Hedgehog pathway and GLI1 isoforms in human cancer

Abstract

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.

Figure 1

Structures of the human GLI1 gene and the encoded full-length GLI1, GLI1ΔN, and tGLI1 isoforms. The full-length GLI1 gene is comprised of 12 exons, including the 5′-untranslated exon 1. The GLI1 coding region spans nt +79 to +3399 with the initiating methionine codon, ATG, at +79 in exon 2 (arrows). Exons are indicated as gray boxes while introns are shown by lines. The known functional domains of full-length GLI1 include the degron degradation signals (Dn and Dc; aa 77–116; 464–469), SUFU-binding domains (SU; aa 111–125 and C-terminus) (Dunaeva et al., 2003), zinc finger domains (ZF; aa 235–387), the nuclear localization signal (NLS; aa 380–420), and the transactivation domain (aa 1020–1091). Alternative splicing of GLI1 RNA can lead to the deletion of exons 1–3 totaling 128 amino acids in the N-terminus, forming the GLI1ΔN variant. The deletion of the entire exon 3 and part of exon 4 totaling 41 amino acids yields the tGLI1 isoform. Notably, tGLI1 retains all the known functional domains of the full-length GLI1.

Figure 2

GLI1 Isoforms and cancer characteristics. GLI1 is induced by Shh, Ras, TGF-β, and PI3-K, leading to induction of PTCH, NANOG, Snail, and other genes. Conversely, GLI1 is negatively regulated by PKA and p53. GLI1 expression is linked to proliferation, EMT, cancer stemness, and tumorigenesis. Shh can induce tGLI1 activity; however, it is unknown whether non-shh pathways can regulate tGLI1. Like GLI1, tGLI1 induces PTCH expression. Importantly, tGLI1 has gained the ability to transcriptionally upregulate expression of CD24, VEGF-A, MMP-2, and MMP-9 genes, and thereby promotes migration, invasion, and angiogenesis.