Lactoferrin inhibits G1 cyclin‐dependent kinases during growth arrest of human breast carcinoma cells

Lactoferrin inhibits cell proliferation and suppresses tumor growth in vivo. However, the molecular mechanisms underlying these effects remain unknown. In this in vitro study, we demonstrate that treatment of breast carcinoma cells MDA‐MB‐231 with human lactoferrin induces growth arrest at the G1 to S transition of the cell cycle. This G1 arrest is associated with a dramatic decrease in the protein levels of Cdk2 and cyclin E correlated with an inhibition of the Cdk2 kinase activity. Cdk4 activity is also significantly decreased in the treated cells and is accompanied by an increased expression of the Cdk inhibitor p21CIP1. Furthermore, we show that lactoferrin maintains the cell cycle progression regulator retinoblastoma protein pRb in a hypophosphorylated form. Additional experiments with synchronized cells by serum depletion confirm the anti‐proliferative activity of human lactoferrin. These effects of lactoferrin occur through a p53‐independent mechanism both in MDA‐MB‐231 cells and other epithelial cell lines such as HBL‐100, MCF‐7, and HT‐29. These findings demonstrate that lactoferrin induces growth arrest by modulating the expression and the activity of key G1 regulatory proteins. J. Cell. Biochem. 74:486–498, 1999. © 1999 Wiley‐Liss, Inc.