SIAH-1 interacts with CtIP and promotes its degradation by the proteasome pathway

Abstract

SIAH-1 and SIAH-2 are the human members of an evolutionary highly conserved E3 ligase family. SIAH-1 is a p53 and p21Waf-1/Cip-1 induced gene during apoptosis and tumor suppression. In stable-transfected clones of MCF-7 cells, SIAH-1 overexpression was associated with apoptosis, mitotic alterations and p21Waf-1/Cip-1 induction of expression. Using a two-hybrid screening, we identified here the transcriptional corepressor CtBP-interacting protein (CtIP) as a SIAH-1-interacting protein. CtIP has been proposed as a regulator of p21Waf-1/Cip-1 gene transcription through a protein complex involving BRCA1. We demonstrate that SIAH-1 associates with CtIP both in vitro and in vivo. This interaction led to CtIP degradation by the ubiquitin-proteasome pathway. As expected, SIAH-1 induced p21Waf-1/Cip-1 transcription in Jurkat-T cell. Surprisingly, a SIAH protein deleted of its RING finger, SIAH-1ΔN, which is able to interact with CtIP but does not promote its degradation, also induced transcription from the p21Waf-1 promoter in a similar extent as did SIAH-1. Our results suggest that p21Waf-1/Cip-1 induction by SIAH-1 could not be mediated by CtIP degradation.