Inhibition of CRM1-dependent nuclear export sensitizes malignant cells to cytotoxic and targeted agents

Abstract

Nuclear-cytoplasmic trafficking of proteins is a significant factor in the development of cancer and drug resistance. Subcellular localization of exported proteins linked to cancer development include those involved in cell growth and proliferation, apoptosis, cell cycle regulation, transformation, angiogenesis, cell adhesion, invasion, and metastasis. Here, we examined the basic mechanisms involved in the export of proteins from the nucleus to the cytoplasm. All proteins over 40 kDa use the nuclear pore complex to gain entry or exit from the nucleus, with the primary nuclear export molecule involved in these processes being chromosome region maintenance 1 (CRM1, exportin 1 or XPO1). Proteins exported from the nucleus must possess a hydrophobic nuclear export signal (NES) peptide that binds to a hydrophobic groove containing an active-site Cys528 in the CRM1 protein. CRM1 inhibitors function largely by covalent modification of the active site Cys528 and prevent binding to the cargo protein NES. In the absence of a CRM1 inhibitor, CRM1 binds cooperatively to the NES of the cargo protein and RanGTP, forming a trimer that is actively transported out of the nucleus by facilitated diffusion. Nuclear export can be blocked by CRM1 inhibitors, NES peptide inhibitors or by preventing post-translational modification of cargo proteins. Clinical trials using the classic CRM1 inhibitor leptomycin B proved too toxic for patients; however, a new generation of less toxic small molecule inhibitors are being used in clinical trials in patients with both hematological malignancies and solid tumors. Additional trials are being initiated using small-molecule CRM1 inhibitors in combination with chemotherapeutics such as pegylated liposomal doxorubicin. In this review, we present evidence that combining the new CRM1 inhibitors with other classes of therapeutics may prove effective in the treatment of cancer. Potential combinatorial therapies discussed include the use of CRM1 inhibitors and the addition of alkylating agents (melphalan), anthracyclines (doxorubicin and daunomycin), BRAF inhibitors, platinum drugs (cisplatin and oxaliplatin), proteosome inhibitors (bortezomib and carfilzomib), or tyrosine-kinase inhibitors (imatinib). Also, the sequence of treatment may be important for combination therapy. We found that the most effective treatment regimen involved first priming the cancer cells with the CRM1 inhibitor followed by doxorubicin, bortezomib, carfilzomib, or melphalan. This order sensitized both de novo and acquired drug-resistant cancer cell lines.

Selinexor (KPT-330) Induces Tumor Suppression through Nuclear Sequestration of IκB and Downregulation of Survivin

“…Export of proteins, such as survivin (4,5), inhibitor of NFkB (IkB), p53 (6,7), Rb (8), and topo II a (9) from the nucleus through nuclear pore complex constitutes a key component of intracellular signaling which regulates cell proliferation and apoptosis (10)(11)(12)(13). Malfunctioning of the nuclear pore complex can cause cancer by stimulating tumor growth and inhibiting apoptosis (2,14) and overexpression of XPO1 has been reported in many types of cancers, including sarcomas (15).…”

“…Malfunctioning of the nuclear pore complex can cause cancer by stimulating tumor growth and inhibiting apoptosis (2,14) and overexpression of XPO1 has been reported in many types of cancers, including sarcomas (15). Because its expression level and activity perpetuate tumor growth, XPO1 is an important target for the development of small molecule inhibitors to treat cancer (10).…”

Magic pills: new oral drugs to treat chronic lymphocytic leukemia

“…It has already been demonstrated that XPO1 is overexpressed and recurrently mutated in CLL cells [96]. Selinexor (KPT-330) is an oral inhibitor of XPO1 and it is active as single agent in different hematologic malignancies including acute myeloid leukemia, NHL, CLL and multiple myeloma [97]. A phase I study exploring the activity of selinexor in combination with ibrutinib in patients with CLL or Richter's transformation is currently ongoing (NCT02303392).…”

Promising therapies in multiple myeloma

“…101 Selinexor (KPT-330), an inhibitor of the nuclear export protein CRM1, functions by maintaining the cellular distribution of tumor suppressors in MMCs. 102,103 Although single-agent activity was not observed in MM, phase 1/2 clinical trials in combination with dexamethasone, liposomal doxorubicin, pomalidomide/dexamethasone, or PIs/dexamethasone are ongoing, with promising interim results (NCT02336815, NCT02186834, NCT02343042, or NCT02199665, respectively). 104 Multiple studies have shown that PIs trigger apoptotic signaling but also induce AKT.…”