Mechanisms for chromium-induced signaling changes in pulmonary epithelial cells

Both human epidemiological and animal studies have associated inhalation of chromium(VI) (Cr(VI)) with increased incidence of pulmonary disease. This thesis examined the hypotheses that Cr(VI) activates cell signaling through kinase cascades and that chromium-stimulated signaling changes the susceptibility of lung cells to injury by producing prolonged changes in transcription factor activity. In A549 cells, Cr(VI) selectively activated c-Jun N-terminal kinase (JNK) and two members of the Src family of kinases (SFK). Activation of the SFKs, Lck and Fyn, by Cr(VI) appeared to be mediated through direct interactions with the kinase protein. Pre-treatment with the antioxidant catalase had little impact on the activation of Fyn by Cr(VI). However, N-acetylcysteine, a donator of thiol groups, attenuated activation of both Fyn and JNK. These data indicated that Cr(VI) activation of the SFKs and mitogen-activated protein kinases (MAPKs) did not require reactive oxygen generation, but may have involved reaction with protein thiols. Activation of SFK and MAPKs were also observed in mice receiving intranasal Cr(VI) over three days and then allowed to recover for 21 days. This suggested that Cr(VI) caused persistent changes in lung cell signaling. Chronic exposure of BEAS-2B cells to Cr(VI) caused a Lck-mediated activation of STAT3beta. This may be important given the role of STAT3 in pulmonary injury, reduced inflammation, and, if activity is prolonged, lung cancer (1). Heme oxygenase-1 (HO-1) is also important in preventing lung injury from oxidative and environmental stresses. The current study showed that Cr(VI) decreased HO-1 mRNA levels in vivo and inhibited arsenite-stimulation of HO-1 in cultured BEAS-2B cells. The mechanism by which Cr(VI) attenuated the arsenite-invoked response occurred at the level of transactivation. Cr(VI) inhibited transactivation by the antioxidant response element, which is stimulated by arsenite. These data indicated that Cr(VI) chronically activated cell signaling cascades, which altered nuclear levels of specific transcription factors. As a result of shifting transcription factor activation, transcription of a cytoprotective gene (HO-1) was inhibited by Cr(VI). These chronic changes in cell signaling and protective responses may underlie the pathophysiology of Cr(VI)-induced pulmonary disease.