Group 1B phospholipase A2-mediated lysophospholipid absorption directly contributes to postprandial hyperglycemia

Abstract

Postprandial hyperglycemia is an early indicator of abnormality in glucose metabolism leading to type 2 diabetes. However, mechanisms that contribute to postprandial hyperglycemia have not been identified. This study showed that mice with targeted inactivation of the group 1B phospholipase A2 (Pla2g1b) gene displayed lower postprandial glycemia than that observed in wild-type mice after being fed a glucose-rich meal. The difference was caused by enhanced postprandial glucose uptake by the liver, heart, and muscle tissues as well as altered postprandial hepatic glucose metabolism in the Pla2g1b-/- mice. These differences were attributed to a fivefold decrease in the amount of dietary phospholipids absorbed as lysophospholipids in Pla2g1b-/- mice compared with that observed in Pla2g1b+/+ mice. Elevating plasma lysophospholipid levels in Pla2g1b-/- mice via intraperitoneal injection resulted in glucose intolerance similar to that exhibited by Pla2g1b+/+ mice. Studies with cultured hepatoma cells revealed that lysophospholipids dose-dependently suppressed insulin-stimulated glycogen synthesis. These results demonstrated that reduction of lysophospholipid absorption enhances insulin-mediated glucose metabolism and is protective against postprandial hyperglycemia.

FIG. 1

Absorption of phosphatidylcholine (PC) and LPC levels in the livers and plasma of postprandial Pla2g1b^+/+ and Pla2g1b^−/− mice. The Pla2g1b^+/+ and Pla2g1b^−/− mice fed control diet or diabetogenic diet were fasted overnight and then fed a 0.1-ml glucose-lipid test meal containing phosphatidyl-[³H]choline. After 2 h, hepatic portal blood was collected from the mesenteric portal vein to determine absorption of lipids associated [³H]choline (A). Additionally, retroorbital blood (B), and liver tissue (C) were collected and assayed for LPC. Increased LPC levels were confirmed by comigration of a radioactive band with an LPC standard by thin-layer chromatography. Data are the means ± SE (n = 4–6) and are representative of two experiments. *P≤ 0.05; **P ≤ 0.01; ***P≤ 0.001.

FIG. 2

Impact of Pla2g1b gene expression on oral glucose tolerance in mice. The Pla2g1b^+/+ and Pla2g1b^−/− mice, age-matched at 15 weeks, were maintained on a basal low-fat diet and then subjected to an oral glucose tolerance test or an intraperitoneal glucose tolerance test. After an overnight fast of 12 h, each group was then fed by oral gavage (A) or injected intraperitoneally (B) with 50% glucose (~2 g/kg) in saline. Glucose tolerance tests were also performed, using Pla2g1b^−/− mice injected with saline or saline plus LPC (32 mg/kg body wt) 5 min before oral (C) or intraperitoneal (D) administration of the glucose test meal. Blood was obtained via the tail vein before and 15, 30, 60, and 120 min after meal feeding for glucose analysis. Data are the means ± SE. Each panel is representative of at least two experiments of n = 5. *Statistical significant difference at P <0.05.

FIG. 3

Tissue glucose uptake and plasma insulin levels in Pla2g1b^+/+ and Pla2g1b^−/− mice. We fed 4-month-old Pla2g1b^+/+ and Pla2g1b^−/− mice control diet and injected them intraperitoneally with glucose (2 g/kg body wt) containing 2-deoxy-[³H]glucose after a fast to measure glucose uptake (A). Liver, heart, white fat, and muscle tissues were collected after 30 min for determination of radioactivity. Plasma insulin levels in Pla2g1b^+/+ and Pla2g1b^−/− mice after administration of a lipid-glucose test meal (B) were determined by radioimmunoassay. Data are the means ± SE (n = 4–7) and are representative of two experiments. *P ≤0.05; **P ≤0.01.

FIG. 4

Effects of LPC on insulin-induced glycogen synthesis. HepG2 (A) and H4- 2E→H4-2E (B) cells were serum starved for 12 h and then treated with [¹⁴C]glucose (0.25 Ci), BSA, or LPC as indicated. All cells were subsequently incubated for 2 h after insulin or empty vehicle where indicated. Cells were washed and lysed, and after glycogen precipitation samples were assayed for radioactivity in a scintillation counter. Shown is a representative of at least three experiments. Data are expressed as the means ± SE (n = 4). Bars with different letters were significantly different at P <0.05.

FIG. 5

Expression of G6Pase and glucose kinase in LPC-treated HepG2 cells (A and C) and in the livers (B and D) of Pla2g1b^+/+ and Pla2g1b^−/− mice. Total RNA was isolated from liver or HepG2 cells after treatment and analyzed for expression of G6Pase (A and B) and glucose kinase (C and D) mRNA by real-time quantitative PCR. Livers were obtained from Pla2g1b^+/+ and Pla2g1b^−/− mice after 18 h of fasting or after 2 h of feeding (after an 18-h fast) with a 0.1-ml glucose-lipid test meal containing 50% glucose (~2 g/kg body wt), 2.6 mmol/l egg phosphatidylcholine, 13.33 mmol/l triolein, and 2.6 mmol/l cholesterol. HepG2 cells were cultured to confluency in 12-well plates and serum-starved for 18 h before experiments. The cells were preincubated in the absence or presence of 100 nmol/l LPC, followed by an additional 2-h incubation with or without 20 mU/l insulin. Results were normalized to cyclophilin mRNA levels and are illustrated relative to wild-type fasting levels. Data are the means ± SE (n = 4). Bars with different letters were significantly different at P <0.05.