engleski

Mechanisms of activation and cellular functions of phosphatidyl-inositol 3-kinase C2beta

The large family of phosphoinositide 3-kinase (PI3K) enzymes can be divided into three distinct classes (I, II and III) based on sequence similarity and lipid products they generate in vitro. Whilst biological roles have been assigned to the class I and class III PI3K enzymes, the significance of class II PI3K enzymes remains elusive. In mammalian cells there are three class II PI3K isoforms, PI3K-C2alfa, PI3K-C2beta and PI3K-C2gamma and they act as downstream targets of growth factor, chemokine and integrin receptors. PI3K-C2beta was cloned from U937 monocyte cDNA library and the enzyme is expressed in mammalian and insect cells. Like other class II PI 3-kinases in vitro, PI3K-C2beta utilizes phosphatidyl-inositol (PtdIns) and PtdIns(4)P but not PtdIns(4, 5)P2 as substrates. The enzyme is sensitive to the inhibitor wortmannin and is not regulated by the small GTP-binding protein Ras. Deletion of C2 domain increased the lipid kinase activity suggesting that it functions as a negative regulator of the catalytic domain. The enzyme has wide tissue distribution and is predominately associated with membrane fractions of the cells. The proline-rich motifs within the N-terminal region of PI3K-C2beta mediate the association of the enzyme with activated EGF receptor and this interaction involves the Grb2 adaptor. In platelets, enzyme is activated following stimulation of integrin alfaIIbbeta3 receptor with fibrinogen and causes transient formation of PtdIns(3)P and generation of PtdIns(3, 4)P2 which activates protein kinase B (PKB). However, the expression of a kinase inactive mutant attenuated phosphorylation of PKB suggesting its role in the generation of PtdIns(3, 4, 5)P3. On the other hand, stimulation of c-Met receptor in basal-lateral plasma membrane of renal cortical tubular cells, causes calpain-mediated activation of the enzyme in the brush-border plasma membrane with concomitant increase in PtdIns(3)P. PtdIns(3)P can interact with FYVE fingers and this motif is commonly found in the proteins which function at endosomes. Furthermore, elevated expression of the enzyme increases cell motility and stimulates cell fusion by a mechanism that involves PtdIns(3)P production. In Drosophila melanogaster, ectopic expression of wild-type and mutated version of the enzyme, which is catalytically inactive, in the larval imaginal disks affected the development of the wing veins, the wing margin, and the number of external sense organs, suggesting a role of the enzyme in signalling pathways affecting pattering. In the membrane-depleted nuclei following partial hepatectomy an increase in PtdIns(3)P formation due to PI3K-C2beta activation was observed, and this was caused by calpain-mediated proteolytic cleavage of the enzyme. Furthermore, when HL-60 cells were allow to progress synchronously through cell cycle above mentioned activation of the enzyme was observed to occur at G2/M phase of the cell cycle. Finally, when human promyelocytic cell line HL-60 was differentiated along neutrophilic pathway by all-trans-retinoic acid, tyrosine phosphorylation and activation of PI3K-C2beta in nuclear envelopes was observed together with intranuclear formation of PtdIns(3)P.

phosphatidyl-inositol 3-kinase C2beta; classes; mammalian cells; nuclei

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