Introduction

BIRC6 (a.k.a. APOLLON, BRUCE) is an exceptionally large protein of 528 kDa belonging to the family of inhibitor of apoptosis (IAP). BIRC6 contains one bacalovirus IAP repeat (BIR) domain that shows homology to the IAP Survivin. Furthermore, BIRC6 is the only IAP with an ubiquitin-conjugating domain further pointing to a particular function of this protein in the IAP family [1]. Several groups reported that BIRC6 executes its function via inhibition of Smac and Caspase-9[2, 3]. Moreover, a study in breast cancer cells revealed that BIRC6 inhibits cell death by p53 destabilization and inactivation of caspase-3[4]. The role of p53-dependent BIRC6 effector functions was confirmed by investigations of Ren et al. in mice and human lung cancer cells [5].

Due to their anti-apoptotic function it was hypothesized that overexpression of IAPs might contribute to tumorigenesis. Survivin, for example, one of the best studied IAPs is up regulated in most tumors and has been associated with their chemoresistence [6, 7]. Recently, Houdt et al. [8] observed BIRC6 overexpression in colon cancer stem cells when compared to more differentiated tumor cells. BIRC6 expression protected colon cancer stem cells from the cytotoxic effects of oxaliplatin and cisplatin. Furthermore, knocking-down BIRC6 led to growth inhibition in several cancer cell lines and xenografted mice and rendered the tumor cells more sensitive to 5-fluoruracil treatment in vivo and in vitro[9]. The significance of IAPs in the pathology of hematological malignancies however remains poorly understood and controversial data regarding an effect on prognosis were published. Carter et al. [10] did not find any prognostic significance for Survivin or XIAP expression in adult AML samples, whereas BIRC6 overexpression is associated with unfavorable clinical features and negatively impacts on the 3-year relapse-free survival in childhood acute myeloid leukemia (AML) [11]. Similar results were obtained by Ismail et al. in childhood AML and acute lymphoblastic leukemia (ALL) [12]. Abe et al. [13] found lower expression of BIRC6 in bone marrow-derived cells of healthy donors compared to de novo AML samples.

To our knowledge, BIRC6 levels in primary AML have never been compared to the respective expression in normal myeloid cells. Thus, we aimed at comparing BIRC6 levels in a large cohort (n = 98) of adult AML patient samples and mature neutrophils from healthy donors. Since a hallmark of AML is a differentiation block of hematopoietic precursors at different developmental stages and since this block can be overcome by treating acute promylocytic leukemia (APL) patients with all-trans retinoic acid (ATRA) and low dose chemotherapy, we also analyzed the role of BIRC6 during neutrophil differentiation of AML cells.

Materials and methods

Primary patient samples

A cohort of 98 samples from patients with a diagnosis of primary AML (FAB M0-M4) were enrolled on HOVON/SAKK (Dutch-Belgian Hematology-Oncology/ Swiss Group for Clinical Cancer Research Cooperative group) protocols −04, -04A, -29, and −42 (available at http://www.hovon.nl) between 1987 and 2006 [14–18]). All patients provided written informed consent in accordance with the Declaration of Helsinki. Patient data are summarized in the Table 1. In vitro differentiation of CD34+ progenitor cells was done as previously described [19].

Table 1 AML patient characteristics from the HOVON/SAKK cohort
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Cell lines and culture conditions

The acute myeloid leukemia (AML) cell lines HL60, FAB M2, the ATRA-resistant subline HL60-R, NB4, FAB M3 (acute promyelocytic leukemia; APL), the ATRA-resistant subline NB4-R2 and HT93 were kept in RPIMI-1640 culture media (Sigma-Aldrich, Buchs, Schweiz) containing 10 % foetal bovine serum (FBS). In order to differentiate the AML cell lines towards granulocytes 1 μM all-trans retinoic acid (ATRA) was added to the cells that were seeded at a density of 0.2x106 cells/ml. Successful neutrophil differentiation was assessed by CD11b surface and by increased granulocyte colony-stimulating factor receptor (G-CSFR; CSF3R) mRNA expression.

Quantitative real-time RT-PCR (qPCR) and TaqMan Low Density Array (LDA)

For RNA extraction the miRCURY RNA Isolation Kit from Exiqon was used. RT-PCR has been described elsewhere [19]. Quantitative measurement of BIRC6 and G-CSFR mRNA was performed using the TaqMan® Gene Expression assays Hs00212288_m1 and Hs00167918_m1, respectively (Applied Biosystems, Rotkreuz, Switzerland). LDA measurements as well as data analysis were done as described [20]. HMBS primers and probes have been described previously [21]. Measurements were carried out on an ABI PRISM 7500 Sequence Detection System (Applied Biosystems, Rotkreuz, Switzerland).

Generation of BIRC6 knock-down cell lines

NB4 cells were transduced with a lentiviral vector (pLKO.1) expressing a small hairpin (sh)RNA targeting the BIRC6 mRNA (NM_016252.x-2281s1c1, Sigma-Aldrich, Buchs, Schweiz). Lentivirus production has been described previously [19]. As a non-targeting control we used the SHC002 lentiviral vector. Two days after transduction, NB4 cells were selected with 1.5ug/ml puromycin (Sigma-Aldrich, Buchs, Schweiz) for one week.

Cell viability assay

For Trypan blue exclusion assay cells were washed with PBS before diluting 1:10 in 0.4 % Trypan blue solution (Sigma-Aldrich, Buchs, Switzerland). For AnnexinV staining 1x105 cells were washed in 500 μl binding buffer (PBS with 0.33 g/L Ca2+) and resuspended in a final volume of 100 μl. 5 μl AnnexinV-PE were added (Immunotools, Germany) and the samples were incubated for 15 minutes at room temperature, before FACS analysis.

Statistical analysis

Differences between two groups were assessed using the non-parametric Mann–Whitney-U test. P-values <0.05 were considered to be statistically significant.

Results

Repression of BIRC6 mRNA in AML patients with particular chromosomal aberrations

To study the BIRC6 expression patterns in normal versus leukemic myeloid cells, we measured BIRC6 levels in a large cohort of 98 primary AML patients (FAB M0-M4), in 24 granulocyte preparations from healthy donors and in 3 CD34+ progenitor cell samples. We were able to detect BIRC6 in 95/98 AML patient, in 14/24 granulocytes and in 3/3 CD34+ progenitor cell samples. Surprisingly, we found significantly lower BIRC6 levels in AML patients with the translocations t(8;21) and t(15;17) as well as in AML patients with a complex karyotype, whereas no significant differences in BIRC6 expression was found in AML patients with inv(16) or normal karyotype as compared to its expression levels in granulocytes from healthy donors (Figure 1). We found the lowest BIRC6 mRNA levels in in CD34+ progenitor cells (Figure 1). Together, our data suggest an association of low BIRC6 expression with an immature myeloid phenotype.

Figure 1
figure 1

BIRC6 mRNA expression is significantly lower in particular AML subtypes as compared to granulocytes. Relative expression levels of BIRC6 mRNA were determined by qPCR analysis. Raw Ct values were normalized to HMBS and ABL1 (−ΔCt) and represent log2 expression levels. BIRC6 expression in AML subtypes FAB M0-M4 and the following molecular lesions: (inv(16), t(8;21), t(15;17), Comp. = Complex Karyotype, Norm. = Normal Karyotype). Relative BIRC6 expression levels were compared to the relative expression in granulocytes from healthy donors (Gran.) and CD34+ progenitor cells. Mann–Whitney U test *p < 0.05, **p < 0.01, ns not significant.

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Significant BIRC6 inductions during ATRA-induced neutrophil differentiation

In order to assess a possible role of BIRC6 in neutrophil differentiation of AML cells, we took advantage of several AML cells line models for neutrophil differentiation. First, we differentiated NB4 APL cells with ATRA towards neutrophils. In accordance with the low BIRC6 expression in undifferentiated AML patients compared to mature neutrophils, BIRC6 mRNA levels were up-regulated 1.8- and 3.6-fold at day 4 and 6 of ATRA treatment, respectively (Figure 2A). In order to exclude a direct effect of ATRA on BIRC6 expression, we also treated ATRA-resistant NB4-R2 APL cells with ATRA. BIRC6 transcripts were only marginally up-regulated in these cells further supporting a particular role of BIRC6 in neutrophil differentiation (Figure 2B). Neutrophil differentiation of NB4 and NB4-R2 cells was assessed by CD11b surface expression (Figure 2C). A similar induction of BIRC6 mRNA expression upon neutrophil differentiation was seen in HT93 APL cells (data not shown). In a further experiment, we determined BIRC6 expression in HL60 and ATRA-resistant HL60-R AML cells during neutrophil differentiation. We observed a significant 1.6- and 2.8-fold increase of BIRC6 transcript expression at day 4 and 6, respectively. No significant change in BIRC6 expression was seen in HL60-R cells upon 4 days of ATRA treatment, whereas at day 6 a minor but significant increase was seen (Figure 2D and E). Neutrophil differentiation in HL60 cells was confirmed by CD11b FACS analysis (Figure 2F). ). Consistent with our data showing increased BIRC6 expression upon neutrophil differentiation of NB4 and HL60 AML cells, we observed a 3.3- and 2.8-fold increase in BIRC6 mRNA expression at day 3 and 6 upon in vitro granulocytic differentiation of CD34+ progenitor cells, respectively (data not shown).

Figure 2
figure 2

Significant induction of BIRC6 mRNA levels upon neutrophil differentiation of AML cells. (A) BIRC6 qPCR analysis of NB4 APL cells treated with 1 μM ATRA for 4 and 6 days. Raw Ct values were normalized to HMBS and to the untreated control of day 4. Results from four independent experiments in duplicates are shown. (B) BIRC6 qPCR analysis of ATRA-resistant NB4-R2 APL cells treated as in A. (C) CD11b surface expression of NB4 and NB4-R APL cells upon six days of ATRA-induced neutrophil differentiation. CD11b was measured by FACS analysis. (D) Neutrophil differentiation of HL60 AML cells with 1 μM ATRA for 4 and 6 days, respectively. BIRC6 mRNA expression was assessed by qPCR as in A. (E) BIRC6 qPCR analysis of ATRA-resistant HL60-R cells treated as in A. (F) The differentiation levels of HL60 and HL60-R were assessed by CD11b surface expression upon 6 days of ATRA-induced neutrophil differentiation. Mann–Whitney U test *p < 0.05, **p < 0.01, ***p < 0.001, ns not significant.

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Overall, our findings clearly show an association of increased BIRC6 expression with neutrophil development of AML and CD34+ progenitor cells.

Knocking down BIRC6 attenuates neutrophil differentiation but not cell death and proliferation of APL cells

To test if specific BIRC6 depletion inhibits neutrophil differentiation of APL cells, we generated NB4 BIRC6 knockdown cells. BIRC6 knockdown efficiency was confirmed in control and ATRA-treated shBIRC6 expressing NB4 cells (Figure 3A). Neutrophil development as determined by G-CSFR mRNA as well as CD11b surface expression was on average 50 % reduced compared to the non-targeting control transduced NB4 cells at day 4 of ATRA treatment (Figure 3B-C).

Figure 3
figure 3

Knocking-down BIRC6 attenuates neutrophil differentiation of APL cells. (A) NB4 BIRC6 knock-down cells (shBIRC6) were generated by lentiviral transduction of shRNAs targeting the BIRC6 mRNA. BIRC6 knockdown efficiency compared to control transduced (SHC002) NB4 cells was determined by qPCR analysis. Values are shown as n-fold regulation compared to control transduced cells. (B,C) NB4 shBIRC6 and SHC002 were treated with 1 μM ATRA for 4 and 6 days, respectively. Neutrophil differentiation was determined by G-CSFR mRNA and CD11b surface expression. The raw Ct values of G-CSFR were normalized to HMBS and to the Ct values of the respective control transduced cells under ATRA treatment. CD11b expression is shown as mean fluorescence intensity (MFI). (D,E) Cell death in BIRC6 knock-down cells at day 4 and 6 of ATRA treatment was assessed by AnnexinV staining and Trypanblue exclusion assays. Data are given as percentage or total number of death cells, respectively. (F) Total cell counts were determined using a hemocytometer. There was no difference in total cell number at day 4 or 6 of ATRA treatment between shBIRC6 or control NB4 cells. Mann–Whitney U test **p < 0.01, ***p < 0.001, ns not significant.

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Since BIRC6 is involved in the inhibition of apoptosis, we were asking if the observed lack of neutrophil differentiation in NB4 BIRC6 knockdown cells is due to increased cell death in these cells. For this purpose we assessed cell death by Trypan blue exclusion and Annexin V flow cyotometry at day 4 and 6 of ATRA treatment. We did not find any significant differences in cell viability between NB4 control and BIRC6 knockdown cells during ATRA-induced neutrophil differentiation (Figure 3D and E). Furthermore, control transduced as well as BIRC6 knockdown NB4 cells showed no significant differences in total cell counts at day 4 and 6 of ATRA treatment (Figure 3F).

In summary, we showed that inhibiting BIRC6 significantly affects neutrophil differentiation of APL cells, but not cell viability.

Discussion

In this report we publish for the first time that AML patient samples with the t(8;21), the t(15;17) or a complex karyotype express BIRC6 significantly lower than normal human granulocytes. Subtype-specific BIRC6 expression in AML is supported by earlier findings published by Ismail et al. [12]. Low expression of the anti-apoptotic IAP BIRC6 in AML may seem controversial given the often high expression of IAPs in cancer. Our findings of high BIRC6 expression in granulocytes versus AML may reflect the cellular differentiation status of these cells rather than a cancer-associated deregulation. This hypothesis is confirmed by our observation that BIRC6 mRNA levels are clearly reduced in CD34+ myeloid precursor cells and increase during granulocyte differentiation. These data are in line with previous findings showing that BIRC6 is down-regulated in bone marrow-derived cells if compared to de novo AML samples [13]. Furthermore, the expression of the BIRC6 related XIAP is induced upon monocyte differentiation and contributes to monocyte cell survival. In the same study, XIAP levels declined during neutrophil differentiation supporting the cell type specific regulation of IAPs in myeloid cells [22]. Consistent with the monocyte specific expression of XIAP, a correlation of XIAP levels with monocytic markers in AML was found [23]. In contrast to these finding in adult AML, in childhood de novo acute myeloid leukemia the levels of XIAP correlated with an immature FAB-subtype [24]. This may suggest different XIAP functions in distinct leukemic entities.

In conclusion, we were able to link increased BIRC6 mRNA expression with neutrophil differentiation and inhibiting BIRC6 resulted in attenuated neutrophil differentiation but did not alter cell survival. In summary, we established a new role for BIRC6 in neutrophil differentiation of AML cells.

Author`s contributions

AS performed the experimental research, interpreted the data and drafted the article. BET provided primary cells and essential reagents, analyzed patient data and revised the article. MFF investigated the initial concept and experimental design and revised the drafted article. MPT designed the project and gave final approval of the submitted manuscript. All authors read and approved the final manuscript.