RTP801 Is Required for Ceramide-Induced Cell-Specific Death in the Murine Lung

Abstract

Key host responses to the stress induced by environmental exposure to cigarette smoke (CS) are responsible for initiating pathogenic effects that may culminate in emphysema development. CS increases lung ceramides, sphingolipids involved in oxidative stress, structural alveolar cell apoptosis, and inhibition of apoptotic cell clearance by alveolar macrophages, leading to the development of emphysemalike pathology. RTP801, a hypoxia and oxidative stress sensor, is also increased by CS, and has been recently implicated in both apoptosis and inflammation. We investigated whether inductions of ceramide and RTP801 are mechanistically linked, and evaluated their relative importance in lung cell apoptosis and airspace enlargement in vivo. As reported, direct lung instillation of either RTP801 expression plasmid or ceramides in mice triggered alveolar cell apoptosis and oxidative stress. RTP801 overexpression up-regulated lung ceramide levels 2.6-fold. In turn, instillation of lung ceramides doubled the lung content of RTP801. Cell sorting after lung tissue dissociation into single-cell suspension showed that ceramide triggers both endothelial and epithelial cell apoptosis in vivo. Interestingly, mice lacking rtp801 were protected against ceramide-induced apoptosis of epithelial type II cells, but not type I or endothelial cells. Furthermore, rtp801-null mice were protected from ceramide-induced alveolar enlargement, and exhibited improved static lung compliance compared with wild-type mice. In conclusion, ceramide and RTP801 participate in alveolar cell apoptosis through a process of mutual up-regulation, which may result in self-amplification loops, leading to alveolar damage.Keywords: emphysema; sphingolipids; apoptosis; cigarette smoke; stress responseThe abnormal enlargement of distal airspaces in the lungs, which occurs through the destruction of the alveolar walls, is the hallmark of emphysema, a disease included in the chronic obstructive pulmonary disease (COPD) spectrum. There is an increased need to identify molecular mechanisms that link the environmental stress induced by cigarette smoke (CS) and air pollution, which are the main causes of alveolar destruction and emphysema in COPD. COPD is a prevalent disease, which carries high morbidity and mortality rates. Although removal of the etiological agent, most commonly the exposure to CS, is an important intervention (1), the ongoing tissue destruction may progress even after CS cessation (2). There is ample evidence that mechanisms of amplification of lung injury lead to destruction of alveolar walls and persistence of inflammatory systemic and lung responses (3, 4). Ceramide and RTP801 have been identified as potential signaling relays that are engaged early by CS exposure, which may interact and therefore amplify alveolar wall injury in emphysema (5-7). Because both ceramide and RTP801 are involved in apoptosis, we sought to investigate if ceramide and RTP801 are mechanistically linked in vivo.RTP801, also known as REDD1, is a stress response protein and si...

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Lung Injury and Lung Cancer Caused by Cigarette Smoke-Induced Oxidative Stress: Molecular Mechanisms and Therapeutic Opportunities Involving the Ceramide-Generating Machinery and Epidermal Growth Factor Receptor

“…Src, the first proto-oncogene described, has previously been observed to be redox-sensitive, becoming activated by exogenous H 2 O 2 (61) and CS (58).Owing to this shared stimuli between Src activation and CS-induced ceramide generation in HAE cells, unanticipated evidence was recently provided, demonstrating that Src activation upon CS/oxidative stress exposure controls nSMase2 function and subsequent ceramide generation at the plasma membrane of HAE cells (29), an accepted hallmark of apoptosis induction and lung injury (56,59,60,114,190).…”

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