Pancreatic-type phospholipase A2 stimulates prostaglandin synthesis in mouse osteoblastic cells (MC3T3-E1) via a specific binding site

Group II phospholipase A2 activates mitogen-activated protein kinase in cultured rat mesangial cells

Group II phospholipase A2 (PLA2) is a mediator of inflammation in various disease including glomerulonephritis. We recently found that urinary excretion of PLA2 was increased in patients with mesangial proliferative glomerulonephritis and that interleukin-1 (IL-1) enhanced platelet derived growth factor-stimulated mesangial cell proliferation through the action of group II PLA2 secreted in response to IL-1 stimuli. Here we report signal transducing mechanism through group II PLA2 in mesangial cells. Group II PLA2 (1-15 U/ml) rapidly activated mitogen-activated protein (MAP) kinase. IL-1 beta activated MAP kinase in two phases and the slow activation in the late phase, proceeding in parallel with increased group II PLA2 secretion elicited by IL-1 treatment, was inhibited by the specific antibody raised against group II PLA2. This suggests that the late phase activation of IL-1-induced MAP kinase was mediated, at least in part, by secreted group II PLA2.

Detection and purification of two 14 kDa phospholipase A2 isoforms in rat kidney: their role in eicosanoid synthesis

Phospholipase A2 (PLA2) activity in the soluble fraction of rat kidney yielded three peaks on DEAE cellulose column chromatography. From these three, we purified two PLA2 isoforms to near-homogeneity. Both had a molecular weight of approx. 14,000 on SDS-PAGE, and immunochemical and enzymological studies indicated that one is a 14 kDa type I PLA2 and the other a 14 kDa type II PLA2. RNA blot analysis confirmed that rat kidney contains both types of PLA2 and that administration of lipopolysaccharides and mercury chloride into rats increased type II PLA2 mRNA levels in kidney. When cultured rat mesangial cells were incubated with purified type I or type II PLA2 in combination with the calcium ionophore A23187 at suboptimal condition, augmentation of prostaglandin E2 production was observed. Type I and type II forms of PLA2 may play a role in arachidonate metabolism in rat kidney.

Structural comparison of phospholipase-A2-binding regions in phospholipase-A2 receptors from various mammals

We determined the nucleotide sequence of a mouse cDNA encoding the receptor for pancreatic group I phospholipase A2 (PLA2-I). Interspecies structural comparison of the mouse receptor with bovine PLA2-I receptor, whose structure had been clarified, revealed that the fourth carbohydrate-recognition domain (CRD)-like domain (CRD-like 4) was the most conserved among the domains in the PLA2-I receptor, suggesting the functional importance of CRD-like 4. A transient expression experiment with a truncated form of the receptor consisting of three CRD-like domains, from the third to the fifth, demonstrated that the PLA2-I-binding site of the receptor is constituted from these three CRD-like domains, supporting the functional indispensability of CRD-like 4 in the receptor. Since the PLA2-I-binding region was thus assigned to be CRD-like domains 3-5, we further analyzed the structures of the PLA2-I-binding regions in the PLA2-I receptors from the rat, rabbit and human. Furthermore, the obtained PLA2-I receptor cDNA fragments from these animals made it possible to examine the tissue expression patterns of this receptor in various mammals. The results, together with the results of the genomic structural analysis of this gene, indicated that a PLA2 receptor recently characterized by Lambeau et al. [Lambeau, G., Ancian, P., Barhanin, J. & Lazdunski, M. (1994) J. Biol. Chem. 269, 1575-1578] is a rabbit counterpart of the PLA2-I receptor although these two PLA2 receptors have distinctive PLA2-binding specificities.

Studies on the induction of cyclooxygenase isozymes by various prostaglandins in mouse osteoblastic cell line with reference to signal transduction pathways

A mouse osteoblastic cell line MC3T3-E1 has a cyclooxygenase enzyme, and produces prostaglandin E2. When the cells were cultured in the presence of iloprost (a stable analogue of prostacyclin) or prostaglandin E1 or F2 alpha, the activity of cyclooxygenase increased in a dose- and time-dependent manner. The increase of the enzyme activity was attributed mostly to the cyclooxygenase isoform-2 because immunoprecipitation using an anti-cyclooxygenase-2 antibody removed the majority of the cyclooxygenase activity from the solubilized enzyme fraction, and the corresponding activity was detected in the immunoprecipitant. In addition, there was a marked increase in the cyclooxygenase-2 protein which was demonstrated by Western blotting. As analyzed by Northern blotting, the cyclooxygenase-2 mRNA increased and reached a maximum 1 and 3 h after the addition of iloprost and prostaglandin F2 alpha (about 15- and 60-fold increase), respectively, whereas the cyclooxygenase-1 mRNA increased slowly and only by about 3-fold. Iloprost and prostaglandin E1 stimulated the production of cAMP by 60-fold over the basal level, whereas the cAMP level was almost unchanged by prostaglandin F2 alpha. In contrast, prostaglandin F2 alpha stimulated IP3 production more efficiently than iloprost and prostaglandin E1. These results suggest that the stimulated syntheses prominently of cyclooxygenase-2 and to a lesser extent of cyclooxygenase-1 are mediated by at least two distinct signal transduction pathways involving the cAMP-synthesis stimulated by iloprost and prostaglandin E1 and the phosphoinositide turnover stimulated by prostaglandin F2 alpha.

Receptor-binding capability of pancreatic phospholipase A2 is separable from its enzymatic activity

Mammalian pancreatic phospholipase A2 (PLA2-I) has its specific receptor through which PLA2-I induces a variety of biological responses. In this study, a fundamental relationship between the enzymatic and the receptor-binding activities of PLA2-I was investigated. The specific binding of PLA2-I to the receptor was found to be independent of Ca2+ which is requisite for the PLA2 activity. On the basis of this observation, we designed and produced mutant PLA2-Is without Ca(2+)-binding abilities in order to demonstrate that the structural requirement for the enzymatic activity of PLA2-I is not identical with that for its receptor-binding reaction. These mutant PLA2-Is lost almost all enzymatic activity through a disturbance at the Ca(2+)-binding site, as expected, but still retained a substantial affinity to the receptor, allowing us to conclude that the receptor-binding reaction of PLA2-I is separable from its catalytic action.