In unstressed normal cells, the well-known tumor suppressor p53 is present at low levels because of rapid degradation by the ubiquitin-dependent proteasome pathway. Mdm2 and Mdm4 (also known as hMDMX) use an a-helical segment to bind the N-terminal domain of p53 thereby regulating its activity. Mdm2 mediates p53 turnover by binding p53 and acting as an E3 ubiquitin ligase. Ubiquitination targets p53 for proteosomal degradation thereby inhibiting its direct pro-apoptotic function as well as its growth inhibition by disrupting the interaction with the transcriptional machinery. Mdm2 overexpression has been found in human cancers, in general correlating with the expression of wild-type p53.
Mdm4, a structural homologue of Mdm2, is also a critical regulator of p53 activity, whose primary function seems to be inhibition of p53-mediated transcriptional activation.1 Mdm4 may also enhance the Mdm2 mediated ubiquitination and subsequent degradation of p53. Similar toMdm2, Mdm4 has been found overexpressed in a significant number of human tumors and tumor cell lines suggesting that Mdm4 contributes to p53 inactivation. This data suggests that Mdm4, in addition to the better known Mdm2, is an attractive target for tumors expressing wild type p53. Proof of concept of the efficacy of inhibiting the Mdm4-p53 interaction has been generated.2
Our fragment library has been screened for ligands for Mdm4. 87 of the hits have been validated and their binding site on Mdm4 determined by protein observed NMR. Two series have been elaborated to single or low µM potency with Ligand Efficiency above 0.3 and 0.25 respectively. In addition, we have full access to biological assays via a local (Leiden, NL) collaborator. This project is primed for rapid success.
- Role of Mdm4 in drug sensitivity of breast cancer cells. Lam, Oncogene, 2010.
- A stapled p53 helix overcomes HDMX-mediated suppression of p53. Bernal, Cancer Cell, 2010.