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(American Journal of Pathology. 2005;167:81-88.)
© 2005 American Society for Investigative Pathology

Proteolysis of Cell-Surface Tissue Transglutaminase by Matrix Metalloproteinase-2 Contributes to the Adhesive Defect and Matrix Abnormalities in Thrombospondin-2-Null Fibroblasts and Mice

From the Department of Biochemistry, School of Medicine, University of Washington, Seattle, Washington

Thrombospondin (TSP)-2-null dermal fibroblasts display an attachment defect that results from increased matrix metalloproteinase (MMP)-2 levels in their conditioned media. To investigate the molecular mechanisms responsible for this defect, we analyzed the activity of tissue transglutaminase (tTG) in TSP-2-null dermal fibroblasts and in tissues of TSP-2-null mice. tTG functions as a co-receptor for ß1 and ß3 integrins and stabilizes extracellular matrix proteins by introduction of isopeptide cross-links. Cell-surface tTG activity was reduced in TSP-2-null cells (0.50 ± 0.05 arbitrary units versus 0.84 ± 0.07 for wild type; P 0.05), and addition of MMP-2 to the culture medium of wild-type cells caused a 35% reduction in cell-surface tTG activity. tTG was susceptible to proteolysis by MMP-2 in vitro, and addition of the MMP inhibitor TIMP-2 to TSP-2-null cells restored tTG activity (0.3 ± 0.08 for untreated cells; 0.71 ± 0.09 with TIMP-2). TSP-2-null mice had reduced tTG activity in skin, as measured by incorporation of fluorescein isothiocyanate-labeled cadaverine, and a threefold increase in acetic acid-extracted dermal collagen. Furthermore, isopeptide cross-links were reduced in both uninjured skin and in excisional wounds of TSP-2-null mice, as determined by morphometric immunohistochemical analysis, indicating that isopeptide cross-links are important for the stabilization of the collagenous matrix in dermis. These findings provide a mechanism for the reduced adhesion of TSP-2-null fibroblasts and an explanation for the increased collagen solubility and fragility of TSP-2-null skin.

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