Differential impact of hepatic deficiency and total body inhibition of MTP on cholesterol metabolism and RCT in mice

Abstract

Cholesterol within apoB-containing lipoproteins is a major risk factor for the development of atherosclerotic CVD ( 1, 2 ). Plasma levels of apoB-containing lipoproteins are determined by hepatic production in the form of VLDLs and intestinal production of chylomicrons, in which cholesterol absorbed from the diet is packaged ( 3-5 ). For the assembly of both forms of apoB-containing lipoproteins, microsomal triglyceride transfer protein (MTP) expression is essential ( 4, 5 ). This is exemplifi ed by reduced secretion of apoBcontaining lipoproteins and the subsequent accumulation of hepatic triglycerides when MTP is absent in rodent models ( 6-9 ) or in human abetalipoproteinemia, a genetic disease characterized by MTP defi ciency ( 10 ). On the other hand, overexpression of MTP increases secretion of apoBcontaining lipoproteins ( 11, 12 ). Deletion of MTP, specifically in the intestine, was shown to reduce plasma cholesterol levels by about 45-50% ( 13, 14 ), illustrating the contribution of the intestine to systemic lipoprotein metabolism. The key role of MTP in determining circulating levels of apoB-containing lipoproteins and their importance for atherosclerotic CVD resulted in the development of systemically active pharmacological inhibitors that have recently entered phase 3 clinical trials ( 15, 16 ). Although a variable increase in hepatic triglyceride content was noticed, MTP inhibitors seem to have a reasonable safety profi le and thus Abstract Because apoB-containing lipoproteins are proatherogenic and their secretion by liver and intestine largely depends on microsomal triglyceride transfer protein (MTP) activity, MTP inhibition strategies are actively pursued. How decreasing the secretion of apoB-containing lipoproteins affects intracellular rerouting of cholesterol is unclear. Therefore, the aim of the present study was to determine the effects of reducing either systemic or liver-specifi c MTP activity on cholesterol metabolism and reverse cholesterol transport (RCT) using a pharmacological MTP inhibitor or a genetic model, respectively. Plasma total cholesterol and triglyceride levels were decreased in both MTP inhibitortreated and liver-specifi c MTP knockout (L-Mttp ؊ / ؊ ) mice (each P < 0.001). With both inhibition approaches, hepatic cholesterol as well as triglyceride content was consistently increased (each P < 0.001), while biliary cholesterol and bile acid secretion remained unchanged. A small but significant decrease in fecal bile acid excretion was observed in inhibitor-treated mice ( P < 0.05), whereas fecal neutral sterol excretion was substantially increased by 75% ( P < 0.001), conceivably due to decreased intestinal absorption. In contrast, in L-Mttp ؊ / ؊ mice both fecal neutral sterol and bile acid excretion remained unchanged. However, while total RCT increased in inhibitor-treated mice ( P < 0.01), it surprisingly decreased in L-Mttp ؊ / ؊ mice ( P < 0.05). These data demonstrate that: i ) pharmacological MTP inhibition increases RCT, an effect that might provide additional cli...

A new model of reverse cholesterol transport: enTICEing strategies to stimulate intestinal cholesterol excretion

“…Fecal neutral sterol excretion was significantly reduced with MTP HKD in NPC1L1 LiverTg but not littermate controls leading to the conclusion that hepatic VLDL secretion is necessary for TICE [107]. Our conclusion was supported by the findings of Dikkers and colleagues who showed that macrophage-to-feces RCT was decreased in mice with hepatic MTP deficiency [108]. Thus, stimulating the secretion of hepatic apoB-containing lipoproteins may promote TICE.…”

Measurement of Intestinal and Peripheral Cholesterol Fluxes by a Dual‐Tracer Balance Method

“…The percentage of cholesterol that is absorbed (fractional absorption) is calculated from dietary cholesterol. The fraction that is not absorbed (1 -fractional absorption) is partially metabolized by the microbiota into neutral sterols such as coprostanol and dihydrocholesterol (Kudchodkar et al, 1972;Dikkers et al, 2014). Neither humans nor their microbiota are capable of degrading the cyclopentanophenanthrene nucleus, which defines the sterol structure (Kudchodkar et al, 1972).…”

“…However, the two cholesterol tracers must (1) be distinguishable from one another with available equipment and (2) not interfere with the optional measurement of cholesterol synthesis. We have successfully used D 5 (2,2,4,4,6-deuterium-cholesterol)-and D 7 (25,26,26,26,27,27,27)labeled cholesterol for assessing cholesterol absorption in combination with cholesterol synthesis (de Vogel-van den Bosch et al, 2008;Freark de Boer et al, 2012;Grefhorst et al, 2012;van der Wulp et al, 2012a,b;Dikkers et al, 2014). In experiments where cholesterol synthesis was not measured, we have successfully used 13 C2 cholesterol as an alternative to D 7 -cholesterol.…”

“…The ultimate design of an animal experiment largely depends on the research question and the specific need for information. In typical experiments designed to assess intestinal cholesterol fluxes and cholesterol synthesis, we have been using a variety of techniques (Brufau and Groen, ; Freark de Boer et al., ; Grefhorst et al., ; Temel and Brown, ; van der Wulp et al., ; Dikkers et al., ). These techniques can be combined into one coherent protocol that can be performed within the same cohort without causing severe discomfort to the animals.…”

L’excrétiontrans-intestinale de cholestérol (TICE)

“…En plus de leur effet inhibiteur sur l'absorption intestinale du cholestérol [22], les phytostérols stimulent significativement le TICE (Figure 3) [23]. [26]. Le rôle du TICE dans le transport inverse du cholestérol demeure donc controversé.…”