2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced reduction of adenosine deaminase activity in vivo and in vitro

Uric acid stones in the urinary bladder of aryl hydrocarbon receptor (AhR) knockout mice

The aryl hydrocarbon receptor (AhR) knockout mice raised in the laboratory of Fujii-Kuriyama have been under investigation for several years because of the presence in their urinary bladder of large, yellowish stones. The stones are composed of uric acid and become apparent in the bladders as tiny stones when mice are 10 wk of age. By the time the mice are 6 mo of age, there are usually two or three stones with diameters of 3-4 mm. The urate concentration in the serum was normal but in the urine the concentration was 40-50 mg/dL, which is 10 times higher than that in the WT littermates. There were no apparent histological pathologies in the kidney or joints and the levels of enzymes involved in elimination of purines were normal. The source of the uric acid was therefore judged to be from degradation of nucleic acids due to a high turnover of cells in the bladder itself. The bladder was fibrotic and the luminal side of the bladder epithelium was filled with eosinophilic granules. There was loss of E-cadherin between some epithelial cells, with an enlarged submucosal area filled with immune cells and sometimes invading epithelial cells. We hypothesize that in the absence of AhR there is loss of detoxifying enzymes, which leads to accumulation of unconjugated cytotoxins and carcinogens in the bladder. The presence of bladder toxins may have led to the increased apoptosis and inflammation as well as invasion of epithelial cells in the bladders of older mice.

On the significance of the role of cellular stress response reactions in the toxic actions of dioxin

Dioxin is known to cause many toxic effects that vary greatly in different tissues, ages, genders, and species. In this review, an attempt has been made to sort out major signaling pathways involved in the expression of the toxicities of dioxin. The major strategy adopted in analyzing its major signaling pathways is to view the toxic actions of dioxin as the result of the Ah receptor-mediated expression of a major cellular emergency stress response signal. Evidence pointing to the similarities between the symptoms of poisoning by dioxin and those produced by chronic administration of typical stressors, particularly lipopolysaccharides (LPS), bacterial endotoxins, has been assembled and analyzed. The common symptoms are wasting syndrome, atherosclerosis, fatty liver, and thymic atrophy. On the other hand, oxidative stress caused by cytochrome P450 induction is one of the typical stresses of dioxin poisoning, but not LPS poisoning. One of the major means through which dioxin triggers stress responses via "stress-activated kinase pathways" is stimulation of the cellular production of cytokines/autocrines, particularly growth factors. In the case of hepatocytes for instance, transforming growth factor-alpha plays a pivotal role in the dioxin-induced activation of the epidermal growth factor receptor and the extracellular signal-related kinase pathway, which acts as a signal to suppress apoptosis induced by cellular stress. These observations as well as additional experimental data support the idea that one of the major functions of the Ah receptor could be the elicitation of cellular stress response reactions. Another key point in understanding the toxic action of dioxin is that, unlike other cases of stressors, dioxin signaling becomes chronically sustained because of its extreme persistence in the human body, its half-life of 7-10 years, and its selective accumulation in fatty target tissues.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes an Ah receptor-dependent and ARNT-independent increase in membrane levels and activity of p60(Src)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to affect various cellular activities including growth factor signal transduction, hormone responses, and cell differentiation. The purpose of this study was to examine more closely the very early effects of TCDD on protein tyrosine kinase activity, specifically p60(Src). We found that TCDD causes rapid changes in the plasma-microsomal membrane levels and activity of p60(Src) in Hepa 1c1c7, Hepa c4 cells, and SR3Y1 cells, a p60(v-Src) overexpressing cell line. Such cellular changes occur within 30 minutes after 10 nM TCDD treatment, as measured by Western blot analysis. TCDD's ability to increase p60(Src) levels was found to be: (1) dose-dependent, with an estimated EC(50) between 10(-10) and 10(-11) M TCDD; (2) Ah receptor (AhR)-dependent, since TCDD's effect was blocked by co-administration with 1 μM α-naphthoflavone, an AhR antagonist; and interestingly (3) ARNT-independent, since TCDD's effect was observed in Hepa c4 cells, an ARNT(-) mutant cell line. Since ARNT is a heterodimerization partner of the AhR required for binding of the ligand-activated AhR to dioxin-responsive elements on DNA in the nucleus to transactivate genes controlled by the AhR, an alternative mechanism for TCDD's action is discussed which does not require ARNT. Along with increased membrane levels of p60(Src), we observed a corresponding increase in the activity of a 60 kDa protein tyrosine kinase using two different kinase detection assays. This effect of TCDD was also found to be AhR-dependent, ARNT-independent, and independent of de novo protein synthesis since cycloheximide was unable to completely abolish TCDD's effect. The present findings provide a potentially important mechanism by which TCDD can alter cell growth and differentiation.