MNK1 signaling induces an ANGPTL4-mediated gene signature to drive melanoma progression.

The BRAFV600E mutation occurs in more than 50% of cutaneous melanomas, and results in the constitutive activation of the mitogen-activated protein kinases (MAPK) pathway. MAP kinase-interacting serine/threonine-protein kinase 1 and 2 (MNK1/2) are downstream effectors of the activated MAPK pathway, and important molecular targets in invasive and metastatic cancer. Despite the well-known role of MNK1 in regulating mRNA translation, little is known concerning the impact of its aberrant activation on gene transcription. Here, we show that changes in the activity, or abundance, of MNK1 result in changes in the expression of pro-oncogenic and pro-invasive genes. Among the MNK1-upregulated genes, we identify Angiopoietin-like 4 (ANGPTL4), which in turn promotes an invasive phenotype via its ability to induce the expression of matrix metalloproteinases (MMPs). Using a pharmacologic inhibitor of MNK1/2, SEL201, we demonstrate that BRAFV600E-mutated cutaneous melanoma cells are reliant on MNK1/2 for invasion and lung metastasis.

MNK1/NODAL Signaling Promotes Invasive Progression of Breast Ductal Carcinoma In Situ.

The mechanisms by which breast cancers progress from relatively indolent ductal carcinoma in situ (DCIS) to invasive ductal carcinoma (IDC) are not well understood. However, this process is critical to the acquisition of metastatic potential. MAPK-interacting serine/threonine-protein kinase 1 (MNK1) signaling can promote cell invasion. NODAL, a morphogen essential for embryogenic patterning, is often reexpressed in breast cancer. Here we describe a MNK1/NODAL signaling axis that promotes DCIS progression to IDC. We generated MNK1 knockout (KO) or constitutively active MNK1 (caMNK1)-expressing human MCF-10A-derived DCIS cell lines, which were orthotopically injected into the mammary glands of mice. Loss of MNK1 repressed NODAL expression, inhibited DCIS to IDC conversion, and decreased tumor relapse and metastasis. Conversely, caMNK1 induced NODAL expression and promoted IDC. The MNK1/NODAL axis promoted cancer stem cell properties and invasion in vitro. The MNK1/2 inhibitor SEL201 blocked DCIS progression to invasive disease in vivo. In clinical samples, IDC and DCIS with microinvasion expressed higher levels of phospho-MNK1 and NODAL versus low-grade (invasion-free) DCIS. Cumulatively, our data support further development of MNK1 inhibitors as therapeutics for preventing invasive disease. SIGNIFICANCE: These findings provide new mechanistic insight into progression of ductal carcinoma and support clinical application of MNK1 inhibitors to delay progression of indolent ductal carcinoma in situ to invasive ductal carcinoma.

Peroxisomes protect lymphoma cells from HDAC inhibitor-mediated apoptosis.

Peroxisomes are a critical rheostat of reactive oxygen species (ROS), yet their role in drug sensitivity and resistance remains unexplored. Gene expression analysis of clinical lymphoma samples suggests that peroxisomes are involved in mediating drug resistance to the histone deacetylase inhibitor (HDACi) Vorinostat (Vor), which promotes ROS-mediated apoptosis. Vor augments peroxisome numbers in cultured lymphoma cells, concomitant with increased levels of peroxisomal proteins PEX3, PEX11B, and PMP70. Genetic inhibition of peroxisomes, using PEX3 knockdown, reveals that peroxisomes protect lymphoma cells against Vor-mediated cell death. Conversely, Vor-resistant cells were tolerant to elevated ROS levels and possess upregulated levels of (1) catalase, a peroxisomal antioxidant, and (2) plasmalogens, ether glycerophospholipids that represent peroxisome function and serve as antioxidants. Catalase knockdown induces apoptosis in Vor-resistant cells and potentiates ROS-mediated apoptosis in Vor-sensitive cells. These findings highlight the role of peroxisomes in resistance to therapeutic intervention in cancer, and provide a novel modality to circumvent drug resistance.

NPM and BRG1 Mediate Transcriptional Resistance to Retinoic Acid in Acute Promyelocytic Leukemia.

Perturbation in the transcriptional control of genes driving differentiation is an established paradigm whereby oncogenic fusion proteins promote leukemia. From a retinoic acid (RA)-sensitive acute promyelocytic leukemia (APL) cell line, we derived an RA-resistant clone characterized by a block in transcription initiation, despite maintaining wild-type PML/RARA expression. We uncovered an aberrant interaction among PML/RARA, nucleophosmin (NPM), and topoisomerase II beta (TOP2B). Surprisingly, RA stimulation in these cells results in enhanced chromatin association of the nucleosome remodeler BRG1. Inhibition of NPM or TOP2B abrogated BRG1 recruitment. Furthermore, NPM inhibition and targeting BRG1 restored differentiation when combined with RA. Here, we demonstrate a role for NPM and BRG1 in obstructing RA differentiation and implicate chromatin remodeling in mediating therapeutic resistance in malignancies. NPM mutations are the most common genetic change in patients with acute leukemia (AML); therefore, our model may be applicable to other more common leukemias driven by NPM.

Triple A therapy: the molecular underpinnings of the unique sensitivity of leukemic promyelocytes to anthracyclines, all-trans-retinoic acid and arsenic trioxide.

If looking for a mnemonic to remember the relevant facts about acute promyelocytic leukemia (APL), one just has to remember that APL is a disease of A's. It is acute and it is highly sensitive to treatment with anthracyclines, all-trans-retinoic acid (RA) and arsenic trioxide (ATO). The presence of fusions involving the retinoic acid receptor alpha (RARA) is without question the central player driving APL and dictating the response of this disease to these therapeutic agents. However, beyond this knowledge, the molecular mechanisms that contribute to the complicated pathogenesis and the response to treatment of APL are not completely defined. As more is understood about this hematological malignancy, there are more opportunities to refine and improve treatment based on this knowledge. In this review article, we discuss the response of APL to these "A" therapies.

The novel arsenical darinaparsin is transported by cystine importing systems.

Darinaparsin (Dar; ZIO-101; S-dimethylarsino-glutathione) is a promising novel organic arsenical currently undergoing clinical studies in various malignancies. Dar consists of dimethylarsenic conjugated to glutathione (GSH). Dar induces more intracellular arsenic accumulation and more cell death than the FDA-approved arsenic trioxide (ATO) in vitro, but exhibits less systemic toxicity. Here, we propose a mechanism for Dar import that might explain these characteristics. Structural analysis of Dar suggests a putative breakdown product: dimethylarsino-cysteine (DMAC). We show that DMAC is very similar to Dar in terms of intracellular accumulation of arsenic, cell cycle arrest, and cell death. We found that inhibition of γ-glutamyl-transpeptidase (γ-GT) protects human acute promyelocytic leukemia cells (NB4) from Dar, but not from DMAC, suggesting a role for γ-GT in the processing of Dar. Overall, our data support a model where Dar, a GSH S-conjugate, is processed at the cell surface by γ-GT, leading to formation of DMAC, which is imported via xCT, xAG, or potentially other cystine/cysteine importing systems. Further, we propose that Dar induces its own import via increased xCT expression. These mechanisms may explain the enhanced toxicity of Dar toward cancer cells compared with ATO.

Expanding PML's functional repertoire through post-translational mechanisms.

Post-translational modifications, such as acetylation and ubiquitination, can greatly expand the functionality of a particular protein. The promyelocytic leukemia (PML) protein is a functionally promiscuous protein with proposed roles in many cellular processes. Its cellular headquarters are the macromolecular structures termed PML nuclear bodies. Post-translational modification of PML is emerging as a defining feature of this protein that regulates its physiological consequences. This review will highlight the expansion of our knowledge about the post-translational modifications of PML.

ERbeta sensitizes breast cancer cells to retinoic acid: evidence of transcriptional crosstalk.

The ability of retinoids to inhibit breast cancer cell growth correlates with estrogen receptor (ER) alpha status, as shown by the antiproliferative effects of retinoids in ERalpha-positive breast cancer cells and their use as chemopreventive agents in premenopausal women. The discovery of ERbeta, also present in breast cancer cells, has added a new level of complexity to this malignancy. To determine the retinoid response in ERbeta-expressing breast cancer cells, we used retroviral transduction of ERbeta in ER-negative MDA-MB-231 cells. Western blot and immunofluorescence confirmed expression and nuclear localization of ERbeta, whereas functionality was shown using an estrogen response element-containing reporter. A significant retinoic acid (RA)-mediated growth inhibition was observed in the transduced ERbeta-positive cells as shown by proliferation assays. Addition of estradiol, tamoxifen, or ICI 182,780 had no effect on cell growth and did not alter RA sensitivity. We observed that retinoids altered ERbeta-mediated transcriptional activity from an estrogen response element, which was confirmed by decreased expression of the pS2 gene, and from an activator protein response element. Conversely, the expression of ERbeta altered RA receptor (RAR) beta expression, resulting in greater induction of RARbeta gene expression on RA treatment, without altered expression of RARalpha. Our data provide evidence of transcriptional crosstalk between ERbeta and RAR in ERbeta-positive breast cancer cells that are growth inhibited by RA.