Cbl functions downstream of Src kinases in Fc gamma RI signaling in primary human macrophages.

Cbl is a cytosolic protein that is rapidly tyrosine phosphorylated in response to Fc receptor activation and binds to the adaptor proteins Grb2, CrkL, and Nck. A few reports describe Cbl interactions in primary human hematopoietic cells. We show evidence that Cbl participates in signaling initiated by Fc gammaRI receptor cross-linking in human primary macrophages, and functions downstream of Src family kinases in this pathway. Fc gammaRI stimulation in human macrophages was associated with rapid and transient tyrosine phosphorylation of the Cbl adaptor protein. Immunoprecipitated Cbl was complexed with several tyrosine phosphorylated proteins, the most prominent of which was a 38-kDa band identified as the CrkL adaptor protein. CrkL associated with tyrosine-phosphorylated Cbl and itself became tyrosine phosphorylated after Fc gammaRI cross-linking. SLP-76, a recently cloned Grb2-associated protein, was strongly tyrosine phosphorylated after Fc gammaRI stimulation and was associated with both Cbl and Grb2. Grb2 and Cbl binding to SLP-76 were inducible after Fc gammaRI stimulation of the macrophages. Nck was inducibly bound to Cbl after Fc gammaRI stimulation, whereas Grb2 was constitutively associated with it. Shc was also inducibly tyrosine phosphorylated and bound to Grb2 after Fc gammaRI stimulation of the macrophages. PP1, a specific inhibitor of Src kinases, inhibited the Fc gammaRI-induced respiratory burst, as well as the tyrosine phosphorylation of Cbl and its inducible association with CrkL. These results suggest a fundamental role for the tyrosine phosphorylation of Cbl, CrkL, SLP-76, and Shc and the association of Cbl with CrkL, SLP-76, and Nck in Fc gammaRI signaling in human macrophages. Experiments performed with PP1, the specific Src kinase inhibitor, demonstrate the first evidence that Cbl and the Cbl-Crkl interaction are downstream targets for myeloid Src kinases required for the activation of myeloid NADPH oxidase activity.

Actin mRNA is not lowered in chronic myeloid leukemic granulocytes.

Chronic myeloid leukemic (CML) granulocytes exhibit defects in several functions, some of which have been associated with changes in the expression of cell surface molecules, actin reorganization and lowered levels of total cellular actin. In this study, we show by northern blotting that the steady-state level of mRNA for actin is not decreased in the CML granulocyte. Our data suggest that the lowered levels of actin protein in the leukemic granulocyte may be due to altered control at the translational/post-translational step, rather than at the level of transcription/post-transcription, implicated in the regulation of expression of the surface molecules, Fc gamma RII, Fc gamma RIII and alkaline phosphatase.

Heterogeneity in the expression of Fc gammaRIII in morphologically mature granulocytes from patients with chronic myeloid leukemia.

Morphologically mature granulocytes from patients with chronic myeloid leukemia exhibit a defect in receptor mediated endocytosis of FITC-conjugated heat-aggregated IgG. In our earlier studies, we have shown that transcripts for Fc gammaRII and Fc gammaRIII are lowered in CML granulocytes, while no such trend was seen for CD11b, CD18 and actin, the other molecules involved in this process. We have also shown by flow cytometry that the number of granulocytes expressing the Fc receptors on their surface is lowered in CML patients. In this report, we show that the lowered steady state level of the mRNA for the Fc receptors is due to their faster degradation and not due to any defect in transcription. A study of the expression of mRNA for Fc gammaRIII in morphologically mature CML granulocytes by in situ hybridization showed that there is heterogeneity in the expression of this receptor, with some cells positive for the message and some not. These results suggest that the Fc gammaRIII transcript reaches a stable RNA pool in only some of the granulocytes, whereas in the rest of the cells, it is probably degraded after it is transcribed and is therefore not detected. The Fc gammaRIII is probably one of the first antigens to be lost from the leukemic granulocyte surface during the transition of the disease from the chronic phase to the accelerated phase and finally to the blast crisis.