The Rad51 family of proteins: Interactions, vitamin D, and implications in head and neck cancer

Protection of the genome from carcinogenic consequences of DNA double-strand breaks (DSBs) is accomplished through the pathways of non-homologous end-joining (NHEJ) or homologous recombinational repair. Five human proteins with homology to Rad51 known as the Rad51 paralogs, Rad51B, Rad51C, Rad51D, XRCC2, and XRCC3, whose loss of function in cell lines leads to high chromosomal instability. Previous studies have shown Rad51C participate in two paralog protein complexes, one containing Rad51B, Rad51C, Rad51D and XRCC2 (BCDX2) and the other containing only Rad51C and XRCC3 (CX3). However, the only structural data available is the crystal structure of RecA, the bacterial homolog, the determination of the N-terminus of human Rad51 by NMR, and the crystal structure of Pyroccocus furious Rad51. Currently the Alvinlla pompejana Rad51C has been cloned, expressed and is currently being crystallized in the Tainer laboratory (UC Berkeley) since the human Rad51C protein has proven too difficult to be utilized. To test functional association of Hs Rad51B and Hs XRCC3 to Ap Rad51C. The human proteins were heterologously expressed in Pichia pastoris and the other expressed in E. coli. The proteins were extracted and interaction was tested through co-immunoprecipitation. Initial results depict weak binding or an unstable interaction between Hs Rad51B and Ap Rad51C. Hs XRCC3 and Ap Rad51C interaction remains unclear and requires further testing. Additionally, we have utilized a cellular model of HNSCC to identify whether the down-regulation of Rad51 after application of VD 3 is concomitant with the down-regulation of NBS1. NBS1 is a DNA repair protein involved in both pathways of DNA double-strand break repair, non-homologous end-joining and homologous recombinational repair. It has recently been demonstrated that NBS1 binds to Rad51 aiding in its localization to sites of DNA damage. VD 3 is a potential chemopreventive agent in the treatment of head and neck cancer. For the in vitro model Rad51 and NBS1 protein were both extracted from SCC25 and MCF-7 cancer cell lines were treated with 100 nM of VD 3 . For the in vivo model hamsters cheek pouch tissue sections with VD 3 treated and DMBA over the course of 14 weeks were used. Rad51 and NBS1 staining is restricted to the nuclei of the basal cell layer of the epithelium in VD 3 treated animals as compared to untreated controls where staining is evident throughout the dysplastic epithelium and is not restricted to nuclei. Unlike the western blot data of Rad51 that shows similar downregulation as the immunocytochemistry, the western blot analysis of NBS1 is unclear. However, the immunocytochemistry suggests that NBS1 is also downregulated by VD 3 in vivo, and therefore, it may be implied that both the HRR and NHEJ pathways are involved in the cellular effects of VD 3 in HNSCC