MDM2 regulates RB levels during genotoxic stress.

The retinoblastoma tumour suppressor protein (RB) regulates a number of diverse cellular functions including differentiation, angiogenesis, chromatin remodelling, senescence and apoptosis. The best-characterised function of RB is cell cycle regulation, and it has been considered a phosphoprotein regulated by cyclin-dependent kinases. In its hypophosphorylated form, RB binds the transcription factor E2F1, arresting the cell cycle in the G1 phase. Here, we show that MDM2 controls the cell cycle through synthesis and degradation of RB protein in a cell cycle condition-dependent fashion. MDM2 induces G1 cell cycle arrest by enhancing the translation of the RB mRNA under genotoxic stress. Translation requires direct interaction between the RB mRNA and the MDM2 protein that accompanies the RB mRNA to the polysomes. However, MDM2 ubiquitinates and degrades RB protein at the G2/M phase under genotoxic stress. The ATM phosphomimetic mutant MDM2(S395D) corroborates that the effect on the RB levels is dependent on the DNA damage. These results provide the basis of a dual regulatory mechanism by which MDM2 controls cell cycle progression during DNA damage.

Expression and purification of the recombinant full-length retinoblastoma protein and characterisation of its interaction with the oncoprotein HDM2.

Retinoblastoma (Rb) was the first tumour suppressor factor described, and it is dysfunctional in several types of cancers. Structurally, Rb is a very large, multifunctional protein organized in different domains connected by intrinsically disordered regions. Due to the complex structure of Rb, biochemical manipulation is difficult. The Rb protein has been implicated in many different cellular processes, such as the cell cycle control, senescence and even apoptosis. The activity of Rb is regulated by phosphorylation, and many different sites of phosphorylation have been described. However, the oncoprotein HDM2, can promote Rb degradation by the proteasome. This form of Rb regulation is largely unknown. Here we report the expression and purification of the full-length Rb protein and its phosphomimetic form, Rb(S567D), in a recombinant system. We also produced and purified the HDM2 protein and its phosphomimetic mutant, HDM2(S395D). The proteins interacted strongly when we used the phosphomimetic mutants, mimicking damaged DNA conditions. The expression of the proteins in E. coli allowed us to control the phosphorylation status of the proteins.

Dual function of MDM2 and MDMX toward the tumor suppressors p53 and RB.

The orchestrated crosstalk between the retinoblastoma (RB) and p53 pathways contributes to preserving proper homeostasis within the cell. The deregulation of one or both pathways is a common factor in the development of most types of human cancer. The proto-oncoproteins MDMX and MDM2 are the main regulators of the well- known tumor suppressor p53 protein. Under normal conditions, MDM2 and MDMX inhibit p53, either via repression of its transcriptional activity by protein-protein interaction, or via polyubiquitination as a result of MDM2-E3 ubiquitin ligase activity, for which MDM2 needs to dimerize with MDMX. Under genotoxic stress conditions, both become positive regulators of p53. The ATM-dependent phosphorylation of MDM2 and MDMX allow them to bind p53 mRNA, these interactions promote p53 translation. MDM2 and MDMX are also being revealed as effective regulators of the RB protein. MDM2 is able to degrade RB by two different mechanisms, that is, by ubiquitin dependent and independent pathways. MDMX enhances the ability of MDM2 to bind and degrade RB protein. However, MDMX also seems to stabilize RB through interaction and competition with MDM2. Here, we will contextualize the findings that suggest that the MDM2 and MDMX proteins have a dual function on both p53 and RB.