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 cross-link hydroxylysylpyridinoline is observed. Hydroxylysylpyridinoline is a typical crosslink of skeletal tissue and is thought to play a major part in the hardening of sclerotic tissue. We investigated whether the increase in hydroxylysylpyridinoline is due to overhydroxylation of lysyl residues in the collagen molecule, which may also be associated with an increase of glycosylated hydroxylysine residues. Furthermore, we determined whether the collagen fibrils in lipodermatosclerosis showed a decrease of the diameter in the tissue as well as in vitro after fibrillogenesis of pepsin-solubilized collagens. Isolated α-chains of pepsin solubilized collagen I showed an increase in lysyl hydroxylation (hyl/(hyl + lys)) as compared with normal control [α1(I): lipodermatosclerosis 0.18 ± 0.01; control 0.12 ± 0.01; α2(I): lipodermatosclerosis 0.36 ± 0.02; control 0.25 ± 0.03, p < 0.001]. Furthermore, the content of enzymatic glycosylated hydroxlysine residues increased. This increase is associated with a decrease of fibril diameter of both tissue and fibrils formed in vitro of pepsin-solubilized collagens. In the same pool of collagens an increase in collagen III content was observed as compared with controls (lipodermatosclerosis 14.5% ± 1.6, control 10.3% ± 1.6, p < 0.001). Our results showed that the overhydroxylation of lysyl residues, which is required for the generation of hydroxylysylpyridinoline, is not only restricted to the telopeptides but also affects 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 observed alteration in the architecture of collagen fibrils in sclerotic skin.