Functional expression of system x(c)- is upregulated by asbestos but not crystalline silica in murine macrophages.
BACKGROUND
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.


CONCLUSIONS
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.
Department of Biological Sciences, Idaho State University, Pocatello, ID 83209, USA. pfaujean@isu.edu
Jean C
JC
Pfau
Todd
T
Seib
Jason J
JJ
Overocker
Jeremy
J
Roe
Aaron S
AS
Ferro
D027182:Amino Acid Transport System y+ / Q000096:biosynthesis*; D000818:Animals; D017636:Asbestos, Amphibole / Q000633:toxicity*; D017638:Asbestos, Crocidolite / Q000633:toxicity*; D001692:Biological Transport; D017610:Calcium Compounds / Q000633:toxicity; D018929:Cell Culture Techniques; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D003545:Cysteine / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005434:Flow Cytometry; D018698:Glutamic Acid / Q000378:metabolism; D005978:Glutathione / Q000378:metabolism; D008264:Macrophages / Q000187:drug effects* / Q000378:metabolism; D051379:Mice; D021122:Protein Subunits; D017640:Silicates / Q000633:toxicity; D012822:Silicon Dioxide / Q000633:toxicity*; D015854:Up-Regulation
D027182:Amino Acid Transport System y+; D017636:Asbestos, Amphibole; D017610:Calcium Compounds; D021122:Protein Subunits; D017640:Silicates; C492255:Slc7a11 protein, mouse; D017638:Asbestos, Crocidolite; D018698:Glutamic Acid; D012822:Silicon Dioxide; D005978:Glutathione; D003545:Cysteine; C031293:calcium silicate