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 p110a 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.

Suppression of MEK/ERK signalling by Myc: role of Bin-1.

We report for the first time that over-expression of Myc suppresses mitogen-activated ERK kinase (MEK)/extracellular regulated kinase (ERK) signalling in chick embryo fibroblasts (CEF). Myc does not interfere with individual components of the signalling cascade, since efficient signal propagation via MEK and ERK in Myc-infected CEF can be seen. However, using the Myc-binding domain (MBD) of Bin-1, which binds to and negatively regulates the activity of Myc, we selectively suppressed Myc-induced apoptosis, without affecting its transforming properties. This was accompanied by a restoration in MEK/ERK signalling, suggesting a critical role for this pathway in regulating apoptosis in these cells. This was also confirmed using a specific pharmacological inhibitor of MEK. Experiments with conditioned media suggest that over-expression of Myc may inhibit autocrine growth factor production, which can be restored by co-expression of MBD. Although the identity of the growth factor(s) is not known, we propose a feedback mechanism whereby Myc interferes with growth factor signalling.

Proinflammatory cytokines increase iron uptake into human monocytes and synovial fibroblasts from patients with rheumatoid arthritis.

BACKGROUND: It has been hypothesized that iron is stored in the synovium of patients with rheumatoid arthritis which perpetuates inflamation by aiding the production of oxygen free radicals. Proinflammatory cytokines are produced by macrophages and lymphocytes present within synovium and by mononuclear cells of in synovial fluid from patients with rheumatoid arthritis. There are two known systems for iron uptake. The first involves binding of iron to transferrin and uptake via transferrin receptors. The second involves uptake by low molecular weight organic anions such as ascorbate and citrate (non-transferrin bound uptake). MATERIALS/METHODS: Proinflammatory cytokines (IL-1, IL-6, TNFalpha and interferon gamma) were added to fibroblasts isolated from patients with rheumatoid arthritis and human monocytes in culture and their effect on 59Fe-transferrin and citrate uptake was determined. RESULTS: Proinflammatory cytokines increase transferrin and non-transferrin bound iron uptake into human monocytes and increase transferrin-bound iron uptake by synovial fibroblasts, but have no effect on non-transferrin bound uptake into fibroblasts. CONCLUSIONS: Proinflammatory cytokines produced in human rheumatoid arthritis synovium and synovial fluid may contribute to the accumulation of iron that occurs in rheumatoid arthritis synovium which may lead to damage to synovial fibroblasts, macrophages and lymphocytes.