The p53 isoforms are differentially modified by Mdm2.

The discovery that the single p53 gene encodes several different p53 protein isoforms has initiated a flurry of research into the function and regulation of these novel p53 proteins. Full-length p53 protein level is primarily regulated by the E3-ligase Mdm2, which promotes p53 ubiquitination and degradation. Here, we report that all of the novel p53 isoforms are ubiquitinated and degraded to varying degrees in an Mdm2-dependent and -independent manner, and that high-risk human papillomavirus can degrade some but not all of the novel isoforms, demonstrating that full-length p53 and the p53 isoforms are differentially regulated. In addition, we provide the first evidence that Mdm2 promotes the NEDDylation of p53ß. Altogether, our data indicates that Mdm2 can distinguish between the p53 isoforms and modify them differently.

MDM4 downregulates p53 transcriptional activity and response to stress during differentiation.

Embryonic stem (ES) cells are invaluable for their therapeutic potential as well as for the study of early development. Their clinical use demands an understanding of ES cell differentiation, particularly with respect to cell proliferation and the maintenance of genomic integrity, processes for which the transcription factor p53 is essential. However, although the function of p53 as a tumor suppressor has been extensively studied, its role in ES cell biology has not been clearly elucidated. To study p53 activity and regulation in differentiating ES cells, we used knock-in constructs to create a novel reporter system that provides a direct readout of p53 transcriptional activity. We thereby determine that the p53 pathway is active in ES cells, but that p53 activity and the p53-dependent stress response decrease upon differentiation. Although p53 protein levels and activity are usually primarily controlled by the ubiquitin ligase MDM2, we identify the MDM2 homolog MDM4 as the key modulator of p53 activity in differentiating ES cells. Our results provide a better understanding of ES cell regulation and could help to optimize ES cell differentiation protocols for their use in regenerative medicine.

Hexamethylene bisacetamide (HMBA) simultaneously targets AKT and MAPK pathway and represses NF kappaB activity: implications for cancer therapy.

Hexamethylene Bisacetamide (HMBA) is a hybrid polar compound originally developed as a differentiation inducing agent. We show in this study that HMBA can inhibit activation of several NF kappaB target genes in both lung and breast cancer cell lines. Furthermore, consistent with its ability to inhibit NF kappaB function, HMBA can also sensitize cells to apoptosis. We show that HMBA mediates inhibition of the Akt and ERK/MAPK cascade, both of which are critical for cell survival and proliferation and are well known regulators of NF kappaB activation. We also show that PTEN negative breast cancer cells which have hyper activation of the PI3K/Akt pathway show increased sensitivity to growth inhibitory effects of combination of HMBA and TNFalpha. Furthermore, HMBA can decrease the kinase activity of the IKK complex leading to defective phosphorylation of I kappaB alpha and Ser536 of p65. This study gives mechanistic insight into the mechanism of action of HMBA, provides the rationale for the potential use of HMBA in combination with various existing kinase inhibitors in combination therapy and also suggests useful biomarkers for monitoring tumor response to HMBA.