Title

Hepatic organic anion transporter expression following induction of hepatocellular proliferation in vivo

Date of Completion

January 2003

Abstract

Hepatocyte proliferation can be induced by either compensatory hyperplasia or by mitogenic stimulation. Compensatory hyperplasia occurs following loss of hepatocytes, which can be induced by partial hepatectomy or administration of a toxic but non-lethal dose of a hepatotoxin. Direct mitogens include growth factors and compounds such as clofibrate. Hepatocytes induced to proliferate by either of these two methods have an increased resistance to the toxic effects of certain hepatotoxins. The mechanisms for the resiliency of proliferating hepatocytes to hepatotoxins is currently unknown. One potential mechanism for this protective effect is an increase in the expression of ATP-dependent organic anion transporters that actively export compounds or their conjugated metabolites, such as Mrp2 and Mrp3. ^ Clofibrate treated mice had no change in liver Mrp2 mRNA expression. Initial studies of acetaminophen or carbon tetrachloride treated mice also showed no relative change in liver Mrp2 mRNA expression. Similarly, Wistar rats treated with either acetaminophen or carbon tetrachloride had no relative changes in liver Mrp2 mRNA expression as assessed over a period of time of up to 72 hours following treatment. Liver Mrp2 protein expression also did not change significantly following the same treatments. Next, expression of liver Mrp3 mRNA was assessed since this transporter has been shown to be inducible and was found to be increased by both acetaminophen and carbon tetrachloride. Both acetaminophen and carbon tetrachloride caused sufficient but non-lethal hepatic centrilobular necrosis at 24 and 48 hours following treatment, which had decreased in severity by 72 hours. Analysis of hepatocyte proliferation by PCNA immunohistochemical staining showed increased numbers of proliferating hepatocytes at 48 and 72 hours. ^ A natural mutant strain of Wistar rats deficient in Mrp2 protein, GY/TR rats (transporter deficient rats), were used as negative controls for the Mrp2 mRNA and protein expression analyses. ^ Hepatocyte proliferation induced by either mitogenic or compensatory stimulation does not affect Mrp2 mRNA and protein expression. However, hepatic compensatory proliferation does increase Mrp3 mRNA expression. Thus, Mrp2 is unlikely to be significantly involved in the mechanisms of protection in proliferating hepatocytes, whereas Mrp3 may have a role. ^