PubMed Health⌕ Search

Biomedical subjects

Michael P Czech

Publications and source records attributed to Michael P Czech.

At least 19 recordsLinked to original sources
PubMed2006

RNAi-based gene silencing in primary mouse and human adipose tissues.

Cultured adipocyte cell lines are a model system widely used to study adipose function, but they exhibit significant physiological differences compared with primary cells from adipose tissue. Here we report short interfering RNA-based methodology to selectively attenuate gene expression in mouse and human primary adipose tissues as a means of rapidly validating findings made in cultured adipocyte cell lines. The method is exemplified by depletion of the PTEN phosphatase in white adipose tissue (WAT) from mouse and humans, which increases Akt phosphorylation as expected. This technology is also shown to silence genes in mouse brown adipose tissue. Previous work revealed upregulation of the mitochondrial protein UCP1 in adipose cells from mice lacking the gene for the transcriptional corepressor RIP140, whereas in cultured adipocytes, loss of RIP140 has a little effect on UCP1 expression. Application of our method to deplete RIP140 in primary mouse WAT elicited markedly increased oxygen consumption and expression of UCP1 that exactly mimics the phenotype observed in RIP140-null mice. This ex-vivo method of gene silencing should be useful in rapid validation studies as well as in addressing the depot- and species-specific functions of genes in adipose biology.

Adaptor Proteins, Signal Transducing↗
PubMed2006

An RNA interference-based screen identifies MAP4K4/NIK as a negative regulator of PPARgamma, adipogenesis, and insulin-responsive hexose transport.

The insulin-regulated glucose transporter GLUT4 is a key modulator of whole body glucose homeostasis, and its selective loss in adipose tissue or skeletal muscle causes insulin resistance and diabetes. Here we report an RNA interference-based screen of protein kinases expressed in adipocytes and identify four negative regulators of insulin-responsive glucose transport: the protein kinases PCTAIRE-1 (PCTK1), PFTAIRE-1 (PFTK1), IkappaB kinase alpha, and MAP4K4/NIK. Integrin-linked protein kinase was identified as a positive regulator of this process. We characterized one of these hits, MAP4K4/NIK, and found that it is unique among mitogen-activated protein (MAP) kinases expressed in cultured adipocytes in attenuating hexose transport. Remarkably, MAP4K4/NIK suppresses expression of the adipogenic transcription factors C/EBPalpha, C/EBPbeta, and PPARgamma and of GLUT4 itself in these cells. RNA interference-mediated depletion of MAP4K4/NIK early in differentiation enhances adipogenesis and triglyceride deposition, and even in fully differentiated adipocytes its loss up-regulates GLUT4. Conversely, conditions that inhibit adipogenesis such as TNF-alpha treatment or depletion of PPARgamma markedly up-regulate MAP4K4/NIK expression in cultured adipocytes. Furthermore, TNF-alpha signaling to down-regulate GLUT4 is impaired in the absence of MAP4K4/NIK, indicating that MAP4K4 expression is required for optimal TNF-alpha action. These results reveal a MAP4K4/NIK-dependent signaling pathway that potently inhibits PPARgamma-responsive gene expression, adipogenesis, and insulin-stimulated glucose transport.

3T3-L1 Cells↗
PubMed2005

Suppression of oxidative metabolism and mitochondrial biogenesis by the transcriptional corepressor RIP140 in mouse adipocytes.

Using an siRNA-based screen, we identified the transcriptional corepressor RIP140 as a negative regulator of insulin-responsive hexose uptake and oxidative metabolism in 3T3-L1 adipocytes. Affymetrix GeneChip profiling revealed that RIP140 depletion upregulates the expression of clusters of genes in the pathways of glucose uptake, glycolysis, TCA cycle, fatty acid oxidation, mitochondrial biogenesis, and oxidative phosphorylation in these cells. Conversely, we show that reexpression of RIP140 in mouse embryonic fibroblasts derived from RIP140-null mice downregulates expression of many of these same genes. Consistent with these microarray data, RIP140 gene silencing in cultured adipocytes increased both conversion of [14C]glucose to CO2 and mitochondrial oxygen consumption. RIP140-null mice, previously reported to resist weight gain on a high-fat diet, are shown here to display enhanced glucose tolerance and enhanced responsiveness to insulin compared with matched wild-type mice upon high-fat feeding. Mechanistically, RIP140 was found to require the nuclear receptor ERRalpha to regulate hexose uptake and mitochondrial proteins SDHB and CoxVb, although it likely acts through other nuclear receptors as well. We conclude that RIP140 is a major suppressor of adipocyte oxidative metabolism and mitochondrial biogenesis, as well as a negative regulator of whole-body glucose tolerance and energy expenditure in mice.

3T3 Cells↗
PubMed2005

Crystal structure of the C2 domain of class II phosphatidylinositide 3-kinase C2alpha.

Phosphatidylinositide (PtdIns) 3-kinase catalyzes the addition of a phosphate group to the 3'-position of phosphatidyl inositol. Accumulated evidence shows that PtdIns 3-kinase can provide a critical signal for cell proliferation, cell survival, membrane trafficking, glucose transport, and membrane ruffling. Mammalian PtdIns 3-kinases are divided into three classes based on structure and substrate specificity. A unique characteristic of class II PtdIns 3-kinases is the presence of both a phox homolog domain and a C2 domain at the C terminus. The biological function of the C2 domain of the class II PtdIns 3-kinases remains to be determined. We have determined the crystal structure of the mCPK-C2 domain, which is the first three-dimensional structural model of a C2 domain of class II PtdIns 3-kinases. Structural studies reveal that the mCPK-C2 domain has a typical anti-parallel beta-sandwich fold. Scrutiny of the surface of this C2 domain has identified three small, shallow sulfate-binding sites. On the basis of the structural features of these sulfate-binding sites, we have studied the lipid binding properties of the mCPK-C2 domain by site-directed mutagenesis. Our results show that this C2 domain binds specifically to PtdIns(3,4)P(2) and PtdIns(4,5)P(2) and that three lysine residues at SBS I site, Lys-1420, Lys-1432, and Lys-1434, are responsible for the phospholipid binding affinity.

Amino Acid Sequence↗
PubMed2005

The v-SNARE Vti1a regulates insulin-stimulated glucose transport and Acrp30 secretion in 3T3-L1 adipocytes.

Regulated exocytosis in adipocytes mediates key functions, exemplified by insulin-stimulated secretion of peptides such as adiponectin and recycling of intracellular membranes containing GLUT4 glucose transporters to the cell surface. Using a proteomics approach, the v-SNARE Vti1a (vps10p tail interacting 1a) was identified by mass spectrometry in purified GLUT4-containing membranes. Insulin treatment of 3T3-L1 adipocytes decreased the amounts of both Vti1a and GLUT4 in these membranes, confirming that Vti1a is a component of insulin-sensitive GLUT4-containing vesicles. In the basal state, endogenous Vti1a colocalizes exclusively with perinuclear GLUT4. Although Vti1a has previously been reported to be a v-SNARE localized in the trans-Golgi network, treatment with brefeldin A failed to significantly modify Vti1a or GLUT4 localization while completely dispersing Golgi and trans-Golgi network marker proteins. Furthermore, depletion of Vti1a protein in cultured adipocytes through small interfering RNA-based gene silencing significantly inhibited both adiponectin secretion and insulin-stimulated deoxyglucose uptake. Taken together, these results suggest that the v-SNARE Vti1a may regulate a step common to both GLUT4 and Acrp30 trafficking in 3T3-L1 adipocytes.

3T3-L1 Cells↗
PubMed2005

PTEN, but not SHIP2, suppresses insulin signaling through the phosphatidylinositol 3-kinase/Akt pathway in 3T3-L1 adipocytes.

Glucose homeostasis is controlled by insulin in part through the stimulation of glucose transport in muscle and fat cells. This insulin signaling pathway requires phosphatidylinositol (PI) 3-kinase-mediated 3'-polyphosphoinositide generation and activation of Akt/protein kinase B. Previous experiments using dominant negative constructs and gene ablation in mice suggested that two phosphoinositide phosphatases, SH2 domain-containing inositol 5'-phosphatase 2 (SHIP2) and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) negatively regulate this insulin signaling pathway. Here we directly tested this hypothesis by selectively inhibiting the expression of SHIP2 or PTEN in intact cultured 3T3-L1 adipocytes through the use of short interfering RNA (siRNA). Attenuation of PTEN expression by RNAi markedly enhanced insulin-stimulated Akt and glycogen synthase kinase 3alpha (GSK-3alpha) phosphorylation, as well as deoxyglucose transport in 3T3-L1 adipocytes. In contrast, depletion of SHIP2 protein by about 90% surprisingly failed to modulate these insulin-regulated events under identical assay conditions. In control studies, no diminution of insulin signaling to the mitogen-activated protein kinases Erk1 and Erk2 was observed when either PTEN or SHIP2 were depleted. Taken together, these results demonstrate that endogenous PTEN functions as a suppressor of insulin signaling to glucose transport through the PI 3-kinase pathway in cultured 3T3-L1 adipocytes.

3T3-L1 Cells↗
PubMed2005

Identification of WNK1 as a substrate of Akt/protein kinase B and a negative regulator of insulin-stimulated mitogenesis in 3T3-L1 cells.

Insulin signaling through protein kinase Akt/protein kinase B (PKB), a downstream element of the phosphatidylinositol 3-kinase (PI3K) pathway, regulates diverse cellular functions including metabolic pathways, apoptosis, mitogenesis, and membrane trafficking. To identify Akt/PKB substrates that mediate these effects, we used antibodies that recognize phosphopeptide sites containing the Akt/PKB substrate motif (RXRXX(p)S/T) to immunoprecipitate proteins from insulin-stimulated adipocytes. Tryptic peptides from a 250-kDa immunoprecipitated protein were identified as the protein kinase WNK1 (with no lysine) by matrix-assisted laser desorption ionization time-of-flight mass spectrometry, consistent with a recent report that WNK1 is phosphorylated on Thr60 in response to insulin-like growth factor I. Insulin treatment of 3T3-L1 adipocytes stimulated WNK1 phosphorylation, as detected by immunoprecipitation with antibody against WNK1 followed by immunoblotting with the anti-phosphoAkt substrate antibody. WNK1 phosphorylation induced by insulin was unaffected by rapamycin, an inhibitor of p70 S6 kinase pathway but abolished by the PI3K inhibitor wortmannin. RNA interference-directed depletion of Akt1/PKB alpha and Akt2/PKB beta attenuated insulin-stimulated WNK1 phosphorylation, but depletion of protein kinase C lambda did not. Whereas small interfering RNA-induced loss of WNK1 protein did not significantly affect insulin-stimulated glucose transport in 3T3-L1 adipocytes, it significantly enhanced insulin-stimulated thymidine incorporation by about 2-fold. Furthermore, depletion of WNK1 promoted serum-stimulated cell proliferation of 3T3-L1 preadipocytes, as evidenced by a 36% increase in cell number after 48 h in culture. These data suggest that WNK1 is a physiologically relevant target of insulin signaling through PI3K and Akt/PKB and functions as a negative regulator of insulin-stimulated mitogenesis.

3T3-L1 Cells↗
PubMed2004

Early growth response gene-2, a zinc-finger transcription factor, is required for full induction of clonal anergy in CD4+ T cells.

Ag-specific immune tolerance results from the induction of cellular mechanisms that limit T cell responses to selective Ags. One of these mechanisms is characterized by attenuated proliferation and decreased IL-2 production in fully stimulated CD4(+) Th cells and is denoted T cell anergy. We report the identification of the early growth response gene (Egr-2; Krox-20), a zinc-finger transcription factor, as a key protein required for induction of anergy in cultured T cells. Gene array screening revealed high Egr-2 expression distinctly persists in anergized but not proliferating murine A.E7 T cells. In contrast, Egr-1, a related family member induced upon costimulation, displays little or no expression in the anergic state. IL-2-mediated abrogation of anergy causes rapid depletion of Egr-2 protein. Full stimulation of anergic A.E7 T cells fails to enhance IL-2 and Egr-1 expression, whereas Egr-2 expression is greatly increased. Silencing Egr-2 gene expression by small interfering RNA treatment of cultured A.E7 T cells before incubation with anti-CD3 alone prevents full induction of anergy. However, small interfering RNA-mediated depletion of Egr-2 5 days after anergy induction does not appear to abrogate hyporesponsiveness to stimulation. These data indicate that sustained Egr-2 expression is necessary to induce a full anergic state through the actions of genes regulated by this transcription factor.

Animals↗
PubMed2004

Structural determinants of phosphoinositide selectivity in splice variants of Grp1 family PH domains.

The pleckstrin homology (PH) domains of the homologous proteins Grp1 (general receptor for phosphoinositides), ARNO (Arf nucleotide binding site opener), and Cytohesin-1 bind phosphatidylinositol (PtdIns) 3,4,5-trisphosphate with unusually high selectivity. Remarkably, splice variants that differ only by the insertion of a single glycine residue in the beta1/beta2 loop exhibit dual specificity for PtdIns(3,4,5)P(3) and PtdIns(4,5)P(2). The structural basis for this dramatic specificity switch is not apparent from the known modes of phosphoinositide recognition. Here, we report crystal structures for dual specificity variants of the Grp1 and ARNO PH domains in either the unliganded form or in complex with the head groups of PtdIns(4,5)P(2) and PtdIns(3,4,5)P(3). Loss of contacts with the beta1/beta2 loop with no significant change in head group orientation accounts for the significant decrease in PtdIns(3,4,5)P(3) affinity observed for the dual specificity variants. Conversely, a small increase rather than decrease in affinity for PtdIns(4,5)P(2) is explained by a novel binding mode, in which the glycine insertion alleviates unfavorable interactions with the beta1/beta2 loop. These observations are supported by a systematic mutational analysis of the determinants of phosphoinositide recognition.

Alternative Splicing↗
PubMed2004

An essential role of the JIP1 scaffold protein for JNK activation in adipose tissue.

The c-Jun NH2-terminal kinase (JNK) is activated during obesity. One consequence of obesity is that JNK phosphorylates the adapter protein insulin receptor substrate 1 (IRS-1) on Ser 307 and inhibits signaling by the insulin receptor. JNK can therefore cause peripheral insulin resistance during obesity and may contribute to the development of type 2 diabetes. Here we report that the JNK-interacting protein 1 (JIP1) scaffold protein, which binds components of the JNK signaling module, is essential for JNK activation in the adipose tissue of obese mice. These data identify JIP1 as a novel molecular target for therapeutic intervention in the development of obesity.

Adaptor Proteins, Signal Transducing↗
PubMed2004

RNAi-based analysis of CAP, Cbl, and CrkII function in the regulation of GLUT4 by insulin.

Stimulation of glucose transport by insulin in cultured adipocytes through translocation of intracellular GLUT4 glucose transporters to the plasma membrane has been suggested to require phosphatidylinositol (PI) 3-kinase-dependent and independent mechanisms. To test the involvement of a PI 3-kinase-independent pathway leading to activation of the TC10 GTPase, the putative intermediates CAP, c-Cbl, Cbl-b, and CrkII were selectively depleted in 3T3-L1 adipocytes using highly efficient small interfering (si) RNAs. Simultaneous depletion of the ubiquitination factors c-Cbl plus Cbl-b in cultured adipocytes had the expected effect of delaying dephosphorylation of EGF receptors upon removal of EGF. However, siRNA-mediated gene silencing of both Cbl isoforms or CAP or CrkII in these cells failed to attenuate insulin-stimulated deoxyglucose transport or Myc-tagged GLUT4-GFP translocation at either sub-maximal or maximal concentrations of insulin. The dose-response relationship for insulin stimulation of deoxyglucose transport in primary adipocytes derived from c-Cbl knock-out mice was also identical to insulin action on adipocytes from wild type mice. These data are consistent with the hypothesis that CAP, Cbl iso-forms, and CrkII are not required components of insulin signaling to GLUT4 transporters.

3T3 Cells↗
PubMed2004

Role of EHD1 and EHBP1 in perinuclear sorting and insulin-regulated GLUT4 recycling in 3T3-L1 adipocytes.

Insulin stimulates glucose transport in muscle and adipose tissues by recruiting intracellular membrane vesicles containing the glucose transporter GLUT4 to the plasma membrane. The mechanisms involved in the biogenesis of these vesicles and their translocation to the cell surface are poorly understood. Here, we report that an Eps15 homology (EH) domain-containing protein, EHD1, controls the normal perinuclear localization of GLUT4-containing membranes and is required for insulin-stimulated recycling of these membranes in cultured adipocytes. EHD1 is a member of a family of four closely related proteins (EHD1, EHD2, EHD3, and EHD4), which also contain a P-loop near the N terminus and a central coiled-coil domain. Analysis of cultured adipocytes stained with anti-GLUT4, anti-EHD1, and anti-EHD2 antibodies revealed that EHD1, but not EHD2, partially co-localizes with perinuclear GLUT4. Expression of a dominant-negative construct of EHD1 missing the EH domain (DeltaEH-EHD1) markedly enlarged endosomes, dispersed perinuclear GLUT4-containing membranes throughout the cytoplasm, and inhibited GLUT4 translocation to the plasma membranes of 3T3-L1 adipocytes stimulated with insulin. Similarly, small interfering RNA-mediated depletion of endogenous EHD1 protein also markedly dispersed perinuclear GLUT4 in cultured adipocytes. Moreover, EHD1 is shown to interact through its EH domain with the protein EHBP1, which is also required for insulin-stimulated GLUT4 movements and hexose transport. In contrast, disruption of EHD2 function was without effect on GLUT4 localization or translocation to the plasma membrane. Taken together, these results show that EHD1 and EHBP1, but not EHD2, are required for perinuclear localization of GLUT4 and reveal that loss of EHBP1 disrupts insulin-regulated GLUT4 recycling in cultured adipocytes.

3T3-L1 Cells↗
PubMed2004

Unconventional myosin Myo1c promotes membrane fusion in a regulated exocytic pathway.

Glucose homeostasis is controlled in part by regulation of glucose uptake into muscle and adipose tissue. Intracellular membrane vesicles containing the GLUT4 glucose transporter move towards the cell cortex in response to insulin and then fuse with the plasma membrane. Here we show that the fusion step is retarded by the inhibition of phosphatidylinositol (PI) 3-kinase. Treatment of insulin-stimulated 3T3-L1 adipocytes with the PI 3-kinase inhibitor LY294002 causes the accumulation of GLUT4-containing vesicles just beneath the cell surface. This accumulation of GLUT4-containing vesicles near the plasma membrane prior to fusion requires an intact cytoskeletal network and the unconventional myosin motor Myo1c. Remarkably, enhanced Myo1c expression under these conditions causes extensive membrane ruffling and overrides the block in membrane fusion caused by LY294002, restoring the display of GLUT4 on the cell exterior. Ultrafast microscopic analysis revealed that insulin treatment leads to the mobilization of GLUT4-containing vesicles to these regions of Myo1c-induced membrane ruffles. Thus, localized membrane remodeling driven by the Myo1c motor appears to facilitate the fusion of exocytic GLUT4-containing vesicles with the adipocyte plasma membrane.

3T3-L1 Cells↗
PubMed2004

Phosphatidylinositol-4,5-bisphosphate-rich plasma membrane patches organize active zones of endocytosis and ruffling in cultured adipocytes.

A major regulator of endocytosis and cortical F-actin is thought to be phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] present in plasma membranes. Here we report that in 3T3-L1 adipocytes, clathrin-coated membrane retrieval and dense concentrations of polymerized actin occur in restricted zones of high endocytic activity. Ultrafast-acquisition and superresolution deconvolution microscopy of cultured adipocytes expressing an enhanced green fluorescent protein- or enhanced cyan fluorescent protein (ECFP)-tagged phospholipase Cdelta1 (PLCdelta1) pleckstrin homology (PH) domain reveals that these zones spatially coincide with large-scale PtdIns(4,5)P2-rich plasma membrane patches (PRMPs). PRMPs exhibit lateral dimensions exceeding several micrometers, are relatively stationary, and display extensive local membrane folding that concentrates PtdIns(4,5)P2 in three-dimensional space. In addition, a higher concentration of PtdIns(4,5)P2 in the membranes of PRMPs than in other regions of the plasma membrane can be detected by quantitative fluorescence microscopy. Vesicular structures containing both clathrin heavy chains and PtdIns(4,5)P2 are revealed immediately beneath PRMPs, as is dense F actin. Blockade of PtdIns(4,5)P2 function in PRMPs by high expression of the ECFP-tagged PLCdelta1 PH domain inhibits transferrin endocytosis and reduces the abundance of cortical F-actin. Membrane ruffles induced by the expression of unconventional myosin 1c were also found to localize at PRMPs. These results are consistent with the hypothesis that PRMPs organize active PtdIns(4,5)P2 signaling zones in the adipocyte plasma membrane that in turn control regulators of endocytosis, actin dynamics, and membrane ruffling.

3T3 Cells↗
PubMed2004

Mitochondrial remodeling in adipose tissue associated with obesity and treatment with rosiglitazone.

Adipose tissue plays a central role in the control of energy homeostasis through the storage and turnover of triglycerides and through the secretion of factors that affect satiety and fuel utilization. Agents that enhance insulin sensitivity, such as rosiglitazone, appear to exert their therapeutic effect through adipose tissue, but the precise mechanisms of their actions are unclear. Rosiglitazone changes the morphological features and protein profiles of mitochondria in 3T3-L1 adipocytes. To examine the relevance of these effects in vivo, we studied white adipocytes from ob/ob mice during the development of obesity and after treatment with rosiglitazone. The levels of approximately 50% of gene transcripts encoding mitochondrial proteins were decreased with the onset of obesity. About half of those genes were upregulated after treatment with rosiglitazone, and this was accompanied by an increase in mitochondrial mass and changes in mitochondrial structure. Functionally, adipocytes from rosiglitazone-treated mice displayed markedly enhanced oxygen consumption and significantly increased palmitate oxidation. These data reveal mitochondrial remodeling and increased energy expenditure in white fat in response to rosiglitazone treatment in vivo and suggest that enhanced lipid utilization in this tissue may affect whole-body energy homeostasis and insulin sensitivity.

3T3-L1 Cells↗