Effects of phospholipid surfactant on apoptosis induction by respirable quartz and kaolin in NR8383 rat pulmonary macrophages

Apoptosis was measured in rat alveolar macrophage NR8383 cells challenged i n vitro with respirable quartz or kaolin dust and with the dusts pretreated with dipalmitoyl phosphatidylcholine (DPPC) to model conditioning of respi red dusts by interaction with a primary phospholipid component of pulmonary surfactant. Quartz dust is known to induce apoptosis in vitro and in vivo. For this study, quartz and kaolin were compared as dusts of similar cytoto xicity in some in vitro assays but of differing pathogenic potential: quart z can cause significant pulmonary fibrosis while kaolin generally does not. NR8383 cells exposed to native quartz at concentrations from 50 to 400 mug /ml for 6 h showed a dose-dependent increase in apoptosis measured by the T dT-mediated dUTP-fluorescein nick end labeling (TUNEL), cell death ELISA, a nd DNA ladder formation assays, while native kaolin induced significant res ponse only at the higher concentrations and only in the TUNEL and ELISA ass ays. For cell challenge from 6 h to 5 days at 100 mug/ml of dust, quartz wa s active at all times while kaolin was active only at 5 days. DPPC pre-trea tment suppressed quartz activity until 3 days and kaolin activity through 5 days. Cellular release of lactate dehydrogenase, measured in parallel expe riments to compare dust apoptotic and necrotic activities, indicated that c omponents of serum as well as surfactant may affect kaolin in vitro express ion of those activities.