Platelet activation—a role for a 40K anti-phospholipase A2 protein indistinguishable from lipocortin

Abstract

Stimulus-response (S-R) coupling in platelets requires an intermediary other than an elevation in cytosolic free calcium ([Ca2+]i). While an increase in [Ca2+]i is essential in S-R coupling, effecting phosphorylation of myosin of relative molecular mass (Mr) 20,000 (20 K), platelet activation is also associated with phosphorylation of a 40K protein, which can occur in the absence of changes in [Ca2+]i. The 40K protein is the substrate for protein kinase C (PKC). Mounting evidence suggests that activation of PKC by diacylglycerol is the other signal involved in S-R coupling. Although phosphorylation of the 40K protein is associated with certain platelet functional responses, no precise role has been accredited to it. Recently, we and others have described several proteins (collectively known as lipocortin) which inhibit phospholipase A2 (PLA2). One of the most conspicuous proteins of this group is a 40K peptide whose inhibitory activity can be suppressed by prior phosphorylation. We hypothesized that the 40K protein described in platelets may possess anti-PLA2 activity and that phosphorylation by PKC, suppressing its inhibitory activity, may represent the mechanism underlying mobilization of arachidonic acid, the precursor of prostaglandins. The results of the present study strongly support this hypothesis.

Isolation of two 67 kDa calcium‐binding proteins from pig lung differing in affinity for phospholipids and in anti‐phospholipase A2 activity

“…This is probably not the case for 67R, which was revealed as a poor substrate for protein kinase C. As recently shown by Touqui et al [33], platelets contain, in addition to LC, a protein eluting at around 67 kDa upon molecular sieving on a TSK-G2000 column, whose activity is not modulated by protein kinase C activation. It is thus tempting to suggest some similarity if not identity between that platelet protein and the 67R protein from lung.…”

Production of lipocortin‐like proteins by cultured human tracheal submucosal gland cells

“…Several reports have shown that the 35 kDa and 67-70 kDa lipocortin-like protein are closely related with respect to sequence, antiserum cross-reactivity and binding of Ca 2+ and phospholipid (see [35] for refs). Lipocortin I is a substrate for both the EGF receptor tyrosyl kinase [36] and the protein kinase C (PKC) [2,37]. Antonicelli et al [32] recently showed that among four proteins (32, 35, 36 and 67-70 kDa) of the lipocortin family, only lipocortin I was the endogenous substrate for the PKC in thyroid-stimulating hormone (TSH) treated cells.…”

1,2‐Dioctanoylglycerol but not 1‐oleoyl‐2‐acetylglycerol inhibits agonist‐induced platelet responses

“…Recent studies [36] have postulated a stimulatory role for C kinase in agonist-induced arachldonate release achieved via phosphorylation of the anti-phospholipase A2 protein, lipocortin, with no mention of the type of agonist this would be applicable to. Our results suggest that where the coupling processes involved in arachidonate release are inhibited by C kinase activation, as seems to be the case with thrombin, this inhibition may supercede any stimulatory effects of phosphorylated lipocortin on arachidonate release.…”