Lysyl hydroxylase 3 (LH3) modifies proteins in the extracellular space, a novel mechanism for matrix remodeling

Lysyl hydroxylase 3 (LH3), the multifunctional enzyme associated with collagen biosynthesis that possesses lysyl hydroxylase and collagen glycosyltransferase activities, has been characterized in the extracellular space in this study. Lysine modifications are known to occur in the endoplasmic reticulum (ER) prior to collagen triple-helix formation, but in this study we show that LH3 is also present and active in the extracellular space. Studies with in vitro cultured cells indicate that LH3, in addition to being an ER resident, is secreted from the cells and is found both in the medium and on the cell surface associated with collagens or other proteins with collagenous sequences. Furthermore, in vivo, LH3 is present in serum. LH3 protein levels correlate with the galactosylhydroxylysine glucosyltransferase (GGT) activity of mouse tissues. This, together with other data, indicates that LH3 is responsible for GGT activity in the tissues and that GGT activity assays can be used to quantify LH3 in tissues. LH3 in vivo is located in two compartments, in the ER and in the extracellular space, and the partitioning varies with tissue type. In mouse kidney the enzyme is located mainly intracellularly, whereas in mouse liver it is located solely in the extracellular space. The extracellular localization and the ability of LH3 to modify lysyl residues of extracellular proteins in their native, nondenaturated conformation reveals a new dynamic in extracellular matrix remodeling, suggesting a novel mechanism for adjusting the amount of hydroxylysine and hydroxylysine-linked carbohydrates in collagenous proteins.