The c-MYC oncoprotein functions as a sequence-specific transcription factor. The ability of c-MYC to activate transcription relies in part on the recruitment of cofactor complexes containing the histone acetyltransferases mammalian GCN5 (mGCN5)/PCAF and TIP60. In addition to acetylating histones, these enzymes have been shown to acetylate other proteins involved in transcription, including sequence-specific transcription factors. This study was initiated in order to determine whether c-MYC is a direct substrate of mGCN5 and TIP60. We report here that mGCN5/PCAF and TIP60 acetylate c-MYC in vivo. By using nanoelectrospray tandem mass spectrometry to examine c-MYC purified from human cells, the major mGCN5-induced acetylation sites have been mapped. Acetylation of c-MYC by either mGCN5/PCAF or TIP60 results in a dramatic increase in protein stability. The data reported here suggest a conserved mechanism by which acetyltransferases regulate c-MYC function by altering its rate of degradation.

No abstract

No abstract

Inactivating mutations within the von Hippel-Lindau (VHL) tumor suppressor gene predispose patients to develop a variety of highly vascularized tumors. pVHL targets ␣ subunits of the heterodimeric transcription factor hypoxia-inducible factor (HIF), a critical regulator of energy metabolism, angiogenesis, hematopoiesis, and oxygen (O 2 ) delivery, for ubiquitin-mediated degradation in an O 2 -dependent manner. To investigate the role of Vhl in cellular proliferation and tumorigenesis, we utilized mouse embryonic fibroblasts (MEFs), a common tool for analyzing cell cycle regulation, and generated Vhl ؊/؊ MEF-derived fibrosarcomas. Surprisingly, growth of both Vhl ؊/؊ MEFs and fibrosarcomas was impaired, although tumor vascularity was increased. Decreased proliferation of Vhl ؊/؊ MEFs was correlated with an overexpression of cyclin kinase inhibitors (CKIs) p21 and p27. The transcription of p21 and p27 is inhibited by c-Myc; therefore, the induction of CKIs was attributed to the ability of HIF to antagonize c-Myc activity. Indeed, p21 mRNA levels were elevated under normoxia in Vhl ؊/؊ MEFs, while c-Myc transcriptional activity was markedly reduced. Gene silencing of HIF-1␣ by small interfering RNA reduced p21 and p27 protein and mRNA levels in Vhl ؊/؊ MEFs. The induction of p21 and p27, mediated by constitutive activation of the HIF pathway, provides a mechanism for the decreased proliferation rates of Vhl ؊/؊ MEFs and fibrosarcomas. These results demonstrate that a loss of pVHL can induce growth arrest in certain cells types, which suggests that additional genetic mutations are necessary for VHL-associated tumorigenesis.