Glucocorticoids are widely used as one of the most efficacious medicines against various inflammatory and immunological diseases such as rheumatoid arthritis, systemic lupus erythematosus and bronchial asthma. Their anti-inflammatory action is mediated by inhibition of arachidonic acid release, a precursor of prostaglandins (PGs), hydroxyeicosatetraenoic acids (HFTEs) and leukotrienes (LTs). Phos-pholipase A, (PLA2) and alkylphospholipids, a precursor of platelet-activating factor (PAF), in biomem-branes have been considered as the key enzyme and the major source of arachidonic acid, respectively. Anti-inflammatory steroids also inhibit the forma tion of both PAF and its inactive lyso-form (deacylated molecule). Since most of the antiinflammatory actions of glucocorticoids are inhibited in vivo and in vitro by inhibitors of protein synthesis as well as mRNA synthesis, it was proposed that the anti-inflammatory action of glucocorticoids is partly, if not exclusively, mediated through the induced synthesis of PLA2 inhibitory protein(s).
Between 1980 and 1984, attempts were made by several groups to isolate a protein 1) whose synthesis is induced by glucocorticoids, 2) that inhibits the activity of PLA2 in vitro and in vivo, and 3) that displays an anti-inflammatory action like glucorti-coids. Several different proteins that inhibit PLA, in vitro were identified and partially purified. These included 'lipomodulin' from rabbit neutrophils (molecular weight = 40 kDa), 'renocortin' from rat kidney cells (molecular weight = 30 kDa) and 'mac-rocortin' from rat macrophages (molecular weight = 15 kDa). Among these proteins, lipomodulin was best characterized: a possibly glycosylated protein with an apparent molecular weight of 40 kDa and a pi value of 9.0. This protein can bind 2 mol of Ca21 mol 1 protein, and the concentration of Ca2 required for half maximal binding is approximately 1 ¡jlm. It was proposed that this protein suppresses PLA2 activity by forming a complex with PLA2 at the Ca2+-binding site(s). Furthermore, various protein kinases including protein kinase C and tyrosine protein kinase, can phosphorylate lipomodulin to modify its function and structure as detected by the inhibitory activity against PLA2 and the susceptibility to proteases.
Despite differences in their chemical properties, all proteins isolated as second messengers of glucocorticoid anti-inflammatory action have similar, if not identical, biological properties: 1) the synthesis of these proteins is induced by glucocorticoids, 2) preparations of these proteins, regardless of their purity, inhibit porcine pancreas and/or snake venom PLA2 in vitro, 3) these protein preparations mimic some actions of glucocorticoids, especially with regard to arachidonic acid release and production of lipid mediators such as prostaglandins, and 4) some monoclonal antibodies raised in one laboratory cross-react with the proteins prepared by the other laboratories. Based upon these findings, it was proposed in 1984 that a family of PLA2 inhibitory proteins whose syntheses are induced by glucocorticoids be named as /¿po(modulin, macro- or reno-)cortins (lipocortins). In 1986, the Biogen group purified two proteins from human placenta whose properties are almost identical to those of lipomodulin. They isolated cDNA clones encoding these proteins, and described their chemical natures as follows: 1) their molecular weights are around 36 kDa, 2) they have multiple phosphorylation sites including one for tyrosine protein kinase and a potential glycosylation site, 3) they have four repeats which can bind acidic phospholipids and Ca2+, and 4) they inhibit PLA2 in vitro. Interestingly, these cDNA clones were almost identical to the simultaneously discovered cDNAs that encode two of the major substrate proteins (calpactins) for the EGF receptor (tyrosine) kinase.
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