Deficiency of group via Phospholipase A2 in OB/OB mice renders protection against hepatic steatosis by reduction of VLDL secretion and alteration of phospholipid fatty acid composition
Introduction: iPLA2β hydrolyzes fatty acyl bond at the sn-2 position of phospholipids (PL) recognized as a key process on homeostatic membrane metabolism. iPLA2β and its product lysophospholipids play a role in lipid synthesis and very low density lipoprotein (VLDL) assembly. Our previous study shows that global whole body deficiency of iPLA2β causes lower body weight as well as lower serum and hepatic lipids when compared with the control littermates (WT). Aims: We herein tested whether global deficiency of iPLA2β would attenuate adiposity and hepatic steatosis in leptin-knockout Ob/Ob (OB) mice. Methods: We performed cross-breeding between leptin (+/-) and iPLA2β (+/-) mice to generate leptin (-/-)/iPLA2β(-/-) double-knockout (PKO-OB) mice. Male and female mice at 6 – 7 months old (n = 5 – 9) were used. Serum transaminases were determined using diagnostic kits. Serum lipoprotein profiles were analyzed by Liposearch service. Lipidomics analyses in mouse livers were studied by using GC-MS and ESI-MS/MS. Formalin-fixed liver sections were stained with hematoxylin-eosin (H&E). Hepatic gene expression was studied by quantitative TaqMan RT-PCR. Results: Compared with WT, OB mice had marked obesity with fatty liver and liver injury. Compared with OB, PKO-OB mice showed significant reduction of body weight (-25%), epididymal fat (-54%), and retroperitoneal fat (-23%). This was concomitant with reduction of serum ALT (-50%), LDH (-55%) as well as cholesterol (Chol) (-44%), non-esterified fatty acids (-72%), and PL (-34%). The total triglycerides (TG) and Chol associated with lipoproteins were decreased in PKO-OB compared with OB. The most significant difference between PKO-OB and OB was the reduction of VLDL in both TG and Chol, particularly those with very small sized particles. GC-MS analyses showed that the total, saturated and mono-unsaturated fatty acids levels (nmol/mg liver) were decreased in PKO-OB. The contents of hepatic cholesterol esters (CE), mainly seen in CE 18:1 and CE 16:1, were decreased in PKO-OB. Among 9 PL classes analyzed by ESI-MS/MS, OB livers contained decreased contents of total LPC, PS and PC, and these PLs that specifically contained omega-3 20:5, 22:5, 22:6 fatty acids were reversed by iPLA2β deficiency indicating an improvement. Histology data revealed that PKO-OB mice exhibited decreased hepatic steatosis, this was concomitant with significant reduction of hepatic mRNA expression of transcription factors PPARγ, SREBP1c and inflammatory MCP-1. Conclusions: Deficiency of iPLA2β in obese mice decreased adiposity and elicited protection against hepatic steatosis, and liver injury. This was associated with significant reduction of total fatty acids, VLDL secretion, hepatic CE, and alteration of PL fatty acid composition. Our data shed lights on the role of iPLA2β on adipose and hepatic lipid syntheses in fatty liver and obesity, and that this gene may be used as a therapeutic target.