IMPAIRED SECRETION OF VERY-LOW-DENSITY-LIPOPROTEIN TRIGLYCERIDES BY APOLIPOPROTEIN-E DEFICIENT MOUSE HEPATOCYTES

To explore mechanisms underlying triglyceride (TG) accumulation in liv ers of chow-fed apo E-deficient mice (Kuipers, F., J.M. van Ree, M.H. Hofker, H. Welters, 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 mi ce, (b) in isolated perfused mouse livers, and (c) in cultured mouse h epatocytes. (a) Hepatic VLDL-TG production rate in vivo, determined af ter Triton WR1339 injection, was reduced by 46% in apo E-deficient mic e compared with controls. To eliminate the possibility that impaired V LDL secretion is caused by aspecific changes in hepatic function due t o hypercholesterolemia, VLDL-TG production rates were also measured in apo E-deficient mice after transplantation of wild-type mouse bone ma rrow. Bone marrow-transplanted ape E-deficient mice, which do not expr ess ape 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 V LDL particles isolated from the perfusate was similar for both groups. (c) Mass VLDL-TG secretion by cultured ape E-deficient hepatocytes wa s reduced by 23% compared with control values in serum-free medium, an d by 61% in the presence of oleate in medium (0.75 mM) to stimulate li pogenesis. Electron microscopic evaluation revealed a smaller average size for VLDL particles produced by apo E-deficient cells compared wit h control cells in the presence of oleate (38 and 49 nm, respectively) . In short-term labeling studies, apo E-deficient and control cells sh owed a similar time-dependent accumulation of [H-3]TG formed from [H-3 ]glycerol, yet secretion of newly synthesized VLDL-associated [H-3]TG by ape 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.