Class III multidrug resistance (MDR) P-glycoproteins (P-gp), mdr2 in mice and MDR3 in man, mediate the translocation of phosphatidylcholine across the canalicular membrane of the hepatocyte. Mice with a disrupted mdr2 gene completely lack biliary phospholipid excretion and develop progressive liver disease, characterized histologically by portal inflammation, proliferation of the bile duct epithelium, and fibrosis. This disease phenotype is very similar to a subtype of progressive familial intrahepatic cholestasis, hallmarked by a high serum γ-glutamyltransferase (γ-GT) activity. We report immunohistochemistry for MDR3 P-gp, reverse transcription-coupled PCR sequence analysis, and genomic DNA analysis of MDR3 from two progressive familial intrahepatic cholestasis patients with high serum γ-GT. Canalicular staining for MDR3 P-gp was negative in liver tissue of both patients. Reverse transcription-coupled PCR sequencing of the first patient’s sequence demonstrated a homozygous 7-bp deletion, starting at codon 132, which results in a frameshift and introduces a stop codon 29 codons downstream. The second patient is homozygous for a nonsense mutation in codon 957 (C → T) that introduces a stop codon (TGA). Our results demonstrate that mutations in the human MDR3 gene lead to progressive familial intrahepatic cholestasis with high serum γ-GT. The histopathological picture in these patients is very similar to that in the corresponding mdr2 (−/−) mouse, in which mdr2 P-gp deficiency induces complete absence of phospholipid in bile.

A growing body of evidence implicates ceramide and/or its glycosphingolipid metabolites in the pathogenesis of insulin resistance. We have developed a highly specific small molecule inhibitor of glucosylceramide synthase, an enzyme that catalyzes a necessary step in the conversion of ceramide to glycosphingolipids. In cultured 3T3-L1 adipocytes, the iminosugar derivative N-(5′-adamantane-1′-yl-methoxy)-pentyl-1-deoxynojirimycin (AMP-DNM) counteracted tumor necrosis factor-α–induced abnormalities in glycosphingolipid concentrations and concomitantly reversed abnormalities in insulin signal transduction. When administered to mice and rats, AMP-DNM significantly reduced glycosphingolipid but not ceramide concentrations in various tissues. Treatment of ob/ob mice with AMP-DNM normalized their elevated tissue glucosylceramide levels, markedly lowered circulating glucose levels, improved oral glucose tolerance, reduced A1C, and improved insulin sensitivity in muscle and liver. Similarly beneficial metabolic effects were seen in high fat–fed mice and ZDF rats. These findings provide further evidence that glycosphingolipid metabolites of ceramide may be involved in mediating the link between obesity and insulin resistance and that interference with glycosphingolipid biosynthesis might present a novel approach to the therapy of states of impaired insulin action such as type 2 diabetes.