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

Progranulin Oncogenic Network in Solid Tumors

Progranulin is a pleiotropic growth factor with important physiological roles in embryogenesis and maintenance of adult tissue homeostasis. While-progranulin deficiency is associated with a broad range of pathological conditions affecting the brain, such as frontotemporal dementia and neuronal ceroid lipofuscinosis, progranulin upregulation characterizes many tumors, including brain tumors, multiple myeloma, leiomyosarcoma, mesothelioma and epithelial cancers such as ovarian, liver, breast, bladder, adrenal, prostate and kidney carcinomas. The increase of progranulin levels in tumors might have diagnostic and prognostic significance. In cancer, progranulin has a pro-tumorigenic role by promoting cancer cell proliferation, migration, invasiveness, anchorage-independent growth and resistance to chemotherapy. In addition, progranulin regulates the tumor microenvironment, affects the function of cancer-associated fibroblasts, and modulates tumor immune surveillance. However, the molecular mechanisms of progranulin oncogenic function are not fully elucidated. In bladder cancer, progranulin action relies on the activation of its functional signaling receptor EphA2. Notably, more recent data suggest that progranulin can also modulate a functional crosstalk between multiple receptor-tyrosine kinases, demonstrating a more complex and context-dependent role of progranulin in cancer. Here, we will review what is currently known about the function of progranulin in tumors, with a focus on its molecular mechanisms of action and regulation.

Tweaking Progranulin Expression: Therapeutic Avenues and Opportunities

Frontotemporal dementia (FTD) is a neurodegenerative disease, leading to behavioral changes and language difficulties. Heterozygous loss-of-function mutations in progranulin (GRN) induce haploinsufficiency of the protein and are associated with up to one-third of all genetic FTD cases worldwide. While the loss of GRN is primarily associated with neurodegeneration, the biological functions of the secreted growth factor-like protein are more diverse, ranging from wound healing, inflammation, vasculogenesis, and metabolic regulation to tumor cell growth and metastasis. To date, no disease-modifying treatments exist for FTD, but different therapeutic approaches to boost GRN levels in the central nervous system are currently being developed (including AAV-mediated GRN gene delivery as well as anti-SORT1 antibody therapy). In this review, we provide an overview of the multifaceted regulation of GRN levels and the corresponding therapeutic avenues. We discuss the opportunities, advantages, and potential drawbacks of the diverse approaches. Additionally, we highlight the therapeutic potential of elevating GRN levels beyond patients with loss-of-function mutations in GRN.

Lysosomal Dysfunction and Other Pathomechanisms in FTLD: Evidence from Progranulin Genetics and Biology

It has been more than a decade since heterozygous loss-of-function mutations in the progranulin gene (GRN) were first identified as an important genetic cause of frontotemporal lobar degeneration (FTLD). Due to the highly diverse biological functions of the progranulin (PGRN) protein, encoded by GRN, multiple possible disease mechanisms have been proposed. Early work focused on the neurotrophic properties of PGRN and its role in the inflammatory response. However, since the discovery of homozygous GRN mutations in patients with a lysosomal storage disorder, investigation into the possible roles of PGRN and its proteolytic cleavage products granulins, in lysosomal function and dysfunction, has taken center stage. In this chapter, we summarize the GRN mutational spectrum and its associated phenotypes followed by an in-depth discussion on the possible disease mechanisms implicated in FTLD-GRN. We conclude with key outstanding questions which urgently require answers to ensure safe and successful therapy development for GRN mutation carriers.

Abnormally localized DLK1 interacts with NCOR1 in non-small cell lung cancer cell nuclear

Delta-like homolog 1 (DLK1) regulates noncanonical Notch signaling pathway as ligand. DLK1 was abnormally expressed in a variety of tumors, affecting tumorigenesis and developments. The biological function of DLK1 toward cell proliferation and signaling activation was controversial across different cell types. Two currently known isoforms of DLK1, which are membrane-tethered isoform and soluble isoform, are believed to be the key of DLK1 dual behaviors. While these isoforms are not enough to explain the phenomena, our observations offer the possibility of a third isoform of DLK1. In the present study, we verified the nuclear localization of DLK1 in lung cancer cells. The nuclear localized DLK1 was observed in 107 of 351 non-small cell lung cancer (NSCLC) samples and was associated with tissue differentiation and tumor size. Through co-immunoprecipitation (co-IP) combined mass spectrometry (MS), we identified nuclear receptor corepressor 1 (NCOR1) as DLK1's novel interaction protein and confirmed their interaction in nuclear. We analyzed the expression of NCOR1 in two independent cohorts and demonstrated that NCOR1 is a tumor suppressor and has prognosis potential in lung squamous carcinomas. At last, we analyzed the colocalization of DLK1 and NCOR1 in 147 NSCLC samples by immunohistochemistry (IHC). The result indicated NCOR1 might participate with nuclear localized DLK1 in regulating cell differentiation.

Progranulin: A conductor of receptors orchestra, a chaperone of lysosomal enzymes and a therapeutic target for multiple diseases

Progranulin (PGRN), a widely expressed glycoprotein with pleiotropic function, has been linked to a host of physiological processes and diverse pathological states. A series of contemporary preclinical disease models and clinical trials have evaluated various therapeutic strategies targeting PGRN, highlighting PGRN as a promising therapeutic target. Herein we summarize available knowledge of PGRN targeting in various kinds of diseases, including common neurological diseases, inflammatory autoimmune diseases, cancer, tissue repair, and rare lysosomal storage diseases, with a focus on the functional domain-oriented drug development strategies. In particular, we emphasize the role of extracellular PGRN as a non-conventional, extracellular matrix bound, growth factor-like conductor orchestrating multiple membrane receptors and intracellular PGRN as a chaperone/co-chaperone that mediates the folding and traffic of its various binding partners.

DLK proteins modulate NOTCH signaling to influence a brown or white 3T3-L1 adipocyte fate

The role of NOTCH signaling in adipogenesis is highly controversial, with data indicating null, positive or negative effects on this differentiation process. We hypothesize that these contradictory results could be due to the different global NOTCH signaling levels obtained in different experimental settings, because of a specific modulation of NOTCH receptors’ activity by their ligands. We have previously demonstrated that DLK1 and DLK2, two non-canonical NOTCH1 ligands that inhibit NOTCH1 signaling in a dose-dependent manner, modulate the adipogenesis process of 3T3-L1 preadipocytes. In this work, we show that over-expression of any of the four NOTCH receptors enhanced adipogenesis of 3T3-L1 preadipocytes. We also determine that DLK proteins inhibit not only the activity of NOTCH1, but also the activity of NOTCH2, 3 and 4 receptors to different degrees. Interestingly, we have observed, by different approaches, that NOTCH1 over-expression seems to stimulate the differentiation of 3T3-L1 cells towards a brown-like adipocyte phenotype, whereas cells over-expressing NOTCH2, 3 or 4 receptors or DLK proteins would rather differentiate towards a white-like adipocyte phenotype. Finally, our data also demonstrate a complex feed-back mechanism involving Notch and Dlk genes in the regulation of their expression, which suggest that a precise level of global NOTCH expression and NOTCH-dependent transcriptional activity of specific targets could be necessary to determine the final phenotype of 3T3-L1 adipocytes.

The lysosomal function of progranulin, a guardian against neurodegeneration

Progranulin (PGRN), encoded by the GRN gene in humans, is a secreted growth factor implicated in a multitude of processes ranging from regulation of inflammation to wound healing and tumorigenesis. The clinical importance of PGRN became especially evident in 2006, when heterozygous mutations in the GRN gene, resulting in haploinsufficiency, were found to be one of the main causes of frontotemporal lobar degeneration (FTLD). FTLD is a clinically heterogenous disease that results in the progressive atrophy of the frontal and temporal lobes of the brain. Despite significant research, the exact function of PGRN and its mechanistic relationship to FTLD remain unclear. However, growing evidence suggests a role for PGRN in the lysosome-most striking being that homozygous GRN mutation leads to neuronal ceroid lipofuscinosis, a lysosomal storage disease. Since this discovery, several links between PGRN and the lysosome have been established, including the existence of two independent lysosomal trafficking pathways, intralysosomal processing of PGRN into discrete functional peptides, and direct and indirect regulation of lysosomal hydrolases. Here, we summarize the cellular functions of PGRN, its roles in the nervous system, and its link to multiple neurodegenerative diseases, with a particular focus dedicated to recent lysosome-related mechanistic developments.

Progranulin: a new avenue towards the understanding and treatment of neurodegenerative disease

Progranulin, a secreted glycoprotein, is encoded in humans by the single GRN gene. Progranulin consists of seven and a half, tandemly repeated, non-identical copies of the 12 cysteine granulin motif. Many cellular processes and diseases are associated with this unique pleiotropic factor that include, but are not limited to, embryogenesis, tumorigenesis, inflammation, wound repair, neurodegeneration and lysosome function. Haploinsufficiency caused by autosomal dominant mutations within the GRN gene leads to frontotemporal lobar degeneration, a progressive neuronal atrophy that presents in patients as frontotemporal dementia. Frontotemporal dementia is an early onset form of dementia, distinct from Alzheimer's disease. The GRN-related form of frontotemporal lobar dementia is a proteinopathy characterized by the appearance of neuronal inclusions containing ubiquitinated and fragmented TDP-43 (encoded by TARDBP). The neurotrophic and neuro-immunomodulatory properties of progranulin have recently been reported but are still not well understood. Gene delivery of GRN in experimental models of Alzheimer's- and Parkinson's-like diseases inhibits phenotype progression. Here we review what is currently known concerning the molecular function and mechanism of action of progranulin in normal physiological and pathophysiological conditions in both in vitro and in vivo models. The potential therapeutic applications of progranulin in treating neurodegenerative diseases are highlighted.

Human soluble delta-like 1 homolog exerts antitumor effects in vitro and in vivo

Proteolysis of delta-like 1 homolog (DLK1), a cell-surface transmembrane protein, produces an active soluble form of DLK1 (sDLK1). Both membrane-bound DLK1 and sDLK1 modulate multiple developmental processes including adipogenesis, osteogenesis, chondrogenesis and myogenesis. However, cancer-related functions of DLK1 have not yet been established. We thus evaluated the roles of extracellular sDLK1, comprising six EGF-like domains and juxtamembrane regions, in human pancreatic cancer MIA PaCa-2 cells in vitro and in vivo. We observed that sDLK1 exerted antitumor effects not only in cancer cell migration and anchorage-independent cell growth but also in in vivo tumor growth.

The proteins DLK1 and DLK2 modulate NOTCH1-dependent proliferation and oncogenic potential of human SK-MEL-2 melanoma cells

NOTCH receptors regulate cell proliferation and survival in several types of cancer cells. Depending on the cellular context, NOTCH1 can function as an oncogene or as a tumor suppressor gene. DLK1 is also involved in the regulation of cell growth and cancer, but nothing is known about the role of DLK2 in these processes. Recently, the proteins DLK1 and DLK2 have been reported to interact with NOTCH1 and to inhibit NOTCH1 activation and signaling in different cell lines. In this work, we focused on the role of DLK proteins in the control of melanoma cell growth, where NOTCH1 is known to exert an oncogenic effect. We found that human DLK proteins inhibit NOTCH signaling in SK-MEL-2 metastatic melanoma cells. Moreover, the proliferation rate of these cells was dependent upon the level of NOTCH activation and signaling as regulated by DLK proteins. In particular, high levels of NOTCH inhibition resulted in a decrease, whereas lower levels of NOTCH inhibition led to an increase in melanoma cell proliferation rates, both in vitro and in vivo. Finally, our data revealed additive NOTCH-mediated effects of DLK proteins and the γ-secretase inhibitor DAPT on cell proliferation. The data presented in this work suggest that a fine regulation of NOTCH signaling plays an important role in the control of metastatic melanoma cell proliferation. Our results open the way to new research on the role of DLK proteins as potential therapeutic tools for the treatment of human melanoma.

Preadipocytes proliferate and differentiate under the guidance of Delta-like 1 homolog (DLK1)

Obesity occurs when an excessive dietary fat intake leads to expansion of adipose tissue, which mainly consists of adipocytes that arise from proliferating and differentiating adipose stem cells, the preadipocytes. Obesity is a consequence of both adipocyte hypertrophy and hyperplasia. Knowledge about preadipocyte differentiation is relatively well established, whereas the mechanism responsible for preadipocyte proliferation is incompletely understood and only in the early stage of comprehension. In this regard, we have recently identified that Delta-like 1 homolog (Dlk1) (also known as Preadipocyte factor 1 [Pref-1]) inhibits preadipocyte proliferation by regulating their entry into G1/S-phase. This novel disclosure, adding to the previous published data on Dlk1 repression of preadipocyte differentiation, has given us the chance to firmly place Dlk1 as a master regulator of preadipocyte homeostasis and adipose tissue expansion. Dlk1 manipulation may, therefore, open new perspectives in obesity treatments.

Dual role of delta-like 1 homolog (DLK1) in skeletal muscle development and adult muscle regeneration

Muscle development and regeneration is tightly orchestrated by a specific set of myogenic transcription factors. However, factors that regulate these essential myogenic inducers remain poorly described. Here, we show that delta-like 1 homolog (Dlk1), an imprinted gene best known for its ability to inhibit adipogenesis, is a crucial regulator of the myogenic program in skeletal muscle. Dlk1-/- mice were developmentally retarded in their muscle mass and function owing to inhibition of the myogenic program during embryogenesis. Surprisingly however, Dlk1 depletion improves in vitro and in vivo adult skeletal muscle regeneration by substantial enhancement of the myogenic program and muscle function, possibly by means of an increased number of available myogenic precursor cells. By contrast, Dlk1 fails to alter the adipogenic commitment of muscle-derived progenitors in vitro, as well as intramuscular fat deposition during in vivo regeneration. Collectively, our results suggest a novel and surprising dual biological function of DLK1 as an enhancer of muscle development, but as an inhibitor of adult muscle regeneration.

Membrane-tethered delta-like 1 homolog (DLK1) restricts adipose tissue size by inhibiting preadipocyte proliferation

Adipocyte renewal from preadipocytes has been shown to occur throughout life and to contribute to obesity, yet very little is known about the molecular circuits that control preadipocyte expansion. The soluble form of the preadipocyte factor (also known as pref-1) delta-like 1 homolog (DLK1(S)) is known to inhibit adipogenic differentiation; however, the impact of DLK1 isoforms on preadipocyte proliferation remains to be determined. We generated preadipocytes with different levels of DLK1 and examined differentially affected gene pathways, which were functionally tested in vitro and confirmed in vivo. Here, we demonstrate for the first time that only membrane-bound DLK1 (DLK1(M)) exhibits a substantial repression effect on preadipocyte proliferation. Thus, by independently manipulating DLK1 isoform levels, we established that DLK1(M) inhibits G1-to-S-phase cell cycle progression and thereby strongly inhibits preadipocyte proliferation in vitro. Adult DLK1-null mice exhibit higher fat amounts than wild-type controls, and our in vivo analysis demonstrates that this may be explained by a marked increase in preadipocyte replication. Together, these data imply a major dual inhibitory function of DLK1 on adipogenesis, which places DLK1 as a master regulator of preadipocyte homeostasis, suggesting that DLK1 manipulation may open new avenues in obesity treatment.

The effects of increased expression of DLK1 gene on the pathogenesis of myelodysplastic syndromes

To study the potential role of Delta-like-1 (DLK1) in myelodysplastic syndromes (MDS), we carried out a series of experiments and found that DLK1 mRNA levels are dysregulated in patients with MDS or acute myelogenous leukemia (AML), and its overexpression leads to dysfunction of 32D and 3T3 cells. We conclude that DLK1 dysfunction may contribute to abnormal hematopoiesis of MDS and may be 1 of the antioncogenes. Delta-like-1 (DLK1) is frequently expressed at elevated levels in CD34(+) cells from patients with MDS. To investigate its role in the pathogenesis of MDS, we tested bone marrow samples from a panel of patients with MDS, AML, or myeloproliferative neoplasms, with real-time polymerase chain reaction (PCR). We show here that DLK1 mRNA levels are higher in MDS patients and lower in AML patients than in healthy individuals. Myeloid progenitor 32D cells overexpressing DLK1 display increased apoptosis, reduced differentiation, and decreased cell number expansion, which is also accompanied by changes in cell cycle progression. Immortalized fibroblastic 3T3 cells can grow into tumors in nude mice but the size of tumors are smaller from those overexpressing DLK1. These observations suggest that DLK1 dysfunction may contribute to the ineffective hematopoiesis of MDS.

Genome-wide analysis of a Wnt1-regulated transcriptional network implicates neurodegenerative pathways

Wnt proteins are critical to mammalian brain development and function. The canonical Wnt signaling pathway involves the stabilization and nuclear translocation of β-catenin; however, Wnt also signals through alternative, noncanonical pathways. To gain a systems-level, genome-wide view of Wnt signaling, we analyzed Wnt1-stimulated changes in gene expression by transcriptional microarray analysis in cultured human neural progenitor (hNP) cells at multiple time points over a 72-hour time course. We observed a widespread oscillatory-like pattern of changes in gene expression, involving components of both the canonical and the noncanonical Wnt signaling pathways. A higher-order, systems-level analysis that combined independent component analysis, waveform analysis, and mutual information-based network construction revealed effects on pathways related to cell death and neurodegenerative disease. Wnt effectors were tightly clustered with presenilin1 (PSEN1) and granulin (GRN), which cause dominantly inherited forms of Alzheimer's disease and frontotemporal dementia (FTD), respectively. We further explored a potential link between Wnt1 and GRN and found that Wnt1 decreased GRN expression by hNPs. Conversely, GRN knockdown increased WNT1 expression, demonstrating that Wnt and GRN reciprocally regulate each other. Finally, we provided in vivo validation of the in vitro findings by analyzing gene expression data from individuals with FTD. These unbiased and genome-wide analyses provide evidence for a connection between Wnt signaling and the transcriptional regulation of neurodegenerative disease genes.

Delta-like 1/fetal antigen-1 (Dlk1/FA1) is a novel regulator of chondrogenic cell differentiation via inhibition of the Akt kinase-dependent pathway

Delta-like 1 (Dlk1, also known as fetal antigen-1, FA1) is a member of Notch/Delta family that inhibits adipocyte and osteoblast differentiation; however, its role in chondrogenesis is still not clear. Thus, we overexpressed Dlk1/FA1 in mouse embryonic ATDC5 cells and tested its effects on chondrogenic differentiation. Dlk1/FA1 inhibited insulin-induced chondrogenic differentiation as evidenced by reduction of cartilage nodule formation and gene expression of aggrecan, collagen Type II and X. Similar effects were obtained either by using Dlk1/FA1-conditioned medium or by addition of a purified, secreted, form of Dlk1 (FA1) directly to the induction medium. The inhibitory effects of Dlk1/FA1 were dose-dependent and occurred irrespective of the chondrogenic differentiation stage: proliferation, differentiation, maturation, or hypertrophic conversion. Overexpression or addition of the Dlk1/FA1 protein to the medium strongly inhibited the activation of Akt, but not the ERK1/2, or p38 MAPK pathways, and the inhibition of Akt by Dlk1/FA1 was mediated through PI3K activation. Interestingly, inhibition of fibronectin expression by siRNA rescued the Dlk1/FA1-mediated inhibition of Akt, suggesting interaction of Dlk1/FA1 and fibronectin in chondrogenic cells. Our results identify Dlk1/FA1 as a novel regulator of chondrogenesis and suggest Dlk1/FA1 acts as an inhibitor of the PI3K/Akt pathways that leads to its inhibitory effects on chondrogenesis.

Cellular effects of progranulin in health and disease

Progranulin is a fascinating multifunctional protein, which has been implicated in cell growth, wound repair, tumorigenesis, inflammation, neurodevelopment, and more recently in neurodegeneration. The mechanism of action of this protein is still largely unknown, but the knowledge about the cellular effects on various cell types is expanding. In the current review, we will summarize what is known about the cell biology of progranulin. A better understanding of the biology of progranulin will impact diverse areas of research.

MicroRNA-15a fine-tunes the level of Delta-like 1 homolog (DLK1) in proliferating 3T3-L1 preadipocytes

Delta like 1 homolog (Dlk1) exists in both transmembrane and soluble molecular forms, and is implicated in cellular growth and plays multiple roles in development, tissue regeneration, and cancer. Thus, DLK1 levels are critical for cell function, and abnormal DLK1 expression can be lethal; however, little is known about the underlying mechanisms. We here report that miR-15a modulates DLK1 levels in preadipocytes thus providing a mechanism for DLK1 regulation that further links it to cell cycle arrest and cancer since miR-15a is deregulated in these processes. In preadipocytes, miR-15a increases with cell density, and peaks at the same stage where membrane DLK1(M) and soluble DLK1(S) are found at maximum levels. Remarkably, miR-15a represses the amount of all Dlk1 variants at the mRNA level but also the level of DLK1(M) protein while it increases the amount of DLK1(S) supporting a direct repression of DLK1 and a parallel effect on the protease that cleaves off the DLK1 from the membrane. In agreement with previous studies, we found that miR-15a represses cell numbers, but additionally, we report that miR-15a also increases cell size. Conversely, anti-miR-15a treatment decreases cell size while increasing cell numbers, scenarios that were completely rescued by addition of purified DLK1(S). Our data thus imply that miR-15a regulates cell size and proliferation by fine-tuning Dlk1 among others, and further emphasize miR-15a and DLK1 levels to play important roles in growth signaling networks.

A novel regulatory mechanism for Fgf18 signaling involving cysteine-rich FGF receptor (Cfr) and delta-like protein (Dlk)

Fibroblast growth factors (FGFs) transduce signals through FGF receptors (FGFRs) and have pleiotropic functions. Besides signal-transducing FGFRs, cysteine-rich FGF receptor (Cfr; Glg1) is also known to bind some FGFs, although its physiological functions remain unknown. In this study, we generated Cfr-deficient mice and found that some of them die perinatally, and show growth retardation, tail malformation and cleft palate. These phenotypes are strikingly similar to those of Fgf18-deficient mice, and we revealed interaction between Cfr and Fgf18 both genetically and physically, suggesting functional cooperation. Consistently, introduction of Cfr facilitated Fgf18-dependent proliferation of Ba/F3 cells expressing Fgfr3c. In addition, we uncovered binding between Cfr and delta-like protein (Dlk), and noticed that Cfr-deficient mice are also similar to Dlk-transgenic mice, indicating that Cfr and Dlk function in opposite ways. Interestingly, we also found that Dlk interrupts the binding between Cfr and Fgf18. Thus, the Fgf18 signaling pathway seems to be finely tuned by Cfr and Dlk for skeletal development. This study reveals a novel regulatory mechanism for Fgf18 signaling involving Cfr and Dlk.

The granulin gene family: from cancer to dementia

The growth factor progranulin (PGRN) regulates cell division, survival, and migration. PGRN is an extracellular glycoprotein bearing multiple copies of the cysteine-rich granulin motif. With PGRN family members in plants and slime mold, it represents one of the most ancient of the extracellular regulatory proteins still extant in modern animals. PRGN has multiple biological roles. It contributes to the regulation of early embryogenesis, to adult tissue repair and inflammation. Elevated PGRN levels often occur in cancers, and PGRN immunotherapy inhibits the growth of hepatic cancer xenografts in mice. Recent studies have demonstrated roles for PGRN in neurobiology. An autosomal dominant mutation in GRN, the gene for PGRN, leads to neuronal atrophy in the frontal and temporal lobes, resulting in the disease frontotemporal lobar dementia. In this review we will discuss current knowledge of the multifaceted biology of PGRN.

An anti-DLK1 monoclonal antibody produced using ELISA and hybridoma techniques

DLK1 is a newly identified prognostic factor associated with liver cancer survival. To prepare specific monoclonal antibody (MAb) against DLK1, cDNA of DLK1 was cloned by RT-PCR and inserted into prokaryotic expression vector pGEX-4T1, respectively. The fusion proteins were expressed in Escherichia coli. Monoclonal antibody against DLK1 was obtained with hybridoma technique and specific ELISA screening. Western blotting and immunohistochemistry assays showed that MAb 6D6 had specific binding ability with DLK1 protein in eukaryotic cells and cancer tissues. This MAb will be a helpful tool for the detection of DLK1 protein in the tissues and serum of liver cancer and other cancer patients.

A role for notch signaling in stromal survival and differentiation during prostate development

Notch1 signaling is involved in epithelial growth and differentiation of prostate epithelia, and we have examined the role that notch signaling plays in the stroma of the developing prostate. We initially observed expression of delta-like 1 (Dlk1) and Notch2 in gene profiling studies of prostatic mesenchyme, and anticipated that they might be expressed in a key subset of inductive mesenchyme. Using quantitative RT-PCR, Northern blotting, and whole mount in situ hybridization, we confirmed that both Dlk1 and Notch2 mRNAs showed a restricted expression pattern within subsets of the stroma during prostate development. Localization of Dlk1 and Notch2 proteins mirrored the transcript expression, and showed both distinct and overlapping expression patterns within the stroma. Dlk1 and Notch2 were coexpressed in condensed inductive mesenchyme of the ventral mesenchymal pad (VMP), and were partially colocalized in the smooth muscle (SM) layer of the urethral stroma. In addition, Dlk1 was not expressed in SM adjacent to the VMP in female urethra. The function of notch signaling was examined using organ cultures of prostate rudiments and a small molecule inhibitor of notch receptor activity. Inhibition of notch signaling led to a loss of stromal tissue in both prostate and female VMP cultures, suggesting that this pathway was required for stromal survival. Inhibition of notch signaling also led to changes in both epithelial and stromal differentiation, which was evident in altered distributions of SM alpha-actin and p63 in prostates grown in vitro. The effects of notch signaling upon the stroma were only evident in the presence of testosterone, in contrast to effects upon epithelial differentiation.

Promoter hypermethylation of the MEG3 (DLK1/MEG3) imprinted gene in multiple myeloma

BACKGROUND

Methylation represents the most studied epigenetic modification and results in the silencing of genes involved in various processes such as differentiation and cell-cycle regulation. MEG3 represents an imprinted gene maternally expressed in humans that encodes a nontranslated product. In this survey, we studied the methylation status of the specific gene in multiple myeloma (MM).

PATIENTS AND METHODS

Twenty-one patients with MM (17 with immunoglobulin [Ig] G, 3 with IgA, and 1 with IgM) were evaluated using methylation-specific polymerase chain reaction (after DNA bisulphite modification).

RESULTS

Promoter hypermethylation was observed in 12 (57.14%) bone marrow samples and in 9 of 14 (64.28%) available peripheral blood samples. A correlation with disease stage was also observed and also with the disease subtype (IgG, 64.7%; IgA, 0; IgM, 100%).

CONCLUSION

We conclude that promoter hypermethylation of the differentially methylated region of the MEG3 imprinted gene is observed in patients with MM.

The serum levels of the EGF-like homeotic protein dlk1 correlate with different metabolic parameters in two hormonally different children populations in Spain

BACKGROUND

The Dlk1 gene encodes for dlk1, a transmembrane protein belonging to the EGF-like repeat-containing family. Dlk1 has been shown to act as a regulator of adipogenesis. Fc-dlk1 transgenic mice show a decrease in adipose tissue and glucose tolerance, hypertriglyceridaemia and lower insulin sensitivity. Dlk1-deficient mice show growth retardation, increased serum lipid metabolites and develop obesity. These data advocate for a role of dlk1 in the maintenance of lipid homeostasis, and suggest that dlk1 levels may influence the development of cardiovascular disease.

AIM AND METHODS

In this study, we analysed whether dlk1 serum levels could be indicative of the different hormonal or metabolic status shown by two Spanish children populations (6-8 years-old), Orense and Murcia. We determined dlk1 serum levels by ELISA assay, using an antibody raised against the recombinant protein, and performed a correlation analysis against measurements of several hormonal and biochemical parameters in samples from 494 subjects.

RESULTS

We found a statistically significant positive correlation between serum levels of dlk1 and those of glucose (P < 0.05), total cholesterol (P < 0.01) and high-density lipoprotein-cholesterol (HDL-C) (P < 0.01) in children from Murcia, but not from Orense's population, where dehydroepiandrosterone-sulphate (DHEA-S) levels were significantly higher (P < 0.01) and dlk1 correlated positively with insulin (P < 0.01), homeostasis model assessment (HOMA) (P < 0.01) and free fatty acids (FFA) (P < 0.05).

CONCLUSIONS

dlk1 serum levels appear related to the anabolic status of the children in association with changes in the levels of DHEA-S, which have been associated with hyperinsulinaemia and diabetes. Monitoring dlk1 levels may be important to evaluate the metabolic and hormonal stage of child development.

Progenitor gene DLK1 might be an independent prognostic factor of liver cancer

BACKGROUND

Delta-like 1 homolog (DLK1) is a marker for progenitor cells of the liver. The gene encoding DLK1 is expressed early during embryonic development but, importantly, it is also expressed in some human liver cancers. However, the prognostic value of the DLK1 gene has not been investigated.

OBJECTIVES

To examine the association between the DLK1 gene and survival time and whether high levels of expression of DLK1 are a prognostic factor for liver cancer.

METHODS

We evaluated 60 cases of primary liver cancer, and investigated the link between the expression of DLK1 and patient survival. Clinical characteristics of the cases used for our study, such as tumor size, differentiation and staging, are statistically evenly distributed. Using RT-PCR, western blotting and immunohistochemistry, we analyzed the expression of DLK1 in the tumor samples and evaluated the results statistically.

RESULTS

DLK1 was expressed in 22 of the 60 cases (36.7%), and analysis of the survival of the patients revealed that DLK1-positive patients had a shorter survival time than DLK1-negative patients. Cox regression analysis also showed that DLK1 is a risk factor. However, DLK1 expression does not seem to correlate with other classic prognostic factors such as alpha-fetoprotein (AFP), tumor-node-metastasis (TNM) and vascular invasion, which implies that it is an independent prognostic factor.

The EGF-like Protein dlk1 Inhibits Notch Signaling and Potentiates Adipogenesis of Mesenchymal Cells

The EGF-like homeotic gene Dlk1 appears to function as an inhibitor of adipogenesis. Overexpression of Dlk1 prevents adipogenesis of 3T3-L1 cells. Dlk1-deficient mice are obese; however, adipose tissue still develops in Fc-dlk1 transgenic mice, suggesting that Dlk1 is not a strict inhibitor of adipogenesis. To clarify the role of Dlk1 in adipogenesis, we studied whether Dlk1 could act differently on this process depending upon the differentiation state of the precursor cells. We found that Dlk1 is a potentiator of adipogenesis for mesenchymal C3H10T1/2 cells. This potentiating effect can be triggered by overexpressing the entire protein or the extracellular EGF-like-containing region, but not by overexpressing the intracellular dlk1 sequence. In addition, coculture of C3H10T1/2 cells with other cells expressing Dlk1, but not with cells lacking Dlk1 expression, enhances their adipogenic response. Potentiation of adipogenesis by Dlk1 was associated with changes in the activation of ERK1/2 after IGFI/insulin induction. Finally, as reported with other cells, dlk1 functioned as a Notch signaling inhibitor in C3H10T1/2 cells, but inhibition of Notch1 expression prevented the potentiating effects of Dlk1 in adipogenesis. These data suggest that Dlk1 may potentiate or inhibit adipogenesis depending upon the cellular context, and that Notch1 expression and activation are important factors in this context.

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.

Identification of a lipase-linked cell membrane receptor for pigment epithelium-derived factor

Pigment epithelium-derived factor (PEDF) is an extracellular multifunctional protein belonging to the serpin superfamily with demonstrable neurotrophic, gliastatic, neuronotrophic, antiangiogenic, and antitumorigenic properties. We have previously provided biochemical evidence for high affinity PEDF-binding sites and proteins in plasma membranes of retina, retinoblastoma, and CNS cells. This study was designed to reveal a receptor involved in the biological activities of PEDF. Using a yeast two-hybrid screening, we identified a novel gene from pigment epithelium of the human retina that codes for a PEDF-binding partner, which we term PEDF-R. The derived polypeptide has putative transmembrane, intracellular and extracellular regions, and a phospholipase domain. Recently, PEDF-R (TTS-2.2/independent phospholipase A(2) (PLA(2))zeta and mouse desnutrin/ATGL) has been described in adipose cells as a member of the new calcium-independent PLA(2)/nutrin/patatin-like phospholipase domain-containing 2 (PNPLA2) family that possesses triglyceride lipase and acylglycerol transacylase activities. Here we describe the PEDF-R gene expression in the retina and its heterologous expression by bacterial and eukaryotic systems, and we demonstrate that its protein product has specific and high binding affinity for PEDF, has a potent phospholipase A(2) activity that liberates fatty acids, and is associated with eukaryotic cell membranes. Most importantly, PEDF binding stimulates the enzymatic phospholipase A(2) activity of PEDF-R. In conclusion, we have identified a novel PEDF-R gene in the retina for a phospholipase-linked membrane protein with high affinity for PEDF, suggesting a molecular pathway by which ligand/receptor interaction on the cell surface could generate a cellular signal.

Transcriptomic and genomic analysis of human hepatocellular carcinomas and hepatoblastomas

This study analyzed gene expression patterns and global genomic alterations in hepatocellular carcinomas (HCC), hepatoblastomas (HPBL), tissue adjacent to HCC and normal liver tissue derived from normal livers and hepatic resections. We found that HCC and adjacent non-neoplastic cirrhotic tissue have considerable overlap in gene expression patterns compared to normal liver. Several genes including Glypican 3, spondin-2, PEG10, EDIL3 and Osteopontin are over-expressed in HCC vs. adjacent tissue whereas Ficolin 3 is the most consistently under-expressed gene. HCC can be subdivided into three clusters based on gene expression patterns. HCC and HPBL have clearly different patterns of gene expression, with genes IGF2, Fibronectin, DLK1, TGFb1, MALAT1 and MIG6 being over-expressed in HPBL versus HCC. In addition, specific areas of the genome appear unstable in HCC, with the same regions undergoing either deletion or increased gene dosage in all HCC. In conclusion, a set of specific genes and areas of genomic instability are found across the board in liver neoplasia.

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.

Is GAS1 a co-receptor for the GDNF family of ligands?

Glial-cell-line-derived neurotrophic factor (GDNF) is a survival and maintenance factor for dopamine-containing neurons and motoneurons. GDNF belongs to a family of structurally related factors that includes neurturin (NRTN), artemin (ARTN) and persephin (PSPN). An initial step in the activation of signaling via the GDNF family of ligands (GFLs) is their binding to their cognate co-receptor GFR alpha. GAS1, an apparently unrelated protein, exhibits homology to GFR alpha and thus we hypothesize that GAS1 can serve as an alternative receptor for GFLs. The functional similarity between GFR alpha and GAS1 extends to their role in embryogenesis, differentiation and glia maintenance, and is substantiated by overlap in their expression profile, subcellular localization and structural details. We propose that the relative expression and localization of the two remote receptors, GFR alpha and GAS1, on the membranes of neuronal and glial cells determines whether these cells survive or undergo apoptotic death.

Expression of DLK1 in hematopoietic cells results in inhibition of differentiation and proliferation

The Delta-like (DLK1) gene is overexpressed in CD34+ cells from myelodysplastic syndrome (MDS) patients. DLK1 encodes an EGF-like homeotic transmembrane protein homologous to the notch/delta/serrate family. Although exogenous DLK1 promotes maintenance of murine hematopoietic stem cells, the functional effects of DLK1 overexpression in hematopoietic cells are unknown. We show that ectopically expressed DLK1 significantly inhibits differentiation and proliferation of human promyelocytic HL-60 cells. Unlike preadipocytes, where proteolytic processing of membrane-bound protein and release of a soluble form mediates differentiation inhibition, proteolytic release of the extracellular domain was not required for inhibition of hematopoietic cell differentiation. However, intracellular domain interactions were critical to this DLK1 function. We conclude that DLK1 overexpression in hematopoietic cells has important functional consequences. Our studies identify novel molecular mechanisms and indicate that DLK1 has activity both as a soluble and a transmembrane expressed protein. Our results support further investigation of the role of DLK1 in abnormal hematopoiesis in MDS.

Differential expression of delta-like gene and protein in neuroblastoma, ganglioneuroblastoma and ganglioneuroma

Neuroblastoma is an extremely malignant solid tumor in children, characterized by spontaneous differentiation and regression. An epidermal growth factor-like homeotic protein, delta-like (dlk), has been involved in differentiation of neuroblastoma cell lines, but is unknown in in vivo expression of neuroblastoma. By using in situ hybridization and immunohistochemistry, dlk mRNA and protein expression were studied in formalin-fixed archival tissues from 10 patients with neuroblastoma, five with ganglioneuroblastoma, and five with ganglioneuroma. Three adrenal tissues from children died of diseases other than adrenal tumors and one from an adult with pheochromocytoma were severed as normal and disease controls. The results showed strong immunoreactive dlk staining in endothelial cells in neuroblastoma, ganglioneuroblastoma and ganglioneuroma. Dlk was detectable in mature neuromatous stroma and gangliocytes of ganglioneuroma, but not in neuroblasts of neuroblastoma and ganglioneuroblastoma, neither in gangliocytes of ganglioneuroblastoma. In contrast, dlk mRNA expression was mainly observed in the gangliocytes, but was less intense in the neuroblasts and neuromatous stroma cells. Endothelial cells were essentially devoid of dlk mRNA expression. The findings indicated that there is differential expression of dlk gene and protein among neuroblastoma, ganglioneuroblastoma and ganglioneuroma. The stronger expression of dlk in gangliocytes in ganglioneuroma, in contrast to weaker or no expression in gangliocytes in ganglioneuroblastoma and neuroblasts in neuroblastoma, suggests upregulation of dlk during differentiation of neuroblastoma into more benign form. Furthermore, higher dlk protein expression in the tumor endothelium than in the endothelium of normal adrenal gland implies that dlk may regulate the endothelial function in neuroblastic tumors.

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.

Progranulin (granulin-epithelin precursor, PC-cell-derived growth factor, acrogranin) mediates tissue repair and tumorigenesis

Progranulin (Pgrn) is a pluripotent secreted growth factor that mediates cell cycle progression and cell motility. It activates the extracellular regulated kinases and phosphatidyl inositol-3 kinase signal cascades, among others, and increases expression of cyclins D and B. Structurally, it belongs to none of the well-established growth factor families. It regulates developmental events as diverse as the onset of cavitation in the preimplantation embryo and male-specific brain differentiation. During wound repair it promotes granulation and neovascularization. It regulates inflammation through a tripartite loop with secretory leukocyte protease inhibitor (SLPI) which protects pgrn from proteolysis, and elastase, which digests it to smaller peptides. Intact pgrn is anti-inflammatory through the inhibition of some of the actions of tumor necrosis factor, while the proteolytic peptides may stimulate the production of proinflammatory cytokines such as interleukin 8. Pgrn is highly expressed in aggressive cancer cell lines and clinical specimens including breast, ovarian, and renal cancers as well as gliomas. In experimental systems it confers an aggressive phenotype on poorly tumorigenic epithelial cancer cells. The malignancy of highly tumorigenic progranulin-expressing cell lines depends on the expression level of the pgrn gene since attenuating pgrn mRNA levels in pgrn-responsive cells greatly inhibits tumor progression. Given its actions in wound repair and tumorigenesis pgrn may prove a useful clinical target, both for prognosis and for therapy.

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.