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-RAR␣ induced the degradation of HIPK2. We discuss the roles of PML and PML-retinoic acid receptor ␣, as well as those of HIPK2 and p300 ubiquitination, in transcriptional regulation and leukemogenesis.

The Pml gene is the target of t(15;17) chromosome translocation in acute promyelocytic leukemia. PML protein is known to localize in discrete nuclear speckles, named PML nuclear bodies (NBs). In NBs, PML interacts with several transcription factors, such as p53 and AML1, and their co-activators, such as HIPK2 and p300. PML activates transcription of their target genes. PML is thought to stabilize transcription factor complex and function as a mediator in transcription activation, but little is known about the molecular mechanism by which PML activates transcription. To clarify the role of PML in transcription regulation, we purified the PML complex and identified a novel F-box protein (FBP), Skp1, and Cullin1 (Cul1) in the PML complex by LC/MS/MS analysis. FBPs form SCF ubiquitin ligase complexes with Skp1, Cul1 and ROC1 and mediate recognition of specific substrates for ubiquitination. We found that the FBP that we identified here also forms a SCF complex with Skp1, Cul1 and ROC1. To identify substrates for the SCF complex, we tested several proteins that could bind to PML, and found that the FBP promotes degradation of HIPK2 and p300. These degradations were inhibited in the presence of a proteasome inhibitor, MG132. The FBP stimulated ubiquitination of HIPK2. These results suggest that the SCF promotes degradation of these proteins by the ubiquitin-proteasome pathway. The fact that the SCF is a part of the PML complex suggests that PML plays a role in the SCF-mediated degradation of HIPK2 and p300 by the ubiquitin-proteasome pathway. In order to clarify the role of PML in degradation of HIPK2 and p300, we tested effects of PML on the degradation and found that PML inhibited the SCF-mediated degradation of HIPK2 and p300 without inhibition of ubiquitination. To clarify roles of HIPK2, PML IV and the FBP in p53-dependent transcription, we performed reporter analysis using the MDM2 promoter in H1299 cells. Since the FBP promotes degradation of HIPK2, we initially thought that the FBP might inhibit activation of p53-dependent transcription by HIPK2 and PML IV. However, the FBP, HIPK2 and PML synergistically stimulated the p53-dependent transcriptional activation. Taken together our data suggest that the SCF-induced ubiquitination of transcription co-activators HIPK2 and p300 plays a critical role in transcriptional regulation, and that PML stimulates transcription by protecting HIPK2 and p300 from ubiquitin-dependent degradation.