Wei Gu, Ph.D.

P53 IN TUMOR SUPPRESSION AND AGING

The primary goal of this laboratory is to understand molecular mechanisms of tumor suppression and cell aging after the cell is insulted by various types of stress. In particular, we are interested in elucidating the roles of acetylation and ubiquitination in the regulation of p53-mediated transcription, apoptosis, DNA repair and senescence.

Mutations within the p53 tumor suppressor gene have been well documented in more than half of all human tumors. p53 exerts anti-proliferative effects, including growth arrest, apoptosis, and senescence. The molecular function of p53 that is required for tumor suppression involves its ability to act as a transcriptional factor in regulating endogenous gene expression. Tight regulation of p53 is essential for its effect on tumorigenesis as well as maintaining normal cell growth. The precise mechanism by which p53 is activated by cellular stress is not completely understood; it is generally thought to involve mainly post-translational modifications of p53 including acetylation, ubiquitination and phophorylation. We are using biochemical methods as well as genetic (transgenic and knock-out) approaches to investigate how p53 is functionally regulated (through the acetylation/phosphorylation pathway) and stabilized (through the ubiquitination pathway) in response to DNA damage and other types of stress.

Early studies demonstrated that CBP/p300, a histone acetyl-transferase (HAT), acts as a coactivator of p53 and potentiates its transcriptional activity and biological function in vivo. Significantly, the observation of functional synergism between p53 and CBP/p300 together with its intrinsic HAT activity led to the discovery of a novel FAT (transcriptional factor acetyl-transferase) activity of CBP/p300 on p53; this finding also predicted that acetylation may represent a general functional modification for non-histone proteins in vivo. In contrast, much less is known about the role of deacetylation in modulating p53 function. We have identified PID/MTA2, a component of a histone deacetylase (HDAC1) complex, which acts as an adaptor protein to play a critical role in p53 deacetylation. PID is homologous to MTA1 (metastasis-associated protein 1), and also highly expressed in tumor cells. We have found that PID expression effectively inhibits p53-mediated transactivation as well as its biological functions including cell growth arrest and apoptosis. Thus, acetylation and decaetylation of p53 have been implicated as a critical event in p53-mediated transcriptional activation, apoptosis and senescence.

p53 is a short-lived protein whose activity is maintained at low levels in normal cells. By serving as a signal for specific cellular protein degradation, ubiquitination plays a critical role in physiological regulation of p53 stability. The ubiquitination of p53 was first discovered in human papilloma virus (HPV)-infected cells through the functions mediated by the viral E6 protein. However, in normal cells, Mdm2 functions as a major ubiquitin ligase (E3) that mediates p53 ubiquitination and subsequent degradation. Stabilization of p53 is critical for its effects on cell growth repression and apoptosis in response to DNA damage as well as other types of stress. Recently, by mass spectrometry of affinity-purified p53-associated factors, we have identified a novel p53-interacting protein, which stabilizes p53 by specifically deubiquitinating p53 both in vitro and in vivo. Thus, our findings reveal an important mechanism by which p53 can be stabilized by deubiquitination even when the Mdm2-mediated ubiquitination pathway is intact and provide the first example of a deubiquitinating enzyme-mediated stabilization of a specific cellular factor in mammalian cells.

Inactivation of p53 functions has been well documented as a common mechanism for tumorigenesis and cell immortalization. Many cancer therapy drugs have been designed based on either reactivating p53 functions or inactivating p53 negative regulators. Thus, the elucidation of these critical steps involved in p53 regulation will provide novel insights into cancer therapy.

Selected Publications:

• Gu, W., Shi, X. L., and Roeder, R. G. (1997) Synergistic activation of transcription by CBP and p53. Nature. 387; 819-823.

• Gu, W., and Roeder, R. G. (1997) Activation of p53 sequence-specific DNA binding by acetylation of the p53 C-terminal domain. Cell. 90; 595-606.

• Gu, W., Malik, S., Ito, M., Yuan, C., Fondell, J.D., Zhang, X., Martinez, M., Qin, J., and Roeder, R. G. (1999). A novel human SRB/MED-containing cofactor complex (SMCC) involved in transcription regulation. Mole. Cell. 3, 97-108.

• Ito, M., Yuan, C-X, Malik, S., Gu, W., Fondell, J.D., Yamamura, S., Fu, Z-Y., Zhang X, Qin, J., and Roeder, R. G. (1999) Structural and functional identities between Thyroid hormone receptor-associated protein (TRAP) and SRB/MED-containing cofactor (SMCC) complexes indicate novel pathways for the function of nuclear receptors and diverse mammalian activators. Mole. Cell. 3; 361-370.

• Malik, S., Gu, W., Wu, W., Qin, J., and Roeder, R.G. (2000) The USA-derived transcriptional coactivator PC2 is a submodule of TRAP/SMCC and acts synergistically with other PCs. Mole. Cell. 5; 753-760.

• Guo, A., Salomoni, P., Luo, J., Shih, A., Zhong, S., Gu, W., and Pandofi, P.P. (2000) Role of PML in p53-dependent apoptosis. Nature Cell Biol. 2, 730-736.

• Luo, J, Su, F., Chen, D., Shiloh, A., and Gu, W. (2000) Deacetylation of p53 modulates its effect on cell growth and apoptosis. Nature 408, 377-381.

• Skowyra, D., Zeremski, M., Neznanov, N., Li, M., Choi, Y., Uesugi, M., Hauser, C. A., Gu, W., Gudkov, A. V., and Qin, J. (2001) Differential association of products of alternative transcripts of the candidate tumor suppressor ING1 with the mSin3/HDAC1 transcriptional corepressor complex J. Biol. Chem. 276, 8734-8739.