Cytosolic phospholipase A₂: physiological function and role in disease

Abstract

The group IV phospholipase A2 (PLA2) family is comprised of six intracellular enzymes (GIVA, -B, -C, -D, -E, and -F) commonly referred to as cytosolic PLA2 (cPLA2)α, -β, -γ, -δ, -ε, and -ζ. They contain a Ser-Asp catalytic dyad and all except cPLA2γ have a C2 domain, but differences in their catalytic activities and subcellular localization suggest unique regulation and function. With the exception of cPLA2α, the focus of this review, little is known about the in vivo function of group IV enzymes. cPLA2α catalyzes the hydrolysis of phospholipids to arachidonic acid and lysophospholipids that are precursors of numerous bioactive lipids. The regulation of cPLA2α is complex, involving transcriptional and posttranslational processes, particularly increases in calcium and phosphorylation. cPLA2α is a highly conserved widely expressed enzyme that promotes lipid mediator production in human and rodent cells from a variety of tissues. The diverse bioactive lipids produced as a result of cPLA2α activation regulate normal physiological processes and disease pathogenesis in many organ systems, as shown using cPLA2α KO mice. However, humans recently identified with cPLA2α deficiency exhibit more pronounced effects on health than observed in mice lacking cPLA2α, indicating that much remains to be learned about this interesting enzyme.

Keywords: Golgi apparatus; arachidonic acid; calcium; cyclooxygenase; inflammation; leukotrienes; lipoxygenase; prostaglandins; protein kinases/MAP kinase.

Fig. 1.

Regulation of cPLA₂α. cPLA₂α-mediated arachidonic acid (AA) release occurs following cell stimulation that results in increases in intracellular Ca²⁺ and kinase activation. In the cytosol, cPLA₂α is bound to p11/annexin A2 and is released from this complex when phosphorylated on S727. Calcium binds to the calcium binding loops (CBL, green) at the membrane binding face of the C2 domain. This increases the hydrophobicity of the C2 domain and promotes preferential binding of cPLA₂α to the Golgi and, at higher levels of intracellular calcium, to the ER and nuclear envelope. Basic residues (orange patch) in the C2 domain are implicated in binding to ceramide-1-P in the Golgi. Association of the catalytic domain with membrane is mediated in part by a tryptophan residue in the catalytic domain (W464, red patch) that stabilizes cPLA₂α on the membrane. Phosphorylation of S505 by MAPKs in the catalytic domain enhances cPLA₂α activity. Interaction of basic residues in the catalytic domain (blue patch) with anionic components in the membrane, such as polyphosphoinositides, optimizes catalytic activity. AA released by cPLA₂α is metabolized by COX-1 and -2, which are primarily localized to the ER, but also occur on the Golgi. Leukotriene production occurs at the nuclear envelope, the site of 5-LO translocation in response to increases in calcium. 5-LO binds to the scaffold protein FLAP, an AA binding protein, which also binds LTC₄ synthase (syn) for production of LTC₄.

Fig. 2.

Ribbon diagram of cPLA₂α structure. Two ions of calcium bind to the calcium-binding loops (CBL) on the membrane binding face of the C2 domain. The catalytic domain with the active site nucleophile S228 contains residues involved in cPLA₂α regulation (pink) that include phosphorylation sites (S505, S727), basic residues (K488, K544, K543), and W464 important for membrane interaction. The residues mutated in humans with cPLA₂α deficiencies are shown in yellow. The residue V707 is predicted to be the start of the deletion in patients with cryptogenic multifocal ulcerating stenosing enteritis.