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

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 viva particle ex posure situation, The use of serum as a supplement to the growth medium of cultured cells is a widely accepted practice, However, little is known abou t whether the various serum proteins may interact with the surfaces of part icles, consequently altering their toxicity, inflammatory properties, or fi brogenicity, etc. observed in vivo. Using a murine alveolar type II cell li ne, MLE-15, we measured the early changes in various chemokine mRNA species following exposure of the cells to silica (cristobalite) in the presence o r absence of serum. Total mRNA was isolated and assayed using an RNase prot ection 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 id entified a specific serum component, apolipoprotein-Al (apo-Al), as a prote in in serum that binds selectively to silica, thus leading to the altered c hemokine response. We also found that ape-Al not only binds to silica but a lso binds to other nonfibrous and fibrous particles such as titanium dioxid e and asbestos, These results demonstrate the importance of culture conditi ons for modifying the outcome of an experiment when performing in vitro par ticle exposure studies. (C) 1999 Academic Press.