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.

Targeting GLI factors to inhibit the Hedgehog pathway

“…Imiquimod ( Figure 1) is a synthetic nucleoside analog of the imidazoquinoline family, recently approved for topical treatment of small superficial BCCs. This molecule counteracts Hh signaling downstream of SMO by inducing PKA activity ( Figure 2) with consequent GLI2/3 phosphorylation and their cleavage into repressor forms [79]. Other compounds have been identified to impinge the Hh pathway by targeting proteins and/or interactors that modulate GLI factors activity.…”

Emerging drugs and combination strategies for basal cell carcinoma

“…Recently Wolff et al showed that imiquimod can directly repress Hh signalling, independently of the TLR/MYD88/NF-kB pathway by negatively modulating Gli activity in BCC and medulloblastoma cells through a mechanism that engages adenosine receptors to control Gli signalling. Pharmacological activation of adenosine receptors with either an adenosine receptor agonist or imiquimod resulted in a PKA-mediated Gli phosphorylation and reduction in Gli activator levels [103]. Makinodan and Marneros demonstrated that PKA activation via the cAMP agonist forskolin is sufficient to completely abolish oncogenic Smo activity in vitro.…”

Radiation takes its Toll

“…IMQ is currently in clinical trials in combination with radiation to treat breast cancer metastasis (5x 6Gy; NCT01421017) or glioma (33x 1.8Gy; NCT01400672) (www.clinicaltrials.gov). IMQ’s actions seem multifaceted, sometimes independent of TLRs, namely through redirecting the adenosine receptor-mediated immune suppression to inhibit oncogenic hedgehog signaling and by driving tumor cell death [115, 116]. A TLR4 agonist is being added to RT of sarcoma (NCT02180698) while TLR9 agonists are very promising as adjuvants to the RT of solid and hematological cancers [117, 118].…”