Characterization of a novel high molecular weight tropomyosin Tpm4.1 involved in invasive behaviors in breast epithelial cells

Abstract

Tropomyosins (Tpm) are a family of actin filament binding proteins that regulates actin cytoskeleton organization and various cellular functions. Alterations in Tpm expression are implicated in various properties of transformed cells. Here I report the identification and characterization of a novel high molecular weight isoform of tropomyosin, Tpm4.1, expressed from the human TPM4 gene. Tpm4.1 expression is down-regulated in breast cancer cells compared with untransformed breast epithelial cells and in highly metastatic breast cancer cell lines derived from poorly metastatic MDA-MD-231 cells. In addition, patients with invasive ductal breast carcinoma show decreased TPM4 expression compared with patients with ductal breast carcinoma in situ, and low TPM4 expression is associated with a poor prognosis. To gain insights into the role of Tpm4.1 in breast cancer, I inhibited its function using siRNA in untransformed breast epithelial cells, MCF10A, and the human breast cancer cell line MDA-MB-231. Loss of Tpm4.1 in MCF10A cells increases cell migration in wound-healing and Boyden chamber assays and invasion out of spheroids as well as disruption of cell-cell adhesions. Down-regulation of Tpm4.1 in poorly metastatic MDA-MB-231 cells led to less stress fibers and increased cell invasion and dissemination from spheroids into collagen gels. Collectively these findings suggest that decreased expression of Tpm4.1 might play a critical role during tumor progression by facilitating the metastatic potential of tumor cells.