Drug Resistance Mechanisms of Acute Myeloid Leukemia Stem Cells

Abstract

Acute myeloid leukemia (AML) is a polyclonal and heterogeneous hematological malignancy. Relapse and refractory after induction chemotherapy are still challenges for curing AML. Leukemia stem cells (LSCs), accepted to originate from hematopoietic stem/precursor cells, are the main root of leukemogenesis and drug resistance. LSCs are dynamic derivations and possess various elusive resistance mechanisms. In this review, we summarized different primary resistance and remolding mechanisms of LSCs after chemotherapy, as well as the indispensable role of the bone marrow microenvironment on LSCs resistance. Through a detailed and comprehensive review of the spectacle of LSCs resistance, it can provide better strategies for future researches on eradicating LSCs and clinical treatment of AML.

Keywords: LSCs remolding; acute myeloid leukemia; clonal heterogeneity; drug resistance; leukemia stem cells; resistant mechanisms.

Figure 1

The schematic of LSCs clonal diversification resulting in AML relapse. The normal HSCs or progenitor cells carry CH-type mutations without proliferative potentials, such as ‘DTA’ mutations, denoted by the number I, and convert to pre-leukemia HSCs or progenitor cells with enhanced self-renewal through acquiring additional genetic mutations, which are divided into linear and branching patterns of clonal evolution. The former stepwise acquires CH-type mutations like TP53, IDH1/2 and AML-related mutations like FLT3, NPM1. The latter parallelly acquires CH-type mutations and AML-related mutations in different cell populations. Different colors of nuclei show heterogeneous LSCs clones. Residual leukemia cells (grey box) after treatment include dominant clones LSCs and subclones leukemia cells. Relapsed AML arises from these two types of cells corresponding to the primary resistance (green box) of LSCs and the secondary resistance (yellow box) of reprogramming leukemia cells via acquiring a new relapse-driven clone. AML, acute myeloid leukemia; LSCs, AML stem cells; HSCs, hematopoietic stem cells; CH, clonal hematopoiesis; BM, bone marrow; ‘DTA’ mutations, mutations in DNMT3A, TET2 and ASXL1.

Figure 2

Signaling pathways of dormancy in LSCs. LSCs arrest the cell cycle at G0 phase in a low level of ROS. The Wnt signaling pathway is activated through overexpression of FOXM1 which stabilizes β-catenin, up-regulated RSPO3 by activating β-catenin and increased lncRNA DANCR which upregulates MYC expression to induce LSCs dormancy. The repression of PI3K/AKT signaling pathway by miR126 mediates the stemness of LSCs and drug resistance. The abnormal activation of the Hh and NOTCH signaling pathways plays an important role in maintaining LSCs quiescence. AML, acute myeloid leukemia; LSCs, AML stem cells; ROS, reactive oxygen species; Hh, Hedgehog; SMO, smoothened; EVI1, Ecotropic virus integration site 1.

Figure 3

Drug efflux and drug resistance. Multiple anti-tumor drugs enter to kill leukemia cells (upper part), but LSCs upregulate ABC transporters to efflux drugs to overcome the cytotoxic effects of multiple chemotherapeutic drugs, even single blocking of MDR1 does not significantly improve chemotherapy outcomes. Additionally, MRP1 mediates the ATP-dependent efflux of GSH to reduce cellular oxidative stress. BRCP transports excessive PPIX out of the cell to maintain porphyrin homeostasis (lower part). MRP1, multidrug resistance-associated protein 1; MDR1, multidrug resistance gene 1; BRCP, breast cancer resistance protein; PPIX, protoporphyrin IX; GSH, glutathione; ATP, adenosine triphosphate; ADP, adenosine diphosphate.

Figure 4

The role of TP53 in apoptosis and senescence. Mutations or inactivation of p53 plays an important role in both apoptosis and senescence resistance of LSCs. While LSCs with mutant P53 are unable to activate downstream P21 to enter the senescent process, they do not target apoptosis pathway proteins including induction of pro-apoptotic proteins like Bax, and apoptosis initiator groups like NOXA and repressing anti-apoptotic members BCL2 and MCL1, eventually fail to initiate apoptosis. MDM2, minute 2 homolog; BCL2, B-cell lymphoma 2; MCL1, myeloid cell leukemia sequence 1; Bax, Bcl-2-associated X protein; NOXA, proapoptotic BH3-only protein.

Figure 5

Metabolic alteration and drug resistance. LSCs are particularly dependent on OXPHOS, which utilizes amino acids and fatty acids rather than glucose as energy substrates to generate ATP. Therapy-resistant LSCs exhibit increased FAO through increasing fatty acid transporters such as CD36, CPT1 and FABP4. Inhibitions of MCL1 or BCL2 inhibit amino acid and fatty acid metabolisms to reduce OXPHOS. The mutant IDH1/2 catalyzes the conversion of α-KG to 2-HG, the latter regulates both epigenetic and metabolic changes. OXPHOS, oxidative phosphorylation; FAO, fatty acid oxidation; TCA, tricarboxylic acid cycle; FFA, free fatty acids; FABP4, fatty acid-binding protein-4; PPARγ, peroxisome proliferator-activated receptor γ; CPT1, carnitine O-palmitoyltransferase 1; α-KG, α-ketoglutaric acid; α-HG, 2-hydroxyglutaric acid; IDH1/2, isocitrate dehydrogenase 1 or 2; cox, cytochrome c oxidase; KDM6A, Lysine Demethylase 6A.

Figure 6

Epigenetic regulation in LSCs. Methylation operates at the DNA, RNA, and histone levels as a ‘reader’, ‘writer’ and ‘eraser’. LSCs showed a hypomethylated state which overexpressed the tumor-promoting gene like HOXA cluster, MYC and MEIS1. Overexpressed HBO1 in LSCs adds acetyl to histone H3K14 up-regulating HOXA gene transcription. Me, methyl; Ac, Acetyl; HMTs, histone methyltransferases; HDMs, histone demethylases; EZH2, zeste homolog 2; HBO1, lysine acetyltransferase 7; HDACs, histone deacetylases; DNMTs, DNA methyltransferases; TETs, ten-eleven translocation; MBD, methyl-CpG binding domain protein 2; m6A, N6-methyladenosine; METTL3/14, methyltransferase-like protein 3/14; FTO, fat mass and obesity-associated protein; ALBH5, ALKB Homolog 5YTHDC1/YTHDF2, YTH family proteins.

Figure 7

BM niches and drug resistance. LSCs utilize the BM niches for increased survival. LSCs highly express CXCR4, binding to its ligand CXCL12, positioned in a hypoxic area. BMSCs also transfer mitochondria to provide additional energy for the survival of LSCs. Adipocytes provide sufficient fatty acids for LSCs metabolic reprogramming. LSCs globally induce immune tolerance via upregulating TIM3 and inducing multiple chemokines and an inflammatory secretome and thereby promoting leukemia progression. BM, bone marrow; MSCs, mesenchymal stromal cells; CXCR4, CXC chemokine receptor-type 4; CXCL12, CXC motif chemokine ligand 12; IL1RAP, interleukin-1 co-receptor; TIM3/Gal9, T cell immunoglobulin and mucin protein 3/galectin-9; NF-κB, nuclear factor κB. EVs, extracellular vesicles.