Proceedings of The Physiological Society
AstraZeneca (2010) Proc Physiol Soc 18, C11 and PC11
Role of lysophosphatidylcholine in GIP secretion by primary K-cells
E. Diakogiannaki1, F. M. Gribble1, F. Reimann1
1. Clinical Biochemistry, University of Cambridge, Cambridge, United Kingdom.
Background and aims: Glucose-dependent insulinotropic polypeptide (GIP) is a hormone secreted by enteroendocrine K-cells found in highest density in duodenal and jejunal epithelium. Apart from being a critical regulator of insulin secretion, GIP modulates pancreatic beta-cell proliferation and survival, and controls dietary fat metabolism. It is secreted in response to the presence of nutrients in the gut lumen and particularly ingested lipids. The aim of this study was to investigate the effects of lipid micelles on GIP secretion by K-cells. Results: Experiments were performed on primary cultures of murine duodenal epithelium and the enteroendocrine model cell line STC-1. To simulate the conditions epithelial cells experience after a lipid rich meal, “post-prandial micelles”, comprised of oleic acid (200µM), 2-monooleoyl glycerol (70µM), L-α-lysophosphatidylcholine (LPC) (70µM), cholesterol (17µM) and taurocholic acid (TC) (700µM), were applied. Both primary and STC-1 cells responded to lipid micelles by secreting enhanced amounts of GIP (9.2 fold and 3.1 fold stimulation, respectively compared to baseline, p<0.001 for both). The stimulation of GIP secretion by lipid micelles was not attributable to cell lysis, as monitored by lactate dehydrogenase activity released into the supernatant. Fluorescence calcium imaging measurements in STC-1 cells, following loading with Fura-2AM, demonstrated elevations in intracellular calcium in response to lipid micelles (R340/380 increased 1.8 fold compared to baseline p<0.001 n=104). To investigate the relative importance of the different micellar lipids for the secretory response a series of experiments was performed omitting individual components. Exclusion of LPC significantly reduced secretory responses in both primary and STC-1 cells (46% in primary cells n=7 p<0.05; 22% in STC-1 n=12, compared to stimulation by micelles containing LPC). Replacement of LPC with phosphatidylcholine (PC) could not compensate (1.14-fold stimulation by micelles containing PC in primary cells; n=4). LPC (in the presence of 700µM TC) promoted the release of GIP in a dose dependent manner over the range of concentrations between 1-100µM. Both in the presence and absence of TC, 70µM LPC stimulated reversible rises in the cytosolic Ca2+ and cAMP concentration monitored in STC-1 cells preloaded with Fura2 or transfected with a Epac2-based FRET-sensor respectively. Conclusion: Lipid micelles stimulate GIP secretion from primary murine cultures and STC-1 cells. One of the components, LPC, enhanced intracellular concentrations of calcium and augmented levels of intracellular cAMP suggesting the involvement of Gs protein- mediated signaling.
Where applicable, experiments conform with Society ethical requirements