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International audience; The flavonoid silibinin is known to intervene in many cellular processes involved in a variety of pathol-ogies, thus appearing a promising therapeutic tool. The molecular mechanisms responsible for these activities, however, have not been clearly defined, and although some of its interactions with proteins have been identified, the relative affinities are often too low to appear relevant in vivo. Here we describe the interaction of silibinin with the protein disulfide isomerase ERp57, characterized by a submicromolar dissociation constant. This interaction enhances the formation of a ERp57/REF-1 complex, and furthermore appears to affect the intracellular distribution of ERp57. This protein is involved in signaling pathways which are also affected by silibinin. This suggests that the ERp57–silibinin interaction might explain at least some of the biological effects caused by the flavonoid.

The protein ERp57/GRP58 is a stress-responsive protein and a component of the protein disulfide isomerase family. Its functions in the endoplasmic reticulum are well known, concerning mainly the proper folding and quality control of glycoproteins, and participation in the assembly of the major histocompatibility complex class 1. However, ERp57 is present in many other subcellular locations, where it is involved in a variety of functions, primarily suggested by its participation in complexes with other proteins and even with DNA. While in some instances these roles need to be confirmed by further studies, a great number of observations support the participation of ERp57 in signal transduction from the cell surface, in regulatory processes taking place in the nucleus, and in multimeric protein complexes involved in DNA repair.

In the melanoma M14 cell line, we found that the antimetastatic protein NM23/nucleoside diphosphate kinase binds to the promoters of the oncogene cMYC and of P53, a gene often mutated in human cancer (Cervoni et al. [2006] J. Cell. Biochem. 98:421-428). In a further study, we find now that IFI16, a transcriptional repressor, in both promoters binds to the G-rich fragment that also binds NM23/NDPK. These fragments possess non-B DNA structures. Moreover, by sequential chromatin immunoprecipitation (re-ChIP) we show that the two proteins (IFI16 and NM23/NDPK) are simultaneously bound in vivo to the same DNA fragments. Since P53 stimulates apoptosis and inhibits cellular growth, and cMYC promotes cell growth and, in several instances, also apoptosis, the presence of NM23 and IFI16 on the same DNA fragments suggests their common involvement in the reduced development of some tumors.