Aberrant constitutive expression of NF-B subunits, reported in more than 90% of breast cancers and multiple other malignancies, plays pivotal roles in tumorigenesis. Higher RelB subunit expression was demonstrated in estrogen receptor alpha (ER␣)-negative breast cancers versus ER␣-positive ones, due in part to repression of RelB synthesis by ER␣ signaling. Notably, RelB promoted a more invasive phenotype in ER␣-negative cancers via induction of the BCL2 gene. We report here that RelB reciprocally inhibits ER␣ synthesis in breast cancer cells, which contributes to a more migratory phenotype. Specifically, RelB is shown for the first time to induce expression of the zinc finger repressor protein Blimp1 (B-lymphocyte-induced maturation protein), the critical mediator of Band T-cell development, which is transcribed from the PRDM1 gene. Blimp1 protein repressed ER␣ (ESR1) gene transcription. Commensurately higher Blimp1/PRDM1 expression was detected in ER␣-negative breast cancer cells and primary breast tumors. Induction of PRDM1 gene expression was mediated by interaction of Bcl-2, localized in the mitochondria, with Ras. Thus, the induction of Blimp1 represents a novel mechanism whereby the RelB NF-B subunit mediates repression, specifically of ER␣, thereby promoting a more migratory phenotype.NF-B is a structurally and evolutionary conserved family of dimeric transcription factors with subunits having an N-terminal region of approximately 300 amino acids that shares homology with the v-Rel oncoprotein (17, 44). The conserved Rel homology domain is responsible for DNA binding, dimerization, nuclear translocation, and interaction with inhibitory proteins of NF-B (IBs). Mammals express five NF-B members, including c-Rel, RelB, RelA (p65), p50, and p52, which can form either homo-or heterodimers. RelB differs from the other members in that it only binds DNA as a heterodimer with either p52 or p50 and interacts only poorly with the inhibitory protein IB␣. In most untransformed cells, other than B lymphocytes, NF-B complexes are sequestered in the cytoplasm bound to specific IB proteins.

The propeptide region of the lysyl oxidase proenzyme (LOX-PP) has been shown to inhibit Ras signaling in NIH 3T3 and lung cancer cells with activated RAS, but its mechanism of action is poorly understood. Here, a yeast two-hybrid assay of LOX-PP-interacting proteins identified a clone encoding the intracellular phosphatase domains of receptor-type protein tyrosine phosphatase kappa (RPTP-), and the interaction of the two proteins in mammalian cells was confirmed. RPTP-is proteolytically processed to isoforms that have opposing effects on ␤-catenin activity. The RPTP-transmembrane P subunit interacts with and sequesters ␤-catenin at the cell membrane, where it can associate with E-cadherin and promote intercellular interactions. At high cell density, further processing of the P subunit yields a phosphatase intracellular portion (PIC) subunit, which chaperones ␤-catenin to the nucleus, where it can function to activate transcription. Lung cancer cells were found to contain higher PIC levels than untransformed lung epithelial cells. In H1299 lung cancer cells, ectopic LOX-PP expression reduced the nuclear levels of PIC by increasing its turnover in the lysosome, thereby decreasing the nuclear levels and transcriptional activity of ␤-catenin while increasing ␤-catenin membrane localization. Thus, LOX-PP is shown to negatively regulate pro-oncogenic ␤-catenin signaling in lung cancer cells.The enzyme lysyl oxidase (LOX) catalyzes oxidative modifications that promote the formation of lysine-derived covalent cross-links needed for the normal structural integrity of the extracellular matrix. LOX is synthesized as a 50-kDa inactive proenzyme (pro-LOX), which is N-and O-glycosylated within sites of the propeptide domain (45), secreted, and then cleaved to the functional C-terminal ϳ30-kDa enzyme and an ϳ18-kDa N-terminal lysyl oxidase propeptide (LOX-PP). Another major function of the LOX gene was discovered with the observation that its expression inhibited the transforming activity of the H-Ras oncogene in NIH 3T3 fibroblasts (6,22). Consistent with this finding, many cancers and derived cell lines display reduced levels of LOX protein or RNA (3,5,12,16,17,22,24,25,38). LOX reexpression was also seen in stable phenotypic revertants of Ras-transfected NIH 3T3 cells following interferon treatment (5, 22). These and other findings led Contente et al. (5) to term the LOX gene the Ras recision gene (rrg). Subsequently, pro-LOX was shown to inhibit the activities of the Akt and Erk kinases and NF-B transcription factors in Ras-transformed NIH 3T3 cells (19), and ectopic expression of the LOX gene in gastric cancer cells resulted in reduced tumor formation in nude mice (21). More recently, our group noted that LOX-PP was sufficient to mediate the rrg activity of the LOX gene in Ras-transformed NIH 3T3 cells (35). Consistent with these findings, Bouez et al. showed that LOX protein expression was absent from human basal and squamous cell carcinomas and that a reduction in LOX mRNA levels but not enzyme activity caused a more in...

The lysyl oxidase proenzyme propeptide region (LOX-PP) is a tumor suppressor protein whose mechanism of action is not completely understood. Here, the Ubiquitously expressed Transcript (UXT) was identified in a yeast two-hybrid assay with LOX-PP as bait and confirmed as a novel LOX-PP associating protein. UXT, a prefoldin-like protein, is ubiquitous in human and mouse. Since UXT modulates androgen receptor transcriptional activity in prostate cancer, we studied its role in breast cancer. Breast tumors and derived cell lines overexpressed UXT. UXT was able to associate with the estrogen receptor alpha (ER) and decrease its transcriptional activity and target gene expression. Conversely, UXT knockdown increased ER element-dependent transcriptional activity. Ectopic LOX-PP relocalized UXT to the cytoplasm and decreased its stability. UXT ubiquitination and depletion in the presence of LOX-PP was rescued by a proteasomal inhibitor. In summary, proteasome-mediated turnover of UXT upon interaction with LOX-PP releases repression of ER transcriptional activity. J. Cell. Biochem. 118: 2347-2356, 2017. © 2017 Wiley Periodicals, Inc.