Chromium compounds are widely used industrial chemicals and are also recognized human carcinogens. In the occupational setting, hexavalent chromium Cr(VI) enters the human body via inhalation and dermal absorption. In the environmental setting it can be absorbed via the ingestion of contaminated water. Recent reports indicate that chronic exposure to Cr(VI) may increase the risk for developing lung diseases among susceptible individuals. Although multiple intracellular targets have been proposed for the toxicity of hexavalent chromium, the exact pathway remains unclear. Recent studies have suggested that some instances of chronic inflammation may contribute to the risk of developing lung cancer. Chronic exposure to Cr(VI) could activate several soluble biochemical mediators of inflammation, therefore it is important to examine the inflammatory response of normal healthy following exposure. We hypothesize that subchronic exposure to low doses of Cr(VI) induces cellular damage and activates mediators of inflammation. This could be, in part, due to enhanced production of reactive oxygen species (ROS) leading to the activation macrophages. The macrophages may trigger transcriptional changes of TNFα, nuclear factor NF-κB, AP1 pathways and result in an elevated expression of the inducible form of cyclooxygenase (COX-2). In the present study, we addressed the question on (a) whether hexavalent chromium alters the architecture of normal lung cells causing cytotoxic effects at low doses and, (b) whether the positive entry of Cr(VI) into the cell nucleus alters the transcription of genes involved in the process of inflammation. Using normal human lung cells we demonstrate that a low dose of 10uM of Cr(VI) induces damage to the cell membrane, generates pinnocytic vesicles around the membrane, causes DNA fragmentation and enhances COX-2 expression. In a similar set of experiments, RNA was extracted from A549 lung cancer cells for Real Time RT-PCR analysis. Using specific primers, the gene transcripts show an increase in the level of expression of IL-6, IL-10, Cox-2, and TNF-alpha. However, there was no change in the expression of IL-8, IL-12, Cox-1, FGFR-1 and GADPH. Only the MMP-2 gene was down regulated by treatment with Cr(VI). Studies are in progress to determine the DNA binding activity of transcription factor NF-κB, AP1 and IL1 in normal and lung cancer cells. Conclusion: Normal and lung cancer cells exposed to low doses of Cr(VI) show enhanced expression of major pro-inflammatory factors compared to the control cells. Understanding the complex networks of mediators of inflammation and their regulation at the gene transcription level will provide better prevention strategies to reduce the risk of lung cancer among chromium workers.
[Fifth AACR International Conference on Frontiers in Cancer Prevention Research, Nov 12-15, 2006]