Regulation of 14 kDa Group II PLA₂ in Rat Mesangial Cells

Abstract

Phospholipase A₂ (PLA₂) is believed to play an essential role in inflammation through the release of arachidonic acid from membrane phospholipids for the production of important lipid mediators such as eicosanoids and platelet activating factor (Van den Bosch, 1980; Glaser et al, 1993). The last years it has become clear that PLA₂S are a heterogeneous family of enzymes that can be classified in two classes based on their molecular weight. There is a class of low molecular weight PLA₂S (14 kDa) and one of the more recently discovered high molecular mass enzymes (85 kDa). The high molecular weight PLA₂, also referred to as cPLA₂, is mainly located in the cytosolic fraction of cells and tissues including human platelets (Takayamo et al, 1991), rat renal mesangial cells (Gronich et al, 1988; Bonventre et al, 1990), and the human monoblast U937 cell line (Clark et al., 1990; Kramer et al, 1991). Although the enzyme has been shown to be 85 kDa by sequence and cloning (Clark et al, 1991; Sharp et al, 1991), it shows a molecular weight on SDS-PAGE of about M_r 110,000 (Leslie et al, 1988; Clark et al, 1990; Kramer et al, 1991). This enzyme preferentially hydrolyzes arachidonic acid from the sn-2-position of phospholipids (Clark et al, 1990), is insensitive for dithiotreitol and has optimal activity at micromolar Ca²⁺-concentrations (Gronich et al, 1990). cPLA₂ also comprises multiple phosphorylation sites, among which a MAP-kinase phosphorylation site (serine-505) that appears to play an important role in enzyme activation (Liscovitch and Cantley, 1994). The 14 kDa PLA₂S can be further divided in two groups, based on their amino acid sequence (Heinrikson et al, 1977). Mammalian group I PLA₂ comprises the pancreatic type of PLA₂ and is characterized by the presence of cys 11. Homologous non-pancreatic group II phospholipase A₂ is lacking cys 11. Type II PLA₂ is often found as a membrane-bound, enzyme (Aarsman et al, 1989; Ono et al., 1988) which has an optimal activity at millimolar Ca²⁺-concentrations (Mizushima et al., 1989) and has no selectivity for arachidonic acid (Schalkwijk et al, 1990).