Loss of Nuclear Profilin 1 Triggers Oncogenic Reprogramming of Mammary Epithelial Cells Through Dysregulated DNA Replication in Breast Cancer.

PURPOSE: Profilin 1 (Pfn1) has been implicated in cytoskeletal regulation; however, its role in breast cancer progression and DNA replication remains unclear. This study investigated the functional significance of Pfn1 nuclear-cytoplasmic shuttling in breast cancer. METHODS: We analyzed Pfn1 expression and its correlation with DNA replication, repair, and oncogenic markers in breast cancer cell lines. Chromatin-bound and soluble Pfn1 levels were quantified by western blotting. The effects of nuclear (nuclear localization sequence-Pfn1) and cytoplasmic (nuclear export sequence-Pfn1) localization on cell growth, DNA replication, and stemness were assessed using colony formation, Alamar blue fluorescence, replication protein A 32-kDa foci staining, and DNA fiber assays. Mouse xenografts of breast cancer cells were used to determine the effect of Pfn1 localization on tumor growth in vivo. We identified the direct interactors of nuclear Pfn1 by immunoprecipitation, and their affinity was determined using bio-layer interferometry. RESULTS: Pfn1 expression was positively correlated with DNA replication, repair, p53, and MYC expression. Chromatin-bound Pfn1 was significantly degraded in breast cancer cell lines compared to that in non-cancerous MCF10a cells. Nuclear Pfn1 inhibited cell growth and DNA replication in SKBR3 cells, while cytoplasmic Pfn1 promoted cell survival and DNA replication in MCF10a cells. Loss of nuclear Pfn1 in SKBR3 cells inhibited their growth in vivo. Additionally, cytoplasmic Pfn1 upregulated stemness markers (c-Myc, B lymphoma Mo-MLV insertion region 1, and Nijmegen breakage syndrome 1). Pfn1 regulated cell stemness by binding to the nucleosome remodeler sucrose non-fermenting 2 homolog. CONCLUSION: Our findings revealed that nuclear Pfn1 acts as a tumor suppressor by inhibiting DNA replication and cell growth, while cytoplasmic Pfn1 promotes tumorigenesis by enhancing stemness and replication efficiency. These results highlight the dual role of Pfn1 in breast cancer progression, governed by its subcellular localization. They suggested that modulating Pfn1 nuclear-cytoplasmic shuttling may be a potential therapeutic strategy.