Abstract
The mitochondrial calcium uniporter (MCU) regulates metabolic reprogramming in lung macrophages and the progression of pulmonary fibrosis. Fibrosis progression is associated with apoptosis resistance in lung macrophages; however, the mechanism(s) by which apoptosis resistance occurs is poorly understood. Here, we found a marked increase in mitochondrial B-cell lymphoma-2 (Bcl-2) in lung macrophages from subjects with idiopathic pulmonary fibrosis (IPF). Similar findings were seen in bleomycin-injured wild-type (WT) mice, whereas Bcl-2 was markedly decreased in mice expressing a dominant-negative mitochondrial calcium uniporter (DN-MCU). Carnitine palmitoyltransferase 1a (Cpt1a), the rate-limiting enzyme for fatty acid β-oxidation, directly interacted with Bcl-2 by binding to its BH3 domain, which anchored Bcl-2 in the mitochondria to attenuate apoptosis. This interaction was dependent on Cpt1a activity. Lung macrophages from IPF subjects had a direct correlation between CPT1A and Bcl-2, whereas the absence of binding induced apoptosis. The deletion of Bcl-2 in macrophages protected mice from developing pulmonary fibrosis. Moreover, mice had resolution when Bcl-2 was deleted or was inhibited with ABT-199 after fibrosis was established. These observations implicate an interplay between macrophage fatty acid β-oxidation, apoptosis resistance, and dysregulated fibrotic remodeling.
Introduction
Pulmonary fibrosis is a chronic disease that consists of aberrant remodeling of lung tissue. Idiopathic pulmonary fibrosis (IPF) is the most common form of pulmonary fibrosis and has a high mortality rate within 3–5 years after diagnosis [1]. The currently approved medications for IPF have limited efficacy based on the absence of changes in quality of life or mortality [2, 3]. Thus, understanding the cellular and molecular mechanisms in the pathogenesis of IPF may lead to more effective therapies.
Both apoptosis and resistance to apoptosis are associated with fibrosis in multiple organ systems [4,5,6]. Increased alveolar epithelial cell (AEC) injury and apoptosis are proposed to be the initiating event in lung fibrosis [1]. In contrast, apoptosis resistance in fibroblasts is critical for aberrant lung remodeling in IPF [7]. Macrophages in chronic disease exhibit apoptosis resistance, and their prolonged survival is generally associated with disease progression [8, 9]. Moreover, conditional macrophage depletion attenuates models of liver and lung injury in vivo [10, 11]. Although prior data demonstrated cleaved caspase-3 was reduced in lung macrophages from IPF subjects [9], the exact mechanism by which this occurs has not been determined.
The B-cell lymphoma-2 (Bcl-2) family of proteins are critical regulators of programmed cell death. The pro-apoptotic Bid gene in AECs [12] and Bcl-2 expression in myofibroblasts are key factors in fibrosis development [13]. The pharmacologic inhibition of Bcl-2 reversed established skin fibrosis by inducing apoptosis of myofibroblasts in vitro [14]. Based on the decisive role of lung macrophages in orchestrating fibrosis development and progression [15, 16], the molecular mechanism(s) by which Bcl-2 is regulated in lung macrophages is critical to understand the pathogenesis of dysregulated fibrotic remodeling.
Metabolic reprogramming that entails fatty acid β-oxidation (FAO) and oxidative phosphorylation is a feature in macrophage activation in multiple diseases [17, 18]. The mitochondrial calcium uniporter (MCU) is known to enhance aerobic glycolysis in cancer [19, 20]; however, we recently showed that MCU regulated metabolic reprogramming to FAO, in part, by increasing expression and activity of Cpt1a [21]. Inhibition of Cpt1a and FAO pharmacologically has been used as a potential therapeutic by inducing apoptosis in breast cancer cells. The mechanism(s) by which the drug-induced apoptosis occurs has not been determined at the molecular level [22]. Prior evidence showed that apoptosis resistance in lung macrophages occurs in fibrosis development [9], but it is not known if MCU regulation of Cpt1a has a direct role in mediating apoptosis resistance in lung macrophages or if this regulation has an impact on the progression of fibrotic remodeling…..
