Molecular basis for adaptive responses during chemically induced hepatotoxicity.

Prior exposure to toxic xenobiotics can elicit changes within liver that confer resistance upon subsequent reexposure— a phenomenon sometimes referred to as autoprotection (Dalhoff et al., 2001; Thakore and Mehendale, 1991). Classically, the mechanisms underlying such responses have been assigned to either of two categories. First, so-called toxicodynamic alterations involve molecular changes within hepatocytes that counteract the deleterious biochemical events whereby hepatotoxicants induce toxicity. An example of this phenomenon is the induction of various antioxidant pathways in rodent liver on exposure to such diverse prooxidants as heavy metals, plasticizers, and hepatotoxic drugs (Bando et al., 2005; Nicholls-Grzemski et al., 2000; O’Brien, et al., 2000). Second, dispositional or toxicokinetic alterations decrease the delivery of chemicals to the liver or otherwise diminish the intracellular concentrations of xenobiotics attained in hepatocytes. In the past, the mechanisms underlying this second class of adaptive changes were often largely unknown. Together with other recent work from the Manautou group and other laboratories, the report by Aleksunes et al. in this issue of Toxicological Sciences shows how our understanding of the molecular basis for these complex phenomena is steadily improving. Studying changes to the abundance of mRNA transcripts during chemical exposure holds promise for illuminating the biochemical mechanisms underlying a wide range of toxicological syndromes (Hayes and Bradfield, 2005). Use of cDNA or oligo microarrays to characterize thousands of mRNA transcripts simultaneously is a common approach, although technologies that focus on gene subsets of known toxicological relevance present a useful alternative strategy. In a companion study to the present work, Aleksunes et al. (2005) used branched DNA signal amplification to quantify mRNAs for one dozen membrane transporters in the livers of mice 6, 24, and 48 h after they received moderately hepatotoxic doses of acetaminophen or carbon tetrachloride (Aleksunes et al., 2005). These classic hepatoxicants induce liver cell injury following conversion to their respective reactive intermediates, N-acetyl-p-quinoneimine and the trichloromethyl radical. Suggesting an attempt by hepatocytes to minimize uptake of noxious substances from the sinusoidal circulation, toxic doses of acetaminophen decreased mRNA transcripts for various basolateral uptake transporters including organic anion-transporting polypeptides (Oatp1a1, Oatp1b2) and the sodium/ taurocholate-cotransporting peptide (Ntcp) (Aleksunes et al., 2005). A concurrent effort by acetaminophenand CCl4intoxicated hepatocytes to increase compound efflux was indicated by the elevated transcripts for genes involved in canalicular transport into bile (Mrp2) and also for returning chemicals to the general circulation (the basolateral efflux transporters Mrp 1, 3, 4) (Aleksunes et al., 2005). Although the power of transcript-profiling is readily apparent, use of this technology to characterize gene profiles in rodent models of chemical toxicity can be confounded by such technical challenges as irreproducibility, unwanted signal variation, and noise (Fielden and Zacharewski, 2001). In a similar manner, the results obtained by Aleksunes et al. (2005) were plagued by an unwanted degree of inconsistency. For example, while hepatic levels of the Oatp1a1 mRNA transcript were strongly suppressed ( 90%) in mice 48 h after they received 400 mg/kg acetaminophen, this transcript was unaltered by this dose at the 24 h time point, despite the fact that a lower dose of acetaminophen (300 mg/kg) reduced this marker by 60% at the earlier time (Aleksunes et al., 2005). Furthermore, in mice that received the top dose of acetaminophen, the Mrp2 and Mrp3 genes were strongly induced at 6 and 48 h, yet were at comparable levels to controls at an intermediate time point (i.e., 24 h, see Fig. 4 in Aleksunes et al., 2005). Together with the more fundamental concern of 1 To whom correspondence should be addressed at Pharmacology Unit, School of Medicine and Pharmacology, QEII Medical Centre, M-Block, Monash Ave, Nedlands, WA 6009, Australia. Fax: 61-8-9346 3469. E-mail: pburcham@meddent.uwa. edu.au.