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Abstract
BackgroundBCA2 is an E3 ligase linked with hormone responsive breast cancers. We have demonstrated previously that the RING E3 ligase BCA2 has autoubiquitination activity and is a very unstable protein. Previously, only Rab7, tetherin, ubiquitin and UBC9 were known to directly interact with BCA2.MethodsHere, additional BCA2 binding proteins were found using yeast two-hybrid and bacterial-II-hybrid screening techniques with Human breast and HeLa cDNA libraries. Co-expression of these proteins was analyzed through IHC of TMAs. Investigation of the molecular interactions and effects were examined through a series of in vivo and in vitro assays.ResultsTen unique BCA2 interacting proteins were identified, two of which were hHR23a and 14-3-3sigma. Both hHR23a and 14-3-3sigma are co-expressed with BCA2 in breast cancer cell lines and patient breast tumors (n = 105). hHR23a and BCA2 expression was significantly correlated (P = < 0.0001 and P = 0.0113) in both nucleus and cytoplasm. BCA2 expression showed a statistically significant correlation with tumor grade. High cytoplasmic hHR23a trended towards negative nodal status. Binding to BCA2 by hHR23a and 14-3-3sigma was confirmed in vitro using tagged partner proteins and BCA2. hHR23a and 14-3-3sigma effect the autoubiquitination and auto-degradation activity of BCA2. Ubiquitination of hHR23a-bound BCA2 was found to be dramatically lower than that of free BCA2, suggesting that hHR23a promotes the stabilization of BCA2 by inactivating its autoubiquitination activity, without degradation of hHR23a. On the other hand, phosphorylated BCA2 protein is stabilized by interaction with 14-3-3sigma both with and without proteasome inhibitor MG-132 suggesting that BCA2 is regulated by multiple degradation pathways.ConclusionsThe interaction between BCA2 and hHR23a in breast cancer cells stabilizes BCA2. High expression of BCA2 is correlated with grade in breast cancer, suggesting regulation of this E3 ligase is important to cancer progression.
Highlights
Breast Cancer Associated gene 2 (BCA2) is an E3 ligase linked with hormone responsive breast cancers
Breast Cancer Associated gene 2 (BCA2) was first identified in an effort to investigate drivers of breast cancer via the subtractive hybridization [1]. These studies aimed to identify differentially expressed genes between Hs578Bst and Hs578T mammary epithelial cell lines derived from adjacent normal and cancerous tissues respectively [1]. These analyses revealed 950 cDNAs enriched in breast cancer cells [1]
Through binding experiments we confirmed that the BCA2 protein bound both to 14-3-3s and hHR23a (Figure 1B and 1C). hHR23a and BCA2 were co-expressed in a mammalian system, while 14-3-3s was expressed in a bacterial system and incubated with recombinant purified BCA2
Summary
We have demonstrated previously that the RING E3 ligase BCA2 has autoubiquitination activity and is a very unstable protein. Breast Cancer Associated gene 2 (BCA2) was first identified in an effort to investigate drivers of breast cancer via the subtractive hybridization [1]. These studies aimed to identify differentially expressed genes between Hs578Bst and Hs578T mammary epithelial cell lines derived from adjacent normal and cancerous tissues respectively [1]. These analyses revealed 950 cDNAs enriched in breast cancer cells [1]. Ubiquitin conjugation to target proteins involves a number of well-coordinated steps, catalyzed by three enzyme types [11,12,13,14]
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