Alveolar Epithelial Type II Cell Metabolism in Health, Hypoxia & Disease.

摘要

Alveolar epithelial type II (ATII) cells constitute 50% of cells composing the alveolar epithelium and are essential to proper lung function. They are the primary producers of pulmonary surfactant, serve as progenitors capable of rapid self-renewal and differentiation, and play roles in immunity and fluid homeostasis, all of which require considerable energy investment. Given their many ATP-demanding functions, ATII cells are expected to be highly metabolically active; however, little is known about the fundamental metabolism of this critical cell type.;ATII cells are normally exposed to uniquely high oxygen concentrations. However, numerous lung diseases including idiopathic pulmonary fibrosis (IPF) lead to pulmonary hypoxia. The role of hypoxia has been extensively investigated in pathologies like cancer and heart disease but has received far less attention in pulmonary disease. Recent findings of lactic acid build-up in IPF lung suggest a role for altered cell metabolism, potentially related to hypoxia.;We investigate the hypothesis that hypoxia alters ATII metabolism, and that similar metabolic change occurs in IPF lung. ATII metabolism was characterized under ambient versus 1.5% O2. Additionally, to understand possible contributions of ATII to lactic acid build-up in disease, the ability of healthy cells to both produce and consume lactate was assessed. Extracellular flux analysis was performed to measure glycolytic and mitochondrial metabolism in a model cell line and ATII isolated from mouse and human lungs, and flux experiments were correlated with metabolite measurements and gene and protein expression.;This work demonstrates that ATII cells are highly metabolic and dependent on mitochondrial metabolism. Hypoxia suppresses ATII mitochondrial metabolism without concurrent change in glycolysis, despite enhanced enzyme expression. Similarly, ATII from IPF patient lung showed low mitochondrial function compared to control, while glycolytic output occurred at near-control rates or higher, generating a highly glycolytic phenotype. In both hypoxia-treated and IPF-derived ATII, reserve mitochondrial capacity was maintained. Additionally, we demonstrate that ATII consume lactate and that this ability is limited by hypoxia. Based on our findings, we propose a hypothetical model by which metabolic cooperation between ATII and other cell types is altered in IPF to favor enhanced lactic acid generation and reduced consumption.