Inhibitory mechanism of FAT4 gene expression in response to actin dynamics during Src-induced carcinogenesis

Oncogenic transformation is characterized by morphological changes resulting from alterations in actin dynamics and adhesive activities. Emerging evidence suggests that the protocadherin FAT4 acts as a tumor suppressor in humans, and reduced <em>FAT4</em> gene expression has been reported in breast and lung cancers and melanoma. However, the mechanism controlling<em> FAT4</em> gene expression is poorly understood. In this study, we show that transient activation of the Src oncoprotein represses <em>FAT4</em> mRNA expression through actin depolymerization in the immortalized normal human mammary epithelial cell line MCF-10A. Src activation causes actin depolymerization via the MEK/Erk/Cofilin cascade. The MEK inhibitor U0126 blocks the inhibitory effect of Src on<em> FAT4</em> mRNA expression and Src-induced actin depolymerization. To determine whether actin dynamics act on the regulation of <em>FAT4</em> mRNA expression, we treated MCF-10A cells with the ROCK inhibitor Y-27632. Y-27632 treatment decreased <em>FAT4</em> mRNA expression. This suppressive effect was blocked by siRNA-mediated knockdown of Cofilin1. Furthermore, simultaneous administration of Latrunculin A (an actin depolymerizing agent), Y-27632, and Cofilin1 siRNA to the cells resulted in a marked reduction of <em>FAT4</em> mRNA expression. Intriguingly, we also found that<em> FAT4</em> mRNA expression was reduced under both low cell density and low stiffness conditions, which suggests that mechanotransduction affects <em>FAT4</em> mRNA expression. Additionally, we show that siRNA-mediated FAT4 knockdown induced the activity of the Hippo effector YAP/TAZ in MCF-10A cells. Taken together, our results reveal a novel inhibitory mechanism of <em>FAT4</em> gene expression through actin depolymerization during Src-induced carcinogenesis in human breast cells.