microRNAs and Acute Myeloid Leukemia Chemoresistance: A Mechanistic Overview

Abstract

Up until the early 2000s, a functional role for microRNAs (miRNAs) was yet to be elucidated. With the advent of increasingly high-throughput and precise RNA-sequencing techniques within the last two decades, it has become well established that miRNAs can regulate almost all cellular processes through their ability to post-transcriptionally regulate a majority of protein-coding genes and countless other non-coding genes. In cancer, miRNAs have been demonstrated to play critical roles by modifying or controlling all major hallmarks including cell division, self-renewal, invasion, and DNA damage among others. Before the introduction of anthracyclines and cytarabine in the 1960s, acute myeloid leukemia (AML) was considered a fatal disease. In decades since, prognosis has improved substantially; however, long-term survival with AML remains poor. Resistance to chemotherapy, whether it is present at diagnosis or induced during treatment is a major therapeutic challenge in the treatment of this disease. Certain mechanisms such as DNA damage response and drug targeting, cell cycling, cell death, and drug trafficking pathways have been shown to be further dysregulated in treatment resistant cancers. miRNAs playing key roles in the emergence of these drug resistance phenotypes have recently emerged and replacement or inhibition of these miRNAs may be a viable treatment option. Herein, we describe the roles miRNAs can play in drug resistant AML and we describe miRNA-transcript interactions found within other cancer states which may be present within drug resistant AML. We describe the mechanisms of action of these miRNAs and how they can contribute to a poor overall survival and outcome as well. With the precision of miRNA mimic- or antagomir-based therapies, miRNAs provide an avenue for exquisite targeting in the therapy of drug resistant cancers.

Keywords: RNA therapy; acute myeloid leukemia; chemotherapy; cytarabine; daunorubicin; drug resistance; microRNA.

Figure 1

The six hallmarks of drug resistance: DNA damage and repair dysregulation, cell cycle dysregulation, cell death evasion, altered drug metabolism, altered drug target, and dysregulated drug trafficking.

Figure 2

microRNAs (miRNAs) regulate DNA damage response by regulating proteins that behave as DNA damage response elements. In the process of generating DNA damage through genotoxic drugs such as the anthracyclines and the cytosine analogs, the upregulation of effector and response proteins such as ataxia telangiectasia mutated (ATM) and Rad51 is likely to occur. The inhibition of ATM through miR-181a targeting allows tolerance for DNA damage. Reduction of Rad51 through miR-128, miR-506, miR-103, and miR-107 reduces DNA damage response and also contributes to DNA damage tolerance.

Figure 3

microRNAs (miRNAs) can dysregulate cell cycling mechanisms by dysregulating several phases of the cell cycle, but the majority of known targeting occurs at the G1 and S phases and at the G1/S transition. The downregulation of the cyclins that would normally signal for cell cycling to proceed can be downregulated. Cyclin D1 and cyclin D3 can be dysregulated by miR-188 and miR-16, cyclin E1 can be knocked down by miR-16 while cyclin E2 can be downregulated by miR-17-92 and finally, cyclin A1 and A2 are downregulated by miR-188 and miR-372, respectively. The cyclin-dependent kinases (CDKs) are also adjustable through miRNA targeting and their targeting reduces cycling as well. CDK2 can be downregulated by miR-638, miR-885-5p, miR-372, and miR-188; CDK4 is downregulated by miR-188, and CDK6 is downregulated by miR-16. Effector proteins such as E2F1, E2F7, and p21 can also be downregulated by miRNAs to lead to differentiation blocks. They can be targeted by miR-223, miR-26a, and miR-17-92, respectively.

Figure 4

The interactions between microRNAs (miRNAs) and cell death-related proteins in drug resistant cells. Within the apoptosis cell death mechanism, proteins part of the intrinsic or extrinsic pathway can respond to miRNAs to inhibit apoptosis or reduce their regulatory signaling of apoptosis. BCL2, an anti-apoptosis gene, will gain signaling when the associated miRNAs such as miR-156, miR-15a/b, miR-16, miR-125b-5p, and miR-139-5p are lost in the drug resistant cell. The gain of BAK1 miRNA targeting through miR-125b or the gain of BIM targeting through miR-32 will lead to the same effect as well. The Fas-ligand can also be suppressed by miR-149-5p thus ending extrinsic apoptosis signaling. P53 suppression through miR-125b and miR-504 will prevent apoptosis as well. Dysregulating autophagy through increased targeting may increase drug resistance through the binding of miR-125b and miR-101 on Atg4D. miR-30a is known to inversely correlate with Beclin1 and Atg5 in leukemia cell lines, but less is known about the outcome of this interaction.

Figure 5

The role of metabolism and microRNA (miRNA) in daunorubicin and cytarabine treatment. While daunorubicin is an active drug, cytosine requires bio-activation. As a cytosine analog, it must undergo three phosphorylation steps to become fully activated and capable of incorporating into the genome. The deactivation of daunorubicin and cytarabine is partially dependent on the cytochrome P450s and they commonly share CYP3A4 in their pathway of degradation. In other cancer, CYP3A4 has been shown to be targeted by miR-27b, miR-298, miR-577, miR-1, miR-532-3p, and miR-627. In the pathway of cytarabine activation, deoxycytidine kinase (DCK) has been shown to be downregulated by miR-330 in other cancers.

Figure 6

microRNAs (miRNAs) have been shown to dysregulate drug efflux mechanisms in both leukemia and other cancer. There are no known miRNA regulators of the drug influx proteins. In leukemia, P-glycoprotein has been demonstrably targeted by miR-27a and miR-331. In other cancers, P-glycoprotein has been shown to be regulated by miR-145, miR-298, miR-451, miR-508-5p, and miR-9. MRP1 has been targeted by miR-1291, miR-873, miR-221, miR-223, and miR-326, while MRP2 has been shown to be targeted my miR-379. The last protein to exhibit miRNA binding in lab setting is BCRP which has been shown to be a target of miR-328 and miR-519c.