Epithelial-mesenchymal transition (EMT) is normal physiological process that regulates tissue development, remodeling, and repair; however, aberrant EMT also elicits disease development in humans, including lung fibrosis, rheumatoid arthritis, and cancer cell metastasis. Transforming growth factor-β (TGF-β) is a master regulator of EMT in normal mammary epithelial cells (MECs), wherein this pleiotropic cytokine also functions as a potent suppressor of mammary tumorigenesis. In contrast, malignant MECs typically evolve resistance to TGF-β-mediated cytostasis and develop the ability to proliferate, invade, and metastasize when stimulated by TGF-β. It therefore stands to reason that establishing how TGF-β promotes EMT may offer new insights into targeting the oncogenic activities of TGF-β in human breast cancers. By monitoring alterations in the actin cytoskeleton and various markers of EMT, we show here that the TGF-β gene target, fibulin-5 (FBLN5), initiates EMT and enhances that induced by TGF-β. While normal MECs contain few FBLN5 transcripts, those induced to undergo EMT by TGF-β show significant upregulation of FBLN5 mRNA, suggesting that EMT and the dedifferentiation of MECs overrides the repression of FBLN5 expression in polarized MECs. We also show that FBLN5 stimulated MMP expression and activity, leading to MEC invasion and EMT, to elevated Twist expression, and to reduced E-cadherin expression. Finally, FBLN5 promoted anchorage-independent growth in normal and malignant MECs, as well as enhanced the growth of 4T1 tumors in mice. Taken together, these findings identify a novel EMT and tumor promoting function for FBLN5 in developing and progressing breast cancers.

The bone morphogenetic protein (BMP) signaling pathways have important roles in embryonic development and cellular homeostasis, with aberrant BMP signaling resulting in a broad spectrum of human disease. We report that BMPs unexpectedly signal through the canonical transforming growth factor β (TGF-β)-responsive Smad2 and Smad3. BMP-induced Smad2/3 signaling occurs preferentially in embryonic cells and transformed cells. BMPs signal to Smad2/3 by stimulating complex formation between the BMP-binding TGF-β superfamily receptors, activin receptor-like kinase (ALK)3/6, and the Smad2/3 phosphorylating receptors ALK5/7. BMP signaling through Smad2 mediates, in part, dorsoventral axis patterning in zebrafish embryos, whereas BMP signaling through Smad3 facilitates cancer cell invasion. Consistent with increased BMP-mediated Smad2/3 signaling during cancer progression, Smad1/5 and Smad 2/3 signaling converge in human cancer specimens. Thus, the signaling mechanisms used by BMPs and TGF-β superfamily receptors are broader than previously appreciated.

NCT01991223 (www.clinicaltrials.gov).