Research ArticleCancer

Reduced abundance of the E3 ubiquitin ligase E6AP contributes to decreased expression of the INK4/ARF locus in non–small cell lung cancer

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Sci. Signal.  10 Jan 2017:
Vol. 10, Issue 461,
DOI: 10.1126/scisignal.aaf8223

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INK4/ARF repression in lung cancer by loss of E6AP

The abundance of the cell cycle decelerator p16INK4a, encoded at the INK4/ARF locus, is decreased in various cancers. Gamell et al. found that the absence of p16INK4a in some patients is explained by the lack of the E3 ubiquitin ligase E6AP. E6AP bound to and inhibited the activity of transcription factor E2F1; a decrease in E6AP abundance enabled E2F1-mediated expression of the cell cycle promoter CDC6, which encodes a transcriptional regulator that represses INK4/ARF expression. The findings identify a tumor-suppressive role for E6AP in lung cancer and suggest that targeting the E2F1-CDC6 pathway might slow tumor growth in some NSCLC patients.

Abstract

The tumor suppressor p16INK4a, one protein encoded by the INK4/ARF locus, is frequently absent in multiple cancers, including non–small cell lung cancer (NSCLC). Whereas increased methylation of the encoding gene (CDKN2A) accounts for its loss in a third of patients, no molecular explanation exists for the remainder. We unraveled an alternative mechanism for the silencing of the INK4/ARF locus involving the E3 ubiquitin ligase and transcriptional cofactor E6AP (also known as UBE3A). We found that the expression of three tumor suppressor genes encoded in the INK4/ARF locus (p15INK4b, p16INK4a, and p19ARF) was decreased in E6AP−/− mouse embryo fibroblasts. E6AP induced the expression of the INK4/ARF locus at the transcriptional level by inhibiting CDC6 transcription, a gene encoding a key repressor of the locus. Luciferase assays revealed that E6AP inhibited CDC6 expression by reducing its E2F1-dependent transcription. Chromatin immunoprecipitation analysis indicated that E6AP reduced the amount of E2F1 at the CDC6 promoter. In a subset of NSCLC samples, an E6AP-low/CDC6-high/p16INK4a-low protein abundance profile correlated with low methylation of the gene encoding p16INK4a (CDKN2A) and poor patient prognosis. These findings define a previously unrecognized tumor-suppressive role for E6AP in NSCLC, reveal an alternative silencing mechanism of the INK4/ARF locus, and reveal E6AP as a potential prognostic marker in NSCLC.

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