Nuclear functions of NME proteins.

The NME family of proteins is composed of 10 isoforms, designated NME1-10, which are diverse in their enzymatic activities and patterns of subcellular localization. Each contains a conserved domain associated with a nucleoside diphosphate kinase (NDPK) function, although not all are catalytically active. Several of the NME isoforms (NME1, NME5, NME7, and NME8) also exhibit a 3'-5' exonuclease activity, suggesting roles in DNA proofreading and repair. NME1 and NME2 have been shown to translocate to the nucleus, although they lack a canonical nuclear localization signal. Binding of NME1 and NME2 to DNA does not appear to be sequence-specific in a strict sense, but instead is directed to single-stranded regions and/or other non-B-form structures. NME1 and NME2 have been identified as potential canonical transcription factors that regulate gene transcription through their DNA-binding activities. Indeed, the NME1 and NME2 isoforms have been shown to regulate gene expression programs in a number of cellular settings, and this regulatory function has been proposed to underlie their well-recognized ability to suppress the metastatic phenotype of cancer cells. Moreover, NME1 and, more recently, NME3, have been implicated in repair of both single- and double-stranded breaks in DNA. This suggests that reduced expression of NME proteins could contribute to the genomic instability that drives cancer progression. Clearly, a better understanding of the nuclear functions of NME1 and possibly other NME isoforms could provide critical insights into mechanisms underlying malignant progression in cancer. Indeed, clinical data indicate that the subcellular localization of NME1 may be an important prognostic marker in some cancers. This review summarizes putative functions of nuclear NME proteins in DNA binding, transcription, and DNA damage repair, and highlights their possible roles in cancer progression.

Identification of a gene expression signature associated with the metastasis suppressor function of NME1: prognostic value in human melanoma.

Although NME1 is well known for its ability to suppress metastasis of melanoma, the molecular mechanisms underlying this activity are not completely understood. Herein, we utilized a bioinformatics approach to systematically identify genes whose expression is correlated with the metastasis suppressor function of NME1. This was accomplished through a search for genes that were regulated by NME1, but not by NME1 variants lacking metastasis suppressor activity. This approach identified a number of novel genes, such as ALDOC, CXCL11, LRP1b, and XAGE1 as well as known targets such as NETO2, which were collectively designated as an NME1-Regulated Metastasis Suppressor Signature (MSS). The MSS was associated with prolonged overall survival in a large cohort of melanoma patients in The Cancer Genome Atlas (TCGA). The median overall survival of melanoma patients with elevated expression of the MSS genes was >5.6 years longer compared with that of patients with lower expression of the MSS genes. These data demonstrate that NMEl represents a powerful tool for identifying genes whose expression is associated with metastasis and survival of melanoma patients, suggesting their potential applications as prognostic markers and therapeutic targets in advanced forms of this lethal cancer.

The HGF/SF Mouse Model of UV-Induced Melanoma as an In Vivo Sensor for Metastasis-Regulating Gene.

Cutaneous malignant melanoma is an aggressive and potentially lethal form of skin cancer, particularly in its advanced and therapy-resistant stages, and the need for novel therapeutics and prognostic tools is acute. Incidence of melanoma has steadily increased over the past few decades, with exposure to the genome-damaging effects of ultraviolet radiation (UVR) well-recognized as a primary cause. A number of genetically-engineered mouse models (GEMMs) have been created that exhibit high incidence of spontaneous and induced forms of melanoma, and a select subset recapitulates its progression to aggressive and metastatic forms. These GEMMs hold considerable promise for providing insights into advanced stages of melanoma, such as potential therapeutic targets and prognostic markers, and as in vivo systems for testing of novel therapies. In this review, we summarize how the HGF/SF transgenic mouse has been used to reveal metastasis-regulating activity of four different genes (CDK4R24C, survivin and NME1/NME2) in the context of UV-induced melanoma. We also discuss how these models can potentially yield new strategies for clinical management of melanoma in its most aggressive forms.

NME1 Drives Expansion of Melanoma Cells with Enhanced Tumor Growth and Metastatic Properties.

Melanoma is a lethal skin cancer prone to progression and metastasis, and resistant to therapy. Metastasis and therapy resistance of melanoma and other cancers are driven by tumor cell plasticity, largely via acquisition/loss of stem-like characteristics and transitions between epithelial and mesenchymal phenotypes (EMT/MET). NME1 is a metastasis suppressor gene that inhibits metastatic potential when its expression is enforced in melanoma and other cancers. Herein, we have unmasked a novel role for NME1 as a driver of melanoma growth distinct from its canonical function as a metastasis suppressor. NME1 promotes expansion of stem-like melanoma cells that exhibit elevated expression of stem cell markers (e.g., Sox2, Sox10, Oct-4, KLF4, and Ccnb-1), enhanced growth as melanoma spheres in culture, and enhanced tumor growth and lung colonizing activities in vivo. In contrast, NME1 expression did not affect the proliferation of melanoma cell lines in monolayer culture conditions. Silencing of NME1 expression resulted in a dramatic reduction in melanoma sphere size, and impaired tumor growth and metastatic activities of melanoma sphere cells when xenografted in immunocompromised mice. Individual cells within melanoma sphere cultures displayed a wide range of NME1 expression across multiple melanoma cell lines. Cell subpopulations with elevated NME1 expression were fast cycling and displayed enhanced expression of stem cell markers. IMPLICATIONS: Our findings suggest the current model of NME1 as a metastasis-suppressing factor requires refinement, bringing into consideration its heterogeneous expression within melanoma sphere cultures and its novel role in promoting the expansion and tumorigenicity of stem-like cells.