Fibrillar collagens of the extracellular matrix are critical for tissue structure and physiology; however, excessive or abnormal deposition of collagens is a defining feature of fibrosis. Regulatory mechanisms that act on collagen fibril assembly potentially offer new targets for antifibrotic treatments. Tissue weakening, altered collagen fibril morphologies, or both, are shared phenotypes of mice lacking matricellular thrombospondins. Thrombospondin-1 (TSP1) plays an indirect role in collagen homeostasis through interactions with matrix metalloproteinases and transforming growth factor–β1 (TGF-β1). We found that TSP1 also affects collagen fibril formation directly. Compared to skin from wild-type mice, skin from Thbs1^−/− mice had reduced collagen cross-linking and reduced prolysyl oxidase (proLOX) abundance with increased conversion to catalytically active LOX. In vitro, TSP1 bound to both the C-propeptide domain of collagen I and the highly conserved KGHR sequences of the collagen triple-helical domain that participate in cross-linking. TSP1 also bound to proLOX and inhibited proLOX processing by bone morphogenetic protein-1. In human dermal fibroblasts (HDFs), TSP1 and collagen I colocalized in intracellular vesicles and on extracellular collagen fibrils, whereas TSP1 and proLOX colocalized only in intracellular vesicles. Inhibition of LOX-mediated collagen cross-linking did not prevent the extracellular association between collagen and TSP1; however, treatment of HDFs with KGHR-containing, TSP1-binding, triple-helical peptides disrupted the collagen-TSP1 association, perturbed the collagen extracellular matrix, and increased myofibroblastic differentiation in a manner that depended on TGF-β receptor 1. Thus, the extracellular KGHR-dependent interaction of TSP1 with fibrillar collagens contributes to fibroblast homeostasis.

细胞外基质的纤维状胶原蛋白对于组织结构和生理学至关重要；然而，胶原蛋白的过度或异常沉积是纤维化的一个决定性特征。作用于胶原纤维组装的调节机制可能为抗纤维化治疗提供新的靶点。组织弱化、胶原纤维形态改变或两者兼而有之，是缺乏基质细胞血小板反应蛋白的小鼠共有的表型。血小板反应蛋白-1 (TSP1) 通过与基质金属蛋白酶和转化生长因子-β1 (TGF-β1) 相互作用，在胶原蛋白稳态中发挥间接作用。我们发现TSP1也直接影响胶原纤维的形成。与野生型小鼠的皮肤相比， Thbs1 ^-/−小鼠的皮肤胶原蛋白交联减少，脯氨酰氧化酶 (proLOX) 丰度减少，同时向催化活性 LOX 的转化增加。在体外，TSP1 与 I 型胶原蛋白的 C 前肽结构域和参与交联的胶原蛋白三螺旋结构域的高度保守的 KGHR 序列结合。 TSP1 还与 proLOX 结合并抑制骨形态发生蛋白 1 对 proLOX 的加工。在人真皮成纤维细胞 (HDF) 中，TSP1 和 I 型胶原蛋白共定位于细胞内囊泡和细胞外胶原纤维上，而 TSP1 和 proLOX 仅共定位于细胞内囊泡中。抑制 LOX 介导的胶原蛋白交联并不能阻止胶原蛋白和 TSP1 之间的细胞外结合；然而，用含有 KGHR、TSP1 结合的三螺旋肽处理 HDF 会破坏胶原蛋白-TSP1 的结合，扰乱胶原细胞外基质，并以依赖于 TGF-β 受体 1 的方式增加肌成纤维细胞分化。 因此，TSP1 与纤维状胶原的细胞外 KGHR 依赖性相互作用有助于成纤维细胞稳态。