Fully processed lysyl oxidase catalyst translocates from the extracellular space into nuclei of aortic smooth-muscle cells

Lysyl oxidase (LO), a secreted protein, was recently identified within the nuclei of vascular smooth‐muscle cells (SMC) and 3T3 fibroblasts. A possible pathway by which LO can enter cell nuclei was explored in the present study. SMC were incubated with purified 32‐kDa bovine aorta LO that had been fluorescently labeled with rhodamine (TRITC‐LO). TRITC‐LO entered the cytosol and then rapidly concentrated within the nuclei of preconfluent cultures of these cells, whereas carbonic anhydrase, a protein of similar molecular weight and similarly labeled, did not enter the cells under these conditions. LO that had been reductively methylated at lysine residues with [14C]HCHO was also taken up into the cytosolic and nuclear compartments. Intracellular uptake and intracellular distribution were not altered by inhibiting LO activity with β‐aminopropionitrile. An excess of native LO but not of carbonic anhydrase competitively inhibited the uptake of the isotopically labeled enzyme. Thus, once secreted and proteolytically processed, mature LO can enter the cells and concentrate within nuclei in a manner that appears to be specific and independent of its catalytic activity. J. Cell. Biochem. 79:576–582, 2000. © 2000 Wiley‐Liss, Inc.