Paul I, Ahmed SF, Bhowmik A, Deb S, Ghosh MK

c-Myc is a proto-oncogenic transcription factor and its rapid turnover mediated by the ubiquitin-proteasome system is critical for maintaining normal cellular homeostasis. Multiple ubiquitin ligases have been assigned for c-Myc regulation till date. However, the available data suggest for the possible existence of additional E3 ligase(s). Here, we report a ...

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new E3 ligase for c-Myc, the carboxyl terminus of Hsc70-interacting protein or CHIP, which is a chaperone-associated Ubox-containing E3 ligase. In this report, we show that CHIP interacts and ubiquitinates c-Myc, thus targeting it for proteasome-mediated degradation. Overexpression of CHIP could accelerate the turnover rate of c-Myc protein. Conversely, knockdown of CHIP by RNAi stabilizes endogenous c-Myc. The interaction between CHIP and c-Myc depends on the N-terminally located tetratricopeptide repeats of CHIP, which has been implicated as a chaperone-binding motif. Inhibition of Hsp90 chaperone activity by 17-N-allylamino-17-demethoxygeldanamycin reduces c-Myc protein level. We found that the association between CHIP and c-Myc is dependent on the chaperones; particularly Hsp70. CHIP antagonizes the transcriptional activity of c-Myc and decreases the abundance of the transcripts of its target genes. Overall, CHIP-knockdown increases malignant behavior of C6 glioma cells. To the best of our knowledge, this is the first report of c-Myc being regulated by a bona-fide chaperone-associated E3 ligase in HEK293 as well as glioma cells. Because CHIP has been reported earlier to be negatively regulating Akt1, BCR-ABL and hTERT, and now c-Myc, the present study may strengthen the view that CHIP acts as a tumor suppressor.Oncogene advance online publication, 30 April 2012; doi:10.1038/onc.2012.144.

Date: Apr. 30, 2012

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