The hallmark of fibrotic processes is an excessive accumulation of collagen. The deposited collagen shows an increase in pyridinoline cross-links, which are derived from hydroxylated lysine residues within the telopeptides. This change in cross-linking is related to irreversible accumulation of collagen in fibrotic tissues. The increase in pyridinoline cross-links is likely to be the result of increased activity of the enzyme responsible for the hydroxylation of the telopeptides (telopeptide lysyl hydroxylase, or TLH). Although the existence of TLH has been postulated, the gene encoding TLH has not been identified. By analyzing the genetic defect of Bruck syndrome, which is characterized by a pyridinoline deficiency in bone collagen, we found two missense mutations in exon 17 of PLOD2, thereby identifying PLOD2 as a putative TLH gene. Subsequently, we investigated fibroblasts derived from fibrotic skin of systemic sclerosis (SSc) patients and found that PLOD2 mRNA is highly increased indeed. Furthermore, increased pyridinoline cross-link levels were found in the matrix deposited by SSc fibroblasts, demonstrating a clear link between mRNA levels of the putative TLH gene (PLOD2) and the hydroxylation of lysine residues within the telopeptides. These data underscore the significance of PLOD2 in fibrotic processes.The biosynthesis of collagen molecules involves several intracellular post-translational modifications followed by excretion and extracellular aggregation of the collagen molecules into fibrils, which are subsequently stabilized by intermolecular cross-links (1, 2). Two related routes are responsible for the formation of these collagen cross-links, namely the allysine route, in which a lysine residue in the telopeptide is converted by lysyl oxidase into the aldehyde allysine, and the hydroxyallysine route, in which a hydroxylysine residue in the telopeptide is converted into the aldehyde hydroxyallysine. Subsequently, the allysine or the hydroxyallysine reacts with a Lys or Hyl residue in the triple helix to form di-, tri-, or tetrafunctional cross-links (3-6). The mature cross-links hydroxylysylpyridinoline or lysylpyridinoline are formed via the hydroxyallysine route and occur in a variety of connective tissues such as bone, tendon, ligaments, and cartilage (7). In contrast, collagen in the skin is mainly cross-linked via the allysine route. Interestingly, in fibrotic skin (lipodermatosclerosis, keloid) and organ fibrosis (lung, liver), which is characterized by an excessive accumulation of collagen, an increase in crosslinks derived from the hydroxyallysine route is found (8 -15). It has been shown that the amount of hydroxyallysine-derived cross-links is related to the irreversible accumulation of collagen in fibrotic tissues, indicating that collagen containing hydroxyallysine-derived cross-links is more difficult to degrade than collagen containing allysine-derived cross-links (10 -12, 14, 15). Inhibition of the formation of hydroxyallysine-derived cross-links in fibrosis is therefore likely to resu...

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The hallmark of fibrosis is the excessive accumulation of collagen. The deposited collagen contains increased pyridinoline cross-link levels due to an overhydroxylation of lysine residues within the collagen telopeptides. Lysyl hydroxylase 2b (LH2b) is the only lysyl hydroxylase consistently up-regulated in several forms of fibrosis, suggesting that an enhanced LH2b level is responsible for the overhydroxylation of collagen telopeptides. The present paper reports the effect of profibrotic cytokines on the expression of collagen, lysyl hydroxylases and lysyl oxidase in normal human skin fibroblasts, as well as the effect on pyridinoline formation in the deposited matrix. All three isoforms of TGF-beta induce a substantial increase in LH2b mRNA levels, also when expressed relatively to the mRNA levels of collagen type I alpha2 (COL1A2). The TGF-beta isoforms also clearly influence the collagen cross-linking pathway, since higher levels of pyridinoline cross-links were measured. Similar stimulatory effects on LH2b/COL1A2 mRNA expression and pyridinoline formation were observed for IL-4, activin A, and TNF-alpha. An exception was BMP-2, which has no effect on LH2b/COL1A2 mRNA levels nor on pyridinoline formation. Our data show for the first time that two processes, i.e., up-regulation of LH2b mRNA levels and increased formation of pyridinoline cross-links, previously recognized to be inherent to fibrotic processes, are induced by various profibrotic cytokines.

including 15 graduate students) and 53 nonmembers (including 16 graduate students). There were 17 invited speakers, with three from the USA, five from the EU and five from the UK.Jean Schwarzbauer (Princeton University, USA) opened the meeting by describing her recent findings concerning the events that control fibronectin (Fn) matrix assembly. The assembly of Fn matrix fibrils is influenced by extracellular factors such as availability of Fn and other matrix molecules, and intracellular signalling pathways mediated by integrin receptors. Jean's group has investigated two kinases downstream of integrin receptors, focal adhesion kinase (FAK) and pp60-Src. Mouse fibroblasts lacking FAK were found to assemble reduced amounts of Fn fibrils. Src family kinases phosphorylate FAK, and it was found that fibroblasts without Src family kinases (SYF cells) or wild-type cells treated with an Src inhibitor (PP1) also lack Fn matrix. The extracellular influence of tenascin-C was also discussed. In fibroblasts on a 3D Fn matrix, FAK was constitutively phosphorylated, whereas in the presence of tenascin-C, FAK is transiently activated, and there is a reduction in the assembly of Fn matrix fibrils. Other aspects of integrin-Fn interactions were also described, including the stimulation of Fn matrix formation by dexamethasone in the tumourgenic cell line HT1080.Karl Kadler (University of Manchester) summarized findings on the molecular and cellular basis of collagen fibrillogenesis. The talk focussed on the sorting and secretion of type-I collagen in organ cultures of embryonic chick tendon. It Int.