The Egyptian Journal of Haematology

ORIGINAL ARTICLE
Year
: 2012  |  Volume : 37  |  Issue : 2  |  Page : 91--95

Detection of Cx43 and P27 in acute myeloid leukemia patients with t(8;21)


Samy B.M. El-Hady1, Eman Almasry2, Ashraf M. ElHefney3,  
1 Department of Clinical Pathology, Faculty of Medicine, Zagazig University, Zagazig, Egypt
2 Department of Biochemistry, Faculty of Medicine, EL_Minia University, EL_Minia, Egypt
3 Department of Medical Oncology, Faculty of Medicine, Zagazig University, Zagazig, Egypt

Correspondence Address:
Samy B.M. El-Hady
Department of Clinical Pathology, Faculty of Medicine, Zagazig University, Zagazig
Egypt

Abstract

Background

Connexin 43 (Cx43) inhibits cell proliferation in gap junction-dependent and gap junction-independent mechanisms and contributes to the pathogenesis of many diseases. The aim of this study was to assess Cx43 expression in acute myeloid leukemia (AML) patients with t(8;21) compared with patients without this translocation and correlate this expression with the cyclin-dependent kinase inhibitor P27.

Patients and methods

Group I comprised 21 patients with de-novo M2 who were positive for t(8;21). Group II comprised 29 patients with de-novo M2 who were negative for t(8;21). Group III comprised 25 apparently normal individuals, with matched age and sex, which served as the control group. In addition to routine laboratory investigations, detection of Cx43 expression on bone marrow mononuclear cells (MNCs) and P27 in MNC lysate was carried out for all patients.

Results

In this study, we found increased expression of Cx43 on leukemic cells carrying t(8;21) compared with leukemic cells without t(8;21). Compared with normal cells, there was decreased expression of Cx43 on leukemic cells carrying t(8;21). Furthermore, we found significant negative correlation between the count of leukemic cells and Cx43 expression. We found increased levels of P27kip1 protein in MNCs extracted from leukemic patients with positive t(8;21) compared with leukemic patients without this translocation. In addition, we found significant positive correlation between Cx43 expression and P27kip1 protein level.

Conclusion

Our experiments show that Cx43 expression increases in M2 with t(8;21) and may contribute to the growth-arresting effect of leukemogenic AML1–eight-twenty one (ETO) fusion protein, possibly by causing the accumulation of P27kip1 protein. These novel discoveries have shed new light on the mechanisms of AML1–ETO-induced growth arrest and have provided a foundation for exploring additional mutagenic ‘hits’ for AML1–ETO-associated AML. We recommend the use of drugs that improve Cx43 expression in treating M2.




How to cite this article:
El-Hady SB, Almasry E, ElHefney AM. Detection of Cx43 and P27 in acute myeloid leukemia patients with t(8;21).Egypt J Haematol 2012;37:91-95


How to cite this URL:
El-Hady SB, Almasry E, ElHefney AM. Detection of Cx43 and P27 in acute myeloid leukemia patients with t(8;21). Egypt J Haematol [serial online] 2012 [cited 2019 Dec 11 ];37:91-95
Available from: http://www.ehj.eg.net/text.asp?2012/37/2/91/135061


Full Text

 Introduction



Gap junctions are specialized cell membrane structures that form ion channels interconnecting the cytoplasm of adjacent cells. The intercellular channels are formed by apposition of hemi-channels or connexons, each one comprising six protein subunits belonging to the connexin family. They are named according to the molecular weights in kDa of the proteins predicted from their cDNAs, for example connexin 43 (Cx43) 1. Molecules of up to 1 kDa can pass through gap junctions, thus allowing for the rapid diffusion of metabolites or second messengers between cells of a given tissue. The control of intercellular communication mediated by gap junctions is achieved by transjunctional potential, intracellular calcium and pH levels, and connexin phosphorylation. Despite the fact that gap junctions are usually not present in circulating blood cells, connexin expression is known to occur in leukocytes during inflammatory reactions in blood vessels 2. In humans, Cx43 has been shown to mediate coupling between bone marrow stromal cells (BMSCs) and CD34+ blood cell precursors 3. Studies on cell lines derived from Cx43 knockout animals that were transfected with Cx43 cDNA confirmed the supportive role of gap junction communication in hematopoiesis 4. In another study, hematopoiesis in Cx43 knockout mice was shown to be deficient in both lymphopoiesis and myelopoiesis 5. In addition to contributing to gap junctional intercellular communication (GJIC), increasing evidence supported that Cx43 also inhibits cell proliferation in gap junction-dependent and independent mechanisms and contributes to pathogenesis of many diseases 6,7. For instance, Cx43 is deregulated in cancers of various origins such as endometrial cancer, ovarian cancer, and lung cancer 8,9. The restoration of Cx43 expression by transfection leads to growth suppression and/or differentiation/apoptosis induction of cancer cells 10. In addition, Cx43 overexpression also regulates various angiogenesis-linked proteins and inhibits the malignant properties of breast cancer cells 11. On the basis of these important findings, Cx43 is regarded as a hypothetical tumor suppressor and possibly acts as a potential antioncogenic target for chemoprevention and/or chemotherapy 12.

P27kip1 is a member of a family of cyclin-dependent kinase (CDK) inhibitors that bind to cyclin/CDK complexes and arrest cell cycle progression from G1 to S phase 13. It is known that a relatively large number of nutritional and chemopreventive anticancer agents specifically upregulate expression of P27 in eukaryotic cells 14,15.

Acute myeloid leukemia (AML) is characterized by the blockage of the maturation/differentiation of myeloid progenitor cells at various stages, depending on the subtype of leukemia, with the uncontrolled proliferation of malignant leukemic cells. The pathogenesis of AML has been attributed to acquired genetic changes, specifically reciprocal chromosome translocations 16. One of the most common translocations in AML is t(8;21)(q22;q22), which is associated with 12% of de-novo AML cases and up to 40% of M2-type AML cases of the French–American–British classification. This translocation rearranges the AML1 gene and ETO gene, creating a fusion gene called AML1–ETO. Its product, AML1–ETO protein, is composed of the 177 N-terminal amino acid residues of AML1 and nearly the entire ETO protein 17. Studies using a tetracycline-inducible AML1–ETO-expressing leukemic U937 cell line as an in-vitro model report that AML1–ETO induces growth arrest. This observation alone is difficult to reconcile with leukemogenesis, because growth arrest would not be favorable for the propagation of cells harboring the t(8;21) translocation 18. Thus, it has been predicted that other oncogenic events promoting proliferation are necessary for the development of t(8;21)-carrying AML. Toward this end, it is imperative to explore the precise molecular mechanisms of AML1–ETO-induced growth arrest to understand these additional mutagenic ‘hits’. The aim of this study was to assess Cx43 expression in AML patients with t(8;21) compared with patients without this translocation and to correlate this expression with the CDK inhibitor P27.

 Patients and methods



This study included 75 patients categorized into three groups:

Group I: Group I comprised 21 patients with de-novo M2. The diagnosis of AML was made according to the French–American–British criteria and the WHO classification. There were 12 males and eight females patients with a mean age of 40.1±8.5 years, all of whom were positive for t(8;21).

Group II: Group II comprised 29 patients with de-novo M2. The diagnosis of AML was made according to the French–American–British criteria and the WHO classification. There were 17 males and 12 females patients with a mean age of 39.2±10.6 years, all of whom were negative for t(8;21).

Group III: Group III comprised 25 apparently normal individuals, with matched age and sex, which served as the control group. There were 15 males and 10 females individuals with a mean age of 41.8±9.1 years.

Methods

All participants were subjected to the following:

Full history taking and complete clinical examination.Routine laboratory investigations including: complete blood count using Sysmex S.F3000 (Kobe, Japan), with examination of Leishman-stained films; liver and kidney function tests using a Dimension Autoanalyzer (Dade Behring, Illinois, USA); bone marrow aspiration with examination of Leishman-stained films; smears were also stained for myeloperoxidase; immunophenotyping of bone marrow performed by FACScan flow cytometry (Becton Dickinson, San Jose, California, USA) using the following panel of monoclonal antibodies (mAbs): CD19, CD10, CD20, CD5, CD13, HLA-DR, CD33, CD7, CD45, CD14, MPO, cyCD79a, and cyCD3. At least 10 000 cells per sample were acquired and analyzed by flow cytometry. Data were collected and analyzed with CellQuest software (Becton Dickinson).Special investigations.

Cx43 expression on bone marrow CD34+ cells (mononuclear cells)

Heparinized bone marrow samples of 2 ml were collected in sterile tubes. Mononuclear cells (MNCs) were separated according to the procedure described in the study by Brach et al. 19. MNCs from bone marrow were separated by ficoll-hypaque density gradient centrifugation (Histopaque 1077, Sigma, Saint Louis, Missouri, USA) at 2000 rpm for 40 min at 20°C. The separated cells were suspended in PBS.

Phenotypic analysis of MNCs was carried out by flow cytometry to determine the expression of Cx43 on CD34+ cells; the MNCs were incubated with 10 µg/ml of mAb for 30 min at 4°C. After washing three times in PBS, the cells were incubated for 30 min at 4°C with PE-conjugated (secondary) goat F(abº)2 anti-mouse IgG. Data pertaining to at least 10 000 cells were collected using FACScan flow cytometry (Becton Dickinson) and analyzed using CellQuest software (Becton Dickinson). Cells positive for both CD34 and Cx43 in a two-color bivariate dot plot were classified as Cx43-positive blast cells. CellQuest software (Becton Dickinson) was used for data acquisition and analysis.

Polymerase chain reaction detection for t(8;21)

Total RNA was isolated from bone marrow with TRIzol reagent (Gibco BRL, Gaithersburg, Maryland, USA). RNA (1 μg) was reverse-transcribed using a cDNA synthesis kit (Pharmacia, Uppsala, Sweden) at 37°C for 1 h. Single-stranded cDNA synthesized from patient and control total RNA was amplified by PCR using the following primer sets: A1 (sense) 5º-AGCTTCACTCTGACCATCAC, E1 (antisense) 3º-TCAGCCTAGATTGCGTCTTC. The PCR reactions were performed using 75 ng of each of the appropriate sense and antisense primers in a 50 µl reaction mixture containing 1 U Taq polymerase (Applied Biosystems Inc., Foster City, California, USA), 0.5 mmol/l of each deoxynucleotide triphosphate 3 mmol/l MgCl2, and 5 μl 10× PCR buffer (670 mmol/l Tris, 100 mmol/l β-mercaptoethanol, 166 mmol/l ammonium sulfate, 67 mmol/l EDTA, and 0.5 mg/ml bovine serum albumin). PCR cycling consisted of an initial denaturation step at 94°C for 3 min, followed by 44 cycles at 95°C for 1 min, 60°C for 1 min, and 72°C for 1 min, with a final extension at 72°C for 10 min. cDNA of 2 µl (equivalent to 0.2 µg of total RNA) was amplified in a 50 µl reaction mixture. PCR product of 20 µl was mixed with 4.0 µl of 5× gel loading buffer and electrophoresed on an agarose gel for 1.7 h at 100 V. The gel was stained in a solution of 1× Tris-acetate/EDTA electrophoresis buffer containing 1 μg/ml ethidium bromide and photographed under ultraviolet light.

Detection of P27 in mononuclear cell lysate: TiterZyme Kits (Gentaur, Av. de l'Armée, Brussels, Belgium)

Rinse MNCs two times with PBS, making sure to remove any remaining PBS after the second rinse. Solubilize cells at 1×107 cells/ml in lysis buffer and allow samples to sit on ice for 15 min. Assay immediately or store at −70°C. Before use, centrifuge samples at 2000g for 5 min and transfer the supernatant to a clean test tube. Sample protein concentration may be quantified using a total protein assay. If needed, further dilutions should be made in IC diluent. Standards and samples (total cell lysates) were added to the wells and incubated. After washing, a yellow antibody was added to the wells and the plate was incubated. The plate was washed, a blue conjugate was added to the wells, and the plate was incubated. After washing the plate, substrate was added to the wells and the plate was incubated. The color development was stopped, and the intensity of the color was measured and compared using a standard curve. Reading was done at 450 nm wavelength. The concentration read from the standard curve must be multiplied by the dilution factor.

Statistical analysis

Results were expressed as mean±SD (X±SD) and were analyzed statistically using analysis of variance. Least significant difference was assessed to test the difference between the different studied groups. Correlation analysis was performed with Pearson’s correlation test. P values below 0.05 were considered significant. Data were tabulated statistically and analyzed using SPSS version 17.0 for Windows (SPSS Inc., Chicago, Illinois, USA).

 Results



All studied patients received an ultimate diagnosis based on clinical and laboratory data. Some laboratory data pertaining to the M2 group with positive t(8;21) (group I), the M2 group with negative t(8;21) (group II), and the control group (group III) are illustrated in [Table 1].{Table 1}

[Table 2] shows that the mean±SD of the percentage of Cx43 expression were 30.1±10.2% for group I, 5.45±3.2% for group II, and 63.1±9.18% for the control group. There were highly significant differences among the studied groups (F=366.3, P<0.0001).{Table 2}

The mean values of P27kip1 levels in cell lysates were 133±55.4 for group I, 44.9±17.5 for group II, and 267±101 pg/ml for the control group [Table 2].

[Table 3] and [Figure 1] and [Figure 2] show that there were significant negative correlations between Cx43 expression and blast cell percentage in group I (r=−0.528, P<0.05). Also, there were significant positive correlations between Cx43 expression and P27kip1 in group I (r=0.508, P<0.05).{Table 3}{Figure 1}{Figure 2}

 Discussion



Cx43 is the principal gap junction protein expressed in hematopoietic tissues 5,20. Cx43 was originally described in the myocardium and is detectable in almost all vertebrate organs in various amounts. Cx43 expression is overwhelmingly upregulated at the endosteal surface of the bone marrow, with minimal expression in deeper areas of the medullary cavity toward the central sinus. It has been suggested that the most immature hematopoietic cells, including pluripotent hematopoietic stem cells and immature progenitors, are located in the endosteal area 21,22. These observations raise the possibility that the most primitive hematopoietic cells are dependent on gap junction expression by their supporting stroma.

The chromosomal translocation t(8;21) is associated with 10–15% of de-novo AML cases and up to 40% of M2-type AML cases of the French–American–British classification. As widely accepted, the AML1–ETO gene inhibits the differentiation of leukemic cells toward granulocytic, monocytic, or erythroid cells 7. However, inducible AML1–ETO expression also induces growth arrest and makes leukemic cells prone to both extrinsic and intrinsic apoptoses in leukemic cell lines 23. AML1–ETO failed to reliably cause overt leukemia in mice. This fact strongly suggests that additional mutagenic ‘hits’ are required for AML1–ETO-related leukemogenesis 24. The aim of our study was to assess the expression of Cx43 in M2 cases that are positive for t(8;21) and to correlate these expressions with P27 levels in these cases.

In this study, we found increased expression of Cx43 on leukemic cells carrying t(8;21) compared with leukemic cells without t(8;21). Compared with normal cells, there is decreased expression of Cx43 on leukemic cells carrying t(8;21). Our results are in line with those of Li et al. 7, whose results showed that inducible AML1–ETO expression in leukemic cells upregulates expression of Cx43. Their results showed that downregulation of Cx43 partially restores the cell growth rates in the presence of AML1–ETO expression, suggesting that the upregulated Cx43 expression exerted a role in AML1–ETO-induced growth arrest.

Liu et al. 25 showed that the expression level of Cx43 and its mRNA in BMSCs in normal cells were higher than in primary acute leukemia BMSCs. They found that the function of GJIC in acute leukemic BMSCs was also weaker than that in normal BMSCs. They concluded that cell–cell communication function is lowered in acute leukemic BMSCs. The function of GJIC in acute leukemic BMSCs was significantly improved following effective chemotherapy. These findings suggest that Cx43 and GJIC might be involved in the occurrence, development, and termination of acute leukemia, and effective chemotherapy could improve Cx43 expression and GJIC function that were dysfunctional before treatment 26. Several studies show that 80% of CD34+ cells do communicate through gap junction with the stroma, suggesting that gap junction might play a role in the maintenance of normal stem cells 5,27.

In contrast, Kawamoto et al. 28 showed that CD34+ cells that display stem cell progenitor properties express very low levels of the 20 connexin isoforms examined, and only Cx37 and Cx43 were detected at the mRNA level.

Furthermore, we found significant negative correlation between the count of leukemic cells and Cx43 expression. The mechanisms by which Cx43 inhibits cell growth have been previously investigated. The Cx43 effect was proposed to be related to modulation of cytokines such as monocyte chemotactic protein 1 or extracellular growth factors including milk fat globule epidermal growth factor 8 29,30. Increased expression of Cx43 in cells with t(8;21) possibly has both a direct and an indirect effect mediated through c-Jun. Gao et al. 31 showed that the c-Jun N-terminal kinase signaling pathway mediates AML1–ETO-induced Cx43 expression.

However, other reports have shown that the growth-inhibiting effect of Cx43 might be mediated through a gap junction-independent pathway 32, of which the most important is posttranscriptional regulation of P27kip1 33,34. There is mounting evidence to show that P27kip1 is important in controlling the proliferation of myeloid cells. For example, Cheng et al. 35 reported that P27kip1 exerts a dominant effect in regulating progenitor proliferation. A critical role for P27kip1 in the regulation of the cell cycle in BCR-ABL-positive leukemic cells was also reported 36. Using a P27kip1 antisense vector, Gadhoum et al. 37 provided direct evidence that P27kip1 is essential for growth arrest induced by ligation of CD44, a cell surface molecule present on AML cells, with specific monoclonal antibodies in leukemic NB4, THP-1, KG1a, and HL60 cell lines and in primary leukemic cells. In our study, we found increased levels of P27kip1 protein in MNCs extracted from leukemic patients with positive t(8;21) compared with leukemic patients without this translocation. In addition, we found significant positive correlation between Cx43 expression and P27kip1 protein level. Our results are in agreement with those of Yan et al. 38 who found that AML1–ETO-infected hematopoietic K562 cells were shown to present increased levels of P27kip1 protein. Further, Li et al. 7 showed that increased Cx43 expression after AML1–ETO induction increased cellular P27kip1 protein by inhibiting its degradation. Of importance, they found that the downregulation of Cx43 after AML1–ETO induction blocked the increase in P27kip1 protein and restored its degradation rate. These results strongly suggest that Cx43 mediates AML1–ETO-induced stability of P27kip1 protein, the latter contributing to growth arrest as a critical CDK inhibitor 39.

In summary, our study shows that Cx43 expression increases in M2 with t(8;21) and may contribute to the growth-arresting effect of leukemogenic AML1–ETO fusion protein, possibly by causing the accumulation of P27kip1 protein. We recommend the use of drugs that improve Cx43 expression in treating M2.[39]

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