Selective inhibition of nuclear export: a promising approach in the shifting treatment paradigms for hematological neoplasms

Abstract

Novel targeted therapeutics alone or in rational combinations are likely to dominate the future management of various hematological neoplasms. However, the challenges currently faced are the molecular heterogeneity in driver lesions and genetic plasticity leading to multiple resistance pathways. Thus, progress has overall been gradual. For example, despite the advent of targeted agents against actionable drivers like FLT3 in acute myeloid leukemia (AML), the prognosis remains suboptimal in newly diagnosed and dismal in the relapsed/refractory (R/R) setting, due to other molecular abnormalities contributing to inherent and acquired treatment resistance. Nuclear export inhibitors are of keen interest because they can inhibit several active tumorigenic processes simultaneously and also synergize with other targeted drugs and chemotherapy. XPO1 (or CRM1, chromosome maintenance region 1) is one of the most studied exportins involved in transporting critical cargoes, including tumor suppressor proteins like p27, p53, and RB1. Apart from the TSP cargo transport and its role in drug resistance, XPO1 inhibition results in retention of master transcription factors essential for cell differentiation, cell survival, and autophagy, rendering cells more susceptible to the effects of other antineoplastic agents, including targeted therapies. This review will dissect the role of XPO1 inhibition in hematological neoplasms, focusing on mechanistic insights gleaned mainly from work with SINE compounds. Future potential combinatorial strategies will be discussed.

Fig. 1. An illustrative picture of XPO1-dependant nuclear transport.

A Nucleo-cytoplasmic shuttling process transports various cargo proteins critical for cellular functions through nuclear pore complexes (NPCs) that facilitate macromolecular exchange. (i), The chromatin-bound nucleotide exchange factor, the regulator of chromosome condensation 1 (RCC1) in the nucleus, aids the conversion of RanGDP to RanGTP. (ii), RanGTP binds with cargo protein-loaded XPO1, causing a conformational change to expose the binding site’s nuclear export signal (NES). The cargo protein’s leucine-rich NES domain interacts with the NES binding site of XPO1. The active complex containing RanGTP, XPO1, and the corresponding cargo protein is docked into the NPC and subsequently shuttled out of the nucleus. (iii), in the cytoplasm, the RanGTP-XPO1-cargo loaded complex is subjected to GTP hydrolysis with RanGAP (GTPase activating protein) along with other protein ligases, including RanBP1/2. First, it releases the RanGTP off the complex and, on hydrolysis, converts RanGTP to RanGDP, eventually maintaining a higher gradient of the latter in the cytoplasm. RanGTP less XPO1-cargo complex aids in releasing the cargo from XPO1. (iv), in the final step of the energy-dependent nucleocytoplasmic shuttling, XPO1 is relocated back to the nucleus. B Demonstrates XPO1 gene locus in chromosome 2, the crystal structure of free XPO1 protein, and the cargo loaded RAN-GTP state [obtained from PROTEIN DATA BANK: 10.2210/pdb4FGV/pdb and 10.2210/pdb3GJX/pdb].

Fig. 2. Mechanistic pathways interrupted by XPO1 inhibition and possible synergies.

XPO1 transports several cellular protein cargoes and RNAs across the nuclear membrane into the cytoplasm. Important cargoes include tumor suppressor proteins like Rb1, p53, APC, and others to apoptosis. Potential synergy with anti-apoptotic inhibitors like Bcl2 (Ven, venetoclax) and MDM2 inhibitors is illustrated. Cell cycle growth regulators shuttled through XPO1 like p21, p27, and cyclin B1 maintain tumorigenesis. FLT3ITD, an oncogene in AML, can be inhibited by combining FLT3 and XPO1 inhibitor. NPM1 (Nucleophosmin) mutation translocate master transcription factor for monocytic differentiation PU.1 along with it to the cytoplasm when mutated. The NPM1c/PU.1 complex export dislocates it from CEBPA/RUNX1 transcription factor essential for granulomonocytic (GM) differentiation. XPO1 inhibition locks NPM1 within the nucleus enabling terminal monocytic differentiation. Upregulated Meis1/Hoxa9 in NPM1 mutant AML is downregulated when NPM1 is retained within the nucleus and can synergize with menin inhibitors. CEBPA/RUNX1 interactome act as co-repressors on differentiation when unbound by NPM1/PU.1 complex and, when inhibited with DNMT1 inhibitors like decitabine, can aid GM terminal differentiation.

Fig. 3. Mechanisms of XPO1 mediated emergent drug resistance pathways.

Increased XPO1 expression mediates nucleocytoplasmic displacement and inactivation of tumor suppressor proteins, leading to tumorigenesis as well as emergent drug resistance. Cytoplasmic dislocation of P53 is implicated in imatinib’s acquired drug resistance and PI3K inhibitor CYH33. Mislocalized topoisomerase 2α (TOP2A) to the cytoplasm is linked to doxorubicin resistance. Ibrutinib resistance is associated with XPO1 mediated nuclear export of inhibitors of NF- κB (IκB), P50, and P65, leading to activation of NF-κB signaling pathway. The nuclear export of FOXO3A is illustrated in acquired ibrutinib resistance, which XPO1 inhibitors can overcome. Platinum resistance is linked to β-catenin, which is regulated by the XPO1 mediated cytoplasmic displacement of Galectin 3.