HOXA5-Mediated Stabilization of IkB? Inhibits the NF-kB Pathway and Suppresses Malignant Transformation of Breast Epithelial Cells

HOXA5 is a transcription factor and tumor suppressor that promotes differentiation of breast epithelial cells and is frequently lost during malignant transformation. HOXA5 loss alone, however, does not confer tumorigenicity. To determine which molecular alterations combined with loss of HOXA5 expression can transform cells, we examined isogenic derivatives of a nonmalignant breast epithelial cell line containing knock-in or knockout mutations in key breast cancer genes. Knockdown (KD) of HOXA5 in cells harboring double knock-in (DKI) of mutated PIK3CA (E545K) and HER2 (V777L) induced epithelial-mesenchymal transition and migration and promoted invasive tumor outgrowth within mouse mammary ducts. The NF-xB pathway was significantly upregulated in DKI cells following HOXA5 KD. HOXA5 KD upregulated multiple NF-xB target genes, including IL6. IxBa protein, but not RNA, expression was reduced in HOXA5-KD cells. HOXA5 bound and stabilized IxBa, forming a nuclear HOXA5-IxBa complex. Chromatin immunoprecipitation sequencing database queries revealed that HOXA5 and IxBa are co-enriched at 528 geno-mic loci. In patients with breast cancer, high coexpression of HOXA5 and IxBa conferred a significantly better overall and progression-free survival. Collectively, these data suggest that HOXA5 suppresses malignancy in breast epithelial cells by blunting NF-xB action via stabilization of its inhibitor IxBa.Significance: Loss of HOXA5 reduces IxBa stability and increases NF-xB signaling to exacerbate breast cancer aggres-siveness, providing new insights into the tumor suppressor functions of HOXA5.