Overhydroxylation of lysyl residues is the initial step for altered collagen cross-links and fibril architecture in fibrotic skin

In fibrotic skin of lipodermatosclerosis a substantial increase of the cros s-link hydroxylysylpyridinoline is observed. Hydroxylysylgyridinoline is a typical cross-link of skeletal tissue and is thought to play a major part i n the hardening of sclerotic tissue. We investigated whether the increase i n hydroxylysylpyridinoline is due to overhydroxylation of lysyl residues in the collagen molecule, which may also be associated with an increase of gl ycosylated hydroxylysine residues. Furthermore, we determined whether the c ollagen fibrils in lipodermatosclerosis showed a decrease of the diameter i n the tissue as well as in vitro after fibrillogenesis of pepsin-solubilize d collagens. Isolated alpha-chains of pepsin solubilized collagen I showed an increase in lysyl hydroxylation (hyl/(hyl + lys)) as compared with norma l control [alpha 1(I): Lipodermatosclerosis 0.18 +/- 0.01; control 0.12 +/- 0.01; alpha 2(I): Lipodermatosclerosis 0.36 +/- 0.02; control 0.25 +/- 0.0 3, p < 0.001]. Furthermore, the content of enzymatic glycosylated hydroxlys ine residues increased. This increase is associated with a decrease of fibr il diameter of both tissue and fibrils formed in vitro of pepsin-solubilize d collagens. In the same pool of collagens an increase in collagen III cont ent was observed as compared with controls (lipodermatosclerosis 14.5% +/- 1.6, control 10.3% +/- 1.6, p < 0.001). Our results showed that the overhyd roxylation of lysyl residues, which is required for the generation of hydro xylysylpyridinoline, is not only restricted to the telopeptides but also af fects the helical part of the molecule, This process is further associated with an increase of glycosylated hydroxylysyl residues. These changes along with the increase in collagen III content seem to be responsible for the o bserved alteration in the architecture of collagen fibrils in sclerotic ski n.