Warburg and Scars? Alveolar Type II Cell Glycolytic Reprogramming and Differential Mitochondrial Dynamics Observed in a Murine Model of Pulmonary Fibrosis

Abstract

Our emerging conceptual understanding of Idiopathic Pulmonary Fibrosis (IPF) highlights the significant role of alveolar epithelial type II cell (AT2) cell dysfunction in underlying susceptibility, disease severity, and disease progression. This multifunctional cell is solely responsible for the synthesis and secretion of pulmonary surfactant, a highly metabolically taxing process. Though mutations in surfactant component genes are one of the recognized etiological causes of IPF, the relationship between the metabolic program of the AT2 cell and IPF is poorly understood. Leveraging a novel preclinical model of IPF, the IER-SP-CI73T mouse, we aimed to characterize the role of altered AT2 metabolism in IPF fibrogenesis and remodeling. Using this model, we characterized the metabolic profile of SftpcI73T AT2 cells at 3 key time points (7,14, and 28 days) post tamoxifen induction corresponding to an initiation phase (0-7 days), development of a multicellular alveolitis (7-14 days), and transition to spontaneous fibrotic remodeling (14-28 days). Endpoints included assessment of intracellular ATP levels, TCA cycle intermediates, lactate, mitochondrial copy number, mitochondrial function, and mitochondrial dynamics. Transcriptional changes underlying the acquired metabolic program were assessed via bulk AT2 RNA-sequencing with key proteins validated by immunoblotting. We identified significant alterations to AT2 metabolism and mitochondrial function beginning in the early inflammatory phase and sustained though fibrogenesis which included: (i) early and significant transcriptional changes reflective of glycolytic reprograming and alterations in PPARg/PGC1a signaling; (ii) a novel metabolic profile marked by increased cellular ATP and accumulate TCA intermediates supporting the glycolytic phenotype; (iii) genomic, biochemical, and morphological changes indicative of impaired mitochondrial biogenesis and network dynamics suggesting sustained mitochondrial dysfunction during injury and fibrotic remodeling; (iv) transcriptional and histochemical evidence of a persistent, highly proliferative AT2 phenotype. In this preclinical model of IPF we observe an AT2 hyperproliferative state supported by glycolytic reprogramming and high ATP levels with features that recapitulate the classical Warburg effect observed in tumorigenesis. Together with observations of altered mitochondrial dynamics and function, these data suggest a significant role for metabolic dysregulation of the AT2 cell in IPF pathogenesis and present novel pathways for disease intervention.