Arsenic (As) is a well established cause of cancer in humans, and increasing evidence indicates that As has deleterious effects on the immune system that are not directly related to carcinogenesis. However, the mechanisms of As toxicity remain poorly understood. Our laboratory previously reported decreased urinary levels of human beta-defensin-1 (HBD1) peptides in As-exposed individuals from two cross-sectional studies based in Nevada and Chile, and confirmed in vitro that As exposure suppressed HBD1 mRNA expression which is encoded by the DEFB1 gene. DEFB1 is constitutively expressed in epithelial tissues, plays a role in both the innate and adaptive branches of the immune system, and is implicated in anti-tumor immunity. Therefore, the objectives of this dissertation are to review the immunotoxicological effects of As, characterize the effects of As on DEFB1 gene and protein expression in relevant in vitro model systems, investigate the molecular mechanisms mediating these effects, and explore the influence of other metals on HBD1 levels.
A comprehensive review of the literature on the immune-related effects associated with As exposure in humans, animals and in vitro models reveals that chronic exposure to As can severely impair various aspects of immune function and consequently result in elevated risk of infections and chronic diseases. However, further investigation is needed to better understand the relationship between As exposure and the development of disease, and several recommendations are discussed to help bridge the gaps in knowledge.
The current research investigated the effects of As exposure on DEFB1 in cells derived from target tissues of toxicity using immortalized non-tumorigenic human HOK-16B keratinocytes and HK-2 kidney epithelial cells. DEFB1 mRNA levels were more abundant in HK-2 cells than in HOK-16B cells, and were suppressed by exposure to arsenite (AsIII) or monomethylarsonous acid (MMAIII), the postulated more toxic metabolite. The suppressive effect of AsIII and MMAIII treatments on DEFB1 transcript levels continued for several passages after removal of As. HBD1 peptide levels were significantly reduced following exposure to AsIII, but were not affected by treatment with lead, cadmium or chromium, suggesting that decreased HBD1 may be a specific response to As. Finally, AsIII treatment was found to suppress DEFB1 promoter activity, indicating that the inhibition of DEFB1 mRNA by As is likely due to transcriptional down-regulation. Taken together, the research presented here provides evidence that our previous findings of decreased urinary HBD1 levels are likely due to a direct effect of As on the kidney, and suggest a novel mechanism by which As exposure may promote cancer development.
This dissertation summarizes the known in vivo and in vitro effects of As on the immune system, characterizes the effects of As on DEFB1 using relevant cell culture models, and establishes DEFB1 as a potentially relevant biomarker of response to As. Future studies should address the role of DEFB1 inhibition in As immunotoxicity and carcinogenicity.