An in vivo and in vitro Investigation of Glycerol Utilization in Intestinal Triacylglycerol Metabolism.

Overproduction of intestinal lipoproteins contribute to postprandial hypertriacylglycerolaemia. Understanding the mechanisms that modulate chylomicron (CM) production in health and disease may lead to prevention and improvement of treatments for atherosclerosis. It is well known that following the hydrolysis of dietary triacylglycerol (TAG) in the intestine, glycerol is taken up by the luminal side of absorptive cells. However, it has recently been shown that in humans, endogenously derived glycerol tracer (administered intravenously) also appears to be incorporated into exogenous (dietary derived) lipoprotein particles. This was investigated further utilising stable isotopic techniques in human volunteers and by an in vitro model utilising the Caco2 cell line. Phospholipids (PL) synthesis was also measured in the Caco2 cells. Five healthy subjects (age 50-65, BMI21-24) were fed identical high fat meals every 2 hours for 11 hours. During this time, two different tracers, one administered intravenously (2H5-glycerol) and another given orally (13C3-glycerol) as part of a high fat meal were used to measure the incorporation of endogenous and exogenous glycerol into CM or very low density lipoproteins (VLDL). 2H5-glycerol was detected in VLDL and CM derived TAG, confirming endogenous glycerol could be incorporated into TAG derived from enterocytes. 13C3-glycerol was found to be predominantly associated with CM. There was no significant difference in the proportion of oral or intravenous tracer used for TAG synthesis. VLDL1 production rate (PR) was significantly higher than VLDL2 PR; CM1 PR was significantly higher than CM2 PR. The incorporation of 13C3-labelled glycerol into TAG and PL by Caco2 cells was measured by the addition of a trace quantity of labelled C3-glycerol to either the apical or basolateral side of cells. As different culture conditions might affect lipoprotein synthesis and secretion, the effect of culturing in different media concentrations of glucose and insulin was also investigated. Incubation with labelled 13C3-glycerol on either the apical or the basolateral surface of the cells resulted in the detection of glycerol enriched TAG and PL in the basolateral media and the cells themselves. Because the enrichment of basolateral media TAG and PL was higher than that in the cells, the presence of unlabelled cellular storage pools was proposed. Insulin had no effect on the synthesis and secretion or cellular enrichment of TAG and PL, but reduced enrichment of TAG and PL in the media. This suggests that insulin increased the incorporation of newly synthesised (labelled) TAG and PL into the cellular storage pool and reduced the amount in the export pool. This study clearly demonstrates that both exogenous and endogenous glycerol can be used to synthesise TAG in enterocytes in both an in vivo and in an in vitro model. The study also confirms the presence of intracellular storage pools of TAG in enterocytes and that enterocytes can regulate CM synthesis and secretion using these pools. Endogenous substrates may have an important role in the overproduction of CM and further studies are needed to understand their role in postprandial hypertriacylglycerolaemia