PML Activates Transcription by Protecting HIPK2 and p300 from SCF^Fbx3-Mediated Degradation

Abstract

PML, a nuclear protein, interacts with several transcription factors and their coactivators, such as HIPK2 and p300, resulting in the activation of transcription. Although PML is thought to achieve transcription activation by stabilizing the transcription factor complex, little is known about the underlying molecular mechanism. To clarify the role of PML in transcription regulation, we purified the PML complex and identified Fbxo3 (Fbx3), Skp1, and Cullin1 as novel components of this complex. Fbx3 formed SCF^Fbx3 ubiquitin ligase and promoted the degradation of HIPK2 and p300 by the ubiquitin-proteasome pathway. PML inhibited this degradation through a mechanism that unexpectedly did not involve inhibition of the ubiquitination of HIPK2. PML, Fbx3, and HIPK2 synergistically activated p53-induced transcription. Our findings suggest that PML stabilizes the transcription factor complex by protecting HIPK2 and p300 from SCF^Fbx3-induced degradation until transcription is completed. In contrast, the leukemia-associated fusion PML-RARα induced the degradation of HIPK2. We discuss the roles of PML and PML-retinoic acid receptor α, as well as those of HIPK2 and p300 ubiquitination, in transcriptional regulation and leukemogenesis.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/ .

ACKNOWLEDGMENTS

We thank Y. Taya (National Cancer Center Research Institute) for kindly providing the cDNAs for p53 cDNA and the MDM2-luc reporter. We also thank Yukiko Aikawa, Noriko Aikawa, and Chikako Hatanaka for technical assistance.

This work was supported in part by grants-in-aid for scientific research from the Ministry of Health, Labor and Welfare and from the Ministry of Education, Culture, Sports, Science and Technology and by the Program for Promotion of Fundamental Studies from the National Institute of Biomedical Innovation of Japan.