Induction of hepatic acute-phase protein transcripts: differential effects of acute and subchronic dimethylnitrosamine exposure in vivo

Acute phase proteins in animals

Acute phase proteins (APP) were first identified in the early 1900s as early reactants to infectious disease. They are now understood to be an integral part of the acute phase response (APR) which is the cornerstone of innate immunity. APP have been shown to be valuable biomarkers as increases can occur with inflammation, infection, neoplasia, stress, and trauma. All animals--from fish to mammals--have demonstrable APP, but the type of major APP differs by species. While the primary application of these proteins in a clinical setting is prognostication, studies in animals have demonstrated relevance to diagnosis and detection and monitoring for subclinical disease. APP have been well documented in laboratory, companion, and large animals. With the advent of standardized and automated assays, these biomarkers are available for use in all fields of veterinary medicine as well as basic and clinical research.

Lack of acute phase response in the livers of mice exposed to diesel exhaust particles or carbon black by inhalation

BACKGROUND

Epidemiologic and animal studies have shown that particulate air pollution is associated with increased risk of lung and cardiovascular diseases. Although the exact mechanisms by which particles induce cardiovascular diseases are not known, studies suggest involvement of systemic acute phase responses, including C-reactive protein (CRP) and serum amyloid A (SAA) in humans. In this study we test the hypothesis that diesel exhaust particles (DEP) - or carbon black (CB)-induced lung inflammation initiates an acute phase response in the liver.

RESULTS

Mice were exposed to filtered air, 20 mg/m3 DEP or CB by inhalation for 90 minutes/day for four consecutive days; we have previously shown that these mice exhibit pulmonary inflammation (Saber AT, Bornholdt J, Dybdahl M, Sharma AK, Loft S, Vogel U, Wallin H. Tumor necrosis factor is not required for particle-induced genotoxicity and pulmonary inflammation., Arch. Toxicol. 79 (2005) 177-182). As a positive control for the induction of an acute phase response, mice were exposed to 12.5 mg/kg of lipopolysaccharide (LPS) intraperitoneally. Quantitative real time RT-PCR was used to examine the hepatic mRNA expression of acute phase proteins, serum amyloid P (Sap) (the murine homologue of Crp) and Saa1 and Saa3. While significant increases in the hepatic expression of Sap, Saa1 and Saa3 were observed in response to LPS, their levels did not change in response to DEP or CB. In a comprehensive search for markers of an acute phase response, we analyzed liver tissue from these mice using high density DNA microarrays. Globally, 28 genes were found to be significantly differentially expressed in response to DEP or CB. The mRNA expression of three of the genes (serine (or cysteine) proteinase inhibitor, clade A, member 3C, apolipoprotein E and transmembrane emp24 domain containing 3) responded to both exposures. However, these changes were very subtle and were not confirmed by real time RT-PCR.

CONCLUSION

Our findings collectively suggest that Sap, Saa1 and Saa3 are not induced in livers of mice exposed to DEP or CB. Despite pulmonary inflammation in these mice, global transcriptional profiling of liver did not reveal any hepatic response following exposure by inhalation.

A critical assessment of some biomarker approaches linked with dietary intake

In this review many examples are given of the complexities involved in using some biomarkers in relation to assessing the effects of dietary exposure, when there is frequently a need to determine changes following long-term low level exposure to dietary components. These range from understanding why the biomarker might be valuable and how best it can be measured, to the pitfalls which can occur in the interpretation of data. Analytical technique is considered in relation to folate and selenium, and flavonoid and carotenoid species are used to illustrate how the metabolism of a compound may alter the validity or adequacy of a marker. Vitamin A is discussed in relation to the difficulties which can arise when there are several biomarkers that may be available to assess exposure to one nutrient. Vitamin B12 is discussed in relation to the dietary choices made by individuals. Possible interactions and the role of measuring total antioxidant capacity is considered in some detail. In contrast to most nutrients, there is a marked lack of biomarkers of either exposure or effect for most non-nutrients. The role of biological effect monitoring is considered for dietary contaminants, fumonisins and polyhalogenated aromatic hydrocarbons. Aflatoxins are discussed to exemplify food contaminants for which the biomarker approach has been extensively studied. Finally some compounds which are deliberately added to foods and some which appear as processing contaminants are each considered briefly in relation to the requirement for a biomarker of exposure to be developed.

Immunotoxicology: role of inflammation in chemical-induced hepatotoxicity

The liver, which is the major organ responsible for the metabolism of drugs and chemicals, is also the primary target organ for many toxic chemicals. Increasing evidence has indicated that inflammatory processes are intimately involved in chemical-induced hepatotoxic processes, and like other inflammatory diseases, such as autoimmunity, are responsible for producing mediators which can effect liver damage or repair. This review will summarize the authors' current understanding of how inflammatory processes influence hepatic pathology and repair following exposure to established hepatotoxic chemicals including carbon tetrachloride (CCl4), an industrial chemical, and acetaminophen (APAP), a widely used analgesic.

Molecular dissection of dimethylnitrosamine (DMN)-induced hepatotoxicity by mRNA differential display

Differential display reverse transcription-polymerase chain reaction (DDRT-PCR) was used to catalogue altered hepatic transcript expression during dimethylnitrosamine (DMN) exposure in vivo. Mice were administered DMN (1.5 or 5 mg/kg) or vehicle (phosphate-buffered saline) i.p. once daily for up to 7 days, and livers were collected 6 h post-injection. Total RNA was reverse transcribed and cDNA subsets were selectively amplified by PCR. DDRT-PCR products were fractionated on denaturing polyacrylamide gels, and differentially expressed bands were excised, reamplified, and subsequently cloned into a plasmid vector. This study identified 23 cDNAs that were induced and 25 cDNAs that were suppressed during DMN exposure. Altered expression during DMN exposure for cDNA clones was confirmed by Northern blotting, RNase protection, or in situ hybridization analyses. DNA sequence information indicated that four cDNAs suppressed during DMN exposure encode cytochrome P450 isoenzyme-cholesterol 7 alpha-hydroxylase (CYP7), a monokine, a myeloid cell differentiation protein, and mouse major urinary protein (MUP). We further observed a DMN-induced increase in transcripts for complement factor 3 (C3) and serum amyloid A (SAA). In contrast, the remaining differentially expressed transcripts detected by DDRT-PCR during DMN exposure demonstrated no similarity to sequences present in Genbank, suggesting that they may encode previously unreported gene products. In situ hybridization showed MUP transcripts to be expressed by hepatic centrilobular areas that undergo necrosis during subchronic DMN exposure. Thus, the utilization of DDRT-PCR has identified several differentially expressed hepatic mRNAs associated with various doses and stages of DMN exposure.