A cold-inducible coactivator of nuclear receptors linked to adaptive thermogenesis

Immortal brown adipocytes from p53-knockout mice: differentiation and expression of uncoupling proteins

Brown adipose tissue (BAT) is the specific site for metabolic heat production in mammals. To establish a novel immortal brown adipocyte cell line, the stromal-vascular fraction containing preadipocytes was obtained from interscapular BAT of mice deficient of a tumor-suppressor gene p53. The p53-deficient cells, tentatively named as HB2 cells, could be cultured in vitro after repeated passages and differentiated into adipocytes in the presence of insulin, T3 and/or troglitazone, expressing some adipocyte-specific genes and accumulating intracellular lipid droplets. The mRNA level of uncoupling protein 1 (UCP1), a mitochondrial protein specifically present in brown adipocytes, was undetectable in HB2 preadipocytes, but increased after adipose differentiation. In HB2 adipocytes, UCP1 mRNA expression was markedly activated after stimulation of the beta-adrenergic receptor pathway. The mRNA of UCP2 and UCP3, recently cloned isoforms of UCP1, were also detected in HB2 adipocytes, but their levels were not influenced by adrenergic stimulation. Thus HB2 cells seem useful for in vitro studies of BAT and UCP functions.

A testis-specific androgen receptor coregulator that belongs to a novel family of nuclear proteins

We have characterized a novel partner for androgen receptor (AR), termed ARIP3, that interacts with the DNA-binding domain/zinc finger region of AR and is predominantly expressed in the testis. Rat ARIP3 is a nuclear protein comprising 572 amino acids. It modulates AR-dependent but not basal transcription, suggesting that ARIP3 acts as an AR transcriptional coregulator. Except for the C-terminal AR-interacting domain, ARIP3 contains distinct regions that are also present in two recently described proteins, a protein inhibitor of activated Stat3 and an RNA helicase II-interacting protein (Gu/RH-II binding protein). Conserved structural features of these proteins indicate the existence of a gene family involved in the regulation of various transcription factors. Collectively, ARIP3 belongs to a novel nuclear protein family and is perhaps the first tissue-specific coregulator of androgen receptor.

The Drosophila beta FTZ-F1 orphan nuclear receptor provides competence for stage-specific responses to the steroid hormone ecdysone

The acquisition of competence is a key mechanism for refining global signals to distinct spatial and temporal responses. The molecular basis of competence, however, remains poorly understood. Here, we show that the beta FTZ-F1 orphan nuclear receptor functions as a competence factor for stage-specific responses to the steroid hormone ecdysone during Drosophila metamorphosis. beta FTZ-F1 mutants pupariate normally in response to the late larval pulse of ecdysone but display defects in stage-specific responses to the subsequent ecdysone pulse in prepupae. The ecdysone-triggered genetic hierarchy that directs these developmental responses is severely attenuated in beta FTZ-F1 mutants, although ecdysone receptor expression is unaffected. This study define beta FTZ-F1 as an essential competence factor for stage-specific responses to a steroid signal and implicates interplay among nuclear receptors as a mechanism for achieving hormonal competence.

N-(2-Benzoylphenyl)-L-tyrosine PPARgamma agonists. 1. Discovery of a novel series of potent antihyperglycemic and antihyperlipidemic agents

We have identified a novel series of antidiabetic N-(2-benzoylphenyl)-L-tyrosine derivatives which are potent, selective PPARgamma agonists. Through the use of in vitro PPARgamma binding and functional assays (2S)-3-(4-(benzyloxy)phenyl)-2-((1-methyl-3-oxo-3-phenylpropenyl)+ ++amin o)propionic acid (2) was identified as a structurally novel PPARgamma agonist. Structure-activity relationships identified the 2-aminobenzophenone moiety as a suitable isostere for the chemically labile enaminone moiety in compound 2, affording 2-((2-benzoylphenyl)amino)-3-(4-(benzyloxy)phenyl)propionic acid (9). Replacement of the benzyl group in 9 with substituents known to confer in vivo potency in the thiazolidinedione (TZD) class of antidiabetic agents provided a dramatic increase in the in vitro functional potency and affinity at PPARgamma, affording a series of potent and selective PPARgamma agonists exemplified by (2S)-((2-benzoylphenyl)amino)-3-¿4-[2-(methylpyridin-2-ylamino+ ++)ethoxy ]phenyl¿propionic acid (18), 3-¿4-[2-(benzoxazol-2-ylmethylamino)ethoxy]phenyl¿-(2S)-((2- benzoylph enyl)amino)propanoic acid (19), and (2S)-((2-benzoylphenyl)amino)-3-¿4-[2-(5-methyl-2-phenyloxazol-4-y l)e thoxy]phenyl¿propanoic acid (20). Compounds 18 and 20 show potent antihyperglycemic and antihyperlipidemic activity when given orally in two rodent models of type 2 diabetes. In addition, these analogues are readily prepared in chiral nonracemic fashion from L-tyrosine and do not show a propensity to undergo racemization in vitro. The increased potency of these PPARgamma agonists relative to troglitazone may translate into superior clinical efficacy for the treatment of type 2 diabetes.

A mitochondrial ketogenic enzyme regulates its gene expression by association with the nuclear hormone receptor PPARalpha

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (mHMG-CoAS) is a key enzyme in ketogenesis, catalyzing the condensation of acetyl-CoA and acetoacetyl-CoA to generate HMG-CoA, which is eventually converted to ketone bodies. Transcription of the nuclear-encoded gene for mHMG-CoAS is stimulated by peroxisome proliferator-activated receptor (PPAR) alpha, a fatty acid-activated nuclear hormone receptor. Here we show that the mHMG-CoAS protein physically interacts with PPARalpha in vitro, and potentiates PPARalpha-dependent transcriptional activation via the cognate PPAR response element of the mHMG-CoAS gene in vivo. Immunofluorescence of transiently transfected cells demonstrated that in the presence of PPARalpha, mHMG-CoAS is translocated into the nucleus. Binding to PPARalpha, stimulation of PPARalpha activity and nuclear penetration require the integrity of the sequence LXXLL in mHMG-CoAS, a motif known to mediate the interaction between nuclear hormone receptors and coactivators. These findings reveal a novel mechanism of gene regulation whereby the product of a PPARalpha-responsive gene, normally resident in the mitochondria, directly interacts with this nuclear hormone receptor to autoregulate its own nuclear transcription.

Receptor-interacting protein 140 interacts with and inhibits transactivation by, peroxisome proliferator-activated receptor alpha and liver-X-receptor alpha

Receptor interacting protein 140 (RIP140), a previously identified putative ligand-dependent coactivator of nuclear hormone receptors, was isolated by yeast two-hybrid cloning as a factor that interacts with peroxisome proliferator-activated receptor alpha (PPARalpha). This interaction in yeast required the integrity of the carboxyl-terminal, ligand-dependent activation domain of PPARalpha. However, protein binding studies carried out in vitro showed that full-length RIP140 bound efficiently to PPARalpha in the absence of exogenously added ligand. RIP140 also bound strongly to the liver-X-receptor (LXRalpha) in the absence of an activator for this receptor. In contrast, a strong interaction of RIP140 with the PPARalpha and LXRalpha heterodimerization partner retinoid-X-receptor alpha (RXRalpha) required the presence of its cognate ligand, 9-cis retinoic acid. Transfection analysis in mammalian cells demonstrated that RIP140 antagonized PPARalpha/RXRalpha- and LXRalpha/RXRalpha-mediated signaling. Our findings identify RIP140 as a novel modulator of transcriptional activation mediated by PPARalpha and LXRalpha and indicate that RIP140 can also bind to nuclear hormone receptors in a ligand-independent manner and repress their activity.

In vitro induction of UCP1 mRNA in preadipocytes from rabbit considered as a model of large mammals brown adipose tissue development: importance of PPARgamma agonists for cells isolated in the postnatal period

In mammals with a lower mass-specific metabolic rate than small laboratory rodents, the brown adipose tissue (BAT) loses its thermogenic activity after birth and undergoes a transformation into white adipose tissue (WAT). Rabbit is a model of these mammals of larger body mass. Preadipocytes from cervical BAT of foetal or newborn rabbits differentiated in a chemically-defined medium and expressed low levels of uncoupled protein-1 (UCP1) mRNA, greatly increased by beta3-adrenergic or retinoic acid stimulations. On the contrary, preadipocytes from 1-month-old animals differentiated in the same conditions with no detectable,expression of UCP1. Peroxisome proliferator-activated receptor gamma (PPARgamma) agonists were necessary to induce UCP1 in these cells from older animals, a synergistic increase being noted in the presence of beta3-adrenergic agonists. In contrast to these results, preadipocytes from perirenal WAT stimulated by PPARgamma agonists never expressed UCPI.

CONCLUSION

preadipocytes in the postnatal period are determined as brown or white preadipocytes. PPARgamma agonists induce UCP1 expression in brown postnatal preadipocytes, but they are unable to trigger the gene in white preadipocytes.

Abnormal nonshivering thermogenesis in mice with inherited defects of fatty acid oxidation

When placed in the cold (4 degreesC), BALB/cByJ mice of both genders rapidly lose body temperature as compared with the control strain, C57BL/6J. This sensitivity to cold resembles that previously described for mice with a defect in nonshivering thermogenesis due to the targeted inactivation of the brown adipocyte-specific mitochondrial uncoupling protein gene, Ucp1. Genetic mapping of the trait placed the gene on chromosome 5 near Acads, a gene encoding the short chain acyl CoA dehydrogenase, which is mutated in BALB/cByJ mice. The analysis of candidate genes in the region indicated a defect only in the expression of Acads. Confirmation of the importance of fatty acid oxidation to thermogenesis came from our finding that mice carrying the targeted inactivation of the long chain acyl CoA dehydrogenase gene (Acadl) are also sensitive to the cold. Both of these mutations attenuate the induction of genes normally responsive to adrenergic signaling in brown adipocytes. These results suggest that the action of fatty acids as regulators of gene expression has been perturbed in the mutant mice. From a clinical perspective, it is important to determine whether defects in thermogenesis may be a phenotype in human neonates with inherited deficiencies in fatty acid beta-oxidation.

Nuclear receptor binding factor-1 (NRBF-1), a protein interacting with a wide spectrum of nuclear hormone receptors

To identify the proteins which may modulate the functions of peroxisome proliferator-activated receptor (PPAR), a rat liver cDNA library was screened by a yeast two-hybrid system, using the mouse PPARalpha as a bait. A protein named nuclear receptor binding factor-1 (NRBF-1) was identified, which interacts not only with PPARalpha, but also with various nuclear hormone receptors in the presence of the respective ligands. Both the hinge and ligand-binding domains of PPARalpha are required for the interaction. NRBF-1 seems to be translocated to the nucleus by a piggyback mechanism, together with PPARalpha. NRBF-1 has a significant homology to the yeast protein MRF1, a putative transcription factor regulating the expression of mitochondrial respiratory proteins. NRBF-1 might be another type of nuclear receptor co-operator.

The nuclear receptor PPARgamma - bigger than fat

Work reported over the past year has provided insights into the mechanisms whereby ligand activation of the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) regulates systemic glucose and lipid homeostasis. PPARgamma has also been implicated recently in the biology of monocytes and in cell-cycle regulation and cancer. Polyunsaturated fatty acids and eicosanoids bind and activate PPARgamma, suggesting that these lipids may serve as hormonal regulators of a variety of biological processes.

Abnormal regulation of the leptin gene in the pathogenesis of obesity

A subset of obese humans has relatively low plasma levels of leptin. This finding has suggested that in some cases abnormal regulation of the leptin gene in adipose tissue is etiologic in the pathogenesis of the obese state. The possibility that a relative decrease in leptin production can lead to obesity was tested by mating animals carrying a weakly expressed adipocyte specific aP2-human leptin transgene to C57BL/6J ob/ob mice (which do not express leptin). The transgene does not contain the regulatory elements of the leptin gene and is analogous to a circumstance in which the cis elements and/or trans factors regulating leptin RNA production are abnormal. The ob/ob mice carrying the transgene had a plasma leptin level of 1. 78 ng/ml, which is approximately one-half that found in normal, nontransgenic mice (3.72 ng/ml, P < 0.01). The ob/ob animals expressing the leptin transgene were markedly obese though not as obese as ob/ob mice without the transgene. The infertility as well as several of the endocrine abnormalities generally evident in ob/ob mice were normalized in the ob/ob transgenic mice. However, the ob/ob transgenic mice had an abnormal response when placed at an ambient temperature of 4 degreesC, suggesting that different thresholds exist for the different biologic effects of leptin. Leptin treatment of the ob/ob transgenic mice resulted in marked weight loss with efficacy similar to that seen after treatment of wild-type mice. In aggregate these data suggest that dysregulation of leptin gene can result in obesity with relatively normal levels of leptin and that this form of obesity is responsive to leptin treatment.

Coactivators and corepressors as mediators of nuclear receptor function: an update

The past 3 years have been an exciting time in the field of hormone receptor research because of the discovery and characterization of novel groups of proteins that mediate the transcriptional activity of steroid receptors. These classes of proteins, called coactivators and corepressors, have greatly enhanced our understanding of how steroid receptors activate or inhibit transcription of their target genes. Multiple coactivators have been identified that fit the definition of a protein that connects or bridges the DNA-bound receptor to proteins in the preinitiation complex and thereby enhance transcription. Besides this bridging function, some coactivators can modify chromatin by histone acetylation and make promoters more accessible for the binding of other transcription factors. This finding explains old data concerning steroid receptor-induced nucleosome displacement and indicates a dual role for coactivators as bridging factors and chromatin remodeling proteins. The opposites of coactivators are corepressors, which are recruited into the receptor-DNA-bound complex in the absence of ligand and actively inhibit transcription of the target gene. Although unliganded steroid receptors are associated with heat shock proteins and do not bind to their response elements, the binding of antagonists to these receptors can result in the recruitment of corepressors. The expression level and repertoire of coactivators and corepressors have become important determinants in the functional activity of steroid hormones and their receptors.

Differential expression of the peroxisome proliferator-activated receptor gamma (PPARgamma) and its coactivators steroid receptor coactivator-1 and PPAR-binding protein PBP in the brown fat, urinary bladder, colon, and breast of the mouse

Peroxisome proliferator-activated receptors (PPARs) regulate genes involved in lipid metabolism and adipocyte differentiation. Steroid receptor coactivator-1 (SRC-1) and PPAR-binding protein (PBP) interact with PPARgamma and act as coactivators to enhance ligand-dependent transcription. We report here that PPARgamma, SRC-1, and PBP are differentially expressed in the brown fat, transitional epithelium of the urinary bladder, colonic mucosa, and mammary epithelium of the adult mouse. PPARgamma and PBP are expressed in the transitional epithelium of urinary bladder and in brown adipose tissue, but not SRC-1. In the colonic mucosa, PPARgamma expression occurs throughout the villi, whereas the expression of both SRC-1 and PBP is confined mostly to the crypts. The expression of both SRC-1 and PBP is prominent in the breast epithelium of nonpregnant, pregnant, and lactating mice, whereas PPARgamma expression appeared prominent during lactation. During early embryonic development, PPARgamma, SRC-1, and PBP are differentially expressed, with only limited cell types displaying overlapping expression. PPARgamma and PBP expression overlapped in the brown fat and urogenital sinus at stage E15.5 of embryogenesis, whereas SRC-1 expression occurred mostly in neuroepithelium and cartilage between stages E9.5 and E13.5 of embryogenesis.

Thyroid hormone receptor coactivators and corepressors

In the absence of triiodothyronine (T3), thyroid hormone receptors (TRs) repress transcription of many genes; in the presence of T3, TRs activate transcription of those same genes. Both of these events are dependent on interactions between TRs and other nuclear proteins. TRs bind to specific DNA sequences, generally found in the 5' flanking regions of target genes. In the unliganded state, TRs interact with one of several corepressor proteins. These proteins, in turn, interact with a series of other proteins, which includes histone deacetylases. Histone deacetylation tightens chromatin structure, thus impairing access of critical transcription factors and thereby repressing transcription. In addition, corepressors may invoke mechanisms of gene repression independent of histone deacetylation. The binding of T3 causes a conformational change in the TR that results in release of the corepressor and recruitment of coactivator proteins. Several coactivator proteins appear to bind the ligand-occupied TR as a multiprotein complex. Opposite to corepressors, coactivators acetylate histones, thereby loosening chromatin structure and facilitating access of key transcription factors. Again, mechanisms independent of histone acetylation also may be involved. Overall, gene activation by T3 is a two-step process; removal of active repression, and induction of transcription to levels above the "neutral" state.

Adaptive thermogenesis: turning on the heat

Brown adipocytes play important roles in the regulation of fat storage and body temperature, by virtue of their ability to uncouple mitochondrial fuel oxidation and ATP synthesis. The discovery of a tissue-specific transcriptional coactivator provides new insights into the regulation of thermogenesis by brown adipocytes.

The nuclear receptor ligand-binding domain: structure and function

In the past few years our understanding of nuclear receptor action has dramatically improved as a result of the elucidation of the crystal structures of the empty (apo) ligand-binding domains of the nuclear receptor and of complexes formed by the nuclear receptor's ligand-binding domain bound to agonists and antagonists. Furthermore, the concomitant identification and functional analysis of co-regulators (transcriptional intermediary factors [TIFs], comprising co-activators and co-repressors) previously predicted from squelching studies, have deepened this understanding. Recent data have provided the structural basis for the specific recognition of ligands and the molecular mechanisms of agonism and antagonism, enabling us to gain a comprehensive view of the early steps of nuclear receptor action.

Functional alpha 1 protease inhibitor produced by a human hepatoma cell line

Alpha 1 protease inhibitor antigen was identified in the culture medium of the human ascites hepatoma cell line SK-HEP-1. Trypsin inhibitory activity and alpha 1 Pl antigen accumulated in serum-free medium concomitantly over a period of several days. Radioactive alpha 1 Pl antigen was detected in conditioned medium from cultures supplemented with 35S-L-methionine, indicating a synthesis and release of the protein. Alpha 1 Pl antigen in conditioned medium appeared to be antigenically identical to that in human plasma, and the newly synthesized (radiolabeled) antigen co-migrated with plasma, alpha 1 Pl after immunoelectrophoresis or SDS-polyacrylamide gel electrophoresis. Moreover, evidence is presented that the synthesized inhibitor exhibits functional activity, since the 35S-labeled alpha 1 Pl in conditioned medium complexes with trypsin. We conclude that SK-HEP-1 cells in culture produce functionally active alpha 1 Pl which may be identical to that in plasma.

[Experience in the management of children with diabetes mellitus]

Aerobic glycolysis, also referred to as the Warburg effect, has been regarded as the dominant metabolic phenotype in cancer cells for a long time. More recently, it has been shown that mitochondria in most tumors are not defective in their ability to carry out oxidative phosphorylation (OXPHOS). Instead, in highly aggressive cancer cells, mitochondrial energy pathways are reprogrammed to meet the challenges of high energy demand, better utilization of available fuels and macromolecular synthesis for rapid cell division and migration. Mitochondrial energy reprogramming is also involved in the regulation of oncogenic pathways via mitochondria-to-nucleus retrograde signaling and post-translational modification of oncoproteins. In addition, neoplastic mitochondria can engage in crosstalk with the tumor microenvironment. For example, signals from cancer-associated fibroblasts can drive tumor mitochondria to utilize OXPHOS, a process known as the reverse Warburg effect. Emerging evidence shows that cancer cells can acquire a hybrid glycolysis/OXPHOS phenotype in which both glycolysis and OXPHOS can be utilized for energy production and biomass synthesis. The hybrid glycolysis/OXPHOS phenotype facilitates metabolic plasticity of cancer cells and may be specifically associated with metastasis and therapy-resistance. Moreover, cancer cells can switch their metabolism phenotypes in response to external stimuli for better survival. Taking into account the metabolic heterogeneity and plasticity of cancer cells, therapies targeting cancer metabolic dependency in principle can be made more effective.