Pericentral hepatocytes translocate hydrophilic bile acids more rapidly than hydrophobic ones

Zonation of multidrug resistance-associated protein 2 in rat liver after induction with dexamethasone

BACKGROUND AND AIM

The present study was aimed to evaluate the hepatic zonation of multidrug resistance-associated protein 2 (mrp2), an important drug transporter, and its potential changes during the induction of its expression by known inducer, dexamethasone (DEX).

METHODS

The hepatic expression of mrp2 was studied by immunohistochemistry with consequent quantification by measurement of integral optical densities of mrp2 staining in the periportal and perivenous areas of the liver acinus in control and DEX-pretreated rats (1 mg/kg daily per os for 4 days). Overall changes in mrp2 expression and function produced by DEX were monitored using Western blotting and an in vivo clearance study of endogenous-conjugated bilirubin, a mrp2 substrate.

RESULTS

In the control animals, a quantitative image analysis revealed the primary periportal localization of mrp2 within the liver acinus with the expression of mrp2 being 16.7-fold of that in the perivenous area. After DEX pretreatment, the expression of mrp2 increased, especially in the perivenous hepatocytes. The overall expression of mrp2 increased 3.2-fold in comparison with the control group. This observation was confirmed by Western blotting, which showed a 1.3-fold increase in the mrp2 protein after DEX pretreatment. The functional consequences of the induced mrp2 protein in the livers of the DEX-pretreated rats were demonstrated by the increased biliary excretion of conjugated bilirubin.

CONCLUSION

In conclusion, these results indicate the zonation of mrp2 protein expression primarily to periportal hepatocytes. The induction by DEX produced spatially disproportional changes with an increase in the mrp2 protein being most prominent in the perivenous hepatocytes.

Zonation of hepatic bile salt transporters

Pericentral and periportal hepatocytes differ in their capacity to eliminate and velocity of eliminating bile acids and other organic anions. We wonder whether differences in the distribution of anion transporters (ntcp [M77479], besp [NM_031760], mrp2 [NM_012833], oatp1 [NM_017111], oatp2 [NM_131906]) cause the differences in bile acid excretion. Therefore, we analyzed the distribution of these anion transporters in periportal and pericentral cells by immunohistology, their mRNA by quantitative PCR, and regulating nuclear factors (NF-kappaB, HNF1, HNF3, HNF4, FXR, PXR) by gel shift assay. We did not find any differences in nuclear factors or regarding the proteins that could explain the zonal differences in anion transport.

The 3alpha-hydroxy-steroid-dehydrogenase-mRNA and -protein are more prevalent in pericentral than in periportal hepatocytes

BACKGROUND

Under physiological conditions, de-novo synthesis and metabolism of bile acids are confined mainly to the pericentral zone of the liver acinus. In the rat, 3alpha-hydroxy-steroid-dehydrogenase (3alpha-HSD) is the major bile acid-binding protein. At the same time, this protein is involved in the de-novo synthesis and metabolism of bile acids. Because bile acid processing is greater in the pericentral than in the periportal region, we investigated whether 3alpha-HSD is more prevalent in the pericentral than in the periportal area.

DESIGN

We determined the 3alpha-HSD-protein and its mRNA in periportal and pericentral rat cells.

METHOD

Rat hepatocytes from the periportal or pericentral areas were isolated using the digitonin perfusion technique. For Northern blotting, a labelled 1.3-kb cDNA insert corresponding to the mRNA sequence of 3alpha-HSD was used. For Western blotting, a polyclonal rabbit antiserum against human 3alpha-HSD was used. Blots were quantified by densitometry using phosphoimaging.

RESULTS

The amounts of the 3alpha-HSD-protein and its mRNA were significantly greater in the pericentral than in the periportal cells.

CONCLUSIONS

The greater occurrence of 3alpha-HSD in pericentral than in periportal hepatocytes is in line with the concept that bile acid synthesis and metabolism take place predominantly in pericentral cells.