Editors' ChoiceCancer

Atypical GLI Activation

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Science Signaling  05 Mar 2013:
Vol. 6, Issue 265, pp. ec55
DOI: 10.1126/scisignal.2004122

Signaling through the Hedgehog (Hh) pathway promotes growth in various types of cancers, including basal cell carcinoma (BCC). Hh-dependent cancers often develop resistance to inhibitors of the transmembrane protein and Hh signal transducer Smoothened (SMO), so it is important to identify additional potential targets for inhibiting Hh signaling. Atwood et al. report that atypical protein kinase C ι/λ (aPKC-ι/λ) coimmunoprecipitated with the scaffold protein missing-in-metastasis (MIM), which promotes activation of Hh-responsive genes by the GLI transcription factors. aPKC-ι/λ is encoded by the protein kinase C iota (Prkci) gene, and treating mouse ASZ001 BCC cells with a short hairpin RNA targeting prkci transcripts or with a peptide inhibitor of aPKC-ι/λ kinase activity inhibited cell proliferation and reduced Hh signaling as measured by transcription of a Hh-responsive target gene. Analysis of the expression of Hh target genes indicated that aPKC-ι/λ was not required for Hh signaling but was necessary for maximal Hh signaling. The abundance of aPKC-ι/λ and phosphorylated (activated) aPKC-ι/λ was increased in primary human BCC cells, including those from tumors resistant to SMO inhibition, compared with normal keratinocytes. Hh signaling stimulated expression of Prkci, and chromatin immunoprecipitation assays indicated that overexpressed GLI1 bound to the Prkci promoter, suggesting that aPKC-ι/λ and GLI1, which is both a mediator and target of Hh signaling, may constitute a positive feedback loop. Experiments with the peptide inhibitor of aPKC-ι/λ and the SMO inhibitor cyclopamine in ASZ001 cells suggested that aPKC-ι/λ enhanced GLI activity, and aPKC-ι/λ phosphorylated GLI1 in vitro. Endogenous GLI1 immunoprecipitated from ASZ001 cells was phosphorylated on serine and threonine residues in the zinc finger domain, and this phosphorylation was reduced in cells treated with the peptide inhibitor of aPKC-ι/λ. Experiments with mutant forms of GLI1 in electrophoretic mobility shift assays indicated that aPKC-ι/λ–mediated phosphorylation of GLI1 increased GLI1’s DNA binding affinity. Topically treating human BCC tumors allografted into mice with the aPKC-ι/λ peptide inhibitor reduced tumor size and resulted in decreased Gli1 expression and increased apoptosis of tumor cells. The aPKC-ι/λ peptide inhibitor also inhibited growth of tumors that were resistant to SMO inhibition. Thus, aPKC-ι/λ is a potential target for developing therapies to treat Hh-dependent and SMO inhibitor–resistant cancers.

S. X. Atwood, M. Li, A. Lee, J. Y. Tang, A. E. Oro, GLI activation by atypical protein kinase C ι/λ regulates the growth of basal cell carcinomas. Nature 494, 484–488 (2013). [PubMed]

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