Influence of plasma free fatty acids on lipoprotein synthesis and diabetic dyslipidemia.

The regulation of hepatic VLDL secretion mainly depends on apolipoprotein (apo) B synthesis, on microsomal triglyceride transfer protein, insulin and the availability of triglycerides, free fatty acids (FFA) and cholesteryl ester. Four sources of fatty acids are used for lipoprotein synthesis: de-novo lipogenesis, cytoplasmic triglyceride stores, fatty acids derived from lipoproteins taken up directly by the liver and plasma FFA. Quantitatively, de-novo lipogenesis plays a minor role in regulating VLDL synthesis, but evidently it is elevated under conditions of high carbohydrate feeding. Cytoplasmic triglyceride stores appear to essentially contribute to VLDL triglycerides. Plasma FFA enter the hepatocytes and are either oxidized or esterified. The relationship between oxidation and esterification appears to be important in regulating the VLDL synthesis. An enhanced esterification is accompanied by increased VLDL secretion. The addition of oleic acid to hepatocytes has been shown to stimulate production of VLDL triglyceride and apoB. In human beings, an acute experimental elevation of plasma FFA stimulates VLDL production. In healthy men strong positive relations were found between the late increases in large triglyceride-rich lipoproteins and plasma FFA concentrations after 6 h following a mixed meal. In contrast, n-3 fatty acids impair VLDL assembly and secretion. Chronic hyperinsulinemia seems to stimulate VLDL production. On the other hand, the short-term addition of insulin has been shown to inhibit VLDL-triglyceride and apoB production in vitro. There is in vivo evidence that acute hyperinsulinemia suppresses VLDL-apoB and VLDL-triglyceride production in insulin-sensitive humans. Part of this action is due to suppression of plasma FFA. In patients with impaired glucose tolerance (IGT), VLDL production was increased when compared with subjects with normal glucose (NGT). When infusing a lipid emulsion, VLDL production could not be further stimulated in IGT patients in contrast to NGT persons. Hypertriglyceridemia in type 2 diabetes mellitus is usually the consequence of a VLDL overproduction. In type 2 diabetic patients, in contrast to normal men, insulin failed to suppress VLDL1 particle release. In normal men, an elevation of blood glucose led to a decrease in fatty acid oxidation and an increase in hepatic triglyceride secretion. Under these conditions, approximately 30% of total VLDL triglycerides coming out of the liver did not originate from plasma FFA. In conclusion, plasma FFA seem to play an important role in stimulating hepatic VLDL production. Other factors such as chronic hyperinsulinemia or nutrition modify this effect.