The alveolar epithelium represents a major site of tissue destruction during
lung injury. It consists of alveolar epithelial type I (ATI) and type II (ATII) cells. ATII cells are capable of self-renewal and exert progenitor function for ATI cells upon alveolar epithelial injury. Cell differentiation pathways enabling this plasticity and allowing for proper repair, however, are poorly understood. Here, we applied proteomics, expression analysis and functional studies in primary murine ATII cells to identify
proteins and molecular mechanisms involved in alveolar epithelial plasticity. Mass spectrometry of cultured ATII cells revealed a reduction of
carbonyl reductase 2 (CBR2) and an increase in
enolase 1 (ENO1) and
protein disulfide-isomerase associated 3 (PDIA3)
protein expression during ATII-to-ATI cell trans-differentiation. This was accompanied by increased Wnt/β-
catenin signaling, as analyzed by qRT-PCR and immunoblotting. Notably, ENO1 and PDIA3, along with T1α (podoplanin; an ATI cell marker), exhibited decreased
protein expression upon pharmacological and molecular Wnt/β-
catenin inhibition in cultured ATII cells, whereas CBR2 levels were stabilized. Moreover, we analyzed primary ATII cells from mice with
bleomycin-induced
lung injury, a model exhibiting activated Wnt/β-
catenin signaling in vivo. We observed reduced CBR2 significantly correlating with
surfactant protein C (SFTPC), whereas ENO1 and PDIA3 along with T1α were increased in injured ATII cells. Finally,
siRNA-mediated knockdown of ENO1, as well as PDIA3, in primary ATII cells led to reduced T1α expression, indicating diminished cell trans-differentiation. Our data thus identified
proteins involved in ATII-to-ATI cell trans-differentiation and suggest a Wnt/β-
catenin-driven functional role of ENO1 and PDIA3 in alveolar epithelial cell plasticity in
lung injury and repair.