Deficiency of calcium-independent phospholipase A2 beta in vivo causes systematic inflammation and hepatocellular carcinoma

Abstract

Introduction: Calcium-independent phospholipaseA2beta (iPLA2beta), which is one of the patatin-like phospholipase domain-containing genes, catalyzes the hydrolysis of phospholipids for cell membrane remodeling. It is known that iPLA2beta produces lysophosphatidylcholine as a 'find me signal' for phagocytes to remove apoptotic cells. We hypothesize that deficiency of iPLA2beta leads an accumulation of apoptotic cells, and that the subsequent inflammatory pressure induces the generation of proliferative hepatocytes which further leads to hepatocellular carcinoma (HCC). Methods: Whole body iPLA2beta-/- (with the deletion of the catalytic domain exon 9) and wild-type (WT) male mice were allowed to age to 20 months old. Histology and immunohistochemistry of livers, spleen and mesenteric lymph nodes were performed. Results: Compared with WT, iPLA2beta-/- mice exhibited reduced body, liver and visceral fat weight, and showed increased serum transaminases. Hepatic steatosis was observed in aged WT, but not in mutant livers. Not seen in WT livers, livers of mutant mice exhibited periportal inflammation, cholangitis, and necrosis with a 50%, 11%, 20% incidence rate, respectively. The mutant livers showed significant fibrosis as seen by Sirius red and α-smooth muscle actin staining. Mutant mice also had significantly decreased cellularity of B cells in the cortex of mesenteric lymph nodes concomitant with increased apoptosis in spleen. Among 34 aged mutant mice, 24% of them had large and dark reddish nodular HCC which showed portal vein obliteration and an invasion front together with decreased epithelial marker E-cadherin, and increased proliferation marker Ki67. The observed loss of cytokeratin 19 in mutant nodular HCC indicated the destruction of bile duct architecture. We also observed small and light color nodules of adenoma or HCC in 10% of 28 aged WT commonly seen in C57/BL6 mice. In another mouse line, the mutation in exon 7 of iPLA2beta gene did not produce any fibrosis nor HCC. Finally, iPLA2beta was expressed in normal human livers with or without steatosis, while being completely absent in human HCC livers without steatosis. Conclusions: In an absence of steatosis, the deficiency of exon 9 in iPLA2beta gene caused systemic inflammation which led to chronic aggressive hepatitis, fibrosis and HCC. Thus, iPLA2beta regulation on apoptosis in liver and immune organs may exacerbate in the development of HCC during aging.