title: Imiquimod directly inhibits Hedgehog signalling by stimulating adenosine receptor/protein kinase A-mediated GLI phosphorylation.
abstract: Imiquimod (IMQ), a nucleoside analogue of the imidazoquinoline family, is used in the topical treatment of basal cell carcinoma (BCC) and other skin diseases. It is reported to be a TLR7 and TLR8 agonist and, as such, initiates a Th1 immune response by activating sentinel cells in the vicinity of the tumour. BCC is a hedgehog (HH)-driven malignancy with oncogenic glioma-associated oncogene (GLI) signalling activated in a ligand-independent manner. Here we show that IMQ can also directly repress HH signalling by negatively modulating GLI activity in BCC and medulloblastoma cells. Further, we provide evidence that the repressive effect of IMQ on HH signalling is not dependent on TLR/MYD88 signalling. Our results suggest a mechanism for IMQ engaging adenosine receptors (ADORAs) to control GLI signalling. Pharmacological activation of ADORA with either an ADORA agonist or IMQ resulted in a protein kinase A (PKA)-mediated GLI phosphorylation and reduction in GLI activator levels. The activation of PKA and HH pathway target gene downregulation in response to IMQ were abrogated by ADORA inhibition. Furthermore, activated Smoothened signalling, which positively signals to GLI transcription factors, could be effectively counteracted by IMQ. These results reveal a previously unknown mode of action of IMQ in the treatment of BCC and also suggest a role for ADORAs in the regulation of oncogenic HH signalling.
authors.affiliation: Department of Molecular Biology, University of Salzburg, Salzburg, Austria.
authors.forename: F
authors.initials: F
authors.lastname: Wolff
authors.affiliation: Department of Molecular Biology, University of Salzburg, Salzburg, Austria.
authors.forename: A
authors.initials: A
authors.lastname: Loipetzberger
authors.affiliation: Department of Molecular Biology, University of Salzburg, Salzburg, Austria.
authors.forename: W
authors.initials: W
authors.lastname: Gruber
authors.affiliation: Department of Laboratory Medicine, Medical University Vienna, Vienna, Austria.
authors.forename: H
authors.initials: H
authors.lastname: Esterbauer
authors.affiliation: Department of Molecular Biology, University of Salzburg, Salzburg, Austria.
authors.forename: F
authors.initials: F
authors.lastname: Aberger
authors.affiliation: Department of Molecular Biology, University of Salzburg, Salzburg, Austria.
authors.forename: A M
authors.initials: AM
authors.lastname: Frischauf
mesh_terms: D000634:Aminoquinolines / Q000494:pharmacology*; D000970:Antineoplastic Agents / Q000494:pharmacology*; D002280:Basal Cell Carcinoma / Q000235:genetics / Q000378:metabolism; D045744:Cell Line, Tumor; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D053823:Hedgehog Proteins / Q000378:metabolism*; D006801:Humans; D000077271:Imiquimod; D051741:Kruppel-Like Transcription Factors / Q000378:metabolism; D008527:Medulloblastoma / Q000235:genetics / Q000378:metabolism; D053594:Myeloid Differentiation Factor 88 / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000378:metabolism; D018047:Receptors, Purinergic P1 / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects*; D051199:Toll-Like Receptor 7 / Q000378:metabolism; D051200:Toll-Like Receptor 8 / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism*; D000071676:Zinc Finger Protein GLI1; D000074290:Zinc Finger Protein Gli3
chemical_list: D000634:Aminoquinolines; D000970:Antineoplastic Agents; C400551:GLI1 protein, human; C065683:GLI3 protein, human; D053823:Hedgehog Proteins; D051741:Kruppel-Like Transcription Factors; D053594:Myeloid Differentiation Factor 88; D009419:Nerve Tissue Proteins; D018047:Receptors, Purinergic P1; D051199:Toll-Like Receptor 7; D051200:Toll-Like Receptor 8; D014157:Transcription Factors; D000071676:Zinc Finger Protein GLI1; D000074290:Zinc Finger Protein Gli3; D017868:Cyclic AMP-Dependent Protein Kinases; D000077271:Imiquimod