Hedgehog Signaling Regulates Telomerase Reverse Transcriptase in Human Cancer Cells

Abstract

The Hedgehog (HH) signaling pathway is critical for normal embryonic development, tissue patterning and cell differentiation. Aberrant HH signaling is involved in multiple human cancers. HH signaling involves a multi-protein cascade activating the GLI proteins that transcriptionally regulate HH target genes. We have previously reported that HH signaling is essential for human colon cancer cell survival and inhibition of this signal induces DNA damage and extensive cell death. Here we report that the HH/GLI axis regulates human telomerase reverse transcriptase (hTERT), which determines the replication potential of cancer cells. Suppression of GLI1/GLI2 functions by a C-terminus truncated GLI3 repressor mutant (GLI3R), or by GANT61, a pharmacological inhibitor of GLI1/GLI2, reduced hTERT protein expression in human colon cancer, prostate cancer and Glioblastoma multiforme (GBM) cell lines. Expression of an N-terminus deleted constitutively active mutant of GLI2 (GLI2ΔN) increased hTERT mRNA and protein expression and hTERT promoter driven luciferase activity in human colon cancer cells while GANT61 inhibited hTERT mRNA expression and hTERT promoter driven luciferase activity. Chromatin immunoprecipitation with GLI1 or GLI2 antibodies precipitated fragments of the hTERT promoter in human colon cancer cells, which was reduced upon exposure to GANT61. In contrast, expression of GLI1 or GLI2ΔN in non-malignant 293T cells failed to alter the levels of hTERT mRNA and protein, or hTERT promoter driven luciferase activity. Further, expression of GLI2ΔN increased the telomerase enzyme activity, which was reduced by GANT61 administration in human colon cancer, prostate cancer, and GBM cells. These results identify hTERT as a direct target of the HH signaling pathway, and reveal a previously unknown role of the HH/GLI axis in regulating the replication potential of cancer cells. These findings are of significance in understanding the important regulatory mechanisms that determine the functions of HH/GLI signaling in cancer cells.

Anti‐cancer stem cell activity of a hedgehog inhibitor GANT61 in estrogen receptor‐positive breast cancer cells

“…It has been indicated that GANT61 inhibits proliferation by its effects on cell cycle progression. Several independent studies have shown that GANT61 induces a G1 arrest, consistent with decreased expression levels of the Hh target Cyclin D and/or increased expression levels of p21 . In the present study, GANT61 was shown to dose‐dependently induce a G1‐S cell cycle retardation in ER‐positive breast cancer cell lines (Fig.…”

GLI1 inhibitor GANT61 exhibits antitumor efficacy in T-cell lymphoma cells through down-regulation of p-STAT3 and SOCS3

“…Moreover, GANT61 serves as a valuable tool to investigate Hh pathway biology. Antitumor activity of GANT61 has been ascribed to its effect on cell viability, proliferation, apoptosis, DNA damage repair, autophagy, cancer stem cells and immune response [ 23 – 26 ]. Studying the mechanisms by which the GANT61 interacts with cancer cells is of great clinical interest for inhibiting growth, metastasis, and recurrence of cancers.…”

GLI1, a master regulator of the hallmark of pancreatic cancer

“…Furthermore, the chemokine receptor CXCR4, which is responsible for CXCL12‐mediated organ‐specific metastasis, is also up‐regulated by GLI1 and GLI2 in PDAC cells . In addition, experiments using other types of cells have demonstrated that GLI1 directly up‐regulates the expression of many other genes, such as telomerase reverse transcriptase ( TERT ), NANOG for immortality and stemness; interleukin ( IL )‐ 6 / 8 , colony stimulating factor 1 ( CSF1 / MCSF ) and C‐C motif chemokine ligand 2 ( CCL2 / MCP1 ) for inflammation (particularly in fibroblastic stroma cells); cysteine rich angiogenic inducer 61 ( CYR61 / CCN1 ) for angiogenesis; and the ATP‐binding cassette transporter gene ABCG2 for multi‐drug resistance. These results support the notion that GLI1 governs the hallmarks of the malignant nature of PDAC through target gene regulation.…”