Silica binds serum proteins resulting in a shift of the dose-response for silica-induced chemokine expression in an alveolar type II cell line

Abstract

There is a growing concern about whether the myriad of culture conditions, cell lines, and doses of nonfibrous and fibrous particles used in vitro are truly representative of the complex environment of the in vivo particle exposure situation. The use of serum as a supplement to the growth medium of cultured cells is a widely accepted practice. However, little is known about whether the various serum proteins may interact with the surfaces of particles, consequently altering their toxicity, inflammatory properties, or fibrogenicity, etc. observed in vivo. Using a murine alveolar type II cell line, MLE-15, we measured the early changes in various chemokine mRNA species following exposure of the cells to silica (cristobalite) in the presence or absence of serum. Total mRNA was isolated and assayed using an RNase protection assay after 6 h of particle exposure. We observed that the addition of serum to the culture media reduced the in vitro silica-induced chemokine response (i.e., shift in the dose-response curve) in MLE-15 cells. Further, using Western blot analysis and protein sequencing techniques, we have identified a specific serum component, apolipoprotein-A1 (apo-A1), as a protein in serum that binds selectively to silica, thus leading to the altered chemokine response. We also found that apo-A1 not only binds to silica but also binds to other nonfibrous and fibrous particles such as titanium dioxide and asbestos. These results demonstrate the importance of culture conditions for modifying the outcome of an experiment when performing in vitro particle exposure studies.