Loss of MLCK leads to disruption of cell-cell adhesions and invasive behavior of breast epithelial cells via increased expression of EGFR and ERK/JNK signaling

Abstract

Myosin light chain kinase (MLCK) expression is down-regulated in breast cancer, including invasive ductal carcinoma compared with ductal breast carcinoma in situ and metastatic breast tumors. However, little is known about how loss of MLCK expression contributes to tumor progression. MLCK is a component of the actin cytoskeleton and its known role is the phosphorylation of the regulatory light chains of myosin II. To gain insights into the role of MLCK in breast cancer, we perturbed its function using siRNA or pharmacological inhibition in untransformed breast epithelial cells (MCF10A). Loss of MLCK by siRNAs led to increased cell migration and invasion, disruption of cell-cell adhesions and enhanced formation of focal adhesions at the leading edge of migratory cells. In addition, down-regulation of MLCK cooperated with HER2 in MCF10A cells to promote cell migration and invasion and low levels of MLCK is associated with a poor prognosis in HER2 positive breast cancer patients. Associated with these altered migratory behaviors were increased expression of EGFR and activation of ERK and JNK signaling pathways in MLCK down-regulated MCF10A epithelial cells. By contrast, inhibition of the kinase function of MLCK using pharmacological agents inhibited cell migration and invasion, and did not affect cellular adhesions. Our results demonstrate loss of MLCK contributes to the migratory properties of epithelial cells resulting from changes in cell-cell and cell-matrix adhesions, and increased EGFR signaling. These findings suggest that decreased expression of MLCK might play a critical role during tumor progression by facilitating the metastatic potential of tumor cells.