Research Accomplishment Reports 2007

Dietary Antioxidants, NF-kB, and Carcinogenesis

H.P. Glauert
Department of Human Environmental Sciences

 

Project Description

We have examined possible mechanisms in the liver tumor promoting activity of PCBs and PBBs. In the first study we determined the effects of PCBs and PBBs on peroxisome proliferator activated receptor-alpha-associated enzyme activities or protein levels. None of the peroxisomal enzyme activities were significantly increased by any of the halogenated biphenyl congeners tested. CYP4A protein contents were not increased by any treatment. The two most acutely toxic halogenated biphenyls significantly diminished the liver content of CYP4A isoenzyme proteins and activities of the peroxisomal enzymes studied. Since a range of congeners with different biologic and toxicologic activities were selected for this study, it may be concluded that the polyhalogenated biphenyls do not induce peroxisome proliferation in the male rat but rather toxic members of this class of compounds down-regulate peroxisome-associated enzymes.

In the second study, we examined if Kupffer cells are important in the promoting activity of PCBs. We studied the effects of Kupffer cell inhibition by dietary glycine (an inhibitor of Kupffer cell secretory activity) in a rat two-stage hepatocarcinogenesis model using PCB-153 and PCB-77 as promoters. Diethylnitrosamine (DEN) was administered to female Sprague Dawley rats, which were then placed on an unrefined diet containing glycine (or casein as nitrogen control). The rats then received a total of 4 PCB injections. Glycine did not significantly affect the number or volume of altered hepatic foci. PCB-153 did not significantly increase the focal volume, but increased the number of foci per liver, but only in the rats not fed glycine; PCB-77 increased both the foci number and their volume in both glycine-fed and control rats. Glycine diminished hepatocyte proliferation in PGST-positive foci, but not in normal tissue. Overall these results do not support the hypothesis that dietary glycine inhibits the promoting activities of PCBs.

In the third study, we conducted a proteome analysis with protein extracts from normal mouse liver and from liver tumors which were induced by a single injection of DEN followed by multiple injections of PCB-77 or PCB-153, or corn oil as a control. Mouse liver tumors frequently harbor mutations in Ha-ras, B-raf, or Ctnnb1 (encoding beta-Catenin). Liver tumors were stratified into 2 classes: they were either mutated in Ctnnb1, and as a result positive for the marker glutamine synthetase (GS+; mainly PCB-153 treated mice), or they lacked Ctnnb1 mutations and were therefore GS-negative (GS-; mainly PCB-77 treated mice). Protein identification by mass spectrometry revealed 105 significantly deregulated proteins, 47 in GS+ and 58 in GS- tumors. Several of the identified enzymes could be assigned to fundamental metabolic or other cellular pathways with characteristically different alterations in GS+ and GS- tumors, such as ammonia and amino acid turnover, cellular energy supply, calcium homeostasis and deregulation of cytoskeletal proteins. Our data suggest that GS+ and GS- tumor cells show a completely different biology.

Impact

These studies show that PCBs likely do not exert their tumor promoting activities by activating PPAR-alpha or by increasing Kupffer cell activity. However, different PCBs were found to alter different metabolic pathways in liver tumors. Therefore prevention strategies for individuals exposed to PCBs likely should not be focused on altering PPAR-alpha activation or Kupffer cell activity. The alteration of the metabolic pathways influenced by PCBs or inhibition of PCB-induced NF-kB activation should be more effective strategies.

Publications

Robertson, L.W., I. Berberian, T. Borges, L.-C. Chen, C.K. Chow, H.P. Glauert, J.G. Filser and H. Thomas, 2007. Suppression of peroxisomal enzyme activities and cytochrome P-4504A isozyme expression by congeneric polybrominated and polychlorinated biphenyls. PPAR Res. 2007, Article ID 15481, 5 pages. http://www.hindawi.com/getarticle.aspx?doi=10.1155/2007/15481

Strathmann, J., K. Paal, C. Ittrich, E. Krause, K.E. Appel, H.P. Glauert, A. Buchmann, and M. Schwarz, 2007. Proteome analysis of chemically induced mouse liver tumors with different genotype. Proteomics 7:3318-3331. http://www3.interscience.wiley.com/cgi-bin/fulltext/115807007/PDFSTART

Bunaciu, R.P., J.C. Tharappel, H.-J. Lehmler, I. Korwel, L.W. Robertson, C. Srinivasan, B.T. Spear, and H.P. Glauert, 2007. The effect of dietary glycine on the hepatic tumor promoting activity of polychlorinated biphenyls (PCBs) in rats. Toxicology 239:147-155. http://dx.doi.org/10.1016/j.tox.2007.06.102