The state of cholesterol metabolism in the liver of patients with primary biliary cirrhosis: the role of MDR3 expression

Abstract

Aim: Because dyslipidemia, such as hypercholesterolemia, is a characteristic of primary biliary cirrhosis (PBC), hepatic lipid metabolism may be disturbed in PBC patients. We examined the expression of lipid metabolism-associated genes in PBC liver.

Methods: All of the patients examined were in stage I or II PBC and without medication. RNA was isolated from liver specimens by needle biopsies of PBC patients and controls. The expression levels of various genes were measured by real-time RT-PCR. Multidrug resistance 3 (MDR3) expression was examined immunohistochemically. Statistical correlations between the gene expression levels and indices of blood testing were calculated.

Results: The expression levels of sterol regulatory element-binding protein (SREBP) 2 and LDL receptor were significantly lower, and those of apolipoprotein B, microsomal triglyceride transfer protein, ATP-binding cassette G5, and liver X receptor α (LXRα) were significantly higher in the PBC liver than in the normal control liver. The expression levels of bile acid synthesis- and excretion-associated genes did not change, and those of farnesoid X receptor, peroxisome proliferator-activated receptor α, and SREBP-1c were similar between the PBC and normal liver. MDR3 gene expression levels in the PBC liver were more than 4-fold higher than those in the control liver. Immunohistochemically, strong canalicular staining for MDR3 was observed in the PBC liver. LXRα expression was positively correlated with MDR3 levels. Serum levels of γ-glutamyl transpeptidase (GGT) and IgM were negatively correlated with MDR3 levels.

Conclusions: Hepatocellular cholesterol metabolism was at least partially disturbed, even in the early stage of PBC. The most characteristic finding was a distinct elevation of MDR3 expression, and the MDR3 levels were negatively correlated with GGT and IgM levels.

Fig. 1

Expression levels of cholesterol uptake-, synthesis-, and secretion-associated genes. SREBP-2 sterol regulatory element-binding protein 2, LDLR LDL receptor, HMGR HMG-CoA reductase, MTP microsomal triglyceride protein, ApoB apolipoprotein B, ABCG5 ATP-binding cassette G5, NPC1L1 Niemann-Pick C1 like 1, LXRα liver X receptor α. * Significant difference between PBC patients and normal control

Fig. 2

Expression levels of bile acid synthesis- and secretion-associated genes and phospholipid synthesis- and secretion-associated genes. CYP7A1 cholesterol 7α-hydroxylase, FXR farnesoid X receptor, BSEP bile salt export pump, MDR3 multidrug resistance 3, PPARα peroxisome proliferator-activated receptor α, SREBP-1c sterol regulatory element-binding protein-1c. * Significant difference between PBC patients and normal control

Fig. 3

Immunohistochemical staining for MDR3 in the normal liver (a) and PBC liver (b). MDR3 multidrug resistance 3. Original magnification: ×100

Fig. 4

Correlations between the expression levels of MDR3 and LXRα (a) or PPARα (b). MDR3 multidrug resistance 3, PPARα peroxisome proliferator-activated receptor α, LXRα liver X receptor α

Fig. 5

Correlations between the expression levels of MDR3 and the serum levels of GGT (a) or IgM (b). MDR3 multidrug resistance 3, GGT γ-glutamyl transpeptidase

Fig. 6

Schema showing the interactions between cholesterol metabolism-associated factors. Arrows (⇑ and ⇓) represent significant difference in expression levels between PBC patients and normal controls. LDLR LDL receptor, SREBP sterol regulatory element-binding protein, NPC1L1 Niemann-Pick C1 like 1, ApoB apolipoprotein B, MTP microsomal triglyceride protein, ABCG5/G8 ATP-binding cassette G5/G8, LXRα liver X receptor α, CYP7A1 cholesterol 7α-hydroxylase, FXR farnesoid X receptor, BSEP bile salt export pump, PPARα peroxisome proliferator-activated receptor α, MDR3 multidrug resistance 3