Inhibition of CRM1-mediated nucleocytoplasmic transport: triggering human melanoma cell apoptosis by perturbing multiple cellular pathways.

Development of multiple drug resistance mechanisms in melanomas necessitates the identification of new drug targets, which when inhibited could impact multiple cellular pathways, thus circumventing potential resistance. By performing complementary DNA microarray analysis, we identified four key components of the nucleocytoplasmic transport machinery-CRM1, RAN (RAN-GTPase), RANGAP1, and RANBP1-to be overexpressed in human melanoma metastases. Chromosome region maintenance 1 (CRM1) inhibition induced a marked depletion of prosurvival/cytoplasmic extracellular signal-regulated kinase 1/2 (Erk1/2) and p90 ribosomal S6 kinase1 and elicited persistent Erk-signaling hyperactivation. Consistently, CRM1 inhibition inflicted extensive apoptosis in melanoma cells while sparing nontransformed melanocytes and primary lung fibroblasts. Apoptosis required both the intrinsic and extrinsic apoptotic pathways and was associated with a nuclear entrapment and downregulation of the antiapoptotic CRM1 target protein, Survivin. Apoptosis was preceded by a G1 cell-cycle arrest, and even though CRM1 inhibition mediated marked p53 and p21 induction in wild-type p53 melanoma cells, the latter's silencing or inactivation failed to alleviate apoptosis. Notably, CRM1 inhibition induced cell line-specific, G1 to S progression-retarding changes in the expression of multiple cell-cycle regulatory proteins, thus potentially explaining p53 dispensability. We propose CRM1 as a potential therapeutic target in human melanoma, whose inhibition induces loss of prosurvival/cytoplasmic Erk1/2, mediates persistent Erk hyperactivation, and initiates a multitude of cell context-dependent molecular events to trigger G1 arrest followed by massive apoptosis.