Optimum Serum Concentration Enhances Migration of MDA-MB-231 Triple-Negative Breast Cancer Cells and Promotes Intracellular Delivery of Proapoptotic Domain via Cell-Penetrating Peptides

Breast cancer remains one of the most prevalent cancers among women, with triple-negative breast cancer (TNBC), lacking estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2, accounting for approximately 15-20% of all patients with breast cancer. TNBC is notably aggressive, with a high invasive, metastatic, and recurrence potential. In this study, we found that the migration and invasion capabilities of MDA-MB-231 cells, derived from human TNBC, were strongly influenced by the serum concentration. Transwell assays revealed that TNBC cell migration varied depending on the fetal bovine serum (FBS) level, with an optimal concentration that substantially enhanced migration and invasion. In contrast, non-TNBC MCF-7 cells exhibited no such serum-dependent migration pattern. In addition to data-independent acquisition (DIA) phosphoproteomic analysis for understanding the mechanisms, the cellular uptake of the flock house virus coat (35-49) peptide, a type of arginine-rich cell-penetrating peptide with serum-dependent cellular uptake efficacy, was significantly increased under the optimal serum conditions, which induces cell migration, leading to efficient delivery of apoptosis-inducible peptide and TNBC-killing activity. Our findings highlight the critical role of serum concentration in regulating TNBC behavior and offer insights into leveraging serum-responsive delivery systems for targeted breast cancer therapy.