Role of Urokinase Plasminogen Activator Receptor–Associated Protein in Mouse Lung

Abstract

Urokinase plasminogen activator receptor-associated protein (uPARAP, or Endo180) is a transmembrane endocytic receptor that mediates collagen internalization and degradation. uPARAP may be a novel pathway for collagen turnover and matrix remodeling in the lung. The function of uPARAP in lung injury has not been described. We analyzed the pulmonary mechanics of uPARAP 2/2 and wild-type mice at baseline and examined their response after bleomycin instillation. We compared collagen internalization in primary mouse lung fibroblasts (MLFs) from wild-type and uPARAP 2/2 mice using flow cytometry and fluorescent microscopy, and we examined the role of cytokines in regulating uPARAP expression and collagen internalization. We show that uPARAP is highly expressed in the lung, and that uPARAP 2/2 mice have increased lung elastance at baseline and after injury. uPARAP 2/2 mice are protected from changes in lung permeability after acute lung injury and have increased collagen content after bleomycin injury. uPARAP is the primary pathway for internalization of collagens in MLFs. Furthermore, collagen internalization through uPARAP does not require matrix metalloproteinase digestion and is independent of integrins. Mediators of lung injury, including transforming growth factor-b, TNF-a, and IL-1, down-regulate both uPARAP expression and collagen internalization. uPARAP is highly expressed in the murine lung, and loss of uPARAP leads to differences in lung mechanics, lung permeability, and collagen content after injury. uPARAP is required for collagen internalization by MLFs. Thus, uPARAP is a novel pathway that regulates matrix remodeling in the lung after injury.Keywords: urokinase plasminogen activator receptor-associated protein; Endo180; collagen internalization; lung fibroblasts; matrix remodeling Urokinase plasminogen activator receptor-associated protein (uPARAP, Endo180, or mannose receptor, C type 2) is a 180-kD transmembrane receptor that can bind and internalize both fibrillar and nonfibrillar collagens (1, 2). After internalization, uPARAP targets collagen to the lysosome for degradation and then uPARAP recycles to the plasma membrane (3). uPARAP is expressed in mesenchymal cells, predominantly fibroblasts. In addition, dermal macrophage and human placental endothelial cells express low levels of uPARAP (1,4,5).We previously demonstrated high expression of uPARAP in the mesenchyme throughout lung development (6). Despite the high expression of uPARAP in the developing lung, development proceeds normally in uPARAP 2/2 mice, and is not associated with any differences in matrix metalloproteinase (MMP), tissue inhibitor of metalloproteinases, or collagen expression in the lung (6). uPARAP 2/2 mice appear phenotypically normal in the unchallenged state, and have a normal lifespan. One possible explanation for the lack of lung phenotype is use of an alternative pathway for collagen internalization. Although previous work demonstrated that uPARAP was the primary receptor for collagen internalization in dermal fibroblast...

Always cleave up your mess: targeting collagen degradation to treat tissue fibrosis

“…Another area that warrants further evaluation is examining whether therapies that augment intracellular pathways of collagen degradation will lead to enhanced extracellular remodeling of fibrotic tissue. There is now convincing evidence that defective intracellular collagen degradation leads to excess fibrosis in experimental mouse models (4,19). These data show that inhibition of key pathways mediating collagen uptake by cells promote extracellular accumulation of fibrillar collagen.…”

Fibroblast Activation Protein (FAP) Accelerates Collagen Degradation and Clearance from Lungs in Mice

“…Although intact collagen may be endocytosed, fragmentation of collagen greatly speeds its rate of internalization and clearance (44,71). Collagen internalization through uPARAP/Endo180 has been implicated in both tumorigenesis and the development of fibrosis in the liver and lung in vivo (46,47,72).…”

Mechanisms of Lung Fibrosis Resolution

“…36 Collagen IV and gelatin internalization were nearly ablated in urokinase plasminogen activator receptoreassociated protein deficient mouse lung fibroblasts in an integrin-independent mechanism. 34 Compared with controls, mice deficient in urokinase plasminogen activator receptoreassociated protein exhibit increased total lung hydroxyproline content 28 days after bleomycin injury than controls without alterations in inflammation, or rates of collagen synthesis.…”

“…Collagens that have been precleaved by collagenases are internalized more efficiently than intact collagen, thereby allowing cells to clear and recycle matrix components. 34 The primary processes mediating intracellular collagen degradation include recognition and internalization of collagen fragments, where they are ultimately degraded into the lysosomal network by cysteine proteinases. Several pathways have been shown to internalize collagen fragments (Figure 2).…”