Structural characterization and alternate splicing of the gene encoding the preadipocyte EGF-like protein pref-1

The Novel Gene EGFL9/Dlk2, Highly Homologous to Dlk1, Functions as a Modulator of Adipogenesis

The Dlk1 gene appears to function as a regulator of adipogenesis. Adult Dlk1-deficient mice are obese, but adipose tissue still develops in transgenic mice overexpressing an Fc-dlk1 fusion protein, and neither type of genetically modified mice displays serious abnormalities. It was therefore possible that one yet unidentified gene might either compensate or antagonize for the absence or for overexpression, respectively, of Dlk1 in those animals. In database searches, we found a novel gene, EGFL9, encoding for a protein whose structural features are virtually identical to those of dlk1, suggesting it may function in a similar way. As dlk1 does, the protein encoded by EGFL9/Dlk2 affects adipogenesis of 3T3-L1 preadipocytes and mesenchymal C3H10T1/2 cells; however, it does so in an opposite way to that of dlk1. In addition, expression levels of both genes appear to be inversely correlated in both cell lines. Moreover, enforced changes in the expression of one gene affect the expression levels of the other. Our data suggest that adipogenesis may be modulated by the coordinated expression of Dlk1 and EGFL9/Dlk2.

Pref-1, a preadipocyte secreted factor that inhibits adipogenesis

Preadipocyte factor 1 (Pref-1) belongs to the Notch/Delta/Serrate family of epidermal growth factor-like repeat-containing proteins. Pref-1 is highly expressed in 3T3-L1 cells but is extinguished during adipocyte differentiation. Pref-1 serves as an excellent marker for preadipocytes. Furthermore, Pref-1 is an inhibitor of adipogenesis. Constitutive expression of Pref-1 inhibits, whereas antisense Pref-1 enhances, 3T3-L1 adipocyte differentiation. We found that Pref-1 is synthesized as a transmembrane protein but processed to generate soluble forms, including a large 50-kDa soluble form and the small soluble forms. Furthermore, only the large soluble form, but not the small soluble or the transmembrane forms of Pref-1, is biologically active to inhibit adipogenesis. We recently elucidated that the 50-kDa soluble form of Pref-1 is released by an ADAM family member, tumor necrosis factor-alpha converting enzyme (ADMA 17). In vivo, mice lacking Pref-1 show accelerated fat deposition; conversely, mice overexpressing soluble Pref-1 in adipose tissue show a decrease in fat mass, reduced expression of adipocyte markers, and lower adipocyte-secreted factors. These findings clearly demonstrate the inhibitory effect of Pref-1 on adipogenesis in vivo.

Alternative splicing of delta-like 1 homolog (DLK1) in the pig and human

Delta-like homolog 1 (DLK1), a paternally imprinted gene with several alternative splicing isoforms, is an important regulator of fetal and postnatal development. We report the sequence of porcine DLK1 (pDLK1) and examine the expression and alternative splicing isoforms in the pig (Sus scrofa) and human. DLK1-A was the sole isoform identified in human tissues and has been shown to be present in mouse and cattle. Surprisingly, DLK1-A was undetected in various tissues from fetal and postnatal pigs. Instead, DLK1-C2 was the most abundant isoform while DLK1-B was expressed to a lesser extent. In fractionated adipose tissue, pDLK1 was most highly expressed in the stromal-vascular cell fraction. In addition, total pDLK1 was highly expressed in fetal adipose tissue but dramatically decreased postnatally. Our data suggests that expression of DLK1-B and -C2 isoforms is sufficient for normal pig development. Furthermore, human and pig samples showed no alterations in species-specific splicing, but expression levels decreased with age, suggesting that regulation of expression, not splicing, is the most likely mechanism controlling the biological function of DLK1.

Ectodomain shedding of preadipocyte factor 1 (Pref-1) by tumor necrosis factor alpha converting enzyme (TACE) and inhibition of adipocyte differentiation

Preadipocyte factor 1 (Pref-1), an epidermal growth factor repeat containing transmembrane protein found in the preadipocytes, inhibits adipocyte differentiation in vitro and in vivo. Here, we examined the processing of membrane form of Pref-1A to release the 50-kDa soluble form that inhibits adipocyte differentiation. The ectodomain cleavage of Pref-1 is markedly enhanced by phorbol 12-myristate 13-acetate in a dose- and time-dependent manner. The basal and stimulated cleavage is inhibited by the broad metalloproteinase inhibitor GM6001, a fact that suggests that cleavage of membrane Pref-1A is dependent on a metalloproteinase. Next, we showed that release of soluble Pref-1A is inhibited by TAPI-0 and by a tissue inhibitor of metalloproteinase-3, TIMP-3, that can inhibit tumor necrosis factor alpha converting enzyme (TACE), but not by TIMP-1 or TIMP-2. On the other hand, overexpression of TACE increases Pref-1 cleavage to produce the 50-kDa soluble form. Furthermore, this cleavage was not detected in cells with TACE mutation or with TACE small interfering RNA. TACE-mediated shedding of Pref-1 ectodomain inhibits adipocyte differentiation of 3T3-L1 cells and in Pref-1-null mouse embryo fibroblasts transduced with Pref-1A. Identification of TACE as the major protease responsible for conversion of membrane-bound Pref-1 to the biologically active diffusible form provides a new insight into Pref-1 function in adipocyte differentiation.

DLK1: increased expression in gliomas and associated with oncogenic activities

DLK1 (delta-like) is a transmembrane and secreted protein in the epidermal growth factor-like homeotic family. Although expressed widely during embryonic development, only a few tissues retain the expression in adults. Neuroendocrine tumors often highly express this protein; therefore, we hypothesized that brain tumors might also express it. This study found that the expression of DLK1 in gliomas was higher than that in normal brain (P < 0.05). After stable transfection of a DLK1 cDNA expression vector into GBM cell lines, their proliferation was increased. Furthermore, they lost contact inhibition, had enhanced anchorage-independent growth in soft agar, and had significantly greater capacity to migrate. Western blot studies showed that expression of cyclin D1, CDK2, and E2F4 were increased, and Rb levels were decreased in these cells. DLK1 was found on the cell surface and secreted in the medium from the transfected GBM cells. DLK1-enriched condition medium stimulated the growth of glioblastoma multiforme cell lines and explants. DLK1 antibody blocked cell growth stimulated by DLK1. In summary, these results suggest that DLK1 may play a role in the formation or progression of gliomas.

Identification of DLK1 variants in pituitary- and neuroendocrine tumors

In a gene chip analysis of common pituitary tumor types, one of the genes with the most impressive tissue-specific expression regulation was delta-like 1 (DLK1), which was strongly expressed in GH-secreting (GH-S) pituitary tumors. In addition to pituitary adenomas, various endocrine tumors were subjected to real-time-quantitative PCR revealing high expression of DLK1 in normal pituitary tissue, in GH-S-, in one prolactin-secreting pituitary adenoma and in pheochromocytomas. Additionally, three DLK1 gene-derived subvariants were identified. The first, lacking 204 bp--coding for epidermal growth factor-like domain 6 and parts of the juxtamembrane region--was named Secredeltin. In the other two splice variants (named Brevideltin and Brevideltinin), a stop codon is introduced due to a frame-shift, leading to truncated proteins of 204 and 213 aas, respectively.

Dlk1 expression marks developing endothelium and sites of branching morphogenesis in the mouse embryo and placenta

The protein product of the Delta-like 1 (Dlk1) gene belongs to the Delta-Notch family of signaling molecules, proteins involved in cell fate determination in many tissues during development. The DLK1 protein is believed to function as a growth factor, maintaining the proliferative state of undifferentiated cells, and is usually down-regulated as immature cells differentiate. The expression pattern of the DLK1 protein has been described in certain human tissues; however, Dlk1 expression is not well understood in the mouse, the most tractable mammalian genetic model system. To better understand the role of Dlk1 in embryonic development, the tissue-specific expression pattern of Dlk1 mRNA during mouse embryogenesis was analyzed by in situ hybridization. In embryonic day 12.5 (e12.5) embryos, high levels of Dlk1 were found in the developing pituitary, pancreas, lung, adrenal, and many mesodermally derived tissues. Strikingly, Dlk1 expression also marks the growing branches of organs that develop through the process of branching morphogenesis. At e16.5, Dlk1 expression is down-regulated in most tissues but remains in the pituitary, the adrenal gland, and in skeletal muscle. In the placenta, expression of Dlk1 is detected in endothelial cells lining the fetal blood vessels of the labyrinth. This pattern is distinct from that seen in the human placenta and suggests a role for Dlk1 in regulating maternal-fetal interactions.

dlk acts as a negative regulator of Notch1 activation through interactions with specific EGF-like repeats

The protein dlk, encoded by the Dlk1 gene, belongs to the Notch epidermal growth factor (EGF)-like family of receptors and ligands, which participate in cell fate decisions during development. The molecular mechanisms by which dlk regulates cell differentiation remain unknown. By using the yeast two-hybrid system, we found that dlk interacts with Notch1 in a specific manner. Moreover, by using luciferase as a reporter gene under the control of a CSL/RBP-Jk/CBF-1-dependent promoter in the dlk-negative, Notch1-positive Balb/c 14 cell line, we found that addition of synthetic dlk EGF-like peptides to the culture medium or forced expression of dlk decreases endogenous Notch activity. Furthermore, the expression of the gene Hes-1, a target for Notch1 activation, diminishes in confluent Balb/c14 cells transfected with an expression construct encoding for the extracellular EGF-like region of dlk. The expression of Dlk1 and Notch1 increases in 3T3-L1 cells maintained in a confluent state for several days, which is associated with a concomitant decrease in Hes-1 expression. On the other hand, the decrease of Dlk1 expression in 3T3-L1 cells by antisense cDNA transfection is associated with an increase in Hes-1 expression. These results suggest that dlk functionally interacts in vivo with Notch1, which may lead to the regulation of differentiation processes modulated by Notch1 activation and signaling, including adipogenesis.

Insulin-like growth factor-1/insulin bypasses Pref-1/FA1-mediated inhibition of adipocyte differentiation

Pref-1 is a highly glycosylated Delta-like transmembrane protein containing six epidermal growth factor-like repeats in the extracellular domain. Pref-1 is abundantly expressed in preadipocytes, but expression is down-regulated during adipocyte differentiation. Forced expression of Pref-1 in 3T3-L1 cells was reported to inhibit adipocyte differentiation. Here we show that efficient and regulated processing of Pref-1 occurs in 3T3-L1 preadipocytes releasing most of the extracellular domain as a 50-kDa heterogeneous protein, previously isolated and characterized as FA1. Unexpectedly, we found that forced expression of the soluble form, FA1, or full-length Pref-1 did not inhibit adipocyte differentiation of 3T3-L1 cells when differentiation was induced by standard treatment with methylisobutylxanthine, dexamethasone, and high concentrations of insulin. However, forced expression of either form of Pref-1/FA1 in 3T3-L1 or 3T3-F442A cells inhibited adipocyte differentiation when insulin or insulin-like growth factor-1 (IGF-1) was omitted from the differentiation mixture. We demonstrate that the level of the mature form of the IGF-1 receptor is reduced and that IGF-1-dependent activation of p42/p44 mitogen-activated protein kinases (MAPKs) is compromised in preadipocytes with forced expression of Pref-1. This is accompanied by suppression of clonal expansion and terminal differentiation. Accordingly, supplementation with insulin or IGF-1 rescued p42/p44 MAPK activation, clonal expansion, and adipocyte differentiation in a dose-dependent manner.

Inhibition of adipogenesis and development of glucose intolerance by soluble preadipocyte factor-1 (Pref-1)

Preadipocyte factor-1 (Pref-1) is a transmembrane protein highly expressed in preadipocytes. Pref-1 expression is, however, completely abolished in adipocytes. The extracellular domain of Pref-1 undergoes two proteolytic cleavage events that generate 50 and 25 kDa soluble products. To understand the function of Pref-1, we generated transgenic mice that express the full ectodomain corresponding to the large cleavage product of Pref-1 fused to human immunoglobulin-gamma constant region. Mice expressing the Pref-1/hFc transgene in adipose tissue, driven by the adipocyte fatty acid-binding protein (aP2, also known as aFABP) promoter, showed a substantial decrease in total fat pad weight. Moreover, adipose tissue from transgenic mice showed reduced expression of adipocyte markers and adipocyte-secreted factors, including leptin and adiponectin, whereas the preadipocyte marker Pref-1 was increased. Pref-1 transgenic mice with a substantial, but not complete, loss of adipose tissue exhibited hypertriglyceridemia, impaired glucose tolerance, and decreased insulin sensitivity. Mice expressing the Pref-1/hFc transgene exclusively in liver under the control of the albumin promoter also showed a decrease in adipose mass and adipocyte marker expression, suggesting an endocrine mode of action of Pref-1. These findings demonstrate the inhibition of adipogenesis by Pref-1 in vivo and the resulting impairment of adipocyte function that leads to the development of metabolic abnormalities.

Differential expression of Dlk-1 in bovine adipose tissue depots

Dlk-1, a type 1 membrane glycoprotein, is a member of the Epidermal Growth Factor-like family of homeotic proteins that are typically involved in cell fate decisions and in mice it has been implicated in the control of differentiation of adipocytes. The aim of this study was to determine whether there were tissue-specific expression patterns of Dlk-1 splice variants in bovine tissues. Only the Dlk-1-C2 variant was expressed in adult bovine tissues while both Dlk-1-C2 and Dlk-1-A variants were expressed in foetal tissues. Quantitative real-time PCR revealed large differences in the relative levels of expression of the Dlk-1-C2 variant in adult adipose tissue depots with no expression in subcutaneous and brisket adipose tissues. Expression was also demonstrated in three adult skeletal muscle samples. The large variation in the level of expression of Dlk-1-C2 in different adult tissues may reflect the relative preadipocyte content of those tissues and consequently their potential for generating new adipocytes. A low abundance soluble glycoprotein (bFA1) was purified from bovine amniotic fluid. Analyses of its amino acid sequence revealed that it corresponded to most of the extracellular domain of bovine Dlk-1 and was derived by proteolytic processing from the full-length Dlk-1 protein encoded by the Dlk-1-A variant. The tissues expressing the Dlk-1-A variant have not been identified but are likely to be foetal in origin. Splice variants of Dlk-1 may have varied functional roles with the foetal Dlk-1-A form capable of generating a protein that undergoes proteolytic processing to release a soluble ecto-domain of Dlk-1. In contrast the Dlk-1-C2 splice variant codes for a protein lacking this processing site and therefore it probably remains bound to the cell membrane.

Mice lacking paternally expressed Pref-1/Dlk1 display growth retardation and accelerated adiposity

Preadipocyte factor 1 (Pref-1/Dlk1) inhibits in vitro adipocyte differentiation and has been recently reported to be a paternally expressed imprinted gene at human chromosome 14q32. Studies on human chromosome 14 deletions and maternal uniparental disomy (mUPD) 14 suggest that misexpression of a yet-to-be-identified imprinted gene or genes present on chromosome 14 causes congenital disorders. We generated Pref-1 knockout mice to assess the role of Pref-1 in growth and in vivo adipogenesis and to determine the contribution of Pref-1 in mUPD. Pref-1-null mice display growth retardation, obesity, blepharophimosis, skeletal malformation, and increased serum lipid metabolites. Furthermore, the phenotypes observed in Pref-1-null mice are present in heterozygotes that harbor a paternally inherited, but not in those with a maternally inherited pref-1-null allele. Our results demonstrate that Pref-1 is indeed paternally expressed and is important for normal development and for homeostasis of adipose tissue mass. We also suggest that Pref-1 is responsible for most of the symptoms observed in mouse mUPD12 and human mUPD14. Pref-1-null mice may be a model for obesity and other pathologies of human mUPD14.

Only the large soluble form of preadipocyte factor-1 (Pref-1), but not the small soluble and membrane forms, inhibits adipocyte differentiation: role of alternative splicing

We originally identified preadipocyte factor-1 (Pref-1) as an inhibitor of adipogenesis by the fact that constitutive expression of full-length Pref-1A inhibits differentiation of 3T3-L1 cells into adipocytes. Subsequently, we found that the membrane form of Pref-1 is proteolytically processed at two sites in the extracellular domain, resulting in the larger (50 kDa) and smaller (25 kDa) soluble forms. A specific form(s) of Pref-1, which is active in inhibiting adipocyte differentiation, has not been elucidated. Here, various artificial constructs and alternative-splicing variants of Pref-1 were stably transfected into 3T3-L1 cells, or conditioned media from COS cells transfected with the various forms were added into differentiating 3T3-L1 cells. Judging by Oil Red O staining for lipid accumulation and expression of adipocyte markers, we determined that, unlike the full-length Pref-1A and the constructed large soluble form, the artificial membrane form of Pref-1 lacking the processing site proximal to the membrane was not effective in inhibiting adipogenesis. Furthermore, conditioned media from COS cells transfected with the construct containing only the first three epidermal growth factor repeats, corresponding to the small soluble form, was not effective in inhibiting adipocyte differentiation. Of the four alternative-splicing products, Pref-1A and Pref-1B, which generate both large and small soluble forms, inhibited adipogenesis, whereas Pref-1C and Pref-1D, which lack the processing site proximal to the membrane and therefore generate only the smaller soluble form, did not show any effect. We conclude that only the large soluble form, and not the transmembrane or the small soluble form, of Pref-1 is biologically active and that alternative splicing therefore determines Pref-1 function in adipocyte differentiation.

The EGF-like homeotic protein dlk affects cell growth and interacts with growth-modulating molecules in the yeast two-hybrid system

Levels of dlk, an EGF-like homeotic protein, are critical for several differentiation processes. Because growth and differentiation are, in general, exclusive of each other, and increasing evidence indicates that Dlk1 expression changes in tumorigenic processes, we studied whether dlk could also affect cell growth. We found that, in response to glucocorticoids, Balb/c 3T3 cells with diminished levels of dlk expression develop foci-like cells that have lost contact inhibition, display altered morphology, and grow faster than control cell lines. Balb/c 3T3 cells spontaneously growing more rapidly are also dlk-negative cells. Moreover, screening by the yeast two-hybrid system, using Dlk1 constructs as baits, resulted in the isolation of GAS1 and acrogranin cDNAs. Interestingly, these proteins are cysteine-rich molecules involved in the control of cell growth. Taken together, these observations suggest that dlk may participate in a network of interactions controlling how the cells respond to growth or differentiation signals.

Analysis of the sheep genome

The identification of genes controlling several traits of interest in sheep has been accomplished by positional candidate cloning. In these studies, the trait is first mapped to a specific chromosomal region by linkage analysis, which requires families that are segregating for the trait and for polymorphic markers. Microsatellite markers are usually used for these analyses because of their extensive genetic variability. Once the location of a trait is determined by linkage to the markers, possible candidate genes controlling the trait can be inferred because of their proximity to linked markers. It is not necessary to map all possible genes in sheep for this strategy to be effective. Rather, a subset of genes that are mapped in humans and mice have also been mapped in sheep; these genes serve as "anchors" across the comparative maps of the different species. Further study of these positional candidates has revealed naturally occurring mutations that produce phenotypes that are unique to sheep. Thus the genetic analysis of sheep traits advances knowledge not only in this species but provides critical information for understanding biological pathways in mammalian species.

Molecular cloning, expression analysis, promoter characterization, and chromosomal localization of the bovine PREF1 gene

Pre-adipocyte factor-1 (pref-1), originally identified in mouse 3T3-L1 pre-adipocytes, is known to play a key role in inhibiting the adipose conversion. As a first step to study the involvement of the PREF1 gene in intramuscular adipose tissue development in cattle, which has an economic importance for beef cattle, we have isolated the bovine PREF1 cDNA and genomic clones and characterized expression in abdominal adipose and promoter region of the bovine PREF1 gene. We have detected two bovine PREF1 splice-isoforms, PREF1C2 and PREF1A, in abdominal fat tissue. The RT-PCR experiment revealed that the two isoforms are identified in neonatal, but no in adult abdominal fat tissue, suggesting age-dependent suppression of the bovine PREF1 gene expression in the form of PREF1C2 and PREF1A in abdominal fat tissue. By mapping the regulatory region of this gene, we have shown that at least two regions within 1121 bp upstream of putative transcription start site are sufficient to confer promoter activity, when accompanied by a short region including the transcription start site. The chromosomal location of the PREF1 gene was determined by fluorescence in situ hybridization (FISH). The PREF1 gene locates on bovine chromosome 21q24.

Synergistic antiadipogenic effects of HIV type 1 protease inhibitors with tumor necrosis factor alpha: suppression of extracellular insulin action mediated by extracellular matrix-degrading proteases

Long-term use of HIV-1 protease inhibitors (PIs) is associated with a lipodystrophy syndrome. To delineate the associated mechanisms, adipogenesis was determined in 3T3-L1 cells in the presence or absence of either indinavir (2-50 microg/ml) or ritonavir (0.4-10 microg/ml). A concentration-dependent decrease in both lipid (4-59%) and triglyceride (11-49%) levels was seen after 10 days of exposure. Simultaneous treatment with TNF-alpha showed a synergistic suppression in lipid levels by 45-95% at 10 U/ml and almost complete suppression at 100 U/ml. The effect of PIs on insulin-induced lipogenesis was monitored by [(14)C)]glucose incorporation into lipids, which was suppressed by 21-86% in a concentration-dependent manner. Insulin-sensitizing agent, troglitazone (80 and 400 nM), effectively blocked the PI-mediated adipogenic suppression. Preadipocyte factor 1 gene (pref-1) expression, as monitored by RT-PCR, was downregulated (4- to 6-fold) within 48 hr after insulin stimulation; however, a smaller decrease (1.2- to 1.8-fold) was observed in PI-exposed cells. The decrease in proteolytic activity of matrix metalloproteases (MMP-2 and MMP-9) during adipogenesis was reversed on exposure to the PIs. Similarly, the plasminolytic activity was increased and plasminogen activator inhibitor (PAI) activity was decreased in supernatants from PI-treated cells. The insulin-mediated induction (3- to 4-fold) of PAI-1 and PAI-2 message was suppressed on exposure to PIs, which was reversed by troglitazone treatment. Thus, the HIV-1 PIs may suppress adipogenesis by disrupting the concerted actions of host proteases that regulate ECM integrity required for initiation of differentiation.

Fetal antigen 1 in healthy adults and patients with pituitary disease: relation to physiological, pathological, and pharmacological GH levels

Immunohistochemical analysis of the distribution of human fetal antigen 1 (FA1) in adult human tissues has demonstrated a strong association between FA1 and (neuro)endocrine structures. In the anterior pituitary gland FA1 was colocalized with GH, and the present study was performed to evaluate a possible relationship between GH and FA1. FA1 and GH levels were measured during a 24-h period at 20-min intervals. In contrast to the known GH peaks during 24-h sampling, there was no detectable FA1 peak. The FA1 responses to placebo were not significantly different from the responses to the combination of pyridostigmine and GHRH. No significant difference was found between basal FA1 (nanograms per ml) levels [median (minimum-maximum)] in healthy adults [n = 40; 28.6 ng/ml (12.5-72.0)], acromegalic patients [n = 11; 31.0 ng/ml (21.6-56.3)], and patients with GH deficiency [n = 22; 32.1 ng/ml (13.4-108.7)]. FA1 levels were significantly reduced, in the six of seven acromegalic GH responders to octreotide, from [median (minimum-maximum)] 30.6 ng/ml (20.0-43.1) to 20.3 (13.9-30.2; P < 0.02). There was no significant change during placebo. FA1 levels were significantly increased compared with placebo values during 3 months of GH therapy. The increase in FA1 levels was significantly higher than the change during placebo (P < 0.003). In conclusion, a common secretory and stimulatory pathway for FA1 and GH in healthy adults has been ruled out. However, we found that pharmacologically induced changes in GH levels during weeks to months had a corresponding direct or indirect effect on FA1 levels in patients with GH deficiency or acromegaly. However, a direct effect of octreotide on FA1 levels, independent of GH levels, has not been ruled out.

Human-ovine comparative sequencing of a 250-kb imprinted domain encompassing the callipyge (clpg) locus and identification of six imprinted transcripts: DLK1, DAT, GTL2, PEG11, antiPEG11, and MEG8

Two ovine BAC clones and a connecting long-range PCR product, jointly spanning approximately 250 kb and representing most of the MULGE5-OY3 marker interval known to contain the clpg locus, were completely sequenced. The resulting genomic sequence was aligned with its human ortholog and extensively annotated. Six transcripts, four of which were novel, were predicted to originate from within the analyzed region and their existence confirmed experimentally: DLK1, DAT, GTL2, PEG11, antiPEG11, and MEG8. RT-PCR experiments performed on a range of tissues sampled from an 8-wk-old animal demonstrated the preferential expression of all six transcripts in skeletal muscle, which suggests that they are under control of common regulatory elements. The six transcripts were also shown to be subject to parental imprinting: DLK1, DAT, and PEG11 were shown to be paternally expressed and GTL2, antiPEG11, and MEG8 to be maternally expressed.

Characterization of a proximal element in the rat preadipocyte factor-1 (Pref-1) gene promoter

Preadipocyte factor-1 (Pref-1) was shown to negatively regulate adipocyte differentiation. We recently reported that ZOG, a rat homolog of Pref-1, was specifically expressed in the adrenal zona glomerulosa. Results of the investigation of Pref-1 expression in preadipocyte and in undifferentiated adrenal cortex suggested that down-regulation of Pref-1 gene was closely correlated with the differentiation process. In this study we demonstrate that an upstream region (from -76 to -47) of the rat Pref-1 gene was essential for its expression in adrenocortical carcinoma-derived H295R cells. A nucleotide sequence found in this region, GCGTGGGCGTGGGCGGGGG (Egr/GC-box), seemed to contain three elements, two early growth response (Egr) elements and one GC-box, overlapping each other. Mutations of four or five nucleotides in a 7-nucleotides-stretch in the midst of the Egr/GC-box eliminated the binding of Sp1/3, abolished the activation by Egr-factor(s) and diminished the Pref-1 promoter activity. When mutations were introduced into the outside of the middle portion, the binding of Sp1/3 to the Egr/GC-box was abolished similarly. However, the decrease in the promoter activity was less than that found with the construct mutated at the middle. These results indicated that an element present at the 7-nucleotides-stretch in the midst of the Egr/GC-box might be important for the Pref-1 promoter activity, and this proximal element was possibly activated by a still-unidentified nuclear factor(s). This element would function as the promoter of the Pref-1 gene in H295R cells, but not in HeLa cells.

Does fetal antigen 1 (FA1) identify cells with regenerative, endocrine and neuroendocrine potentials? A study of FA1 in embryonic, fetal, and placental tissue and in maternal circulation

Fetal antigen 1 (FA1) is a circulating EGF multidomain glycoprotein. FA1 and its membrane-associated precursor is defined by the mRNAs referred to as delta-like (dlk), preadipocyte factor 1 (pref-1) or zona glomerulosa-specific factor (ZOG). Using a polyclonal antibody recognising both forms, the localisation of FA1/dlk was analysed in embryonic and fetal tissues between week 5 to 25 of gestation and related to germinal origin and development. FA1 was observed in endodermally derived hepatocytes, glandular cells of the pancreas anlage, and in respiratory epithelial cells. FA1 was also present in mesodermally derived cells of the renal proximal tubules, adrenal cortex, Leydig and Hilus cells of the testes and ovaries, fetal chondroblasts, and skeletal myotubes. Ectodermally derived neuro- and adenohypophysial cells, cells in the floor of the 3rd ventricle and plexus choroideus were also FA1 positive. The number of cells expressing FA1 decreased during fetal development where the expression became restricted to specific functional cells. Epidermis, gut epithelium, gall bladder, blood cells, spleen, thyroid gland, salivary glands, and smooth muscle cells were FA1 negative. Analysis of extra-embryonic tissues from normal and pathological pregnancies revealed FA1 in stromal cells surrounding the blood islands of the yolk sac as well as in placental fibroblasts where the expression was most pronounced in diploid, androgenic complete hydatidiform moles. However, as measured by ELISA, the circulating maternal FA1 levels in complete moles were not different from normal pregnancies. The results presented suggest that FA1 is a growth and/or differentiation factor extensively expressed in immature cells and down-regulated during fetal development. FA1 down-regulation was associated with a shift in the subcellular localisation indicating differential post-translational/post-transcriptional modifications during fetal development. FA1 may be a new marker of cellular subtypes with a regenerative potential and of specific cells with endocrine or neuroendocrine functions.

Genomic organization and genetic mapping of the bovine PREF-1 gene

As a potential regulator of nutrient partitioning in beef cattle, we have cloned and genetically mapped the bovine PREF-1 gene. A full-length PREF-1 cDNA was isolated by iterative purification from a mixed-tissue cDNA library to which adipose contributed mRNA. Analysis of partial cDNAs from this library revealed that the 3'-terminal exon of the bovine PREF-1 mRNA is spliced in a manner analogous to its murine ortholog. However, we have also detected a PREF-1 splice form apparently unique to cattle. Aside from this alternative selection of a splice donor in the bovine fifth exon, the exon/intron junctions of the bovine PREF-1 gene recapitulate those observed for mice. The sequences proximal to the bovine PREF-1 transcription start site are homologous to the mouse PREF-1 promoter. Importantly, the sequence experimentally identified as critical to PREF-1 "suppression in adipocyte differentiation" is conserved in the bovine gene. The bovine PREF-1 gene was mapped to the telomeric end of BTA 21 by virtue of a physically linked microsatellite with seven alleles and 285 informative meiosis.

Transcriptional repression of pref-1 by glucocorticoids promotes 3T3-L1 adipocyte differentiation

Pref-1 is an epidermal growth factor-like domain-containing transmembrane protein that is cleaved to generate a soluble factor. It is abundant in 3T3-L1 preadipocytes but absent in mature adipocytes. Constitutive expression of pref-1 or the addition of its ectodomain inhibits adipogenesis. We find that the pref-1 gene is an early target of dexamethasone, a component of the dexamethasone/methylisobutylxanthine differentiation mixture used routinely for adipoconversion. The time course of the decrease in pref-1 mRNA by dexamethasone reflected the pref-1 mRNA half-life determined by actinomycin D treatment. Nuclear run-on assays showed that dexamethasone attenuates pref-1 transcription. We demonstrate a correlation between pref-1 down-regulation and adipoconversion by varying the time period and concentration of dexamethasone. Increasing the dexamethasone treatment from 2 to 4 days resulted in a time-dependent pref-1 down-regulation and increased differentiation as measured by adipocyte marker mRNAs. The dexamethasone concentration between 1 and 10 nM showed a dose-dependent decrease in pref-1 mRNA and an enhancement of adipogenesis. To test the hypothesis that dexamethasone initiation of adipoconversion may be via down-regulation of pref-1, we lowered endogenous pref-1 mRNA levels by stably transfecting 3T3-L1 preadipocytes with antisense pref-1. At 1 microM, antisense cells had enhanced adipose conversion; a similar degree of differentiation occurred with 2 nM dexamethasone, a concentration that does not support differentiation of control 3T3-L1 cells. We conclude that dexamethasone-mediated repression of pref-1 contributes to the mechanisms whereby glucocorticoids promote adipogenesis.

Adipocyte differentiation is modulated by secreted delta-like (dlk) variants and requires the expression of membrane-associated dlk

Previous studies demonstrate that the delta-like (dlk) and preadipocyte factor 1 (Pref-1) genes encode similar proteins. Pref-1 is downregulated during adipocyte differentiation, and expression of ectopic Pref-1 inhibits adipogenesis. We explored whether dlk functions similarly to Pref-1 and studied the role of alternately spliced dlk variants encoding membrane-associated or -secreted forms. We also studied whether enforced downregulation of dlk/Pref-1 may enhance the differentiation response of non-committed cells. Ectopic expression of a potentially secreted dlk variant, conditioned media from dlk expressing cells or several individual epidermal-growth-factor-dlk peptides inhibited 3T3-L1 differentiation. This demonstrates that dlk and Pref-1 are functionally equivalent. dlk gene mRNA encoding for secreted variants decreased much faster than total dlk gene mRNA during differentiation of 3T3-L1 cells. In fact, total dlk or membrane-associated dlk protein expression increased during the first hours of differentiation. Cells sorted for lowest levels of dlk protein diminished or lost their ability to differentiate. These data suggest that membrane and secreted dlk protein variants play opposite roles in the control of adipogenesis. In addition, enforced downregulation of dlk protein expression in the weakly adipogenic Balb/c 3T3 cell line dramatically enhanced adipogenesis in response to insulin. These results indicate that dlk protein not only participates in processes leading to inhibition of adipogenesis but that the control of its expression and different spliced variants is essential for the adipogenic response to extracellular signals.

Transcriptional control of the pref-1 gene in 3T3-L1 adipocyte differentiation. Sequence requirement for differentiation-dependent suppression

Preadipocyte factor-1 (Pref-1) is a transmembrane epidermal growth factor-like domain-containing protein highly expressed in 3T3-L1 preadipocytes, but is undetectable in mature fat cells; this down-regulation is required for adipocyte differentiation. We show here that pref-1 transcription is markedly suppressed during adipose conversion and results in decreased Pref-1 RNA levels. Using 3T3-L1 cells stably transfected with Pref-1 5'-deletion constructs truncated at -6000, -2100, -1300, -692, -300, -235, -193, -183, -170, -93, and -45 base pairs, we determined that the -183 to -170 region is responsible for the suppression of the pref-1 gene during adipogenesis. This is distinct from the -93 to -45 sequence important for pref-1 promoter activity in preadipocytes. The placement of a 40-base pair -193 to -154 pref-1 sequence containing the putative SAD (suppression in adipocyte differentiation) element upstream of the SV40 promoter decreased promoter activity by 85% upon adipocyte differentiation, compared with 40% observed with the SV40 promoter alone. The SAD element is therefore sufficient for adipocyte differentiation-dependent down-regulation of a heterologous promoter. A DNA-protein complex was observed when the -193 to -174 sequence was used with 3T3-L1 nuclear extracts in gel mobility shift assays. Competition with oligonucleotides harboring base substitution mutations identified a core sequence of -183AAAGA-179 as crucial for DNA-protein complex formation. UV cross-linking predicts that an approximately 63-kDa protein specifically binds the SAD element.

Expression of alternatively spliced human latent transforming growth factor beta binding protein-1

Latent transforming growth factor beta binding protein-1 (LTBP1) is important in regulating the localisation and activation of transforming growth factor beta(TGFbeta). Three forms of LTBP1 mRNA have previously been described, LTBP1L, LTBP1S and LTBPdelta53. Here, we have analysed the LTBP1 coding sequence and identified two other spliced forms, LTBP1delta55 and LTBP1delta41. LTBP1delta55 is a short form of LTBPIL which lacks 55 amino acids including two consensus N-glycosylation sites and LTBP1delta41 is a form of LTBP1 which lacks the 12th EGF-like repeat. Furthermore, sequencing of genomic clones showed that splicing to generate LTBP1L occurs using an intra-exonic 3' splice acceptor site in the first coding exon of LTBP1S and that LTBP1delta55 arises from the alternative use of an exonic 3' splice acceptor site at the end of the following intron. LTBP1delta41 arises from skipping the exon which encodes the 12th EGF-like repeat. LTBP1delta55 and LTBP1delta41 mRNA are expressed in a wide variety of human tissues but the proportions of each splice form vary in the tissues.

Understanding adipocyte differentiation

The adipocyte plays a critical role in energy balance. Adipose tissue growth involves an increase in adipocyte size and the formation of new adipocytes from precursor cells. For the last 20 years, the cellular and molecular mechanisms of adipocyte differentiation have been extensively studied using preadipocyte culture systems. Committed preadipocytes undergo growth arrest and subsequent terminal differentiation into adipocytes. This is accompanied by a dramatic increase in expression of adipocyte genes including adipocyte fatty acid binding protein and lipid-metabolizing enzymes. Characterization of regulatory regions of adipose-specific genes has led to the identification of the transcription factors peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and CCAAT/enhancer binding protein (C/EBP), which play a key role in the complex transcriptional cascade during adipocyte differentiation. Growth and differentiation of preadipocytes is controlled by communication between individual cells or between cells and the extracellular environment. Various hormones and growth factors that affect adipocyte differentiation in a positive or negative manner have been identified. In addition, components involved in cell-cell or cell-matrix interactions such as preadipocyte factor-1 and extracellular matrix proteins are also pivotal in regulating the differentiation process. Identification of these molecules has yielded clues to the biochemical pathways that ultimately result in transcriptional activation via PPAR-gamma and C/EBP. Studies on the regulation of the these transcription factors and the mode of action of various agents that influence adipocyte differentiation will reveal the physiological and pathophysiological mechanisms underlying adipose tissue development.

Regulation of fat synthesis and adipose differentiation

Adipocytes have highly specialized function of accumulating fat as stored energy that can be used during periods of food deprivation. The process of fat synthesis and development of adipose tissue are under hormonal and nutritional control. This review first describes transcription of the two critical enzymes involved in fat synthesis, fatty acid synthase and mitochondrial glycerol-3-phosphate acyltransferase, is decreased to an undetectable level during fasting. Food intake, especially a high carbohydrate, fat-free diet, subsequent to fasting causes dramatic increase in transcription of these genes. Insulin secretion is increased during feeding, having a positive effect, whereas cAMP, which mediates the effect of glucagon which increases during fasting, has a negative effect on transcription of these genes. Using adipocytes in culture and in transgenic mice that express liciferase driven by the fatty acid synthase promoter, cis-acting and trans-acting factors that may mediate the transcriptional regulation were examined. Upstream stimulatory factors (USFs) that bind to -65 E-box are required for insulin-mediated transcriptional activation of the fatty acid synthase gene. This review next describes how pref-1 is a novel inhibitor of adipose differentiation and is a plasma membrane protein containing six EGF-repeats in the extracellular domain. Pref-1 is highly expressed in 3T3-L1 preadipocytes, but is not detectable in mature fat cells. Down regulation of pref-1 is required for adipose differentiation, and constitutive expression of pref-1 inhibits adipogenesis. Moreover, the ectodomain of pref-1 is cleaved to generate a biologically active 50 kDa soluble form. There are four major forms of membrane pref-1 resulting from alternate splicing, but two of the forms with a larger deletion do not produce biologically active soluble form, indicating that alternate splicing determines the range of action, juxtacrine or paracrine, of the pref-1.

Growth hormone and prolactin stimulate the expression of rat preadipocyte factor-1/delta-like protein in pancreatic islets: molecular cloning and expression pattern during development and growth of the endocrine pancreas

GH and PRL have been shown to stimulate proliferation and insulin production in islets of Langerhans. To identify genes regulated by GH/PRL in islets, we performed differential screening of a complementary DNA library from neonatal rat islets cultured for 24 h with human GH (hGH). One hGH-induced clone had 96% identity with mouse preadipocyte factor-1 (Pref-1, or delta-like protein (Dlk)]. The size of Pref-1 messenger RNA (mRNA) in islets was 1.6 kilobases, with two less abundant mRNAs of 3.7 and 6.2 kilobases. The Pref-1 mRNA content of islets from adult rats was only 1% of that in neonatal islets. Pref-1 mRNA was markedly up-regulated in islets from pregnant rats from day 12 to term compared with those from age-matched female rats. Two peaks in mRNA expression were observed during gestation, one on day 14 and the other at term, whereafter it decreased to nonpregnant levels. Pref-1 mRNA was up-regulated 3- to 4-fold in neonatal rat islets of Langerhans after 48-h culture with hGH, as found also with bovine GH or ovine PRL. During the development of pancreas from embryonic day 12 (E12) to postnatal day 4, we observed a 2-fold increase in Pref-1 mRNA on E17 and a 5-fold increase at birth, followed by a rapid decline on postnatal day 4. Pref-1 immunoreactivity was found in a subpopulation of insulin cells of neonatal islets of Langerhans. At an early embryonal stage (E13), most cells of the pancreatic anlage were Pref-1 positive, becoming predominantly restricted to the insulin-producing cells during development. In conclusion, these findings suggest that Pref-1 is involved in both differentiation and growth of beta-cells.

TNF alpha-mediated inhibition and reversal of adipocyte differentiation is accompanied by suppressed expression of PPARgamma without effects on Pref-1 expression

Tumor necrosis factor alpha (TNF alpha) is a polypeptide hormone with pleiotropic effects on cellular proliferation and differentiation. To investigate how TNF alpha inhibits and reverses adipocyte differentiation, we studied the expression of two factors involved in the adipocyte differentiation process. Peroxisome proliferator-activated receptor gamma (PPARgamma) is a positive regulator of adipogenesis, whereas preadipocyte factor 1 (Pref-1) inhibits adipocyte differentiation. The expression patterns of both PPARgamma and Pref-1 change during early stages of adipocyte differentiation. Decreased expression of Pref-1 and increased expression of PPARgamma occur 1 day and 2 days, respectively, after 3T3-L1 cells reach confluence. During TNF alpha-mediated inhibition of adipocyte differentiation, PPARgamma messenger RNA (mRNA) expression stays at low levels. In contrast, TNF alpha treatment has no effect on the normal decrease in Pref-1 gene expression that occurs during adipogenesis. We observed that certain cytokine and growth factors [such as TNF alpha, basic fibroblast growth factor, transforming growth factor beta, and protein kinase C-activating agents plus calcium ionophore], when added to differentiated adipocytes, cause rapid down-regulation of PPARgamma mRNA expression with concomitant decrease in adipocyte-specific gene expression but fail to increase Pref-1 mRNA expression. Moreover, addition of TNF alpha to fully differentiated adipocytes results in the rapid disappearance of PPARgamma protein expression and the rapid loss of PPARgamma DNA-binding activity. Therefore, Pref-1 seems to function as a nonreversible molecular checkpoint whose expression is insensitive to TNF alpha-generated signals, whereas PPARgamma expression remains sensitive to TNF alpha at all stages of the adipogenesis program. Our results support the notion that dedifferentiated adipocytes and preadipocytes are not identical, though they share many similar morphological and gene expression patterns.

Cleavage of membrane-associated pref-1 generates a soluble inhibitor of adipocyte differentiation

pref-1 is an epidermal growth factor-like repeat protein present on the surface of preadipocytes that functions in the maintenance of the preadipose state. pref-1 expression is completely abolished during 3T3-L1 adipocyte differentiation. Bypassing this downregulation by constitutive expression of full-length transmembrane pref-1 in preadipocytes drastically inhibits differentiation. For the first time, we show processing of cell-associated pref-1 to generate both a soluble pref-1 protein of approximately 50 kDa that corresponds to the ectodomain and also smaller products of 24 to 25 kDa and 31 kDa. Furthermore, while all four of the alternately spliced forms of pref-1 produce cell-associated protein, only the two largest of the four alternately spliced isoforms undergo cleavage in the juxtamembrane region to release the soluble 50-kDa ectodomain. We demonstrate that addition of Escherichia coli-expressed pref-1 ectodomain to 3T3-L1 preadipocytes blocks differentiation, thus overriding the adipogenic actions of dexamethasone and methylisobutylxanthine. The inhibitory effects of the pref-1 ectodomain are blocked by preincubation of the protein with pref-1 antibody. That the ectodomain alone is sufficient for inhibition demonstrates that transmembrane pref-1 can be processed to generate an inhibitory soluble form, thereby greatly extending its range of action. Furthermore, we present evidence that alternate splicing is the mechanism that governs the production of transmembrane versus soluble pref-1, thereby determining the mode of action, juxtacrine or paracrine, of the pref-1 protein.

FA1 immunoreactivity in endocrine tumours and during development of the human fetal pancreas; negative correlation with glucagon expression

Fetal antigen 1 (FA1) is a glycoprotein containing six epidermal growth factor (EGF)-like repeats. It is closely similar to the protein translated from the human delta-like (dlk) cDNA and probably constitutes a proteolytically processed form of dlk. dlk is homologous to the Drosophila homeotic proteins delta and notch and to the murine preadipocyte differentiation factor Pref-1. These proteins participate in determining cell fate choices during differentiation. We now report that FA1 immunoreactivity is present in a number of neuroectodermally derived tumours as well as in pancreatic endocrine tumours. A negative correlation between FA1 and glucagon immunoreactants in these tumours prompted a reexamination of FA1 immunoreactants during fetal pancreatic development. At the earliest stages of development, FA1 was expressed by most of the non-endocrine parenchymal cells and, with ensuing development, gradually disappeared from these cells and became restricted to insulin-producing beta cells. Throughout development FA1 was not detected in endocrine glucagon, somatostatin or pancreatic polypeptide cells. Moreover, developing insulin cells that coexpressed glucagon were negative for FA1. Thus, there was a negative correlation between FA1 and glucagon both in tumours and during development. These results, together with FA1/dlk's similarity with homeotic proteins, point to a role of FA1 in islet cell differentiation.

CRP-ductin: a gene expressed in intestinal crypts and in pancreatic and hepatic ducts

BACKGROUND

A subtraction screen isolated CRP-ductin (CRP), a gene expressed in intestinal crypts.

METHODS

DNA sequencing, in situ hybridization, immunostaining, Western and Northern blotting were used to characterize murine CRP.

RESULTS

CRP is restricted to the intestine and its associated glands. In the small intestine, CRP mRNA is expressed in crypt cells at all stages of differentiation from the stem cells to the terminally differentiating cells of the crypt top, but not in the mature cells of the villus. In the colon, CRP mRNA is most heavily expressed in the mid-crypt. Expression is also seen in the pancreas and pancreatic ducts, and in the epithelium lining larger hepatic ducts, but not in the liver parenchyma or stomach. CRP protein is localized to the lumenal aspect of crypt cells in the small intestine. In the colon, the protein is seen in the lumenal aspect of surface epithelial cells. CRP protein is similarly found in the lumenal aspect of epithelial cells lining the pancreatic duct system and the larger hepatic ducts. Two cDNA variants, CRP-alpha and CRP-beta, were cloned from mouse jejunal epithelium. Their 3'-sequence differs in an 82-bp domain unique to CRP-beta.

CONCLUSIONS

The CRP-alpha sequence predicts a protein with a short cytoplasmic region, a transmembrane domain, and a large extracellular region composed of many repeats (8 scavenger receptor domains, 5 CUB-domains, 1 ZP-domain, and 6 copies of a previously unreported domain which we call the CRP-domain). The structure of the CRP protein suggests a role in ligand interaction; possible functions are discussed.

Control of adipocyte differentiation

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dlk, pG2 and Pref-1 mRNAs encode similar proteins belonging to the EGF-like superfamily. Identification of polymorphic variants of this RNA

dlk encodes a transmembrane protein member of the EGF-like family of homeotic proteins. dlk is expressed in the same type of neuroendocrine tissues and tumors as pG2, a gene cloned because of its differential expression in human pheochromocytomas versus neuroblastomas. Human dlk and pG2 cDNAs are around 98% similar in sequence, but the predicted proteins encoded by those genes are apparently unrelated. This fact suggested the existence of polymorphic variants of the same gene. We have sequenced again several pG2 and dlk clones in parallel. We identified a pG2 cDNA species corresponding to an alternatively spliced dlk mRNA, as well as several other variant forms of dlk mRNA. One of the pG2 clones resulted to be identical to human dlk and encode the same EGF-like protein. Pref-1, a cDNA isolated from 3T3-L1 fibroblasts, encodes a putative protein possessing an extracellular EGF-like domain similar to dlk, but a different intracellular region. Analysis of sequence data from different clones obtained in our laboratory confirmed some of the differences between dlk and Pref-1. However, the putative difference in the intracellular regions of dlk and Pref-1 was due to sequence artifacts. These data suggest that dlk, pG2 and Pref-1 are variant products of the same gene.