The Role of Mitochondrial DNA in Mediating Alveolar Epithelial Cell Apoptosis and Pulmonary Fibrosis

Abstract

Convincing evidence has emerged demonstrating that impairment of mitochondrial function is critically important in regulating alveolar epithelial cell (AEC) programmed cell death (apoptosis) that may contribute to aging-related lung diseases, such as idiopathic pulmonary fibrosis (IPF) and asbestosis (pulmonary fibrosis following asbestos exposure). The mammalian mitochondrial DNA (mtDNA) encodes for 13 proteins, including several essential for oxidative phosphorylation. We review the evidence implicating that oxidative stress-induced mtDNA damage promotes AEC apoptosis and pulmonary fibrosis. We focus on the emerging role for AEC mtDNA damage repair by 8-oxoguanine DNA glycosylase (OGG1) and mitochondrial aconitase (ACO-2) in maintaining mtDNA integrity which is important in preventing AEC apoptosis and asbestos-induced pulmonary fibrosis in a murine model. We then review recent studies linking the sirtuin (SIRT) family members, especially SIRT3, to mitochondrial integrity and mtDNA damage repair and aging. We present a conceptual model of how SIRTs modulate reactive oxygen species (ROS)-driven mitochondrial metabolism that may be important for their tumor suppressor function. The emerging insights into the pathobiology underlying AEC mtDNA damage and apoptosis is suggesting novel therapeutic targets that may prove useful for the management of age-related diseases, including pulmonary fibrosis and lung cancer.

Keywords: Sirtuin 3; alveolar epithelial cell; mitochondrial DNA damage; oxidative stress; pulmonary fibrosis.

Figure 1

Hypothetical model whereby mtDNA damage induces diverse degenerative diseases. MtDNA damage and mutation can be induced by cell stress from environmental particulates and/or DNA abnormality. MtDNA damage/mutation cause mitochondrial dysfunction reducing bioenergeneric metabolism that can promote degenerative diseases, metabolic dysfunction, aging, apoptosis and cancer. Red-up arrow, increase; red-down arrow, decrease.

Figure 2

Proposed model of mtDNA damage in mediating AEC intrinsic apoptosis and pulmonary fibrosis. Oxidative stress-induced mtROS induces mtDNA damage by decreasing SIRT3, ACo-2 and mtOGG1 in AEC. MtDNA damage causes a defective ETC that can promote mitochondrial dysfunction, AEC apoptosis, and pulmonary fibrosis. I, II, III, IV, four different complex of ETC in mitochondria. Red-up arrow, increase; red-down arrow, decrease.

Figure 3

SIRT3 mitochondrial protein deactylation targets modulate cell metabolism and longevity. Red-up arrow, increase.