Karlsruhe Institute of Technology, Institute of Toxicology and Genetics, Hermann-von-Helmholtz Platz 1, Eggenstein-Leopoldshafen 76344, Germany.
BMC Cancer. 2013 Mar 1;13:96. doi: 10.1186/1471-2407-13-96.
The Bag (Bcl-2 associated athanogene) family of proteins consists of 6 members sharing a common, single-copied Bag domain through which they interact with the molecular chaperone Hsp70. Bag5 represents an exception in the Bag family since it consists of 5 Bag domains covering the whole protein. Bag proteins like Bag1 and Bag3 have been implicated in tumor growth and survival but it is not known whether Bag5 also exhibits this function.
Bag5 mRNA and protein expression levels were investigated in prostate cancer patient samples using real-time PCR and immunoblot analyses. In addition immunohistological studies were carried out to determine the expression of Bag5 in tissue arrays. Analysis of Bag5 gene expression was carried out using one-way ANOVA and Bonferroni's Multiple Comparison test. The mean values of the Bag5 stained cells in the tissue array was analyzed by Mann-Whitney test. Functional studies of the role of Bag5 in prostate cancer cell lines was performed using overexpression and RNA interference analyses.
Our results show that Bag5 is overexpressed in malignant prostate tissue compared to benign samples. In addition we could show that Bag5 levels are increased following endoplasmic reticulum (ER)-stress induction, and Bag5 relocates from the cytoplasm to the ER during this process. We also demonstrate that Bag5 interacts with the ER-resident chaperone GRP78/BiP and enhances its ATPase activity. Bag5 overexpression in 22Rv.1 prostate cancer cells inhibited ER-stress induced apoptosis in the unfolded protein response by suppressing PERK-eIF2-ATF4 activity while enhancing the IRE1-Xbp1 axis of this pathway. Cells expressing high levels of Bag5 showed reduced sensitivity to apoptosis induced by different agents while Bag5 downregulation resulted in increased stress-induced cell death.
We have therefore shown that Bag5 is overexpressed in prostate cancer and plays a role in ER-stress induced apoptosis. Furthermore we have identified GRP78/BiP as a novel interaction partner of Bag5.
Bag（Bcl-2 相关抗凋亡基因）蛋白家族由 6 个成员组成，它们通过共同的单个 Bag 结构域与分子伴侣 Hsp70 相互作用。Bag5 是 Bag 家族中的一个例外，因为它由 5 个 Bag 结构域组成，覆盖整个蛋白。Bag 蛋白，如 Bag1 和 Bag3，已被牵连到肿瘤生长和存活中，但尚不清楚 Bag5 是否也具有这种功能。
使用实时 PCR 和免疫印迹分析研究前列腺癌患者样本中的 Bag5 mRNA 和蛋白表达水平。此外，还进行了免疫组织化学研究以确定组织芯片中 Bag5 的表达。使用单因素方差分析和 Bonferroni 多重比较检验分析 Bag5 基因表达。通过 Mann-Whitney 检验分析组织芯片中 Bag5 染色细胞的平均值。使用过表达和 RNA 干扰分析研究 Bag5 在前列腺癌细胞系中的作用。
我们的结果表明，与良性样本相比，Bag5 在恶性前列腺组织中过度表达。此外，我们还表明，在内质网（ER）应激诱导后，Bag5 水平增加，并且在此过程中 Bag5 从细胞质重新定位到 ER。我们还证明 Bag5 与 ER 驻留伴侣 GRP78/BiP 相互作用并增强其 ATP 酶活性。在 22Rv.1 前列腺癌细胞中过表达 Bag5 通过抑制 PERK-eIF2-ATF4 活性而增强该途径的 IRE1-Xbp1 轴，从而抑制 ER 应激诱导的细胞凋亡。表达高水平 Bag5 的细胞对不同药物诱导的凋亡的敏感性降低，而 Bag5 下调导致应激诱导的细胞死亡增加。
因此，我们已经表明 Bag5 在前列腺癌中过度表达，并在 ER 应激诱导的细胞凋亡中发挥作用。此外，我们已经确定 GRP78/BiP 是 Bag5 的一个新的相互作用伙伴。
