Immunoglobulin Superfamily Member 8 Is Indispensable for Myeloid Leukemia Via Wnt/β-Catenin Signaling Pathway

Immunoglobulin superfamily member 8 (IGSF8, also known as EWI-2, PGRL, and CD316), is a cell surface protein containing 4 immunoglobulin domains. IGSF8 directly binds to the tetraspanin molecules, CD9 and CD81, and modulates cell adhesion, migration, and growth. Previous studies demonstrated that IGSF8 was associated with prognosis and metastasis in several solid tumors. However, the role of IGSF8 in normal hematopoiesis and myeloid leukemia is still unclear.

First, we examined the expression levels of Igsf8 in various hematopoietic fraction of wild-type murine bone marrow cells, and found that Igsf8 is expressed in all hematopoietic lineages. To investigate hematopoietic functions of Igsf8, we generated hematopoietic cells specific Igsf8 deleted mice (Igsf8^fl/fl; Vav-Cre) and tamoxifen induced Igsf8 deleted mice (Igsf8^fl/fl; Rosa26-CreERT). Igsf8^fl/fl, Vav-Cre (denoted as Igsf8^-/-) mice represented normal maturation. Deletion of Igsf8 did not significantly affect adult hematopoiesis in peripheral blood and bone marrow. Igsf8^-/- long-term hematopoietic stem cells (LT-HSCs: CD34^- Flk2^- c-Kit⁺ Sca-1⁺ Lineage^- cells) reduced colony forming ability in vitro, and serial competitive transplantation assay showed comparable donor chimerism by 3 months, but led to decrease Igsf8^-/- donor chimerism at 4 months and those after second transplantation in vivo. These results suggest that Igsf8 does not affect the adult hematopoiesis, but it can affect their proliferative and reconstitutive capacity of HSCs.

To investigate the effects of Igsf8 on myeloid leukemia, we generated MLL-AF9 and NRAS^G12V-driven acute myelogenous leukemia (AML), or BCR-ABL and NUP98-HOXA9-driven blast crisis of chronic myelogenous leukemia (CML-BC) mice models. Igsf8^-/- led to a dramatic reduction in the number of leukemic colonies formed in vitro (Figure 1A). Igsf8^-/- leukemia mice showed significantly longer survival in vivo (Figure 1B). This effect was also observed by eliminating Igsf8 expression after leukemia establishment using conditionally deletion. Igsf8^-/- AML cells showed decreased S phase fraction. Igsf8^-/- leukemia stem cells (LSCs: c-Kit⁺ Lineage^- cells) triggered an increment of the apoptosis, which contribute to significantly lower proportion of LSCs in spleen of Igsf8^-/- leukemic mice. Given that Igsf8^-/- did not affect homing ability of leukemia cells, these results indicate that Igsf8 is required for propagation of myeloid leukemia and maintenance of LSC.

To understand the Igsf8-mediated regulation of myeloid leukemia, we conducted RNA sequencing analysis of LT-HSCs, and LSCs of AML and CML-BC. Gene set enrichment analysis exhibited increase apoptosis related genes and decrease Wnt/β-catenin related genes in Igsf8^-/- leukemic cells, but not in LT-HSCs (Figure 1C). Increment of pro-apoptosis genes, and decrement of anti-apoptosis genes and Wnt/β-catenin target genes in Igsf8^-/- AML stem cells were validated in quantitative polymerase chain reaction analysis. Further, expression levels of β-catenin protein in Igsf8^-/- leukemic cells were significantly lower compared to Igsf8^+/+ leukemic cells, but not in normal hematopoietic stem and progenitor cells (Figure 1D). These results suggest that Igsf8 might be critical for myeloid leukemia maintenance via Wnt/β-catenin signaling pathway.

Then, we investigated the effects of IGSF8 on human myeloid leukemia. We confirmed IGSF8 expression in several human myeloid leukemia cell line and primary patient-derived leukemia cells. Knockdown of IGSF8 by small hairpin RNA in myeloid leukemia cell lines (THP-1, MV4-11, SKM-1, and K562) and primary patient-derived AML cells exhibited reduced numbers of colony forming cells in vitro. Knockdown of IGSF8 also caused decrease expression of β-CATENIN in AML cell lines. These results indicate that IGSF8 is also required for propagation of human myeloid leukemia cells.

Taken together, our present study reveals that Igsf8 is indispensable for myeloid leukemia, but not adult hematopoiesis, suggesting that IGSF8 inhibition should be considered for targeting myeloid leukemia.

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