New G-CSF-Dependent Signaling Pathway and Its Role In Patients with Severe Congenital Neutropenia and Acute Myeloid Leukemia

Abstract 385

We demonstrated that stimulation of hematopoietic CD34⁺ cells with granulocyte-colony stimulating factor (G-CSF) leads to phosphorylation of hematopoietic cell-specific Lyn substrate 1 (HCLS1) protein and HCLS1 interaction with G-CSF receptor-associated tyrosine kinases Lyn and Syk. Activated HCLS1 binds to lymphoid-enhancer factor 1 (LEF-1) protein, inducing LEF-1-dependent autoregulation of the LEF-1 gene promoter. Transcription factor LEF-1 is a downstream effector of Wnt signaling and a master regulator of myeloid differentiation. In patients with severe congenital neutropenia (CN), inherited mutations in the HCLS1-associated protein × 1 (HAX1) lead to profound defects in the G-CSF-triggered expression and phosphorylation of HCLS1 protein and subsequently, to reduced expression of LEF-1 and abrogated granulopoiesis. Using immunoprecipitation assay, we identified endogenous HAX1 protein in the complex with LEF-1 and HCLS1 proteins. HAX1 is known as a mitochondrial protein, however using cell fractionation experiments of different myeloid cell lines (NB4, HL60, THP-1) and of primary CD34⁺ cells we identified endogenous HAX1 protein in different intracellular compartments. Thus, HAX1 protein was expressed in mitochondria, cytoplasm, nucleus, cytosol and in membranous fraction. Expression levels of HAX1 protein varied between different cell lines and between maturation stage of the cells. In vitro inhibition of HCLS1 or HAX1 by shRNA severely disrupted granulocytic differentiation of CD34⁺ cells, due to a lack of LEF-1 and C/EBPa protein expression and subsequently abolished activation of LEF-1 and C/EBPa gene promoters. Intriguingly, LEF-1 in turn induced HCLS1 mRNA and protein expression by direct binding to the HCLS1 gene promoter and inhibition of LEF-1 in CD34⁺ cells leads to severe downregulation of HCLS1 protein levels. These findings demonstrated a reciprocal feed-back regulation between LEF-1 and HCLS1 proteins. Moreover, HCLS1^-/- mice are neutropenic due to a lack of LEF-1 protein expression and defective in vitro G-CSF-triggered F-actin rearrangement in bone marrow myeloid progenitor cells, as compared to WT mice. At the same time, in bone marrow biopsies of 88.46 % of tested patients (46 out of 52 patients) with acute myeloid leukemia (AML), we found significantly elevated levels of HCLS1 protein. Inhibition of HCLS1 in the AML cell lines and primary blasts of AML patients resulted in reduced proliferation and increased apoptosis. Taken together, we described a novel G-CSFR downstream signaling pathway, with dose-dependent effects on myelopoiesis: diminished expression of HCLS1 and LEF-1 resulted in a “maturation arrest” of granulopoiesis and severe neutropenia, but hyperactivation of one of these proteins led to a hyperproliferation of myeloid progenitors and AML.