CD133⁺ Acute Myeloid Leukemia (AML) Cells Exhibit Higher Clonogenic Capacity and Higher Levels of pAKT and Bcl-2 Proteins Than Their Negative Counterpart

CD133 is a transmembrane protein expressed by hematopoietic stem cells and other cell types, whose physiological functions remain unknown. To determine its potential prognostic value in AML, we analyzed its expression on blasts from 76 patients at diagnosis. Moreover, since leukemic stem cells play a critical role in the resistance to chemotherapeutic drugs and may be responsible for relapse, we compared the growth potential and activated cellular pathways of CD133+ and CD133- cells from 20 AML patients.

First, bone marrow samples were collected at diagnosis from 76 AML patients (median age: 65, range: 1–84). CD133 expression was evaluated on blasts cells and CD34⁺ cells using 6-color flow cytometry (FC) and the monoclonal antibody AC133. The mean fluorescence intensity ratio (MFIR) was calculated by dividing the MFI of CD133 by the MFI of its isotypic control. Second, purified CD34⁺, CD34⁺CD133⁺ and CD34⁺CD133^- cells were isolated from 20 AML bone marrows using the magnetic separation system Minimacs (Miltenyi Biotec). These fractions were submitted to an in vitro colony forming assay during 14 days. Finally, the level of expression of survival proteins involved in the PI3K/AKT, Mitogen-Activated Protein Kinase (MAPK) and Bcl-2 pathways, as well as CXCR4 and Focal Adhesion Kinase (FAK) migration proteins, was analyzed by FC in these fractions.

The level of CD133 expression on blasts cells was significantly different between AML patients with favorable (n=12), intermediate (n=38) and unfavorable karyotypes (n=23) (p=0.007). In particular, it was significantly higher in patients with unfavorable karyotype than in patients with intermediate karyotype (p=0.032) or favorable karyotype (p=0.005). Moreover, when considering NPM1 and FLT3 mutation status in patients with intermediate karyotype, the level of CD133 expression was significantly higher in patients with an intermediate molecular profile than in patients with a favorable molecular profile (p=0.004). Most interestingly, we showed that the level of CD133 expression has an impact on overall survival in multivariate analysis, independently of leukocytosis, karyotype and achievement of complete remission after induction therapy (p=0.04). Second, in 20 patients, we found that AML progenitor cells (AML CFU) were present in all sorted fractions, but the CD34⁺CD133⁺ fraction gave rise to larger and more numerous colonies (74.9±39.6) than the CD34⁺CD133^- fraction (11,25±6). Moreover, the CD34⁺CD133⁺ fraction was the only one able to form secondary colonies (52.1±31.8). Finally, we found that the expression of PI3K (mean percentage of positive cells: 78±20.1), pAKT (80±19.8), pERK (80.1±19), CXCR4 (71.6±22.7), pFAK (81.2±19.8), Bcl-2 (72.9±26.1), Bcl-xL (45.7±24.8), and pBad (63.9±36.1) was significantly upregulated in CD34⁺CD133⁺ cells compared to CD34⁺CD133^- cells, whereas Bax was downregulated.

CD133 is known to be expressed in hematopoietic progenitors, but its role in the resistance to chemotherapy is not well studied. In our work, the level of expression of CD133 on AML blasts appeared as an independent immunophenotypic prognostic factor, encouraging its further evaluation in a larger number of cases. We also showed that CD133⁺ cells have greater replicative properties than CD133^- cells, and exhibit activation of various signaling pathways. Since CD133⁺ cells might escape from apoptosis through activation of PI3K/AKT and ERK, these proteins could represent potential therapeutic targets. In conclusion, our results revealed that CD133⁺ cells exhibit more aggressive behavior than CD133^- ones, and that CD133⁺ AML progenitors might contribute to resistance to chemotherapy through preferential activation of PI3K/AKT, MAPK and Bcl-2 survival response.

No relevant conflicts of interest to declare.