A paracrine requirement for hedgehog signalling in cancer

Small-molecule modulators of the Sonic Hedgehog signaling pathway

Sonic hedgehog (Shh) is the most widely characterized of the three vertebrate Hedgehog homologs, and is essential for proper embryonic development. Shh binds to its receptor, Patched (Ptch1), resulting in the de-repression of Smoothened (Smo). This leads to the activation of Gli2, which regulates the transcription of target genes that include Gli1 and Ptch1. Several synthetic and naturally occurring small-molecule modulators of Smo have been discovered. Shh-signaling antagonists that bind to Smo include cyclopamine, SANT1, and Cur-61414. Shh signaling agonists that bind to Smo include the synthetic small molecules purmorphamine and SAG. Small molecules that inhibit Shh signaling downstream of Smo, GANT58 and GANT61 have also been reported. Robotnikinin inhibits the Shh pathway by directly targeting Shh. Although progress has been made in understanding and modulating Shh signaling, fundamental aspects of Shh signal transduction remain obscure, including the mechanism(s) whereby Ptch1 regulates Smo activity. Small-molecule modulators of Shh signaling provide a means to regulate the activity of a pathway implicated in medulloblastoma, basal cell carcinoma (BCC), pancreatic cancer, prostate cancer and developmental disorders. Several Shh inhibitors have not succeeded in the clinic for unknown reasons, but clinical trials in BCC and pancreatic cancer with the promising Smo antagonists GDC-0449 and IPI-926 are currently underway.

Discovery of a potent and orally active hedgehog pathway antagonist (IPI-926)

Recent evidence suggests that blocking aberrant hedgehog pathway signaling may be a promising therapeutic strategy for the treatment of several types of cancer. Cyclopamine, a plant Veratrum alkaloid, is a natural product antagonist of the hedgehog pathway. In a previous report, a seven-membered D-ring semisynthetic analogue of cyclopamine, IPI-269609 (2), was shown to have greater acid stability and better aqueous solubility compared to cyclopamine. Further modifications of the A-ring system generated three series of analogues with improved potency and/or solubility. Lead compounds from each series were characterized in vitro and evaluated in vivo for biological activity and pharmacokinetic properties. These studies led to the discovery of IPI-926 (compound 28), a novel semisynthetic cyclopamine analogue with substantially improved pharmaceutical properties and potency and a favorable pharmacokinetic profile relative to cyclopamine and compound 2. As a result, complete tumor regression was observed in a Hh-dependent medulloblastoma allograft model after daily oral administration of 40 mg/kg of compound 28.

Primary cilia can both mediate and suppress Hedgehog pathway-dependent tumorigenesis

Primary cilia are present on most mammalian cells and are implicated in transducing Hedgehog (Hh) signals during development; however, the prevalence of cilia on human tumors remains unclear, and the role of cilia in cancer has not been examined. Here we show that human basal cell carcinomas (BCCs) are frequently ciliated, and we test the role of cilia in BCC by conditionally deleting Kif3a (encoding kinesin family member 3A) or Ift88 (encoding intraflagellar transport protein 88), genes required for ciliogenesis, in two Hh pathway-dependent mouse tumor models. Ciliary ablation strongly inhibited BCC-like tumors induced by an activated form of Smoothened. In contrast, removal of cilia accelerated tumors induced by activated Gli2, a transcriptional effector of Hh signaling. These seemingly paradoxical effects are consistent with a dual role for cilia in mediating both the activation and the repression of the Hh signaling pathway. Our findings demonstrate that cilia function as unique signaling organelles that can either mediate or suppress tumorigenesis depending on the nature of the oncogenic initiating event.

Phage-display selection on tumor histological specimens with laser capture microdissection

A method was developed to obtain phage-display ligands that bind to a select population of cells in histological specimens of freshly harvested solid human cancers. It combines phage-display panning with laser capture microdissection (LCM). This method allows selection of phage ligands bound to subpopulations of specific cells contained in tumor tissue on histological sections. Naïve phage scFv library was incubated directly on a histological section of human breast cancer that was snap frozen immediately after surgical resection. Tumor and stromal cells were captured by LCM and bound phages were recovered by bacterial infection. Individual phage clones selected after panning were evaluated for their binding ability by immunofluorescence staining on tumor tissue from the same patient. One phage-display antibody clone selected on tumor stroma showed selective binding on tumor stroma but did not bind to malignant cell population. The expressed scFv of this clone showed no significant binding to normal tissue, or 13 other breast cancers, or 4 colon cancer samples. Using the same method, phage display antibody clones were selected on tumor cells which showed binding to tumor cells and normal tissue. This method is applicable for selection of ligands to virtually any portion of a histological specimen amenable to LCM. This may speed the process of generating ligands to any subset of cells or noncellular feature present on histological specimens.

GDC-0449-a potent inhibitor of the hedgehog pathway

SAR for a wide variety of heterocyclic replacements for a benzimidazole led to the discovery of functionalized 2-pyridyl amides as novel inhibitors of the hedgehog pathway. The 2-pyridyl amides were optimized for potency, PK, and drug-like properties by modifications to the amide portion of the molecule resulting in 31 (GDC-0449). Amide 31 produced complete tumor regression at doses as low as 12.5mg/kg BID in a medulloblastoma allograft mouse model that is wholly dependent on the Hh pathway for growth and is currently in human clinical trials, where it is initially being evaluated for the treatment of BCC.

Activation of hedgehog signaling inhibits osteoblast differentiation of human mesenchymal stem cells

Mesenchymal stem cells within the bone are responsible for the generation of osteoblasts, chondrocytes, and adipocytes. In rodents, Indian hedgehog has been shown to play a role in osteoblast differentiation. However, evidence for a direct function of hedgehog (Hh) in human osteoblastic differentiation is missing. Using different models of human mesenchymal stem cells we show that Hh signaling decreases during osteoblast differentiation. This is associated with a decrease in Smoothened expression, a key partner that triggers Hh signaling, and in the number of cells displaying a primary cilium, an organelle necessary for Hh signaling. Remarkably, treatment of human mesenchymal stem cells with sonic hedgehog or two molecules able to activate Hh signaling inhibits osteoblast differentiation. This inhibition is visualized through a decrease in mineralization and in the expression of osteoblastic genes. In particular, activation of Hh signaling induces a decrease in Runx2 expression, a key transcriptional factor controlling the early stage of osteoblast differentiation. Consistently, the activation of Hh signaling during the first days of differentiation is sufficient to inhibit osteoblast differentiation, whereas differentiated osteoblasts are not affected by Hh signaling. In summary, we show here, using various inducers of Hh signaling and mesenchymal stem cells of two different origins, that Hh signaling inhibits human osteoblast differentiation, in sharp contrast to what has been described in rodent cells. This species difference should be taken into account for screening for pro-osteogenic molecules.

Small-molecule inhibitors reveal multiple strategies for Hedgehog pathway blockade

Inappropriate activation of the Hedgehog (Hh) signaling pathway has been implicated in a diverse spectrum of cancers, and its pharmacological blockade has emerged as an anti-tumor strategy. While nearly all known Hh pathway antagonists target the transmembrane protein Smoothened (Smo), small molecules that suppress downstream effectors could more comprehensively remediate Hh pathway-dependent tumors. We report here four Hh pathway antagonists that are epistatic to the nucleocytoplasmic regulator Suppressor of Fused [Su(fu)], including two that can inhibit Hh target gene expression induced by overexpression of the Gli transcription factors. Each inhibitor has a unique mechanism of action, and their phenotypes reveal that Gli processing, Gli activation, and primary cilia formation are pharmacologically targetable. We further establish the ability of certain compounds to block the proliferation of cerebellar granule neuron precursors expressing an oncogenic form of Smo, and we demonstrate that Hh pathway inhibitors can have tissue-specific activities. These antagonists therefore constitute a valuable set of chemical tools for interrogating downstream Hh signaling mechanisms and for developing chemotherapies against Hh pathway-related cancers.

Paracrine Hedgehog signaling in cancer

Ligand-dependent and ligand-independent activation of the Hedgehog (Hh) signaling pathway is involved in tumorigenesis. The importance of mutations in Hh pathway components leading to constitutive signaling has been well established in basal cell carcinoma and medulloblastoma. However, the role of ligand-driven Hh pathway activation in cancer remains to be established. Three recent articles support a model in which, in the absence of mutations in the Hh pathway, Hh ligands expressed by a subset of epithelial cancers, including colon, pancreatic, and ovarian cancer, promote tumor growth indirectly by activating Hh signaling in the surrounding stroma, which, in turn, provides a more favorable environment for tumor growth. These data have important implications for the use of Hh pathway inhibitors currently in development and for selection of tumors likely to respond to such inhibitors.

Reduced level of smoothened suppresses intestinal tumorigenesis by down-regulation of Wnt signaling

BACKGROUND & AIMS

Although the Hedgehog (Hh) pathway regulates development and progression of several types of cancer, its involvement in colon cancer remains unclear. We aimed to clarify the roles of Hh signaling in intestinal tumorigenesis.

METHODS

We studied expression of the Hh signaling components in the intestinal tumors of Apc(+/Delta716) mouse, a model for familial adenomatous polyposis. We used small interfering RNAs against Smoothened (SMO), which encodes the major signal transducer of the Hh pathway, to knockdown SMO expression and explore its function in human colon cancer cell lines. We also compared the intestinal tumor phenotypes of Apc(+/Delta716)Smo(+/-) mice with those of Apc(+/Delta716) mice.

RESULTS

Expression of Smo was markedly increased in the intestinal adenoma epithelium of Apc(+/Delta716) mice. Importantly, SMO knockdown in human colon cancer cell lines suppressed proliferation in culture; cells arrested at the G1/S phase. Furthermore, Apc(+/Delta716)Smo(+/-) mice had decreased numbers of polyps in the large size class (Phi >or= 1-2 mm) and recessed polyp morphology, accompanied by reduced proliferation of the tumor epithelial cells. Unexpectedly, reduced expression of Smo suppressed beta-catenin-dependent transcription, rather than Hh-responsive Gli-dependent transcription. Interestingly, SMO knockdown reduced protein levels of active beta-catenin and induced its nuclear exclusion.

CONCLUSIONS

Smo contributes to intestinal tumorigenesis by increasing Wnt signaling. SMO might be a good therapeutic target for patients with colorectal polyps and carcinomas, even in the absence of Hh signal activation.

Therapeutic options for triple-negative breast cancers with defective homologous recombination

Breast cancer is the most common malignancy among women in developed countries, affecting more than a million women per year worldwide. Over the last decades, our increasing understanding of breast cancer biology has led to the development of endocrine agents against hormone receptor-positive tumors and targeted therapeutics against HER2-expressing tumors. However, no targeted therapy is available for patients with triple-negative breast cancer, lacking expression of hormone receptors and HER2. Overlap between BRCA1-mutated breast cancers and triple-negative tumors suggests that an important part of the triple-negative tumors may respond to therapeutics targeting BRCA1-deficient cells. Here, we review the features shared between triple-negative, basal-like and BRCA1-related breast cancers. We also discuss the development of novel therapeutic strategies to target BRCA1-mutated tumors and triple-negative tumors with BRCA1-like features. Finally, we highlight the utility of mouse models for BRCA1-mutated breast cancer to optimize (combination) therapy and to understand drug resistance.

Pancreatic cancer--could it be that simple? A different context of vulnerability

In a recent issue of Science, Olive and colleagues document that inhibition of Hedgehog (Hh) signaling in a genetically engineered mouse model of pancreatic cancer can enhance the intratumor concentration of certain anticancer drugs. Could this finding provide us with a new method to attack pancreatic cancer?

Structural insights into hedgehog ligand sequestration by the human hedgehog-interacting protein HHIP

Hedgehog (Hh) morphogens have fundamental roles in development, whereas dysregulation of Hh signaling leads to disease. Multiple cell-surface receptors are responsible for transducing and/or regulating Hh signals. Among these, the Hedgehog-interacting protein (Hhip) is a highly conserved, vertebrate-specific inhibitor of Hh signaling. We have solved a series of crystal structures for the human HHIP ectodomain and Desert hedgehog (DHH) in isolation, as well as HHIP in complex with DHH (HHIP-DHH) and Sonic hedgehog (Shh) (HHIP-Shh), with and without Ca2+. The interaction determinants, confirmed by biophysical studies and mutagenesis, reveal previously uncharacterized and distinct functions for the Hh Zn2+ and Ca2+ binding sites--functions that may be common to all vertebrate Hh proteins. Zn2+ makes a key contribution to the Hh-HHIP interface, whereas Ca2+ is likely to prevent electrostatic repulsion between the two proteins, suggesting an important modulatory role. This interplay of several metal binding sites suggests a tuneable mechanism for regulation of Hh signaling.

The structure of SHH in complex with HHIP reveals a recognition role for the Shh pseudo active site in signaling

Hedgehog (Hh) signaling is crucial for many aspects of embryonic development, whereas dysregulation of this pathway is associated with several types of cancer. Hedgehog-interacting protein (Hhip) is a surface receptor antagonist that is equipotent against all three mammalian Hh homologs. The crystal structures of human HHIP alone and bound to Sonic hedgehog (SHH) now reveal that HHIP is comprised of two EGF domains and a six-bladed beta-propeller domain. In the complex structure, a critical loop from HHIP binds the pseudo active site groove of SHH and directly coordinates its Zn2+ cation. Notably, sequence comparisons of this SHH binding loop with the Hh receptor Patched (Ptc1) ectodomains and HHIP- and PTC1-peptide binding studies suggest a 'patch for Patched' at the Shh pseudo active site; thus, we propose a role for Hhip as a structural decoy receptor for vertebrate Hh.

Tumor-promoting phenotype of CD90hi prostate cancer-associated fibroblasts

BACKGROUND

Cancer-associated stroma contributes to the malignant behavior of adenocarcinomas of the prostate and other organs. CD90 is a marker of mesenchymal stem cells (MSCs) and its expression is higher in prostate cancer stroma compared to normal tissue. Cultured prostate cancer-associated fibroblasts (CAFs) expressing high versus low levels of CD90 were analyzed for an MSC-like or tumor-promoting phenotype.

METHODS

CD90(hi) and CD90(lo) cells were collected by fluorescence-activated cell sorting (FACS). Expression of genes associated with MSCs and/or tumor-promoting activities was measured by quantitative polymerase chain reaction (qPCR). Effects of stromal cell co-culture or conditioned media were tested on BPH-1 epithelial cells.

RESULTS

The pattern of gene expression did not support the hypothesis that CD90(hi) cells were MSCs. However, CD90(hi) cells expressed higher levels of many genes associated with tumor promotion, including cytokines, angiogenic factors, hedgehog signaling components, and transforming growth factor (TGF)-beta. Co-culture or conditioned medium from CD90(hi) cells increased CXCR4 expression in BPH-1 cells, at least in part due to TGF-beta, and protected BPH-1 cells from apoptosis.

CONCLUSIONS

Our results suggest that the elevated expression of CD90 previously observed in the cancer-associated stroma of the human prostate is biologically significant. Although our results do not support the idea that CD90(hi) cells cultured from the cancer stroma are MSCs, our findings suggest that the phenotype of these cells is more tumor-promoting than that of cells expressing low CD90.

Inhibition of Hedgehog signaling enhances delivery of chemotherapy in a mouse model of pancreatic cancer

Pancreatic ductal adenocarcinoma (PDA) is among the most lethal human cancers in part because it is insensitive to many chemotherapeutic drugs. Studying a mouse model of PDA that is refractory to the clinically used drug gemcitabine, we found that the tumors in this model were poorly perfused and poorly vascularized, properties that are shared with human PDA. We tested whether the delivery and efficacy of gemcitabine in the mice could be improved by coadministration of IPI-926, a drug that depletes tumor-associated stromal tissue by inhibition of the Hedgehog cellular signaling pathway. The combination therapy produced a transient increase in intratumoral vascular density and intratumoral concentration of gemcitabine, leading to transient stabilization of disease. Thus, inefficient drug delivery may be an important contributor to chemoresistance in pancreatic cancer.

Inhibition of Hedgehog signaling enhances delivery of chemotherapy in a mouse model of pancreatic cancer

Pancreatic ductal adenocarcinoma (PDA) is among the most lethal human cancers in part because it is insensitive to many chemotherapeutic drugs. Studying a mouse model of PDA that is refractory to the clinically used drug gemcitabine, we found that the tumors in this model were poorly perfused and poorly vascularized, properties that are shared with human PDA. We tested whether the delivery and efficacy of gemcitabine in the mice could be improved by coadministration of IPI-926, a drug that depletes tumor-associated stromal tissue by inhibition of the Hedgehog cellular signaling pathway. The combination therapy produced a transient increase in intratumoral vascular density and intratumoral concentration of gemcitabine, leading to transient stabilization of disease. Thus, inefficient drug delivery may be an important contributor to chemoresistance in pancreatic cancer.

Interplay of tumor microenvironment cell types with parenchymal cells in pancreatic cancer development and therapeutic implications

INTRODUCTION

The process of "induction," namely, the formation of a tissue by the functional interaction between the epithelial layer and the stroma, is key for the development of many organs, in particular to the pancreas.

DISCUSSION

In diseases like pancreatic cancer, most studies performed to date, in the area of pancreatic cancer, have focused on studying epithelial cells and their contribution to this disease. Strikingly, until recently, the stroma that surrounds cancer cells in pancreatic tumors (desmoplastic reaction-tumor microenvironment) has remained an underrepresented area of research. However, several laboratories are increasingly posing questions as what is the role of this tumor microenvironment in the development and progression of this fatal disease. Therefore, in the current article, we define and describe the components of this desmoplastic reaction and the pancreatic tumor microenvironment and briefly review advances being made. More importantly, we highlight the urgent need of research in this field.

CONCLUSION

We anticipate that, because of the paucity of knowledge on this subject, studies on the pancreatic tumor microenvironment will bring new concepts which will ultimately impact in designing new diagnosis and treatment for this disease.

Hedgehog signaling is dispensable for adult murine hematopoietic stem cell function and hematopoiesis

We report the unexpected finding that loss of Hh signaling through conditional deletion of Smoothened (Smo) in the adult hematopoietic compartment has no apparent effect on adult hematopoiesis, including peripheral blood count, number or cell-cycle status of stem or progenitor cells, hematopoietic colony-forming potential, long-term repopulating activity in competitive repopulation assays, or stress response to serial 5-fluorouracil treatment. Furthermore, pharmacologic inhibition of Hh signaling with a potent and selective small molecule antagonist has no substantive effect on hematopoiesis in the mouse. In addition, Hh signaling is not required for the development of MLL-AF9-mediated acute myeloid leukemia (AML). Taken together, these data demonstrate that Hh signaling is dispensable for normal hematopoietic development and hematopoietic stem cell function, indicating that targeting of Hh signaling in solid tumors is not likely to result in hematopoietic toxicity. Furthermore, the Hh pathway may not be a compelling target in certain hematopoietic malignancies.

Depletion of the colonic epithelial precursor cell compartment upon conditional activation of the hedgehog pathway

BACKGROUND & AIMS

The intestinal epithelium is a homeostatic system in which differentiated cells are in dynamic equilibrium with rapidly cycling precursor cells. Wnt signaling regulates intestinal epithelial precursor cell fate and proliferation. Homeostatic systems exist by virtue of negative feedback loops, and we have previously identified the Hedgehog (Hh) pathway as a potential negative feedback signal in the colonic epithelium. Indian hedgehog (Ihh) is produced by the differentiated enterocytes and negatively regulates Wnt signaling in intestinal precursor cells. We studied the role of members of the Hh signaling family in the intestine using a conditional genetic approach.

METHODS

We inactivated the Hh receptor Patched1 (Ptch1) in adult mice, resulting in constitutive activation of the Hh signaling pathway. Effects on colonic mucosal homeostasis were examined. Colon tissues were examined by immunohistochemistry, in situ hybridization, transmission electron microscopy, and real-time polymerase chain reaction.

RESULTS

Ihh but not Sonic hedgehog (Shh) was expressed in colonic epithelium. Expression of Ptch1 and Gli1 was restricted to the mesenchyme. Constitutive activation of Hh signaling resulted in accumulation of myofibroblasts and colonic crypt hypoplasia. A reduction in the number of epithelial precursor cells was observed with premature development into the enterocyte lineage and inhibition of Wnt signaling. Activation of Hh signaling resulted in induction of the expression of bone morphogenetic proteins (Bmp) and increased Bmp signaling in the epithelium.

CONCLUSIONS

Hh signaling acts in a negative feedback loop from differentiated cells via the mesenchyme to the colonic epithelial precursor cell compartment in the adult mouse.

The hedgehog pathway as a therapeutic target for treatment of breast cancer

The Hedgehog (Hh) signaling pathway plays a key role in a variety of processes, such as embryogenesis and maintenance of adult tissue homeostasis. It is also becoming increasingly clear that this pathway can have a crucial role in tumorigenesis. Most recently, the Hh signaling pathway has been implicated in the development and maintenance of breast cancer. Here we review Hh signaling, advances in small molecule and antibody-based inhibitors targeting the Hh pathway, and dysregulation of the Hh signaling pathway in breast cancer.

The Hedgehog receptor Patched1 regulates myeloid and lymphoid progenitors by distinct cell-extrinsic mechanisms

Hedgehog (Hh) ligands bind to the Patched1 (Ptch1) receptor, relieving repression of Smoothened, which leads to activation of the Hh signaling pathway. Using conditional Ptch1 knockout mice, the aim of this study was to determine the effects of activating the Hh signaling pathway in hematopoiesis. Surprisingly, hematopoietic-specific deletion of Ptch1 did not lead to activation of the Hh signaling pathway and, consequently, had no phenotypic effect. In contrast, deletion of Ptch1 in nonhematopoietic cells produced 2 distinct hematopoietic phenotypes. First, activation of Hh signaling in epithelial cells led to apoptosis of lymphoid progenitors associated with markedly elevated levels of circulating thymic stromal lymphopoietin. Second, activation of Hh signaling in the bone marrow cell niche led to increased numbers of lineage-negative c-kit(+) Sca-1(+) bone marrow cells and mobilization of myeloid progenitors associated with a marked loss of osteoblasts. Thus, deletion of Ptch1 leads to hematopoietic effects by distinct cell-extrinsic mechanisms rather than by direct activation of the Hh signaling pathway in hematopoietic cells. These findings have important implications for therapeutics designed to activate the Hh signaling pathway in hematopoietic cells including hematopoietic stem cells.

Inhibition of Hedgehog signaling enhances delivery of chemotherapy in a mouse model of pancreatic cancer

Pancreatic ductal adenocarcinoma (PDA) is among the most lethal human cancers in part because it is insensitive to many chemotherapeutic drugs. Studying a mouse model of PDA that is refractory to the clinically used drug gemcitabine, we found that the tumors in this model were poorly perfused and poorly vascularized, properties that are shared with human PDA. We tested whether the delivery and efficacy of gemcitabine in the mice could be improved by coadministration of IPI-926, a drug that depletes tumor-associated stromal tissue by inhibition of the Hedgehog cellular signaling pathway. The combination therapy produced a transient increase in intratumoral vascular density and intratumoral concentration of gemcitabine, leading to transient stabilization of disease. Thus, inefficient drug delivery may be an important contributor to chemoresistance in pancreatic cancer.

Mechanisms of Hedgehog pathway activation in cancer and implications for therapy

The Hedgehog (Hh) signaling pathway regulates body patterning and organ development during embryogenesis. In adults the Hh pathway is mainly quiescent, with the exception of roles in tissue maintenance and repair, and its inappropriate reactivation has been linked to several disparate human cancers. In addition to cancers with mutations in components of the Hh pathway, Hh ligand-dependent cancers have been proposed to respond to Hh in an autocrine manner. More recent findings that Hh might instead signal in a paracrine manner from the tumor to the surrounding stroma or in cancer stem cells alter our understanding of Hh mechanisms in cancer, with important implications for choice of preclinical tumor models, drug screening, patient selection and therapeutic intervention. We review here the roles of the Hh pathway in cancer, Hh pathway inhibitors (HPIs) and early clinical trial results using a novel small molecule HPI, GDC-0449.

The imidazopyridine derivative JK184 reveals dual roles for microtubules in Hedgehog signaling

Eradicating hedgehogs: The title molecule has been previously identified as a potent inhibitor of the Hedgehog signaling pathway, which gives embryonic cells information needed to develop properly. This molecule is shown to modulate Hedgehog target gene expression by depolymerizing microtubules, thus revealing dual roles of the cytoskeleton in pathway regulation (see figure).

Hedgehog signaling is restricted to the stromal compartment during pancreatic carcinogenesis

The Hedgehog (Hh) pathway has been implicated in pancreatic cancer but its role remains controversial. To delineate the cell populations able to respond to Hh ligand stimulation, we expressed an oncogenic allele of Smoothened (SmoM2) to cell autonomously activate Hh signaling in the mouse pancreas. Surprisingly, we found that expression of SmoM2 in epithelial cells was not able to activate the pathway and had no impact on pancreatic development or neoplasia. In contrast, activation of Smo in the mesenchyme led to Hh pathway activation, indicating that only the tumor stroma is competent to transduce the Hh signal. Using a Ptc-LacZ reporter mouse, we show that Hh signaling is active in stromal cells surrounding Hh-expressing tumor epithelium in various mouse pancreatic cancer models. Activation of the Hh pathway in the tumor stroma of human pancreatic and metastatic cancer specimens was confirmed by quantitative RT-PCR of microdissected tissue samples. These data support a paracrine model of Hh-mediated tumorigenesis, in which tumor cells secrete Hh ligand to induce tumor-promoting Hh target genes in adjacent stroma.

Hedgehog signalling in breast cancer

Breast cancer is the most common cause of cancer death among women worldwide. In order to improve the treatment of this disease, a more complete understanding of its biological basis is necessary. Since the Hedgehog (Hh) pathway was recently found to be required for growth and propagation of a number of different cancers, we discuss here the possible involvement of this pathway in the normal biology and development of cancer in the mammary gland. The use of mouse mammary cancer models has assisted the process of dissecting the mechanisms behind Hh-driven mammary tumour formation and growth. Based on recent studies, we conclude that the inhibition of Hh signalling in breast tumours may interfere with the maintenance of a putative cancer stem cell compartment and the abnormal stimulation of tumour stroma. Therefore, the components of the Hh signalling cascade may provide a set of drug targets, which could be implemented into novel combinatorial strategies for the treatment of breast cancer.

MMTV-Wnt1 and -DeltaN89beta-catenin induce canonical signaling in distinct progenitors and differentially activate Hedgehog signaling within mammary tumors

Canonical Wnt/β-catenin signaling regulates stem/progenitor cells and, when perturbed, induces many human cancers. A significant proportion of human breast cancer is associated with loss of secreted Wnt antagonists and mice expressing MMTV-Wnt1 and MMTV-ΔN89β-catenin develop mammary adenocarcinomas. Many studies have assumed these mouse models of breast cancer to be equivalent. Here we show that MMTV-Wnt1 and MMTV-ΔN89β-catenin transgenes induce tumors with different phenotypes. Using axin2/conductin reporter genes we show that MMTV-Wnt1 and MMTV-ΔN89β-catenin activate canonical Wnt signaling within distinct cell-types. ΔN89β-catenin activated signaling within a luminal subpopulation scattered along ducts that exhibited a K18⁺ER⁻PR⁻CD24^highCD49f^low profile and progenitor properties. In contrast, MMTV-Wnt1 induced canonical signaling in K14⁺ basal cells with CD24/CD49f profiles characteristic of two distinct stem/progenitor cell-types. MMTV-Wnt1 produced additional profound effects on multiple cell-types that correlated with focal activation of the Hedgehog pathway. We document that large melanocytic nevi are a hitherto unreported hallmark of early hyperplastic Wnt1 glands. These nevi formed along the primary mammary ducts and were associated with Hedgehog pathway activity within a subset of melanocytes and surrounding stroma. Hh pathway activity also occurred within tumor-associated stromal and K14⁺/p63⁺ subpopulations in a manner correlated with Wnt1 tumor onset. These data show MMTV-Wnt1 and MMTV-ΔN89β-catenin induce canonical signaling in distinct progenitors and that Hedgehog pathway activation is linked to melanocytic nevi and mammary tumor onset arising from excess Wnt1 ligand. They further suggest that Hedgehog pathway activation maybe a critical component and useful indicator of breast tumors arising from unopposed Wnt1 ligand.

Cancer-associated fibroblasts and tumor growth--bystanders turning into key players

Novel mechanisms, and molecular mediators, of the pro-tumorigenic effects of cancer-associated fibroblasts (CAFs) have been identified. These include CXCL12/SDF-1-mediated recruitment of bone marrow-derived endothelial precursor cell and pro-metastatic effects of CCL5. Co-culture experiments also suggest that CAFs can influence the drug-sensitivity of cancer cells. Comparisons of CAFs from different tumors have started to identify tumor-type specific differences in CAF gene expression and marker protein profiling indicates the existence of multiple distinct co-existing CAF-subsets. Studies in animal models have demonstrated that CAFs can be derived from bone marrow-derived cells or from epithelial or endothelial cells undergoing mesenchymal transition. The genetic status of CAFs remains controversial following conflicting findings. Meanwhile, analyses of CAFs from human tumors have revealed consistent epigenetic changes. An increasing number of translational studies have emphasized the prognostic significance of different CAF-related tumor characteristics. Clinical studies aiming at CAF-targeting can now be envisioned based on findings from experimental intervention studies with agents targeting, for example FAP or PDGF-, TGF-beta- or hedgehog-signaling.

Synthesis and structure revision of nakiterpiosin

This manuscript describes a convergent synthesis and the revision of the relative stereochemistry of nakiterpiosin, a marine C-nor-D-homosteroid. Our synthesis features a late-stage carbonylative Stille cross-coupling reaction and a photo-Nazarov cyclization reaction that deliver the complete nakiterpiosin skeleton efficiently.

Sonic hedgehog in gastric physiology and neoplastic transformation: friend or foe?

PURPOSE OF REVIEW

To understand the role of sonic hedgehog (Shh) in normal gastric physiology and neoplastic transformation.

RECENT FINDINGS

Emerging evidence shows that gastric epithelial cells produce Shh ligand, which subsequently targets the mesenchyme. This paracrine signaling event is recapitulated by Shh-producing tumors that signal to the supporting stroma to encourage growth. Primary cilia contain components of the hedgehog signaling apparatus, and thus are typically found on responding stromal cells.

SUMMARY

In the stomach, Shh is produced in epithelial cells and received by responding cells in the mesenchyme. In vitro, Shh enhances gastric acid secretion and induces mucin expression. It remains to be determined whether the canonical signaling pathway mediates the observed epithelial effects. Shh expression and signaling is reduced in chronic gastritis, and Shh(-/-) embryos exhibit hyperplasia and metaplastic changes in the gastric mucosa. After its loss in the corpus, Shh is re-expressed in some gastric carcinomas typically arising in the distal stomach or antrum, suggesting that it promotes tumor growth.

GLI1 is regulated through Smoothened-independent mechanisms in neoplastic pancreatic ducts and mediates PDAC cell survival and transformation

Pancreatic ductal adenocarcinoma (PDAC) is characterized by the deregulation of the hedgehog signaling pathway. The Sonic Hedgehog ligand (Shh), absent in the normal pancreas, is highly expressed in pancreatic tumors and is sufficient to induce neoplastic precursor lesions in mouse models. We investigated the mechanism of Shh signaling in PDAC carcinogenesis by genetically ablating the canonical bottleneck of hedgehog signaling, the transmembrane protein Smoothened (Smo), in the pancreatic epithelium of PDAC-susceptible mice. We report that multistage development of PDAC tumors is not affected by the deletion of Smo in the pancreas, demonstrating that autocrine Shh-Ptch-Smo signaling is not required in pancreatic ductal cells for PDAC progression. However, the expression of Gli target genes is maintained in Smo-negative ducts, implicating alternative means of regulating Gli transcription in the neoplastic ductal epithelium. In PDAC tumor cells, we find that Gli transcription is decoupled from upstream Shh-Ptch-Smo signaling and is regulated by TGF-beta and KRAS, and we show that Gli1 is required both for survival and for the KRAS-mediated transformed phenotype of cultured PDAC cancer cells.

Hedgehog signaling in development and cancer

The Hedgehog (Hh) family of secreted proteins governs a wide variety of processes during embryonic development and adult tissue homeostasis. Here we review the current understanding of the molecular and cellular basis of Hh morphogen gradient formation and signal transduction, and the multifaceted roles of Hh signaling in development and tumorigenesis. We discuss how the Hh pathway has diverged during evolution and how it integrates with other signaling pathways to control cell growth and patterning.

Androgenic regulation of hedgehog signaling pathway components in prostate cancer cells

Hedgehog signaling is thought to play a role in several human cancers including prostate cancer. Although prostate cancer cells express many of the gene products involved in hedgehog signaling, these cells are refractory to the canonical signaling effects of exogenous hedgehog ligands or to activated Smoothened, the hedgehog-regulated mediator of Gli transcriptional activation. Here, we show that the expression of hedgehog ligands and some hedgehog target genes are regulated by androgen in the human prostate cancer cell line, LNCaP and its more metastatic variants (C4-2 and C4-2B). Androgen (R1881) strongly suppressed the expression of hedgehog ligands in these cells and their prolonged maintenance in androgen-deficient medium upregulated Sonic and Indian hedgehog mRNA and protein levels by up to 30,000-fold. Hedgehogs were released into the conditioned medium of androgen-deprived LNCaP cells and this medium was able to increase hedgehog target gene expression in hedgehog-responsive mouse fibroblasts (MC3T3-E1). Moreover, this activity was accompanied by increased expression of Gli target genes, Patched 1 and Gli2, in LNCaP that could be suppressed by cyclopamine, indicating that chronic androgen-deprivation also re-awakens the autocrine responsiveness of the cancer cells to hedgehog. In contrast to the suppressive effects of R1881 on hedgehog ligand and Gli2 expression, we found that Gli1 expression in LNCaP cells was induced by R1881. Given the ability of androgen to modulate the expression and release of hedgehog ligands and the activity of the autocrine hedgehog signaling pathway in these prostate cancer cells, our results imply that chronic androgen deprivation therapy (ADT) for prostate cancer might create a hedgehog signaling environment in the region of the tumor that could ultimately impact on the long term effectiveness of this treatment. This consideration supports the idea of clinically testing hedgehog-blocking drugs in conjunction with ADT in patients with advanced prostate cancer.

Analysis of germline GLI1 variation implicates hedgehog signalling in the regulation of intestinal inflammatory pathways

BACKGROUND

Ulcerative colitis (UC) and Crohn's disease (CD) are polygenic chronic inflammatory bowel diseases (IBD) of high prevalence that are associated with considerable morbidity. The hedgehog (HH) signalling pathway, which includes the transcription factor glioma-associated oncogene homolog 1 (GLI1), plays vital roles in gastrointestinal tract development, homeostasis, and malignancy. We identified a germline variation in GLI1 (within the IBD2 linkage region, 12q13) in patients with IBD. Since this IBD-associated variant encodes a GLI1 protein with reduced function and our expression studies demonstrated down-regulation of the HH response in IBD, we tested whether mice with reduced Gli1 activity demonstrate increased susceptibility to chemically induced colitis.

METHODS AND FINDINGS

Using a gene-wide haplotype-tagging approach, germline GLI1 variation was examined in three independent populations of IBD patients and healthy controls from Northern Europe (Scotland, England, and Sweden) totalling over 5,000 individuals. On log-likelihood analysis, GLI1 was associated with IBD, predominantly UC, in Scotland and England (p < 0.0001). A nonsynonymous SNP (rs2228226C-->G), in exon 12 of GLI1 (Q1100E) was strongly implicated, with pooled odds ratio of 1.194 (confidence interval = 1.09-1.31, p = 0.0002). GLI1 variants were tested in vitro for transcriptional activity in luciferase assays. Q1100E falls within a conserved motif near the C terminus of GLI1; the variant GLI protein exhibited reduced transactivation function in vitro. In complementary expression studies, we noted the colonic HH response, including GLI1, patched (PTCH), and hedgehog-interacting protein (HHIP), to be down-regulated in patients with UC. Finally, Gli1(+/lacZ) mice were tested for susceptibility to dextran sodium sulphate (DSS)-induced colitis. Clinical response, histology, and expression of inflammatory cytokines and chemokines were recorded. Gli1(+/lacZ) mice rapidly developed severe intestinal inflammation, with considerable morbidity and mortality compared with wild type. Local myeloid cells were shown to be direct targets of HH signals and cytokine expression studies revealed robust up-regulation of IL-12, IL-17, and IL-23 in this model.

CONCLUSIONS

HH signalling through GLI1 is required for appropriate modulation of the intestinal response to acute inflammatory challenge. Reduced GLI1 function predisposes to a heightened myeloid response to inflammatory stimuli, potentially leading to IBD.

Co-evolution of tumor cells and their microenvironment

Increasing evidence indicates that tumor-stromal cell interactions have a crucial role in tumor initiation and progression. These interactions modify cellular compartments, leading to the co-evolution of tumor cells and their microenvironment. Although the importance of microenvironmental alterations in tumor development is recognized, the molecular mechanisms underlying these changes are only now beginning to be understood. Epigenetic and gene expression changes have consistently been reported in cancer-associated stromal cells and the influence of the host genotype on tumorigenesis is also well documented. However, the presence of clonally selected somatic genetic alterations within the tumor microenvironment has been controversial. A thorough understanding of the co-evolution of these two cellular compartments will require carefully executed molecular studies combined with mathematical modeling.

Therapeutic inhibition of Hedgehog-GLI signaling in cancer: epithelial, stromal, or stem cell targets?

Hedgehog (HH)-GLI signaling is a developmental patterning pathway used by many tumors for bulk proliferation that has been shown also to regulate cancer stem cell self-renewal and survival. Surprisingly, a recent study by Yauch et al. (2008) proposes that HH-GLI signaling acts only on the tumor stroma. The mode of action of HH-GLI signaling in cancer may shape the development of therapeutic antagonists.