ARTICLE ABSTRACTEndocrine therapy is the standard treatment for advanced prostate cancer; however, relapse occurs in most patients with few treatment options available after recurrence. To overcome this therapeutic hurdle, the identification of new molecular targets is a critical issue. The capability to proliferate in three-dimensional (3D) conditions is a characteristic property of cancer cells. Therefore, factors that regulate 3D growth are considered rational targets for cancer therapy. Here, we applied a functional genomic approach to the 3D spheroid cell culture model and identified TRIB1, a member of the Trib family of serine/threonine kinase-like proteins, as an essential factor for prostate cancer cell growth and survival. RNAi-mediated silencing of TRIB1 suppressed prostate cancer cell growth selectively under the 3D conditions. This effect was rescued by ectopic expression of an RNAi-resistant TRIB1 exogene. Gene signature–based analysis revealed that TRIB1 was related to endoplasmic reticulum (ER) pathways in prostate cancer and was required for expression of the ER chaperone GRP78, which is critical for prostate tumorigenesis. Of note, GRP78 was expressed preferentially in a subpopulation of prostate cancer cells that possess tumor-propagating potential, and these tumor-propagating cells were highly sensitive to TRIB1 and GRP78 depletion. In a xenograft model of human prostate cancer, TRIB1 depletion strongly inhibited tumor formation. Supporting these observations, we documented frequent overexpression of TRIB1 in clinical specimens of prostate cancer. Overall, our results indicated that the TRIB1–ER chaperone axis drives prostate tumorigenesis and the survival of the tumor-propagating cells. Cancer Res; 74(17); 4888–97. ©2014 AACR.