of TAG in these tissues. These mice had impaired thermogenesis and decreased oxygen consumption. Notably, BAT in these mice was drastically enlarged, and resembled WAT, indicating that desnutrin may be important in the conversion of WAT to BAT.

While the classic model of the endocrine regulation of lipolysis by catecholamines and insulin has been extensively studied, the local regulation of lipolysis in adipose tissue by autocrine/paracrine factors is not well understood. Identification of adipose-specific phospholipase A2 (AdPLA) revealed a surprisingly important and dominant role for adipocyte-derived PGE2 in the autocrine/paracrine regulation of lipolysis. As a PLA2, AdPLA catalyzes the release of fatty acids from the sn-2 position of phospholipids that is typically enriched in arachidonic acid, providing substrate for the initial, rate-limiting step in the synthesis of eicosanoids. To investigate the role of AdPLA in vivo, AdPLA null mice were generated. Through the PGE2/EP3/cAMP pathway, AdPLA dominantly regulates lipolysis in adipose tissue in an autocrine/paracrine manner. The unrestrained adipocyte lipolysis in AdPLA null mice resulted in resistance to both diet-induced and genetic obesity. Consistent with results from aP2-desnutrin mice, despite elevated lipolysis in adipose tissue, serum NEFA levels were not elevated in AdPLA null mice and fatty acid oxidation was increased in WAT, resulting in higher energy expenditure and resistance to obesity. Taken together, findings from aP2-desnutrin mice, desnutrin ASKO mice, and AdPLA null mice have led to the discovery of novel players in the lipolytic cascade, and revealed an unexpected link between lipolysis and the ability of adipose tissue to oxidize fatty acids and release heat, rather than to provide useful energy for work.

    Cover page: Lipolysis in Adipose Tissue: Roles of Desnutrin and Adipose-specific phospholipase A2
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2013)
    Excessive caloric intake leading to obesity is associated with insulin resistance and dysfunction of islet β cells. High-fat feeding decreases desnutrin (also called ATGL/PNPLA2) levels in islets. Here we show that desnutrin ablation via RIP-Cre (βKO) or RIP-CreER results in hyperglycemia with impaired glucose-stimulated insulin secretion (GSIS). Due to decreased lipolysis, islets have higher TAG content but lower free FA levels. βKO islets exhibit impaired mitochondrial respiration and lower production of ATP required for GSIS, along with decreased expression of PPARδ target genes involved in mitochondrial oxidation. Furthermore, synthetic PPARδ, but not PPARα, agonist restores GSIS and expression of mitochondrial oxidative genes in βKO mice, revealing that desnutrin-catalyzed lipolysis generates PPARδ ligands. Finally, adenoviral expression of desnutrin in βKO islets restores all defects of βKO islet phenotype and function, including GSIS and mitochondrial defects, demonstrating the critical role of the desnutrin-PPARδ-mitochondrial oxidation axis in regulating islet β cell GSIS.
      Cover page: Desnutrin/ATGL activates PPARδ to promote mitochondrial function for insulin secretion in islet β cells.
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2015)
      Preadipocyte factor-1 (Pref-1) is made as a transmembrane protein containing EGF-repeats at the extracellular domain that can be cleaved to generate a biologically active soluble form. Pref-1 is found in islet β-cells and its level has been reported to increase in neonatal rat islets upon growth hormone treatment. We found here that Pref-1 can promote growth of pancreatic tumor derived AR42J cells. To examine Pref-1 function in pancreatic islets in vivo, we generated transgenic mouse lines overexpressing the Pref-1/hFc in islet β-cells using rat insulin II promoter (RIP). These transgenic mice exhibit an increase in islet mass with higher proportion of larger islets in pancreas compared to wild-type littermates. This is in contrast to pancreas from Pref-1 null mice that show higher proportion of smaller islets. Insulin expression and insulin secretion from pancreatic islets from RIP-Pref-1/hFc transgenic mice are increased also. Thus, RIP-Pref-1/hFc transgenic mice show normal glucose levels but with higher plasma insulin levels in both fasting and fed conditions. These mice show improved glucose tolerance. Taken together, we conclude Pref-1 as a positive regulator of islet β-cells and insulin production.
        Cover page: Overexpression of Pref-1 in pancreatic islet β-cells in mice causes hyperinsulinemia with increased islet mass and insulin secretion
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2020)
        Catecholamines stimulate the mobilization of stored triglycerides in adipocytes to provide fatty acids (FAs) for other tissues. However, a large proportion is taken back up and either oxidized or re-esterified. What controls the disposition of these FAs in adipocytes remains unknown. Here, we report that catecholamines redirect FAs for oxidation through the phosphorylation of signal transducer and activator of transcription 3 (STAT3). Adipocyte STAT3 is phosphorylated upon activation of β-adrenergic receptors, and in turn suppresses FA re-esterification to promote FA oxidation. Adipocyte-specific Stat3 KO mice exhibit normal rates of lipolysis, but exhibit defective lipolysis-driven oxidative metabolism, resulting in reduced energy expenditure and increased adiposity when they are on a high-fat diet. This previously unappreciated, non-genomic role of STAT3 explains how sympathetic activation can increase both lipolysis and FA oxidation in adipocytes, revealing a new regulatory axis in metabolism.
          Cover page: Catecholamines suppress fatty acid re-esterification and increase oxidation in white adipocytes via STAT3
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2014)
          Fibroblast growth factor 1 (FGF1) is an autocrine/paracrine regulator whose binding to heparan sulphate proteoglycans effectively precludes its circulation. Although FGF1 is known as a mitogenic factor, FGF1 knockout mice develop insulin resistance when stressed by a high-fat diet, suggesting a potential role in nutrient homeostasis. Here we show that parenteral delivery of a single dose of recombinant FGF1 (rFGF1) results in potent, insulin-dependent lowering of glucose levels in diabetic mice that is dose-dependent but does not lead to hypoglycaemia. Chronic pharmacological treatment with rFGF1 increases insulin-dependent glucose uptake in skeletal muscle and suppresses the hepatic production of glucose to achieve whole-body insulin sensitization. The sustained glucose lowering and insulin sensitization attributed to rFGF1 are not accompanied by the side effects of weight gain, liver steatosis and bone loss associated with current insulin-sensitizing therapies. We also show that the glucose-lowering activity of FGF1 can be dissociated from its mitogenic activity and is mediated predominantly via FGF receptor 1 signalling. Thus we have uncovered an unexpected, neomorphic insulin-sensitizing action for exogenous non-mitogenic human FGF1 with therapeutic potential for the treatment of insulin resistance and type 2 diabetes.
            Cover page: Endocrinization of FGF1 produces a neomorphic and potent insulin sensitizer
            • Article
            • Peer Reviewed
            UC San Diego Previously Published Works (2017)
            Numerous studies indicate an inflammatory link between obesity and type 2 diabetes. The inflammatory kinases IKKɛ and TBK1 are elevated in obesity; their inhibition in obese mice reduces weight, insulin resistance, fatty liver and inflammation. Here we studied amlexanox, an inhibitor of IKKɛ and TBK1, in a proof-of-concept randomized, double-blind, placebo-controlled study of 42 obese patients with type 2 diabetes and nonalcoholic fatty liver disease. Treatment of patients with amlexanox produced a statistically significant reduction in Hemoglobin A1c and fructosamine. Interestingly, a subset of drug responders also exhibited improvements in insulin sensitivity and hepatic steatosis. This subgroup was characterized by a distinct inflammatory gene expression signature from biopsied subcutaneous fat at baseline. They also exhibited a unique pattern of gene expression changes in response to amlexanox, consistent with increased energy expenditure. Together, these data suggest that dual-specificity inhibitors of IKKɛ and TBK1 may be effective therapies for metabolic disease in an identifiable subset of patients.
              Cover page: Inhibition of IKKɛ and TBK1 Improves Glucose Control in a Subset of Patients with Type 2 Diabetes
              • Article
              • Peer Reviewed
              UC San Diego Previously Published Works (2018)
              Brown adipose tissue (BAT) adaptively transfers energy from glucose and fat into heat by inducing a gene network that uncouples mitochondrial electron transport. However, the innate transcription factors that enable the rapid adaptive response of BAT are unclear. Here, we identify estrogen-related receptor gamma (ERRγ) as a critical factor for maintaining BAT identity. ERRγ is selectively expressed in BAT versus WAT, in which, in the absence of PGC1α, it drives a signature transcriptional network of thermogenic and oxidative genes in the basal (i.e., thermoneutral) state. Mice lacking ERRγ in adipose tissue (ERRγKO mice) display marked downregulation of BAT-selective genes that leads to a pronounced whitening of BAT. Consistent with the transcriptional changes, the thermogenic capacity of ERRγKO mice is severely blunted, such that they fail to survive an acute cold challenge. These findings reveal a role for ERRγ as a critical thermoneutral maintenance factor required to prime BAT for thermogenesis.
                Cover page: ERRγ Preserves Brown Fat Innate Thermogenic Activity
                • Article
                • Peer Reviewed
                UC San Diego Previously Published Works (2015)
                The search for effective treatments for obesity and its comorbidities is of prime importance. We previously identified IKK-ε and TBK1 as promising therapeutic targets for the treatment of obesity and associated insulin resistance. Here we show that acute inhibition of IKK-ε and TBK1 with amlexanox treatment increases cAMP levels in subcutaneous adipose depots of obese mice, promoting the synthesis and secretion of the cytokine IL-6 from adipocytes and preadipocytes, but not from macrophages. IL-6, in turn, stimulates the phosphorylation of hepatic Stat3 to suppress expression of genes involved in gluconeogenesis, in the process improving glucose handling in obese mice. Preliminary data in a small cohort of obese patients show a similar association. These data support an important role for a subcutaneous adipose tissue-liver axis in mediating the acute metabolic benefits of amlexanox on glucose metabolism, and point to a new therapeutic pathway for type 2 diabetes.
                  Cover page: A subcutaneous adipose tissue–liver signalling axis controls hepatic gluconeogenesis
                  • Article
                  • Peer Reviewed
                  UC San Diego Previously Published Works (2022)
                  The dysregulation of energy homeostasis in obesity involves multihormone resistance. Although leptin and insulin resistance have been well characterized, catecholamine resistance remains largely unexplored. Murine β3-adrenergic receptor expression in adipocytes is orders of magnitude higher compared with that of other isoforms. While resistant to classical desensitization pathways, its mRNA (Adrb3) and protein expression are dramatically downregulated after ligand exposure (homologous desensitization). β3-Adrenergic receptor downregulation also occurs after high-fat diet feeding, concurrent with catecholamine resistance and elevated inflammation. This downregulation is recapitulated in vitro by TNF-α treatment (heterologous desensitization). Both homologous and heterologous desensitization of Adrb3 were triggered by induction of the pseudokinase TRIB1 downstream of the EPAC/RAP2A/PI-PLC pathway. TRIB1 in turn degraded the primary transcriptional activator of Adrb3, CEBPα. EPAC/RAP inhibition enhanced catecholamine-stimulated lipolysis and energy expenditure in obese mice. Moreover, adipose tissue expression of genes in this pathway correlated with body weight extremes in a cohort of genetically diverse mice and with BMI in 2 independent cohorts of humans. These data implicate a signaling axis that may explain reduced hormone-stimulated lipolysis in obesity and resistance to therapeutic interventions with β3-adrenergic receptor agonists.
                    Cover page: β3-adrenergic receptor downregulation leads to adipocyte catecholamine resistance in obesity
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