Carl Maki, Ph.D.

Regulated Activity and Degradation of p53 and p53-pathway Proteins

Research Summary

P53 is a tumor suppressor protein and key regulator of cell growth. P53 inhibits growth by activating the expression of genes that cause either growth arrest or death (apoptosis). Inactivation of the p53 pathway is essential for the development of most human cancers. It is therefore important to determine how the p53 pathway is normally regulated, and how this regulation is altered in cancer. We are currently studying three p53 pathway proteins: p53, p73, and p21. Wild-type p53 is a transcription factor that must localize in the nucleus to function. However, in a high percentage of breast cancers and neuroblastomas, p53 is inactivated by being exported from the nucleus and held in the cytoplasm. We are examining how this nuclear export occurs, and the effect of DNA damage and other stresses on p53 nuclear export. P73 is a p53-family protein that is very similar to p53 in both structure and amino acid sequence. We are investigating the mechanisms that control p73 localization and activity. P21 is a cyclin-dependent kinase (cdk) inhibitor and potent cell growth inhibitor. P53 activates p21 gene expression in response to stress, and increased levels of p21 protein bind and inhibit G1-phase cdks, causing a G1 cell-cycle arrest. We are examining how stability of the p21 protein is regulated. Our preliminary data indicate p21-binding proteins can both stabilize p21, as well as promote p21 degradation.

In other studies, we are attempting to increase cancer sensitivity to radiation through altering the p53 response. Activation of p53 could have two different effects on radiation sensitivity. First, p53 activation might decrease radiation sensitivity by causing a growth arrest and allowing DNA repair. Second, p53 activation might enhance radiation sensitivity by increasing death through apoptosis. Our preliminary data indicate certain chemical agents can decrease p53 levels in breast cancer cells increase the sensitivity of these cells to radiation-induced killing. We will grow human breast tumors in mice and test if these chemical agents can enhance radiation therapy and radiation-induced killing of these tumors.

Selected Papers

Alarcon R, Koumenis C, Geyer RK, Maki CG and Giaccia AJ. (1999). Hypoxia induces p53 accumulation through MDM2 down-regulation and inhibition of E6-mediated degradation. Cancer Res, 59: 6046-6051.

Maki CG. (1999). Oligomerization is required for p53 to be efficiently ubiquitinated by MDM2. J Biol Chem, 274: 16531-16535.

Geyer RK, Nagasawa H, Little JB and Maki CG. (2000). Role and regulation of p53 during an ultraviolet radiation-induced G1 cell cycle arrest. Cell Growth Differ, 11: 149-156.

Yu ZK, Geyer RK and Maki CG. (2000). MDM2-dependent ubiquitination of nuclear and cytoplasmic P53. Oncogene, 19: 5892-5897.

Geyer RK, Yu ZK and Maki CG. (2000). The MDM2 RING-finger domain is required to promote p53 nuclear export. Nat Cell Biol, 2: 569-573.

Inoue T, Geyer RK, Yu ZK and Maki CG. (2001). Downregulation of MDM2 stabilizes p53 by inhibiting p53 ubiquitination in response to specific alkylating agents. FEBS Lett, 490: 196-201.

Inoue T, Geyer RK, Howard D, Yu ZK and Maki CG. (2001). MDM2 can promote the ubiquitination, nuclear export, and degradation of p53 in the absence of direct binding. J Biol Chem, 276: 45255-45260.

Inoue T, Stuart J, Leno R and Maki CG. (2002). Nuclear import and export signals in control of the p53-related protein p73. J Biol Chem, 277: 15053-15060.

Wei X, Yu ZK, Ramalingam A, Grossman SR, Yu JH, Bloch DB Maki CG. (2002). Physical and functional interactions between PML and MDM2. J Biol Chem, 278: 29288-29297.

Jardine LJ and Maki CG. (2003). Stability and ubiquitination of the tumor suppressor protein p53. Methods Mol Biol, 223: 27-38.

Zhu H, Wu L and Maki CG. (2003). MDM2 and promyelocytic leukemia antagonize each other through their direct interaction with p53. J Biol Chem, 278: 49286-49292.

Wu L, Zhu H, Nie L, Maki CG. (2004). A link between p73 transcriptional activity and p73 degradation. Oncogene, (in press).