Large parts of mammalian genomes are transcriptionally inactive and enriched with various classes of interspersed and tandem repeats. Here we show that the tumor suppressor protein p53 cooperates with DNA methylation to maintain silencing of a large portion of the mouse genome. Massive transcription of major classes of short, interspersed nuclear elements (SINEs) B1 and B2, both strands of near-centromeric satellite DNAs consisting of tandem repeats, and multiple species of noncoding RNAs was observed in p53-deficient but not in p53 wild-type mouse fibroblasts treated with the DNA demethylating agent 5-aza-2'-deoxycytidine. The abundance of these transcripts exceeded the level of β-actin mRNA by more than 150-fold. Accumulation of these transcripts, which are capable of forming double-stranded RNA (dsRNA), was accompanied by a strong, endogenous, apoptosis-inducing type I IFN response. This phenomenon, which we named "TRAIN" (for "transcription of repeats activates interferon"), was observed in spontaneous tumors in two models of cancer-prone mice, presumably reflecting naturally occurring DNA hypomethylation and p53 inactivation in cancer. These observations suggest that p53 and IFN cooperate to prevent accumulation of cells with activated repeats and provide a plausible explanation for the deregulation of IFN function frequently seen in tumors. Overall, this work reveals roles for p53 and IFN that are key for genetic stability and therefore relevant to both tumorigenesis and the evolution of species.interferon alpha-beta receptor | RNA sequencing | epigenetic repression | microarray hybridization

Multiple myeloma is a form of plasma cell neoplasm that accounts for approximately 10% of all hematological malignancies. Recently, several novel drugs have been discovered that almost doubled the overall survival of multiple myeloma patients. One of these drugs, the first-in-class proteasome inhibitor bortezomib (Velcade) has demonstrated remarkable response rates in multiple myeloma patients, and yet, currently this disease remains incurable. The major factor undermining the success of multiple myeloma treatment is a rapidly emerging resistance to the available therapy. Thus, the development of stand-alone or adjuvant anti-myeloma agents becomes of paramount importance. Overproduction of intracellular reactive oxygen species (ROS) often accompanies malignant transformation due to oncogene activation and/or enhanced metabolism in tumor cells. As a result, these cells possess higher levels of ROS and lower levels of antioxidant molecules compared to their normal counterparts. Unbalanced production of ROS leads to oxidative stress which, if left unchecked, could be toxic for the cell. In multiple myeloma cells where high rates of immunoglobulin synthesis is an additional factor contributing to overproduction of ROS, further induction of oxidative stress can be an effective strategy to cope with this disease. Here we will review the available data on the role of oxidative stress in the cytotoxicity of proteasome inhibitors and the use of ROS-inducing compounds as anti-myeloma agents.

Cellular responses to the loss of genomic stability are well-established, while how mammalian cells respond to chromatin destabilization is largely unknown. We previously found that DNA demethylation on p53-deficient background leads to transcription of repetitive heterochromatin elements, followed by an interferon response, a phenomenon we named TRAIN (Transcription of Repeats Activates INterferon). Here, we report that curaxin, an anticancer small molecule, destabilizing nucleosomes via disruption of histone/DNA interactions, also induces TRAIN. Furthermore, curaxin inhibits oncogene-induced transformation and tumor growth in mice in an interferon-dependent manner, suggesting that anticancer activity of curaxin, previously attributed to p53-activation and NF-kappaB-inhibition, may also involve induction of interferon response to epigenetic derepression of the cellular ‘repeatome’. Moreover, we observed that another type of drugs decondensing chromatin, HDAC inhibitor, also induces TRAIN. Thus, we proposed that TRAIN may be one of the mechanisms ensuring epigenetic integrity of mammalian cells via elimination of cells with desilenced chromatin.

Although antioxidants promote melanoma metastasis, the role of reactive oxygen species (ROS) in other stages of melanoma progression is controversial. Moreover, genes regulating ROS have not been functionally characterized throughout the entire tumor progression in mouse models of cancer. To address this question, we crossed mice-bearing knock-out of Klf9, an ubiquitous transcriptional regulator of oxidative stress, with two conditional melanocytic mouse models: BrafCA mice, where BrafV600E causes premalignant melanocytic hyperplasia, and BrafCA/Pten−/− mice, where BrafV600E and loss of Pten induce primary melanomas and metastases. Klf9 deficiency inhibited premalignant melanocytic hyperplasia in BrafCA mice but did not affect formation and growth of BrafCA/Pten−/− primary melanomas. It also, as expected, promoted BrafCA/Pten−/− metastasis. Treatment with antioxidant N-acetyl cysteine phenocopied loss of Klf9 including suppression of melanocytic hyperplasia. We were interested in a different role of Klf9 in regulation of cell proliferation in BrafCA and BrafCA/Pten−/− melanocytic cells. Mechanistically, we demonstrated that BRAFV600E signaling transcriptionally upregulated KLF9 and that KLF9-dependent ROS were required for full-scale activation of ERK1/2 and induction of cell proliferation by BRAFV600E. PTEN depletion in BRAFV600E-melanocytes did not further activate ERK1/2 and cell proliferation, but rendered these phenotypes insensitive to KLF9 and ROS. Our data identified an essential role of KLF9-dependent ROS in BRAFV600E signaling in premalignant melanocytes, offered an explanation to variable role of ROS in premalignant and transformed melanocytic cells and suggested a novel mechanism for suppression of premalignant growth by topical antioxidants.