Sequential Activation of Protein Kinase C Isoforms by Organic Dust Is Mediated by Tumor Necrosis Factor

Abstract

Dust samples collected from Nebraska swine confinement facilities (hog dust extract [HDE]) are known to elicit proinflammatory cytokine release from human bronchial epithelial (HBE) cells in vitro. This response involves the activation of two protein kinase C (PKC) isoforms: PKCα and PKCε. Experiments were designed to investigate the relationship between the two isoenzymes and the degree to which each is responsible for cytokine release in HBE. Experiments also examined the contribution of TNF-α to IL-6 and IL-8 release. PKCα and PKCε activities were inhibited using isoform-specific pharmacologic inhibitors and genetically modified dominant-negative (DN) expressing cell lines. Release of the proinflammatory cytokines IL-6, IL-8, and TNF-α was measured and PKC isoform activities assessed. We found that HDE stimulates PKCα activity by 1 hour, and within 6 hours the activity returns to baseline. PKCα-specific inhibitor or PKCαDN cells abolish this HDE-mediated effect. Both IL-6 and IL-8 release are likewise diminished under these conditions compared with normal HBE, and treatment with TNF-α–neutralizing antibody does not further inhibit cytokine release. In contrast, PKCε activity was enhanced by 6 hours after HDE treatment. TNF-α blockade abrogated this effect. HDE-stimulated IL-6, but not IL-8 release in PKCεDN cells. The concentration of TNF-α released by HDE-stimulated HBE is sufficient to have a potent cytokine-eliciting effect. A time course of TNF-α release suggests that TNF-α is produced after PKCα activation, but before PKCε. These results suggest a temporal ordering of events responsible for the release of cytokines, which initiate and exacerbate inflammatory events in the airways of people exposed to agricultural dust.