The Non-phagocytic Route of Collagen Uptake

Abstract

The degradation of collagens, the most abundant proteins of the extracellular matrix, is involved in numerous physiological and pathological conditions including cancer invasion. An important turnover pathway involves cellular internalization and degradation of large, soluble collagen fragments, generated by initial cleavage of the insoluble collagen fibers. We have previously observed that in primary mouse fibroblasts, this endocytosis of collagen fragments is dependent on the receptor urokinase plasminogen activator receptor-associated protein (uPARAP)/Endo180. Others have identified additional mechanisms of collagen uptake, with different associated receptors, in other cell types. These receptors include ␤1-integrins, being responsible for collagen phagocytosis, and the mannose receptor. We have now utilized a newly developed monoclonal antibody against uPARAP/Endo180, which down-regulates the receptor protein level on treated cells, to examine the role of uPARAP/Endo180 as a mediator of collagen internalization by a wide range of cultured cell types. With the exception of macrophages, all cells that proved capable of efficient collagen internalization were of mesenchymal origin and all of these utilized uPARAP/Endo180 for their collagen uptake process. Macrophages internalized collagen in a process mediated by the mannose receptor, a protein belonging to the same protein family as uPARAP/ Endo180. ␤1-Integrins were found not to be involved in the endocytosis of soluble collagen, irrespectively of whether this was mediated by uPARAP/Endo180 or the mannose receptor. This further distinguishes these pathways from the phagocytic uptake of particulate collagen.Remodeling of the extracellular matrix is required for a wide range of physiological and pathological conditions such as morphogenesis, organ growth, wound healing, arthritis, fibrosis and tumor growth, and metastasis (1-4). Collagens are the most abundant components of the extracellular matrix with collagen type I as the quantitatively dominating subtype. Thus, collagen constitutes about 25-30% of the dry weight of a human (5). The collagen in the body is undergoing continuous renewal and normally the collagen turnover rate is carefully controlled. Depending on the tissue type or extraneous events the collagen turnover rate can change dramatically. Therefore, highly efficient biological systems are needed in both the formation and degradation of collagen throughout life.In normal healthy tissue, collagen is fully hydroxylated and forms insoluble, cross-linked fibers and sheets of triple helical structures that are resistant to attack by most proteases (6). A number of proteases are nevertheless potentially capable of initiating the collagen degradation process through the cleavage of intact collagen fibers. These proteases are the matrix metalloproteinases (MMPs) 3 MMP-1, MMP-2, MMP-8, MMP-13, MMP-14, MMP-15, and MMP-16 and the cysteine protease cathepsin K (7-9). So far, most studies of collagen turnover have focused on the extracellular collagen degradat...

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