Melanoma Suppressor Functions of the Carcinoma Oncogene FOXQ1.

Lineage-specific regulation of tumor progression by the same transcription factor is understudied. We find that levels of the FOXQ1 transcription factor, an oncogene in carcinomas, are decreased during melanoma progression. Moreover, in contrast to carcinomas, FOXQ1 suppresses epithelial-to-mesenchymal transition, invasion, and metastasis in melanoma cells. We find that these lineage-specific functions of FOXQ1 largely depend on its ability to activate (in carcinomas) or repress (in melanoma) transcription of the N-cadherin gene (CDH2). We demonstrate that FOXQ1 interacts with nuclear ß-catenin and TLE proteins, and the ß-catenin/TLE ratio, which is higher in carcinoma than melanoma cells, determines the effect of FOXQ1 on CDH2 transcription. Accordingly, other FOXQ1-dependent phenotypes can be manipulated by altering nuclear ß-catenin or TLE proteins levels. Our data identify FOXQ1 as a melanoma suppressor and establish a mechanism underlying its inverse lineage-specific transcriptional regulation of transformed phenotypes.

Nrf2 amplifies oxidative stress via induction of Klf9.

Reactive oxygen species (ROS) activate NF-E2-related transcription factor 2 (Nrf2), a key transcriptional regulator driving antioxidant gene expression and protection from oxidant injury. Here, we report that in response to elevation of intracellular ROS above a critical threshold, Nrf2 stimulates expression of transcription Kruppel-like factor 9 (Klf9), resulting in further Klf9-dependent increases in ROS and subsequent cell death. We demonstrated that Klf9 independently causes increased ROS levels in various types of cultured cells and in mouse tissues and is required for pathogenesis of bleomycin-induced pulmonary fibrosis in mice. Mechanistically, Klf9 binds to the promoters and alters the expression of several genes involved in the metabolism of ROS, including suppression of thioredoxin reductase 2, an enzyme participating in ROS clearance. Our data reveal an Nrf2-dependent feedforward regulation of ROS and identify Klf9 as a ubiquitous regulator of oxidative stress and lung injury.

A purine nucleotide biosynthesis enzyme guanosine monophosphate reductase is a suppressor of melanoma invasion.

Melanoma is one of the most aggressive types of human cancers, and the mechanisms underlying melanoma invasive phenotype are not completely understood. Here, we report that expression of guanosine monophosphate reductase (GMPR), an enzyme involved in de novo biosynthesis of purine nucleotides, was downregulated in the invasive stages of human melanoma. Loss- and gain-of-function experiments revealed that GMPR downregulates the amounts of several GTP-bound (active) Rho-GTPases and suppresses the ability of melanoma cells to form invadopodia, degrade extracellular matrix, invade in vitro, and grow as tumor xenografts in vivo. Mechanistically, we demonstrated that GMPR partially depletes intracellular GTP pools. Pharmacological inhibition of de novo GTP biosynthesis suppressed whereas addition of exogenous guanosine increased invasion of melanoma cells as well as cells from other cancer types. Our data identify GMPR as a melanoma invasion suppressor and establish a link between guanosine metabolism and Rho-GTPase-dependent melanoma cell invasion.

Ribonucleotide reductase and thymidylate synthase or exogenous deoxyribonucleosides reduce DNA damage and senescence caused by C-MYC depletion.

The down-regulation of dominant oncogenes, including C-MYC, in tumor cells often leads to the induction of senescence via mechanisms that are not completely identified. In the current study, we demonstrate that MYC-depleted melanoma cells undergo extensive DNA damage that is caused by the underexpression of thymidylate synthase (TS) and ribonucleotide reductase (RR) and subsequent depletion of deoxyribonucleoside triphosphate pools. Simultaneous genetic inhibition of TS and RR in melanoma cells induced DNA damage and senescence phenotypes very similar to the ones caused by MYC-depletion. Reciprocally, overexpression of TS and RR in melanoma cells or addition of deoxyribo-nucleosides to culture media substantially inhibited DNA damage and senescence-associated phenotypes caused by C-MYC depletion. Our data demonstrate the essential role of TS and RR in C-MYC-dependent suppression of senescence in melanoma cells.

Preferential induction of apoptotic cell death in melanoma cells as compared with normal keratinocytes using a non-thermal plasma torch.

Selective induction of apoptosis in melanoma cells is optimal for therapeutic development. To achieve this goal, a non-thermal helium plasma torch was modified for use on cultured cells in a temperature-controlled environment. Melanoma cells were targeted with this torch (1) in parallel cultures with keratinocytes, (2) in co-culture with keratinocytes and (3) in a soft agar matrix. Melanoma cells displayed high sensitivity to reactive oxygen species generated by the torch and showed a 6-fold increase in cell death compared with keratinocytes. The extent of cell death was compared between melanoma cells and normal human keratinocytes in both short-term (5 min) co-culture experiments and longer assessments of apoptotic cell death (18-24 h). Following a 10 sec plasma exposure there was a 4.9-fold increase in the cell death of melanoma vs. keratinocytes as measured after 24 h at the target site of the plasma beam. When the treatment time was increased to 30 sec, a 98% cell death was reported for melanoma cells, which was 6-fold greater than the extent of cell death in keratinocytes. Our observations further indicate that this preferential cell death is largely due to apoptosis.. In addition, we report that this non-thermal plasma torch kills melanoma cells growing in soft agar, suggesting that the plasma torch is capable of inducing melanoma cell death in 3D settings. We demonstrate that the presence of gap junctions may increase the area of cell death, likely due to the "bystander effect" of passing apoptotic signals between cells. Our findings provide a basis for further development of this non-invasive plasma torch as a potential treatment for melanoma.

KLF9 is a novel transcriptional regulator of bortezomib- and LBH589-induced apoptosis in multiple myeloma cells.

Bortezomib, a therapeutic agent for multiple myeloma (MM) and mantle cell lymphoma, suppresses proteosomal degradation leading to substantial changes in cellular transcriptional programs and ultimately resulting in apoptosis. Transcriptional regulators required for bortezomib-induced apoptosis in MM cells are largely unknown. Using gene expression profiling, we identified 36 transcription factors that displayed altered expression in MM cells treated with bortezomib. Analysis of a publically available database identified Kruppel-like family factor 9 (KLF9) as the only transcription factor with significantly higher basal expression in MM cells from patients who responded to bortezomib compared with nonresponders. We demonstrated that KLF9 in cultured MM cells was up-regulated by bortezomib; however, it was not through the induction of endoplasmic reticulum stress. Instead, KLF9 levels correlated with bortezomib-dependent inhibition of histone deacetylases (HDAC) and were increased by the HDAC inhibitor LBH589 (panobinostat). Furthermore, bortezomib induced binding of endogenous KLF9 to the promoter of the proapoptotic gene NOXA. Importantly, KLF9 knockdown impaired NOXA up-regulation and apoptosis caused by bortezomib, LBH589, or a combination of theses drugs, whereas KLF9 overexpression induced apoptosis that was partially NOXA-dependent. Our data identify KLF9 as a novel and potentially clinically relevant transcriptional regulator of drug-induced apoptosis in MM cells.