How Aging and Oxidative Stress Influence the Cytopathic and Inflammatory Effects of SARS-CoV-2 Infection: The Role of Cellular Glutathione and Cysteine Metabolism

Abstract

SARS-CoV-2 infection can cause a severe respiratory distress syndrome with inflammatory and thrombotic complications, the severity of which increases with patients' age and presence of comorbidity. The reasons for an age-dependent increase in the risk of severe COVID-19 could be many. These include defects in the homeostatic processes that control the cellular redox and its pivotal role in sustaining the immuno-inflammatory response to the host and the protection against oxidative stress and tissue degeneration. Pathogens may take advantage of such age-dependent abnormalities. Alterations of the thiol redox balance in the lung tissue and lining fluids may influence the risk of infection, and the host capability to respond to pathogens and to avoid severe complications. SARS-CoV-2, likewise other viruses, such as HIV, influenza, and HSV, benefits in its replication cycle of pro-oxidant conditions that the same viral infection seems to induce in the host cell with mechanisms that remain poorly understood. We recently demonstrated that the pro-oxidant effects of SARS-CoV-2 infection are associated with changes in the cellular metabolism and transmembrane fluxes of Cys and GSH. These appear to be the consequence of an increased use of Cys in viral protein synthesis and to ER stress pathway activation that interfere with transcription factors, as Nrf2 and NFkB, important to coordinate the metabolism of GSH with other aspects of the stress response and with the pro-inflammatory effects of this virus in the host cell. This narrative review article describes these cellular and molecular aspects of SARS-CoV-2 infection, and the role that antivirals and cytoprotective agents such as N-acetyl cysteine may have to limit the cytopathic effects of this virus and to recover tissue homeostasis after infection.

Keywords: SARS-CoV-2; aging; cellular redox; cytokines; glutathione; inflammation; lung diseases; oxidative stress; thiols.

Figure 1

SARS-CoV-2 infection interferes with the bidirectional flux of cellular thiols (cystine-cysteine cycle) and glutathione metabolism. Cells actively control their redox state and the redox homeostasis of the extracellular environment, synthesizing the tripeptide glutathione and maintaining it in its reduced form (GSH). This latter process relies on the glycolytic activity of the Pentose Phosphate Shunt (PPP) that supports reducing equivalents (under the form of the pyridine coenzyme NADPH) to GSH-reductase (GR). The de novo biosynthesis process of glutathione depends on the cellular availability of Cys. Adequate Cys levels are provided through the diet and the trans-sulfuration pathway and reach the cells by means of the cystine-cysteine cycle. This cycle and the efflux mechanisms of cellular thiols maintain GSH, Cys, and the other LMW thiols available in the extracellular milieu for the reaction with protein thiols (left insert) and for the intercellular metabolism of GSH. At the same time, this cycle participates in the homeostatic control of the redox gradient existing between cells and extracellular fluids of tissues (right insert). The viral replication process implies an increased incorporation of cellular Cys in the viral proteins, thus reducing its availability for the biosynthesis of GSH and the redistribution of cellular thiols to the extracellular compartment. Aging conspires with SARS-CoV-2 infection to promote these changes in the metabolism of thiols, which may increase the risk of degenerative complications and severe COVID-19.

Figure 2

Effects of SARS-CoV-2 infection on the Cys/GSH system and its interplay with signal transduction and transcriptional mechanisms that sustain the pro-inflammatory and cytopathic outcomes of this infection in the host cell. Key steps in the molecular and cellular effects that link these events together are identified from experimental data obtained in Vero E6 epithelial cells [8,13]: 1. signal transduction effects are produced post-infection by the induction of ER stress that follows the remodelling of this subcellular compartment by the abnormal synthesis and assembly of viral particles; 2. redox abnormalities are induced by the increased incorporation of Cys into viral proteins that inhibits the synthesis of cellular GSH (2a), and by the increased flux of cellular ROS sustained in the different cellular organelles by the viral replication, as ER and mitochondria. These alterations include effects on the extracellular redox by the reduced efficacy of redox buffering systems that rely on the efflux of GSH and other cellular thiols (2b); 3. transcriptional effects are induced by the effect of ER stress signalling and a defective metabolism of cellular GSH, which modulate the activity of transcriptional proteins, such as NFkB and Nrf2, during the viral replication cycle; 4. cellular effects develop as a consequence of these events. In fact, GSH metabolism inhibition, ER stress signalling and the activation of inflammatory genes may converge in the receptor-dependent and protein kinase-mediated activation of inflammatory and death pathways of the host cell, ultimately sustaining the cytopathic effect of the viral infection. These changes and the inhibition of GSH metabolism may also help the virus to evade innate immunity mechanism of the host cell. Red connectors highlight the main effects of SARS-CoV-2 infection on cellular pathways.

Figure 3

Age-dependent changes of tissue redox and risk of severe COVID-19. Aging promotes a progressive decline of physiological processes that modulate the tissue redox, also identified with the all-encompassing definition of “adaptive homeostasis” [29]. These processes include the homeostatic control of the levels and redox buffering properties of cellular thiols. Defects in these fundamental components, increase the risk of oxidative stress and immune dysfunction in the diseased host, ultimately reiterating the risk of SARS-CoV-2 infection and complications. In the infected cell, a decreased efficiency of the cellular metabolism and redox function of GSH is observed [8] (marked with the symbol * and discussed in Section 4) and this interferes with the redox balance of extracellular thiols, further sustaining the risk of infections and the age-related defect of thiol metabolism in the infected tissue and in the whole organism. SARS-CoV-2 infection also sustains stress kinase signalling and NFkB activation, thus stimulating the production of inflammatory cytokines in the host cell [13,58]. These pro-inflammatory events observed at the cellular level conspire with the redox defects of tissues to increase the risk of immune dysfunction and inflammatory complications that characterize the comorbidity of severe COVID-19.

Figure 4

Signal transduction, transcriptional and metabolic effects of SARS-CoV-2 infection in Vero E6 cells. The time-course of the main effects shown in the chart include the modulation of stress response and apoptotic pathways, inflammatory genes and innate immunity of the host cell that were identified according to [8,13]. The reported time-points were 6 h post infection (hpi) and 24 hpi; these indicate the earliest and late phase of the viral replication process in which the cytopathic effect (CPE) of the virus develops, respectively. Nelfinavir (Nel) and N-acetyl cysteine (NAC) reverse the reprogramming effect of SARS-CoV-2 infection on the transcriptional and metabolic parameters of the host cell, acting early during the viral replication process to prevent ER reprogramming and Cys consumption for viral protein synthesis (shown in Figure 1 and Figure 2).