Olf-1/early B cell factor is a regulator of glut4 gene expression in 3T3-L1 adipocytes

Nipping Adipocyte Inflammation in the Bud

Adipose tissue inflammation continues to represent a significant area of research in immunometabolism. We have identified a transcription factor, EBF1, which crucially regulates the expression of numerous inflammatory loci in adipocytes. However, EBF1 appears to do so without physically binding to these inflammatory genes. Our research is currently focused on understanding this discrepancy, and we believe that future findings could pave the road for drug development aimed to block adipose inflammation at its source.

Early B-cell factor-1 (EBF1) is a key regulator of metabolic and inflammatory signaling pathways in mature adipocytes

EBF1 plays a crucial role in early adipogenesis; however, despite high expression in mature adipocytes, its function in these cells is currently unknown. To identify direct and indirect EBF1 targets in fat, we undertook a combination of transcriptional profiling of EBF1-deficient adipocytes and genome-wide EBF1 location analysis. Our results indicate that many components of metabolic and inflammatory pathways are positively and directly regulated by EBF1, including PI3K/AKT, MAPK, and STAT1 signaling. Accordingly, we observed significant reduction of multiple signaling events in EBF1 knockdown cells as well as a reduction in insulin-stimulated glucose uptake and lipogenesis. Inflammatory signaling, gene expression, and secretion of inflammatory cytokines were also significantly affected by loss of EBF1 in adipocytes, although ChIP-sequencing results suggest that these actions are indirect. We also found that EBF1 occupies some 35,000 sites in adipocytes, most of which occur in enhancers. Significantly, comparison with three other published EBF1 ChIP-sequencing data sets in B-cells reveals both gene- and cell type-specific patterns of EBF1 binding. These results advance our understanding of the transcriptional mechanisms regulating signaling pathways in mature fat cells and indicate that EBF1 functions as a key integrator of signal transduction, inflammation, and metabolism.

EBF proteins participate in transcriptional regulation of Xenopus muscle development

EBF proteins have diverse functions in the development of multiple lineages, including neurons, B cells and adipocytes. During Drosophila muscle development EBF proteins are expressed in muscle progenitors and are required for muscle cell differentiation, but there is no known function of EBF proteins in vertebrate muscle development. In this study, we examine the expression of ebf genes in Xenopus muscle tissue and show that EBF activity is necessary for aspects of Xenopus skeletal muscle development, including somite organization, migration of hypaxial muscle anlagen toward the ventral abdomen, and development of jaw muscle. From a microarray screen, we have identified multiple candidate targets of EBF activity with known roles in muscle development. The candidate targets we have verified are MYOD, MYF5, M-Cadherin and SEB-4. In vivo overexpression of the ebf2 and ebf3 genes leads to ectopic expression of these candidate targets, and knockdown of EBF activity causes downregulation of the endogenous expression of the candidate targets. Furthermore, we found that MYOD and MYF5 are likely to be direct targets. Finally we show that MYOD can upregulate the expression of ebf genes, indicating the presence of a positive feedback loop between EBF and MYOD that we find to be important for maintenance of MYOD expression in Xenopus. These results suggest that EBF activity is important for both stabilizing commitment and driving aspects of differentiation in Xenopus muscle cells.

Transcriptional targets in adipocyte biology

The global burden of metabolic disease demands that we develop new therapeutic strategies. Many of these approaches may center on manipulating the behavior of adipocytes, which contribute directly and indirectly to a host of disease processes including obesity and type 2 diabetes. One way to achieve this goal will be to alter key transcriptional pathways in fat cells, such as those regulating glucose uptake, lipid handling, or adipokine secretion. In this review, we look at what is known about how adipocytes govern their physiology at the gene expression level, and discuss novel ways that we can accelerate our understanding of this area.

Ebf1-dependent control of the osteoblast and adipocyte lineages

Ebf1 is a transcription factor essential for B cell fate specification and function and important for the development of olfactory sensory neurons. We show here that Ebf1 also plays an important role in regulating osteoblast and adipocyte development in vivo. Ebf1 mRNA and protein is expressed in MSCs, in OBs at most stages of differentiation, and in adipocytes. Tibiae and femora from Ebf1(-/-) mice had a striking increase in all bone formation parameters examined including the number of OBs, osteoid volume, and bone formation rate. Serum osteocalcin, a marker of bone formation, was significantly elevated in mutant mice. The numbers of osteoclasts in bone were normal in younger (4 week-old) Ebf1(-/-) mice but increased in older (12 week-old) Ebf1(-/-) mice. This correlated well with in vitro osteoclast development from bone marrow cells. In addition to the increased osteoblastogenesis, there was a dramatic increase in adipocyte numbers in the bone marrow of Ebf1(-/-) mice. Increased adiposity was also seen histologically in the liver but not in the spleen of these mice, and accompanied by decreased deposition of adipose to subcutaneous sites. Thus Ebf1-deficient mice appear to be a new model of lipodystrophy. Ebf1 is a rare example of a transcription factor that regulates both the osteoblast and adipocyte lineages similarly.

Early B cell factor: Regulator of B lineage specification and commitment

B lymphocytes are generated from hematopoietic stem cells in a series of steps controlled by transcription factors. One of the most important regulators of this process is early B cell factor (EBF). Multiple lines of evidence indicate that expression of EBF is a principle determinant of the B cell fate. In the absence of EBF, progenitor cells fail to express classical markers of B cells, including immunoglobulins. EBF drives B cell differentiation by activating the Pax5 gene and other genes required for the pre-B and B cell receptors. New evidence suggests that expression of EBF in common lymphoid progenitors directs B cell fate decisions. Specification and commitment of cells to the B cell lineage are further established by Pax5, which increases expression of EBF. Recently, it was demonstrated that both EBF and Pax5 contribute to the commitment of cells to the B lineage. Together, these studies confirm that EBF is a keystone in the regulatory network that coordinates B cell lineage specification and commitment.

Age related obesity-induced shortening of GLUT4 mRNA poly(A) tail length in rat gastrocnemius skeletal muscle

Obese insulin resistant animals and humans have shown reduced GLUT4 gene expression. Yet, in skeletal muscle, discrepancy between mRNA and protein regulation has been frequently observed, suggesting a post-transcriptional modulation. We investigated the GLUT4 expression in adipose tissue and muscle of obese 12-month-old (12-mo) rats, comparing with lean 2-month-old (2-mo) animals. Obesity was accompanied by insulin resistance, and 65% reduction (P<0.01) in GLUT4 mRNA and protein in adipose tissue. However, in muscle, despite increased (P<0.05) mRNA content, GLUT4 protein was unchanged. RNase H and poly(A) test assays showed a reduction (P<0.01) of approximately 80 adenines in the GLUT4 mRNA poly(A) tail of muscle from 12-mo rats, recognizing that the poly(A) tail length correlates with translation efficiency. Concluding, age related obesity of 12-mo rats involves suppression of GLUT4 expression in adipose tissue; however, in muscle, GLUT4 mRNA content increases, but with a shorter poly(A) tail, thus unchanging the protein content.

Immunologic regulation of bone development

A regulatory network comprised of transcription factors PU.1, Ikaros, E2A, EBF, and Pax5 control B cell fate specification and differentiation. Early B Cell Factor-1 (EBF-1) is essential for B cell fate specification while Pax5 is required for B cell development. Mice deficient in Pax5 or EBF-1 have a developmental arrest of B cell differentiation at the pro-B cell stage, which results in the absence of mature B cells. We analyzed the bone phenotype of Pax5 and EBF-1 wild-type (+/+) and homozygous mutant (-/-) mice to determine if the loss of these transcription factors regulated bone cell development. Bones from Pax5-/- mice were strikingly osteopenic 15 days after birth, with increased numbers of osteoclasts, and decreased trabecular number. The number of osteoblasts in Pax5-/- bones and their function in vitro were not different from controls. In addition, Pax5 was not expressed by wild-type osteoblasts. To investigate the origin of the in vivo increase in osteoclasts, Pax5-/- or +/+ spleen cells were cultured with M-CSF and RANKL and multinucleated, TRAP' cells counted. Cells from Pax5-/- spleen produced 5-10 times more osteoclasts than did controls. Tibia from EBF-1-/- mice had a striking increase in osteoblasts lining bone surfaces. Consistent with this was an increase in osteoid thickness and in the bone formation rate. This correlated with a 2-fold increase in serum osteocalcin. However, in vitro proliferation and ALP of mutant osteoblasts did not differ from control. In contrast, osteoclast number was similar in 4 week-old +/+ and -/- mice; however, at 12 weeks the number of osteoclasts was more than twice that of controls These data correlated with an increase in bone volume at 12 weeks of age. The most striking aspect of the EBF-1-/- bones was the presence of adipocytes, which filled the marrow space. The adipocytes in the marrow were present at both 4 and 12 weeks of age. Increased fat was also seen in the liver of mutant mice. However, subcutaneous fat was almost absent in EBF-1-/- mice. Importantly, EBF-1 mRNA was expressed in wild-type osteoblasts and in adipocytes. Loss of EBF-1 and Pax5 causes distinct, non-overlapping bone phenotypes. It is important to understand why this network of transcription factors, which are so important for B cell development, have such striking effects on bone cell growth and development.

Differential gene expression profile in omental adipose tissue in women with polycystic ovary syndrome

CONTEXT

The polycystic ovary syndrome (PCOS) is frequently associated with visceral obesity, suggesting that omental adipose tissue might play an important role in the pathogenesis of the syndrome.

OBJECTIVE

The objective was to study the expression profiles of omental fat biopsy samples obtained from morbidly obese women with or without PCOS at the time of bariatric surgery.

DESIGN

This was a case-control study.

SETTINGS

We conducted the study in an academic hospital.

PATIENTS

Eight PCOS patients and seven nonhyperandrogenic women submitted to bariatric surgery because of morbid obesity.

INTERVENTIONS

Biopsy samples of omental fat were obtained during bariatric surgery.

MAIN OUTCOME MEASURE

The main outcome measure was high-density oligonucleotide arrays.

RESULTS

After statistical analysis, we identified changes in the expression patterns of 63 genes between PCOS and control samples. Gene classification was assessed through data mining of Gene Ontology annotations and cluster analysis of dysregulated genes between both groups. These methods highlighted abnormal expression of genes encoding certain components of several biological pathways related to insulin signaling and Wnt signaling, oxidative stress, inflammation, immune function, and lipid metabolism, as well as other genes previously related to PCOS or to the metabolic syndrome.

CONCLUSION

The differences in the gene expression profiles in visceral adipose tissue of PCOS patients compared with nonhyperandrogenic women involve multiple genes related to several biological pathways, suggesting that the involvement of abdominal obesity in the pathogenesis of PCOS is more ample than previously thought and is not restricted to the induction of insulin resistance.

B cells and osteoblast and osteoclast development

The molecules that regulate bone cell development, particularly at the early stages of development, are only partially known. Data are accumulating that indicate a complex relationship exists between B cells and bone cell differentiation. Although the exact nature of this relationship is still evolving, it takes at least two forms. First, factors that regulate B-cell growth and development have striking effects on osteoclast and osteoblast lineage cells. Similarly, factors that regulate bone cell development influence B-cell maturation. Second, a series of transcription factors required for B-cell differentiation have been identified, and these factors function in a developmentally ordered circuit. These transcription factors have unpredicted, pronounced, and non-overlapping effects on osteoblast and/or osteoclast development. These data indicate that at least a regulatory relationship exists between B lymphopoiesis, osteoclastogenesis, and osteoblastogenesis.

Exercise increases MEF2- and GEF DNA-binding activity in human skeletal muscle

Overexpression of GLUT4 exclusively in skeletal muscle enhances insulin action and improves glucose homeostasis. Transgenic studies have discovered two regions on the GLUT4 promoter conserved across several species that are required for normal GLUT4 expression in skeletal muscle. These regions contain binding motifs for the myocyte enhancer factor 2 (MEF2) family and GLUT4 enhancer factor (GEF). A single bout of exercise increases both GLUT4 transcription and mRNA abundance; however, the molecular mechanisms mediating this response remain largely unexplored. Thus, the aim of this study was to determine whether a single, acute bout of exercise increased the DNA-binding activities of MEF2 and GEF in human skeletal muscle. Seven subjects performed 60 min of cycling at approximately 70% of VO2peak. After exercise, the DNA-binding activities of both the MEF2A/D heterodimer and GEF were increased (P<0.05). There was no change in nuclear MEF2D or GEF abundance after exercise, but nuclear MEF2A abundance was increased (P<0.05). These data demonstrate that exercise increases MEF2 and GEF DNA binding and imply that these transcription factors could be potential targets for modulating GLUT4 expression in human skeletal muscle.

Gene expression analysis suggests that EBF-1 and PPARγ2 induce adipogenesis of NIH-3T3 cells with similar efficiency and kinetics

Differentiation of multipotent mesenchymal stem cells into lipid-accumulating adipocytes is a physiological process induced by transcription factors in combination with hormonal stimulation. We have used Affymetrix microarrays to compare the adipogenic differentiation pathways of NIH-3T3 fibroblasts induced to undergo in vitro differentiation by ectopic expression of early B cell factor (EBF)-1 or peroxisome proliferator-activated receptor (PPAR)γ2. These experiments revealed that commitment to the adipogenic pathway in the NIH-3T3 cells was not reflected in gene expression until 4 days after induction of differentiation. Furthermore, gene expression patterns at the earlier time points after stimulation indicated that EBF-1 and PPARγ2 induced different sets of genes, while the similarities increased upon differentiation, and that several genes linked to adipocyte differentiation were also transiently induced in the vector-transduced cells. These data suggest that the initial activation of genes associated with adipocyte development is independent of commitment to the adipogenic pathway and that EBF-1 and PPARγ2 induce adipocyte differentiation with comparable kinetics and efficiency.

Mechanisms regulating GLUT4 glucose transporter expression and glucose transport in skeletal muscle

Skeletal muscle is a major glucose-utilizing tissue in the absorptive state and the major glucose transporter expressed in muscle in adulthood is GLUT4. GLUT4 expression is exquisitely regulated in muscle and this seems important in the regulation of insulin-stimulated glucose uptake by this tissues. Thus, muscle GLUT4 overexpression in transgenic animals ameliorates insulin resistance associated with obesity or diabetes. Recent information indicates that glut4 gene transcription is regulated by a number of factors in skeletal muscle that include MEF2, MyoD myogenic proteins, thyroid hormone receptors, Kruppel-like factor KLF15, NF1, Olf-1/Early B cell factor and GEF/HDBP1. In addition, studies in vivo indicate that under normal conditions the activity of the muscle-specific GLUT4 enhancer is low in adult skeletal muscle compared with the maximal potential activity that it can attain at high levels of the MRF transcription factors, MEF2, and TRalpha1. This finding indicates that glut4 transcription may be greatly up-regulated via activation of this enhancer through an increase in the levels of expression or activity of these transcription factors. Understanding the molecular basis of the expression of glut4 will be useful for the appropriate therapeutic design of treatments for insulin-resistant states. The nature of the intracellular signals that mediate the stimulation of glucose transport in response to insulin or exercise is also reviewed.

Reduction in Gsalpha induces osteogenic differentiation in human mesenchymal stem cells

We hypothesized that a decrease in Gsalpha expression occurs with osteogenic differentiation and that when Gsalpha expression was decreased by antisense oligonucleotides or direct inhibition of protein kinase A there was a concomitant increase in Runx2/Cbfa1. We also investigated the mechanism involved in the change in Runx2/Cbfa1 levels and whether the expression of other genes known to be involved in bone formation was altered. There was a decrease in Gsalpha expression with osteogenic differentiation and antisense oligonucleotides, and protein kinase A inhibition led to increased expression and DNA binding of the osteoblast-specific Runx2/Cbfa1. Additionally, with decreased Gsalpha expression or protein kinase A inhibition, Runx2/Cbfa1 protein was serine phosphorylated and ubiquitinated less. Microarray analysis, after the addition of antisense Gsalpha, showed a more than 10-fold increase in collagen Type I Alpha 2 mRNA (a target of Runx2/Cbfa1). These data show that reduced expression of Gsalpha can induce an osteoblast-like phenotype. The results also indicate a potential pathophysiologic role in patients with heterozygous inactivating mutations in GNAS1, the gene for the alpha chain (Gsalpha) of the heterotrimeric G protein, present in three disorders with ectopic intramembranous bone: Albright's hereditary osteodystrophy, progressive osseous heteroplasia, and osteoma cutis.

GLUT4 expression in 3T3-L1 adipocytes is repressed by proteasome inhibition, but not by inhibition of calpains

Because of recent studies showing linkage of type 2 diabetes with the calpain 10 gene, we investigated the ability of calpains to regulate GLUT4 expression in 3T3-L1 adipocytes. Treatment of 3T3-L1 adipocytes with the calpain inhibitor ALLN significantly decreased the mRNA and protein expression of GLUT4. GLUT4 expression was not affected by treatment with the more selective calpain inhibitors PD150606, calpeptin, or a calpastatin peptide. In contrast, treatment with the proteasome inhibitors lactacystin or MG132 repressed GLUT4 mRNA level to 35% (10 microM lactacystin) and 12% (10 microM MG132) of control levels. Therefore, the expression of GLUT4 in 3T3-L1 adipocytes was repressed by proteasome inhibition, but not by inhibition of calpains; the effect of ALLN was due to its ability to inhibit proteasome function, rather than its action to inhibit calpains. Concomitant with the repression of GLUT4 mRNA levels, proteasome inhibition decreased GLUT4 protein levels in 3T3-L1 adipocytes. The decrease in GLUT4 expression occurred at the transcriptional level, as treatment with proteasome inhibitors decreased GLUT4 transcription measured by a nuclear run-on assay. Thus, these data demonstrate a new pathway for the regulation of GLUT4 expression that involves proteasomal degradation of factors that regulate GLUT4 expression.

Nuclear factor 1 regulates adipose tissue-specific expression in the mouse GLUT4 gene

Previous studies demonstrated that an adipose tissue-specific element(s) (ASE) of the murine GLUT4 gene is located between -551 and -506 in the 5'-flanking sequence and that a high-fat responsive element(s) for down-regulation of the GLUT4 gene is located between bases -701 and -552. A binding site for nuclear factor 1 (NF1), that mediates insulin and cAMP-induced repression of GLUT4 in 3T3-L1 adipocytes is located between bases -700 and -688. To examine the role of NF1 in the regulation of GLUT4 gene expression in white adipose tissues (WAT) in vivo, we created two types of transgenic mice harboring mutated either 5' or 3' half-site of NF1-binding sites in GLUT4 minigene constructs. In both cases, the GLUT4 minigene was not expressed in WAT, while expression was maintained in brown adipose tissue, skeletal muscle, and heart. This was an unexpected finding, since a -551 GLUT4 minigene that did not have the NF1-binding site was expressed in WAT. We propose a model that explains the requirement for both the ASE and the NF1-binding site for expression of GLUT4 in WAT.

Evolutionary origins of lymphocytes: ensembles of T cell and B cell transcriptional regulators in a cartilaginous fish

The evolutionary origins of lymphocytes can be traced by phylogenetic comparisons of key features. Homologs of rearranging TCR and Ig (B cell receptor) genes are present in jawed vertebrates, but have not been identified in other animal groups. In contrast, most of the transcription factors that are essential for the development of mammalian T and B lymphocytes belong to multigene families that are represented by members in the majority of the metazoans, providing a potential bridge to prevertebrate ancestral roles. This work investigates the structure and regulation of homologs of specific transcription factors known to regulate mammalian T and B cell development in a representative of the earliest diverging jawed vertebrates, the clearnose skate (Raja eglanteria). Skate orthologs of mammalian GATA-3, GATA-1, EBF-1, Pax-5, Pax-6, Runx2, and Runx3 have been characterized. GATA-3, Pax-5, Runx3, EBF-1, Spi-C, and most members of the Ikaros family are shown throughout ontogeny to be 1) coregulated with TCR or Ig expression, and 2) coexpressed with each other in combinations that for the most part correspond to known mouse T and B cell patterns, supporting conservation of function. These results indicate that multiple components of the gene regulatory networks that operate in mammalian T cell and B cell development were present in the common ancestor of the mammals and the cartilaginous fish. However, certain factors relevant to the B lineage differ in their tissue-specific expression patterns from their mouse counterparts, suggesting expanded or divergent B lineage characteristics or tissue specificity in these animals.

Olf/EBF proteins are expressed in neuroblastoma cells: Potential regulators of theChromogranin A andSCG10 promoters

The childhood malignancy neuroblastoma is derived from developmentally arrested sympathetic nervous system precursor cells. To obtain further insight into the molecular processes involved in the formation of these tumors, we decided to investigate the functional role of Olf/EBF (O/E) transcription factors in human neuroblastoma cells. We here report that O/E-1 and O/E-2 are expressed at variable levels in neuroblastoma cell lines and that O/E proteins could be identified by electrophoretic mobility shift assays. To identify potential neuronal target genes for O/E proteins in neuroblastoma cells we investigated the ability of a set of neuronal promoters to interact with O/E-1 in electrophoretic mobility shift assays. This analysis suggested that the Chromogranin A (CgA) and SCG10 promoters both contained binding sites for O/E-1. O/E-1 was able to activate the CgA promoter in vivo and mutation of the O/E-1 binding site in the CgA promoter reduced the functional activity of the element to about 60% of the wild-type in neuroblastoma cells, supporting the idea that O/E proteins may be involved in the control of the CgA promoter. Furthermore, overexpression of O/E-1 in hippocampal progenitor cells led to neurite outgrowth, indicative of a role for O/E proteins in neuronal differentiation.

Regulation of the human GLUT4 gene promoter: interaction between a transcriptional activator and myocyte enhancer factor 2A

The GLUT4 gene is subject to complex tissue-specific and metabolic regulation, with a profound impact on insulin-mediated glucose disposal. We have shown, by using transgenic mice, that the human GLUT4 promoter is regulated through the cooperative function of two distinct regulatory elements, domain 1 and the myocyte enhancer factor 2 (MEF2) domain. The MEF2 domain binds transcription factors MEF2A and MEF2D in vivo. Domain I binds a transcription factor, GLUT4 enhancer factor (GEF). In this report, we show a restricted pattern of GEF expression in human tissues, which overlaps with MEF2A only in tissues expressing high levels of GLUT4, suggesting the hypothesis that GEF and MEF2A function together to activate GLUT4 transcription. Data obtained from transiently transfected cells support this hypothesis. Neither GEF nor MEF2A alone significantly activated GLUT4 promoter activity, but increased promoter activity 4- to 5-fold when expressed together. Deletion of the GEF-binding domain (domain I) and the MEF2-binding domain prevented activation, strengthening the conclusion that promoter regulation occurs through these elements. GEF and MEF2A, isolated from nuclei of transfected cells, bound domain I and the MEF2 domain, respectively, which is consistent with activation through these regulatory elements. Finally, GEF and MEF2A coimmunoprecipitated in vivo, strongly supporting a mechanism of GLUT4 transcription activation that depends on this protein-protein interaction.

Regulatory sequence elements of mouse GLUT4 gene expression in adipose tissues

Ablation of GLUT4 in adipose tissues results in whole body insulin resistance and high-fat feeding down-regulates GLUT4 mRNA in white adipose tissues. Previous studies demonstrated that adipose tissue specific element(s) (ASE) of the murine GLUT4 gene is located between -551 and -442 relative to transcription start site and that high-fat responsive element(s) (HFRE) for down-regulation of the GLUT4 gene is located between bases -1001 and -442. To further characterize these regulatory elements, the regulation of GLUT4 minigenes containing -701, -551, and -506 bp of 5(')-flanking region was studied in transgenic mice. GLUT4 minigene mRNA from -506 transgenic mice did not express in adipose tissues, indicating that ASE located within 45 bp is located between bases -551 and -506. An 80-kDa of nuclear DNA binding protein was found to bind to a -TCCTCGTGGGAAGCG- element located between bases -551 and -537. High-fat diet feeding down-regulated GLUT4 minigene mRNA in -701 transgenic mice, but not in -551 transgenic mice, indicating that HFRE is located within 150 bp between bases -701 and -551 of the GLUT4 gene and is distinct from ASE.

Convergence of peroxisome proliferator-activated receptor gamma and Foxo1 signaling pathways

The forkhead factor Foxo1 (or FKHR) was identified in a yeast two-hybrid screen as a peroxisome proliferator-activated receptor (PPAR) gamma-interacting protein. Foxo1 antagonized PPARgamma activity and vice versa indicating that these transcription factors functionally interact in a reciprocal antagonistic manner. One mechanism by which Foxo1 antagonizes PPARgamma activity is through disruption of DNA binding as Foxo1 inhibited the DNA binding activity of a PPARgamma/retinoid X receptor alpha heterodimeric complex. The Caenorhabditis elegans nuclear hormone receptor, DAF-12, interacted with the C. elegans forkhead factor, DAF-16, paralleling the interaction between PPARgamma and Foxo1. daf-12 and daf-16 have been implicated in C. elegans insulin-like signaling pathways, and PPARgamma and Foxo1 likewise have been linked to mammalian insulin signaling pathways. These results suggest a convergence of PPARgamma and Foxo1 signaling that may play a role in insulin action and the insulinomimetic properties of PPARgamma ligands. A more general convergence of nuclear hormone receptor and forkhead factor pathways may be important for multiple biological processes and this convergence may be evolutionarily conserved.

Enhanced basal activation of mitogen-activated protein kinases in adipocytes from type 2 diabetes: potential role of p38 in the downregulation of GLUT4 expression

Serine and threonine kinases may contribute to insulin resistance and the development of type 2 diabetes. To test the potential for members of the mitogen-activated protein (MAP) kinase family to contribute to type 2 diabetes, we examined basal and insulin-stimulated Erk 1/2, JNK, and p38 phosphorylation in adipocytes isolated from healthy and type 2 diabetic individuals. Maximal insulin stimulation increased the phosphorylation of Erk 1/2 and JNK in healthy control subjects but not type 2 diabetic patients. Insulin stimulation did not increase p38 phosphorylation in either healthy control subjects or type 2 diabetic patients. In type 2 diabetic adipocytes, the basal phosphorylation status of these MAP kinases was significantly elevated and was associated with decreased IRS-1 and GLUT4 in these fat cells. To determine whether MAP kinases were involved in the downregulation of IRS-1 and GLUT4 protein levels, selective inhibitors were used to inhibit these MAP kinases in 3T3-L1 adipocytes treated chronically with insulin. Inhibition of Erk 1/2, JNK, or p38 had no effect on insulin-stimulated reduction of IRS-1 protein levels. However, inhibition of the p38 pathway prevented the insulin-stimulated decrease in GLUT4 protein levels. In summary, type 2 diabetes is associated with an increased basal activation of the MAP kinase family. Furthermore, upregulation of the p38 pathway might contribute to the loss of GLUT4 expression observed in adipose tissue from type 2 diabetic patients.

Roles of EBF and Pax-5 in B lineage commitment and development

B lymphocyte development is regulated by the nuclear proteins Early B cell factor (EBF) and Pax-5. EBF and Pax-5 work separately and in concert to activate genes required for B cell differentiation. Recent studies have defined mechanisms by which these two factors control transcription, including chromatin remodeling activities and recruitment of partner proteins. This review addresses the structures, functions, and roles of these proteins in early B cell commitment and development, as well as in later stages of B cell differentiation.

Early B-cell factor (O/E-1) is a promoter of adipogenesis and involved in control of genes important for terminal adipocyte differentiation

Olf-1/early B-cell factor (O/E-1) is a transcription factor important for B-lymphocyte and neuronal gene regulation. Here we report that all three known O/E genes (O/E-1, -2, and -3) are expressed in mouse adipose tissue and are upregulated during adipocyte differentiation. Forced expression of O/E-1 in either the preadipocyte cell line 3T3-L1 or mouse embryonic fibroblasts augmented adipogenesis, and constitutive expression of O/E-1 in uncommitted NIH 3T3 fibroblasts led to initiation of adipocyte differentiation. Furthermore, a dominant negative form of O/E-1 partially suppressed 3T3-L1 adipogenesis, indicating that expression from endogenous O/E target genes is required for 3T3-L1 terminal differentiation. Thus, our data point to the importance of O/E target genes for adipocyte differentiation and suggest a novel role for O/E-1 as an initiator and stimulator of adipogenesis.