Jan Kitajewski

Stromal Elements Act to Restrain, Rather Than Support, Pancreatic Ductal Adenocarcinoma

Sonic hedgehog (Shh), a soluble ligand overexpressed by neoplastic cells in pancreatic ductal adenocarcinoma (PDAC), drives formation of a fibroblast-rich desmoplastic stroma. To better understand its role in malignant progression, we deleted Shh in a well-defined mouse model of PDAC. As predicted, Shh-deficient tumors had reduced stromal content. Surprisingly, such tumors were more aggressive and exhibited undifferentiated histology, increased vascularity, and heightened proliferation--features that were fully recapitulated in control mice treated with a Smoothened inhibitor. Furthermore, administration of VEGFR blocking antibody selectively improved survival of Shh-deficient tumors, indicating that Hedgehog-driven stroma suppresses tumor growth in part by restraining tumor angiogenesis. Together, these data demonstrate that some components of the tumor stroma can act to restrain tumor growth.

Endothelial cells are essential for the self-renewal and repopulation of Notch-dependent hematopoietic stem cells

Bone marrow endothelial cells (ECs) are essential for reconstitution of hematopoiesis, but their role in self-renewal of long-term hematopoietic stem cells (LT-HSCs) is unknown. We have developed angiogenic models to demonstrate that EC-derived angiocrine growth factors support in vitro self-renewal and in vivo repopulation of authentic LT-HSCs. In serum/cytokine-free cocultures, ECs, through direct cellular contact, stimulated incremental expansion of repopulating CD34(-)Flt3(-)cKit(+)Lineage(-)Sca1(+) LT-HSCs, which retained their self-renewal ability, as determined by single-cell and serial transplantation assays. Angiocrine expression of Notch ligands by ECs promoted proliferation and prevented exhaustion of LT-HSCs derived from wild-type, but not Notch1/Notch2-deficient, mice. In transgenic notch-reporter (TNR.Gfp) mice, regenerating TNR.Gfp(+) LT-HSCs were detected in cellular contact with sinusoidal ECs. Interference with angiocrine, but not perfusion, function of SECs impaired repopulation of TNR.Gfp(+) LT-HSCs. ECs establish an instructive vascular niche for clinical-scale expansion of LT-HSCs and a cellular platform to identify stem cell-active trophogens.

Differential regulation of midbrain dopaminergic neuron development by Wnt-1, Wnt-3a, and Wnt-5a

The Wnts are a family of glycoproteins that regulate cell proliferation, fate decisions, and differentiation. In our study, we examined the contribution of Wnts to the development of ventral midbrain (VM) dopaminergic (DA) neurons. Our results show that β-catenin is expressed in DA precursor cells and that β-catenin signaling takes place in these cells, as assessed in TOPGAL [Tcf optimal-promoter β-galactosidase] reporter mice. We also found that Wnt-1, -3a, and -5a expression is differentially regulated during development and that partially purified Wnts distinctively regulate VM development. Wnt-3a promoted the proliferation of precursor cells expressing the orphan nuclear receptor-related factor 1 (Nurr1) but did not increase the number of tyrosine hydroxylase-positive neurons. Instead, Wnt-1 and -5a increased the number of rat midbrain DA neurons in rat embryonic day 14.5 precursor cultures by two distinct mechanisms. Wnt-1 predominantly increased the proliferation of Nurr1+ precursors, up-regulated cyclins D1 and D3, and down-regulated p27 and p57 mRNAs. In contrast, Wnt-5a primarily increased the proportion of Nurr1+ precursors that acquired a neuronal DA phenotype and up-regulated the expression of Ptx3 and c-ret mRNA. Moreover, the soluble cysteine-rich domain of Frizzled-8 (a Wnt inhibitor) blocked endogenous Wnts and the effects of Wnt-1 and -5a on proliferation and the acquisition of a DA phenotype in precursor cultures. These findings indicate that Wnts are key regulators of proliferation and differentiation of DA precursors during VM neurogenesis and that different Wnts have specific and unique activity profiles.

SEL-10 Is an Inhibitor of Notch Signaling That Targets Notch for Ubiquitin-Mediated Protein Degradation

ABSTRACT Notch receptors and their ligands play important roles in both normal animal development and pathogenesis. We show here that the F-box/WD40 repeat protein SEL-10 negatively regulates Notch receptor activity by targeting the intracellular domain of Notch receptors for ubiquitin-mediated protein degradation. Blocking of endogenous SEL-10 activity was done by expression of a dominant-negative form containing only the WD40 repeats. In the case of Notch1, this block leads to an increase in Notch signaling stimulated by either an activated form of the Notch1 receptor or Jagged1-induced signaling through Notch1. Expression of dominant-negative SEL-10 leads to stabilization of the intracellular domain of Notch1. The Notch4 intracellular domain bound to SEL-10, but its activity was not increased as a result of dominant-negative SEL-10 expression. SEL-10 bound Notch4 via the WD40 repeats and bound preferentially to a phosphorylated form of Notch4 in cells. We mapped the region of Notch4 essential for SEL-10 binding to the C-terminal region downstream of the ankyrin repeats. When this C-terminal fragment of Notch4 was expressed in cells, it was highly labile but could be stabilized by the expression of dominant-negative SEL-10. Ubiquitination of Notch1 and Notch4 intracellular domains in vitro was dependent on SEL-10. Although SEL-10 interacts with the intracellular domains of both Notch1 and Notch4, these proteins respond differently to interference with SEL-10 function. Thus, SEL-10 functions to promote the ubiquitination of Notch proteins; however, the fates of these proteins may differ.

New targets of beta-catenin signaling in the liver are involved in the glutamine metabolism

Inappropriate activation of the Wnt/β-catenin signaling has been implicated in the development of hepatocellular carcinoma (HCC), but exactly how β-catenin works remains to be elucidated. To identify, in vivo, the target genes of β-catenin in the liver, we have used the suppression subtractive hybridization technique and transgenic mice expressing an activated β-catenin in the liver that developed hepatomegaly. We identified three genes involved in glutamine metabolism, encoding glutamine synthetase (GS), ornithine aminotransferase (OAT) and the glutamate transporter GLT-1. By Northern blot and immunohistochemical analysis we demonstrated that these three genes were specifically induced by activation of the β-catenin pathway in the liver. In different mouse models bearing an activated β-catenin signaling in the liver known to be associated with hepatocellular proliferation we observed a marked up-regulation of these three genes. The cellular distribution of GS and GLT-1 parallels β-catenin activity. By contrast no up-regulation of these three genes was observed in the liver in which hepatocyte proliferation was induced by a signal-independent of β-catenin. In addition, the GS promoter was activated in the liver of GS+/LacZ mice by adenovirus vector-mediated β-catenin overexpression. Strikingly, the overexpression of the GS gene in human HCC samples was strongly correlated with β-catenin activation. Together, our results indicate that GS is a target of the Wnt/β-catenin pathway in the liver. Because a linkage of the glutamine pathway to hepatocarcinogenesis has already been demonstrated, we propose that regulation of these three genes of glutamine metabolism by β-catenin is a contributing factor to liver carcinogenesis.

Vascular patterning defects associated with expression of activated Notch4 in embryonic endothelium

Notch proteins function as receptors for membrane-bound ligands (Jagged and Delta-like) to regulate cell-fate determination. We have investigated the role of Notch signaling in embryonic endothelium of the mouse by expressing an activated form of the Notch4 protein in vasculature under the regulation of the Flk1 (VEGFR) locus. Expression of activated Notch4 results in a growth and developmental delay and embryonic lethality at about 10 days postcoitum. The extent of the developing vasculature in mutant embryos was restricted, fewer small vessels were seen, and vascular networks were disorganized. The brain periphery of mutant embryos contained large dilated vessels with evidence of compromised vessel-wall integrity and large areas of necrosis; yolk-sac vasculature was abnormal. Expression of an activated form of Notch4 in embryonic vasculature leads to abnormal vessel structure and patterning, implicating the Notch pathway in phases of vascular development associated with vessel patterning and remodeling.

Bile Acid and Inflammation Activate Gastric Cardia Stem Cells in a Mouse Model of Barrett-Like Metaplasia

Esophageal adenocarcinoma (EAC) arises from Barrett esophagus (BE), intestinal-like columnar metaplasia linked to reflux esophagitis. In a transgenic mouse model of BE, esophageal overexpression of interleukin-1β phenocopies human pathology with evolution of esophagitis, Barrett-like metaplasia and EAC. Histopathology and gene signatures closely resembled human BE, with upregulation of TFF2, Bmp4, Cdx2, Notch1, and IL-6. The development of BE and EAC was accelerated by exposure to bile acids and/or nitrosamines, and inhibited by IL-6 deficiency. Lgr5(+) gastric cardia stem cells present in BE were able to lineage trace the early BE lesion. Our data suggest that BE and EAC arise from gastric progenitors due to a tumor-promoting IL-1β-IL-6 signaling cascade and Dll1-dependent Notch signaling.

A Foxo/Notch pathway controls myogenic differentiation and fiber type specification

Forkhead box O (Foxo) transcription factors govern metabolism and cellular differentiation. Unlike Foxo-dependent metabolic pathways and target genes, the mechanisms by which these proteins regulate differentiation have not been explored. Activation of Notch signaling mimics the effects of Foxo gain of function on cellular differentiation. Using muscle differentiation as a model system, we show that Foxo physically and functionally interacts with Notch by promoting corepressor clearance from the Notch effector Csl, leading to activation of Notch target genes. Inhibition of myoblast differentiation by constitutively active Foxo1 is partly rescued by inhibition of Notch signaling while Foxo1 loss of function precludes Notch inhibition of myogenesis and increases myogenic determination gene (MyoD) expression. Accordingly, conditional Foxo1 ablation in skeletal muscle results in increased formation of MyoD-containing (fast-twitch) muscle fibers and altered fiber type distribution at the expense of myogenin-containing (slow-twitch) fibers. Notch/Foxo1 cooperation may integrate environmental cues through Notch with metabolic cues through Foxo1 to regulate progenitor cell maintenance and differentiation.

Notch signaling regulates tumor angiogenesis by diverse mechanisms

The Notch signaling pathway is fundamental to proper cardiovascular development and is now recognized as an important player in tumor angiogenesis. Two key Notch ligands have been implicated in tumor angiogenesis, Delta-like 4 and Jagged1. We introduce the proteins and how they work in normal developing vasculature and then discuss differing models describing the action of these Notch ligands in tumor angiogenesis. Endothelial Dll4 expression activates Notch resulting in restriction of new sprout development; for instance, in growing retinal vessels. In agreement with this activity, inhibition of Dll4-mediated Notch signaling in tumors results in hypersprouting of nonfunctional vasculature. This Dll4 inhibition may paradoxically lead to increased angiogenesis but poor tumor growth because the newly growing vessels are not functional. In contrast, Jagged1 has been described as a Notch ligand expressed in tumor cells that can have a positive influence on tumor angiogenesis, possibly by activating Notch on tumor endothelium. A novel Notch inhibitor, the Notch1 decoy, which blocks both Dll4 and Jagged1 has been recently shown to restrict tumor vessel growth. We discuss these models and speculate on therapeutic approaches.

Notch–RBP-J signaling regulates the transcription factor IRF8 to promote inflammatory macrophage polarization

Emerging concepts suggest that the functional phenotype of macrophages is regulated by transcription factors that define alternative activation states. We found that RBP-J, the main nuclear transducer of signaling via Notch receptors, augmented Toll-like receptor 4 (TLR4)-induced expression of key mediators of classically activated M1 macrophages and thus of innate immune responses to Listeria monocytogenes. Notch–RBP-J signaling controlled expression of the transcription factor IRF8 that induced downstream M1 macrophage–associated genes. RBP-J promoted the synthesis of IRF8 protein by selectively augmenting kinase IRAK2–dependent signaling via TLR4 to the kinase MNK1 and downstream translation-initiation control through eIF4E. Our results define a signaling network in which signaling via Notch–RBP-J and TLRs is integrated at the level of synthesis of IRF8 protein and identify a mechanism by which heterologous signaling pathways can regulate the TLR-induced inflammatory polarization of macrophages.