The interaction of the SRA domain of ICBP90 with a novel domain of DNMT1 is involved in the regulation of VEGF gene expression

Inverted CCAAT box-binding protein of 90 kDa (ICBP90) is over-expressed in several types of cancer, including breast, prostate and lung cancers. In search for proteins that interact with the set and ring-associated (SRA) domain of ICBP90, we used the two-hybrid system and screened a placental cDNA library. Several clones coding for a new domain of DNMT1 were found. The interaction, between the ICBP90 SRA domain and the DNMT1 domain, has been confirmed with purified proteins by glutathione-S-transferase pull-down experiments. We checked whether ICBP90 and DNMT1 are present in the same macromolecular complexes in Jurkat cells and immortalized human vascular smooth muscle cells (HVTs-SM1). Coimmunoprecipitation experiments showed that ICBP90 and DNMT1 are present in the same molecular complex, which was further confirmed by co-localization experiments as assessed by immunocytochemistry. Downregulation of ICBP90 and DNMT1 decreased VEGF gene expression, a major pro-angiogenic factor, whereas those of p16(INK4A) gene and RB1 gene were significantly enhanced. Together, these results indicate that DNMT1 and ICBP90 are involved in VEGF gene expression, possibly via an interaction of the SRA domain of ICBP90 with a novel domain of DNMT1 and an upregulation of p16(INK4A). They further suggest a new role of ICBP90 in the relationship between histone ubiquitination and DNA methylation in the context of tumoral angiogenesis and tumour suppressor genes silencing.