Molecular Mechanisms by Which the Novel Tumor Suppressor Phospholipase C-b3 Inhibits Hematopoietic Stem Cell Proliferation and Myeloid Differentiation

Abstract 2446

Given its catalytic activity to generate diacylglycerol and inositol 1,4,5-trisphosphate, phospholipase C (PLC) is implicated in promoting cell growth. However, we recently found that PLC-b3-deficient mice develop myeloproliferative neoplasm (MPN), lymphoma, and other tumors (Xiao et al., Cancer Cell, 2009). The mutant mice had increased numbers of hematopoietic stem cells (HSC) with increased proliferative, survival, and myeloid-differentiative abilities. These properties were dependent on the transcription factor Stat5 and could be antagonized by the SH2 domain-containing protein tyrosine phosphatase SHP-1. Stat5-dependent cooperative transformation by active c-Myc and PLC-b3 deficiency was shown for mouse lymphomas in PLC-b3^−/− and in Em-myc;PLC-b3^+/− mice and human Burkitt's lymphoma cells. The same mechanism for malignant transformation seemed to be operative in other human lymphoid and myeloid malignancies. Thus, PLC-b3 was shown to be a novel tumor suppressor (Xiao et al., Cancer Cell, 2009). Similar to PLC-b3-deficient mice, Lyn (Src family kinase)-deficient mice develop MPN. Lyn/PLC-b3 doubly deficient lyn^−/−;PLC-β3^−/− mice developed a fatal myelodysplastic/myeloproliferative neoplasm (MDS/MPN), which was similar to human chronic myelomonocytic leukemia (CMML). In HSCs of lyn^−/−;PLC-β3^−/− mice that caused the CMML-like disease, phosphorylation of SHP-1 at Tyr⁵³⁶ and Tyr⁵⁶⁴ was abrogated, resulting in reduced phosphatase activity and constitutive activation of Stat5. Furthermore, SHP-1 phosphorylation at Tyr⁵⁶⁴ by Lyn was indispensable for maximal phosphatase activity and for suppression of the CMML-like disease in these mice. On the other hand, Tyr⁵³⁶ in SHP-1 could be phosphorylated by Lyn and another kinase(s) and was necessary for efficient interaction with Stat5. Therefore, we identified a novel Lyn/PLC-b3-mediated regulatory mechanism of SHP-1 and Stat5 activities (Xiao et al., Blood 2010).

PLC-b3 could form the multimolecular SPS complex together with SHP-1, Stat5, and Lyn (Xiao et al., Cancer Cell, 2009; Xiao et al., Blood 2010). The close physical proximity of SHP-1 and Stat5 brought about by interacting with the C-terminal segment of PLC-b3 (PLC-b3-CT) accelerated SHP-1-mediated dephosphorylation of Stat5. More recently, we identified the minimal sequences within PLC-b3-CT required for its tumor suppressor function. Two of the three Stat5-binding noncontiguous regions, one of which also bound SHP-1, substantially inhibited in vitro proliferation of Ba/F3 cells. Surprisingly, an 11-residue Stat5-binding peptide (residues 988–998) suppressed Stat5 activity in Ba/F3 cells and in vivo proliferation and myeloid differentiation of hematopoietic stem/progenitor cells. Therefore, this study further defines PLC-b3-CT as the Stat5- and SHP-1-binding domain by identifying minimal functional sequences of PLC-b3 for its tumor suppressor function and implies their potential utility in the control of hematopoietic malignancies.

This study is supported in part by a grant from the MPN Foundation.