The transcription factors p53 and p73 are critical to the induction of apoptotic cell death, particularly in response to cell stress that activates c-Jun N-terminal kinase (JNK). Mutations in the DNA-binding domain of p53, which are commonly seen in cancers, result in conformational changes that enable p53 to interact with and inhibit p73, thereby suppressing apoptosis. In contrast, wild-type p53 reportedly does not interact with p73. We found that JNK-mediated phosphorylation of Thr in the proline-rich domain (PRD) of p53 enabled wild-type p53, as well as mutant p53, to form a complex with p73. Structural algorithms predicted that phosphorylation of Thr exposes the DNA-binding domain in p53 to enable its binding to p73. The dimerization of wild-type p53 with p73 facilitated the expression of apoptotic target genes [such as those encoding p53-up-regulated modulator of apoptosis (PUMA) and Bcl-2-associated X protein (BAX)] and, subsequently, the induction of apoptosis in response to JNK activation by cell stress in various cells. Thus, JNK phosphorylation of mutant and wild-type p53 promotes the formation of a p53/p73 complex that determines cell fate: apoptosis in the context of wild-type p53 or cell survival in the context of the mutant. These findings refine our current understanding of both the mechanistic links between p53 and p73 and the functional role for Thr phosphorylation.

Metastasis of cancer cells to distant organ systems is a complex process that is initiated with the programming of cells in the primary tumor. The formation of distant metastatic foci is correlated with poor prognosis and limited effective treatment options. We and others have correlated Mouse double minute 2 (Mdm2) with metastasis; however, the mechanisms involved have not been elucidated. Here, it is reported that shRNA-mediated silencing of Mdm2 inhibits epithelial-mesenchymal transition (EMT) and cell migration. analysis demonstrates that silencing Mdm2 in both post-EMT and basal/triple-negative breast cancers resulted in decreased primary tumor vasculature, circulating tumor cells, and metastatic lung foci. Combined, these results demonstrate the importance of Mdm2 in orchestrating the initial stages of migration and metastasis. Mdm2 is the major factor in the initiation of metastasis. .

Mouse double minute 2 (Mdm2) and MdmX dimerize in response to low levels of genotoxic stress to function in a ubiquitinating complex, which signals for destabilization of p53. Under growth conditions, Mdm2 functions as a neddylating ligase, but the importance and extent of MdmX involvement in this process are largely unknown. Here we show that when Mdm2 functions as a neddylating enzyme, MdmX is stabilized. Furthermore, we demonstrate that under growth conditions, MdmX enhances the neddylation activity of Mdm2 on p53 and is a substrate for neddylation itself. Importantly, MdmX knockdown in MCF-7 breast cancer cells resulted in diminished neddylated p53, suggesting that MdmX is important for Mdm2-mediated neddylation. Supporting this finding, the lack of MdmX in transient assays or in p53/MdmX-/- MEFs results in decreased or altered neddylation of p53 respectively; therefore, MdmX is a critical component of the Mdm2-mediated neddylating complex. c-Src is the upstream activator of this Mdm2-MdmX neddylating pathway and loss of Src signaling leads to the destabilization of MdmX that is dependent on the RING (Really Interesting New Gene) domain of MdmX. Treatment with a small molecule inhibitor of neddylation, MLN4924, results in the activation of Ataxia Telangiectasia Mutated (ATM). ATM phosphorylates Mdm2, converting Mdm2 to a ubiquitinating enzyme which leads to the destabilization of MdmX. These data show how distinct signaling pathways engage neddylating or ubiquitinating activities and impact the Mdm2-MdmX axis.

Cell fusion is a physiological process utilized by a variety of biological systems during fertilization, embryogenesis, and cellular differentiation and morphogenesis. During the malignant transformation of breast and bone tissues, normal and malignant cells become fused to form hybrid structures. These enhanced phenotypes acquire genetic characteristics which support initiation, progression, and oncogenesis. In breast cancer and osteosarcoma cell lines, cell fusion generates chromosomal instability, aneuploidy, and DNA damage while maintaining the characteristics of enhanced migration, invasion, and chemoresistance. We have found that mouse double minute 2 (Mdm2) significantly induces cell fusion and the generation of large multinucleated cells in MDA468, MDA231, MDA157, T47D breast cancer and MG63 osteosarcoma cell lines under normoxia when compared to silenced Mdm2 controls (P<0.0005). Interestingly, silencing mutant p53 in the MDA468 cell line resulted in significantly higher fusion events (P<0.0004) suggesting mutant p53 may play a regulatory role in reducing Mdm2-associated cell fusion. Silenced Mdm2 MDA231 cells showed a three-fold increase under hypoxia whereas MG63 cells showed similar levels of cell fusion events as in normoxic conditions suggesting cell specific dependency to environmental stimulus. The variation in proportions of cell fusion among each cell type suggests this phenomenon is dependent on Mdm2 expression and cell type. CD133+ MDA468 cells have increased fractions of polyploidal cell events suggesting stemness may influence the polyploidal giant cancer cell (PGCC) dynamics. The mechanism by which cell fusion occurs in breast and osteosarcoma is poorly understood. Collectively these data suggest the interplay of Mdm2, p53, and stemness in supporting the formation of fused polypoidal cells and may serve as relevant targets for cancer therapy. Citation Format: David J. Olivos, Mikhail Chtcherbinine, Eric R. Wolf, Ciarán McAtarsney, Lindsey D. Mayo. MDM2 induces cell fusion in breast cancer and osteosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2552. doi:10.1158/1538-7445.AM2017-2552