The control of phosphatidylinositol 3,4-bisphosphate concentrations by activation of the Src homology 2 domain containing inositol polyphosphate 5-phosphatase 2, SHIP2

Activation of class Ia PI3K (phosphoinositide 3-kinase) produces PtdInsP₃, a vital intracellular mediator whose degradation generates additional lipid signals. In the present study vanadate analogues that inhibit PTPs (protein tyrosine phosphatases) were used to probe the mechanisms which regulate the concentrations of these molecules allowing their independent or integrated function. In 1321N1 cells, which lack PtdInsP₃ 3-phosphatase activity, sodium vanadate or a cell permeable derivative, bpV(phen) [potassium bisperoxo(1,10-phenanthroline)oxovanadate (V)], increased the recruitment into anti-phosphotyrosine immunoprecipitates of PI3K activity and of the p85 and p110α subunits of class Ia PI3K and enhanced the recruitment of PI3K activity stimulated by PDGF (platelet-derived growth factor). However, neither inhibitor much increased cellular PtdInsP₃ concentrations, but both diminished dramatically the accumulation of PtdInsP₃ stimulated by PDGF or insulin and markedly increased the control and stimulated concentrations of PtdIns(3,4)P₂. These actions were accounted for by the ability of PTP inhibitors to stimulate the activity of endogenous PtdInsP₃ 5-phosphatase(s), particularly SHIP2 (Src homology 2 domain containing inositol polyphosphate 5-phosphatase 2) and to inhibit types I and II PtdIns(3,4)P₂ 4-phosphatases. Thus bpV(phen) promoted the translocation of SHIP2 from the cytosol to a Triton X-100-insoluble fraction and induced a marked (5–10-fold) increase in SHIP2 specific activity mediated by enhanced tyrosine phosphorylation. The net effect of these inhibitors was, therefore, to switch the signal output of class I PI3K from PtdInsP₃ to PtdIns(3,4)P₂. A key component controlling this shift in the balance of lipid signals is the activation of SHIP2 by increased tyrosine phosphorylation, an effect observed in HeLa cells in response to both PTP inhibitors and epidermal growth factor.