Inhibitory Modulation of B Cell Receptor-mediated Ca2+ Mobilization by Src Homology 2 Domain-containing Inositol 5′-Phosphatase (SHIP)

Abstract

2؉ influx (SOC) elicited by thapsigargin-induced store depletion was not affected by SHIP. These results indicate that the primary target pathway of SHIP is the Ca 2؉release from the stores, and that Ca 2؉ influx by the SOC mechanism is secondarily controlled by the level of Ca (4,5). BCR activation also results in the activation of phosphoinositide 3-kinase, which converts PIP 2 to phosphatidylinositol 3,4,5-trisphosphate (PIP 3 ).In addition to BCR, B lymphocytes express another class of immunoreceptors, Fc␥RIIB. Co-cross-linking of BCR and Fc␥RIIB attenuates the BCR-induced Ca 2ϩ response (6-10). The attenuation of the Ca 2ϩ response requires the activation of the SH2 domain-containing inositol polyphosphate 5Ј-phosphatase (SHIP) (11-14), which catalyzes dephosphorylation of PIP 3 and inositol 1,3,4,5-tetrakisphosphate at their 5-position of the inositol ring in vitro (15,16).One of the possible target mechanisms of SHIP in the attenuation of the Ca 2ϩ response is the SOC activity. Since the Fc␥RIIB-mediated inhibition of Ca 2ϩ signaling was more prominent in the presence of extracellular Ca 2ϩ than in its absence (8 -10), SHIP was postulated to attenuate Ca 2ϩ influx via the SOC channel (11)(12)(13)17). Another potential target of SHIP is the PLC␥ activity. Since PIP 3 was reported to stimulate Bruton's tyrosine kinase (Btk), which in turn tyrosine-phosphorylates PLC␥ (18 -20), degradation of PIP 3 upon activation of SHIP by the co-cross-linking of Fc␥RIIB would decrease PLC␥ activity and hence inhibit Ca 2ϩ release from the stores. However, direct demonstration that SHIP affects SOC or Ca 2ϩ release in Fc␥RIIB-mediated signaling remains to be reported.It has been shown that cross-linking of BCR alone leads to tyrosine phosphorylation of SHIP (21, 22). Furthermore, SHIP seems to inhibit the BCR-mediated Ca 2ϩ response even without co-cross-linking of Fc␥RIIB, because SHIP-deficient cells exhibit prolongation of the [Ca 2ϩ ] i transient upon BCR activation (23). While SHIP is recruited to the immunoreceptor tyrosine-based inhibitory motif on the intracellular region of Fc␥RIIB upon activation of this receptor (11), there is no immunoreceptor tyrosine-based inhibitory motif on the BCR complex. Therefore, SHIP is not a unique molecule for Fc␥RIIB-mediated inhibitory signaling, but plays an important role in BCR-mediated signaling; the mechanisms of the recruitment of SHIP seem different in the absence or presence of Fc␥RIIB-mediated signaling. It remains to be clarified how Ca 2ϩ mobilization is attenuated by SHIP in BCR-mediated signaling.In an effort to understand the role of SHIP in BCR-mediated signaling, we addressed the following questions in this work: 1) how are the patterns of Ca 2ϩ mobilization elicited by BCR stimulation modulated by SHIP, and 2) which Ca 2ϩ mobilization pathway is affected by SHIP: Ca 2ϩ release or SOC. To clarify these points, we compared the patterns of [Ca 2ϩ ] i mobilization in control and SHIP-deficient DT40 cells at the single * This work was supported in part by grants ...

SH2 Domain-Containing Inositol Polyphosphate 5′-Phosphatase Is the Main Mediator of the Inhibitory Action of the Mast Cell Function-Associated Antigen

“…More significantly, these results show that the degree of secretion was markedly lower in cells overexpressing wt SHIP, whereas the overexpression of only the SH2 domain of SHIP led to higher degranulation. There results are consistent with recent findings showing that SHIP has a key role in regulating the secretory response of mast cells (38).…”

“…SHIP Ϫ/Ϫ mast cells were found to have ϳ4-fold higher secretory response to the Fc⑀RI stimulus than those of SHIP ϩ/Ϫ or SHIP ϩ/ϩ cells. Thus, as already stated, the critical role of SHIP is to establish a threshold for mast cell secretory response (32)(33)(34)(35)(36)(37)(38) .…”

Transient Translocation of the B Cell Receptor and Src Homology 2 Domain-Containing Inositol Phosphatase to Lipid Rafts: Evidence Toward a Role in Calcium Regulation

“…It is important to note that in the studies reported here FcR␥IIB is not involved because F(abЈ) 2 antiIgM was always used for BCR stimulation. However, SHIP is also involved in regulating BCR signals in the absence of FcR␥IIB engagement (54,55). Evidence indicates that it is the early, inositol 1,4,5-triphosphate-dependent phase of calcium flux that is primarily regulated by SHIP (55).…”

“…However, SHIP is also involved in regulating BCR signals in the absence of FcR␥IIB engagement (54,55). Evidence indicates that it is the early, inositol 1,4,5-triphosphate-dependent phase of calcium flux that is primarily regulated by SHIP (55). Therefore, we examined low-density insoluble fractions of Ramos lysates for the presence of SHIP before and after BCR stimulation and found that SHIP was detectable at very low levels in rafts before stimulation, but rapidly accumulated there upon BCR cross-linking (Fig.…”

Essential Role for the C-Terminal Noncatalytic Region of SHIP in FcγRIIB1-Mediated Inhibitory Signaling

“…SHIP specifically dephosphorylates phosphatidylinositol-3,4,5-trisphosphate (PIP3), a major product of phosphoinositide-3-kinase (PI3K) enzymatic action, as well as inositoltetrakisphosphate (IP4), both in vitro (28,36) and in vivo (53). The requirement for SHIP in Fc␥RIIB1-mediated inhibition of BCR signaling has been well established (4,5,14,20,32,44,48,53). Recruitment of enzymatically active SHIP to the receptor complex results in potent inhibition of intracellular calcium flux (12,30,44), diminished activation of the serine-threonine kinase Akt (1, 3, 17, 27), inhibition of the Ras/mitogen-activated protein kinase pathway (56), and the regulation of apoptosis (2, 38, 47).…”