We have shown that the glycation

We have shown that the glycation of collagen hinders collagen-DDR2 binding. McCarthy et al. previously showed reduced binding of rat osteosarcoma-derived cells (UMR106) to glycated collagen. The authors speculated that glycation of collagen attenuates collagen binding to collagen-specific integrins on these cells. They showed, however, that glycated collagen does not compete with RGD and DGEA peptides (specific sequence for α1β1 and α2β1 integrins), and proposed that glycation of collagen interferes with different collagen-integrin FITC, Fluorescein isothiocyanate [44]. Based on our current study, we suggest that the loss of collagen interaction with DDR2 under diabetic conditions could potentially also contribute to the observed loss of cell adhesion. It is interesting that although carboxymethylation of lysine residues in our collagen preparations was about 5%, a strong inhibition of collagen stimulation of lysyl oxidase was found. DDR2 binding to collagens occurs at a consensus sequence of GVMGFO where O is hydroxyproline [45]. We speculate that regions of collagens in the vicinity of this sequence are likely to be highly susceptible to chemical modifications of lysine residues, suggesting that they may be more exposed to solvent conditions than other regions of collagen, or that other regions of collagen may also bind and activate DDRs [46]. If true, this notion could have important physiological implications with respect to mechanisms of AGE-dependent extracellular matrix complications, including effects on lysyl oxidase regulation and collagen homeostasis in diabetes: disproportionately high AGE modification of regions of collagen which consist of DDR2 ligands would result in a strong down regulation of lysyl oxidase levels. Global inhibition of integrin signaling with EDTA here suggests that integrin signaling potentially cooperates with DDR2 in collagen induction of lysyl oxidase. This notion is supported by a recent study that shows DDR2 cooperation with specific integrins in cell adhesion to collagen [47]. It is of interest that collagen-specific α1 integrin-ablations in mice manifested as no overt phenotype or anatomical anomalies [48] although bone fracture healing in these mice was impaired [49]. Homozygous mice deficient for collagen-specific α10β1 integrin showed after birth growth plate defects with no morphological skeletal deformities [50]. Integrin α2 deletion in mice revealed no striking anatomical phenotype [51], [52]. In a recent study, mice ablated in α2β1-, α11β1-, or both α2β1–α11β1-integrins were born with no bone phenotype; however, long bone post-natal growth was reduced in mice lacking α11β1- and α2β1–α11β1-integrins. The authors demonstrated that a proportional dwarfism in these mice is a consequence of insufficient systemic IGF-1 levels, and neither osteoblast dysfunction nor growth plate alterations contributed to this phenotype [51]. Thus, collagen-integrin axis activation can be considered to indirectly regulate bone growth and development [51], whereas collagen-activation of DDR2 appears to regulate activities of bone cells which directly synthesize bone extracellular matrix.