Regulation of very-low-density-lipoprotein lipid secretion in hepatocyte cultures derived from diabetic animals

Abstract

Hepatocytes were derived from 2-3-day streptozotocin-diabetic rats and maintained in culture for up to 3 days. Compared with similar cultures from normal animals, these hepatocytes secreted less very-low-density-lipoprotein (VLDL) triacylglycerol, but the decrease in the secretion of VLDL non-esterified and esterified cholesterol was not so pronounced. This resulted in the secretion of relatively cholesterol-rich VLDL particles by the diabetic hepatocytes. Addition of insulin for a relatively short period (24 h) further decreased the low rates of VLDL triacylglycerol secretion from the diabetic hepatocytes. The secretion of VLDL esterified and non-esterified cholesterol also declined. These changes occurred irrespective of whether or not exogenous fatty acids were present in the culture medium. Little or no inhibitory effect of insulin was observed after longer-term (24-48 h) exposure to the hormone. Both dexamethasone and a mixture of lipogenic precursors (lactate plus pyruvate) stimulated VLDL triacylglycerol and cholesterol secretion, but not to the levels observed in hepatocytes from normal animals. The low rate of hepatic VLDL secretion in diabetes contrasts with the increase in whole-body VLDL production rate. This suggests that the intestine is a major source of plasma VLDL in insulin-deficient diabetes.

Impaired hepatic apolipoprotein B and E translation in streptozotocin diabetic rats.

“…Studies in experimental models of diabetes demonstrate that insulin deficiency lowers hepatic apo B (28, 29) and VLDL-triglyceride secretion (40,41), and with long-term insulin treatment in vivo, hepatic apo B production normalizes (28). Our studies suggest a positive correlation between plasma insulin levels and hepatic triglyceride-rich lipoprotein production, and such a correlation is supported by studies which raise ambient plasma levels by insulin injection (72), continuous insulin infusion (73), and sucrosefeeding (74).…”

“…VLDL composition is altered in streptozotocin-induced diabetes with a marked reduction in apo E content (29,31,36) which may be a contributory factor to the catabolic defect, as apo E is an important ligand for hepatic lipoprotein receptors (37). Hepatic secretion of VLDL lipid (36,(38)(39)(40)(41) and apo B and E is substantially reduced in both primary cultures of hepatocytes (28,29) and perfused livers derived from diabetic rats (28,36). These findings suggest that the hyperlipidemia of experimental diabetes is primarily a result of the accumulation of intestinally derived chylomicrons and VLDL, and suggest further that the contribution of apo B-containing lipoproteins to the plasma by the liver in the hypoinsulinemic diabetic state is relatively small.…”

“…Considering the unique role of apo B as a lipid binding and transport protein, cotranslational modification of apo B might play an important role in controlling translational efficiency, as lipid assembly may be obligatory for efficient apo B translation. There are known changes in diabetes, including decreased lipid synthesis (30,41) and increased apo B phosphorylation (68), which could play a part in the control of apo B translational efficiency.…”

Plasminogen activator inhibitor-1 synthesis in the human hepatoma cell line Hep G2. Metformin inhibits the stimulating effect of insulin.

“…Physiological concentrations of insulin have been shown to increase the cellular content of triacylglycerol in hepatocytes from normal and diabetic rats (44,45). We first confirmed that insulin stimulated PAI-1 synthesis in a doseand time-dependent manner.…”

Transcriptional co-factors and hepatic energy metabolism

“…In addition, stimulation of hepatic lipase by insulin facilitates the uptake of FFA derived from chylomicron remnants, their subsequent esterification to triglycerides and packaging into very low density lipoprotein (VLDL) particles that are exported to other tissues. [9][10][11] [12]. Importantly, apart from its stimulatory effect on glycogen formation and lipogenesis, insulin effectively blocks hepatic glycogenolysis and gluconeogenesis in the fed state [13,14].…”