Impaired secretion of very low density lipoprotein-triglycerides by apolipoprotein E- deficient mouse hepatocytes.

Abstract

To explore mechanisms underlying triglyceride (TG) accumulation in livers of chow-fed apo E-deficient mice (Kuipers, F., J.M. van Ree, M.H. Hofker, H. Wolters, G. In't Veld, R.J. Vonk, H.M.G. Princen, and L.M. Havekes. 1996. Hepatology. 24:241-247), we investigated the effects of apo E deficiency on secretion of VLDL-associated TG (a) in vivo in mice, (b) in isolated perfused mouse livers, and (c) in cultured mouse hepatocytes. (a) Hepatic VLDL-TG production rate in vivo, determined after Triton WR1339 injection, was reduced by 46% in apo E-deficient mice compared with controls. To eliminate the possibility that impaired VLDL secretion is caused by aspecific changes in hepatic function due to hypercholesterolemia, VLDL-TG production rates were also measured in apo E-deficient mice after transplantation of wild-type mouse bone marrow. Bone marrow- transplanted apo E-deficient mice, which do not express apo E in hepatocytes, showed normalized plasma cholesterol levels, but VLDL-TG production was reduced by 59%. (b) VLDL-TG production by isolated perfused livers from apo E-deficient mice was 50% lower than production by livers from control mice. Lipid composition of nascent VLDL particles isolated from the perfusate was similar for both groups. (c) Mass VLDL-TG secretion by cultured apo E-deficient hepatocytes was reduced by 23% compared with control values in serum-free medium, and by 61% in the presence of oleate in medium (0. 75 mM) to stimulate lipogenesis. Electron microscopic evaluation revealed a smaller average size for VLDL particles produced by apo E-deficient cells compared with control cells in the presence of oleate (38 and 49 nm, respectively). In short-term labeling studies, apo E-deficient and control cells showed a similar time-dependent accumulation of [3H]TG formed from [3H]glycerol, yet secretion of newly synthesized VLDL-associated [3H]TG by apo E-deficient cells was reduced by 60 and 73% in the absence and presence of oleate, respectively. We conclude that apo E, in addition to its role in lipoprotein clearance, has a physiological function in the VLDL assembly-secretion cascade.

Mea6 controls VLDL transport through the coordinated regulation of COPII assembly

“…ApoE and ApoB are lipid-associated proteins that are required for VLDL assembly [39][40][41][42][43]. The elevated levels of both proteins in Mea6 cKO livers (Figures 3 and 5) correlated with lipid accumulation, suggesting that VLDL is assembled normally.…”

Homocysteine-induced endoplasmic reticulum stress causes dysregulation of the cholesterol and triglyceride biosynthetic pathways

“…CBS +/+ mice fed control or hyperhomocysteinemic diets for 2 weeks were fasted overnight and hepatic VLDL-triglyceride secretion rates quantified by the intravenous administration of Triton WR1339 (500 mg/kg body weight; Sigma Chemical Co.) as described previously (34).…”

“…To assess whether the accumulation of hepatic cholesterol and triglycerides were due to impaired lipid export, VLDL-triglyceride secretion rate was determined by injecting CBS +/+ mice that had been fasted overnight with Triton WR1339. Because Triton WR1339 blocks conversion of VLDL to LDL, the resulting increase in plasma triglyceride and cholesterol concentrations reflect hepatic secretion of VLDL in the absence of LDL uptake (34). Figure 10 shows that the hepatic triglyceride secretion rate was significantly higher in CBS +/+ mice fed hyperhomocysteinemic diets, compared with CBS +/+ mice fed control diet.…”

Hormone-sensitive Lipase Deficiency in Mice Changes the Plasma Lipid Profile by Affecting the Tissue-specific Expression Pattern of Lipoprotein Lipase in Adipose Tissue and Muscle

“…In Vivo Synthesis of VLDL-Hepatic TG synthesis rates were determined after blocking the lipolytic degradation of TG-rich lipoproteins with Triton WR1339 (28) according to a standard protocol (29). Anesthetized fasted mice were injected intravenously with 500 mg of Triton WR1339 (Sigma)/kg of body weight as a 15 g/dl solution in 0.9% NaCl.…”