Mechanisms acting on Hedgehog-GLI pathway and their therapeutic potential

Abstract: Hedgehog signaling is crucial for diverse aspects of animal development, essential in regulating many cellular processes and is largely implicated in various forms of human cancer. However, many aspects of Hedgehog signaling are not completely understood. This thesis aims to contribute towards a better understanding of the mechanisms acting on Hedgehog-GLI signaling and explore their possible therapeutic potential. PAPER I. We demonstrate that the small molecule RITA, a p53 activator, downregulates Hedgehog signaling in human medulloblastoma and rhabdomyosarcoma cells via JNK kinase and irrespective of p53. In vitro RITA enhanced the anti-proliferative effects of the GLI antagonist GANT61. RITA was more potent than GANT61 in downregulating Hedgehog-GLI signaling in rhabdomyosarcoma subcutaneous xenograft tumors with the dual drug administration almost completely blocking the Hedgehog signaling response in vivo, suggesting a certain antagonism of the two drugs. Notably, RITA and GANT61 co-administration decreased cell proliferation and elicited a broader response of pathways involved in cancer cell growth, providing a plausible interpretation for tumor reduction in the absence of Hedgehog signaling downregulation. PAPER II. We address the possible therapeutic role of Hedgehog-GLI1 signaling for targeting and prognosis of ER-alpha positive breast cancer. We showed that expression of the Hedgehog signaling effector protein GLI1 is higher in tamoxifen resistant relative to tamoxifen sensitive cells. In both cell types GLI1 depletion mitigated cell proliferation and ER-alpha activity, irrespective of estrogen stimulation. Tamoxifen cytotoxicity was enhanced by GANT61 co-treatment, both in tamoxifen resistant and sensitive breast cancer cells, reflecting a crosstalk between ER-alpha and Hedgehog-GLI1 signaling. We have observed a positive correlation between GLI1 and ER-alpha/ER-alpha target gene expression, while high GLI1 expression was associated with poor distant metastasis-free survival in breast cancer patients. PAPER III. We identified a signature of GLI1 target genes via a combination of RNA-seq analyses of GLI1 overexpression and depletion datasets supplemented with in-depth validation in human cancer cell lines. Additionally, we found that RNA editing of GLI1 can modulate its effects on GLI1 target genes. Markedly, one of the highly upregulated targets, FOXS1, was found to engage in feedback mechanisms limiting the capacity of GLI1 to act as a proliferation factor in medulloblastoma and rhabdomyosarcoma cells. FOXS1 was both highly expressed and positively correlated with GLI1 in SHH medulloblastoma, further arguing for the existence of a FOXS1-GLI1 interplay in human tumors. PAPER IV (Manuscript). In this ongoing work we address the role of circRNAs in the context of Hedgehog signaling activation and Hedgehog-linked SHH medulloblastoma tumors. Via modified RNA-seq protocols we have determined the circRNA transcriptome of Daoy medulloblastoma and human embryonic palatal mesenchyme HEPM cells, following activation of Hedgehog pathway with SHH ligand or Smoothened agonist SAG. In total, 29 selected circRNAs were independently validated by Sanger sequencing and RT-PCR assays. Of these circRNAs, 10 were apparently regulated by Hedgehog signaling activation, however to a much lesser extent compared with known target genes of the pathway, e.g. GLI1 and HHIP. 7 circRNAs had reduced expression in human medulloblastoma tumors in comparison to normal cerebellum, while the linear mRNAs originating from the same genes did not exhibit a reduced expression. These findings highlight distinct regulatory mechanisms acting on the BACH1, CDYL, FKBP8, GLIS1, OGDH, SMARCA5 and ZKSCAN1 circRNAs and deserve further analysis for possible contribution to the development of medulloblastoma.