Monoacylglycerol metabolism in human intestinal Caco-2 cells: evidence for metabolic compartmentation and hydrolysis.

Free fatty acids (FFA) and sn-2-monoacylglycerol (MG), the two major hydrolysis products of dietary triacylglycerol (TG), are absorbed from the lumen into polarized enterocytes that line the small intestine. Intensive studies regarding FFA metabolism in the intestine have been published; however, little is known regarding the metabolism of MG. In these studies, we examined the metabolism of sn-2-monoolein (sn-2-18:1) by human intestinal Caco-2 cells. To mimic the physiological presentation of MG to the enterocyte, the metabolism of [(3)H]sn-2-monoolein was examined by adding taurocholate-mixed sn-2-18:1 and albumin-bound sn-2-18:1 at the apical (AP) and basolateral (BL) surfaces of the Caco-2 cell, respectively. The results demonstrate that more sn-2-18:1 was incorporated into TG from AP taurocholate-mixed sn-2-18:1, whereas more phospholipid was synthesized from BL albumin-bound sn-2-18:1. The TG:phospholipid ratio was approximately 5-fold higher for AP relative to BL MG incubation. Qualitatively similar results were observed for bovine serum albumin-bound MG added at the apical surface. It was also found that substantial sn-2-18:1 radioactivity was recovered in the FFA fraction, suggesting that sn-2-18:1 may be directly hydrolyzed within the Caco-2. We therefore used reverse transcription-PCR with primers designed from the murine MG lipase (MGL) gene, and detected the presence of MG lipase mRNA in Caco-2. The human MGL gene was cloned and found to be 83% identical to the murine MGL, and identical to a previously described lysophospholipase-like protein. Northern blot analysis showed the expression of MGL throughout Caco-2 differentiation. Thus, MG metabolism in Caco-2 cells may include not only well established anabolic processes, but also catabolic processes. Further, the observed polarity of MG metabolism suggests that, as for fatty acids, separate precursor and/or product pools of lipid may exist in the intestinal enterocyte.