Charles Vinson

The fat-derived hormone adiponectin reverses insulin resistance associated with both lipoatrophy and obesity.

Adiponectin is an adipocyte-derived hormone. Recent genome-wide scans have mapped a susceptibility locus for type 2 diabetes and metabolic syndrome to chromosome 3q27, where the gene encoding adiponectin is located. Here we show that decreased expression of adiponectin correlates with insulin resistance in mouse models of altered insulin sensitivity. Adiponectin decreases insulin resistance by decreasing triglyceride content in muscle and liver in obese mice. This effect results from increased expression of molecules involved in both fatty-acid combustion and energy dissipation in muscle. Moreover, insulin resistance in lipoatrophic mice was completely reversed by the combination of physiological doses of adiponectin and leptin, but only partially by either adiponectin or leptin alone. We conclude that decreased adiponectin is implicated in the development of insulin resistance in mouse models of both obesity and lipoatrophy. These data also indicate that the replenishment of adiponectin might provide a novel treatment modality for insulin resistance and type 2 diabetes.

Leptin Inhibits Bone Formation through a Hypothalamic Relay: A Central Control of Bone Mass

Gonadal failure induces bone loss while obesity prevents it. This raises the possibility that bone mass, body weight, and gonadal function are regulated by common pathways. To test this hypothesis, we studied leptin-deficient and leptin receptor-deficient mice that are obese and hypogonadic. Both mutant mice have an increased bone formation leading to high bone mass despite hypogonadism and hypercortisolism. This phenotype is dominant, independent of the presence of fat, and specific for the absence of leptin signaling. There is no leptin signaling in osteoblasts but intracerebroventricular infusion of leptin causes bone loss in leptin-deficient and wild-type mice. This study identifies leptin as a potent inhibitor of bone formation acting through the central nervous system and therefore describes the central nature of bone mass control and its disorders.

Surgical implantation of adipose tissue reverses diabetes in lipoatrophic mice

In lipoatrophic diabetes, a lack of fat is associated with insulin resistance and hyperglycemia. This is in striking contrast to the usual association of diabetes with obesity. To understand the underlying mechanisms, we transplanted adipose tissue into A-ZIP/F-1 mice, which have a severe form of lipoatrophic diabetes. Transplantation of wild-type fat reversed the hyperglycemia, dramatically lowered insulin levels, and improved muscle insulin sensitivity, demonstrating that the diabetes in A-ZIP/F-1 mice is caused by the lack of adipose tissue. All aspects of the A-ZIP/F-1 phenotype including hyperphagia, hepatic steatosis, and somatomegaly were either partially or completely reversed. However, the improvement in triglyceride and FFA levels was modest. Donor fat taken from parametrial and subcutaneous sites was equally effective in reversing the phenotype. The beneficial effects of transplantation were dose dependent and required near-physiological amounts of transplanted fat. Transplantation of genetically modified fat into A-ZIP/F-1 mice is a new and powerful technique for studying adipose physiology and the metabolic and endocrine communication between adipose tissue and the rest of the body.

Liver Peroxisome Proliferator-activated Receptor γ Contributes to Hepatic Steatosis, Triglyceride Clearance, and Regulation of Body Fat Mass

Peroxisome proliferator-activated receptor γ (PPARγ) is a nuclear receptor that mediates the antidiabetic effects of thiazolidinediones. PPARγ is present in adipose tissue and becomes elevated in fatty livers, but the roles of specific tissues in thiazolidinedione actions are unclear. We studied the function of liver PPARγ in both lipoatrophic A-ZIP/F-1 (AZIP) and wild type mice. In AZIP mice, ablation of liver PPARγ reduced the hepatic steatosis but worsened the hyperlipidemia, triglyceride clearance, and muscle insulin resistance. Inactivation of AZIP liver PPARγ also abolished the hypoglycemic and hypolipidemic effects of rosiglitazone, demonstrating that, in the absence of adipose tissue, the liver is a primary and major site of thiazolidinedione action. In contrast, rosiglitazone remained effective in non-lipoatrophic mice lacking liver PPARγ, suggesting that adipose tissue is the major site of thiazolidinedione action in typical mice with adipose tissue. Interestingly, mice without liver PPARγ, but with adipose tissue, developed relative fat intolerance, increased adiposity, hyperlipidemia, and insulin resistance. Thus, liver PPARγ regulates triglyceride homeostasis, contributing to hepatic steatosis, but protecting other tissues from triglyceride accumulation and insulin resistance.

A Dominant-Negative Inhibitor of CREB Reveals that It Is a General Mediator of Stimulus-Dependent Transcription of c-fos

ABSTRACT Several studies have characterized the upstream regulatory region of c-fos, and identified cis-acting elements termed the cyclic AMP (cAMP) response elements (CREs) that are critical for c-fos transcription in response to a variety of extracellular stimuli. Although several transcription factors can bind to CREs in vitro, the identity of the transcription factor(s) that activates the c-fos promoter via the CRE in vivo remains unclear. To help identify the trans-acting factors that regulate stimulus-dependent transcription of c-fos via the CREs, dominant-negative (D-N) inhibitor proteins that function by preventing DNA binding of B-ZIP proteins in a dimerization domain-dependent fashion were developed. A D-N inhibitor of CREB, termed A-CREB, was constructed by fusing a designed acidic amphipathic extension onto the N terminus of the CREB leucine zipper domain. The acidic extension of A-CREB interacts with the basic region of CREB forming a coiled-coil extension of the leucine zipper and thus prevents the basic region of wild-type CREB from binding to DNA. Other D-N inhibitors generated in a similar manner with the dimerization domains of Fos, Jun, C/EBP, ATF-2, or VBP did not block CREB DNA binding activity, nor did they inhibit transcriptional activation of a minimal promoter containing a single CRE in PC12 cells. A-CREB inhibited activation of CRE-mediated transcription evoked by three distinct stimuli: forskolin, which increases intracellular cAMP; membrane depolarization, which promotes Ca2+ influx; and nerve growth factor (NGF). A-CREB completely inhibited cAMP-mediated, but only partially inhibited Ca2+- and NGF-mediated, transcription of a reporter gene containing 750 bp of the native c-fos promoter. Moreover, glutamate induction of c-fos expression in primary cortical neurons was dependent on CREB. In contrast, induction of c-fos transcription by UV light was not inhibited by A-CREB. Lastly, A-CREB attenuated NGF induction of morphological differentiation in PC12 cells. These results suggest that CREB or its closely related family members are general mediators of stimulus-dependent transcription of c-fos and are required for at least some of the long-term actions of NGF.

Classification of human B-ZIP proteins based on dimerization properties.

B-ZIP transcription factors (98) are exclusively eukaryotic proteins that bind to sequence-specific double-stranded DNA as homodimers or heterodimers to either activate or repress gene transcription (34). We have examined both of the recently published DNA sequences of the human genome (51, 95) and identified 56 genes that contain the B-ZIP motif. Three sequences were identical, giving a total of 53 unique B-ZIP domains with the potential to form 2,809 dimers. This creates the possibility for a tremendous range of transcriptional control (23, 50, 52). While significant effort has been directed at identifying dimerization partners of B-ZIP proteins, the full complement of dimerization partners remains to be elucidated. This review highlights two topics: (i) the known structural rules that regulate leucine zipper dimerization specificity and (ii) experimental data addressing mammalian B-ZIP dimerization partners. We have annotated the leucine zippers of all human B-ZIP domains, highlighting amino acids in the a, d, e, and g positions that appear critical for leucine zipper dimerization specificity. These data were used to group B-ZIP proteins into 12 families with similar dimerization properties: (i) those that strongly favor homodimerization within the family (PAR, CREB, Oasis, and ATF6), (ii) those that have the ability to both homodimerize and heterodimerize with similar affinities (C/EBP, ATF4, ATF2, JUN, and the small MAFs), and (iii) those that favor heterodimerization with other families (FOS, CNC, and large MAFs).

Transcription Factor AP1 Potentiates Chromatin Accessibility and Glucocorticoid Receptor Binding

Ligand-dependent transcription by the nuclear receptor glucocorticoid receptor (GR) is mediated by interactions with coregulators. The role of these interactions in determining selective binding of GR to regulatory elements remains unclear. Recent findings indicate that a large fraction of genomic GR binding coincides with chromatin that is accessible prior to hormone treatment, suggesting that receptor binding is dictated by proteins that maintain chromatin in an open state. Combining DNaseI accessibility and chromatin immunoprecipitation with high-throughput sequencing, we identify the activator protein 1 (AP1) as a major partner for productive GR-chromatin interactions. AP1 is critical for GR-regulated transcription and recruitment to co-occupied regulatory elements, illustrating an extensive AP1-GR interaction network. Importantly, the maintenance of baseline chromatin accessibility facilitates GR recruitment and is dependent on AP1 binding. We propose a model in which the basal occupancy of transcription factors acts to prime chromatin and direct inducible transcription factors to select regions in the genome.

A Dominant Negative to Activation Protein-1 (AP1) That Abolishes DNA Binding and Inhibits Oncogenesis

We describe a dominant negative (DN) to activation protein-1 (AP1) that inhibits DNA binding in an equimolar competition. AP1 is a heterodimer of the oncogenes Fos and Jun, members of the bZIP family of transcription factors. The DN, termed A-Fos, consists of a newly designed acidic amphipathic protein sequence appended onto the N-terminus of the Fos leucine zipper, replacing the normal basic region critical for DNA binding. The acidic extension and the Jun basic region form a heterodimeric coiled coil structure that stabilizes the complex over 3000-fold and prevents the basic region of Jun from binding to DNA. Gel shift assays indicate that A-Fos can inactivate the DNA binding of a Fos:Jun heterodimer in an equimolar competition. Transient transfection assays indicate that A-Fos inhibits Jun-dependent transactivation. Both the acidic extension and the Fos leucine zipper are critical for this inhibition. Expression of A-Fos in mouse fibroblasts inhibits focus formation more than colony formation, reflecting the ability of A-Fos to interfere with the AP1 biological functions in mammalian cells. This reagent is more potent than a deletion of either the Fos or Jun transactivation domain, which has been used previously as a dominant negative to AP1 activity.

A thermodynamic scale for leucine zipper stability and dimerization specificity: e and g interhelical interactions.

The leucine zipper is a dimeric coiled‐coil protein structure composed of two amphipathic alpha‐helices with the hydrophobic surfaces interacting to create the dimer interface. This structure has been found to mediate the dimerization of two abundant classes of DNA binding proteins: the bZIP and bHLH‐Zip proteins. Several workers have reported that amino acids in the e and g positions of the coiled coil can modulate dimerization stability and specificity. Using the bZIP protein VBP as a host molecule, we report a thermodynamic scale (delta delta G) for 27 interhelical interactions in 35 proteins between amino acids in the g and the following e positions (g<==>e’) of a leucine zipper coiled coil. We have examined the four commonly occurring amino acids in the e and g positions of bZIP proteins, lysine (K), arginine (R), glutamine (Q), glutamic acid (E), as well as the only other remaining charged amino acid aspartic acid (D), and finally alanine (A) as a reference amino acid. These results indicate that E<==>R is the most stable interhelical pair, being 0.35 kcal/mol more stable than E<==>K. A thermodynamic cycle analysis shows that the E<==>R pair is 1.33 kcal/mol more stable than A<==>A with ‐1.14 kcal/mol of coupling energy (delta delta Gint) coming from the interaction of E with R. The E<==>K coupling energy is only ‐0.14 kcal/mol. E interacts with more specificity than Q. The R<==>R pair is less stable than the K<==>K by 0.24 kcal/mol. R interacts with more specificity than K. Q forms more stable pairs with the basic amino acids K and R rather than with E. Changing amino acids in the e position to A creates bZIP proteins that form tetramers.

An Essential Role for a MEK-C/EBP Pathway during Growth Factor-Regulated Cortical Neurogenesis

Mammalian neurogenesis is determined by an interplay between intrinsic genetic mechanisms and extrinsic cues such as growth factors. Here we have defined a signaling cascade, a MEK-C/EBP pathway, that is essential for cortical progenitor cells to become postmitotic neurons. Inhibition of MEK or of the C/EBP family of transcription factors inhibits neurogenesis while expression of a C/EBPbeta mutant that is a phosphorylation-mimic at a MEK-Rsk site enhances neurogenesis. C/EBP mediates this positive effect by direct transcriptional activation of neuron-specific genes such as Talpha1 alpha-tubulin. Conversely, inhibition of C/EBP-dependent transcription enhances CNTF-mediated generation of astrocytes from the same progenitor cells. Thus, activation of a MEK-C/EBP pathway enhances neurogenesis and inhibits gliogenesis, thereby providing a mechanism whereby growth factors can selectively bias progenitors to become neurons during development.