Treating Hyperlipidemias by Targeting the Microsomal Triglyceride Transfer Protein.

Abstract

Microsomal triglyceride transfer protein (MTP) is a target to reduce plasma lipids because of its indispensable role in triglyceride-rich lipoprotein biosynthesis. Genetic ablation/chemical inhibition of MTP in western diet fed mice and monkeys decreased plasma triglycerides and cholesterol while increasing plasma transaminases (ALT and AST), which are indicators of hepatic injury. MTP deficiency also demonstrated significant accumulations of triglycerides and free cholesterol in the liver. Subcellular localization studies revealed free cholesterol and triglyceride accretion in the endoplasmic reticulum (ER) and mitochondria. Reductions in hepatic free cholesterol attenuated plasma transaminase elevations, indicating that free cholesterol, and not triglyceride, is related to MTP inhibitor linked adverse events. Further, there was a significant positive correlation between free cholesterol and plasma transaminase concentrations. Mechanistic studies revealed increased free cholesterol in the ER and mitochondria, which activated ER and oxidative stress pathways, respectively, without causing inflammation or cell death. Alleviation of hepatic ER stress by molecular chaperones or free cholesterol lowering therapies blunted changes in plasma ALT/AST levels in MTP deficient animals. In the absence of cell death as a mechanism for plasma transaminase release in MTP deficient animals and cells, we identified increased transcription of the GPT/GOT1 genes, the genes that code for the ALT1 and AST1 proteins, respectively. We identified IRE1α, an ER stress signal transducer, as a positive effector of GPT and GOT1 transcription. Signaling cascades activated by IRE1α led to the association of the cJun transcription factor with the GPT/GOT1 promoters, which mediated their transcriptional up-regulation. Increased transcription was associated with augmented cellular synthesis of ALT1 and AST1 and subsequent release into the extracellular environment as evidenced by metabolic labeling studies. Therefore, our studies indicate that increased hepatic free cholesterol as a result of MTP inhibition/ablation elicits an integrated sub-cellular response involving transcriptional up-regulation of GPT/GOT1 via activation of Ire1α and cJun. This response can be avoided by co-administering cellular triglyceride/free cholesterol lowering agents to MTP inhibitor therapy. These studies provide mechanistic insight into the toxicities associated with MTP deficiency and may shed some light into the cause of increased plasma transaminases in other models of hepatic steatosis.