Ubiquitin-mediated protein degradation and methylation-induced gene silencing cooperate in the inactivation of the INK4/ARF locus in Burkitt's lymphoma cell lines

Burkitt's lymphoma is one of the most aggressive tumors affecting humans. Together with the characteristic chromosomal translocation that constitutively activates the c-Myc oncogene, alterations in cellular tumor suppressor pathways are additionally required in order to allow the cells to overcome anti-oncogenic barriers and proliferate in an uncontrolled manner. The INK4a/ARF locus on chromosome 9p21 is considered a safeguard locus since it encodes the two important tumor suppressor proteins, p14ARF and p16INK4a. By regulating the p53 and Rb pathways p14ARF and p16INK4a respectively act as pro-apoptotic and cell cycle inhibitor proteins. The importance of the INK4a/ARF locus has been well documented in several human tumors as well as in Burkitt's lymphoma. Although the mechanisms responsible for the transcriptional regulation of the INK4a/ARF locus have been thoroughly characterized, less is known about its posttranscriptional control. In this study we found that p16INK4a and p14Arf are concurrently inactivated in a panel of BL cell lines. We demonstrate that along with the epigenetic silencing of the p16INK4a gene, the complete inactivation of the locus is achieved by the improper turnover of INK4/ARF proteins by the ubiquitin-proteasome system (UPS), as the proteasome inhibitor MG-132 blocks p14ARF degradation and induces a dramatic stabilization of the p16INK4a protein. We establish that the simultaneous deregulation of both DNA methylation patterns and the ubiquitin-dependent proteolysis system is required to completely inactive the INK4/ARF locus, opening new prospects for the understanding and treatment of Burkitt's lymphoma. © 2011 Landes Bioscience