Mitchell A. Lazar

The hormone resistin links obesity to diabetes

Diabetes mellitus is a chronic disease that leads to complications including heart disease, stroke, kidney failure, blindness and nerve damage. Type 2 diabetes, characterized by target-tissue resistance to insulin, is epidemic in industrialized societies and is strongly associated with obesity; however, the mechanism by which increased adiposity causes insulin resistance is unclear. Here we show that adipocytes secrete a unique signalling molecule, which we have named resistin (for resistance to insulin). Circulating resistin levels are decreased by the anti-diabetic drug rosiglitazone, and increased in diet-induced and genetic forms of obesity. Administration of anti-resistin antibody improves blood sugar and insulin action in mice with diet-induced obesity. Moreover, treatment of normal mice with recombinant resistin impairs glucose tolerance and insulin action. Insulin-stimulated glucose uptake by adipocytes is enhanced by neutralization of resistin and is reduced by resistin treatment. Resistin is thus a hormone that potentially links obesity to diabetes.

Cofactor Dynamics and Sufficiency in Estrogen Receptor–Regulated Transcription

Many cofactors bind the hormone-activated estrogen receptor (ER), yet the specific regulators of endogenous ER-mediated gene transcription are unknown. Using chromatin immunoprecipitation (ChIP), we find that ER and a number of coactivators rapidly associate with estrogen responsive promoters following estrogen treatment in a cyclic fashion that is not predicted by current models of hormone activation. Cycles of ER complex assembly are followed by transcription. In contrast, the anti-estrogen tamoxifen (TAM) recruits corepressors but not coactivators. Using a genetic approach, we show that recruitment of the p160 class of coactivators is sufficient for gene activation and for the growth stimulatory actions of estrogen in breast cancer supporting a model in which ER cofactors play unique roles in estrogen signaling.

The Many Faces of PPARγ

In an era marked by the increasing prevalence of obesity, diabetes, and cardiovascular disease, the nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) has emerged as a transcriptional regulator of metabolism whose activity can be modulated by direct binding of small molecules. As the master regulator of fat-cell formation, PPARγ is required for the accumulation of adipose tissue and hence contributes to obesity. Yet PPARγ ligands are clinically effective antidiabetic drugs, although side effects limit their utility. Can PPARγ be targeted with greater benefit and with less risk to patients? The answer depends upon the basic biology of PPARγ, and the possibility of selectively modulating the activity of this nuclear receptor in a tissue- and target-gene-specific manner.

Resistin Is an Inflammatory Marker of Atherosclerosis in Humans

Background—Resistin, a plasma protein, induces insulin resistance in rodents. Recent reports suggest that circulating levels of resistin are elevated in obese and insulin-resistant rodents and humans. Whereas rodent resistin is made in adipocytes, macrophages are a major source of human resistin. Given the convergence of adipocyte and macrophage function, resistin may provide unique insight into links between obesity, inflammation, and atherosclerosis in humans. Methods and Results—We examined whether plasma resistin levels were associated with metabolic and inflammatory markers, as well as with coronary artery calcification (CAC), a quantitative index of atherosclerosis, in 879 asymptomatic subjects in the Study of Inherited Risk of Coronary Atherosclerosis. Resistin levels were positively associated with levels of inflammatory markers, including soluble tumor necrosis factor-&agr; receptor-2 (P<0.001), interleukin-6 (P=0.04), and lipoprotein-associated phospholipase A2 (P=0.002), but not measures of insulin resistance in multivariable analysis. Resistin levels also were associated (odds ratio and 95% confidence interval in ordinal regression) with increasing CAC after adjustment for age, sex, and established risk factors (OR, 1.23 [CI, 1.03 to 1.52], P=0.03) and further control for metabolic syndrome and plasma C-reactive protein (CRP) levels (OR, 1.25 [CI, 1.04 to 1.50], P=0.01). In subjects with metabolic syndrome, resistin levels further predicted CAC, whereas CRP levels did not. Conclusions—Plasma resistin levels are correlated with markers of inflammation and are predictive of coronary atherosclerosis in humans, independent of CRP. Resistin may represent a novel link between metabolic signals, inflammation, and atherosclerosis. Further studies are needed to define the relationship of resistin to clinical cardiovascular disease.

A novel therapy for colitis utilizing PPAR-gamma ligands to inhibit the epithelial inflammatory response.

Peroxisome proliferator-activated receptor gamma (PPAR-gamma), a member of the nuclear hormone receptor superfamily originally shown to play a critical role in adipocyte differentiation and glucose homeostasis, has recently been implicated as a regulator of cellular proliferation and inflammatory responses. Colonic epithelial cells, which express high levels of PPAR-gamma protein, have the ability to produce inflammatory cytokines that may play a role in inflammatory bowel disease (IBD). We report here that PPAR-gamma ligands dramatically attenuate cytokine gene expression in colon cancer cell lines by inhibiting the activation of nuclear factor-kappaB via an IkappaB-alpha-dependent mechanism. Moreover, thiazolidinedione ligands for PPAR-gamma markedly reduce colonic inflammation in a mouse model of IBD. These results suggest that colonic PPAR-gamma may be a therapeutic target in humans suffering from IBD.

International union of pharmacology. LXI. Peroxisome proliferator-activated receptors

The three peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors of the nuclear hormone receptor superfamily. They share a high degree of structural homology with all members of the superfamily, particularly in the DNA-binding domain and ligand- and cofactor-binding domain. Many cellular and systemic roles have been attributed to these receptors, reaching far beyond the stimulation of peroxisome proliferation in rodents after which they were initially named. PPARs exhibit broad, isotype-specific tissue expression patterns. PPARα is expressed at high levels in organs with significant catabolism of fatty acids. PPARβ/δ has the broadest expression pattern, and the levels of expression in certain tissues depend on the extent of cell proliferation and differentiation. PPARγ is expressed as two isoforms, of which PPARγ2 is found at high levels in the adipose tissues, whereas PPARγ1 has a broader expression pattern. Transcriptional regulation by PPARs requires heterodimerization with the retinoid X receptor (RXR). When activated by a ligand, the dimer modulates transcription via binding to a specific DNA sequence element called a peroxisome proliferator response element (PPRE) in the promoter region of target genes. A wide variety of natural or synthetic compounds was identified as PPAR ligands. Among the synthetic ligands, the lipid-lowering drugs, fibrates, and the insulin sensitizers, thiazolidinediones, are PPARα and PPARγ agonists, respectively, which underscores the important role of PPARs as therapeutic targets. Transcriptional control by PPAR/RXR heterodimers also requires interaction with coregulator complexes. Thus, selective action of PPARs in vivo results from the interplay at a given time point between expression levels of each of the three PPAR and RXR isotypes, affinity for a specific promoter PPRE, and ligand and cofactor availabilities.

Differential Activation of Peroxisome Proliferator-activated Receptors by Eicosanoids

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that regulate gene transcription in response to peroxisome proliferators and fatty acids. PPARs also play an important role in the regulation of adipocyte differentiation. It is unclear, however, what naturally occurring compounds activate each of the PPAR subtypes. To address this issue, a screening assay was established using heterologous fusions of the bacterial tetracycline repressor to several members of the peroxisome proliferator-activated receptor (PPAR) family. This assay was employed to compare the activation of PPAR family members by known PPAR activators including peroxisome proliferators and fatty acids. Interestingly, the activation of PPARs by fatty acids was partially inhibited by the cyclooxygenase inhibitor indomethacin, which prevents prostaglandin synthesis. Indeed, prostaglandins PGA1 and 2, PGD1 and 2, and PGJ2-activated PPARs, while a number of other prostaglandins had no effect. We also screened a variety of hydroxyeicosatetraenoic acids (HETEs) for the ability to activate PPARs. 8(S)-HETE, but not other (S)-HETEs, was a strong activator of PPARα. Remarkably, PPAR activation by 8(S)-HETE was stereoselective. In addition, 8(S)-HETE was able to induce differentiation of 3T3-L1 preadipocytes. These results indicate that PPARs are differentially activated by naturally occurring eicosanoids and related molecules.

Forming functional fat: a growing understanding of adipocyte differentiation

Adipose tissue, which is primarily composed of adipocytes, is crucial for maintaining energy and metabolic homeostasis. Adipogenesis is thought to occur in two stages: commitment of mesenchymal stem cells to a preadipocyte fate and terminal differentiation. Cell shape and extracellular matrix remodelling have recently been found to regulate preadipocyte commitment and competency by modulating WNT and RHO-family GTPase signalling cascades. Adipogenic stimuli induce terminal differentiation in committed preadipocytes through the epigenomic activation of peroxisome proliferator-activated receptor-γ (PPARγ). The coordination of PPARγ with CCAAT/enhancer-binding protein (C/EBP) transcription factors maintains adipocyte gene expression. Improving our understanding of these mechanisms may allow us to identify therapeutic targets against metabolic diseases that are rapidly becoming epidemic globally.

Peroxisome proliferator-activated receptor gamma activators inhibit gene expression and migration in human vascular smooth muscle cells

Migration of vascular smooth muscle cells (VSMCs) plays an important role in atherogenesis and restenosis after arterial interventions. The expression of matrix metalloproteinases (MMPs), particularly MMP-9, contributes to VSMC migration. This process requires degradation of basal laminae and other components of the arterial extracellular matrix. Peroxisome proliferator-activated receptors (PPARs), members of the nuclear receptor family, regulate gene expression after activation by various ligands. Recent studies have suggested opposing effects of PPAR gamma (PPARgamma) activation on atherogenesis. The present study tested the hypotheses that human VSMCs express PPAR alpha (PPARalpha) and PPARgamma and that PPAR agonists in VSMCs modulate MMP-9 expression and activity, as well as VSMC migration. Human VSMCs expressed PPARalpha and PPARgamma mRNA and protein. Treatment of VSMCs with the PPARgamma ligands troglitazone and the naturally occurring 15-deoxy-Delta12, 14-prostaglandin J2 (15d-PGJ2) decreased phorbol 12-myristate 13-acetate-induced MMP-9 mRNA and protein levels, as well as MMP-9 gelatinolytic activity in the supernatants in a concentration-dependent manner. Six different PPARalpha activators lacked such effects. Addition of prostaglandin F2alpha, known to limit PPARgamma activity, diminished the MMP-9 inhibition seen with either troglitazone or 15d-PGJ2, further implicating PPARgamma in these effects. Finally, troglitazone and 15d-PGJ2 inhibited the platelet-derived growth factor-BB-induced migration of VSMCs in vitro in a concentration-dependent manner. PPARgamma activation may regulate VSMC migration and expression and activity of MMP-9. Thus, PPARgamma activation in VSMCs, via the antidiabetic agent troglitazone or naturally occurring ligands, may act to counterbalance other potentially proatherosclerotic PPARgamma effects.

Enzymatic activity associated with class II HDACs is dependent on a multiprotein complex containing HDAC3 and SMRT/N-CoR.

Histone deacetylases (HDACs) play a key role in regulating eukaryotic gene expression. The HDAC domain, homologous to the yeast repressors RPD3 and HDA1, is considered necessary and sufficient for enzymatic activity. Here, we show that the catalytic domain of HDAC4 interacts with HDAC3 via the transcriptional corepressor N-CoR/SMRT. All experimental conditions leading to the suppression of HDAC4 binding to SMRT/N-CoR and to HDAC3 result in the loss of enzymatic activity associated with HDAC4. In vitro reconstitution experiments indicate that HDAC4 and other class II HDACs are inactive in the context of the SMRT/N-CoR-HDAC3 complex and do not contribute to its enzymatic activity. These observations indicate that class II HDACs regulate transcription by bridging the enzymatically active SMRT/N-CoR-HDAC3 complex and select transcription factors independently of any intrinsic HDAC activity.