Search

Insulin resistance is a fundamental pathogenic factor that characterizes various metabolic disorders, including obesity and type 2 diabetes. Adipose tissue contributes to the development of obesity-related insulin resistance through increased release of fatty acids, altered adipokine secretion, and/or macrophage infiltration and cytokine release. Here, we aimed to analyze the participation of the cyclin-dependent kinase 4 (CDK4) in adipose tissue biology. We determined that white adipose tissue (WAT) from CDK4-deficient mice exhibits impaired lipogenesis and increased lipolysis. Conversely, lipolysis was decreased and lipogenesis was increased in mice expressing a mutant hyperactive form of CDK4 (CDK4R24C). A global kinome analysis of CDK4-deficient mice following insulin stimulation revealed that insulin signaling is impaired in these animals. We determined that insulin activates the CCND3-CDK4 complex, which in turn phosphorylates insulin receptor substrate 2 (IRS2) at serine 388, thereby creating a positive feedback loop that maintains adipocyte insulin signaling. Furthermore, we found that CCND3 expression and IRS2 serine 388 phosphorylation are increased in human obese subjects. Together, our results demonstrate that CDK4 is a major regulator of insulin signaling in WAT.

Contains fulltext : 80913.pdf (publisher's version ) (Closed access), The insulin/insulin-like growth factor 1 (IGF-1) signaling pathway is a major regulator of adipose tissue growth and differentiation. We recently demonstrated that human protein tyrosine phosphatase (PTP) L1, a large cytoplasmic phosphatase also known as PTP-BAS/PTPN13/PTP-1E, is a negative regulator of IGF-1R/IRS-1/Akt pathway in breast cancer cells. This triggered us to investigate the potential role of PTPL1 in adipogenesis. To evaluate the implication of PTP-BL, the mouse orthologue of PTPL1, in adipose tissue biology, we analyzed PTP-BL mRNA expression in adipose tissue in vivo and during proliferation and differentiation of 3T3-L1 pre-adipocytes. To elucidate the role of PTP-BL and of its catalytic activity during adipogenesis we use siRNA techniques in 3T3-L1 pre-adipocytes, and mouse embryonic fibroblasts that lack wildtype PTP-BL and instead express a variant without the PTP domain (Delta P/Delta P MEFs). Here we show that PTP-BL is strongly expressed in white adipose tissue and that PTP-BL transcript and protein levels increase during proliferation and differentiation of 3T3-L1 pre-adipocytes. Strikingly, knockdown of PTP-BL expression in 3T3-L1 adipocytes caused a dramatic decrease in adipogenic gene expression levels (PPAR gamma, aP2) and lipid accumulation but did not interfere with the insulin/Akt pathway. Delta P/Delta P MEFs differentiate into the adipogenic lineage as efficiently as wildtype MEFs. However, when expression of either PTP-BL or PTP-BL Delta P was inhibited a dramatic reduction in the number of MEF-derived adipocytes was observed. These findings demonstrate a key role for PTP-BL in 3T3-L1 and MEF-derived adipocyte differentiation that is independent of its enzymatic activity.

Adipocyte differentiation is coordinatedly regulated by several transcription factors. C/EBP beta, C/EBP delta and ADD-1/SREBP-1 are active early during the differentiation process and induce the expression and/or activity of the peroxisome proliferator activated receptor-gamma (PPAR gamma), the pivotal coordinator of the adipocyte differentiation process. Activated PPAR gamma induces exit from the cell cycle and triggers the expression of adipocyte-specific genes, resulting in increased delivery of energy to the cells. C/EBP alpha, whose expression coincides with the later stages of differentiation, cooperates with PPAR gamma in inducing additional target genes and sustains a high level of PPAR gamma in the mature adipocyte as part of a feedforward loop. Altered activity and/or expression of these transcription factors might underlie the pathogenesis of disorders characterized by increased or decreased adipose tissue depots.

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors, initially described as molecular targets for synthetic compounds inducing peroxisome proliferation. PPAR-gamma, the best characterized of the PPARs, plays a crucial role in adipogenesis and insulin sensitization. Furthermore, PPAR-gamma has been reported to affect cell proliferation/differentiation pathways in various malignancies. We discuss in the present review recent advances in the understanding of the function of PPAR-gamma in both cell proliferation and adipocyte differentiation.

The peroxisome proliferator-activated receptor gamma is a nuclear hormone receptor playing a crucial role in adipogenesis and insulin sensitization. Prostaglandin J2 derivatives and the antidiabetic thiazolidinediones are its respective natural and synthetic ligands. The RXR/PPAR gamma heterodimer has also been reported to have important immunomodulatory activities and its pleiotropic functions suggest wide-ranging medical implications.

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors, initially described as molecular targets for synthetic compounds that induce peroxisome proliferation. PPARgamma is the best characterized of the PPARs. The heterodimer of PPARgamma with the retinoid X receptor (RXR) plays a crucial role in adipogenesis and insulin sensitization. The RXR/PPARgamma heterodimer furthermore has been reported to have important immunomodulatory activities and to affect cell proliferation/differentiation pathways in various malignancies. PPARgamma is activated by a number of naturally occurring fatty acid derivatives and by several synthetic compounds, including the thiazolidinediones and L-tyrosine-based insulin sensitizers. This review gives an overview of the pleiotropic functions of PPARgamma and discusses the wide-ranging medical implications that modulation of PPARgamma activity might have for various diseases, ranging from obesity and type 2 diabetes to cancer and inflammation.

The nuclear peroxisome proliferator-activated receptor gamma (PPARgamma) activates the transcription of multiple genes involved in intra- and extracellular lipid metabolism. Several cofactors are crucial for the stimulation or the silencing of nuclear receptor transcriptional activities. The two homologous cofactors p300 and CREB-binding protein (CBP) have been shown to co-activate the ligand-dependent transcriptional activities of several nuclear receptors as well as the ligand-independent transcriptional activity of the androgen receptor. We show here that the interaction between p300/CBP and PPARgamma is complex and involves multiple domains in each protein. p300/CBP not only bind in a ligand-dependent manner to the DEF region of PPARgamma but also bind directly in a ligand-independent manner to a region in the AB domain localized between residue 31 to 99. In transfection experiments, p300/CBP could thereby enhance the transcriptional activities of both the activating function (AF)-1 and AF-2 domains. p300/CBP displays itself at least two docking sites for PPARgamma located in its N terminus (between residues 1 and 113 for CBP) and in the middle of the protein (between residues 1099 and 1460)., info:eu-repo/semantics/published

PPARgamma is a member of the PPAR subfamily of nuclear receptors. In this work, the structure of the human PPARgamma cDNA and gene was determined, and its promoters and tissue-specific expression were functionally characterized. Similar to the mouse, two PPAR isoforms, PPARgamma1 and PPARgamma2, were detected in man. The relative expression of human PPARgamma was studied by a newly developed and sensitive reverse transcriptase-competitive polymerase chain reaction method, which allowed us to distinguish between PPARgamma1 and gamma2 mRNA. In all tissues analyzed, PPARgamma2 was much less abundant than PPARgamma1. Adipose tissue and large intestine have the highest levels of PPARgamma mRNA; kidney, liver, and small intestine have intermediate levels; whereas PPARgamma is barely detectable in muscle. This high level expression of PPARgamma in colon warrants further study in view of the well established role of fatty acid and arachidonic acid derivatives in colonic disease. Similarly as mouse PPARgammas, the human PPARgammas are activated by thiazolidinediones and prostaglandin J and bind with high affinity to a PPRE. The human PPARgamma gene has nine exons and extends over more than 100 kilobases of genomic DNA. Alternate transcription start sites and alternate splicing generate the PPARgamma1 and PPARgamma2 mRNAs, which differ at their 5'-ends. PPARgamma1 is encoded by eight exons, and PPARgamma2 is encoded by seven exons. The 5'-untranslated sequence of PPARgamma1 is comprised of exons A1 and A2, whereas that of PPARgamma2 plus the additional PPARgamma2-specific N-terminal amino acids are encoded by exon B, located between exons A2 and A1. The remaining six exons, termed 1 to 6, are common to the PPARgamma1 and gamma2. Knowledge of the gene structure will allow screening for PPARgamma mutations in humans with metabolic disorders, whereas knowledge of its expression pattern and factors regulating its expression could be of major importance in understanding its biolog, info:eu-repo/semantics/published