Antidiabetic actions of a non-agonist PPARγ ligand blocking Cdk5-mediated phosphorylation

PPARγ as a therapeutic target in cystic fibrosis

Cystic fibrosis (CF) is characterized by a proinflammatory pulmonary condition that may result from increased infections and altered intracellular metabolism in CFTR-deficient cells. The lipid-activated transcription factor peroxisome proliferator-activated receptor-γ (PPARγ) has well-established roles in immune cell function and inflammatory modulation and has been demonstrated to play an important role in the heightened inflammatory response in CF cells. Here, we summarize current literature describing PPARγ-dependent alterations of CF cells and discuss the potential of PPARγ ligands for treating CF.

Controlling a master switch of adipocyte development and insulin sensitivity: covalent modifications of PPARγ

Adipocytes are highly specialized cells that play a central role in lipid homeostasis and the maintenance of energy balance. Obesity, an excessive accumulation of adipose tissue, is a major risk factor for the development of Type 2 diabetes mellitus (T2DM), cardiovascular disease, and hypertension. A variety of studies suggest that obesity and T2DM can be linked to a breakdown in the regulatory mechanisms that control the expression and transcriptional activity of PPARγ. PPARγ is a nuclear hormone receptor that functions as a master switch in controlling adipocyte differentiation and development. Also important in controlling glucose homeostasis and insulin sensitivity, PPARγ is a ligand-dependent transcription factor that is the functional receptor for the anti-diabetic thiazolidinediones (TZDs). In the last fifteen years, a variety of covalent modifications of PPARγ activity have been identified and studied. These covalent modifications include phosphorylation, ubiquitylation, O-GlcNAcylation and SUMOylation. Covalent modifications of PPARγ represent key regulatory mechanisms that control both PPARγ protein stability and transcriptional activity. A variety of PPARγ transgenic models, including mice heterozygous for PPARγ, have demonstrated the importance of PPARγ expression in glucose homeostasis and insulin resistance. In the following review, we have highlighted the regulation of PPARγ by covalent modifications, the interplay between these interactions and how these post-translational modifications impact metabolic disease states.

Red carpeting the newer antidiabetics

The rapidly increasing prevalence of diabetes on a global scale beseeches an urgent need for newer and better treatment options. Our better understanding of the pathophysiology of diabetes has enabled a continual churn out of newer antidiabetic agents with varying modes of action. Sodium-Glucose Transport Proteins-2 inhibitors, dipeptidyl peptidase IV inhibitors, glucagon-like peptide analogues, glucokinase activators, dual peroxisome proliferator-activated receptor agonists, monoclonal antibodies, and dopamine-2 receptor agonists either as monotherapy or combination therapy with the existing oral hypoglycemic agents compound our fight against diabetes. A review of the newer drugs targeting various aspects in the management of diabetes is presented.

PPARγ agonists induce a white-to-brown fat conversion through stabilization of PRDM16 protein

Brown adipose tissue dissipates energy through heat and functions as a defense against cold and obesity. PPARγ ligands have been shown to induce the browning of white adipocytes; however, the underlying mechanisms remain unclear. Here, we show that PPARγ ligands require full agonism to induce a brown fat gene program preferentially in subcutaneous white adipose. These effects require expression of PRDM16, a factor that controls the development of classical brown fat. Depletion of PRDM16 blunts the effects of the PPARγ agonist rosiglitazone on the induced brown fat gene program. Conversely, PRDM16 and rosiglitazone synergistically activate the brown fat gene program in vivo. This synergy is tightly associated with an increased accumulation of PRDM16 protein, due in large measure to an increase in the half-life of the protein in agonist treated cells. Identifying compounds that stabilize PRDM16 protein may represent a plausible therapeutic pathway for the treatment of obesity and diabetes.

Mode of peroxisome proliferator-activated receptor γ activation by luteolin

The peroxisome proliferator-activated receptor γ (PPARγ) is a target for treatment of type II diabetes and other conditions. PPARγ full agonists, such as thiazolidinediones (TZDs), are effective insulin sensitizers and anti-inflammatory agents, but their use is limited by adverse side effects. Luteolin is a flavonoid with anti-inflammatory actions that binds PPARγ but, unlike TZDs, does not promote adipocyte differentiation. However, previous reports suggested variously that luteolin is a PPARγ agonist or an antagonist. We show that luteolin exhibits weak partial agonist/antagonist activity in transfections, inhibits several PPARγ target genes in 3T3-L1 cells (LPL, ORL1, and CEBPα) and PPARγ-dependent adipogenesis, but activates GLUT4 to a similar degree as rosiglitazone, implying gene-specific partial agonism. The crystal structure of the PPARγ ligand-binding domain (LBD) reveals that luteolin occupies a buried ligand-binding pocket (LBP) but binds an inactive PPARγ LBD conformer and occupies a space near the β-sheet region far from the activation helix (H12), consistent with partial agonist/antagonist actions. A single myristic acid molecule simultaneously binds the LBP, suggesting that luteolin may cooperate with other ligands to bind PPARγ, and molecular dynamics simulations show that luteolin and myristic acid cooperate to stabilize the Ω-loop among H2', H3, and the β-sheet region. It is noteworthy that luteolin strongly suppresses hypertonicity-induced release of the pro-inflammatory interleukin-8 from human corneal epithelial cells and reverses reductions in transepithelial electrical resistance. This effect is PPARγ-dependent. We propose that activities of luteolin are related to its singular binding mode, that anti-inflammatory activity does not require H12 stabilization, and that our structure can be useful in developing safe selective PPARγ modulators.

Physiological responses to acute psychological stress are reduced by the PPARγ agonist rosiglitazone

Physiological reactions to psychological stress are positively associated with several important chronic conditions including cardiovascular and neurodegenerative diseases and are linked to increased mortality. As such, the identification of cellular and molecular pathways that act to reduce stress responding may represent important targets for therapeutic intervention. Here we report that acute treatment with the peroxisome-proliferator activated receptor-γ (PPARγ) agonist rosiglitazone (RSG) blunts systemic responses to acute psychological stress in rats. Rats that had previously received oral RSG for 5 d exhibited a 40% reduction in the initial heart rate response to an acute restraint stress, compared with vehicle-treated controls, suggesting that increased PPARγ signaling blunts the acute autonomic response to stress. Rats previously treated with RSG likewise had a blunted hormonal response to this stressor, exhibiting a 30% reduction in peak corticosterone levels compared with controls. Moreover, stress-induced expression of c-Fos, a marker of early neuronal activation, was similarly reduced in the paraventricular hypothalamus, a key site for brain stress integration, facilitating both autonomic and hypothalamic-pituitary-adrenocortical responses to stress. Taken as a whole, these data suggest that PPARγ stimulation potently inhibits physiological responses to psychological stress, prescribing a novel role for PPARγ signaling in the regulation of brain stress integration.

The role of peroxisome proliferator-activated receptors in carcinogenesis and chemoprevention

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that are involved in regulating glucose and lipid homeostasis, inflammation, proliferation and differentiation. Although all of these functions might contribute to the influence of PPARs in carcinogenesis, there is a distinct need for a review of the literature and additional experimentation to determine the potential for targeting PPARs for cancer therapy and cancer chemoprevention. As PPAR agonists include drugs that are used for the treatment of metabolic diseases, a more complete understanding of the roles of PPARs in cancer will aid in determining any increased cancer risk for patients undergoing therapy with PPAR agonists.

Derepressing nuclear receptors for metabolic adaptation

NCoR is a corepressor of several transcription factors, including the PPAR family of nuclear receptors in fat and muscle. By specifically deleting NCoR in these tissues, Li et al. and Yamamoto et al. now uncover an important role for NCoR in regulating lipid homeostasis through the coordinated control of different nuclear receptors.

Couple dynamics: PPARγ and its ligand partners

Ligand-regulated transcriptional activity is the most important property of nuclear receptors, including PPARγ. In this issue of Structure, Hughes et al. determined how the dynamic conformations of ligands and the receptor contribute to the degree of ligand-dependent activation of PPARγ, which provide further insights into design of PPARγ-based anti-diabetic drugs.

STAT6 phosphorylation inhibitors block eotaxin-3 secretion in bronchial epithelial cells

The STAT6 (signal transducer and activator of transcription 6) protein facilitates T-helper cell 2 (Th2) mediated responses that control IgE-mediated atopic diseases such as asthma. We have identified compounds that bind to STAT6 and inhibit STAT6 tyrosine phosphorylation induced by IL-4. In the bronchial epithelial cell line BEAS-2B, compound (R)-84 inhibits the secretion of eotaxin-3, a chemokine eliciting eosinophil infiltration. (R)-84 appears to prevent STAT6 from assuming the active dimer configuration by directly binding the protein and inhibiting tyrosine phosphorylation.