Hrvoje Banfíƈ

The aggregation of human platelets is an important physiological hemostatic event contingent upon receptor-dependent activation of the surface integrin alphaIIbbeta3 and subsequent binding of fibrinogen. Aggregating platelets form phosphatidylinositol 3, 4-bisphosphate (PtdIns(3,4)P2), which has been reported to stimulate in vitro the activity of the proto-oncogenic protein kinase PKB/Akt, as has phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3). It has been assumed that PtdIns(3,4)P2 is synthesized by either 5-phosphatase-catalyzed hydrolysis of PtdIns(3,4,5)P3 produced by phosphoinositide 3-kinase (PI3K) or phosphorylation by PI3K of PtdIns4P. We investigated the route(s) by which PtdIns(3,4)P2 is formed after directly activating alphaIIbbeta3 with anti-ligand-induced binding site Fab fragment and report that aggregation does not lead to the generation of PtdIns(3,4,5)P3, but to transient formation of PtdIns3P and generation of PtdIns(3,4)P2, the latter primarily by PtdIns3P 4-kinase. Both this novel pathway and the activation of PKB/Akt are inhibited by the PI3K inhibitor, wortmannin, and the calpain inhibitor, calpeptin, constituting the first evidence that PtdIns(3,4)P2 can stimulate PKB/Akt in vivo in the absence of PtdIns(3,4,5)P3. Integrin-activated generation of the second messenger PtdIns(3,4)P2 thus depends upon a route distinct from that known to be utilized initially by growth factors. This pathway is of potential general relevance to the function of integrins.

Highly purified nuclei were prepared from livers and kidneys of rats undergoing compensatory hepatic or renal growth, the former being predominantly by cellular proliferation, and the latter mostly by cellular enlargement. In liver, an increase in nuclear diacylglycerol (DAG) concentration occurred between 16 and 30 h, peaking at around 20 h. At the peak of nuclear DAG production a specific translocation of protein kinase C to the nucleus could be detected; no such changes occurred in kidney. There was no detectable change in whole-cell DAG levels in liver, and the increase in DAG was only measurable in nuclei freed of their nuclear membrane. Overall, these results suggest that there is a stimulation of intranuclear DAG production, possibly through the activation of an inositide cycle [Divecha, Banfić and Irvine (1991) EMBO J. 10, 3207-3214] during cell proliferation in vivo.

We have observed that aggregation of human platelets, caused by activation of integrin alphaIIb beta3 and its consequent binding of fibrinogen, stimulates a novel pathway for synthesis of phosphatidylinositol 3,4bisphosphate, thereby activating protein kinase B/Akt. Such synthesis depends upon both the generation of phosphatidylinositol 3-phosphate (PtdIns3P), which is sensitive to wortmannin (IC50 7 nM) and calpain inhibitors, and the phosphorylation of PtdIns3P by PtdIns3P 4-kinase. We now report that a recently characterized C2 domain-containing phosphoinositide 3-kinase isoform (HsC2-PI3K) is present in platelets and a leukemic cell line (CHRF-288) derived from megakaryoblasts, and is likely to be responsible for the stimulated synthesis of PtdIns3P observed in platelets. HsC2-PI3K, identifiable by Western blotting and immunoprecipitatable activity, is sensitive to wortmannin (IC50 6-10 nM), requires Mg2+, and shows strong preference for PtdIns over PtdIns4P or phosphatidylinositol 4,5-bisphosphate as substrate. HsC2-PI3K is activated severalfold when platelets aggregate in an alphaIIb beta3-dependent manner or when platelet or CHRF-288 lysates are incubated with Ca2+. Activation is prevented by calpain inhibitors. CHRF-288, which cannot undergo activation of alphaIIb beta3 and thereby aggregate in response to platelet agonists, do not generate PtdIns3P or activate HsC2-PI3K under conditions that stimulate other phosphoinositide 3-kinases. HsC2-PI3K may thus be an important effector for integrin-dependent signaling.