Matiullah Khan

Role of PML and PML-RARα in Mad-Mediated Transcriptional Repression

Abstract Fusion of the promyelocytic leukemia (PML) protein to the retinoic acid receptor-α (RARα) generates the transforming protein of acute promyelocytic leukemias. PML appears to be involved in multiple functions, including apoptosis and transcriptional activation by RAR, whereas PML-RARα blocks these functions of PML. However, the mechanisms of leukemogenesis by PML-RARα remain elusive. Here we show that PML interacts with multiple corepressors (c-Ski, N-CoR, and mSin3A) and histone deacetylase 1, and that this interaction is required for transcriptional repression mediated by the tumor suppressor Mad. PML-RARα has the two corepressor-interacting sites and inhibits Mad-mediated repression, suggesting that aberrant binding of PML-RARα to the corepressor complexes may lead to abrogation of the corepressor function. These mechanisms may contribute to events leading to leukemogenesis.

Viral Ski Inhibits Retinoblastoma Protein (Rb)-mediated Transcriptional Repression in a Dominant Negative Fashion

The mechanism by which the viral oncogene ski (v-ski) transforms chicken embryo fibroblasts is currently unknown. Recently, the c-ski gene product (c-Ski) was found to bind to N-CoR (nuclear hormone receptor co-repressor), an element implicated in transcriptional repression mediated by multiple transcriptional repressors including the nuclear hormone receptors and Mad. c-Ski is required for transcriptional repression mediated by Mad involved in negative regulation of cellular proliferation. v-Ski abrogates Mad-induced transcriptional repression in a dominant negative fashion. Here we report that v-Ski also inhibits transcriptional repression mediated by Rb, another tumor suppressor gene product. Rb forms a complex with c-Ski, Sin3A, and histone deacetylase (HDAC) via direct binding to c-Ski and HDAC. c-Ski is required for the transcriptional repression mediated by Rb. These results suggest that inhibition of Rb activity contributes, at least partly, to transformation by v-Ski.

Requirement of the co-repressor homeodomain-interacting protein kinase 2 for ski-mediated inhibition of bone morphogenetic protein-induced transcriptional activation.

Multiple co-repressors such as N-CoR/SMRT, mSin3, and the c-ski proto-oncogene product (c-Ski) mediate the transcriptional repression induced by Mad and the thyroid hormone receptor by recruiting the histone deacetylase complex. c-Ski also binds directly to Smad proteins, which are transcriptional activators in the transforming growth factor-β (TGF-β)/bone morphogenetic protein (BMP) signaling pathways, and inhibits TGF-β/BMP-induced transcriptional activation. However, it remains unknown whether other co-repressor(s) are also involved with Ski in the negative regulation of the TGF-β/BMP signaling pathways. Here, we report that the co-repressor homeodomain-interacting protein kinase 2 (HIPK2) directly binds to both c-Ski and Smad1. HIPK2 efficiently inhibited Smad1/4-induced transcription from the Smad site-containing promoter. A dominant negative form of HIPK2, in which the ATP binding motif in the kinase domain and the putative phosphorylation sites were mutated, enhanced Smad1/4-dependent transcription and the BMP-induced expression of alkaline phosphatase. Furthermore, the c-Ski-induced inhibition of the Smad1/4-dependent transcription was suppressed by a dominant negative form of HIPK2. The HIPK2 co-repressor activity may be regulated by an uncharacterized HIPK2 kinase. These results indicate that HIPK2, together with c-Ski, plays an important role in the negative regulation of BMP-induced transcriptional activation.