Novel human glioma-associated oncogene 1 (GLI1) splice variants reveal distinct mechanisms in the terminal transduction of the hedgehog signal

Genetic variations and alternative splicing: the Glioma associated oncogene 1, GLI1

Alternative splicing is a post-transcriptional regulatory process that is attaining stronger recognition as a modulator of gene expression. Alternative splicing occurs when the primary RNA transcript is differentially processed into more than one mature RNAs. This is the result of a variable definition/inclusion of the exons, the sequences that are excised from the primary RNA to form the mature RNAs. Consequently, RNA expression can generate a collection of differentially spliced RNAs, which may distinctly influence subsequent biological events, such as protein synthesis or other biomolecular interactions. Still the mechanisms that control exon definition and exon inclusion are not fully clarified. This mini-review highlights advances in this field as well as the impact of single nucleotide polymorphisms in affecting splicing decisions. The Glioma-associated oncogene 1, GLI1, is taken as an example in addressing the role of nucleotide substitutions for splicing regulation.

Novel mechanism of action on Hedgehog signaling by a suppressor of fused carboxy terminal variant

The Suppressor of Fused (SUFU) protein plays an essential role in the Hedgehog (HH) signaling pathway, by regulation of the GLI transcription factors. Two major isoforms of human SUFU are known, a full-length (SUFU-FL) and a carboxy-terminal truncated (SUFU- ΔC) variant. Even though SUFU- ΔC is expressed at an equivalent level as SUFU-FL in certain tissues, the function of SUFU-ΔC and its impact on HH signal transduction is still unclear. In two cell lines from rhabdomyosarcoma, a tumor type associated with deregulated HH signaling, SUFU-ΔC mRNA was expressed at comparable levels as SUFU-FL mRNA, but at the protein level only low amounts of SUFU-ΔC were detectable. Heterologous expression provided support to the notion that the SUFU-ΔC protein is less stable compared to SUFU-FL. Despite this, biochemical analysis revealed that SUFU-ΔC could repress GLI2 and GLI1ΔN, but not GLI1FL, transcriptional activity to the same extent as SUFU-FL. Moreover, under conditions of activated HH signaling SUFU-ΔC was more effective than SUFU-FL in inhibiting GLI1ΔN. Importantly, co-expression with GLI1FL indicated that SUFU-ΔC but not SUFU-FL reduced the protein levels of GLI1FL. Additionally, confocal microscopy revealed a co-localization of GLI1FL with SUFU-ΔC but not SUFU-FL in aggregate structures. Moreover, specific siRNA mediated knock-down of SUFU-ΔC resulted in up-regulation of the protein levels of GLI1FL and the HH signaling target genes PTCH1 and HHIP. Our results are therefore suggesting the presence of novel regulatory controls in the HH signaling pathway, which are elicited by the distinct mechanism of action of the two alternative spliced SUFU proteins.

Hedgehog pathway and GLI1 isoforms in human cancer

The Hedgehog signaling pathway regulates normal cell growth and differentiation. When deregulated, the Hedgehog pathway leads to tumorigenesis and supports more aggressive phenotypes of human cancers, such as progression, metastasis, and therapeutic resistance. The glioma-associated oncogene homolog 1 (GLI1) family of zinc finger transcription factors is the nuclear mediator of the Hedgehog pathway that regulates genes essential for various stages of tumor development and progression. Consequently, several components of the Hedgehog pathway are major targets of cancer therapy, including GLI1 and smoothened. Although the GLI1 gene was initially identified as an amplified gene in glioblastoma, its amplification was found to be relatively rare. No somatic mutations have been reported in the GLI1 gene. Notably, two decades after the discovery of the GLI1 gene, the GLI1 transcript was recently found to undergo alternative splicing forming two shorter isoforms, an N-terminal deletion variant (GLI1ΔN) and a truncated GLI1 (tGLI1). These variants appear to have different patterns of tissue expression and functions. Most notably, the tGLI1 isoform behaves as a gain-of-function GLI1 that can induce expression of genes not regulated by GLI1 and promotes more aggressive cancer phenotypes. Therefore, this review will focus on the structural and functional differences between these isoforms, and also on their contributions to important cancer cell characteristics, including proliferation, motility, invasion, and angiogenesis.

Identification, functional characterization, and pathobiological significance of GLI1 isoforms in human cancers

Glioma-associated oncogene homolog 1 (GLI1) is the nuclear mediator of Hedgehog signaling that activates gene transcription via its zinc finger DNA-binding and transactivation domains. GLI1 plays a critical role in several cellular processes, including embryonic development, tumorigenesis, and tumor growth and progression. The human GLI1 gene was identified in 1987 as an amplified gene in glioblastoma. Somatic mutations have never been reported in the GLI1 gene in any cell or tumor type. Very recently in 2008-2009, the full-length GLI1 transcript was discovered to undergo alternative splicing to form two shorter isoforms, namely N-terminal deletion variant (GLI1ΔN) and truncated GLI1 (tGLI1). Emerging evidence suggests that the three structurally different GLI1 isoforms are distinctly different in their expression patterns and functions in the context of human cancers. The tGLI1 isoform, in particular, has been shown to gain the ability to modulate expression of the genes that are not regulated by GLI1 and to support the biology of more aggressive cancer. Consequently, a key focus of this chapter is to summarize and compare the properties of the three GLI1 isoforms and their relations to malignant biology of human cancers.

Expression of hedgehog pathway components in prostate carcinoma microenvironment: shifting the balance towards autocrine signalling

AIMS

The hedgehog (Hh) signalling pathway has been implicated in the pathogenesis and aggressiveness of prostate cancer through epithelial-mesenchymal interactions. The aim of this study was to elucidate the cell-type partitioned expression of the Hh pathway biomarkers in the non-neoplastic and tumour microenvironments and to correlate it with the grade and stage of prostate cancer.

METHODS AND RESULTS

Expression of the Hh pathway components (Shh, Smo, Ptch, Gli1) in the microenvironment of non-neoplastic peripheral zone (n = 119), hormone-naive primary prostate carcinoma (n = 141) and castrate-resistant bone marrow metastases (n = 53) was analysed using immunohistochemistry in tissue microarrays and bone marrow sections. Results showed that epithelial Shh, Smo and Ptch expression was up-regulated, whereas stromal Smo, Ptch, and Gli1 expression was down-regulated in prostate carcinomas compared to non-neoplastic peripheral zone tissue. Ptch expression was modulated further in high-grade and high-stage primary tumours and in bone marrow metastases. Hh signalling correlated with ki67 and vascular endothelial growth factor (VEGF) but not with CD31 expression.

CONCLUSION

Our results highlight the importance of Hh-mediated epithelial-mesenchymal interactions in the non-neoplastic prostate and imply that shifting the balance from paracrine towards autocrine signalling is important in the pathogenesis and progression of prostate carcinoma.

The Human Glioma-Associated Oncogene Homolog 1 (GLI1) Family of Transcription Factors in Gene Regulation and Diseases

Sonic hedgehog (Shh) signaling is critically important for embryogenesis and other cellular processes in which GLI transcription factors mediate the terminal effects of the pathway. GLI1, in particular, plays a significant role in human cancers. Consequently, GLI1 and its upstream positive regulator Smoothened (SMO) are important targets of anti-cancer therapy and several SMO-targeted small molecule inhibitors are being evaluated clinically. Emerging exciting evidence reveals a high level of complexity that lies within the GLI1-mediated pathway. For example, a recent study provided evidence linking the polymorphic GLI1 variants Q1100/E1100 to chronic inflammatory bowel diseases. Two recent reports uncovered the existence of two novel human GLI1 isoforms that differ structurally and functionally from the wild-type GLI1 identified over two decades ago. Interestingly, although both are products of alternative splicing, GLI1∆N and tGLI1 (truncated GLI1) isoforms are predominantly expressed in normal and malignant tissues, respectively. In addition to these important discoveries, gene expression profiling studies have identified a number of novel wild-type GLI1 and tGLI1 target genes, linking wild-type GLI1 to tumor progression and therapeutic resistance, and tGLI1 to tumor invasion and migration. In light of these new insights, this review will provide a comprehensive overview on GLI1 polymorphisms and the three members of the GLI1 family of proteins, and their impacts on human diseases, including, cancers.

Upregulation of VEGF-A and CD24 gene expression by the tGLI1 transcription factor contributes to the aggressive behavior of breast cancer cells

The Hedgehog signaling pathway is one of the most dysregulated pathways in human cancers. The glioma-associated oncogene homolog 1 (GLI1) transcription factor is the terminal effector of the Hedgehog pathway, frequently activated in human breast cancers and an emerging target of breast cancer therapy. While somatic mutations in the human GLI1 gene have never been reported in any cell or tumor type, we recently uncovered the existence of a novel alternatively spliced, truncated GLI1 (tGLI1) that has an in-frame deletion of 41 codons spanning the entire exon 3 and part of exon 4 of the GLI1 gene. Using glioblastoma models, we showed that tGLI1 has gained the ability to promote glioblastoma migration and invasion via its gain-of-function transcriptional activity. However, the pathological impact of tGLI1 on breast cancer remains undefined. Here, we report that tGLI1 is frequently expressed in human breast cancer cell lines and primary specimens we have examined to date, but is undetectable in normal breast tissues. We found for the first time that tGLI1, but not GLI1, binds to and enhances the human vascular endothelial growth factor-A (VEGF-A) gene promoter, leading to its upregulation. Consequently, tGLI1-expressing MDA-MB-231 breast cancer cells secret higher levels of VEGF-A and contain a higher propensity, than the isogenic cells with control vector and GLI1, to stimulate in vitro angiogenesis of human vascular endothelial cells. We further showed that tGLI1 has gained the ability to enhance the motility and invasiveness of breast cancer cells in a proliferation-independent fashion and that this functional gain is associated with increased expression of migration/invasion-associated genes, CD24, MMP-2 and MMP-9. tGLI1 has also acquired the property to facilitate anchorage-independent growth of breast cancer cells. Collectively, our results define tGLI1 as a gain-of-function GLI1 transcription factor and a novel mediator of the behavior of clinically more aggressive breast cancer.

GLI1 confers profound phenotypic changes upon LNCaP prostate cancer cells that include the acquisition of a hormone independent state

The GLI (GLI1/GLI2) transcription factors have been implicated in the development and progression of prostate cancer although our understanding of how they actually contribute to the biology of these common tumours is limited. We observed that GLI reporter activity was higher in normal (PNT-2) and tumourigenic (DU145 and PC-3) androgen-independent cells compared to androgen-dependent LNCaP prostate cancer cells and, accordingly, GLI mRNA levels were also elevated. Ectopic expression of GLI1 or the constitutively active ΔNGLI2 mutant induced a distinct cobblestone-like morphology in LNCaP cells that, regarding the former, correlated with increased GLI2 as well as expression of the basal/stem-like markers CD44, β1-integrin, ΔNp63 and BMI1, and decreased expression of the luminal marker AR (androgen receptor). LNCaP-GLI1 cells were viable in the presence of the AR inhibitor bicalutamide and gene expression profiling revealed that the transcriptome of LNCaP-GLI1 cells was significantly closer to DU145 and PC-3 cells than to control LNCaP-pBP (empty vector) cells, as well as identifying LCN2/NGAL as a highly induced transcript which is associated with hormone independence in breast and prostate cancer. Functionally, LNCaP-GLI1 cells displayed greater clonal growth and were more invasive than control cells but they did not form colonies in soft agar or prostaspheres in suspension suggesting that they do not possess inherent stem cell properties. Moreover, targeted suppression of GLI1 or GLI2 with siRNA did not reverse the transformed phenotype of LNCaP-GLI1 cells nor did double GLI1/GLI2 knockdowns activate AR expression in DU145 or PC-3 cells. As such, early targeting of the GLI oncoproteins may hinder progression to a hormone independent state but a more detailed understanding of the mechanisms that maintain this phenotype is required to determine if their inhibition will enhance the efficacy of anti-hormonal therapy through the induction of a luminal phenotype and increased dependency upon AR function.

Redundant and differential roles of transcription factors Gli1 and Gli2 in the development of mouse fetal Leydig cells

Appearance of mouse fetal Leydig cells requires activation of the Hedgehog pathway. Upon binding to the membrane-bound receptor patched, Hedgehog ligands induce intracellular responses via a combined effect of Gli transcription factors. Szczepny et al. (Biol Reprod 2009; 80:258-263) found that Gli1, one of the three Gli transcription factors, is present in the fetal testis and that its expression is suppressed by the Hedgehog inhibitor cyclopamine. In this study, we investigated the involvement of the Gli1 and Gli2 factors in mouse fetal Leydig cell differentiation. The Gli1 and Gli2 transcription factors showed an overlapping expression pattern in the testis interstitium at the time when fetal Leydig cells appear. Despite their similar expression, Gli1 and Gli2 patterns were differentially regulated. Initial Gli1 and Gli2 expression depends upon an active Hedgehog pathway; however, maintenance of only Gli1, but not Gli2, expression requires activation of the pathway. Inactivation of either the Gli1 or Gli2 gene did not affect fetal Leydig cell development and testis morphology, suggesting a functional redundancy. When the transcriptional activity of both GLI1 and GLI2 was suppressed by a selective inhibitor, GANT61, in cultured fetal testes before the appearance of fetal Leydig cells, Gli1 and Gli2 expression and steroidogenic marker activity were completely abolished. However at later stages when Leydig cells were already present, GANT61 treatment inhibited Gli1 expression but had no effects on Gli2 expression and fetal Leydig cell appearance. Our results reveal overlapping and redundant Gli1 and Gli2 roles in fetal Leydig cell differentiation and a novel regulation of Gli2 expression in the fetal testis.

Disturbed expression of splicing factors in renal cancer affects alternative splicing of apoptosis regulators, oncogenes, and tumor suppressors

Background

Clear cell renal cell carcinoma (ccRCC) is the most common type of renal cancer. One of the processes disturbed in this cancer type is alternative splicing, although phenomena underlying these disturbances remain unknown. Alternative splicing consists of selective removal of introns and joining of residual exons of the primary transcript, to produce mRNA molecules of different sequence. Splicing aberrations may lead to tumoral transformation due to synthesis of impaired splice variants with oncogenic potential. In this paper we hypothesized that disturbed alternative splicing in ccRCC may result from improper expression of splicing factors, mediators of splicing reactions.

Methodology/Principal Findings

Using real-time PCR and Western-blot analysis we analyzed expression of seven splicing factors belonging to SR proteins family (SF2/ASF, SC35, SRp20, SRp75, SRp40, SRp55 and 9G8), and one non-SR factor, hnRNP A1 (heterogeneous nuclear ribonucleoprotein A1) in 38 pairs of tumor-control ccRCC samples. Moreover, we analyzed splicing patterns of five genes involved in carcinogenesis and partially regulated by analyzed splicing factors: RON, CEACAM1, Rac1, Caspase-9, and GLI1.

Conclusions/Significance

We found that the mRNA expression of splicing factors was disturbed in tumors when compared to paired controls, similarly as levels of SF2/ASF and hnRNP A1 proteins. The correlation coefficients between expression levels of specific splicing factors were increased in tumor samples. Moreover, alternative splicing of five analyzed genes was also disturbed in ccRCC samples and splicing pattern of two of them, Caspase-9 and CEACAM1 correlated with expression of SF2/ASF in tumors. We conclude that disturbed expression of splicing factors in ccRCC may possibly lead to impaired alternative splicing of genes regulating tumor growth and this way contribute to the process of carcinogenesis.

Genetic variations regulate alternative splicing in the 5' untranslated regions of the mouse glioma-associated oncogene 1, Gli1

Background

Alternative splicing is one of the key mechanisms that generate biological diversity. Even though alternative splicing also occurs in the 5' and 3' untranslated regions (UTRs) of mRNAs, the understanding of the significance and the regulation of these variations is rather limited.

Results

We investigated 5' UTR mRNA variants of the mouse Gli1 oncogene, which is the terminal transcriptional effector of the Hedgehog (HH) signaling pathway. In addition to identifying novel transcription start sites, we demonstrated that the expression ratio of the Gli1 splice variants in the 5' UTR is regulated by the genotype of the mouse strain analyzed. The GT allele, which contains the consensus intronic dinucleotides at the 5' splice site of intron 1B, favors exon 1B inclusion, while the GC allele, having a weaker 5' splice site sequence, promotes exon 1B skipping. Moreover, the alternative Gli1 5' UTRs had an impact on translational capacity, with the shorter and the exon 1B-skipped mRNA variants being most effective.

Conclusions

Our findings implicate novel, genome-based mechanisms as regulators of the terminal events in the mouse HH signaling cascade.

A novel splice variant of GLI1 that promotes glioblastoma cell migration and invasion

The family of GLI zinc finger transcription factors regulates the expression of genes involved in many important cellular processes, notably embryonal development and cellular differentiation. The glioma-associated oncogene homologue 1 (GLI1) isoform, in particular, has attracted much attention because of its frequent activation in many human cancers and its interactions with other signaling pathways, such as those mediated by K-RAS, transforming growth factor-beta, epidermal growth factor receptor, and protein kinase A. Here, we report the identification of a novel truncated GLI1 splice variant, tGLI1, with an in-frame deletion of 123 bases (41 codons) spanning the entire exon 3 and part of exon 4 of the GLI1 gene. Expression of tGLI1 is undetectable in normal cells but is high in glioblastoma multiforme (GBM) and other cancer cells. Although tGLI1 undergoes nuclear translocalization and transactivates GLI1-binding sites similar to GLI1, unlike GLI1, it is associated with increased motility and invasiveness of GBM cells. Using microarray analysis, we showed >100 genes to be differentially expressed in tGLI1-expressing compared with GLI1-expressing GBM cells, although both cell types expressed equal levels of known GLI1-regulated genes, such as PTCH1. We further showed one of the tGLI1 up-regulated genes, CD24, an invasion-associated gene, to be required for the migratory and invasive phenotype of GBM cells. These data provide conclusive evidence for a novel gain-of-function GLI1 splice variant that promotes migration and invasiveness of GBM cells and open up a new research paradigm on the role of the GLI1 pathway in malignancy.

Mouse hitchhiker mutants have spina bifida, dorso-ventral patterning defects and polydactyly: identification of Tulp3 as a novel negative regulator of the Sonic hedgehog pathway

The mammalian Sonic hedgehog (Shh) signalling pathway is essential for embryonic development and the patterning of multiple organs. Disruption or activation of Shh signalling leads to multiple birth defects, including holoprosencephaly, neural tube defects and polydactyly, and in adults results in tumours of the skin or central nervous system. Genetic approaches with model organisms continue to identify novel components of the pathway, including key molecules that function as positive or negative regulators of Shh signalling. Data presented here define Tulp3 as a novel negative regulator of the Shh pathway. We have identified a new mouse mutant that is a strongly hypomorphic allele of Tulp3 and which exhibits expansion of ventral markers in the caudal spinal cord, as well as neural tube defects and preaxial polydactyly, consistent with increased Shh signalling. We demonstrate that Tulp3 acts genetically downstream of Shh and Smoothened (Smo) in neural tube patterning and exhibits a genetic interaction with Gli3 in limb development. We show that Tulp3 does not appear to alter expression or processing of Gli3, and we demonstrate that transcriptional regulation of other negative regulators (Rab23, Fkbp8, Thm1, Sufu and PKA) is not affected. We discuss the possible mechanism of action of Tulp3 in Shh-mediated signalling in light of these new data.

A GLI1-p53 inhibitory loop controls neural stem cell and tumour cell numbers

How cell numbers are determined is not understood. Hedgehog-Gli activity is involved in precursor cell proliferation and stem cell self-renewal, and its deregulation sustains the growth of many human tumours. However, it is not known whether GLI1, the final mediator of Hh signals, controls stem cell numbers, and how its activity is restricted to curtail tumourigenesis. Here we have altered the levels of GLI1 and p53, the major tumour suppressor, in multiple systems. We show that GLI1 expression in Nestin+ neural progenitors increases precursor and clonogenic stem cell numbers in vivo and in vitro. In contrast, p53 inhibits GLI1-driven neural stem cell self-renewal, tumour growth and proliferation. Mechanistically, p53 inhibits the activity, nuclear localisation and levels of GLI1 and in turn, GLI1 represses p53, establishing an inhibitory loop. We also find that p53 regulates the phosphorylation of a novel N' truncated putative activator isoform of GLI1 in human cells. The balance of GLI1 and p53 functions, thus, determines cell numbers, and prevalence of p53 restricts GLI1-driven stem cell expansion and tumourigenesis.