Cross-talk between α4β1/α5β1 and c-Kit results in opposing effect on growth and survival of hematopoietic cells via the activation of focal adhesion kinase, mitogen-activated protein kinase, and Akt signaling pathways

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Abstract
The SH2-containing inositol-5′-phosphatase, SHIP, restrains bone marrow–derived mast cell (BMMC) degranulation, at least in part, by hydrolyzing phosphatidylinositol (PI)-3-kinase generated PI-3,4,5-P3 (PIP3) to PI-3,4-P2. To determine which domains within SHIP influence its ability to hydrolyze PIP3, bone marrow from SHIP−/− mice was retrovirally infected with various SHIP constructs. Introduction of wild-type SHIP into SHIP−/− BMMCs reverted the Steel factor (SF)-induced increases in PIP3, calcium entry, and degranulation to those observed in SHIP+/+ BMMCs. A 5′-phosphatase dead SHIP, however, could not revert the SHIP−/− response, whereas a SHIP mutant in which the 2 NPXY motifs were converted to NPXFs (2NPXF) could partially revert the SHIP−/− response. SF stimulation of BMMCs expressing the 2NPXF, which could not bind Shc, led to the same level of mitogen-activated protein kinase (MAPK) phosphorylation as that seen in BMMCs expressing the other constructs. Surprisingly, C-terminally truncated forms of SHIP, lacking different amounts of the proline rich C-terminus, could not revert the SHIP−/− response at all. These results suggest that the C-terminus plays a critical role in enabling SHIP to hydrolyze PIP3 and inhibit BMMC degranulation.