Role of peroxisome proliferator-activated receptor-gamma in maintenance of the characteristics of mature 3T3-L1 adipocytes

Peroxisome proliferator-activated receptor (PPAR)-gamma plays an important role in adipogenesis. However, the functions of PPAR-gamma in differentiated adipocytes have remained unclear. The role of PPAR-gamma in mature 3T3-L1 adipocytes was therefore investigated by overexpression of a dominant negative mutant of this protein (PPAR-gamma-DeltaC) that lacks the 16 COOH-terminal amino acids and that has been shown to prevent the thiazolidinedione-induced differentiation of 3T3-L1 cells into adipocytes. Overexpression of PPAR-gamma-DeltaC in mature 3T3-L1 adipocytes by adenovirus gene transfer resulted in a decrease in both cell size and intracellular triglyceride content, an increase in the extent of lipolysis, and a reduction in the rate of free fatty acid uptake. Furthermore, overexpression of this mutant reduced the abundance of mRNAs for several key enzymes that contribute to triglyceride and free fatty acid metabolism as well as the amounts of GLUT4, insulin receptor, insulin receptor substrate (IRS), and C/EBPalpha mRNAs. It also reduced both the concentration of IRS2 and the insulin-stimulated glucose uptake. These results suggest that PPAR-gamma plays an important role in mature 3T3-L1 adipocytes at least in part by maintaining the expression of genes that confer the characteristics of mature adipocytes.

Induction of granulocytic differentiation by 2 pathways

The CCAAT enhancer binding protein alpha (C/EBP alpha) transcription factor plays a critical role in granulocytopoiesis. Mice with a disruption of the C/EBP alpha gene demonstrate an early block in granulocytic differentiation, and disruption of C/EBP alpha function is a common theme in many types of human acute myelogenous leukemia, which is characterized by a block in myeloid development. To characterize further the nature of this block, we derived cell lines from the fetal liver of C/EBP alpha-deficient animals. These lines resembled morphologically the immature myeloid blasts observed in C/EBP alpha(-/-) fetal livers and did not express messenger RNA encoding early myeloid genes such as myeloperoxidase. Similarly, granulocytic markers such as Mac-1 and Gr-1 were not expressed; nor were erythroid and lymphoid surface antigens. Introduction of an inducible C/EBP alpha gene into the line revealed that conditional expression of C/EBP alpha induced the C/EBP family members C/EBP beta and C/EBP epsilon and subsequent granulocyte differentiation. Similar results were obtained when C/EBP alpha(-/-) cells were stimulated with the cytokines interleukin-3 and granulocyte-macrophage colony-stimulating factor, but not with all-trans retinoic acid, supporting a model of at least 2 pathways leading to the differentiation of myeloid progenitors to granulocytes and implicating induction of other C/EBP family members in granulopoiesis.

Trans-10, cis-12, but not cis-9, trans-11 CLA isomer, inhibits brown adipocyte thermogenic capacity

Conjugated linoleic acid (CLA) is reported to have health benefits, including reduction of body fat. Previous studies have shown that brown adipose tissue (BAT) is particularly sensitive to CLA-supplemented diet feeding. Most of them use mixtures containing several CLA isomers, mainly cis-9, trans-11 and trans-10, cis-12 in equal concentration. Our aim was to characterize the separate effects of both CLA isomers on thermogenic capacity in cultured brown adipocytes. The CLA isomers showed opposite effects. Hence, on the one hand, trans-10, cis-12 inhibited uncoupling protein (UCP) 1 induction by norepinephrine (NE) and produced a decrease in leptin mRNA levels. These effects were associated with a blockage of CCAAT-enhancer-binding protein-alpha and peroxisome proliferator-activated receptor-gamma(2) mRNA expression. On the other hand, cis-9, trans-11 enhanced the UCP1 elicited by NE, an effect reported earlier for polyunsaturated fatty acids and also observed here for linoleic acid. These findings could explain, at least in part, the effects observed in vivo when feeding a CLA mixture supplemented diet as a result of the combined action of CLA isomers (reduction of adipogenesis and defective BAT thermogenesis that could be through trans-10, cis-12 and enhanced UCP1 thermogenic capacity through cis-9, trans-11).

Adipogenesis and aging: does aging make fat go MAD?

In advanced old age, fat depot size declines while lipid is redistributed to muscle, bone marrow, and other tissues. Decreased fat depot size is related to reduced fat cell size and function and impaired differentiation of preadipocytes into fat cells. Reduced differentiation-dependent gene expression results from decreased abundance of the adipogenic transcription factors, CCAAT/enhancer binding alpha (C/EBPalpha) and peroxisome proliferator activated receptor gamma (PPARgamma). Increased expression of anti-adipogenic C/EBP family members contributes, perhaps due to cellular stress response pathway activation with aging. Hence, dysfunctional adipocyte-like cells appear in adipose tissue that are smaller and less insulin responsive than fully differentiated fat cells. Adipogenesis can be restored by overexpressing adipogenic transcription factors in preadipocytes from old animals. Redistribution of lipid to extra-adipose sites with aging could result from loss of lipid storage capacity in fat depots, altered fatty acid handling resulting in lipid accumulation, dysdifferentiation of mesenchymal precursors, such as muscle satellite cells and osteoblast precursors, into a partial adipocyte phenotype, or a combination of these mechanisms. Thus, accumulation of mesenchymal adipocyte-like default (MAD) cells in fat depots, muscle, bone marrow, and elsewhere is a potentially reversible process that could contribute to maldistribution of fat in old age.

Activation of an adipogenic program in adult myoblasts with age

Myoblasts isolated from mouse hindlimb skeletal muscle demonstrated increased adipogenic potential as a function of age. Whereas myoblasts from 8-month-old adult mice did not significantly accumulate terminal markers of adipogenesis regardless of culture conditions, myoblasts from 23-month-old mice accumulated fat and expressed genes characteristic of differentiated adipocytes, such as the fatty acid binding protein aP2. This change in differentiation potential was associated with a change in the abundance of the mRNA encoding the transcription factor C/EBPalpha, and in the relative abundance of PPARgamma2 to PPARgamma1 mRNAs. Furthermore, PPARgamma activity appeared to be regulated at the level of phosphorylation, being more highly phosphorylated in myoblasts isolated from younger animals. Although adipogenic gene expression in myoblasts from aged animals was activated, presumably in response to PPARgamma and C/EBPalpha, unexpectedly, myogenic gene expression was not effectively repressed. The Wnt signaling pathway may also alter differentiation potential in muscle with age. Wnt-10b mRNA was more abundantly expressed in muscle tissue and cultured myoblasts from adult compared with aged mice, resulting in stabilization of cytosolic beta-catenin, that may potentially contribute to inhibition of adipogenic gene expression in adult myoblasts. The changes reported here, together with those reported in bone marrow stroma with age, suggest that a default program may be activated in mesenchymal cells with increasing age resulting in a more adipogenic-like phenotype. Whether this change in differentiation potential contributes to the increased adiposity in muscle with age remains to be determined.

CCAAT/enhancer binding protein alpha uses distinct domains to prolong pituitary cells in the growth 1 and DNA synthesis phases of the cell cycle

BACKGROUND

A number of transcription factors coordinate differentiation by simultaneously regulating gene expression and cell proliferation. CCAAT/enhancer binding protein alpha (C/EBPalpha) is a basic/leucine zipper transcription factor that integrates transcription with proliferation to regulate the differentiation of tissues involved in energy balance. In the pituitary, C/EBPalpha regulates the transcription of a key metabolic regulator, growth hormone.

RESULTS

We examined the consequences of C/EBPalpha expression on proliferation of the transformed, mouse GHFT1-5 pituitary progenitor cell line. In contrast to mature pituitary cells, GHFT1-5 cells do not contain C/EBPalpha. Ectopic expression of C/EBPalpha in the progenitor cells resulted in prolongation of both growth 1 (G1) and the DNA synthesis (S) phases of the cell cycle. Transcription activation domain 1 and 2 of C/EBPalpha were required for prolongation of G1, but not of S. Some transcriptionally inactive derivatives of C/EBPalpha remained competent for G1 and S phase prolongation. C/EBPalpha deleted of its leucine zipper dimerization functions was as effective as full-length C/EBPalpha in prolonging G1 and S.

CONCLUSION

We found that C/EBPalpha utilizes mechanistically distinct activities to prolong the cell cycle in G1 and S in pituitary progenitor cells. G1 and S phase prolongation did not require that C/EBPalpha remained transcriptionally active or retained the ability to dimerize via the leucine zipper. G1, but not S, arrest required a domain overlapping with C/EBPalpha transcription activation functions 1 and 2. Separation of mechanisms governing proliferation and transcription permits C/EBPalpha to regulate gene expression independently of its effects on proliferation.

Expression of C/EBPbeta from the C/ebpalpha gene locus is sufficient for normal hematopoiesis in vivo

CCAAT/enhancer-binding proteins (C/EBPs) are critical transcriptional regulators of differentiation of hematopoietic cells. Previous studies have shown that targeted disruption of the C/ebpalpha gene results in a lack of granulocytic differentiation with an arrest at the stage of immature myeloblasts. By using a gene replacement strategy in which C/EBPbeta was expressed from the C/ebpalpha gene locus of C/EBPalpha-null mice, we have evaluated the ability of C/EBPbeta to function for C/EBPalpha in directing differentiation along the granulocytic pathway. We show that the morphology and the differential cell counts of the bone marrow and peripheral blood cells from C/EBPbeta knockin mice are indistinguishable from those of their wild-type littermates, indicating that hematopoiesis occurs normally in these animals. Additionally, we analyzed expression of 21 myeloid-specific genes, including markers for distinct stages of granulocytic differentiation, and found no significant differences in their levels of expression in the bone marrow of C/EBPbeta knockin and wild-type mice. These results imply that C/EBPbeta can substitute for C/EBPalpha during hematopoiesis when expressed from the C/ebpalpha gene locus.

C/EBPalpha triggers proteasome-dependent degradation of cdk4 during growth arrest

CCAAT/enhancer binding protein alpha (C/EBPalpha) causes growth arrest via direct interaction with the cyclin-dependent kinases cdk2 and cdk4. In this paper, we present evidence showing that C/EBPalpha enhances a proteasome-dependent degradation of cdk4 during growth arrest in liver of newborn mice and in cultured cells. Overexpression of C/EBPalpha in several biological systems leads to a reduction of cdk4 protein levels, but not mRNA levels. Experiments with several tissue culture models reveal that C/EBPalpha enhances the formation of cdk4-ubiquitin conjugates and induces degradation of cdk4 through a proteasome-dependent pathway. As a result, the half-life of cdk4 is shorter and protein levels of cdk4 are reduced in cells expressing C/EBPalpha. Gel filtration analysis of cdk4 complexes shows that a chaperone complex cdk4-cdc37-Hsp90, which protects cdk4 from degradation, is abundant in proliferating livers that lack C/EBPalpha, but this complex is weak or undetectable in livers expressing C/EBPalpha. Our studies show that C/EBPalpha disrupts the cdk4-cdc37-Hsp90 complex via direct interaction with cdk4 and reduces protein levels of cdk4 by increasing proteasome-dependent degradation of cdk4.

Mutations in the gene encoding the transcription factor CCAAT/enhancer binding protein alpha in myelodysplastic syndromes and acute myeloid leukemias

The CCAAT/enhancer binding protein alpha (C/EBPalpha) protein is essential for proper lung and liver function and granulocytic and adipose tissue differentation. It was hypothesized that abnormalties in C/EBPalpha function contribute to the development of malignancies in a variety of tissues. To test this, genomic DNA from 408 patient samples and 5 cell lines representing 11 different cancers was screened for mutations in the C/EBPalpha gene. Two silent polymorphisms termed P1 and P2 were present at frequencies of 13.5% and 2.2%, respectively. Of the 12 mutations detected in 10 patients, silent changes were identified in one nonsmall cell lung cancer, one prostate cancer, and one acute myelogenous leukemia (AML) subtype M4. The 9 remaining mutations were detected in 1 of 92 (1.1%) myelodysplastic syndrome (MDS) samples and 6 of 78 (7.7%) AML (AML-M2 and AML-M4) samples. Some mutations truncated the predicted protein with loss of the DNA-binding (basic region) and dimerization (leucine zipper [ZIP]) domains by either deletions or nonsense codons. Also, inframe deletions or insertions in the fork region located between the leucine zipper and basic region, or within the leucine zipper, disrupted the alpha-helical phase of the bZIP domain. The inframe deletion and insertion mutations abrogated the transcriptional activation function of C/EBPalpha on the granulocyte colony-stimulating factor receptor promoter. These mutants localized properly to the nucleus, but were unable to bind to the C/EBP site in the promoter and did not possess dominant-negative activity. The mutations in the MDS patient and one AML-M2 patient were biallelic, indicating a loss of C/EBPalpha function. These results suggest that mutation of C/EBPalpha is involved in specific subtypes of AML and in MDS, but may occur rarely in other types of leukemias or nonhematologic malignancies.

Interaction of C/EBPalpha and the glucocorticoid receptor in vivo and in nontransformed human cells

Belonging to the family of steroid hormones, glucocorticoids are essential for development and survival of vertebrates. The cellular response to glucocorticoids is attributed to the glucocorticoid receptor, which functions as a transcription factor. However, the majority of glucocorticoid-modulated genes lack a DNA binding site for the glucocorticoid receptor, raising the question of which mechanism mediates the responses to glucocorticoids. It has been suggested that besides direct DNA binding of the glucocorticoid receptor, interaction with members of other transcription factor families modulates the effect of the glucocorticoid receptor. However, the significance of such transcription factor interaction is not clear. In cultured human mesenchymal cells and peripheral blood leukocytes of human volunteers treated with glucocorticoids, we detected the formation of a complex between the GR and the CCAAT/enhancer binding protein alpha. In in vitro experiments, this interaction turned out to be responsible for the inhibitory action of glucocorticoids on lymphocytic and mesenchymal cell proliferation. Our results suggest that complex formation of the GR with C/EBPalpha accounts for a novel pathway of glucocorticoid action.

Changes in UCP2, PPARgamma2, and c/EBPalpha gene expression induced by a neuropeptide Y (NPY) related receptor antagonist in overweight rats

Neuropeptide Y (NPY), a peptide released by nervous cells, appears to contribute to adiposity regulation by increasing food intake and inhibiting lipolysis. New NPY receptor related antagonists such as S.A.0204 are being developed as potential anti-obesity drugs affecting adipocyte lipid metabolism and thermogenesis. In this sense, those animals fed on a high-energy yielding (cafeteria) diet decreased body fat weight as compared to overweight controls, when they were administered with S.A.0204, and increased body temperature, which statistically correlated with high UCP2 mRNA expression levels in white adipose tissue. In addition, the in vivo NPY-antagonist administration was able to prevent white adipose tissue growth in animals fed the cafeteria (high-fat) diet by impairing PPARy and CIEBPalpha mRNA expression in white fat cells. In summary, this novel NPY related-antagonist S.A.0204 may regulate body fat deposition by affecting both energy dissipation and white adipose tissue deposition, representing a potential new pharmacological strategy for obesity management.

Down-regulation and antiproliferative role of C/EBPalpha in lung cancer

The transcription factor, CCAAT/enhancer binding protein alpha (C/EBPalpha) is important in the terminal differentiation of granulocytes, hepatocytes, and adipocytes, and recurrent mutations of C/EBPalpha were described in acute myeloid leukemia. In the lung, C/EBPalpha is expressed in bronchial cells and type II pneumocytes. Abnormal proliferation of the latter cell type was reported in C/EBPalpha knockout mice. We determined the expression of C/EBPalpha by Northern blot analysis in 30 lung cancer cell lines and found significant down-regulation in 24 cell lines. Immunohistochemical study of primary tumor specimens showed undetectable or low expression of C/EBPalpha in 23 of 53 specimens. Its expression was more frequently down-regulated in adenocarcinoma and poorly differentiated cancer specimens than in squamous cell cancers. A higher frequency of reduced expression was found in more advanced stages. To investigate the consequences of C/EBPalpha expression in lung cancer cells, we stably transfected two cell lines that do not express the gene (Calu1 and H358) with a plasmid allowing for induction of C/EBPalpha protein expression. Induction of C/EBPalpha led to significant growth reduction attributable to proliferation arrest, morphological changes characteristic of differentiation, and apoptosis. These results suggest that C/EBPalpha is down-regulated in a large proportion of lung cancers and that it has growth-inhibitory properties in airway epithelial cells. Genetic analysis of the C/EBPalpha gene is in progress to fully evaluate its role as a novel tumor suppressor in lung cancer.

BCR-ABL suppresses C/EBPalpha expression through inhibitory action of hnRNP E2

The arrest of differentiation is a feature of both chronic myelogenous leukemia cells in myeloid blast crisis and myeloid precursors that ectopically express the p210BCR-ABL oncoprotein; however, its underlying mechanisms remain poorly understood. Here we show that expression of BCR-ABL in myeloid precursor cells leads to transcriptional suppression of the granulocyte colony-stimulating factor receptor G-CSF-R (encoded by CSF3R), possibly through down-modulation of C/EBPalpha-the principal regulator of granulocytic differentiation. Expression of C/EBPalpha protein is barely detectable in primary marrow cells taken from individuals affected with chronic myeloid leukemia in blast crisis. In contrast, CEBPA RNA is clearly present. Ectopic expression of C/EBPalpha induces granulocytic differentiation of myeloid precursor cells expressing BCR-ABL. Expression of C/EBPalpha is suppressed at the translational level by interaction of the poly(rC)-binding protein hnRNP E2 with CEBPA mRNA, and ectopic expression of hnRNP E2 in myeloid precursor cells down-regulates both C/EBPalpha and G-CSF-R and leads to rapid cell death on treatment with G-CSF (encoded by CSF3). Our results indicate that BCR-ABL regulates the expression of C/EBPalpha by inducing hnRNP E2-which inhibits the translation of CEBPA mRNA.

C/EBPalpha induces adipogenesis through PPARgamma: a unified pathway

PPARgamma and C/EBPalpha are critical transcription factors in adipogenesis, but the precise role of these proteins has been difficult to ascertain because they positively regulate each other's expression. Questions remain about whether these factors operate independently in separate, parallel pathways of differentiation, or whether a single pathway exists. PPARgamma can promote adipogenesis in C/EBPalpha-deficient cells, but the converse has not been tested. We have created an immortalized line of fibroblasts lacking PPARgamma, which we use to show that C/EBPalpha has no ability to promote adipogenesis in the absence of PPARgamma. These results indicate that C/EBPalpha and PPARgamma participate in a single pathway of fat cell development with PPARgamma being the proximal effector of adipogenesis.

Molecular cloning, genetic mapping, and expression analysis of four zebrafish c/ebp genes

The CCAAT/enhancer binding protein family (C/EBP) are transcription factors that play integral roles in the development and function of many organ systems, including hematopoietic cells, adipose tissues, and liver. We have identified and characterized putative zebrafish orthologs of mammalian C/EBP alpha, beta, gamma, and delta using low-stringency hybridization screening and computer searches of the GenBank EST database. c/ebpa and g were mapped within 1 cM of each other on linkage group (LG) 7, syntenic with human CEBPA and G genes on chromosome 19. c/ebpb was mapped to LG8, and c/ebpd was mapped to LG24, on the same LG as a recently identified unique c/ebp in zebrafish, c/ebp1. The mapping of these genes established new syntenic relationships between LG8 and human chromosome 20, extended existing synteny between LG7 and human chromosome 19, and confirmed the synteny between LG24 and human chromosome 8. In addition, these syntenies between zebrafish and human chromosomes are also conserved in the mouse genome. To characterize the expression of these genes, RNA in situ hybridization in embryos of wild type and a hematopoietic mutant, cloche, was performed. The results showed that zebrafish c/ebpa, b, g, and d were expressed in many embryonic tissues. c/ebpa and b were expressed in a subset of hematopoietic cells in a region consistent with myeloid expression. In addition, there was expression of c/ebpa and b in the liver and c/ebpa, b, and d in regions of the gastrointestinal tract. The expression of the c/ebps may serve as important markers for analysis of myelopoiesis, hepatic development, and other developmental processes in the future.

Expression of CCAAT/enhancer binding protein family genes in monolayer and sandwich culture of hepatocytes: induction of stress-inducible GADD153

Removal of hepatocytes from their physiological environment for experimentation in vitro activates loss of liver-specific phenotype. Hepatocytes cultured in a sandwich configuration reportedly maintain greater expression of certain liver-specific genes than hepatocytes in monolayer cultures. We show that sandwich culture of rat hepatocytes improves retention of expression of a liver-enriched transcription factor, C/EBPalpha (CCAAT/enhancer binding protein alpha), which regulates many liver-specific genes. However, we also demonstrate increased expression of a stress-responsive C/EBP homologue, GADD153 (growth arrest and DNA damage gene 153), during monolayer culture, which may promote dedifferentiation. Induction of GADD153 was not prevented in sandwich cultured hepatocytes. Activation of a homologue of the mouse GADD153 target gene, doc1, was observed in monolayer and sandwich culture, suggesting that GADD153 was transcriptionally active. We suggest that the capability of sandwich cultures to maintain hepatocyte phenotype may be limited by the altered profile of transcription factor activity.

Cooperation between C/EBPalpha TBP/TFIIB and SWI/SNF recruiting domains is required for adipocyte differentiation

Chromatin remodeling is an important step in promoter activation during cellular lineage commitment and differentiation. We show that the ability of the C/EBPalpha transcription factor to direct adipocyte differentiation of uncommitted fibroblast precursors and to activate SWI/SNF-dependent myeloid-specific genes depends on a domain, C/EBPalpha transactivation element III (TE-III), that binds the SWI/SNF chromatin remodeling complex. TE-III collaborates with C/EBPalpha TBP/TFIIB interaction motifs during induction of adipogenesis and adipocyte-specific gene expression. These results indicate that C/EBPalpha acts as a lineage-instructive transcription factor through SWI/SNF-dependent modification of the chromatin structure of lineage-specific genes, followed by direct promoter activation via recruitment of the basal transcription-initiation complex, and provide a mechanism by which C/EBPalpha can mediate differentiation along multiple cellular lineages.

Downregulation of repressive CUP/AP-2 isoforms during adipocyte differentiation

Activation of transcription of the C/EBPalpha (CCAAT/enhancer binding protein alpha) gene is a critical event in the differentiation of 3T3-L1 preadipocytes into adipocytes. The kinetics of this process parallels a decline of AP-2alpha protein (also referred to as CUP, C/EBP undifferentiated protein) and decreased binding of CUP/AP-2alpha to the C/EBPalpha promoter. Mutation of the CUP/AP-2 binding sites in the C/EBPalpha promoter results in increased C/EBPalpha expression. Based on these findings, it appears that decline in AP-2alpha expression is an important early event in the adipocyte differentiation program. In the studies presented here, we identify three mRNAs that encode the repressive CUP/AP-2alpha isoforms expressed in undifferentiated 3T3-L1 preadipocytes. We demonstrate that the kinetics of the decline of these isoforms' expression over the course of differentiation parallels both the decrease in CUP/AP-2alpha DNA binding activity and the increase in C/EBPalpha protein observed in previous studies.

McBindall--a better name for CCAAT/enhancer binding proteins?

C/EBP-related transcription factors regulate the balance between cell proliferation and mitotic growth arrest during terminal differentiation. Three new studies give evidence that this regulation is mediated by protein:protein interactions completely distinct from the role of C/EBPs in gene expression.

Multipotent hematopoietic cell lines derived from C/EBPalpha(-/-) knockout mice display granulocyte macrophage-colony-stimulating factor, granulocyte- colony-stimulating factor, and retinoic acid-induced granulocytic differentiation

The transcription factor C/EBPalpha is an important mediator of granulocyte differentiation and regulates the expression of multiple granulocyte-specific genes including the granulocyte-colony-stimulating factor (G-CSF) receptor, neutrophil elastase, and myeloperoxidase. Indeed C/EBPalpha knockout mice display a profound block in granulocyte differentiation. To study this block in granulocytic differentiation in more detail, retroviral vector-mediated transduction of a dominant-negative retinoic acid receptor was used to establish hematopoietic growth factor-dependent, lympho-myeloid progenitor cell lines from the fetal livers of both the C/EBPalpha knockout animals (C/EBPalpha(-/-)) and their heterozygous littermates (C/EBPalpha(+/-)). Surprisingly, the C/EBPalpha(-/-) cell lines displayed significant spontaneous granulocytic differentiation, and this differentiation was markedly enhanced when the cells were stimulated with granulocyte macrophage (GM)-CSF. This GM-CSF-mediated differentiation was associated with the up-regulation of G-CSF receptor mRNA, and the combination of GM-CSF and G-CSF generated more than 95% mature neutrophils in the C/EBPalpha(-/-) cultures. The addition of all-trans retinoic acid also enhanced this granulocytic differentiation of the cultured C/EBPalpha(-/-) cells, indicating that the activated retinoic acid receptors can enhance granulocytic differentiation through a molecular pathway that is independent of C/EBPalpha. These studies clearly indicate that terminal granulocytic differentiation associated with the up-regulation of C/EBPalpha-responsive genes can occur in the absence of C/EBPalpha, and they indicate the existence of multiple independent molecular pathways potentially used by primitive hematopoietic precursors that can lead to the development of mature granulocytes.

CCAAT/enhancer binding protein alpha assembles essential cooperating factors in common subnuclear domains

The transcription factor CCAAT/enhancer binding protein alpha (C/EBP alpha) is the DNA binding subunit of a multiprotein complex that regulates the pituitary-specific GH promoter. C/EBP alpha is absent from the GHFT1-5 pituitary progenitor cell line in which ectopic C/EBP alpha expression leads to activation of the otherwise dormant GH promoter. Transcriptional regulatory complexes are commonly envisaged as assembling from components that evenly diffuse throughout the nucleoplasm. We show that C/EBP alpha, expressed in GHFT1-5 cells as a fusion with color variants of the green fluorescent protein (GFP), concentrated specifically at peri-centromeric chromosomal domains. Although we found the CREB-binding protein (CBP) to activate C/EBP alpha-dependent transcription, CBP was absent from the pericentromeric chromatin. C/EBP alpha expression was accompanied by the translocation of endogenous and ectopically expressed CBP to pericentromeric chromatin. The intranuclear recruitment of CBP required the transcriptional activation domains of C/EBP alpha. C/EBP alpha also caused GFP-tagged TATA binding protein (TBP) to relocate to the Hoechst-stained domains. The altered intranuclear distribution of critical coregulatory factors defines complexes formed upon C/EBP alpha expression. It also identifies an organizational activity, which we label "intranuclear marshalling," that may regulate gene expression by determining the cooperative and antagonistic interactions available at specific nuclear sites.