The Role of Secretory Phospholipase A2 Group IIA in Obesity and Metabolism

Secretory phospholipase A2 group IIA (PLA2G2A) is a member of a family of secretory phospholipases previously implicated in inflammation, atherogenesis, and antibacterial actions. These enzymes hydrolyze glycerophospholipids at the sn-2 position releasing lysophospholipids and fatty acids. Though studies have shown PLA2G2A is pro-inflammatory and promotes atherosclerosis, no research has analyzed the role of this enzyme in obesity and metabolism. Studies in the past 5-10 years utilizing various knock- out or over-expression mouse models have analyzed the role of different secretory phospholipase A2s (sPLA2) in metabolic diseases. From these studies, it is known that at least seven of the 11 sPLA2 isozymes contribute to the prevention or causation of metabolic diseases such as obesity and Type 2 diabetes. Increased energy expenditure, insulin sensitivity, and glucose tolerance are common characteristics of a mouse with a beneficial metabolic phenotype. We studied the impact of a high-fat diet on wild-type C57BL/6 and C57BL/6 mice over-expressing the human PLA2G2A gene (IIA+ mice). Despite no differences in food intake, the IIA+ mice weighed less, had increased oxygen consumption and energy expenditure, increased glucose tolerance and insulin sensitivity, and significantly less fat mass. There was a significant induction of uncoupling protein 1 (UCP1) in the brown adipose tissue (BAT) of IIA+ mice, suggesting enhanced mitochondrial uncoupling in their adipose tissue to drive this metabolic phenotype. Previous work in Dr. Park’s laboratory suggested that hepatic PLA2G2A expression is negatively regulated by acute thyroid hormone administration. We also examined the regulation of PLA2G2A and the metabolic changes that occur in response to variations in thyroid status. The impact of PLA2G2A on brown adipose tissue thermogenic gene expression was explored. C57BL/6 and IIA+ mice were made hypothyroid over a 10-week period or treated with thyroid hormone (T3) for 5 weeks. There were no significant changes in PLA2G2A abundance in response to thyroid status. The hypothyroid IIA+ mice did not increase their energy expenditure even with T3 administration. However, the energy expenditure, substrate utilization, insulin sensitivity and glucose tolerance were all elevated in the IIA+ mice given T3. Moreover, white adipocytes from IIA+ mice were much more prone to ‘beiging,’ including increased expression of brown adipose thermogenic markers such as uncoupling protein 1 (UCP1), PR domain containing 16 (PRDM-16), and early B cell factor 2 (EBF2). Finally, the brown adipose tissue of IIA+ mice had increased UCP1. Further metabolic analysis of C57BL/6 and IIA+ mice revealed that even at thermoneutrality the IIA+ mice display elevated oxygen consumption and energy expenditure. Moreover, citrate synthase activity was increased in the brown adipose tissue but not muscle in IIA+ animals, suggesting that the elevated thermogenesis is adipose tissue-specific. Finally, RNA microarray analysis of brown adipose tissue from C57BL/6 and IIA+ mice showed induction of numerous genes related to fuel substrate transport and metabolism, indicating an increase in substrate flux to fuel brown adipose tissue activity the IIA+ mice. These data illustrate that PLA2G2A elevates thermogenesis in an adipose tissue-specific manner, in part through elevated fuel utilization and increased mitochondrial content in BAT