Nitric oxide inhibits dioxin action for the stimulation of Cyp1a1 promoter activity

Since it is known that hypoxia increases inducible nitric oxide synthase (iNOS) gene expression through the hypoxia responsive element, it was hypothesized that nitric oxide could be a mediator of hypoxia to inhibit Cyp1a1 promoter activity. In order to test this hypothesis, we have undertaken a study to examine the effects of hypoxia and nitric oxide on Cyp1a1 promoter activity in Hepa I cells. Mouse Cyp1a1 5' flanking DNA, 1.6kb, was cloned into pGL3 expression vector in order to construct pmCyp1a1-Luc. Hepa I cells were transfected with pmCyp1a1-Luc and were treated with 10(-9)M 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the presence or absence of various hypoxic agents such as 10(-6)-10(-4) cobalt chloride or 10(-6)-10(-4)M picolinic acid or 10(-6)-10(-4) M desferrioxamine. The luciferase activity of the reporter gene was measured from pmCyp1a1-Luc transfected Hepa I cell lysate which contains 2 microg total protein using luciferin as a substrate. Hypoxic agents such as cobalt chloride, picolinic acid, and desferrioxamine showed inhibition of luciferase activity that was induced by 10(-9) M TCDD treatment in a dose dependent manner. Concomitant treatment of 1 mM N(G)-nitro-l-arginine with 10(-6)-10(-4) M cobalt chloride or 10(-6)-10(-4) M desferrioxamine or 10(-6)-10(-4) M picolinic acid or 10(-6)-10(-4) M sodium nitroprusside recovered luciferase activity from the TCDD induced luciferase activity that was inhibited by hypoxic agents. These data demonstrated that nitric oxide might be a mediator of iron chelating agents and hypoxic agents to inhibit dioxin induced Cyp1a1 promoter activity in Hepa I cells.