Identification of DLK1 variants in pituitary- and neuroendocrine tumors

Targeting the Notch signaling pathway and the Notch ligand, DLL3, in small cell lung cancer

Small cell lung cancer (SCLC) is a highly aggressive and poorly differentiated cancer with high-grade neuroendocrine (NE) features, accounting for approximately 15 % of all lung cancers. For decades, chemotherapy and radiotherapy have predominated the treatment strategy for SCLC, but relapses ensue quickly and result in poor survival of patients. Immunotherapy has brought novel insights, yet the efficacy is still restricted to a limited population with SCLC. Notch signaling is identified to play a key role in the initiation and development of SCLC, and the Notch ligand, Delta-like ligand 3 (DLL3) is found broadly and specifically expressed in SCLC cells. Thus, Notch signaling is under active exploration as a potential therapeutic target in SCLC. Herein, we summarized and updated the functional relevance of Notch signaling in SCLC, discussed Notch signaling-targeted therapy for SCLC and the correspondent preclinical and clinical trials, and investigated the promising synergy effects of Notch signaling targeted therapy and immune checkpoint inhibitors (ICIs) treatment.

Emerging Roles of DLK1 in the Stem Cell Niche and Cancer Stemness

DLK1 is a maternally imprinted, paternally expressed gene coding for the transmembrane protein Delta-like homologue 1 (DLK1), a non-canonical NOTCH ligand with well-described roles during development, and tumor-supportive functions in several aggressive cancer forms. Here, we review the many functions of DLK1 as a regulator of stem cell pools and tissue differentiation in tissues such as brain, muscle, and liver. Furthermore, we review recent evidence supporting roles for DLK1 in the maintenance of aggressive stem cell characteristics of tumor cells, specifically focusing on central nervous system tumors, neuroblastoma, and hepatocellular carcinoma. We discuss NOTCH -dependent as well as NOTCH-independent functions of DLK1, and focus particularly on the complex pattern of DLK1 expression and cleavage that is finely regulated from a spatial and temporal perspective. Progress in recent years suggest differential functions of extracellular, soluble DLK1 as a paracrine stem cell niche-secreted factor, and has revealed a role for the intracellular domain of DLK1 in cell signaling and tumor stemness. A better understanding of DLK1 regulation and signaling may enable therapeutic targeting of cancer stemness by interfering with DLK1 release and/or intracellular signaling.

Insights into Mechanisms of Tumorigenesis in Neuroendocrine Neoplasms

Genomic studies have identified some of the most relevant genetic players in Neuroendocrine Neoplasm (NEN) tumorigenesis. However, we are still far from being able to draw a model that encompasses their heterogeneity, elucidates the different biological effects consequent to the identified molecular events, or incorporates extensive knowledge of molecular biomarkers and therapeutic targets. Here, we reviewed recent insights in NEN tumorigenesis from selected basic research studies on animal models, highlighting novel players in the intergenic cooperation and peculiar mechanisms including splicing dysregulation, chromatin stability, or cell dedifferentiation. Furthermore, models of tumorigenesis based on composite interactions other than a linear progression of events are proposed, exemplified by the involvement in NEN tumorigenesis of genes regulating complex functions, such as MEN1 or DAXX. Although limited by interspecies differences, animal models have proved helpful for the more in-depth study of every facet of tumorigenesis, showing that the identification of driver mutations is only one of the many necessary steps and that other mechanisms are worth investigating.

Gonadotroph tumours with a low SF-1 labelling index are more likely to recur and are associated with enrichment of the PI3K-AKT pathway

AIMS

The gonadotroph tumour (GT) is the most frequently resected pituitary neuroendocrine tumour. Although many symptomatic GT are successfully resected, some recur. We sought to identify histological biomarkers that may predict recurrence and explore biological mechanisms that explain this difference in behaviour.

METHODS

SF-1 immunohistochemistry of 51 GT, a subset belonging to a longitudinal prospective cohort study (n = 25), was reviewed. Four groups were defined: Group 1-recently diagnosed GT (n = 20), Group 2-non-recurrent GT with long-term follow up (n = 11), Group 3-initial resections of GT that recur (n = 7) and Group 4-recurrent GT (n = 13). The percentage of SF-1 immunolabelling in the lowest staining fields (SF-1 labelling index (SLI)) was assessed and RNA sequencing was performed on 5 GT with SLI <80% and 5 GT with SLI >80%.

RESULTS

Diffuse, strong SF-1 immunolabelling was the most frequent pattern in Groups 1/2, whereas patchy SF-1 staining predominated in Groups 3/4. There was a lower median SLI in Groups 3/4 than 1/2. Overall, GT with SLI <80% recurred earlier than GT with SLI >80%. Differential expression analysis identified 89 statistically significant differentially expressed genes (FDR <0.05) including over-expression of pituitary stem cell genes (SOX2, GFRA3) and various oncogenes (e.g. BCL2, ERRB4) in patchy SF-1 GT. Gene set enrichment analysis identified significant enrichment of genes involved in the PI3K-AKT pathway.

CONCLUSIONS

We speculate that patchy SF-1 labelling in GT reflects intratumoural heterogeneity and are less differentiated tumours than diffusely staining GT. SF-1 immunolabelling patterns may have prognostic significance in GT, but confirmatory studies are needed for further validation.

Splicing Machinery is Dysregulated in Pituitary Neuroendocrine Tumors and is Associated with Aggressiveness Features

Pituitary neuroendocrine tumors (PitNETs) constitute approximately 15% of all brain tumors, and most have a sporadic origin. Recent studies suggest that altered alternative splicing and, consequently, appearance of aberrant splicing variants, is a common feature of most tumor pathologies. Moreover, spliceosome is considered an attractive therapeutic target in tumor pathologies, and the inhibition of SF3B1 (e.g., using pladienolide-B) has been shown to exert antitumor effects. Therefore, we aimed to analyze the expression levels of selected splicing-machinery components in 261 PitNETs (somatotropinomas/non-functioning PitNETS/corticotropinomas/prolactinomas) and evaluated the direct effects of pladienolide-B in cell proliferation/viability/hormone secretion in human PitNETs cell cultures and pituitary cell lines (AtT-20/GH3). Results revealed a severe dysregulation of splicing-machinery components in all the PitNET subtypes compared to normal pituitaries and a unique fingerprint of splicing-machinery components that accurately discriminate between normal and tumor tissue in each PitNET subtype. Moreover, expression of specific components was associated with key clinical parameters. Interestingly, certain components were commonly dysregulated throughout all PitNET subtypes. Finally, pladienolide-B reduced cell proliferation/viability/hormone secretion in PitNET cell cultures and cell lines. Altogether, our data demonstrate a drastic dysregulation of the splicing-machinery in PitNETs that might be associated to their tumorigenesis, paving the way to explore the use of specific splicing-machinery components as novel diagnostic/prognostic and therapeutic targets in PitNETs.

Identification of Important Invasion-Related Genes in Non-functional Pituitary Adenomas

Non-functioning pituitary adenomas (NFPAs) are locally invasive with high morbidity. The objective of this study was to diagnose important genes and pathways related to the invasiveness of NFPAs and gain more insights into the underlying molecular mechanisms of NFPAs. The gene expression profiles of GSE51618 were downloaded from the Gene Expression Omnibus database with 4 non-invasive NFPA samples, 3 invasive NFPA samples, and 3 normal pituitary gland samples. Differentially expressed genes (DEGs) are screened between invasive NFPA samples and normal pituitary gland samples, followed by pathway and ontology (GO) enrichment analyses. Subsequently, a protein-protein interaction (PPI) network was constructed and analyzed for these DEGs, and module analysis was performed. In addition, a target gene-miRNA network and target gene-TF (transcription factor) network were analyzed for these DEGs. A total of 879 DEGs were obtained. Among them, 439 genes were upregulated and 440 genes were downregulated. Pathway enrichment analysis indicated that the upregulated genes were significantly enriched in cysteine biosynthesis/homocysteine degradation (trans-sulfuration) and PI3K-Akt signaling pathway, while the downregulated genes were mainly associated with docosahexaenoate biosynthesis III (mammals) and chemokine signaling pathway. GO enrichment analysis indicated that the upregulated genes were significantly enriched in animal organ morphogenesis, extracellular matrix, and hormone activity, while the downregulated genes were mainly associated with leukocyte chemotaxis, dendrites, and RAGE receptor binding. Subsequently, ESR1, SOX2, TTN, GFAP, WIF1, TTR, XIST, SPAG5, PPBP, AR, IL1R2, and HIST1H1C were diagnosed as the top hub genes in the upregulated and downregulated PPI networks and modules. In addition, HS3ST1, GPC4, CCND2, and SCD were diagnosed as the top hub genes in the upregulated and downregulated target gene-miRNA networks, while CISH, ISLR, UBE2E3, and CCNG2 were diagnosed as the top hub genes in the upregulated and downregulated target gene-TF networks. The new important DEGs and pathways diagnosed in this study may serve key roles in the invasiveness of NFPAs and indicate more molecular targets for the treatment of NFPAs.

Identification of differentially coexpressed genes in gonadotrope tumors and normal pituitary using bioinformatics methods

To investigate the underlying molecular mechanisms of pituitary tumor by using the microarray expression profiles of pituitary tumor and normal tissue samples. The gene expression profile of GSE26966 was downloaded from Gene Expression Omnibus, including nine normal samples and 14 pituitary tumor samples. The differentially coexpressed genes (DEGs) were identified by Affy package in R Software. The functional and pathway enrichment analysis of the screened DEGs were performed by DAVID. Then, differential coexpression networks were contructed and further analyzed. Functional and pathway enrichment analysis of the 1220 identified DEGs revealed that phosphatidylinositol signaling system, p53 signaling pathway and inositol phosphate metabolism were disturbed in pituitary tumors. The degree of DLK1, CDKN2A and ITGA4 in the constructed differential coexpression network was 46, 45 and 44, respectively. In addition, MPP2 and ASAP2 were the obvious hub genes in the constructed differential coexpression network. Through exploring genes in the differential coexpression networks, the results suggested that DLK1, CDKN2A, ITGA4, MPP2 and ASAP2 may potentially be used as biomarkers for pituitary tumor.

Transcriptome analysis of MENX-associated rat pituitary adenomas identifies novel molecular mechanisms involved in the pathogenesis of human pituitary gonadotroph adenomas

Gonadotroph adenomas comprise 15-40% of all pituitary tumors, are usually non-functioning and are often large and invasive at presentation. Surgery is the first-choice treatment, but complete resection is not always achieved, leading to high recurrence rates. As gonadotroph adenomas poorly respond to conventional pharmacological therapies, novel treatment strategies are needed. Their identification has been hampered by our incomplete understanding of the molecular pathogenesis of these tumors. Recently, we demonstrated that MENX-affected rats develop gonadotroph adenomas closely resembling their human counterparts. To discover new genes/pathways involved in gonadotroph cells tumorigenesis, we performed transcriptome profiling of rat tumors versus normal pituitary. Adenomas showed overrepresentation of genes involved in cell cycle, development, cell differentiation/proliferation, and lipid metabolism. Bioinformatic analysis identified downstream targets of the transcription factor SF-1 as being up-regulated in rat (and human) adenomas. Meta-analyses demonstrated remarkable similarities between gonadotroph adenomas in rats and humans, and highlighted common dysregulated genes, several of which were not previously implicated in pituitary tumorigenesis. Two such genes, CYP11A1 and NUSAP1, were analyzed in 39 human gonadotroph adenomas by qRT-PCR and found to be up-regulated in 77 and 95% of cases, respectively. Immunohistochemistry detected high P450scc (encoded by CYP11A1) and NuSAP expression in 18 human gonadotroph tumors. In vitro studies demonstrated for the first time that Cyp11a1 is a target of SF-1 in gonadotroph cells and promotes proliferation/survival of rat pituitary adenoma primary cells and cell lines. Our studies reveal clues about the molecular mechanisms driving rat and human gonadotroph adenomas development, and may help identify previously unexplored biomarkers for clinical use.

Pituitary phenotypes of mice lacking the notch signalling ligand delta-like 1 homologue

The Notch signalling pathway ligand delta-like 1 homologue (Dlk1, also named Pref1) is expressed throughout the developing pituitary and becomes restricted to mostly growth hormone (GH) cells within the adult gland. We have investigated the role of Dlk1 in pituitary development and function from late embryogenesis to adulthood using a mouse model completely lacking the expression of Dlk1. We confirm that Dlk1-null mice are shorter and weigh less than wild-type littermates from late gestation, at parturition and in adulthood. A loss of Dlk1 leads to significant reduction in GH content throughout life, whereas other pituitary hormones are reduced to varying degrees depending on sex and age. Both the size of the pituitary and the proportion of hormone-producing cell populations are unchanged, suggesting that there is a reduction in hormone content per cell. In vivo challenge of mutant and wild-type littermates with growth hormone-releasing hormone and growth hormone-releasing hexapeptide shows that reduced GH secretion is unlikely to account for the reduced growth of Dlk1 knockout animals. These data suggest that loss of Dlk1 gives rise to minor pituitary defects manifesting as an age- and sex-dependent reduction in pituitary hormone contents. However, Dlk1 expression in other tissue is most likely responsible for the weight and length differences observed in mutant animals.

Possible roles of DLK1 in the Notch pathway during development and disease

The Delta-Notch pathway is an evolutionarily conserved signaling pathway which controls a broad range of developmental processes including cell fate determination, terminal differentiation and proliferation. In mammals, four Notch receptors (NOTCH1-4) and five activating canonical ligands (JAGGED1, JAGGED2, DLL1, DLL3 and DLL4) have been described. The precise function of noncanonical Notch ligands remains unclear. Delta-like 1 homolog (DLK1), the best studied noncanonical Notch ligand, has been shown to act as an inhibitor of Notch signaling in vitro, but its function in vivo is poorly understood. In this review we summarize Notch signaling during development and highlight recent studies in DLK1expression that reveal new insights into its function.

Identification of growth arrest and DNA-damage-inducible gene beta (GADD45beta) as a novel tumor suppressor in pituitary gonadotrope tumors

Gonadotrope and null cell pituitary tumors cause significant morbidity, often presenting with signs of hypogonadism together with visual disturbances due to mass effects. Surgery and radiation are the only therapeutic options to date. To identify dysregulated genes and pathways that may play a role in tumorigenesis and/or progression, molecular profiling was performed on 14 gonadotrope tumors, with nine normal human pituitaries obtained at autopsy serving as controls. Bioinformatic analysis identified putative downstream effectors of tumor protein 53 (p53) that were consistently repressed in gonadotrope pituitary tumors, including RPRM, P21, and PMAIP1, with concomitant inhibition of the upstream p53 regulator, PLAGL1(Zac1). Further analysis of the growth arrest and DNA damage-inducible (GADD45) family revealed no change in the p53 target, GADD45α, but identified repression of GADD45β in pituitary tumors in addition to the previously reported inhibition of GADD45γ. Overexpression of GADD45β in LβT2 mouse gonadotrope cells blocked tumor cell proliferation and increased rates of apoptosis in response to growth factor withdrawal. Stable gonadotrope cell transfectants expressing increased GADD45β showed decreased colony formation in soft agar, confirming its normal role as a tumor suppressor. Unlike previous studies of GADD45γ in pituitary tumors and α and β in other tumors, bisulfite sequencing showed no evidence of hypermethylation of the GADD45β promoter in human pituitary tumor samples to explain the repression of its expression. Thus, GADD45β is a novel pituitary tumor suppressor whose reexpression blocks proliferation, survival, and tumorigenesis. Together these studies identify new targets and mechanisms to explore in pituitary tumor initiation and progression.

Role of the non-canonical notch ligand delta-like protein 1 in hormone-producing cells of the adult male mouse pituitary

To better understand the role of the non-canonical Notch ligand delta-like protein 1 (DLK1), in hormone-producing cells, we studied the cell distribution and subcellular localisation of DLK1 in the pituitary of male adult 129/SvJ mice, and analysed the variations in the hormone-producing cells associated with the lack of this gene in Dlk1 knockout mice. The results obtained showed the presence of DLK1-immunoreactive (ir) cells in all hormone-producing cells of the anterior pituitary. Immunoelectron microscopy showed DLK1-ir in the rough endoplasmic reticulum and inside secretory vesicles, suggesting that DLK1 is released together with pituitary hormones. Moreover, we found that prolactin (PRL)-DLK1-ir cells are in intimate contact with follicle-stimulating hormone (FSH)-ir-DLK1-negative cells. In Dlk1 knockout mice, we detected a significantly lower number of gowth hormone (GH)-ir cells, a reduction in the FSH and PRL immunostaining intensity, and a significant decrease in FSH mRNA expression compared to wild-type mice. An increase in pituitary GH mRNA expression and serum leptin levels was also found. These findings provide evidence supporting several regulatory functions of DLK1 in the pituitary gland.

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.

Analysis on cDNA sequence, mRNA expression and imprinting status of Dlk1 gene in goats

Dlk1 (Delta-like homolog 1) is a cell surface transmembrane glycoprotein belonging to the epidermal growth factor like family of homeotic proteins and plays an important role in regulating fetal and postnatal development. Increased expression of Dlk1 is the primary cause of muscle hypertrophy in the callipyge sheep exhibiting overgrowth of fast-twitch muscles and reduced adiposity. However, the function of Dlk1 in goats remains unknown. In this study, a fragment of 864 bp of goat Dlk1 cDNA, encoding 287 amino acids, was amplified, which has a high homology both in nucleotide sequence and amino acid sequence with the corresponding region of pig, cattle and sheep Dlk1. The Dlk1 was found to be expressed in most tissues of goat fetuses, and in the adrenal gland, pancreas and thymus of adult goats. Two alternative transcripts of Dlk1-C and Dlk1-C2 were expressed in both fetuses and adult goats. One C/T transition in the coding region of goat Dlk1 was identified and by genotyping one segregating goat family and the expressed allele in the tissues of the offspring, Dlk1 was found to be paternally expressed.

Pituitary progenitor cells tracked down by side population dissection

The pituitary gland represents the endocrine core, governing the body's hormonal landscape by adapting its cellular composition to changing demands. It is assumed that stem/progenitor cells are involved in this remodeling. Recently, we uncovered a candidate stem/progenitor cell population in the anterior pituitary. Here, we scrutinized this "side population" (SP) and show that, unexpectedly, not the subset expressing high levels of "stem cell antigen-1" (Sca1(high)) but the remainder non-Sca1(high) fraction clusters the pituitary progenitor cells. Transcriptomal interrogation revealed in the non-Sca1(high) SP upregulated expression of the pituitary stem/progenitor cell markers Sox2 and Sox9, and of multiple factors critically involved in pituitary embryogenesis. The non-Sca1(high) SP encloses the cells that generate spheres and display multipotent hormone differentiation capacity. In culture conditions selecting for the non-Sca1(high) subset within the SP, stem cell growth factors that induce SP expansion, affect transcription of embryonic factors, suggesting impact on a developmental program that unfolds within this SP compartment. Non-Sca1(high) SP cells, revealed by Sox2 expression, are observed in the postulated periluminal stem/progenitor cell niche, but also in small groups scattered over the gland, thereby advocating the existence of multiple niches. In early postnatal mice undergoing a pituitary growth wave, Sox2(+) cells are more abundant than in adults, concordant with a larger SP and higher non-Sca1(high) proportion. Together, we tracked down pituitary progenitor cells by SP phenotype, and thus provide a straightforward method to isolate and scrutinize these cells from the plastic pituitary ex vivo, as well as a culture system for in-depth exploration of their regulatory network.

Induction of Dlk1 by PTTG1 inhibits adipocyte differentiation and correlates with malignant transformation

Pituitary tumor-transforming gene-1 (PTTG1) is an oncogene highly expressed in a variety of endocrine, as well as nonendocrine-related cancers. Several tumorigenic mechanisms for PTTG1 have been proposed, one of the best characterized being its capacity to act as a transcriptional activator. To identify novel downstream target genes, we have established cell lines with inducible expression of PTTG1 and a differential display approach to analyze gene expression changes after PTTG1 induction. We identified dlk1 (also known as pref-1) as one of the most abundantly expressed PTTG1 targets. Dlk1 is known to participate in several differentiation processes, including adipogenesis, adrenal gland development, and wound healing. Dlk1 is also highly expressed in neuroendocrine tumors. Here, we show that PTTG1 overexpression inhibits adipogenesis in 3T3-L1 cells and that this effect is accomplished by promoting the stability and accumulation of Dlk1 mRNA, supporting a role for PTTG1 in posttranscriptional regulation. Moreover, both pttg1 and dlk1 genes show concomitant expression in fetal liver and placenta, as well as in pituitary adenomas, breast adenocarcinomas, and neuroblastomas, suggesting that PTTG1 and DLK1 are involved in cell differentiation and transformation.

Discovery of transcriptional regulators and signaling pathways in the developing pituitary gland by bioinformatic and genomic approaches

We report a catalog of the mouse embryonic pituitary gland transcriptome consisting of five cDNA libraries including wild type tissue from E12.5 and E14.5, Prop1(df/df) mutant at E14.5, and two cDNA subtractions: E14.5 WT-E14.5 Prop1(df/df) and E14.5 WT-E12.5 WT. DNA sequence information is assembled into a searchable database with gene ontology terms representing 12,009 expressed genes. We validated coverage of the libraries by detecting most known homeobox gene transcription factor cDNAs. A total of 45 homeobox genes were detected as part of the pituitary transcriptome, representing most expected ones, which validated library coverage, and many novel ones, underscoring the utility of this resource as a discovery tool. We took a similar approach for signaling-pathway members with novel pituitary expression and found 157 genes related to the BMP, FGF, WNT, SHH and NOTCH pathways. These genes are exciting candidates for regulators of pituitary development and function.

Delta-like 1 (Dlk-1), a novel marker of prostate basal and candidate epithelial stem cells, is downregulated by notch signalling in intermediate/transit amplifying cells of the human prostate

BACKGROUND

There is a lack of understanding of the processes that regulate differentiation in the prostate.

OBJECTIVE

To determine localisation, activity, and regulation of cytodifferentiation-modulatory proteins in the human adult prostate.

DESIGN, SETTINGS, AND PARTICIPANTS

Eighteen volunteering patients with organ-confined prostate cancer were prospectively enrolled at a single university hospital.

INTERVENTION

All patients underwent radical prostatectomy, and normal/benign tissue was excised and obtained from the transition zone.

MEASUREMENTS

Expression and activity of Notch-protein family members, including the Notch-homologous protein Delta-like 1 (Dlk-1/Pref1), were investigated immunohistochemically in normal/benign tissue and explant cultures. The effect of the Notch inhibitor L-685,458 on Dlk-1 expression and cell number was investigated in primary cell cultures, and data were analysed with Student t test.

RESULTS AND LIMITATIONS

Mature luminal cells were found to co-express Notch-1 and its ligand Jagged1, but epithelia in normal/benign tissue showed no active Notch signalling. The basal cell layer, rare candidate epithelial stem cells, and a subpopulation of neuroendocrine cells expressed the differentiation protein Dlk-1. In explant cultures, luminal cells and Jagged1 expression were lost, whereas intermediate cells downregulated Dlk-1 concomitant with Notch-1 upregulation and activation. Notch inhibition in primary cell cultures led to lower cell densities (p<0.001) and suppressed downregulation of Dlk-1. This is a small study; current results need to be confirmed in larger investigations.

CONCLUSIONS

We demonstrate that Notch-1 is upregulated in differentiation of prostate epithelia, and that the novel prostate progenitor marker Dlk-1 is downregulated by Notch signalling in intermediate cells. The identification of Dlk-1-expressing candidate stem and neuroendocrine cells suggests a hierarchical relationship.

Wnt pathway inhibitors are strongly down-regulated in pituitary tumors

The etiology of sporadic pituitary tumors is currently unknown. The Wnt pathways have been implicated in the pathogenesis of a variety of human tumors, but the role of these pathways in pituitary tumors is unclear. Microarray analysis using the Affymetrix HG U133 plus 2.0 GeneChips identified four secreted frizzled-related protein (sFRP) family members of Wnt pathway inhibitors that were differentially expressed in both nonfunctioning and clinically functioning pituitary tumors (n = 20) compared with normal pituitary controls (n = 3). Reduced tumor expression of Wnt inhibitory factor-1 (WIF1), sFRP2, and sFRP4 mRNA was confirmed by real-time quantitative RT-PCR (P <0.001 and P = 0.002 and 0.013, respectively) in all pituitary subtypes. Hypermethylation of the WIF1 promoter was present in 88% of the pituitary tumors (n = 41). Seventy-six percent of pituitary tumors demonstrated absent or weak cytoplasmic WIF1 staining by immunohistochemistry (n = 41), although preserved staining was seen in some functioning tumors, with strong staining in 92% of normal pituitary controls (n = 13). The Wnt pathway target gene cyclin D1 was found to be up-regulated specifically in the nonfunctioning pituitary tumors compared with controls at both mRNA and protein level, supportive of activation of the Wnt-beta-catenin pathway. Nuclear accumulation of beta-catenin, however, was not observed in any pituitary tumors (n = 70). By transfecting GH3 cells with WIF1, decreased cell proliferation and colony formation was observed compared with empty vector controls. In conclusion, our data suggest that WIF1 may be a tumor suppressor, specifically in nonfunctioning pituitary tumors, and that the Wnt pathways are important in pituitary tumorigenesis.

Linkage on chromosome 14 in a genome-wide linkage study of a broad anxiety phenotype

Several linkage studies on anxiety have been carried out in samples ascertained through probands with panic disorder. The results indicated that using a broad anxiety phenotype instead of a DSM-IV anxiety disorder diagnosis might enhance the chance of finding a linkage signal. In the current study, a genome-wide linkage analysis was performed on anxiety measured with a self-report questionnaire whose scores are highly correlated with DSM-IV anxiety disorders. The self-report questionnaire was included in five surveys of a longitudinal study of the Netherlands Twin Register. Genotype and phenotype data were available for 1602 twins and siblings. To estimate identity by descent , additional genotype data for 564 parents and 22 siblings were used. Linkage analyses were carried out using MERLIN-regress on the average anxiety scores across time. A linkage signal (logarithm of odds score 3.4, empirical P-value 0.07) was obtained at chromosome 14 for marker D14S65 at 105 cM (90% confidence interval, 99-115 cM bounded by markers D14S1434 and D14S985). This finding replicates a linkage finding for a broad anxiety phenotype in a clinically based sample, indicating that the region might harbor a quantitative trait locus associated with the whole spectrum of general anxiety, that is from the normal to the clinical range. Moreover, genome-wide linkage and association studies on emotionality in mice obtained significant results in a syntenic region on mouse chromosome 12. Two homolog genes lie in this region -Dlk1 (delta-like 1 homolog, Drosophila) and Rtl1 (retrotransposon-like 1). Future association studies of these genes are warranted.

Analysis of the callipyge phenotype through skeletal muscle development; association of Dlk1 with muscle precursor cells

The callipyge mutation in sheep in the form of the paternal heterozygote results in skeletal muscle hypertrophy, which is most pronounced in the hindquarters. Overexpression of one of the genes in the region of the causative single-nucleotide polymorphism, Dlk1, is postulated to be a primary cause of the muscle hypertrophy although the mechanism is not clear. This study examined the expression of Dlk1 mRNA and its encoded protein in skeletal muscles of callipyge and wild-type sheep. The muscles examined included those that demonstrate hypertrophy in callipyge sheep as well as an unaffected muscle. The expression pattern of Dlk1 protein in these muscles was also measured over a developmental time course ranging from 80 days of gestation to 12 weeks after birth. Quantitative reverse transcription-polymerase chain reaction demonstrated that Dlk1 mRNA was significantly increased in affected, but not unaffected, muscles from callipyge sheep at 120 days of gestation through to 12 weeks of age. Immuno-localization of Dlk1 was pronounced in the interstitial connective tissue of fetal muscle but was less intense at later ages. No clear difference in Dlk1 immuno-localization was noted between genotypes in the fetal samples. Strong myofiber-specific Dlk1 immuno-localization was observed in hypertrophied callipyge muscles at 12 weeks of age. This staining was exclusively associated with fast type II myofibers and these had a significantly larger mean cross-sectional area, compared with fast type II myofibers in control sheep that did not overexpress Dlk1. In addition, Dlk1 immuno-localization was associated with a sub-population of Pax7-positive mononucleated cells in all skeletal muscles examined during fetal development and at birth, but this was not apparent at 12 weeks. There were no genotype-dependent alterations in the mRNA expression patterns of a number of promyogenic transcription factors indicating that the callipyge mutation was not affecting muscle cell differentiation per se. We postulate that Dlk1 is implicated in the commitment and/or proliferation of fetal myoblasts as well as in the maintenance of hypertrophy in fully differentiated myofibers.

Regulation of growth hormone expression by Delta-like protein 1 (Dlk1)

Delta-like protein 1 (Dlk1) is a transmembrane protein characterized by epidermal growth factor (EGF)-like repeats. Dlk1, which is also known as preadipocyte factor 1 (pref-1) because of its ability to inhibit preadipocyte differentiation, regulates the differentiation of several other cell types through unknown mechanisms. To elucidate Dlk1 functions, identification of Dlk1-regulated target genes is critical. The observation that Dlk1 is expressed in many endocrine tissues suggests that Dlk1 may have endocrine-related functions. Because Dlk1 is expressed in GH producing cells, we hypothesize that one function of Dlk1 is to regulate GH expression. We found that GH mRNA, protein, and secretion were significantly decreased in GH3 pituitary cell clones that stably express Dlk1. In contrast, Dlk1 expression was unable to alter prolactin expression. Co-transfection of GH3 cells with a GH promoter-regulated reporter gene showed that Dlk1 repressed GH promoter activity. Deletion and mutation analysis of the GH promoter indicated that Pit-1 binding sites in the GH promoter are required for Dlk1-mediated repression. Furthermore, Dlk1 expression represses Pit-1-mediated transcription when both proteins are co-expressed in MCF-7 cells. Deletion analysis of Dlk1 revealed that the ability of Dlk1 to regulate GH promoter activity is independent of both its EGF-like repeats and its ability to modulate MAP kinase activity. The observation that Dlk1 regulates GH expression identifies the first endocrine function of Dlk1, establishes GH as a Dlk1-regulated target gene, and provides a model system to facilitate studies of Dlk1-mediated signaling.

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.