Effects of Insulin and Glucose on Very Low Density Lipoprotein Triglyceride Secretion by Cultured Rat Hepatocytes

Abstract

A B S T R A C T The effect of insulin on hepatic triglyceride synthesis and secretion is controversial. Previously, we have described a cell culture system of adult rat hepatocytes that synthesize and secrete very low density lipoprotein (VLDL) triglycerides with small and irreproducible effects of insulin on triglyceride metabolism. To study the primary effects of insulin on hepatic triglyceride metabolism a method was developed utilizing fibronectin-coated culture dishes that allowed adhesion, spreading, and maintenance of hepatocytes for 2-3 d in the absence of serum and insulin. This culture system allowed mass measurements of both cellular and secreted VLDL triglycerides for long time periods after the addition of physiological concentrations of insulin to hormone-free culture medium. In the absence of insulin and after an initial 4 h in culture, the medium was replenished and triglyceride mass was measured at the end of 18-h incubations. VLDL triglyceride accumulated in the culture medium at a linear rate over this time-course with increasing accumulation as the medium glucose concentration was raised from 2.5 to 25 mM glucose (1.77±0.24 to 3.09±0.76 yg triglyceride/mg cell protein per h). There was no apparent significant lipolysis or hepatocellular reuptake of secreted VLDL triglycerides. In the absence of insulin cellular triglyceride levels were unchanged between 3 and 24 h in culture while insulin (50-500 gU/ml) significantly increased cellular triglyceride content at all glucose concentrations tested (0-25 mM). The addition of insulin to the culture medium progressively reduced the rate of VLDL triglyceride secretion accompanied by an increase in cellular triglyceride at insulin concentrations > 50 ,U/ml. Most or all of the observed increase in cell triglyceride content could in all experiments be accounted for by the insulin-induced inhibition of VLDL secretion. Incorporation of [2-3H]glycerol into cellular and VLDL triglycerides as a function of insulin concentration was also measured. Glycerol incorporation data at 20-22 h after plating of the cells closely paralleled the insulin-induced changes in cellular and VLDL triglyceride as determined by mass analysis. The observed effects of insulin occurred at concentrations close to the physiological range and suggest that the direct hepatic effect is to suppress VLDL secretion although the net effect in vivo will clearly reflect many additional accompanying changes.

Acute meal‐induced changes in hepatic glycerolipid metabolism are unimpaired in severely diabetic rats: implications for the role of insulin

“…This scenario would appear to fit in with the observations (see e.g. [4][5][6][7]) that insulin actually inhibits TAG and apoB secretion by cultured hepatocytes. Consequently, it has been suggested that the prandial increase in portal insulin concentration is sufficient to inhibit VLDL secretion by the liver [8][9][10].…”

Influence of insulin resistance, secretion, and clearance on serum cholesterol, triglycerides, lipoprotein cholesterol, and blood pressure in healthy men.

“…38 - 40 A further factor that could contribute to the association between increased triglyceride concentrations and insulin resistance could be resistance of adipose tissue lipoh/sis to inhibition by insulin. This and the resistance of glucose elimination to stimulation by insulin may be linked conditions.…”

Role of insulin in hepatic fatty acid partitioning: emerging concepts

“…Thus, whereas experiments conducted with isolated perfused rat liver showed that insulin stimulates both synthesis and secretion of TAG [136,137], many subsequent studies conducted on cultured hepatocytes have found that insulin, although promoting synthesis of TAG, inhibits its secretion [6,8,133,138,139]. The reasons for this discrepancy between the two sets of data may reside in the use of different experimental systems (perfused liver, cultured cells) and in the choice of the precise perfusion or culture conditions, by different laboratories.…”