Editors' ChoiceCancer

Resistance Through cAMP Signaling

Sci. Signal.  10 Dec 2013:
Vol. 6, Issue 305, pp. ec305
DOI: 10.1126/scisignal.2004979

When trying to understand the molecular mechanisms that produce resistance to targeted cancer drugs, there is a tendency to look for alterations within the pathway targeted by the drug. Johannessen et al. took an unbiased approach and expressed more than 15,000 open reading frames in a melanoma cell line with the catalytically activating V600E mutation in the kinase BRAF, which is normally activated by receptor tyrosine kinases. By exposing the cells to drugs targeting the upstream kinase in this pathway (RAF), its target MEK, or the MEK target ERK, or to combinations of RAF and MEK inhibitors, they determined which of the encoded proteins conferred resistance to inhibition of the mitogen-activated protein kinase (MAPK) pathway. This approach identified several genes encoding proteins in pathways not directly related to the targeted MAPK pathway, including multiple G protein–coupled receptors (GPCRs), the adenylyl cyclase ADCY9, and the catalytic subunit of protein kinase A (PKA). This PKA pathway resembled that of one important for melanocyte lineage specification, suggesting that reactivation of this developmental pathway may contribute to drug resistance in cancer patients. Consistent with the genetic resistance data, treatments that increase cyclic adenosine 3′,5′-monophosphate (cAMP) conferred resistance to MAPK pathway inhibition. CREB and ATF1 are transcription factors activated by the cAMP-PKA pathway, and both of these were phosphorylated (indicating activation) in patient tumor samples before treatment targeting the MAPK pathway. The phosphorylation of each was reduced in the tumors from patients treated with MAPK inhibitors; however, in tumors of patients that had relapsed, the phosphorylation status was similar to those of the pretreatment samples. Consistent with reactivation of a lineage pathway, the activating phosphorylation of the transcription factor MITF (a protein necessary for lineage specification and a melanoma oncoprotein) was reduced and the protein was ultimately undetectable in the melanoma cell line after exposure to a MEK inhibitor, whereas increasing cAMP increased MITF mRNA. Knockdown of MITF reduced the ability of cAMP signaling to mediate resistance to MAPK pathway inhibitors in the melanoma cell line. Because MITF expression is reduced by inhibition of histone deacetylases (HDACs) and HDAC inhibitors prevented cAMP-mediated resistance in the melanoma cell line, the authors suggested that combining HDAC inhibitors with MAPK pathway inhibitors may overcome resistance in BRAFV600E melanoma patients. Thus, the unbiased approach identified previously unknown pathways to target in a particularly relapse-prone cancer.

C. M. Johannessen, L. A. Johnson, F. Piccioni, A. Townes, D. T. Frederick, M. K. Donahue, R. Narayan, K. T. Flaherty, J. A. Wargo, D. E. Root, L. A. Garraway, A melanocyte lineage program confers resistance to MAP kinase pathway inhibition. Nature 504, 138–142 (2013). [PubMed]

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