The G12 family of heterotrimeric G proteins and Rho GTPase mediate Sonic hedgehog signalling

RhoA/ROCK signaling pathway and astrocytes in ischemic stroke

Ischemic stroke is one of the most common and undertreated cerebral diseases with high mortality and disability rate. Various intrinsic and extrinsic factors regulate the onset, severity, and progression of ischemic stroke. As an integral part of the neuronal glia system, astrocytes provide many housekeeping functions in nervous system, and perform multiple functions both beneficial and detrimental for neuronal survival after ischemic stroke. In addition, the small GTPase Rho and its downstream Rho kinase (ROCK) are associated with various neuronal functions such as dendrite development, migration and axonal extension, and numerous central nervous system (CNS) diseases. The aim of this review is to summarize the role of RhoA/ROCK signaling pathway and astrocytes on neurological function after ischemic stroke. We also discuss the interaction of RhoA/ROCK signaling pathway and astrocytes on the tissue repair after brain injury.

Hedgehog/GLI Signaling Pathway: Transduction, Regulation, and Implications for Disease

Simple Summary

The Hedgehog/GLI (Hh/GLI) pathway plays a major role during development and it is commonly dysregulated in many diseases, including cancer. This highly concerted series of ligands, receptors, cytoplasmic signaling molecules, transcription factors, and co-regulators is involved in regulating the biological functions controlled by this pathway. Activation of Hh/GLI in cancer is most often through a non-canonical method of activation, independent of ligand binding. This review is intended to summarize our current understanding of the Hh/GLI signaling, non-canonical mechanisms of pathway activation, its implication in disease, and the current therapeutic strategies targeting this cascade.

Abstract

The Hh/GLI signaling pathway was originally discovered in Drosophila as a major regulator of segment patterning in development. This pathway consists of a series of ligands (Shh, Ihh, and Dhh), transmembrane receptors (Ptch1 and Ptch2), transcription factors (GLI1–3), and signaling regulators (SMO, HHIP, SUFU, PKA, CK1, GSK3β, etc.) that work in concert to repress (Ptch1, Ptch2, SUFU, PKA, CK1, GSK3β) or activate (Shh, Ihh, Dhh, SMO, GLI1–3) the signaling cascade. Not long after the initial discovery, dysregulation of the Hh/GLI signaling pathway was implicated in human disease. Activation of this signaling pathway is observed in many types of cancer, including basal cell carcinoma, medulloblastoma, colorectal, prostate, pancreatic, and many more. Most often, the activation of the Hh/GLI pathway in cancer occurs through a ligand-independent mechanism. However, in benign disease, this activation is mostly ligand-dependent. The upstream signaling component of the receptor complex, SMO, is bypassed, and the GLI family of transcription factors can be activated regardless of ligand binding. Additional mechanisms of pathway activation exist whereby the entirety of the downstream signaling pathway is bypassed, and PTCH1 promotes cell cycle progression and prevents caspase-mediated apoptosis. Throughout this review, we summarize each component of the signaling cascade, non-canonical modes of pathway activation, and the implications in human disease, including cancer.

G Protein-Coupled receptors and heterotrimeric G proteins as cancer drivers

G protein-coupled receptors (GPCRs) and heterotrimeric G proteins play central roles in a diverse array of cellular processes. As such, dysregulation of GPCRs and their coupled heterotrimeric G proteins can dramatically alter the signalling landscape and functional state of a cell. Consistent with their fundamental physiological functions, GPCRs and their effector heterotrimeric G proteins are implicated in some of the most prevalent human diseases, including a complex disease such as cancer that causes significant morbidity and mortality worldwide. GPCR/G protein-mediated signalling impacts oncogenesis at multiple levels by regulating tumour angiogenesis, immune evasion, metastasis, and drug resistance. Here, we summarize the growing body of research on GPCRs and their effector heterotrimeric G proteins as drivers of cancer initiation and progression, and as emerging antitumoural therapeutic targets.

Hedgehog Signaling: Implications in Cancers and Viral Infections

The hedgehog (SHH) signaling pathway is primarily involved in embryonic gut development, smooth muscle differentiation, cell proliferation, adult tissue homeostasis, tissue repair following injury, and tissue polarity during the development of vertebrate and invertebrate organisms. GLIoma-associated oncogene homolog (GLI) family of zinc-finger transcription factors and smoothened (SMO) are the signal transducers of the SHH pathway. Both SHH ligand-dependent and independent mechanisms activate GLI proteins. Various transcriptional mechanisms, posttranslational modifications (phosphorylation, ubiquitination, proteolytic processing, SUMOylation, and acetylation), and nuclear-cytoplasmic shuttling control the activity of SHH signaling pathway proteins. The dysregulated SHH pathway is associated with bone and soft tissue sarcomas, GLIomas, medulloblastomas, leukemias, and tumors of breast, lung, skin, prostate, brain, gastric, and pancreas. While extensively studied in development and sarcomas, GLI family proteins play an essential role in many host-pathogen interactions, including bacterial and viral infections and their associated cancers. Viruses hijack host GLI family transcription factors and their downstream signaling cascades to enhance the viral gene transcription required for replication and pathogenesis. In this review, we discuss a distinct role(s) of GLI proteins in the process of tumorigenesis and host-pathogen interactions in the context of viral infection-associated malignancies and cancers due to other causes. Here, we emphasize the potential of the Hedgehog (HH) pathway targeting as a potential anti-cancer therapeutic approach, which in the future could also be tested in infection-associated fatalities.

Hedgehog signaling promotes angiogenesis directly and indirectly in pancreatic cancer

INTRODUCTION

The inhibition of Hedgehog (Hh) signaling in pancreatic ductal adenocarcinoma (PDAC) reduces desmoplasia and promotes increased vascularity. In contrast to these findings, the Hh ligand Sonic Hedgehog (SHH) is a potent proangiogenic factor in non-tumor models. The aim of this study was to determine the molecular mechanisms by which SHH affects the tumor stroma and angiogenesis.

METHODS

Mice bearing three different xenografted human PDAC (n = 5/group) were treated with neutralizing antibodies to SHH. After treatment for 7 days, tumors were evaluated and the expression of 38 pro- and antiangiogenic factors was assessed in the tumor cells and their stroma. The effect of SHH on the regulation of pro- and antiangiogenic factors in fibroblasts and its impact on endothelial cells was then further assessed in in vitro model systems.

RESULTS

Inhibition of SHH affected tumor growth, stromal content, and vascularity. Its effect on the Hh signaling pathway was restricted to the stromal compartment of the three cancers. SHH-stimulated angiogenesis indirectly through the reduction of antiangiogenic THBS2 and TIMP2 in stromal cells. An additional direct effect of SHH on endothelial cells depended on the presence of VEGF.

CONCLUSION

Inhibition of Hh signaling reduces tumor vascularity, suggesting that Hh plays a role in the maintenance or formation of the tumor vasculature. Whether the reduction in tumor growth and viability seen in the epithelium is a direct consequence of Hh pathway inhibition, or indirectly caused by its effect on the stroma and vasculature, remains to be evaluated.

Heterotrimeric Gα12/13 proteins in kidney injury and disease

Gα₁₂ and Gα₁₃ are ubiquitous members of the heterotrimeric guanine nucleotide-binding protein (G protein) family that play central and integrative roles in the regulation of signal transduction cascades within various cell types in the kidney. Gα₁₂/Gα₁₃ proteins enable the kidney to adapt to an ever-changing environment by transducing stimuli from cell surface receptors and accessory proteins to effector systems. Therefore, perturbations in Gα₁₂/Gα₁₃ levels or their activity can contribute to the pathogenesis of various renal diseases, including renal cancer. This review will highlight and discuss the complex and expanding roles of Gα₁₂/Gα₁₃ proteins on distinct renal pathologies, with emphasis on more recently reported findings. Deciphering how the different Gα₁₂/Gα₁₃ interaction networks participate in the onset and development of renal diseases may lead to the discovery of new therapeutic strategies.

A conserved molecular switch in Class F receptors regulates receptor activation and pathway selection

Class F receptors are considered valuable therapeutic targets due to their role in human disease, but structural changes accompanying receptor activation remain unexplored. Employing population and cancer genomics data, structural analyses, molecular dynamics simulations, resonance energy transfer-based approaches and mutagenesis, we identify a conserved basic amino acid in TM6 in Class F receptors that acts as a molecular switch to mediate receptor activation. Across all tested Class F receptors (FZD_4,5,6,7, SMO), mutation of the molecular switch confers an increased potency of agonists by stabilizing an active conformation as assessed by engineered mini G proteins as conformational sensors. Disruption of the switch abrogates the functional interaction between FZDs and the phosphoprotein Dishevelled, supporting conformational selection as a prerequisite for functional selectivity. Our studies reveal the molecular basis of a common activation mechanism conserved in all Class F receptors, which facilitates assay development and future discovery of Class F receptor-targeting drugs.

Class F receptors are therapeutic targets in human disease and understanding their structural changes during receptor activation may provide important pharmacological insight. Here, the authors combine computational and experimental methods to identify a molecular switch in TM6/7 of Class F receptors that mediates receptor activation.

Targeting the Oncoprotein Smoothened by Small Molecules: Focus on Novel Acylguanidine Derivatives as Potent Smoothened Inhibitors

Hedgehog-GLI (HH) signaling was originally identified as a critical morphogenetic pathway in embryonic development. Since its discovery, a multitude of studies have reported that HH signaling also plays key roles in a variety of cancer types and in maintaining tumor-initiating cells. Smoothened (SMO) is the main transducer of HH signaling, and in the last few years, it has emerged as a promising therapeutic target for anticancer therapy. Although vismodegib and sonidegib have demonstrated effectiveness for the treatment of basal cell carcinoma (BCC), their clinical use has been hampered by severe side effects, low selectivity against cancer stem cells, and the onset of mutation-driven drug resistance. Moreover, SMO antagonists are not effective in cancers where HH activation is due to mutations of pathway components downstream of SMO, or in the case of noncanonical, SMO-independent activation of the GLI transcription factors, the final mediators of HH signaling. Here, we review the current and rapidly expanding field of SMO small-molecule inhibitors in experimental and clinical settings, focusing on a class of acylguanidine derivatives. We also discuss various aspects of SMO, including mechanisms of resistance to SMO antagonists.

A computational study of hedgehog signalling involved in basal cell carcinoma reveals the potential and limitation of combination therapy

BACKGROUND

The smoothened (SMO) receptor is an essential component of the Sonic hedgehog (SHH) signalling, which is associated with the development of skin basal cell carcinoma (BCC). SMO inhibitors are indicated for BCC patients when surgical treatment or radiation therapy are not possible. Unfortunately, SMO inhibitors are not always well tolerated due to severe side effects, and their therapeutical success is limited by resistance mutations.

METHODS

We investigated how common are resistance-causing mutations in two genomic databases which are not linked to BCC or other cancers, namely 1000 Genomes and ExAC. To examine the potential for combination therapy or other treatments, we further performed knowledge-based simulations of SHH signalling, in the presence or absence of SMO and PI3K/Akt inhibitors.

RESULTS

The database analysis revealed that of 18 known mutations associated with Vismodegib-resistance, three were identified in the databases. Treatment of individuals carrying such mutations is thus liable to fail a priori. Analysis of the simulations suggested that a combined inhibition of SMO and the PI3K/Akt signalling pathway may provide an effective reduction in tumour proliferation. However, the inhibition dosage of SMO and PI3K/Akt depended on the activity of phosphodiesterases (PDEs). Under high PDEs activities, SMO became the most important control node of the network. By applying PDEs inhibition, the control potential of SMO decreased and PI3K appeared as a significant factor in controlling tumour proliferation.

CONCLUSIONS

Our systems biology approach employs knowledge-based computer simulations to help interpret the large amount of data available in public databases, and provides application-oriented solutions for improved cancer resistance treatments.

Overlap in signaling between Smoothened and the α subunit of the heterotrimeric G protein G13

The Hedgehog family of morphogens has long been known to utilize, through the 7-transmembrane protein Smoothened (Smo), the heterotrimeric G protein Gi in both canonical and noncanonical forms of signaling. Other G proteins, while not specifically utilized by Smo, may nonetheless provide access to some of the events controlled by it. We reported several years ago that the G protein G13 activates one or more forms of the Gli family of transcription factors. While the Gli transcription factors are well known targets for Smo, the uncertain mechanism of activation by G13 and the identity of the targeted Gli(s) limited predictions as to the extent to which G13 might mimic Smo's actions. We evaluate here the potential for overlap in G13 and Smo signaling using C3H10T1/2 and 3T3-L1 cells as models of osteogenesis and adipogenesis, respectively. We find in C3H10T1/2 cells that a constitutively active form of Gα13 (Gα13QL) increases Gli1 mRNA, as does a constitutively active form of Smo (SmoA1). We find as well that Gα13QL induces alkaline phosphatase activity, a marker of osteogenesis, albeit the induction is far less substantial than that achieved by SmoA1. In 3T3-L1 cells both Gα13QL and SmoA1 markedly suppress adipogenic differentiation as determined by triglyceride accumulation. RNA sequencing reveals that Gα13QL and SmoA1 regulate many of the same genes but that quantitative and qualitative differences exist. Differences also exist, we find, between SmoA1 and purmorphamine, an agonist for Smo. Therefore, while comparisons of constitutively active proteins are informative, extrapolations to the setting of agonists require care.

The cell adhesion molecule CHL1 interacts with patched-1 to regulate apoptosis during postnatal cerebellar development

The immunoglobulin superfamily adhesion molecule close homolog of L1 (CHL1) plays important roles during nervous system development. Here, we identified the hedgehog receptor patched-1 (PTCH1) as a novel CHL1-binding protein and showed that CHL1 interacts with the first extracellular loop of PTCH1 via its extracellular domain. Colocalization and co-immunoprecipitation of CHL1 with PTCH1 suggest an association of CHL1 with this major component of the hedgehog signaling pathway. The trans-interaction of CHL1 with PTCH1 promotes neuronal survival in cultures of dissociated cerebellar granule cells and of organotypic cerebellar slices. An inhibitor of the PTCH1-regulated hedgehog signal transducer, smoothened (SMO), and inhibitors of RhoA and Rho-associated kinase (ROCK) 1 and 2 prevent CHL1-dependent survival of cultured cerebellar granule cells and survival of cerebellar granule and Purkinje cells in organotypic cultures. In histological sections from 10- and 14-day-old CHL1-deficient mice, enhanced apoptosis of granule, but not Purkinje, cells was observed. The results of the present study indicate that CHL1 triggers PTCH1-, SMO-, RhoA- and ROCK-dependent signal transduction pathways to promote neuronal survival after cessation of the major morphogenetic events during mouse cerebellar development.

Differential role for PAK1 and PAK4 during the invadopodia lifecycle

PAK1 and PAK4 are members of the p-21 activated kinase family of serine/threonine kinases. PAK1 has previously been implicated in both the formation and disassembly of invasive cell protrusions, termed invadopodia. We recently reported a novel role for PAK4 during invadopodia maturation and confirmed a specific role for PAK1 in invadopodia formation; findings we will review here. Moreover, we found that PAK4 induction of maturation is delivered via interaction with the RhoA regulator PDZ-RhoGEF. We can now reveal that loss of PAK4 expression leads to changes in invadopodia dynamics. Ultimately we propose that PAK4 but not PAK1 is a key mediator of RhoA activity and provide further evidence that modulation of PAK4 expression levels leads to changes in RhoA activity.

PAK4 suppresses PDZ-RhoGEF activity to drive invadopodia maturation in melanoma cells

Cancer cells are thought to use actin rich invadopodia to facilitate matrix degradation. Formation and maturation of invadopodia requires the co-ordained activity of Rho-GTPases, however the molecular mechanisms that underlie the invadopodia lifecycle are not fully elucidated. Previous work has suggested a formation and disassembly role for Rho family effector p-21 activated kinase 1 (PAK1) however, related family member PAK4 has not been explored. Systematic analysis of isoform specific depletion using in vitro and in vivo invasion assays revealed there are differential invadopodia-associated functions. We consolidated a role for PAK1 in the invadopodia formation phase and identified PAK4 as a novel invadopodia protein that is required for successful maturation. Furthermore, we find that PAK4 (but not PAK1) mediates invadopodia maturation likely via inhibition of PDZ-RhoGEF. Our work points to an essential role for both PAKs during melanoma invasion but provides a significant advance in our understanding of differential PAK function.

Determinant role for the gep oncogenes, Gα12/13, in ovarian cancer cell proliferation and xenograft tumor growth

Recent studies have shown that the gip2 and gep oncogenes defined by the α-subunits of Gi2 and G12 family of G proteins, namely Gαi2 and Gα12/13, stimulate oncogenic signaling pathways in cancer cells including those derived from ovarian cancer. However, the critical α-subunit involved in ovarian cancer growth and progression in vivo remains to be identified. Using SKOV3 cells in which the expressions of individual Gα-subunits were silenced, we demonstrate that the silencing of Gα12 and Gα13 drastically attenuated serum- or lysophosphatidic acid-stimulated proliferation. In contrast, the invasive migration of these cells were reduced only by the silencing of Gαi2 or Gα13. Analyses of the xenograft tumors derived from these Gα-silenced cells indicated that only the silencing of Gα13 drastically reduced xenograft tumor growth and prolonged the survival of the mice. Similar, but albeit reduced, effect was seen with the silencing of Gα12. On the contrary, the silencing of Gαi2 or Gαq failed to exert such effect. Thus, our studies establish for the first time that Gα12/13, the putative gep oncogenes, are the determinant α-subunits involved in ovarian cancer growth in vivo and their increased oncogenicity can be correlated with its ability to stimulate both proliferation and invasive migration.

Canonical and non-canonical Hedgehog signalling and the control of metabolism

Obesity and diabetes represent key healthcare challenges of our day, affecting upwards of one billion people worldwide. These individuals are at higher risk for cancer, stroke, blindness, heart and cardiovascular disease, and to date, have no effective long-term treatment options available. Recent and accumulating evidence has implicated the developmental morphogen Hedgehog and its downstream signalling in metabolic control. Generally thought to be quiescent in adults, Hedgehog is associated with several human cancers, and as such, has already emerged as a therapeutic target in oncology. Here, we attempt to give a comprehensive overview of the key signalling events associated with both canonical and non-canonical Hedgehog signalling, and highlight the increasingly complex regulatory modalities that appear to link Hedgehog and control metabolism. We highlight these key findings and discuss their impact for therapeutic development, cancer and metabolic disease.

The role of astrocytes in mediating exogenous cell-based restorative therapy for stroke

Astrocytes have not been a major therapeutic target for the treatment of stroke, with most research emphasis on the neuron. Given the essential role that astrocytes play in maintaining physiological function of the central nervous system and the very rapid and sensitive reaction astrocytes have in response to cerebral injury or ischemic insult, we propose to replace the neurocentric view for treatment with a more nuanced astrocytic centered approach. In addition, after decades of effort in attempting to develop neuroprotective therapies, which target reduction of the ischemic lesion, there are no effective clinical treatments for stroke, aside from thrombolysis with tissue plasminogen activator, which is used in a small minority of patients. A more promising therapeutic approach, which may affect nearly all stroke patients, may be in promoting endogenous restorative mechanisms, which enhance neurological recovery. A focus of efforts in stimulating recovery post stroke is the use of exogenously administered cells. The present review focuses on the role of the astrocyte in mediating the brain network, brain plasticity, and neurological recovery post stroke. As a model to describe the interaction of a restorative cell-based therapy with astrocytes, which drives recovery from stroke, we specifically highlight the subacute treatment of stroke with multipotent mesenchymal stromal cell therapy.

Trimeric G protein-CARMA1 axis links smoothened, the hedgehog receptor transducer, to NF-κB activation in diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL) is the most common lymphoid malignancy in adults. Aberrant activation of Hedgehog (Hh) and nuclear factor (NF)-κB pathways is ubiquitously observed and known to mediate tumor growth, survival, and chemoresistance in DLBCL. Here, we find that activation of Hh signaling is positively correlated with NF-κB pathway in DLBCL tumors, and that smoothened (SMO), the signal transducer subunit of Hh pathway, contributes to NF-κB activation through recruiting G protein subunits Gαi and Gα12 to activate PKCβ/CARMA1/TRAF6/NEMO signaling axis followed by assembling of the CARMA1/BCL10/MALT1/TRAF6 complex to SMO. Moreover, functional inhibition of SMO enhances the cytotoxic effects of NF-κB inhibitor. Altogether, our study reveals a noncanonical Hh signaling pathway in which SMO activates trimeric G proteins and CARMA1-associated signaling complex, leading to NF-κB activation. This signaling cascade contributes to the survival of DLBCL and may serve as a potential target for combination therapies in DLBCL.

Regulator of G-protein signaling - 5 (RGS5) is a novel repressor of hedgehog signaling

Hedgehog (Hh) signaling plays fundamental roles in morphogenesis, tissue repair, and human disease. Initiation of Hh signaling is controlled by the interaction of two multipass membrane proteins, patched (Ptc) and smoothened (Smo). Recent studies identify Smo as a G-protein coupled receptor (GPCR)-like protein that signals through large G-protein complexes which contain the Gα_i subunit. We hypothesize Regulator of G-Protein Signaling (RGS) proteins, and specifically RGS5, are endogenous repressors of Hh signaling via their ability to act as GTPase activating proteins (GAPs) for GTP-bound Gα_i, downstream of Smo. In support of this hypothesis, we demonstrate that RGS5 over-expression inhibits sonic hedgehog (Shh)-mediated signaling and osteogenesis in C3H10T1/2 cells. Conversely, signaling is potentiated by siRNA-mediated knock-down of RGS5 expression, but not RGS4 expression. Furthermore, using immuohistochemical analysis and co-immunoprecipitation (Co-IP), we demonstrate that RGS5 is present with Smo in primary cilia. This organelle is required for canonical Hh signaling in mammalian cells, and RGS5 is found in a physical complex with Smo in these cells. We therefore conclude that RGS5 is an endogenous regulator of Hh-mediated signaling and that RGS proteins are potential targets for novel therapeutics in Hh-mediated diseases.

Genetic interaction screens identify a role for hedgehog signaling in Drosophila border cell migration

BACKGROUND

Cell motility is essential for embryonic development and physiological processes such as the immune response, but also contributes to pathological conditions such as tumor progression and inflammation. However, our understanding of the mechanisms underlying migratory processes is incomplete. Drosophila border cells provide a powerful genetic model to identify the roles of genes that contribute to cell migration.

RESULTS

Members of the Hedgehog signaling pathway were uncovered in two independent screens for interactions with the small GTPase Rac and the polarity protein Par-1 in border cell migration. Consistent with a role in migration, multiple Hh signaling components were enriched in the migratory border cells. Interference with Hh signaling by several different methods resulted in incomplete cell migration. Moreover, the polarized distribution of E-Cadherin and a marker of tyrosine kinase activity were altered when Hh signaling was disrupted. Conservation of Hh-Rac and Hh-Par-1 signaling was illustrated in the wing, in which Hh-dependent phenotypes were enhanced by loss of Rac or par-1.

CONCLUSIONS

We identified a pathway by which Hh signaling connects to Rac and Par-1 in cell migration. These results further highlight the importance of modifier screens in the identification of new genes that function in developmental pathways.

Astrocyte-derived sonic hedgehog contributes to angiogenesis in brain microvascular endothelial cells via RhoA/ROCK pathway after oxygen-glucose deprivation

The human adult brain possesses intriguing plasticity, including neurogenesis and angiogenesis, which may be mediated by the activated sonic hedgehog (Shh). By employing a coculture system, brain microvascular endothelial cells (BMECs) cocultured with astrocytes, which were incubated under oxygen-glucose deprivation (OGD) condition, we tested the hypothesis that Shh secreted by OGD-activated astrocytes promotes cerebral angiogenesis following ischemia. The results of this study demonstrated that Shh was mainly secreted by astrocytes and the secretion was significantly upregulated after OGD. The proliferation, migration, and tube formation of BMECs cocultured with astrocytes after OGD were significantly enhanced, but cyclopamine (a Shh antagonist) or 5E1 (an antibody of Shh) reversed the change. Furthermore, silencing Ras homolog gene family, member A (RhoA) of BMECs by RNAi and blocking Rho-dependent kinase (ROCK) by Y27632, a specific antagonist of ROCK, suppressed the upregulation of proliferation, migration, and tube formation of BMECs after OGD. These findings suggested that Shh derived from activated astrocytes stimulated RhoA/ROCK pathway in BMECs after OGD, which might be involved in angiogenesis in vitro.

Phosphorylation of Gli by cAMP-dependent protein kinase

Hedgehog (Hh) signaling plays a central role in many developmental processes. Hh protein is a developmental morphogen that elicits a graded cellular response depending on the distance between the recipient cell and the ligand-secreting cell. Gli transcription factors are effectors that induce the expression of downstream target genes. The outline of this cascade from Hh to Gli has been elucidated, and many components have been identified. However, the interpretation of graded ligand stimulation remains to be resolved. Among the components, adenosine 3'5'-cyclic monophosphate-dependent protein kinase (PKA) functions as a negative regulator that phosphorylates a specific region of Gli, thereby inducing proteolytic cleavage to generate the repressor form. In addition, recent studies have identified different mechanisms that are followed by PKA phosphorylation of Gli. In this review, we examine Hh signaling and PKA phosphorylation and propose a possible interaction between the multiple mechanisms regulated by PKA and the gradient-dependent response.

The alpha subunit of the G protein G13 regulates activity of one or more Gli transcription factors independently of smoothened

Smoothened (Smo) is a seven-transmembrane (7-TM) receptor that is essential to most actions of the Hedgehog family of morphogens. We found previously that Smo couples to members of the G(i) family of heterotrimeric G proteins, which in some cases are integral although alone insufficient in the activation of Gli transcription factors through Hedgehog signaling. In response to a report that the G(12/13) family is relevant to Hedgehog signaling as well, we re-evaluated the coupling of Smo to one member of this family, G(13), and investigated the capacity of this and other G proteins to activate one or more of forms of Gli. We found no evidence that Smo couples directly to G(13). We found nonetheless that Gα(13) and to some extent Gα(q) and Gα(12) are able to effect activation of Gli(s). This capacity is realized in some cells, e.g. C3H10T1/2, MC3T3, and pancreatic cancer cells, but not all cells. The mechanism employed is distinct from that achieved through canonical Hedgehog signaling, as the activation does not involve autocrine signaling or in any other way require active Smo and does not necessarily involve enhanced transcription of Gli1. The activation by Gα(13) can be replicated through a G(q)/G(12/13)-coupled receptor, CCK(A), and is attenuated by inhibitors of p38 mitogen-activated protein kinase and Tec tyrosine kinases. We posit that G proteins, and perhaps G(13) in particular, provide access to Gli that is independent of Smo and that they thus establish a basis for control of at least some forms of Gli-mediated transcription apart from Hedgehogs.

Sonic Hedgehog-induced proliferation requires specific Gα inhibitory proteins

Proliferation of cerebellar granular neuronal precursors (CGNPs) is mediated by Sonic Hedgehog (Shh), which activates the Patched and Smoothened (Smo) receptor complex. Although its protein sequence suggests that Smo is a G protein coupled receptor (GPCR), the evidence that this receptor utilizes heterotrimeric G proteins as downstream effectors is controversial. In Drosophila, Gα(i) is required for Hedgehog (Hh) activity, but the involvement of heterotrimeric G proteins in vertebrate Shh signaling has not yet been established. Here, we show that Shh-induced proliferation of rat CGNPs is enhanced strongly by the expression of the active forms of Gα(i/o) proteins (Gα(i1), Gα(i2), Gα(i3), and Gα(o)) but not by members of another class (Gα(12)) of heterotrimeric G proteins. Additionally, the mRNAs of these different Gα(i) members display specific expression patterns in the developing cerebellum; only Gα(i2) and Gα(i3) are substantially expressed in the outer external granular layer, where CGNPs proliferate. Consistent with this, Shh-induced proliferation of CGNPs is reduced significantly by knockdowns of Gα(i2) and Gα(i3) but not by silencing of other members of the Gα(i/o) class. Finally, our results demonstrate that Gα(i2) and Gα(i3) locate to the primary cilium when expressed in CGNP cultures. In summary, we conclude that the proliferative effects of Shh on CGNPs are mediated by the combined activity of Gα(i2) and Gα(i3) proteins.

Hedgehog signaling in the normal and neoplastic mammary gland

The hedgehog signal transduction network is a critical regulator of metazoan development. Inappropriate activation of this network is implicated in several different cancers, including breast. Genetic evidence in mice as well as molecular biological studies in human cells clearly indicate that activated signaling can lead to mammary hyperplasia and, in some cases, tumor formation. However, the exact role(s) activated hedgehog signaling plays in the development or progression of breast cancer also remain unclear. In this review, we have discussed recent data regarding the mechanism(s) by which the hedgehog network may signal in the mammary gland, as well as the data implicating activated signaling as a contributing factor to breast cancer development. Finally, we provide a brief update on the available hedgehog signaling inhibitors with respect to ongoing clinical trials, some of which will include locally advanced or metastatic breast cancers. Given the growing intensity with which the hedgehog signaling network is being studied in the normal and neoplastic mammary gland, a more complete understanding of this network should allow more effective targeting of its activities in breast cancer treatment or prevention.

Sonic hedgehog regulates discrete populations of astrocytes in the adult mouse forebrain

Astrocytes are an essential component of the CNS, and recent evidence points to an increasing diversity of their functions. Identifying molecular pathways that mediate distinct astrocyte functions, is key to understanding how the nervous system operates in the intact and pathological states. In this study, we demonstrate that the Hedgehog (Hh) pathway, well known for its roles in the developing CNS, is active in astrocytes of the mature mouse forebrain in vivo. Using multiple genetic approaches, we show that regionally distinct subsets of astrocytes receive Hh signaling, indicating a molecular diversity between specific astrocyte populations. Furthermore, we identified neurons as a source of Sonic hedgehog (Shh) in the adult forebrain, suggesting that Shh signaling is involved in neuron-astrocyte communication. Attenuation of Shh signaling in postnatal astrocytes by targeted removal of Smoothened, an obligate Shh coreceptor, resulted in upregulation of GFAP and cellular hypertrophy specifically in astrocyte populations regulated by Shh signaling. Collectively, our findings demonstrate a role for neuron-derived Shh in regulating specific populations of differentiated astrocytes.

GLI1 facilitates the migration and invasion of pancreatic cancer cells through MUC5AC-mediated attenuation of E-cadherin

The Krüppel-like zinc-finger protein GLI1 functions as a downstream transcription factor of Hedgehog signaling and plays a pivotal role in the cellular proliferation of many types of tumors, including pancreatic ductal adenocarcinoma (PDA). PDA develops from dysplastic lesions called pancreatic intraepithelial neoplasia (PanIN) through a multistep carcinogenesis process that changes its cellular characteristics, including a mucin expression profile. Increased expression of a gel-forming mucin, MUC5AC, was previously revealed as a major biomarker for the poor prognosis of PDA patients, but the molecular mechanisms responsible for its expression and correlation with poor prognosis are not fully understood. Here we show that MUC5AC is a direct transcriptional target of GLI1 in PDA cells. Overexpression of GLI1 enhanced MUC5AC expression, and a double knockdown of GLI1 and GLI2 suppressed endogenous MUC5AC expression in PDA cells. Luciferase reporter assays revealed that GLI1 and GLI2 can activate the MUC5AC promoter through its conserved CACCC-box-like cis-regulatory elements. We also found that GLI1-upregulated MUC5AC was expressed in the intercellular junction between cultured PDA cells and interfered with the membrane localization of E-cadherin, leading to decreased E-cadherin-dependent cell-cell adhesion and promoting the migration and invasion of PDA cells. Consistently, GLI1 induced the nuclear accumulation and target gene expression of β-catenin in a MUC5AC-dependent manner. Finally, immunohistochemical analysis revealed that GLI1 expression statistically correlated with MUC5AC expression and also with altered subcellular localization of E-cadherin and β-catenin in PanIN lesions and PDA. This evidence revealed a new aspect of GLI1 function in modulating E-cadherin/β-catenin-regulated cancer cell properties through the expression of a gel-forming mucin.

Sonic hedgehog induces angiogenesis via Rho kinase-dependent signaling in endothelial cells

The morphogen Sonic hedgehog (Shh) promotes neovascularization in adults by inducing pro-angiogenic cytokine expression in fibroblasts; however, the direct effects of Shh on endothelial cell (EC) function during angiogenesis are unknown. Our findings indicate that Shh promotes capillary morphogenesis (tube length on Matrigel increased to 271+/-50% of the length in untreated cells, p=0.00003), induces EC migration (modified Boyden chamber assay, 191+/-35% of migration in untreated cells, p=0.00009), and increases EC expression of matrix metalloproteinase 9 (MMP-9) and osteopontin (OPN) mRNA (real-time RT-PCR), which are essential for Shh-induced angiogenesis both in vitro and in vivo. Shh activity in ECs is mediated by Rho, rather than through the "classic" Shh signaling pathway, which involves the Gli transcription factors. The Rho dependence of Shh-induced EC angiogenic activity was documented both in vitro, with dominant-negative RhoA and Rho kinase (ROCK) constructs, and in vivo, with the ROCK inhibitor Y27632 in the mouse corneal angiogenesis model. Finally, experiments performed in MMP-9- and OPN-knockout mice confirmed the roles of the ROCK downstream targets MMP-9 and OPN in Shh-induced angiogenesis. Collectively, our results identify a "nonclassical" pathway by which Shh directly modulates EC phenotype and angiogenic activity.

Activation of Erk by sonic hedgehog independent of canonical hedgehog signalling

Hedgehog (Hh) signalling is mediated through the Patched-1 (Ptch1) receptor. Hh-binding to Ptch1 blocks the inhibitory effects of Ptch1 on the activity of the transmembrane protein, Smoothened (Smo), resulting induction of target genes by the Gli-family of transcription factors. We demonstrate here that Hh-binding to Ptch1 stimulates activation of Erk1/2. This activation is insensitive to the small molecule Smo antagonists and occurs in a cell line that does not express Smo. Specifically, the C-terminus of Ptch1 harbours motifs encoding Class I and II SH3-binding sites. SH3-domain binding activity was verified using GST-c-src(SH3), -Grb2(SH3) and -p85beta(SH3) fusion-proteins. Ectopically expressed Grb2 or p85beta could also be co-immunoprecipitated with the Ptch1 C-terminus. Addition of Shh to serum-starved human mammary epithelial cells and Shh Light II fibroblasts stimulated phosphorylation of Erk1/2. Erk1/2 activation was observed in cells where Smo activity had been inhibited using cyclopamine and in the breast epithelial cell line, MCF10A, that does not express Smo. These data reveal novel binding activities for the C-terminal region of Ptch1 and define a signalling pathway stimulated by the Hh-ligands operating independently of pathways requiring Smo.

Microparticles harbouring Sonic Hedgehog: role in angiogenesis regulation

Sonic Hedgehog (Shh) is a morphogen involved in embryonic development of nervous system. Also, it has been shown that recombinant Shh can modulate angiogenesis under ischemic conditions. However, angiogenic effects of endogenous Shh have not been completely elucidated. Using small membrane-derived vesicles expressing Shh (MPs(Shh+)), we have shown that, although MPs(Shh+) decrease endothelial cell proliferation and migration, they are able to favour angiogenesis through the increase of both endothelial cell adhesion and expression of pro-angiogenenic factors. Activation of proteins implicated in cell adhesion, such as Rho A, as well as upregulation of pro-angiogenic factors were sensitive to inhibition of Shh pathway. Although whole composition of MPs(Shh+) needs to be characterized to understand potential effects of MPs(Shh+), these results highlight a new role of MPs(Shh+) in vascular pathophysiology and may have significant implication for therapy in pathologies associated with altered angiogenesis in order to re-address angiogenic switch.

Regulation and physiological functions of G12/13-mediated signaling pathways

Accumulating data indicate that G12 subfamily (Galpha12/13)-mediated signaling pathways play pivotal roles in a variety of physiological processes, while aberrant regulation of this pathway has been identified in various human diseases. It has been demonstrated that Galpha12/13-mediated signals form networks with other signaling proteins at various levels, from cell surface receptors to transcription factors, to regulate cellular responses. Galpha12/13 have slow rates of nucleotide exchange and GTP hydrolysis, and specifically target RhoGEFs containing an amino-terminal RGS homology domain (RH-RhoGEFs), which uniquely function both as a GAP and an effector for Galpha12/13. In this review, we will focus on the mechanisms regulating the Galpha12/13 signaling system, particularly the Galpha12/13-RH-RhoGEF-Rho pathway, which can regulate a wide variety of cellular functions from migration to transformation.

Microparticles harboring Sonic Hedgehog promote angiogenesis through the upregulation of adhesion proteins and proangiogenic factors

Microparticles (MPs) are small fragments generated from the plasma membrane after cell stimulation or apoptosis. We have recently shown that MPs harboring the morphogen Sonic Hedgehog (MPs(Shh+)) correct endothelial injury by release of nitric oxide from endothelial cells [Agouni, Mostefai, Porro, Carusio, Favre, Richard, Henrion, Martínez and Andriantsitohaina (2007) FASEB J., 21, 2735-2741]. Here, we show that MPs(Shh+) induce the formation of capillary-like structures in an in vitro model using human endothelial cells, although they inhibited cell migration. Besides, MPs(Shh+) regulate cell proliferation. Both cell adhesion and expression of proteins involved in this process such as Rho A and phosphorylation of focal-activated kinase were increased by MPs(Shh+), via a Rho-associated coiled-coil-containing protein kinase inhibitor-sensitive pathway. We demonstrate that MPs(Shh+) increase messenger RNA and protein levels of proangiogenic factors as measured by quantitative reverse transcription-polymerase chain reaction and western blot. In spite of vascular endothelial growth factor expression, conditioned media from endothelial cells treated avec MPs(Shh+) reduces angiogenesis. Interestingly, the effects induced by MPs(Shh+) on the formation of capillary-like structures, expression of adhesion molecules and proangiogenic factors were reversed after silencing of the Shh receptor, using small interfering RNA or when Sonic Hedgehog (Shh) signaling was pharmacologically inhibited with cyclopamine. Taken together, we show that Shh carried by MPs(Shh+) regulate angiogenesis probably through both a direct and an indirect mechanisms, and we propose that MPs harboring Shh may contribute to the generation of a vascular network in pathologies associated with tumor growth.

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.

G protein Galphai functions immediately downstream of Smoothened in Hedgehog signalling

The hedgehog (Hh) signalling pathway has an evolutionarily conserved role in patterning fields of cells during metazoan development, and is inappropriately activated in cancer. Hh pathway activity is absolutely dependent on signalling by the seven-transmembrane protein smoothened (Smo), which is regulated by the Hh receptor patched (Ptc). Smo signals to an intracellular multi-protein complex containing the Kinesin related protein Costal2 (Cos2), the protein kinase Fused (Fu) and the transcription factor Cubitus interruptus (Ci). In the absence of Hh, this complex regulates the cleavage of full-length Ci to a truncated repressor protein, Ci75, in a process that is dependent on the proteasome and priming phosphorylations by Protein kinase A (PKA). Binding of Hh to Ptc blocks Ptc-mediated Smo inhibition, allowing Smo to signal to the intracellular components to attenuate Ci cleavage. Because of its homology with the Frizzled family of G-protein-coupled receptors (GPCR), a likely candidate for an immediate Smo effector would be a heterotrimeric G protein. However, the role that G proteins may have in Hh signal transduction is unclear and quite controversial, which has led to widespread speculation that Smo signals through a variety of novel G-protein-independent mechanisms. Here we present in vitro and in vivo evidence in Drosophila that Smo activates a G protein to modulate intracellular cyclic AMP levels in response to Hh. Our results demonstrate that Smo functions as a canonical GPCR, which signals through Galphai to regulate Hh pathway activation.

SCL/TAL1 interrupting locus derepresses GLI1 from the negative control of suppressor-of-fused in pancreatic cancer cell

As a physically binding protein of GLI1 transcription factor, Suppressor-of-Fused (SUFU) has been placed in the center of negative regulation of Hedgehog (Hh) signaling. SUFU tethers GLI1 in cytoplasm, and in some circumstances, it moves into the nucleus in association with GLI1, leading to the suppression of GLI1 target gene expression by recruiting a corepressor complex. The activated transcriptional function of GLI1 is important for cellular proliferation in a variety of human cancers. However, it has not been revealed how GLI1 is derepressed from SUFU-mediated suppression. Here, we show SCL/TAL1 interrupting locus (SIL) product, a cytoplasmic protein overexpressed in pancreatic ductal adenocarcinoma (PDA), is responsible for the derepression of GLI1. We found SIL associated with the carboxyl terminus of SUFU, one of two distinct GLI1-binding domains, and this association was responsible for cytoplasmic tethering of SUFU. Overexpressed SIL attenuated SUFU-mediated cytoplasmic tethering and target gene suppression of GLI1. Knockdown of SIL in PDA cells conversely induced the nuclear accumulation of SUFU in association with GLI1 and the transcriptional suppression of GLI1 target genes. Importantly, we also showed that oncogenic K-RAS, and not Sonic hedgehog, enhanced the SIL association with the amino-terminus of SUFU, the other GLI1-binding domain that led to further increase of nuclear translocation of GLI1. These results uncover the role of SIL in derepressing GLI1 from the negative control of SUFU, which is a crucial step for activating Hh signaling in cancer cells.

The decoupling of Smoothened from Galphai proteins has little effect on Gli3 protein processing and Hedgehog-regulated chick neural tube patterning

The Hedgehog (Hh) signal is transmitted by two receptor molecules, Patched (Ptc) and Smoothened (Smo). Ptc suppresses Smo activity, while Hh binds Ptc and alleviates the suppression, which results in activation of Hh targets. Smo is a seven-transmembrane protein with a long carboxyl terminal tail. Vertebrate Smo has been previously shown to be coupled to Galpha(i) proteins, but the biological significance of the coupling in Hh signal transduction is not clear. Here we show that although inhibition of Galpha(i) protein activity appears to significantly reduce Hh pathway activity in Ptc(-/-) mouse embryonic fibroblasts and the NIH3T3-based Shh-light cells, it fails to derepress Shh- or a Smo-agonist-induced inhibition of Gli3 protein processing, a known in vivo indicator of Hh signaling activity. The inhibition of Galpha(i) protein activity also cannot block the Sonic Hedgehog (Shh)-dependent specification of neural progenitor cells in the neural tube. Consistent with these results, overexpression of a constitutively active Galpha(i) protein, Galpha(i2)QL, cannot ectopically specify the neural cell types in the spinal cord, whereas an active Smo, SmoM2, can. Thus, our results indicate that the Smo-induced Galpha(i) activity plays an insignificant role in the regulation of Gli3 processing and Shh-regulated neural tube patterning.

Transferring genes into cultured mammalian embryos by electroporation

Mammalian whole embryo culture (WEC) was developed long before transgenic and gene targeted animals are widely used. Electroporation (EP) into cultured rodent embryos has expanded the potential to analyze gene functions in mammalian embryos by transferring exogenous plasmid vectors or small nucleotides in region- and stage-specific ways. This method is quite simple, and therefore enables us to analyze gene functions more quickly than genetic manipulation. In this review, we introduce combinatorial methods of WEC and EP, and summarize various applications in developmental neurobiology.

Pathways of signal transduction employed by vertebrate Hedgehogs

Signalling by Hh (Hedgehog) proteins is among the most actively studied receptor-mediated phenomena relevant to development and post-embryonic homoeostatic events. The impact of signalling by the Hh proteins is profound, and work pertaining to the presentation of these proteins and the pathways engaged by them continues to yield unique insights into basic aspects of morphogenic signalling. We review here the mechanisms of signalling relevant to the actions of Hh proteins in vertebrates. We emphasize findings within the past several years on the recognition of, in particular, Sonic hedgehog by target cells, pathways of transduction employed by the seven-pass transmembrane protein Smoothened and end points of action, as manifest in the regulation of the Gli transcription factors. Topics of extended interest are those regarding the employment of heterotrimeric G-proteins and G-protein-coupled receptor kinases by Smoothened. We also address the pathways, insofar as known, linking Smoothened to the expression and stability of Gli1, Gli2 and Gli3. The mechanisms by which Hh proteins signal have few, if any, parallels. It is becoming clear in vertebrates, however, that several facets of signalling are shared in common with other venues of signalling. The challenge in understanding both the actions of Hh proteins and the overlapping forms of regulation will be in understanding, in molecular terms, both common and divergent signalling events.

The cell biology of Smo signalling and its relationships with GPCRs

The Smoothened (Smo) signalling pathway participates in many developmental processes, contributing to the regulation of gene expression by controlling the activity of transcription factors belonging to the Gli family. The key elements of the pathway were identified by means of genetic screens carried out in Drosophila, and subsequent analysis in other model organisms revealed a high degree of conservation in both the proteins involved and in their molecular interactions. Recent analysis of the pathway, using a combination of biochemical and cell biological approaches, is uncovering the intricacies of Smo signalling, placing its elements in particular cellular compartments and qualifying the molecular processes involved. These include the synthesis, secretion and diffusion of the ligand, the activation of the receptor and the modifications in the activity of nuclear effectors. In this review we discuss recent advances in understanding biochemical and cellular aspects of Smo signalling, with particular focus in the similarities in the mechanism of signal transduction between Smo and other transmembrane proteins belonging to the G-Protein coupled receptors superfamily (GPCR).

G-protein-coupled receptors and cancer

G-protein-coupled receptors (GPCRs), the largest family of cell-surface molecules involved in signal transmission, have recently emerged as crucial players in tumour growth and metastasis. Malignant cells often hijack the normal physiological functions of GPCRs to survive, proliferate autonomously, evade the immune system, increase their blood supply, invade their surrounding tissues and disseminate to other organs. This Review will address our current understanding of the many roles of GPCRs and their signalling circuitry in tumour progression and metastasis. We will also discuss how interfering with GPCRs might provide unique opportunities for cancer prevention and treatment.

The hedgehog signaling network, mammary stem cells, and breast cancer: connections and controversies

Several signal transduction networks have been implicated in the regulation of mammary epithelial stem cell self-renewal and maintenance (Kalirai and Clarke 2006; Liu et al. 2005). These signaling networks include those of the Wnt, Notch, TGFO, EGF, FGF, IGF, and most recently, the Hedgehog (Hh) families of secreted ligands. However, we currently know very little about the cellular and molecular mechanisms by which these signaling pathways function to regulate normal epithelial stem/progenitor cells. What is clear is that the regulatory signaling networks thought to control normal stem/progenitor cell self-renewal and maintenance are, with the current sole exception of the hedgehog network, well-documented to have contributory roles in mammary cancer development and disease progression when misregulated. In this review, genetic regulation of mammary gland development by hedgehog network genes is outlined, highlighting a developing controversy as to whether activated hedgehog signaling regulates normal regenerative mammary epithelial stem cells or, indeed, whether activated hedgehog signaling functions at all in ductal development. In addition, the question of whether inappropriate hedgehog network activation influences breast cancer development is addressed, with emphasis on the prospects for using hedgehog signaling antagonists clinically for breast cancer treatment or prevention.

Identification and characterization of Hedgehog modulator properties after functional coupling of Smoothened to G15

The seven-transmembrane receptor Smoothened (Smo) transduces the signal initiated by Hedgehog (Hh) morphogen binding to the receptor Patched (Ptc). We have reinvestigated the pharmacological properties of reference molecules acting on the Hh pathway using various Hh responses and a novel functional assay based on the coexpression of Smo with the alpha subunit of the G15 protein in HEK293 cells. The measurement of inositol phosphate (IP) accumulation shows that Smo has constitutive activity, a response blocked by Ptc which indicates a functional Hh receptor complex. Interestingly, the antagonists cyclopamine, Cur61414, and SANT-1 display inverse agonist properties and the agonist SAG has no effect at the Smo-induced IP response, but converts Ptc-mediated inactive forms of Smo into active ones. An oncogenic Smo mutant does not mediate an increase in IP response, presumably reflecting its inability to reach the cell membrane. These studies identify novel properties of molecules displaying potential interest in the treatment of various cancers and brain diseases, and demonstrate that Smo is capable of signaling through G15.

HAN11 binds mDia1 and controls GLI1 transcriptional activity

BACKGROUND

The Hedgehog pathway is important in normal and diseased skin. One of the key transcription factors in the pathway is GLI1. GLI1-dependent transcription is positively regulated by DYRK1A, which is reported to bind HAN11. HAN11 is the human homologue of AN11, which controls flavonoid synthesis in plants.

OBJECTIVE

We wanted to identify other binding partners of HAN11 and investigate whether HAN11 regulates GLI1-dependent transcription.

METHODS

We used TAP-tag purification and GST pull down to identify protein-protein interactions and performed luciferase assays of transcriptional activity. We used immunofluorescence microscopy to examine the subcellular distribution of HAN11, mDia1 and GLI1. We performed in situ hybridisation to compare expression of HAN11 with GLI1 and patched in mouse embryos.

RESULTS

We identified the cytoskeletal regulator mDia1 as a binding partner of HAN11. We showed that HAN11 binds the FH2 actin binding domain of mDia1 and confirmed that HAN11 also interacts with DYRK1A. Overexpression of mDia1 or active RhoA caused translocation of HAN11 from nucleus to cytoplasm. HAN11 and mDia1 repressed DYRK1A-dependent GLI1 transcriptional activity. HAN11 overexpression decreased SZ95 sebocyte proliferation and increased cytoplasmic GLI1. AN11 was highly expressed in E10.5 mouse embryo limb buds, in an overlapping pattern with Ptc and GLI1.

CONCLUSION

These results suggest that AN11 may be a physiological regulator of GLI1 transcriptional activity.

Smoothened signal transduction is promoted by G protein-coupled receptor kinase 2

Deregulation of the Sonic hedgehog pathway has been implicated in an increasing number of human cancers. In this pathway, the seven-transmembrane (7TM) signaling protein Smoothened regulates cellular proliferation and differentiation through activation of the transcription factor Gli. The activity of mammalian Smoothened is controlled by three different hedgehog proteins, Indian, Desert, and Sonic hedgehog, through their interaction with the Smoothened inhibitor Patched. However, the mechanisms of signal transduction from Smoothened are poorly understood. We show that a kinase which regulates signaling by many "conventional" 7TM G-protein-coupled receptors, G protein-coupled receptor kinase 2 (GRK2), participates in Smoothened signaling. Expression of GRK2, but not catalytically inactive GRK2, synergizes with active Smoothened to mediate Gli-dependent transcription. Moreover, knockdown of endogenous GRK2 by short hairpin RNA (shRNA) significantly reduces signaling in response to the Smoothened agonist SAG and also inhibits signaling induced by an oncogenic Smoothened mutant, Smo M2. We find that GRK2 promotes the association between active Smoothened and beta-arrestin 2. Indeed, Gli-dependent signaling, mediated by coexpression of Smoothened and GRK2, is diminished by beta-arrestin 2 knockdown with shRNA. Together, these data suggest that GRK2 plays a positive role in Smoothened signaling, at least in part, through the promotion of an association between beta-arrestin 2 and Smoothened.

Activation of heterotrimeric G proteins by Smoothened

The mechanisms by which the activation of Smoothened (Smo), a protein essential to the actions of the Hedgehog family of secreted proteins, is translated into signals that converge on the Gli transcription factors are not fully understood. The seven-transmembrane structure of Smo has long implied the utilization of heterotrimeric GTP-binding regulatory proteins (G proteins); however, evidence in this regard has been indirect and contradictory. In the current study we evaluated the capacity of mammalian Smo to couple to G proteins directly. We found that Smo, by virtue of what appears to be constitutive activity, activates all members of the G(i) family but does not activate members of the G(s), G(q), and G(12) families. The activation is suppressed by cyclopamine and other inhibitors of Hedgehog signaling and is enhanced by the Smo agonist purmorphamine. Activation of G(i) by Smo is essential in the activation of Gli in fibroblasts, because disruption of coupling to G(i) with pertussis toxin inhibits the activation of Gli by Sonic hedgehog and a constitutively active form of Smo (SmoM2). However, G(i) does not provide a sufficient signal because a truncated form of Smo, although capable of activating G(i), does not effect activation of Gli. Rescue of pertussis toxin-inhibited activation of Gli by Sonic hedgehog can be achieved with a constitutively active Galpha(i)-subunit. The data suggest that Smo is in fact the source of two signals relevant to the activation of Gli: one involving G(i) and the other involving events at Smo's C-tail independent of G(i).

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.

Independent roles of Drosophila Moesin in imaginal disc morphogenesis and hedgehog signalling

The three ERM proteins (Ezrin, Radixin and Moesin) form a conserved family required in many developmental processes involving regulation of the cytoskeleton. In general, the molecular function of ERM proteins is to link specific membrane proteins to the actin cytoskeleton. In Drosophila, loss of moesin (moe) activity causes incorrect localisation of maternal determinants during oogenesis, failures in rhabdomere differentiation in the eye and alterations of epithelial integrity in the wing imaginal disc. Some aspects of Drosophila Moe are related to the activity of the small GTPase RhoA, because the reduction of RhoA activity corrects many phenotypes of moe mutant embryos and imaginal discs. We have analysed the phenotype of moesin loss-of-function alleles in the wing disc and adult wing, and studied the effects of reduced Moesin activity on signalling mediated by the Notch, Decapentaplegic, Wingless and Hedgehog pathways. We found that reductions in Moesin levels in the wing disc cause the formation of wing-tissue vesicles and large thickenings of the vein L3, corresponding to breakdowns of epithelial continuity in the wing base and modifications of Hedgehog signalling in the wing blade, respectively. We did not observe any effect on signalling pathways other than Hedgehog, indicating that the moe defects in epithelial integrity have not generalised effects on cell signalling. The effects of moe mutants on Hedgehog signalling depend on the correct gene-dose of rhoA, suggesting that the requirements for Moesin in disc morphogenesis and Hh signalling in the wing disc are mediated by its regulation of RhoA activity. The mechanism linking Moesin activity with RhoA function and Hedgehog signalling remains to be elucidated.