MRC2-mediated collagen internalization is reduced in fibrotic lung fibroblasts and increased upon phenotypic dedifferentiation

Idiopathic pulmonary fibrosis (IPF) is characterized by parenchymal scarring reflecting an imbalance between collagen deposition by myofibroblasts (MFs) and its turnover. Although collagen clearance is essential for fibrosis resolution, this process and its potential for therapeutic modulation in IPF are poorly understood. Here we evaluated internalization of degraded collagen and the role of its requisite endocytic receptor mannose receptor C-type 2 (MRC2), in lung tissue and MFs from IPF patients and bleomycin-injured mice. Fibrotic human and murine lung tissue exhibited an accumulation of degraded collagen, highlighting a failure of its clearance. MFs from fibrotic lung demonstrated a reduced capacity to internalize extracellular degraded collagen, with a concomitant reduction in MRC2 expression and endolysosomal activity. Both diminished collagen uptake and MRC2 expression recovered to baseline levels during spontaneous resolution of bleomycin fibrosis. In vitro treatment of IPF or TGF-β-elicited MFs with a variety of mechanistically distinct agents known to effect phenotypic dedifferentiation restored defective collagen internalization. Although enhanced uptake was MRC2-dependent, it involved increased endolysosomal activity rather than increased MRC2 expression. These results implicate defective MRC2-dependent collagen internalization and endolysosomal function in MFs as important factors contributing to fibrosis that may be therapeutically targeted to promote resolution.