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Male Yucatan swine were allocated to four groups (n = 5-6 pigs per group): low fat (3%) fed control, high fat/2% cholesterol (CH) fed (HF), high fat/CH fed with alloxan-induced diabetes (DF) and DF pigs that were treated with atorvastatin (80 mg/day; DF+A). Pigs were fed two meals per day and daily insulin injections were used in diabetic pigs to maintain plasma glucose between 250 and 350 mg/dl. Diabetic dyslipidemic (DF) pigs exhibited greater coronary atherosclerosis and increased collagen deposition in internal mammary artery compared with normoglycemic hyperlipidemic pigs. Although total and LDL CH concentrations did not differ, triglyceride (TG) were increased in DF pigs and FPLC analysis indicated that the LDL/HDL CH ratio was significantly increased in DF compared with HF pigs. The LDL fraction of DF pigs contained larger, lipid enriched particles resembling IDL. Consumption of the high fat/CH diet caused a moderate increase in the percentage of 14:0 fatty acids in plasma lipids and this was compensated by small-moderate declines in several unsaturated fatty acids. There was a significant increase in phospholipid arachidonic acid in DF compared with HF pigs. Atorvastatin protected diabetic pigs from atherosclerosis and decreased total and VLDL TG, but exerted minimal effects on the FPLC lipoprotein and plasma fatty acid profiles and plasma concentrations of total and LDL CH, vitamin A, vitamin E, and lysophosphatidylcholine. Across all groups the plasma CH concentration was positively correlated with hepatic CH concentration. These findings suggest that atorvastatin's protection against coronary artery atherosclerosis in diabetes may involve effects on plasma VLDL TG concentration. Lack of major effects on other lipid parameters, including the LDL/HDL ratio, suggests that atorvastatin may have yet other anti-atherogenic effects, possibly directly in the vessel wall.

The transport of apolipoprotein B (apoB) between the endoplasmic reticulum (ER) and Golgi was studied in puromycin-synchronized HepG2 cells, using an antibody that could distinguish between apoB in ER and Golgi compartments. In cells with normal ER-to-Golgi transport, both albumin and apoB colocalized throughout the ER and appeared as intense, compact signals in Golgi. When ER-to-Golgi transport was blocked with brefeldin A, apoB and albumin remained colocalized in the ER network and three-dimensional constructed images showed more intense signals for both proteins in a central, perinuclear region of the ER. When protein synthesis was stopped in cells with brefeldin A-inhibited ER-to-Golgi transport, apoB degradation was visualized as a homogeneous decrease in fluorescence signal intensity throughout the ER that could be slowed with clasto-lactacystin β-lactone, a proteasome inhibitor. Incubation of cells with CP-10447, an inhibitor of microsomal triglyceride transfer protein, inhibited apoB, but not albumin, transport from ER to Golgi. Nanogold immunoelectron microscopy of digitonin-permeabilized cells showed proteasomes in close proximity to the cytosolic side of the ER membrane. Thus, newly synthesized apoB is localized throughout the entire ER and degraded homogeneously, most likely by neighboring proteasomes located on the cytosolic side of the ER membrane. Although albumin is colocalized with apoB in the ER, as expected, it was not targeted for ER-associated proteasomal degradation. —Sakata, N., T. E. Phillips, and J. L. Dixon. Distribution, transport, and degradation of apolipoprotein B-100 in HepG2 cells. J. Lipid Res. 2001. 42: 1947–1958.

A major theme of this review is that apoB secretion is regulated post-translationally, and that apoB secretion reacts rapidly to the current state of lipid metabolism in the cell. Therefore, as discussed by Fungwe et al. (122), the metabolism of triglyceride and of cholesteryl ester, in so far as both can be used as core lipids for apoB-containing LPs, are inextricably linked, and the shortage of one or both of these lipids could, by "allowing" increased intracellular degradation in the ER, inhibit the secretion of apoB. Another theme in this review is that the regulation of apoB secretion may be quite different in rat hepatocytes compared to cultured cells (HepG2) used as a model for human hepatocytes. Exogenous fatty acids appear to modulate the rate of apoB secretion in HepG2 cells, whereas they have only minimal effects on apoB secretion in rat hepatocytes or liver. Increased dietary cholesterol, on the other hand, appears to be an important modulator of apoB secretion in rats, but the evidence for effects of cholesterol on apoB secretion in HepG2 cells is less convincing. Finally, because HepG2 cells are an immortalized cell line, there could be many differences between these cells and human hepatocytes in vivo. Therefore, many of the results obtained with HepG2 cells should be corroborated in primary cultures of human hepatocytes. However, investigators utilizing primary human hepatocytes should be sure that the culture conditions are adequate to maintain the continued transcription of liver specific genes and to prevent the dedifferentiation of these cells in culture (85, 86).