Targeting PI3K/Akt/mTOR in AML: Rationale and Clinical Evidence

Abstract

Acute myeloid leukemia (AML) is a highly heterogeneous hematopoietic malignancy characterized by excessive proliferation and accumulation of immature myeloid blasts in the bone marrow. AML has a very poor 5-year survival rate of just 16% in the UK; hence, more efficacious, tolerable, and targeted therapy is required. Persistent leukemia stem cell (LSC) populations underlie patient relapse and development of resistance to therapy. Identification of critical oncogenic signaling pathways in AML LSC may provide new avenues for novel therapeutic strategies. The phosphatidylinositol-3-kinase (PI3K)/Akt and the mammalian target of rapamycin (mTOR) signaling pathway, is often hyperactivated in AML, required to sustain the oncogenic potential of LSCs. Growing evidence suggests that targeting key components of this pathway may represent an effective treatment to kill AML LSCs. Despite this, accruing significant body of scientific knowledge, PI3K/Akt/mTOR inhibitors have not translated into clinical practice. In this article, we review the laboratory-based evidence of the critical role of PI3K/Akt/mTOR pathway in AML, and outcomes from current clinical studies using PI3K/Akt/mTOR inhibitors. Based on these results, we discuss the putative mechanisms of resistance to PI3K/Akt/mTOR inhibition, offering rationale for potential candidate combination therapies incorporating PI3K/Akt/mTOR inhibitors for precision medicine in AML.

Keywords: AML; LSC; PI3K/Akt; combination treatment strategy; drug resistance; mTOR; targeted therapy.

Figure 1

Schematic overview of the activation and regulation of the PI3K/Akt/mTOR signaling pathway. Activation of PI3K is stimulated by binding of an extracellular ligand (e.g., hormones, growth factors, and cytokines) to a cell surface receptor such as the receptor tyrosine kinase (RTK) in the plasma membrane. Activated RTK recruits adaptor proteins, which bind to the regulatory p85 subunit of PI3K and subsequently activate the catalytic subunits for full PI3K activation. PI3K is also activated by G protein-coupled receptors (GPCR) or small GTPase Ras, which bind PI3K directly. Activated PI3K catalyzes the phosphorylation of phosphatidylinositol-4,5-phosphate (PIP₂) to generate phosphatidylinositol-3,4,5-phosphate (PIP₃). PIP₃ recruits phosphoinositide-dependent kinase 1 (PDK1) and Akt to the plasma membrane inducing Akt phosphorylation by PDK1 at T308. Akt activation is completed by phosphorylation at S473 by mTOR complex 2 (mTORC2). The mTOR complex includes two distinct protein complexes, mTORC1 and mTORC2. mTORC1 comprises of mTOR, proline-rich Akt substrate 40 kDa (PRAS40), regulatory-associated protein of mTOR (Raptor), mammalian lethal with Sec13 protein 8 (mLST8, also known as GβL), and DEP-domain-containing mTOR-interacting protein (Deptor) [44]. mTORC2 comprises of mTOR, mLST8, Deptor, protein observed with Rictor-1 (Protor), rapamycin-insensitive companion of mTOR (Rictor), and mammalian stress-activated protein kinase interacting protein (mSin1) [45]. Akt indirectly activates mTORC1 by phosphorylation and inhibition of tuberous sclerosis complex 2 (TSC2) at S939 and T1462, releasing the inhibitory effects of this complex on Ras-related GTPase Rheb, an activator of mTORC1. Akt also directly controls activation of mTORC1 in a TSC2-independent manner via phosphorylation of PRAS40 at T246. The extracellular signal-regulated kinase (ERK)/90 kDa ribosomal S6 kinase (RSK) and liver kinase B1/AMP-activated protein kinase (LKB1/AMPK) signaling pathways impinge on several nodes of the PI3K/Akt/mTOR pathway and can modulate mTORC1 activity. Both ERK and RSK modulate mTORC1 activity by phosphorylation of TSC2 at S664 and S540 (ERK) and S1798 (RSK). ERK1/2 can also control mTORC1 activation by phosphorylation of Raptor at S8, S696, and S863. Master metabolic regulator AMPK inhibits mTORC1 activity in two different pathways, the first by phosphorylation of TSC2 at T1271 and S1387 and the second by phosphorylation of Raptor at S722 and S792. Activated mTORC1 promotes cap-dependent mRNA translation via phosphorylation of eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) at T37 and T46, which is a priming event required for subsequent phosphorylation of several carboxy-terminal serum-sensitive sites to release 4E-BP1 from eIF4E. Ribosomal protein S6 kinase beta-1 (S6K1) is a downstream target of mTORC1, activated by phosphorylation at T389 by mTORC1 as well as T229 phosphorylation mediated by PDK1. S6K1 in turn activates ribosomal protein S6 (rpS6), which is dispensable for cell growth and protein synthesis. RSK can also directly activate rpS6 via phosphorylation at S235 and S236. The black arrows represent positive regulation (activation), whereas the red blunt-ended lines indicate negative regulation (inhibition). IRS-1 = insulin receptor substrate 1, PTEN = phosphatase and tensin homolog, GDP = guanosine diphosphate, GTP = guanosine triphosphate, JAK = Janus kinase, STAT = signal transducer and activator of transcription. Created with BioRender.com.

Figure 2

A summary diagram of Akt downstream target molecules. (From left to right) Fully activated Akt controls numerous effectors implicated in cell growth, proliferation, differentiation, metabolism, and survival, of which some are highlighted. Akt regulates G1/S cell cycle progression by phosphorylation and inactivation of GSK3/cyclin D1, p21, and p27. Akt was found to regulate cell metabolism by mediating lipogenesis and glucose uptake through phosphorylation and inhibition of GSK3, which inhibits glycogen synthesis. Akt controls apoptosis by phosphorylation and inhibition of FoxO and pro-apoptotic Bcl-2 family member BAD. Akt promotes cell growth by activation of mTORC1 though phosphorylation of PRAS40, which prevents its inhibition of mTORC1. Akt can also induce mTORC1 activation through phosphorylation and inhibition of TSC2, relieving the inhibitory effects of the TSC1/TSC2 complex on mTORC1. Akt enhances MDM2-mediated ubiquitination and proteasomal-dependent degradation of p53. Akt can inhibit apoptosis and promote cell survival by activating NF-κB. The arrows represent positive regulation (induction/activation), whereas the blunt-ended lines indicate negative regulation (inhibition/inactivation). The red cross represents inhibition caused by Akt-mediated negative regulation. RTK = receptor tyrosine kinase, IRS-1 = insulin receptor substrate 1, PIP₂ = phosphatidylinositol-4,5-phosphate, PIP₃ = phosphatidylinositol-3,4,5-phosphate, PDK1 = phosphoinositide-dependent kinase-1, MDM2 = mouse double minute 2 homolog, GSK3 = glycogen synthase kinase 3, TSC2 = tuberous sclerosis complex 2, mTORC1 = mTOR complex 1, FoxO = forkhead box O, Bim = Bcl-2-like protein 11, BAD = BCL-2 associated agonist of cell death, Bcl-2 = B-cell lymphoma 2, IKK = IκB kinase, NF-κB = nuclear factor kappa-light-chain-enhancer of activated B cells. Created with BioRender.com.

Figure 3

Targeting the PI3K/Akt/mTOR pathway in AML. The PI3K/Akt/mTOR pathway is commonly dysregulated in AML caused by mutations in membrane-bound proteins such as receptor tyrosine kinases (RTKs) and small GTPase Ras. Activating mutations in fms-like tyrosine kinase 3 (FLT3), such as the FLT3-internal tandem duplication (FLT3-ITD), are an important mechanism leading to dysregulation of PI3K/Akt/mTOR signaling. The ITD mutation causes ligand-independent activation of the FLT3 receptor, leading to constitutive activation of the PI3K/Akt/mTOR pathway. Numerous small-molecule inhibitors of this pathway include FLT3 inhibitors (FLT3i), dual PI3K/mTORi, allosteric mTORi, pan-class I and isoform-specific PI3Ki, ATP-competitive and allosteric Akti, and ATP-competitive mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) inhibitors (mTORC1i and mTORC2i). The red arrow indicates elevated Akt phosphorylation, whereas the red blunt-ended lines represent negative regulation (inhibition). IRS-1 = insulin receptor substrate 1, PIP₂ = phosphatidylinositol-4,5-phosphate, PIP₃ = phosphatidylinositol-3,4,5-phosphate, PTEN = phosphatase and tensin homolog, PDK1 = phosphoinositide-dependent kinase-1, TSC 1/2 = tuberous sclerosis complex 1/2, ATP = adenosine triphosphate, GDP = guanosine diphosphate, GTP = guanosine triphosphate, Rheb = Ras homolog enriched in brain, 4E-BP1 = eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1, S6K1 = ribosomal protein S6 kinase beta-1, rpS6 = ribosomal protein S6. The black blunt-ended lines indicate the main targets for therapeutic intervention. Created with BioRender.com.

Figure 4

Pie chart presenting the classification of interventions for AML therapy evaluated in clinical trials. Data was obtained from ClinicalTrials.gov and filtered for “recruiting” and “active, not recruiting” status in adult and elderly AML and study phase (1b-3) with start date from 1 January 2018. Numbers present frequency within 88 studies. * other = frequency <3 include dihydroorotate dehydrogenase (DHODH) inhibitor, proteasome inhibitor, p53 activator, hedgehog signaling inhibitor, CDK inhibitor, E-selectin inhibitor, nuclein export inhibitor, menin-MLL binding interaction inhibitor, CXCR4 antagonist, proteasome inhibitor, corticosteroid, thrombopoietin receptor agonist, and isoprenyl transferase inhibitor. MDM2 = mouse double minute 2 homolog, NAE = NEDD8-activating enzyme, Bcl-2 = B-cell lymphoma 2.