Tumor cell-selective regulation of NOXA by c-MYC in response to proteasome inhibition

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Abstract
The proteasome controls a plethora of survival factors in all mammalian cells analyzed to date. Therefore, it is puzzling that proteasome inhibitors such as bortezomib can display a preferential toxicity toward malignant cells. In fact, proteasome inhibitors have the salient feature of promoting a dramatic induction of the proapoptotic protein NOXA in a tumor cell-restricted manner. However, the molecular determinants that control this specific regulation of NOXA are unknown. Here, we show that the induction of NOXA by bortezomib is directly dependent on the oncogene c-MYC. This requirement for c-MYC was found in a variety of tumor cell types, in marked contrast with dispensable roles of p53, HIF-1alpha, and E2F-1 (classical proteasomal targets that can regulate NOXA mRNA under stress). Conserved MYC-binding sites identified at the NOXA promoter were validated by ChIP and reporter assays. Down-regulation of the endogenous levels of c-MYC abrogated the induction of NOXA in proteasome-defective tumor cells. Conversely, forced expression of c-MYC enabled normal cells to accumulate NOXA and subsequently activate cell death programs in response to proteasome blockage. c-MYC is itself a proteasomal target whose levels or function are invariably up-regulated during tumor progression. Our data provide an unexpected function of c-MYC in the control of the apoptotic machinery, and reveal a long sought-after oncogenic event conferring sensitivity to proteasome inhibition.