Abstract 5524: Altered Hepatic Triacylglycerol Metabolism and VLDL Production in the Mice with Targeted Disruption Of Ca2+-independent Phospholipase A2 (iPLA2β)
Previously we showed that chemical inhibition of iPLA2β in rat hepatoma McA-RH7777 cells attenuated hepatic VLDL assembly/secretion. To gain an insight into the role that iPLA2β plays in vivo, we generated C57BL/6 mice with targeted disruption of iPLA2β gene. The iPLA2β−/− mice developed normally to their maturity. Under chow diet conditions, female iPLA2β−/− and wild-type littermate (10-week-old) displayed no discernable difference in plasma lipid concentration or apolipoprotein levels, nor was their plasma lipoprotein profiling changed as determined by rate floatation ultracentrifugation. However, iPLA2β−/− mice liver accumulated 20% less triacylglycerol (TG) than wt controls, whereas phosphatidylcholine (PC) and phosphatidylethanolamine (PE) levels were not significantly different. Determination of VLDL-TG production rate in vivo using mice injected with poloxamer 407 (to block the lipoprotein lipase activity) revealed that accumulation of plasma TG in iPLA2β−/− mice was 70% of that in wild-type mice. Metabolic labeling studies using primary hepatocytes showed that incorporation of [3H]glycerol into TG in iPLA2β−/− cells was also 70% of that in wild-type cells. These in vivo and ex vivo data suggest that iPLA2β indeed plays a role in hepatic TG synthesis and secretion. However, the level of expression of genes involved in lipogenesis such as SREBP-1c, FAS, GPAT1, DGAT1 and DGAT2, was not altered by iPLA2β inactivation. In iPLA2β−/− hepatocytes, incorporation of [3H]glycerol into 3H-PC was increased by 60% during metabolic labeling. Pulse-chase studies (using [3H]oleate as tracer) showed that iPLA2β inactivation abolished turnover of [3H]oleate-labeled PC, confirming that iPLA2β mediates the turnover of hepatic PC. These data suggest that hepatic phospholipids turnover is intimately linked to TG synthesis and secretion, and diminishing phospholipids turnover catalyzed by iPLA2β may represent a novel pharmaceutical target for attenuating hepatic VLDL overproduction without causing hepatosteatosis.