Gli genes in development and cancer

Frequent deregulations in the hedgehog signaling network and cross-talks with the epidermal growth factor receptor pathway involved in cancer progression and targeted therapies

The hedgehog (Hh)/glioma-associated oncogene (GLI) signaling network is among the most important and fascinating signal transduction systems that provide critical functions in the regulation of many developmental and physiological processes. The coordinated spatiotemporal interplay of the Hh ligands and other growth factors is necessary for the stringent control of the behavior of diverse types of tissue-resident stem/progenitor cells and their progenies. The activation of the Hh cascade might promote the tissue regeneration and repair after severe injury in numerous organs, insulin production in pancreatic beta-cells, and neovascularization. Consequently, the stimulation of the Hh pathway constitutes a potential therapeutic strategy to treat diverse human disorders, including severe tissue injuries; diabetes mellitus; and brain, skin, and cardiovascular disorders. In counterbalance, a deregulation of the Hh signaling network might lead to major tissular disorders and the development of a wide variety of aggressive and metastatic cancers. The target gene products induced through the persistent Hh activation can contribute to the self-renewal, survival, migration, and metastasis of cancer stem/progenitor cells and their progenies. Moreover, the pivotal role mediated through the Hh/GLI cascade during cancer progression also implicates the cooperation with other oncogenic products, such as mutated K-RAS and complex cross-talk with different growth factor pathways, including tyrosine kinase receptors, such as epidermal growth factor receptor (EGFR), Wnt/beta-catenin, and transforming growth factor-beta (TGF-beta)/TGF-beta receptors. Therefore, the molecular targeting of distinct deregulated gene products, including Hh and EGFR signaling components and other signaling elements that are frequently deregulated in highly tumorigenic cancer-initiating cells and their progenies, might constitute a potential therapeutic strategy to eradicate the total cancer cell mass. Of clinical interest is that these multitargeted approaches offer great promise as adjuvant treatments for improving the current antihormonal therapies, radiotherapies, and/or chemotherapies against locally advanced and metastatic cancers, thereby preventing disease relapse and the death of patients with cancer.

Gli-similar (Glis) Krüppel-like zinc finger proteins: insights into their physiological functions and critical roles in neonatal diabetes and cystic renal disease

GLI-similar (Glis) 1-3 proteins constitute a subfamily of the Krüppel-like zinc finger transcription factors that are closely related to the Gli family. Glis1-3 play critical roles in the regulation of a number of physiological processes and have been implicated in several pathologies. Mutations in GLIS2 have been linked to nephronophthisis, an autosomal recessive cystic kidney disease. Loss of Glis2 function leads to renal atrophy and fibrosis that involves epithelial-mesenchymal transition (EMT) of renal tubule epithelial cells. Mutations in human GLIS3 have been implicated in a syndrome characterized by neonatal diabetes and congenital hypothyroidism (NDH) and in some patients accompanied by polycystic kidney disease, glaucoma, and liver fibrosis. In addition, the GLIS3 gene has been identified as a susceptibility locus for the risk of type 1 and 2 diabetes. Glis3 plays a key role in pancreatic development, particularly in the generation of ß-cells and in the regulation of insulin gene expression. Glis2 and Glis3 proteins have been demonstrated to localize to the primary cilium, a signaling organelle that has been implicated in several pathologies, including cystic renal diseases. This association suggests that Glis2/3 are part of primary cilium-associated signaling pathways that control the activity of Glis proteins. Upon activation in the primary cilium, Glis proteins may translocate to the nucleus where they subsequently regulate gene transcription by interacting with Glis-binding sites in the promoter regulatory region of target genes. In this review, we discuss the current knowledge of the Glis signaling pathways, their physiological functions, and their involvement in several human pathologies.

SUMOylation by Pias1 regulates the activity of the Hedgehog dependent Gli transcription factors

BACKGROUND

Hedgehog (Hh) signaling, a vital signaling pathway for the development and homeostasis of vertebrate tissues, is mediated by members of the Gli family of zinc finger transcription factors. Hh signaling increases the transcriptional activity of Gli proteins, at least in part, by inhibiting their proteolytic processing. Conversely, phosphorylation by cAMP-dependent protein kinase (PKA) inhibits Gli transcriptional activity by promoting their ubiquitination and proteolysis. Whether other post-translational modifications contribute to the regulation of Gli protein activity has been unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Here we provide evidence that all three Gli proteins are targets of small ubiquitin-related modifier (SUMO)-1 conjugation. Expression of SUMO-1 or the SUMO E3 ligase, Pias1, increased Gli transcriptional activity in cultured cells. Moreover, PKA activity reduced Gli protein SUMOylation. Strikingly, in the embryonic neural tube, the forced expression of Pias1 increased Gli activity and induced the ectopic expression of the Gli dependent gene Nkx2.2. Conversely, a point mutant of Pias1, that lacks ligase activity, blocked the endogenous expression of Nkx2.2.

CONCLUSIONS/SIGNIFICANCE

Together, these findings provide evidence that Pias1-dependent SUMOylation influences Gli protein activity and thereby identifies SUMOylation as a post-translational mechanism that regulates the hedgehog signaling pathway.

Combinatorial chemoprevention reveals a novel smoothened-independent role of GLI1 in esophageal carcinogenesis

Reflux-induced injury promotes esophageal adenocarcinoma, one of the most rapidly increasing, highly lethal cancers in Western countries. Here, we investigate the efficacy of a combinatorial chemoprevention strategy for esophageal adenocarcinoma and characterize the underlying molecular mechanisms. Specifically, our approach involves the use of ursodeoxycholic acid (Urso) due to its ability to decrease injury-inducing bile salts in combination with Aspirin to mitigate the consequences of injury. We find that Urso-Aspirin combination reduces the risk of adenocarcinoma in vivo in animals with reflux, decreases the proliferation of esophageal adenocarcinoma cells, and downregulates a key cell cycle regulator, CDK2. Mechanistically, using cell growth, luciferase reporter, expression, and chromatin immunoprecipitation assays, we identify GLI1, a Hedgehog-regulated transcription factor, as a novel target of Urso-Aspirin combination. We show that GLI1 is upregulated during esophageal carcinogenesis, and GLI1 can bind to the CDK2 promoter and activate its expression. Although the Urso-Aspirin combination downregulates GLI1, the GLI1 overexpression not only abrogates the effect of this combination on proliferation but it also restores CDK-2 expression. These findings support that the chemopreventive effect of the Urso-Aspirin combination occurs, at least in part, through a novel GLI1-CDK2-dependent mechanism. To further understand the regulation of CDK2 by GLI1, both pharmacologic and RNAi-mediated approaches show that GLI1 is a transcriptional activator of CDK2, and this regulation occurs independent of Smoothened, the central transducer of the Hedgehog canonical pathway. Collectively, these results identify a novel GLI1-to-CDK2 pathway in esophageal carcinogenesis, which is a bona fide target for effective combinatorial chemoprevention with Urso and Aspirin.

Sonic hedgehog functions through dynamic changes in temporal competence in the developing forebrain

Morphogens act during development to provide graded spatial information that controls patterning and cell lineage specification in the nervous system. The role of morphogen signaling in instructing the expression of downstream effector transcription factors has been well established. However, a key requirement for morphogen signaling is the existence of functional intracellular machinery able to mediate the appropriate response in target cells. Here we suggest that dynamic changes in the temporal responses to Shh in the developing ventral telencephalon occur through alterations in progenitor competence. We suggest these developmental changes in competence are mediated by a transcriptional mechanism that intrinsically integrates information from the distinct signaling pathways that act to pattern the telencephalic neuroepithelium.

Characterization of the tandem CWCH2 sequence motif: a hallmark of inter-zinc finger interactions

Background

The C2H2 zinc finger (ZF) domain is widely conserved among eukaryotic proteins. In Zic/Gli/Zap1 C2H2 ZF proteins, the two N-terminal ZFs form a single structural unit by sharing a hydrophobic core. This structural unit defines a new motif comprised of two tryptophan side chains at the center of the hydrophobic core. Because each tryptophan residue is located between the two cysteine residues of the C2H2 motif, we have named this structure the tandem CWCH2 (tCWCH2) motif.

Results

Here, we characterized 587 tCWCH2-containing genes using data derived from public databases. We categorized genes into 11 classes including Zic/Gli/Glis, Arid2/Rsc9, PacC, Mizf, Aebp2, Zap1/ZafA, Fungl, Zfp106, Twincl, Clr1, and Fungl-4ZF, based on sequence similarity, domain organization, and functional similarities. tCWCH2 motifs are mostly found in organisms belonging to the Opisthokonta (metazoa, fungi, and choanoflagellates) and Amoebozoa (amoeba, Dictyostelium discoideum). By comparison, the C2H2 ZF motif is distributed widely among the eukaryotes. The structure and organization of the tCWCH2 motif, its phylogenetic distribution, and molecular phylogenetic analysis suggest that prototypical tCWCH2 genes existed in the Opisthokonta ancestor. Within-group or between-group comparisons of the tCWCH2 amino acid sequence identified three additional sequence features (site-specific amino acid frequencies, longer linker sequence between two C2H2 ZFs, and frequent extra-sequences within C2H2 ZF motifs).

Conclusion

These features suggest that the tCWCH2 motif is a specialized motif involved in inter-zinc finger interactions.

Varying phenotypes in swine versus murine transgenic models constitutively expressing the same human Sonic hedgehog transcriptional activator, K5-HGLI2 Delta N

This study was undertaken to characterize the effects of constitutive expression of the hedgehog transcriptional activator, Gli2, in porcine skin. The keratinocyte-specific human transgene, K5-hGli2 Delta N, was used to produce transgenic porcine lines via somatic cell nuclear transfer techniques. In mice, K5-hGli2 Delta N induces epithelial downgrowths resembling basal cell carcinomas. Our porcine model also developed these basal cell carcinoma-like lesions, however gross tumor development was not appreciated. In contrast to the murine model, diffuse epidermal changes as well as susceptibility to cutaneous infections were seen in the swine model. Histologic analysis of transgenic piglets revealed generalized epidermal changes including: epidermal hyperplasia (acanthosis), elongated rete ridges, parakeratotic hyperkeratosis, epidermal neutrophilic infiltration, capillary loop dilation and hypogranulosis. By 2 weeks of age, the transgenic piglets developed erythematic and edematous lesions at high contact epidermal areas and extensor surfaces of distal limb joints. Despite antibiotic treatment, these lesions progressed to a deep bacterial pyoderma and pigs died or were euthanized within weeks of birth. Non-transgenic littermates were phenotypically normal by gross and histological analysis. In summary, constitutive expression of the human hGli2 Delta N in keratinocytes, results in cutaneous changes that have not been reported in the K5-hGli2 Delta N murine model. These findings indicate a need for a multiple species animal model approach in order to better understand the role of Gli2 in mammalian skin.

The primary cilium as a Hedgehog signal transduction machine

The Hedgehog (Hh) signal transduction pathway is essential for the development and patterning of numerous organ systems, and has important roles in a variety of human cancers. Genetic screens for mouse embryonic patterning mutants first showed a connection between mammalian Hh signaling and intraflagellar transport (IFT), a process required for construction of the primary cilium, a small cellular projection found on most vertebrate cells. Additional genetic and cell biological studies have provided very strong evidence that mammalian Hh signaling depends on the primary cilium. Here, we review the evidence that defines the integral roles that IFT proteins and cilia play in the regulation of the Hh signal transduction pathway in vertebrates. We discuss the mechanisms that control localization of Hh pathway proteins to the cilium, focusing on the transmembrane protein Smoothened (Smo), which moves into the cilium in response to Hh ligand. The phenotypes caused by loss of cilia-associated proteins are complex, which suggests that cilia and IFT play active roles in mediating Hh signaling rather than serving simply as a compartment in which pathway components are concentrated. Hh signaling in Drosophila does not depend on cilia, but there appear to be ancient links between cilia and components of the Hh pathway that may reveal how this fundamental difference between the Drosophila and mammalian Hh pathways arose in evolution.

delta-EF1 is a negative regulator of Ihh in the developing growth plate

Indian hedgehog (Ihh) regulates proliferation and differentiation of chondrocytes in the growth plate. Although the biology of Ihh is currently well documented, its transcriptional regulation is poorly understood. delta-EF1 is a two-handed zinc finger/homeodomain transcriptional repressor. Targeted inactivation of mouse delta-EF1 leads to skeletal abnormalities including disorganized growth plates, shortening of long bones, and joint fusions, which are reminiscent of defects associated with deregulation of Ihh signaling. Here, we show that the absence of delta-EF1 results in delayed hypertrophic differentiation of chondrocytes and increased cell proliferation in the growth plate. Further, we demonstrate that delta-EF1 binds to the putative regulatory elements in intron 1 of Ihh in vitro and in vivo, resulting in down-regulation of Ihh expression. Finally, we show that delta-EF1 haploinsufficiency leads to a postnatal increase in trabecular bone mass associated with enhanced Ihh expression. In summary, we have identified delta-EF1 as an in vivo negative regulator of Ihh expression in the growth plate.

Wnt signaling stimulates transcriptional outcome of the Hedgehog pathway by stabilizing GLI1 mRNA

Wnt and Hedgehog signaling pathways play central roles in embryogenesis, stem cell maintenance, and tumorigenesis. However, the mechanisms by which these two pathways interact are not well understood. Here, we identified a novel mechanism by which Wnt signaling pathway stimulates the transcriptional output of Hedgehog signaling. Wnt/beta-catenin signaling induces expression of an RNA-binding protein, CRD-BP, which in turn binds and stabilizes GLI1 mRNA, causing an elevation of GLI1 expression and transcriptional activity. The newly described mode of regulation of GLI1 seems to be important to several functions of Wnt, including survival and proliferation of colorectal cancer cells.

An Hh-dependent pathway in lateral plate mesoderm enables the generation of left/right asymmetry

Breaking bilateral symmetry is critical for vertebrate morphogenesis. In the mouse, directional looping of the heart and rotation of the embryo, the first overt evidence of left/right asymmetry (L/R), are observed at early somite stages ( approximately E8.5) [1, 2]. Activation of a Nodal-Pitx2 regulatory pathway specifically within the left lateral plate mesoderm (LPM) is critical for these events [3-10]. Asymmetric expression of Nodal is thought to be triggered by left-oriented, cilia-generated flow within the ventral, midline node [11, 12]. Genetic removal of Hedgehog (Hh) signaling in the mouse demonstrates a requirement for Hedgehog signals in the symmetry-breaking process [13], and analysis of node trafficking has suggested a mechanism of directional transport in the node that might relate to symmetry breaking in the LPM [14]. Here we provide evidence that Hedgehog signaling in the node is not essential for breaking bilateral symmetry. In contrast, direct Hh signaling in the LPM is critical. Evidence is presented that Sonic and Indian hedgehog signals act together, through a Foxf1/Bmp4 pathway, to enable the initiation and propagation of Nodal signaling within the LPM, regulating the competence of that tissue to respond to the Nodal pathway.

Establishing and interpreting graded Sonic Hedgehog signaling during vertebrate neural tube patterning: the role of negative feedback

The secreted protein Sonic Hedgehog (SHH) acts in graded fashion to pattern the dorsal-ventral axis of the vertebrate neural tube. This is a dynamic process in which increasing concentrations and durations of exposure to SHH generate neurons with successively more ventral identities. Interactions between the receiving cells and the graded signal underpin the mechanism of SHH action. In particular, negative feedback, involving proteins transcriptionally induced or repressed by SHH signaling, plays an essential role in shaping the graded readout. On one hand, negative feedback controls, in a noncell-autonomous manner, the distribution of SHH across the field of receiving cells. On the other, it acts cell-autonomously to convert different concentrations of SHH into distinct durations of intracellular signal transduction. Together, these mechanisms exemplify a strategy for morphogen interpretation, which we have termed temporal adaptation that relies on the continuous processing and refinement of the cellular response to the graded signal.

Expression of sonic hedgehog signaling molecules in normal, hyperplastic and carcinomatous endometrium

The aim of the present study was to determine the expression profile of the hedgehog (Hh) signaling molecules in normal, hyperplastic, and carcinomatous uterine endometrium. For this purpose, 271 endometrial tissue samples, (62 of normal endometrium, 127 of endometrial hyperplasias, and 82 endometrial adenocarcinomas) were studied using antibodies recognizing Hh-related signaling proteins, such as, sonic hedgehog (Shh), Patched (PTCH), Smoothened (Smo), Suppressor of fused [Su(Fu)], Gli-1, Gli-2, and Gli-3 by immunohistochemistry. The mRNA expression of these molecules was also assessed on reverse transcription-polymerase chain reaction. In the normal endometrium, the expression of Hh signaling molecules was generally downregulated except for Su(Fu), Gli-2, and Shh. In particular, the expression of both PTCH and Smo was very low or almost absent. Overall expression of Hh signaling molecules increased in hyperplastic endometrium; in particular, PTCH and Smo were significantly highly expressed in complex and atypical hyperplasia. In carcinoma samples extensive alterations were observed in the expression pattern of the signaling molecules. Nuclear Gli-2, cytoplasmic Gli-3, and Su(Fu) were overexpressed, whereas Shh, PTCH, and Smo expression were significantly reduced compared with the hyperplastic endometrium. The results suggest that the alteration of Hh signaling may be implicated in tumorigenesis of the endometrium.

Role of FGFR2-signaling in the pathogenesis of acne

It is the purpose of this review to extend our understanding of the fibroblast growth factor (FGF) receptor-2b-signaling network in the pathogenesis of acne. A new concept of the role of FGFR2b-signaling in dermal-epithelial interaction for skin appendage formation, pilosebaceous follicle homeostasis, comedogenesis, sebaceous gland proliferation and lipogenesis is presented. The FGFR2-gain-of-function mutations in Apert syndrome and unilateral acneiform nevus are most helpful model diseases pointing the way to androgen-dependent dermalepithelial FGFR2-signaling in acne. Androgen-mediated upregulation of FGFR2b-signaling in acne-prone skin appears to be involved in the pathogenesis of acne vulgaris. In organotypic skin cultures, keratinocyte-derived interleukin-1alpha stimulated fibroblasts to secrete FGF7 which stimulated FGFR2b-mediated keratinocyte proliferation. Postnatal deletion of FGFR2b in mice resulted in severe sebaceous gland atrophy. The importance of FGFR2b in sebaceous gland physiology is further supported by the mode of action of anti-acne agents which have been proposed to attenuate FGFR2b-signaling. Downregulation of FGFR2b-signaling by isotretinoin explains its therapeutic effect in acne. Downregulation of FGFR2b-signaling during the first trimester of pregnancy disturbs branched morphogenesis and explains retinoid embryotoxicity. Insulin-like growth factor-1 (IGF-1), the mediator of growth hormone during puberty, intracts with androgen-dependent FGFR2b-signaling and links androgen- and FGF-mediated signal transduction important in sebaceous gland homeostasis. The search for a follicular defect in the dermalepithelial regulation of growth factor-signaling in acne-prone skin appears to be a most promising approach to clarify the pathogenesis of acne.

Sonic hedgehog negatively regulates pre-TCR-induced differentiation by a Gli2-dependent mechanism

Hedgehog signaling regulates differentiation, survival, and proliferation of the earliest double-negative (DN) thymocytes, but its importance at later stages of T-cell development is controversial. Here we use loss- and gain-of-function mouse models to show that Shh, by signaling directly to the developing thymocyte, is a negative regulator of pre-TCR-induced differentiation from DN to double-positive (DP) cell. When hedgehog signaling was reduced, in the Shh(-/-) and Gli2(-/-) thymus, or by T lineage-specific transgenic expression of a transcriptional-repressor form of Gli2 (Gli2DeltaC(2)), differentiation to DP cell after pre-TCR signal transduction was increased. In contrast, when Hh signaling was constitutively activated in thymocytes, by transgenic expression of a constitutive transcriptional-activator form of Gli2 (Gli2DeltaN(2)), the production of DP cells was decreased. Gene expression profiling showed that physiologic Hh signaling in thymocytes maintains expression of the transcription factor FoxA2 on pre-TCR signal transduction.

Pattern formation in the vertebrate neural tube: a sonic hedgehog morphogen-regulated transcriptional network

Neuronal subtype specification in the vertebrate neural tube is one of the best-studied examples of embryonic pattern formation. Distinct neuronal subtypes are generated in a precise spatial order from progenitor cells according to their location along the anterior-posterior and dorsal-ventral axes. Underpinning this organization is a complex network of multiple extrinsic and intrinsic factors. This review focuses on the molecular mechanisms and general strategies at play in ventral regions of the forming spinal cord, where sonic hedgehog-based morphogen signaling is a key determinant. We discuss recent advances in our understanding of these events and highlight unresolved questions.

Expression of Indian Hedgehog signaling molecules in breast cancer

PURPOSE

To investigate the clinicopathological significance and expression pattern of Hedgehog (Hh) signaling molecules in breast normal glands and invasive ductal carcinoma.

MATERIALS AND METHODS

A total of 142 cases, including 21 of normal breast and 121 of invasive ductal carcinoma of the breast, were immunohistochemically analyzed for Ihh, Ptch, Smo, Gli-1, Gli-2, and Gli-3 protein expression.

RESULTS

All of Hh signaling molecules were greatly enhanced in invasive ductal carcinoma compared with the normal breast epithelia. The expressions of Ihh, Smo, and Gli-2 were increased in PR negative cases, and the expressions of Ihh, Ptch, and Gli-1/2/3 were statistically correlated with increased proliferating index of Ki-67 in invasive ductal carcinoma. Ihh and Gli-1/2/3 expressions were correlated with node metastasis. Additionally, the protein expressions of Ihh, Ptch, and Gli-2 were correlated with the clinical stage of breast cancer.

CONCLUSIONS

Hedgehog signaling molecules play an important role in the progression of invasive ductal carcinoma of breast.

CD spectra show the relational style between Zic-, Gli-, Glis-zinc finger protein and DNA

Zic family proteins have five C2H2-type zinc finger motifs. The Zic-zinc finger domains show high homology to the corresponding domains of the Gli and Glis families, which also contain five C2H2-type zinc finger motifs. The zinc finger motifs of the proteins of these three protein families form an alpha-helix conformation in solution. The addition of oligo DNA that included a Gli-binding sequence increased the alpha-helix content estimated by using circular dichroism spectroscopy. Comparison of the Zic-, Gli-, and Glis-zinc fingers indicated that the alpha-helix content after the addition of oligo DNA correlated well with the affinity of each zinc finger for the oligo DNA (correlation coefficient, 0.85). The importance of the zinc ion for protein folding was reflected in a reduction in the alpha-helix content upon removal of the zinc ion. Owing to the compact globular structure, the alpha-helix structure of the proteins of these three protein families is extremely thermally stable. These results suggest that the alpha-helix structure is important for DNA binding and profoundly related to functional and structural diversity among the three families.

Wnt canonical pathway restricts graded Shh/Gli patterning activity through the regulation of Gli3 expression

Dorsoventral patterning of the vertebrate nervous system is achieved by the combined activity of morphogenetic signals secreted from dorsal and ventral signalling centres. The Shh/Gli pathway plays a major role in patterning the ventral neural tube; however, the molecular mechanisms that limit target gene responses to specific progenitor domains remain unclear. Here, we show that Wnt1/Wnt3a, by signalling through the canonical beta-catenin/Tcf pathway, control expression of dorsal genes and suppression of the ventral programme, and that this role in DV patterning depends on Gli activity. Additionally, we show that Gli3 expression is controlled by Wnt activity. Identification and characterization of highly conserved non-coding DNA regions around the human Gli3 gene revealed the presence of transcriptionally active Tcf-binding sequences. These indicated that dorsal Gli3 expression might be directly regulated by canonical Wnt activity. In turn, Gli3, by acting as a transcriptional repressor, restricted graded Shh/Gli ventral activity to properly pattern the spinal cord.

Role of GLI2 transcription factor in growth and tumorigenicity of prostate cells

Aberrant activation of the Hedgehog (Hh) signaling pathway has been reported in various cancer types including prostate cancer. The GLI2 transcription factor is a primary mediator of Hh signaling. However, its relative contribution to development of prostate tumors is poorly understood. To establish the role of GLI2 in maintaining the tumorigenic properties of prostate cancer cells, we developed GLI2-specific small hairpin RNA. Knockdown of GLI2 in these cells resulted in significant down-regulation of the Hh signaling pathway, followed by inhibition of colony formation, anchorage-independent growth, and growth of xenografts in vivo. Conversely, ectopic expression of Gli2 in nontumorigenic prostate epithelial cells resulted in accelerated cell cycle progression, especially transition through G(2)-M, and augmented proliferation. Altogether, our findings suggest that GLI2 plays a critical role in the malignant phenotype of prostate cancer cells, and GLI2 may potentially become an attractive therapeutic target for the treatment of prostate cancer.

The sonic hedgehog signaling network in development and neoplasia

The sonic hedgehog (SHH) pathway was first defined genetically in fruit flies. Subsequently, the SHH network has been shown to be critical for normal mammalian development, by mediating interactions between stromal and epithelial cells. Recent evidence suggests that, deregulation of SHH signaling is important in the pathogenesis of cancer. Further, some observations suggest that a SHH paracrine mechanism mediating tumor-mesenchymal interactions may contribute to the metastatic capacity of cancer. Preclinical studies demonstrate that tumor cells in which SHH is deregulated are dependent on signaling through this pathway for the maintenance of proliferation and viability. SHH antagonists have been identified and show promise in inhibiting tumor growth in preclinical studies. The utility of these agents in the management of cancer patients awaits the outcome of ongoing and future clinical trials.

Sonic hedgehog signalling in T-cell development and activation

The production of mature functional T cells in the thymus requires signals from the thymic epithelium. Here, we review recent experiments showing that one way in which the epithelium controls the production of mature T cells is by the secretion of sonic hedgehog (SHH). We consider the increasing evidence that SHH-induced signalling is not only important for the differentiation and proliferation of early thymocyte progenitors, but also for modulating T-cell receptor signalling during repertoire selection, with implications for positive selection, CD4 versus CD8 lineage commitment, and clonal deletion of autoreactive cells. We also review the influence of hedgehog signalling in peripheral T-cell activation.

PTHrP regulates growth plate chondrocyte differentiation and proliferation in a Gli3 dependent manner utilizing hedgehog ligand dependent and independent mechanisms

Growth plate chondrocytes undergo a tightly regulated process of differentiation, allowing for the longitudinal growth of bones. Although it is known that parathyroid hormone related protein (PTHrP) and Indian hedgehog regulate the differentiation of growth plate chondrocytes, how these pathways interact to regulate chondrocyte development is not fully elucidated. We examined how the interaction between PTHrP and the hedgehog activated transcription factors, Gli2 and Gli3, regulates growth plate chondrocyte differentiation and proliferation. Analysis of fetal limbs showed that Gli2 is a negative regulator and Gli3 a positive regulator of type X collagen expression. Limb explant cultures showed that PTHrP treatment inhibited type X collagen expression and increased chondrocyte proliferation. This effect was substantially enhanced in Gli2-/- limbs, was blocked in Gli3-/- limbs, and was only partially inhibited by hedgehog ligand blockade. PTHrP negatively regulated Gli mediated transcription in cell cultures, and regulated the level of the repressor form of Gli3 in a PKA dependent manner. These results show that PTHrP regulates growth plate chondrocyte proliferation and differentiation in part through the activity of Gli3, suggesting a crucial role for Gli3 in growth plate chondrocyte development.

Molecular analysis of coordinated bladder and urogenital organ formation by Hedgehog signaling

The urogenital and reproductive organs, including the external genitalia, bladder and urethra, develop as anatomically aligned organs. Descriptive and experimental embryology suggest that the cloaca, and its derivative, the urogenital sinus, contribute to the formation of these organs. However, it is unknown how the primary tissue lineages in, and adjacent to, the cloaca give rise to the above organs, nor is bladder formation understood. While it is known that sonic hedgehog (Shh) is expressed by the cloacal epithelia, the developmental programs that regulate and coordinate the formation of the urogenital and reproductive organs have not been elucidated. Here we report that Shh mutant embryos display hypoplasia of external genitalia, internal urethra (pelvic urethra) and bladder. The importance of Shh signaling in the development of bladder and external genitalia was confirmed by analyzing a variety of mutant mouse lines with defective hedgehog signaling. By genetically labeling hedgehog-responding tissue lineages adjacent to the cloaca and urogenital sinus, we defined the contribution of these tissues to the bladder and external genitalia. We discovered that development of smooth muscle myosin-positive embryonic bladder mesenchyme requires Shh signaling, and that the bladder mesenchyme and dorsal (upper) external genitalia derive from Shh-responsive peri-cloacal mesenchyme. Thus, the mesenchymal precursors for multiple urogenital structures derive from peri-cloacal mesenchyme and the coordination of urogenital organ formation from these precursors is orchestrated by Shh signals.

Odontogenic keratocysts arise from quiescent epithelial rests and are associated with deregulated hedgehog signaling in mice and humans

Odontogenic keratocysts in humans are aggressive, noninflammatory jaw cysts that may harbor PTCH1 mutations, leading to constitutive activity of the embryonic Hedgehog (Hh) signaling pathway. We show here that epithelial expression of the Hh transcriptional effector Gli2 is sufficient for highly penetrant keratocyst development in transgenic mice. Mouse and human keratocysts expressed similar markers, leading to tooth misalignment, bone remodeling, and craniofacial abnormalities. We detected Hh target gene expression in epithelial cells lining keratocysts from both species, implicating deregulated Hh signaling in their development. Most mouse keratocysts arose from rests of Malassez--quiescent, residual embryonic epithelial cells that remain embedded in the periodontal ligament surrounding mature teeth. In Gli2-expressing mice, these rests were stimulated to proliferate, stratify, and form a differentiated squamous epithelium. The frequent development of keratocysts in Gli2-expressing mice supports the idea that GLI transcription factor activity mediates pathological responses to deregulated Hh signaling in humans. Moreover, Gli2-mediated reactivation of quiescent epithelial rests to form keratocysts indicates that these cells retain the capacity to function as progenitor cells on activation by an appropriate developmental signal.

Enhanced expression of hedgehog signaling molecules in squamous cell carcinoma of uterine cervix and its precursor lesions

The hedgehog (Hh)-signaling pathway plays an essential role in normal development. Deregulation of this pathway is responsible for several types of cancers. The aim of this study was to determine the expression pattern and the extent of Hh-signaling molecules in squamous cell carcinoma of uterine cervix and its precursor lesions. A total of 106 uterine cervical cancers and related lesions (37 squamous cell carcinomas, 23 cervical intraepithelial neoplasia (CIN) III, 10 CIN II, four CIN I, 32 normal cervical epithelia) were immunohistochemically analyzed with anti-Shh, Indian Hh (Ihh), Patched (PTCH), Smoothened (Smo), Gli-1, Gli-2, Gli-3 antibodies on paraffin blocks. The results showed that the expression of all the Hh-signaling molecules was greatly enhanced in uterine cervical tumors, including carcinoma and its precursor lesions. The staining pattern was mainly cytoplasmic except for Gli-1/2, whose expression was observed in both cytoplasm and nucleus. In case of Ihh, PTCH, Smo and Gli-1, their expression in normal epithelium was completely absent or rare. The expression of all the seven Hh-signaling molecules mentioned above was significantly increased in CIN II/III and carcinoma, compared with that in normal epithelium (P < 0.05). The expression of Shh was increased by double; the first increase occurred in normal epithelium-CIN transition, and the second, during the progression of CIN to carcinoma. These results strongly suggest that the Hh-signaling pathways were extensively activated in carcinoma and CIN of uterine cervix. In conclusion, the Hh-signaling pathways may be involved in carcinogenesis of squamous cell carcinoma of uterine cervix and can be considered as a potential therapeutic target.

Signaling from Smo to Ci/Gli: conservation and divergence of Hedgehog pathways from Drosophila to vertebrates

Although the framework of the Hedgehog (Hh) signaling pathway is evolutionarily conserved, recent studies indicate that fundamental differences exist between Drosophila and vertebrates in the way signals are transduced from the membrane protein Smoothened (Smo) to the Ci/Gli transcription factors. For example, Smo structure and the roles of fused and Suppressor of fused have diverged. Recently, many vertebrate-specific components have been identified that act between Smo and Gli. These include intra-flagellar transport proteins, which link vertebrate Hh signaling to cilia. Because abnormal Hh signaling can cause birth defects and cancer, these vertebrate-specific components may have roles in human health.

Gli2 is targeted for ubiquitination and degradation by beta-TrCP ubiquitin ligase

The Hedgehog (Hh) signaling pathway plays a crucial role in embryogenesis and has been linked to the development of several human malignancies. The transcription factor Gli2 plays a key role in the transduction of Hh signals by modulating transcription of some Hh target genes, yet the mechanisms that control Gli2 protein expression are largely unknown. Here we report that beta-transducin repeat-containing protein (beta-TrCP) E3 ubiquitin ligase is required for Gli2 degradation. beta-TrCP2 directly binds wild type Gli2 and promotes its ubiquitination. Single amino acid substitution in Gli2 putative binding site inhibits its interaction with beta-TrCP2, its ubiquitination, and stabilizes the Gli2 protein. Stable Gli2 mutant is expressed in higher levels and is more potent in the activation of Gli-dependent transcription as compared with wild type Gli2. We also found that GLI2 protein is expressed highly in prostate cancer cell lines and primary tumors, whereas the level of GLI2 mRNA is not appreciably different in normal and neoplastic prostate. These data identify beta-TrCP2 as a pivotal regulator of Gli2 expression and point to an important role for posttranslational modulation of GLI2 protein levels in Hh pathway-associated human prostate cancer.

Sonic hedgehog mRNA expression by real-time quantitative PCR in normal and tumor tissues from colorectal cancer patients

Shh is expressed in the early stages of embryogenesis and in the foregut development. Although Shh has been shown to be overexpressed in brain, pancreas, gastric and lung cancers, its role in the development of colorectal cancer has not been examined. We used real-time quantitative PCR to assess Shh mRNA expression levels in tumor and matched normal tissue from 57 colorectal cancer patients and correlated the results with patient clinicopathological characteristics. Shh expression levels were higher in tumor tissue than in normal tissue from the same patient (P=0.00001). Higher levels of Shh expression were associated with early stage disease (P=0.02). Shh overexpression may influence the development of colorectal cancer.

Regulatory role for Krüppel-like zinc-finger protein Gli-similar 1 (Glis1) in PMA-treated and psoriatic epidermis

In this study, we analyze the expression and potential function of the Krüppel-like zinc-finger protein Gli-similar protein 1 (Glis1) in normal and inflammatory skin and in the differentiation of epidermal keratinocytes. Glis1 mRNA is not expressed in normal human epidermis, but is significantly induced in psoriatic epidermis and in mouse skin upon treatment with the tumor promoter phorbol-12-myristate-13-acetate (PMA). The expression of Glis1 is restricted to the suprabasal layers. These observations suggest that Glis1 expression is associated with hyperplastic, inflammatory epidermis. Consistent with these findings, Glis1 mRNA is not expressed in undifferentiated or differentiated normal human epidermal keratinocytes (NHEK) in culture, but is dramatically induced after the addition of PMA or interferon gamma. A similar induction of Glis1 mRNA by PMA treatment was observed in the immortalized epidermal keratinocyte cell line NHEK-HPV, whereas PMA did not induce Glis1 in HaCaT cells or in several squamous cell carcinoma cell lines. To obtain insight into its function, Glis1 and a C-terminal deletion mutant Glis1DeltaC were expressed in NHEK-HPV cells and changes in epidermal differentiation and gene expression examined. Microarray analysis revealed that Glis1DeltaC promoted PMA-induced epidermal differentiation, as indicated by increased expression of many differentiation-specific genes. This, in association with its induction in psoriasis, suggests that transcriptional factor Glis1 is involved in the regulation of aberrant differentiation observed in psoriatic epidermis.

Medulloblastoma: mouse models and novel targeted therapies based on the Sonic hedgehog pathway

Understanding molecular pathways, signaling cascades, and genetic alterations activated during tumorigenesis is essential for the development of targeted cancer treatments. In children, tumors of the central nervous system are thought to arise from progenitor cells that show considerable temporal and spatial heterogeneity in a developmental environment that is different from that of the adult. Investigating the molecular basis of pediatric tumors is critical because it is likely to generate novel treatments. Animal models have brought many important advances in this field. In this review the authors discuss the mouse models based on the Sonic hedgehog pathway, which have provided a better knowledge of the genetic and molecular alterations of medulloblastoma.

A previously unidentified amino-terminal domain regulates transcriptional activity of wild-type and disease-associated human GLI2

Zinc finger-containing Gli proteins mediate responsiveness to Hedgehog (Hh) signaling, with Gli2 acting as the major transcriptional activator in this pathway in mice. The discovery of disease-associated mutations points to a critical role for GLI2 in human Hh signaling as well. Here, we show that human GLI2 contains previously undescribed 5' sequence, extending the amino-terminus an additional 328 amino acids. In vitro, transcriptional activity of full-length GLI2 is up to 30 times lower than that of GLI2DeltaN (previously thought to represent the entire GLI2 protein), revealing the presence of an amino-terminal repressor domain in the full-length protein. GLI2DeltaN also exhibits potent transcriptional activity in vivo: overexpression in mouse skin leads to the formation of Hh-independent epithelial downgrowths resembling basal cell carcinomas, which in humans are associated with constitutive Hh signaling. The discovery of this additional, functionally relevant GLI2 sequence led us to re-examine several pathogenic human GLI2 mutants, now containing the entire amino-terminal domain. On the basis of the functional domains affected by the mutations, mutant GLI2 proteins exhibited either loss-of-function or dominant-negative activity. Moreover, deletion of the amino-terminus abrogated dominant-negative activity of mutant GLI2, revealing that this domain is required for transcriptional repressor activity of pathogenic GLI2. Our results establish the presence of an amino-terminal transcriptional repressor domain that plays a critical role in modulating the function of wild-type GLI2 and is essential for dominant-negative activity of a GLI2 mutant associated with human disease.

Hedgehog signaling in murine vasculogenesis and angiogenesis

Vasculogenesis-the formation of blood vessels de novo from endothelial cells-and angiogenesis-the process of blood vessel remodeling-are regulated by a number of signal transduction pathways, some specific to the vascular system and others used more broadly during embryogenesis. Recent evidence in both zebrafish and mouse suggests a role for Hedgehog (Hh) signaling in both vasculogenesis and angiogenesis. Hh signaling can target endothelial cells directly or can stimulate blood vessel support cells to produce vascular growth factors. Current studies are aimed at determining how the Hh cascade interacts with the other signaling pathways to promote vessel differentiation.

The transcription factor Gli3 regulates differentiation of fetal CD4- CD8- double-negative thymocytes

Glioblastoma 3 (Gli3) is a transcription factor involved in patterning and oncogenesis. Here, we demonstrate a role for Gli3 in thymocyte development. Gli3 is differentially expressed in fetal CD4- CD8- double-negative (DN) thymocytes and is most highly expressed at the CD44+ CD25- DN (DN1) and CD44- CD25- (DN4) stages of development but was not detected in adult thymocytes. Analysis of null mutants showed that Gli3 is involved at the transitions from DN1 to CD44+ CD25+ DN (DN2) cell and from DN to CD4+ CD8+ double-positive (DP) cell. Gli3 is required for differentiation from DN to DP thymocyte, after pre-T-cell receptor (TCR) signaling but is not necessary for pre-TCR-induced proliferation or survival. The effect of Gli3 was dose dependent, suggesting its direct involvement in the transcriptional regulation of genes controlling T-cell differentiation during fetal development.

MIM/BEG4, a Sonic hedgehog-responsive gene that potentiates Gli-dependent transcription

Sonic hedgehog (Shh) signaling plays a critical role during development and carcinogenesis. While Gli family members govern the transcriptional output of Shh signaling, little is known how Gli-mediated transcriptional activity is regulated. Here we identify the actin-binding protein Missing in Metastasis (MIM) as a new Shh-responsive gene. Together, Gli1 and MIM recapitulate Shh-mediated epidermal proliferation and invasion in regenerated human skin. MIM is part of a Gli/Suppressor of Fused complex and potentiates Gli-dependent transcription using domains distinct from those used for monomeric actin binding. These data define MIM as both a Shh-responsive gene and a new member of the pathway that modulates Gli responses during growth and tumorigenesis.

Activation of the BCL2 promoter in response to Hedgehog/GLI signal transduction is predominantly mediated by GLI2

Aberrant activation of the Hedgehog (HH)/GLI signaling pathway has been implicated in the development of basal cell carcinoma (BCC). The zinc finger transcription factors GLI1 and GLI2 are considered mediators of the HH signal in epidermal cells, although their tumorigenic nature and their relative contribution to tumorigenesis are only poorly understood. To shed light on the respective role of these transcription factors in epidermal neoplasia, we screened for genes preferentially regulated either by GLI1 or GLI2 in human epidermal cells. We show here that expression of the key antiapoptotic factor BCL2 is predominantly activated by GLI2 compared with GLI1. Detailed promoter analysis and gel shift assays identified three GLI binding sites in the human BCL2 cis-regulatory region. We found that one of these binding sites is critical for conferring GLI2-specific activation of the human BCL2 promoter and that the selective induction of BCL2 expression depends on the zinc finger DNA binding domain of GLI2. In vivo, GLI2 and BCL2 were coexpressed in the outer root sheath of hair follicles and BCC and in plasma cells that infiltrated BCC tumor islands. On the basis of the latter observation, we analyzed plasma cell-derived tumors and found strong expression of GLI2 and BCL2 in neoplastic cells of plasmacytoma patients, implicating HH/GLI signaling in the development of plasma cell-derived malignancies. The results reveal a central role for GLI2 in activating the prosurvival factor BCL2, which may represent an important mechanism in the development or maintenance of cancers associated with inappropriate HH signaling.

Using RNAi to catch Drosophila genes in a web of interactions: insights into cancer research

The completion of whole-genome sequencing of various model organisms and the recent explosion of new technologies in the field of Functional Genomics and Proteomics is poised to revolutionize the way scientists identify and characterize gene function. One of the most significant advances in recent years has been the application of RNA interference (RNAi) as a means of assaying gene function. In the post-genomic era, advances in the field of cancer biology will rely upon the rapid identification and characterization of genes that regulate cell growth, proliferation, and apoptosis. Significant efforts are being directed towards cancer therapy and devising efficient means of selectively delivering drugs to cancerous cells. In this review, we discuss the promise of integrating genome-wide RNAi screens with proteomic approaches and small-molecule chemical genetic screens, towards improving our ability to understand and treat cancer.

Molecular cloning, structure, expression, and chromosomal localization of the human Osterix (SP7) gene

We report the isolation of the human orthologue of the mouse Osterix (Osx/Sp7) gene, a C2H2 zinc finger transcription factor of the SP gene family and putative "master" regulator of bone cell differentiation. The human SP7 cDNA encodes a putative 431 amino acid protein that contains three consecutive C2H2 zinc finger repeats. The SP7 protein is highly conserved between mice and humans with an overall sequence identity of 95%. The expression of a SP7 mRNA transcript of approximately 3.2 kb is restricted to bone-derived cell lines in vitro but undetectable in any adult tissues including mandibular bone by Northern blot hybridization. The specific expression of SP7 mRNA in osteoblasts in vivo was further confirmed by in situ hybridization on human embryonic tissues. The highly restricted expression pattern and the divergence of the sequence outside of the zinc finger region distinguish SP7 as a unique member of the SP family. The SP7 gene consists of two exons, with exon 2 containing most of the protein coding sequence. The gene locus was mapped to chromosome 12q13.13 by fluorescent in situ hybridization (FISH). The identification and initial characterization of the SP7 gene will facilitate the study of the molecular regulation of osteoblast differentiation in humans.

The role of Zic genes in neural development

The Zic family of zinc-finger proteins plays a crucial role in neural development. Zic genes are vertebrate homologs of odd-paired, the Drosophila pair-rule gene. Their gene products have zinc-finger domains similar to those of Gli proteins, which act as transcriptional regulators in hedgehog signaling. Recent studies of human, mouse, frog, fish and ascidian Zic homologs have provided evidence that Zic genes are involved in a variety of developmental processes, including neurogenesis, myogenesis, skeletal patterning, and left-right axis establishment. Zic genes appear to have multiple roles in neural development. They control the initial phase during which ectoderm differentiates into neuroectoderm, and they may act as bridges between secreted neural tissue induction signals and the basic-helix-loop-helix class of neurogenesis-inducing transcriptional regulatory factors. Studies of loss-of-function mutations with differing Zic gene subtypes show that the Zic family of genes controls the process of neurulation. Mutations result in neural tube defects, which are seen at different rostrocaudal levels depending on which Zic gene subtype has been affected. Development of holoprosencephaly, forebrain anomalies, and cerebellar dysgenesis indicate that region-specific morphogenesis of the CNS is also controlled by Zic genes. The underlying molecular actions of Zic gene products, which allow them to control development, remain a mystery. Recent molecular characterization has shown that Zic proteins are able to bind Gli-binding DNA sequences in a sequence-specific manner, but with lower affinity than Gli proteins. Zic proteins also can activate transcription from several promoters. Furthermore, Zic and Gli proteins interact physically via their zinc-finger domains, raising the possibility that Zic proteins can act as transcriptional cofactors and modulate the hedgehog-signaling pathway. Clarification of the specific cooperating factors is therefore required in each case. Other evidence also suggests that Zic proteins can inhibit neuronal differentiation by activating Notch signals. This association might be is a clue toward understanding of the multifunctional property of Zic proteins because Notch signaling also is implicated in the control of several developmental processes.

Gonadal mosaicism in severe Pallister-Hall syndrome

Pallister-Hall syndrome (PHS, MIM #146510) is characterized by central and postaxial polydactyly, hypothalamic hamartoma (HH), bifid epiglottis, imperforate anus, renal abnormalities, and pulmonary segmentation anomalies. It is inherited in an autosomal dominant pattern. Here, we describe a family with two affected children manifesting severe PHS with mental retardation, behavioral problems, and intractable seizures. Both parents are healthy, with normal intelligence, and have no malformations on physical, laryngoscopic, and cranial MRI exam. The atypical presentation of these children and the absence of parental manifestations suggested an autosomal recessive mode of inheritance or gonadal mosaicism. Sequencing of GLI3 revealed a two nucleotide deletion in exon 15 (c.3385_3386delTT) predicting a frameshift and premature stop at codon 1129 (p.F1129X) in the children while both parents have wild type alleles. Genotyping with GLI3 intragenic markers revealed that both children inherited the abnormal allele from their mother thus supporting gonadal mosaicism as the underlying mechanism of inheritance (paternity was confirmed). This is the first reported case of gonadal mosaicism in PHS. The severe CNS manifestations of these children are reminiscent of children with non-syndromic HH who often have progressive mental retardation with behavioral problems and intractable seizures. We conclude that the phenotypic spectrum of PHS can include severe CNS manifestations and that recurrence risks for PHS should include a proviso for gonadal mosaicism, though the frequency cannot be calculated from a single case report. Published 2003 Wiley-Liss, Inc.

The FU gene and its possible protein isoforms

Background

FU is the human homologue of the Drosophila gene fused whose product fused is a positive regulator of the transcription factor Cubitus interruptus (Ci). Thus, FU may act as a regulator of the human counterparts of Ci, the GLI transcription factors. Since Ci and GLI are targets of Hedgehog signaling in development and morphogenesis, it is expected that FU plays an important role in Sonic, Desert and/or Indian Hedgehog induced cellular signaling.

Results

The FU gene was identified on chromosome 2q35 at 217.56 Mb and its exon-intron organization determined. The human developmental disorder Syndactyly type 1 (SD1) maps to this region on chromosome 2 and the FU coding region was sequenced using genomic DNA from an affected individual in a linked family. While no FU mutations were found, three single nucleotide polymorphisms were identified. The expression pattern of FU was thoroughly investigated and all examined tissues express FU. It is also clear that different tissues express transcripts of different sizes and some tissues express more than one transcript. By means of nested PCR of specific regions in RT/PCR generated cDNA, it was possible to verify two alternative splicing events. This also suggests the existence of at least two additional protein isoforms besides the FU protein that has previously been described. This long FU and a much shorter isoform were compared for the ability to regulate GLI1 and GLI2. None of the FU isoforms showed any effects on GLI1 induced transcription but the long form can enhance GLI2 activity. Apparently FU did not have any effect on SUFU induced inhibition of GLI.

Conclusions

The FU gene and its genomic structure was identified. FU is a candidate gene for SD1, but we have not identified a pathogenic mutation in the FU coding region in a family with SD1. The sequence information and expression analyses show that transcripts of different sizes are expressed and subjected to alternative splicing. Thus, mRNAs may contain different 5'UTRs and encode different protein isoforms. Furthermore, FU is able to enhance the activity of GLI2 but not of GLI1, implicating FU in some aspects of Hedgehog signaling.

GLI2 is expressed in normal human epidermis and BCC and induces GLI1 expression by binding to its promoter

Sonic hedgehog (Shh) binds to its receptor patched (PTCH), leading to the activation and repression of target genes via the GLI family of zinc-finger transcription factors. Deregulation of the Shh pathway is associated with basal cell carcinoma (BCC) due to upregulation of GLI1 and GLI2. We recently demonstrated a positive feedback loop between GLI1 and GLI2, which revealed that GLI1 may be a direct target of GLI2. Using band shift and luciferase reporter assays, we now show that GLI2 binds the GLI-binding consensus sequence in the GLI1 promoter. These data suggest that GLI2 directly activates GLI1 and that retrovirally expressed GLI2 induces expression of endogenous GLI1 in human primary keratinocytes. Finally, using in situ hybridization, we show that GLI2 is expressed in the interfollicular epidermis and the outer root sheath of hair follicles in normal skin as well as in BCC tumor islands. These results suggest an important role for GLI2 in regulating epidermal proliferation and skin tumorigenesis.

Oncogenic aberrations of cullin-dependent ubiquitin ligases

Accumulating evidence points to a key role of the ubiquitin-proteasome pathway in oncogenesis. Aberrant proteolysis of substrates involved in cellular processes such as the cell division cycle, gene transcription, the DNA damage response and apoptosis has been reported to contribute significantly to neoplastic transformation. Cullin-dependent ubiquitin ligases (CDLs) form a class of structurally related multisubunit enzymes central to the ubiquitin-mediated proteolysis of many important biological substrates. In this review, we describe the role of CDLs in the ubiquitinylation of cancer-related substrates and discuss how altered ubiquitinylation by CDLs may contribute to tumor development.

Hedgehog signaling regulates sebaceous gland development

Epithelial progenitor cells in skin give rise to multiple lineages, comprising the hair follicle, an associated sebaceous gland, and overlying epidermis; however, the signals that regulate sebocyte development are poorly understood. We tested the potential involvement of the Hedgehog pathway in sebaceous gland development using transgenes designed to either block or stimulate Hedgehog signaling in cutaneous keratinocytes in vivo. Whereas inhibition of the Hedgehog pathway selectively suppressed sebocyte development, Hedgehog pathway activation led to a striking increase both in size and number of sebaceous glands. Remarkably, ectopic Hedgehog signaling also triggered the formation of sebaceous glands from footpad epidermis, in regions normally devoid of hair follicles and associated structures. These ectopic sebaceous glands expressed molecular markers of sebocyte differentiation and were functional, secreting their contents directly onto the skin's surface instead of into a hair canal. The Hedgehog pathway thus plays a key role in sebocyte cell fate decisions and is a potential target for treatment of skin disorders linked to abnormal sebaceous gland function, such as acne.

A Requirement for Retinoic Acid-Mediated Transcriptional Activation in Ventral Neural Patterning and Motor Neuron Specification

The specification of neuronal fates in the ventral spinal cord depends on the regulation of homeodomain (HD) and basic-helix-loop-helix (bHLH) proteins by Sonic hedgehog (Shh). Most of these transcription factors function as repressors, leaving unresolved the link between inductive signaling pathways and transcriptional activators involved in ventral neuronal specification. We show here that retinoid signaling and the activator functions of retinoid receptors are required to pattern the expression of HD and bHLH proteins and to specify motor neuron identity. We also show that fibroblast growth factors (FGFs) repress progenitor HD protein expression, implying that evasion of FGF signaling and exposure to retinoid and Shh signals are obligate steps in the emergence of ventral neural pattern. Moreover, joint exposure of neural progenitors to retinoids and FGFs suffices to induce motor neuron differentiation in a Shh-independent manner.

Dysregulated Sonic hedgehog signaling and medulloblastoma consequent to IFN-alpha-stimulated STAT2-independent production of IFN-gamma in the brain

The type I IFNs (IFN-alpha and IFN-beta), which are crucial in antiviral defense and immune regulation, signal via the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway with activation of STAT1 and STAT2. Here, the function of STAT2 was studied in transgenic mice (termed GIFN/STAT2-/-) with CNS production of IFN-alpha. Surprisingly, GIFN/STAT2-/-, but not GIFN/STAT1-null, transgenic mice, with CNS production of IFN-alpha, died prematurely with medulloblastoma. An immune response also induced in the brain of the GIFN/STAT2-/- mice was associated with IFN-gamma gene expression by CD3+ T cells and the activation of the STAT1, STAT3, STAT4, and STAT5 molecules. Expression of the Sonic hedgehog (Shh) and the downstream transcriptional factor Gli-1 genes, implicated in the pathogenesis of medulloblastoma, was found to be significantly increased and cotranscribed in cerebellar granule neurons of the GIFN/STAT2-/- mice. IFN-gamma, but not IFN-alpha, induced STAT1-dependent expression of the Shh gene in cultured cerebellar granule neurons. Thus, there is an unexpected and extraordinarily adverse biological potency of IFN-alpha in the CNS when the primary signal transduction molecule STAT2 is absent. Moreover, a hitherto unknown role is indicated for the immune system in the pathogenesis of developmental disorders and tumorigenesis of the CNS via dysregulated Shh signaling mediated by IFN-gamma.

Gli2 is required for normal Shh signaling and oligodendrocyte development in the spinal cord

Recent studies have demonstrated that oligodendrocyte progenitor (OLP) cells are induced from the ventral neural tube by the ventral midline signal, Sonic hedgehog (Shh). In this study, we investigated the role of Gli2 signal transducer in Shh induction of oligodendrocytes by studying oligodendrocyte development in Gli2-null mutants. In the absence of Gli2, the Olig1/2+ oligodendrogenic domain in the ventral spinal neuroepithelium is markedly reduced, and the initial production of OLP cells from the ventral neuroepithelium is much decreased and delayed. However, at late gestation stages, there is no discernible difference in the steady-state number of OLPs between the wild type and mutants. Interestingly, the initial delay and reduction of OLP production in the mutants is associated with a delayed expression of myelin-specific genes and oligodendrocyte differentiation. In contrast to oligodendrogenesis in the spinal cord, oligodendrocyte development in the forebrain is unaffected by Gli2 mutation. Together, our studies have suggested that Gli2 plays an important role in regulating oligodendrocyte specification and differentiation in the caudal neural tube.