Characterization of FKGK18 as Inhibitor of Group VIA Ca2+-Independent Phospholipase A2 (iPLA2β): Candidate Drug for Preventing Beta-Cell Apoptosis and Diabetes

Ongoing studies suggest an important role for iPLA(2)beta in a multitude of biological processes and it has been implicated in neurodegenerative, skeletal and vascular smooth muscle disorders, bone formation, and cardiac arrhythmias. Thus, identifying an iPLA(2)beta inhibitor that can be reliably and safely used in vivo is warranted. Currently, the mechanism-based inhibitor bromoenol lactone (BEL) is the most widely used to discern the role of iPLA(2)beta in biological processes. While BEL is recognized as a more potent inhibitor of iPLA(2) than of cPLA(2) or sPLA(2), leading to its designation as a "specific" inhibitor of iPLA(2), it has been shown to also inhibit non-PLA(2) enzymes. A potential complication of its use is that while the S and R enantiomers of BEL exhibit preference for cytosol-associated iPLA(2)beta and membrane-associated iPLA(2)gamma, respectively, the selectivity is only 10-fold for both. In addition, BEL is unstable in solution, promotes irreversible inhibition, and may be cytotoxic, making BEL not amenable for in vivo use. Recently, a fluoroketone (FK)-based compound (FKGK18) was described as a potent inhibitor of iPLA(2)beta. Here we characterized its inhibitory profile in beta-cells and find that FKGK18: (a) inhibits iPLA(2)beta with a greater potency (100-fold) than iPLA(2)gamma, (b) inhibition of iPLA(2)beta is reversible, (c) is an ineffective inhibitor of alpha-chymotrypsin, and (d) inhibits previously described outcomes of iPLA(2)beta activation including (i) glucose-stimulated insulin secretion, (ii) arachidonic acid hydrolysis; as reflected by PGE2 release from human islets, (iii) ER stress-induced neutral sphingomyelinase 2 expression, and (iv) ER stress-induced beta-cell apoptosis. These findings suggest that FKGK18 is similar to BEL in its ability to inhibit iPLA(2)beta. Because, in contrast to BEL, it is reversible and not a non-specific inhibitor of proteases, it is suggested that FKGK18 is more ideal for ex vivo and in vivo assessments of iPLA(2)beta role in biological functions.