Josep A. Villena

New insights into PGC-1 coactivators: redefining their role in the regulation of mitochondrial function and beyond.

Members of the PGC‐1 family of coactivators have been revealed as key players in the regulation of energy metabolism. Early gain‐ and loss‐of‐function studies led to the conclusion that all members of the PGC‐1 family (PGC‐1α, PGC‐1β and PRC) play redundant roles in the control of mitochondrial biogenesis by regulating overlapping gene expression programs. Regardless of this, all PGC‐1 coactivators also appeared to differ in the stimuli to which they respond to promote mitochondrial gene expression. Although PGC‐1α was found to be induced by different physiological or pharmacological cues, PGC‐1β appeared to be unresponsive to such stimuli. Consequently, it has long been widely accepted that PGC‐1α acts as a mediator of mitochondrial biogenesis induced by cues that signal high‐energy needs, whereas the role of PGC‐1β is restricted to the maintenance of basal mitochondrial function. By contrast, the function of PRC appears to be restricted to the regulation of gene expression in proliferating cells. However, recent studies using tissue‐specific mouse models that lack or overexpress different PGC‐1 coactivators have provided emerging evidence not only supporting new roles for PGC‐1s, but also redefining some of the paradigms related to the precise function and mode of action of PGC‐1 coactivators in mitochondrial biogenesis. The present review discusses some of the new findings regarding the control of mitochondrial gene expression by PGC‐1 coactivators in a tissue‐specific context, as well as newly‐uncovered functions of PGC‐1s beyond mitochondrial biogenesis, and their link to pathologies, such as diabetes, muscular dystrophies, neurodegenerative diseases or cancer.

Rosiglitazone-Induced Mitochondrial Biogenesis in White Adipose Tissue Is Independent of Peroxisome Proliferator-Activated Receptor γ Coactivator-1α

Background Thiazolidinediones, a family of insulin-sensitizing drugs commonly used to treat type 2 diabetes, are thought to exert their effects in part by promoting mitochondrial biogenesis in white adipose tissue through the transcriptional coactivator PGC-1α (Peroxisome Proliferator-Activated Receptor γ Coactivator-1α). Methodology/Principal Findings To assess the role of PGC-1α in the control of rosiglitazone-induced mitochondrial biogenesis, we have generated a mouse model that lacks expression of PGC-1α specifically in adipose tissues (PGC-1α-FAT-KO mice). We found that expression of genes encoding for mitochondrial proteins involved in oxidative phosphorylation, tricarboxylic acid cycle or fatty acid oxidation, was similar in white adipose tissue of wild type and PGC-1α-FAT-KO mice. Furthermore, the absence of PGC-1α did not prevent the positive effect of rosiglitazone on mitochondrial gene expression or biogenesis, but it precluded the induction by rosiglitazone of UCP1 and other brown fat-specific genes in white adipose tissue. Consistent with the in vivo findings, basal and rosiglitazone-induced mitochondrial gene expression in 3T3-L1 adipocytes was unaffected by the knockdown of PGC-1α but it was impaired when PGC-1β expression was knockdown by the use of specific siRNA. Conclusions/Significance These results indicate that in white adipose tissue PGC-1α is dispensable for basal and rosiglitazone-induced mitochondrial biogenesis but required for the rosiglitazone-induced expression of UCP1 and other brown adipocyte-specific markers. Our study suggests that PGC-1α is important for the appearance of brown adipocytes in white adipose tissue. Our findings also provide evidence that PGC-1β and not PGC-1α regulates basal and rosiglitazone-induced mitochondrial gene expression in white adipocytes.

The db/db mouse: a useful model for the study of diabetic retinal neurodegeneration.

Background To characterize the sequential events that are taking place in retinal neurodegeneration in a murine model of spontaneous type 2 diabetes (db/db mouse). Methods C57BLKsJ-db/db mice were used as spontaneous type 2 diabetic animal model, and C57BLKsJ-db/+ mice served as the control group. To assess the chronological sequence of the abnormalities the analysis was performed at different ages (8, 16 and 24 weeks). The retinas were evaluated in terms of morphological and functional abnormalities [electroretinography (ERG)]. Histological markers of neurodegeneration (glial activation and apoptosis) were evaluated by immunohistochemistry. In addition glutamate levels and glutamate/aspartate transporter (GLAST) expression were assessed. Furthermore, to define gene expression changes associated with early diabetic retinopathy a transcriptome analyses was performed at 8 week. Furthermore, an additional interventional study to lower blood glucose levels was performed. Results Glial activation was higher in diabetic than in non diabetic mice in all the stages (p<0.01). In addition, a progressive loss of ganglion cells and a significant reduction of neuroretinal thickness were also observed in diabetic mice. All these histological hallmarks of neurodegeneration were less pronounced at week 8 than at week 16 and 24. Significant ERG abnormalities were present in diabetic mice at weeks 16 and 24 but not at week 8. Moreover, we observed a progressive accumulation of glutamate in diabetic mice associated with an early downregulation of GLAST. Morphological and ERG abnormalities were abrogated by lowering blood glucose levels. Finally, a dysregulation of several genes related to neurotransmission and oxidative stress such as UCP2 were found at week 8. Conclusions Our results suggest that db/db mouse reproduce the features of the neurodegenerative process that occurs in the human diabetic eye. Therefore, it seems an appropriate model for investigating the underlying mechanisms of diabetes-induced retinal neurodegeneration and for testing neuroprotective drugs.

Mice lacking PGC-1β in adipose tissues reveal a dissociation between mitochondrial dysfunction and insulin resistance

Proper development and function of white adipose tissue (WAT), which are regulated by multiple transcription factors and coregulators, are crucial for glucose homeostasis. WAT is also the main target of thiazolidinediones, which are thought to exert their insulin-sensitizing effects by promoting mitochondrial biogenesis in adipocytes. Besides being expressed in WAT, the role of the coactivator PGC-1β in this tissue has not been addressed. To study its function in WAT, we have generated mice that lack PGC-1β in adipose tissues. Gene expression profiling analysis of WAT reveals that PGC-1β regulates mitochondrial genes involved in oxidative metabolism. Furthermore, lack of PGC-1β prevents the induction of mitochondrial genes by rosiglitazone in WAT without affecting the capacity of thiazolidinediones to enhance insulin sensitivity. Our findings indicate that PGC-1β is important for basal and rosiglitazone-induced mitochondrial function in WAT, and that induction of mitochondrial oxidative capacity is not essential for the insulin-sensitizing effects of thiazolidinediones.

Pharmacological induction of mitochondrial biogenesis as a therapeutic strategy for the treatment of type 2 diabetes

Defects in mitochondrial oxidative function have been associated with the onset of type 2 diabetes. Although the causal relationship between mitochondrial dysfunction and diabetes has not been fully established, numerous studies indicate that improved glucose homeostasis achieved via lifestyle interventions, such as exercise or calorie restriction, is tightly associated with increased mitochondrial biogenesis and oxidative function. Therefore, it is conceivable that potentiating mitochondrial biogenesis by pharmacological means could constitute an efficacious therapeutic strategy that would particularly benefit those diabetic patients who cannot adhere to comprehensive programs based on changes in lifestyle or that require a relatively rapid improvement in their diabetic status. In this review, we discuss several pharmacological targets and drugs that modulate mitochondrial biogenesis as well as their potential use as treatments for insulin resistance and diabetes.

HMGA1 overexpression in adipose tissue impairs adipogenesis and prevents diet-induced obesity and insulin resistance

High-Mobility-Group-A1 (HMGA1) proteins are non-histone proteins that regulate chromatin structure and gene expression during embryogenesis, tumourigenesis and immune responses. In vitro studies suggest that HMGA1 proteins may be required to regulate adipogenesis. To examine the role of HMGA1 in vivo, we generated transgenic mice overexpressing HMGA1 in adipose tissues. HMGA1 transgenic mice showed a marked reduction in white and brown adipose tissue mass that was associated with downregulation of genes involved in adipogenesis and concomitant upregulation of preadipocyte markers. Reduced adipogenesis and decreased fat mass were not associated with altered glucose homeostasis since HMGA1 transgenic mice fed a regular-chow diet exhibited normal glucose tolerance and insulin sensitivity. However, when fed a high-fat diet, overexpression of HMGA1 resulted in decreased body-weight gain, reduced fat mass, but improved insulin sensitivity and glucose tolerance. Although HMGA1 transgenic mice exhibited impaired glucose uptake in adipose tissue due to impaired adipogenesis, the increased glucose uptake observed in skeletal muscle may account for the improved glucose homeostasis. Our results indicate that HMGA1 plays an important function in the regulation of white and brown adipogenesis in vivo and suggests that impaired adipocyte differentiation and decreased fat mass is not always associated with impaired whole-body glucose homeostasis.

Targeting mitochondrial biogenesis to treat insulin resistance.

Over the last century, the prevalence of type 2 diabetes has dramatically increased, reaching the status of epidemic. Because insulin resistance is considered the primary cause of type 2 diabetes, the identification of the cellular processes and gene networks that lead to an impairment of insulin action in target tissues is of crucial importance for the development of new drugs and therapeutic strategies to treat or prevent the disease. Numerous studies in humans and animal models have shown that insulin resistance is frequently associated to reduced mitochondrial mass or oxidative function in insulin sensitive tissues, leading to the hypothesis that defective overall mitochondrial activity could play a relevant role in the etiology of insulin resistance and, therefore, in type 2 diabetes. Although the causal relationship between mitochondrial dysfunction and insulin resistance is still controversial, numerous studies show that lifestyle or pharmacological interventions that improve insulin sensitivity are frequently associated to an increase in mitochondrial function and whole body energy expenditure. Therefore, increasing mitochondrial mass and oxidative activity is viewed as a potential therapeutic approach for the treatment of insulin resistance. Here, we review the current knowledge on the role of mitochondria in the pathogenesis of insulin resistance and discuss some of the potential therapeutic strategies and pharmacological targets for the treatment of insulin resistance based on the activation of mitochondrial biogenesis and the increase of mitochondrial oxidative function.

Expression of Adenine Nucleotide Translocase (ANT) Isoform Genes Is Controlled by PGC-1α Through Different Transcription Factors

Adenine nucleotide translocase (ANT) isoforms are mitochondrial proteins encoded by nuclear DNA that catalyze the exchange of ATP generated in the mitochondria for ADP produced in the cytosol. The aim of this study was to determine the role of the transcriptional coactivator PGC‐1α (peroxisome proliferator‐activated receptor‐γ [PPAR‐γ] coactivator 1α), a master regulator of mitochondrial oxidative metabolism, in the regulation of the expression of ANT isoform genes and to identify the transcription factors involved. We found that PGC‐1α overexpression induced the expression of all ANT human and mouse isoforms but to different degrees. The transcription factor ERRα was involved in PGC‐1α‐induced expression of all human ANT isoforms (hANT1‐3) in HeLa cells as well as in the regulation of mouse isoforms (mANT1‐2) in C2C12 myotubes and 3T3‐L1 adipocytes, even though ANT isoforms have important physiological differences and are regulated in a tissue‐specific manner. In addition to ERRα, PPARδ and mTOR pathways were involved in the induction of mANT1‐2 by PGC‐1α in C2C12 myotubes, while PPARγ was involved in PGC‐1α‐regulation of mANT1‐2 in 3T3‐L1 adipocytes. Furthermore, the regulation of mANT genes by PGC‐1α was also observed in vivo in knockout mouse models lacking PGC‐1α. In summary, our results show that the regulation of genes encoding ANT isoforms is controlled by PGC‐1α through different transcription factors depending on cell type. J. Cell. Physiol. 229: 2126–2136, 2014.

Kidney Androgen-Regulated Protein (KAP) Transgenic Mice Are Protected Against High-Fat Diet Induced Metabolic Syndrome

Metabolic Syndrome (MS) is reaching epidemic proportions with significant social and economical burden worldwide. Since the molecular basis of MS remains poorly defined, we investigated the impact of KAP, a kidney specific androgen-regulated gene, in the development of high fat-diet (hfd)-induced MS. Tg mice overexpressing KAP specifically in proximal tubule cells of the kidney exhibited reduced body weight and lower liver and adipose tissue weight compared to control littermates when fed a hfd. KAP Tg mice showed diminished adipocyte hypertrophy and reduced hepatic steatosis, significantly correlating with expression of relevant molecular markers and lower lipid content in liver. KAP transgenic were protected from hfd-induced insulin resistance, increased blood pressure and exhibited lower IL-6 serum levels and diminished expression of inflammatory markers in the adipose. Moreover, KAP was localized in the secretory pathway of proximal tubule cells and it is released to the extracellular media, preventing IL-6 induction and STAT-3 activation upon TNFα stimulation. We conclude that KAP, which might act as a hormone-like product in extra-renal tissues, protects Tg mice against hfd-induced MS by preventing inflammatory related events that are mediated, in part, through the IL-6 pathway.