The marked deficiency of peroxisomal organelle assembly in the PEX2 ؊/؊ mouse model for Zellweger syndrome provides a unique opportunity to developmentally and biochemically characterize hepatic disease progression and bile acid products. The postnatal survival of homozygous mutants enabled us to evaluate the response to bile acid replenishment in this disease state. PEX2 mutant liver has severe but transient intrahepatic cholestasis that abates in the early postnatal period and progresses to steatohepatitis by postnatal day 36. We confirmed the expected reduction of mature C24 bile acids, accumulation of C27-bile acid intermediates, and low total bile acid level in liver and bile from these mutant mice. Treating the PEX2 ؊/؊ mice with bile acids prolonged postnatal survival, alleviated intrahepatic cholestasis and intestinal malabsorption, reduced C27-bile acid intermediate production, and prevented older mutants from developing severe steatohepatitis. However, this therapy exacerbated the degree of hepatic steatosis and worsened the already severe mitochondrial and cellular damage in peroxisome-deficient liver. Both untreated and bile acid-fed PEX2 ؊/؊ mice accumulated high levels of predominantly unconjugated bile acids in plasma because of altered expression of hepatocyte bile acid transporters. Significant amounts of unconjugated bile acids were also found in the liver and bile of PEX2 mutants, indicating a generalized defect in bile acid conjugation. Conclusion: Peroxisome deficiency widely disturbs bile acid homeostasis and hepatic functioning in mice, and the high sensitivity of the peroxisome-deficient liver to bile acid toxicity limits the effectiveness of bile acid therapy for preventing hepatic disease. (HEPATOLOGY 2007;45:982-997.)

Regulation of hepatic cholesterol biosynthesis, lipogenesis, and insulin signaling intersect at the transcriptional level by control of SREBP and Insig genes. We previously demonstrated that peroxisome-deficient PEX2 ؊/؊ mice activate SREBP-2 pathways but are unable to maintain normal cholesterol homeostasis. In this study, we demonstrate that oral bile acid treatment normalized hepatic and plasma cholesterol levels and hepatic cholesterol synthesis in early postnatal PEX2 mutants, but SREBP-2 and its target gene expressions remained increased. SREBP-2 pathway induction was also observed in neonatal and longer surviving PEX2 mutants, where hepatic cholesterol levels were normal. Abnormal expression patterns for SREBP-1c and Insig-2a, and novel regulation of Insig-2b, further demonstrate that peroxisome deficiency widely affects the regulation of related metabolic pathways. We have provided the first demonstration that peroxisome deficiency activates hepatic endoplasmic reticulum (ER) stress pathways, especially the integrated stress response mediated by PERK and ATF4 signaling. Our studies suggest a mechanism whereby ER stress leads to dysregulation of the endogenous sterol response mechanism and concordantly activates oxidative stress pathways. Several metabolic derangements in peroxisome-deficient PEX2 ؊/؊ liver are likely to trigger ER stress, including perturbed flux of mevalonate metabolites, altered bile acid homeostasis, changes in fatty acid levels and composition, and oxidative stress.

Peroxisome biogenesis disorders, of which Zellweger syndrome is the most severe, result in severe neurological dysfunction associated with abnormal CNS neuronal migrations due to the lack of functional peroxisomes. The PEX2-/- mouse model for Zellweger syndrome has enabled us to evaluate the role of peroxisomes in the development and functioning of the nervous system. These studies have shown that, in addition to disturbances in neuronal migration in developing cerebral cortex and cerebellum, defects in neuronal differentiation, proliferation and survival may also contribute to the CNS malformations. However, owing to the multiorgan dysfunction in peroxisomal disorders, it has been difficult to clearly define an intrinsic role for the peroxisome in brain cells. The use of several in vitro cell culture assays to evaluate the migration and differentiation of cerebellar neurons demonstrates a persistence of defects in peroxisome-deficient neurons. The absence of potential systemically derived, extrinsic factors in these in vitro systems indicates that CNS intrinsic defects contribute to the pathogenesis of disease in these disorders. However, bile acid treatment also increases the survival and growth of PEX2-/- mice and improves some aspects of cerebellar development, indicating that extrinsic factors also affect the developing peroxisome-deficient brain. Therefore, the final phenotype of nervous system dysfunction in peroxisomal disorders will reflect a combination of both CNS intrinsic and extrinsic factors.

Disruption of the Pex2 gene leads to peroxisome deficiency and widespread metabolic dysfunction. We previously demonstrated that peroxisomes are critical for maintaining cholesterol homeostasis, using peroxisome-deficient Pex2−/− mice on a hybrid Swiss Webster×129S6/SvEv (SW/129) genetic background. Peroxisome deficiency activates hepatic endoplasmic reticulum (ER) stress pathways, leading to dysregulation of the endogenous sterol response mechanism. Herein, we demonstrate a more profound dysregulation of cholesterol homeostasis in newborn Pex2−/− mice congenic on a 129S6/SvEv (129) genetic background, and substantial differences between newborn versus postnatal Pex2−/− mice in factors that activate ER stress. These differences extend to relationships between activation of genes regulated by SREBP-2 versus PPARα. The SREBP-2 pathway is induced in neonatal Pex2−/− livers from 129 and SW/129 strains, despite normal hepatic cholesterol levels. ER stress markers are increased in newborn 129 Pex2−/− livers, which occurs in the absence of hepatic steatosis or accumulation of peroxins in the ER. Moreover, the induction of SREBP-2 and ER stress pathways is independent of PPARα activation in livers of newborn 129 and SW/129 Pex2−/− mice. Two-week-old wild-type mice treated with the peroxisome proliferator WY-14,643 show strong induction of PPARα-regulated genes and decreased expression of SREBP-2 and its target genes, further demonstrating that SREBP-2 pathway induction is not dependent on PPARα activation. Lastly, there is no activation of either SREBP-2 or ER stress pathways in kidney and lung of newborn Pex2−/− mice, suggesting a parallel induction of these pathways in peroxisome-deficient mice. These findings establish novel associations between SREBP-2, ER stress and PPARα pathway inductions.

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