Signal transduction pathways involved in the upregulation of cyclooxygenase-2 by 2,2',4,4'-tetrachlorobiphenyl

Polychlorinated biphenyls (PCBs) are ubiquitous, persistent environmental contaminants that affect a number of cellular systems, including neutrophils. PCBs can be divided into two broad classes based on the presence or absence of chlorine atoms in the ortho-position. Congeners that lack chlorines in the ortho-positions are coplanar and bind with high affinity to the aryl hydrocarbon receptor to induce changes in cellular function. Congeners containing chlorines substituted at the ortho-positions cannot attain a coplanar configuration and are, in general, poor ligands for the aryl hydrocarbon receptor. Among the effects caused by noncoplanar PCBs is the alteration in function of polymorphonuclear neutrophils. These alterations include increases in superoxide anion production, activation of phospholipase A2 and subsequent release of arachidonic acid (AA). Accordingly, the overall goal of the work outlined in this dissertation was to test the hypothesis that 2,2',4,4',-tetrachlorobiphenyl (2244-TCB) increases COX-2 expression in granulocytic HL-60 cells, and that this increase depends on activation of intracellular signaling pathways including arachidonic acid release, superoxide anion production and activation of the p38 mitogen-activated protein (MAP) kinase. 2244-TCB increased superoxide anion production, AA release and levels of COX-2 mRNA, protein and enzyme activity. Neither superoxide anion nor free AA was involved in the 2244-TCB-mediated increase in COX-2 mRNA. Nonetheless, the increases in AA occur at the same time as the increases in COX-2 protein, raising the possibility that the AA may be used by the enzyme for eicosanoid production. The 2244-TCB-mediated increases in COX-2 mRNA, protein and enzyme activity were prevented by pretreatment with inhibitors of p38 mitogen-activated protein (MAP) kinase. 2244-TCB also caused the nuclear levels of NF-kappaB and C/EBP beta to increase. These increases were also prevented by inhibition of p38 MAP kinase. Additionally, inhibition of NF-kappaB prevented the 2244-TCB-mediated increase in COX-2 mRNA. Inhibition of transcription with actinomycin D reduced the 2244-TCB-mediated increase in COX-2 mRNA. Inhibition of p38 MAP kinase before exposure to actinomycin D caused a further reduction in COX-2 mRNA levels. Taken together, these results suggest that the 2244-TCB-mediated upregulation of COX-2 involves increased transcription of the COX-2 gene via a pathway requiring p38 MAP kinase and NF-kappaB, as well as the possibility that p38 is increasing COX-2 mRNA by modulating mRNA stability.