MMP9 and breast cancer: Piperine, an alkaloid found in black paper (Piper nigrum), inhibits the growth of human BC cells and xenografts in immune-deficient mice, cell cycle progression, matrix metalloproteinases 2 and 9 (MMP2 and MMP9) mRNA expression, BC cell migration, and induces caspase-dependent apoptosis via mitochondrial pathway [117].