Formin Proteins FHOD1 and INF2 in Triple-Negative Breast Cancer: Association With Basal Markers and Functional Activities

Basal-like breast cancer is an aggressive form of breast cancer with limited treatment options. The subgroup can be identified immunohistochemically, by lack of hormone receptor expression combined with expression of basal markers such as CK5/6 and/or epidermal growth factor receptor (EGFR). In vitro, several regulators of the actin cytoskeleton are essential for efficient invasion of basal-like breast cancer cell lines. Whether these proteins are expressed in vivo determines the applicability of these findings in clinical settings. The actin-regulating formin protein FHOD1 participates in invasion of the triple-negative breast cancer cell line MDA-MB-231. Here, we measure the expression of FHOD1 protein in clinical triple-negative breast cancers by using immunohistochemistry and further characterize the expression of another formin protein, INF2. We report that basal-like breast cancers frequently overexpress formin proteins FHOD1 and INF2. In cell studies using basal-like breast cancer cell lines, we show that knockdown of FHOD1 or INF2 interferes with very similar processes: maintenance of cell shape, migration, invasion, and proliferation. Inhibition of EGFR, PI3K, or mitogen-activated protein kinase activity does not alter the expression of FHOD1 and INF2 in these cell lines. We conclude that the experimental studies on these formins have implications in the clinical behavior of basal-like breast cancer.