Determination of cell uptake pathways for tumor inhibitor lysyl oxidase propeptide

The lysyl oxidase propeptide (LOX-PP) is derived from pro-lysyl oxidase (Pro-LOX) by extracellular biosynthetic proteolysis. LOX-PP inhibits breast and prostate cancer xenograft tumor growth and has tumor suppressor activity. Although, several intracellular targets and molecular mechanisms of action of LOX-PP have been identified, LOX-PP uptake pathways have not been reported. Here we demonstrate that the major uptake pathway for recombinant LOX-PP (rLOX-PP) is PI3K-dependent macropinocytosis in PWR-1E, PC3, SCC9, MDA-MB-231 cell lines. A secondary pathway appears to be dynamin- and caveola dependent. The ionic properties of highly basic rLOX-PP provide buffering capacity at both high and low pHs. We suggest that the buffering capacity of rLOX-PP, which serves to limit endosomal acidification, sustains PI3K-dependent macropinocytosis in endosomes which in turn is likely to facilitate LOX-PP endosomal escape into the cytoplasm and its observed interactions with cytoplasmic targets and nuclear uptake.