Molecular Insights into Profilin Regulation and Its Role in Breast Cancer

Dysregulation of actin-regulatory proteins is a hallmark feature of tumor malignancy. Along this line, it has been shown that actin-binding protein profilin-1 (Pfn1) is downregulated in human breast cancer. Contrary to its pro-migratory role in most physiological contexts, loss of Pfn1 promotes migration and invasion of breast cancer cells, and conversely, restoring Pfn1 expression in breast cancer cells by genetic overexpression suppresses these pro-metastatic traits. Based on these previous findings, we set out to a) determine whether Pfn1 regulates breast cancer cell motility through vascular endothelial cells (an obligatory step for intravasation and systemic dissemination of cancer cells) [Aim 1], b) identify small molecules that can elevate Pfn1 expression and suppress motility of breast cancer cells in a Pfn1-dependent manner [Aim 2], and c) gain fundamental insight into how Pfn1 expression is regulated in cells [Aim 3]. We established that loss of Pfn1 promotes transendothelial cell migration of breast cancer cells through enhancing VEGF secretion and barrier disruption of endothelial cells. Next, through a small-scale screen of small molecules, we identified several molecules that are capable of elevating Pfn1 expression including a broad-range kinase inhibitor Tyrphostin A9. The anti-migratory action of Tyrphostin A9 was further shown to be Pfn1-dependent. Finally, we identified megakaryoblastic leukemia (MKL), a transcriptional co-activator of serum response factor (SRF), to be a major co-regulator of the expression of Pfn1, and its closely related isoform Pfn2 (also implicated in breast cancer), and established a novel MKL/STAT (signal transducer and activator of transcription) signaling axis in the context of Pfn regulation.