Triple-negative breast cancer (TNBC) is particularly aggressive and difficult to treat. For example, the
transforming growth factor-β (TGF-β) pathway is implicated in TNBC progression and
metastasis, but its opposing role in
tumor suppression in healthy tissues and early-stage lesions makes it a challenging target. Therefore, additional molecular characterization of TNBC may lead to improved patient prognosis by informing the development and optimum use of targeted
therapies. We found that musculoaponeurotic
fibrosarcoma (MAF) oncogene family
protein K (MAFK), a member of the small MAF family of
transcription factors that are induced by the TGF-β pathway, was abundant in human TNBC and aggressive mouse mammary tumor cell lines. MAFK promoted tumorigenic growth and
metastasis by 4T1 cells when implanted subcutaneously in mice. Overexpression of MAFK in mouse breast epithelial NMuMG cells induced epithelial-mesenchymal transition (EMT) phenotypes and promoted
tumor formation and invasion in mice. MAFK induced the expression of the gene encoding the transmembrane
glycoprotein nmb (GPNMB). Similar to MAFK, GPNMB overexpression in NMuMG cells induced EMT,
tumor formation, and invasion, in mice, whereas knockdown of MAFK in
tumor cells before implantation suppressed
tumor growth and progression. MAFK and GPNMB expression correlated with poor prognosis in TNBC patients. These findings suggest that MAFK and its target gene GPNMB play important roles in the malignant progression of TNBC cells, offering potentially new therapeutic targets for TNBC patients.