MAGE-A Cancer/Testis Antigens Inhibit MDM2 Ubiquitylation Function and Promote Increased Levels of MDM4

Abstract

Melanoma antigen A (MAGE-A) proteins comprise a structurally and biochemically similar sub-family of Cancer/Testis antigens that are expressed in many cancer types and are thought to contribute actively to malignancy. MAGE-A proteins are established regulators of certain cancer-associated transcription factors, including p53, and are activators of several RING finger-dependent ubiquitin E3 ligases. Here, we show that MAGE-A2 associates with MDM2, a ubiquitin E3 ligase that mediates ubiquitylation of more than 20 substrates including mainly p53, MDM2 itself, and MDM4, a potent p53 inhibitor and MDM2 partner that is structurally related to MDM2. We find that MAGE-A2 interacts with MDM2 via the N-terminal p53-binding pocket and the RING finger domain of MDM2 that is required for homo/hetero-dimerization and for E2 ligase interaction. Consistent with these data, we show that MAGE-A2 is a potent inhibitor of the E3 ubiquitin ligase activity of MDM2, yet it does not have any significant effect on p53 turnover mediated by MDM2. Strikingly, however, increased MAGE-A2 expression leads to reduced ubiquitylation and increased levels of MDM4. Similarly, silencing of endogenous MAGE-A expression diminishes MDM4 levels in a manner that can be rescued by the proteasomal inhibitor, bortezomid, and permits increased MDM2/MDM4 association. These data suggest that MAGE-A proteins can: (i) uncouple the ubiquitin ligase and degradation functions of MDM2; (ii) act as potent inhibitors of E3 ligase function; and (iii) regulate the turnover of MDM4. We also find an association between the presence of MAGE-A and increased MDM4 levels in primary breast cancer, suggesting that MAGE-A-dependent control of MDM4 levels has relevance to cancer clinically.

Expression of MAGE‐A and NY‐ESO‐1 cancer/testis antigens is enriched in triple‐negative invasive breast cancers

“…Through association with p53 coexpression, our findings imply that NY‐ESO‐1‐positive tumours reflect these chromosomal abnormalities, and have a proliferative advantage. However, we observed no association between MAGE‐A and p53, which, perhaps, could be attributed to MAGE‐A inhibiting its function; one possible suggested mechanism is blocking of the interaction between p53 and chromatin, thus making p53 unable to regulate tumour cell proliferation and apoptosis . Furthermore, we found a strong association between MAGE‐A expression and mitosis‐independent expression of the spindle assembly checkpoint protein TTK, which is crucial for chromosomal alignment and centrosome duplication, and is a marker for CIN in TNBC …”

“…However, we observed no association between MAGE-A and p53, which, perhaps, could be attributed to MAGE-A inhibiting its function; 57 one possible suggested mechanism is blocking of the interaction between p53 and chromatin, thus making p53 unable to regulate tumour cell proliferation and apoptosis. 57,58 Furthermore, we found a strong association between MAGE-A expression and mitosis-independent expression of the spindle assembly checkpoint protein TTK, which is crucial for chromosomal alignment and centrosome duplication, and is a marker for CIN in TNBC. 35,59,60 Re-expression of CTAs in cancer is thought to give cancer cells stem cell-like properties.…”

Case report: whole exome sequencing of primary cardiac angiosarcoma highlights potential for targeted therapies

“…Interestingly, a recent report describes an association between MAGE-A and MDM2 oncogene, E3 ubiquitin protein ligase (MDM2)/MDM4 complex in vitro in a human cell line model [53]. Specifically, MAGE-A was shown to compete with MDM4 for binding to MDM2 leading to elevated levels of free MDM4.…”

Epigenetic regulation of MAGE family in human cancer progression-DNA methylation, histone modification, and non-coding RNAs

“…And the interaction of MAGE-A2 with MDM2 inhibits the E3 ubiquitin ligase activity of MDM2, thus increasing the level of MDM4. However, it does not affect p53 turnover mediated by MDM2 [ 36 ]. MAGE-A11 interacts with Skp2, the substrate recognition protein of the Skp1-Cullin1-F-box E3 ubiquitin ligase, and increases Skp2-mediated degradation of cyclin A and p130, but decreases Skp2-mediated degradation of E2F1 and Skp2 self-ubiquitination by sequestering and inactivating Skp2 via forming an E2F1-MAGE-A11-Skp2 complex [ 37 ].…”