Background: Microsomal triglyceride transfer protein (MTP) inhibition augments plasma transaminases, however, mechanisms are unknown. Results: MTP inhibition increased endoplasmic reticulum (ER) stress and induced GPT/GOT1 transcription through the Ire1␣/cJun pathway. Conclusion: Transcriptional up-regulation and increased synthesis contribute to augmentations in plasma ALT/AST. Significance: Increased transcription of the transaminase genes may reflect a mechanism for hepatocyte survival after ER stress.

Background The use of microsomal triglyceride transfer protein (MTP) inhibitors is limited to severe hyperlipidemias due to associated hepatosteatosis and gastrointestinal adverse effects. Comprehensive knowledge about the structure-function of MTP might help design new molecules that avoid steatosis. Characterization of mutations in MTP causing abetalipoproteinemia have revealed that the central α-helical and C-terminal β-sheet domains are important for protein disulfide isomerase (PDI) binding and lipid transfer activity. Our aim was to identify and characterize mutations in the N-terminal domain to understand its function. Methods and Results We identified a novel missense mutation (D169V) in a 4-month old Turkish male with severe signs of ABL. To study the effect of this mutation on MTP function, we created mutants via site-directed mutagenesis. Although D169V was expressed in the endoplasmic reticulum and interacted with apoB17, it was unable to bind PDI, transfer lipids, and support apoB secretion. Computational modeling suggested that D169 could form an internal salt bridge with K187 and K189. Mutagenesis of these lysines to leucines abolished PDI heterodimerization, lipid transfer, and apoB secretion, without affecting apoB17 binding. Further, mutants with preserved charges (D169E, K187R, K189R) rescued these activities. Conclusions D169V is detrimental because it disrupts an internal salt bridge leading to loss of PDI binding and lipid transfer activities; however, it does not affect apoB-binding. Thus, the N-terminal domain of MTP is also important for its lipid transfer activity.

Objectives ω-3 Fatty acids (FAs), natural ligands for the peroxisome proliferator-activated receptor–α (PPAR-α), attenuate parenteral nutrition–associated liver disease (PNALD). However, the mechanisms underlying the protective role of ω-3 FAs are still unknown. The aim of this study was to determine the effects of ω-3 FAs on hepatic triglyceride (TG) accumulation in a murine model of PNALD and to investigate the role of PPAR-α and microsomal triglyceride transfer protein (MTP) in this experimental setting. Methods 129S1/SvImJ wild-type or 129S4/SvJaePparatm/Gonz/J PPAR-α knockout mice were fed chow and water (controls); oral, fat-free PN solution only (PN-O); PN-O plus intraperitoneal (IP) ω-6 FA-predominant supplements (PN–ω-6); or PN-O plus IP ω-3 FA (PN–ω-3). Control and PN-O groups received sham IP injections of 0.9% NaCl. Hepatic histology, TG and cholesterol, MTP activity, and PPAR-α messenger RNA were assessed after 19 days. Results In all experimental groups, PN feeding increased hepatic TG and MTP activity compared with controls. Both PN-O and PN–ω-6 groups accumulated significantly greater amounts of TG when compared with PN–ω-3 mice. Studies in PPAR-α null animals showed that PN feeding increases hepatic TG as in wild-type mice. PPAR-α null mice in the PN-O and PN–ω-6 groups demonstrated variable degrees of hepatic steatosis, whereas no evidence of hepatic fat accumulation was found after 19 days of oral PN plus IP ω-3 FAs. Conclusions PN induces TG accumulation (steatosis) in wild-type and PPAR-α null mice. In PN-fed wild-type and PPAR-α null mice given IP ω-3 FAs, reduced hepatic TG accumulation and absent steatosis are found. Prevention of steatosis by ω-3 FAs results from PPAR-α–independent pathways.