lncRNA BORG:TRIM28 Complexes Drive Metastatic Progression by Inducing a6 Integrin/CD49f Expression in Breast Cancer Stem Cells.

Triple-negative breast cancer (TNBC) is the most lethal subtype of breast cancer, with its aggressive phenotype being attributed to chemotherapy resistance, metastatic dissemination, and rapid disease recurrence. Breast cancer stem cells (BCSCs) are significant contributors to tumor initiation, as well as to the acquisition of aggressive tumorigenic phenotypes, namely due to their ability to self-replicate and to produce heterogeneous differentiated tumor cells. To elucidate the underlying mechanisms that drive BCSC tumorigenicity in TNBC, we identified the long noncoding RNA BMP/OP-Responsive Gene (BORG) as an enhancer of BCSC phenotypes. Indeed, we found BORG expression to (i) correlate with stem cell markers Nanog, Aldh1a3, and Itga6 (alpha6 integrin/CD49f); (ii) enhance stem cell phenotypes in murine and human TNBC cells, and (iii) promote TNBC tumor initiation in mice. Mechanistically, BORG promoted BCSC phenotypes through its ability to interact physically with the E3 SUMO ligase TRIM28. Moreover, TRIM28 binding was observed in the promoter region of Itga6, whose genetic inactivation prevented BORG:TRIM28 complexes from (i) inducing BCSC self-renewal and expansion in vitro, and (ii) eliciting BCSC metastatic outgrowth in the lungs of mice. Collectively, these findings implicate BORG:TRIM28 complexes as novel drivers of BCSC phenotypes in developing and progressing TNBCs. Implications: This work establishes the lncRNA BORG as a driver of BCSC phenotypes and the aggressive behaviors of TNBCs, events critically dependent upon the formation of BORG:TRIM28 complexes and expression of alpha6 integrin.