Lysyl hydroxylase 2 (LH2) catalyzes the hydroxylation of
lysine residues in the telopeptides of
fibrillar collagens, which leads to the formation of stable
collagen cross-links. Recently we reported that LH2 enhances the metastatic propensity of
lung cancer by increasing the amount of stable
hydroxylysine aldehyde-derived
collagen cross-links (HLCCs), which generate a stiffer
tumor stroma (Chen, Y., et al. (2015) J. Clin. Invest. 125, 125, 1147-1162). It is generally accepted that LH2 modifies
procollagen α chains on the endoplasmic reticulum before the formation of triple helical
procollagen molecules. Herein, we report that LH2 is also secreted and modifies
collagen in the extracellular space. Analyses of
lung cancer cell lines demonstrated that LH2 is present in the cell lysates and the
conditioned media in a dimeric, active form in both compartments. LH2 co-localized with
collagen fibrils in the extracellular space in human
lung cancer specimens and in orthotopic lung
tumors generated by injection of a LH2-expressing human
lung cancer cell line into nude mice. LH2 depletion in MC3T3 osteoblastic cells impaired the formation of HLCCs, resulting in an increase in the unmodified
lysine aldehyde-derived
collagen cross-link (LCC), and the addition of recombinant LH2 to the media of LH2-deficient MC3T3 cells was sufficient to rescue HLCC formation in the extracellular matrix. The finding that LH2 modifies
collagen in the extracellular space challenges the current view that LH2 functions solely on the endoplasmic reticulum and could also have important implications for
cancer biology.