Macrophage-specific Pla2G6 deficiency modulates bone marrow Ly6C levels and APAP liver injury in a sex-dependent manner

Group VIA iPLA2 (iPLA2b) or PLA2G6 catalyzes hydrolysis of phospholipids at the sn-2 position, releasing a fatty acid and a lysophospholipid which has been shown to mediate neutrophil adhesion and monocyte migration in immune cells. Moreover, PLA2G6 single nucleotide polymorphism is associated with serum C-reactive protein suggesting PLA2G6 role in inflammation. Therefore, we aim to investigate whether Pla2G6 deficiency in macrophages could modulate liver injury by altering differentiation programs of lymphocyte antigen 6 complex (Ly6C) in the bone marrow.

Methods:

Macrophage-specific (lysMCre) Pla2G6 KO mice with exon 6 – 8 deletion were generated. Bone marrow-derived macrophages (BMDM) from control and KO mice were prepared. Acute liver injury was induced by intraperitoneal injection of acetaminophen (APAP 300 mg/kg), and mice were killed 24h later. Liver injury and gene expression analyses were assessed. BMDM phospholipids were measured by LC-MS. CD45+CD11b+CD115+Ly6G-Ly6C+ monocytes were measured by FACS.

Results:

BMDM of KO mice showed the absence of Pla2G6 protein and mRNA. Livers of KO mice still showed Pla2G6 protein expression indicating specific deletion in macrophages. By LC-MS analysis, BMDM from KO mice showed a significant decrease in monounsaturated lysophosphatidylcholine (LPC) and 18:2 LPC, and an increase of phosphatidylinositol (PI) 40:5 and phosphatidylserine (PS) 40:5. Upon APAP treatment, male KO mice showed a significant increase of AST, and a trend increase of ALT, LDH, and caspase3 activity. BMDM and livers of male KO mice respectively showed increased IL-6 mRNA and protein expression. The analyses of Ly6C mRNA expression and % ly6C+ monocytes by FACS showed no difference between male control and KO. On the other hand, female KO mice showed no worsening liver injury. However BMDM of female mutants showed a significant increase of ly6C mRNA expression, a trend increase of the M2 markers, Retnla and Chi3l3. Consistently, FACS results of BMDM showed a significant increase of % ly6C+ monocytes in these female KO mice.

Conclusions:

Pla2G6 has specificity in the hydrolysis of PI and PS in BMDM. Male KO BMDM showed no alteration of ly6C while female KO BMDM showed an increase of ly6C. As ly6C is thought to be involved in inflammation and resolution and that male KO mice showing worsen liver injury than female KO mice, hence deficiency of Pla2G6 in macrophages may modulate acute liver injury in a ly6C- and sex-dependent manner.