Functional expression of system x_c⁻ is upregulated by asbestos but not crystalline silica in murine macrophages

Abstract

Context: Inhalation of asbestos or silica is associated with chronic and progressive diseases, including fibrosis, cancer, and increased risk of systemic autoimmunity. Because there is a need for treatment options for these diseases, a better understanding of their mechanistic etiologies is essential. While oxidative stress in macrophages is an early consequence of these exposures, it may also serve as a signaling mechanism involved in downstream immune dysregulation. The system x_c⁻ exchange protein is induced by oxidative stress, and exchanges equimolor levels of extracellular cystine for intracellular glutamate. Cystine is subsequently reduced to cysteine, the rate-limiting precursor for glutathione synthesis.

Objective: As the primary transporter responsible for cystine/glutamate exchange on macrophages, system x_c− was hypothesized to be inducible in response to asbestos and silica, and to increase viability through protection from oxidative stress.

Results: When challenged with amphibole asbestos, but not crystalline silica, RAW 264.7 macrophages increased expression of xCT and the rate of cystine/glutamate exchange in sodium-free conditions. This upregulation was prevented with N-acetylcysteine, implicating oxidative stress. Cystine protected the macrophages from asbestos-induced oxidative stress and cell death, supporting the hypothesis that imported cystine was used for synthesis of cellular antioxidants. System x_c⁻ inhibitors, glutamate and S-4-carboxyphenylglycine ((S)-4-CPG), significantly increased oxidative stress and cell death of asbestos-treated macrophages.

Conclusion: System x_c⁻ plays a critical role in survival of macrophages exposed to asbestos, but not silica. These data demonstrate a very early difference in the cellular response to these silicates that may have important downstream implications in the pathologic outcome of exposure.

Keywords::

• Glutamate
• cystine
• glutathione
• asbestos
• silica

Acknowledgements

The Authors gratefully acknowledge the Center for Structural and Functional Neuroscience (P20 RR015583 (CoBRE) and the Center for Environmental Health Sciences P20 NCRR 017670 (CoBRE) at the University of Montana for their support of this project. The work was also supported by these grants from NIH: INBRE P20 RR016454, R15 ES018986-01.

Declaration of interest

The Authors report no declarations of interest.