Pathways of oncogene-induced senescence in human melanocytic cells

Bioinformatics procedure for investigating senolytic (anti-aging) agents: A digital signal processing technique

Objective

Cell growth involves cell division. This stops after reaching a certain limit. Some cells become inactive and unable to undergo apoptosis (programmed cell death). These cells accumulate at sites of tissue damage or disease, thus accelerating aging. They are called senescent cells. Therapeutic interventions that can either eliminate senescent cells (senolytics) or suppress their harmful effects (senomorphics) have been developed. Senescence (aging) is caused by the inter- and intramolecular interactions between the domains of forkhead (FHD) and transactivation (TAD), as well as C-terminal region 3 (CR3) and DNA binding (DBD). On the other hand, anti-senescent/senolytic (anti-aging) activities are achieved by disrupting these interactions with CR3- and forkhead box protein O4 (FOXO4)-based peptides, such as ES2 and DRI, respectively. In this study, we use a computerized procedure based on digital signal processing to systematically analyze the inter-molecular interactions between senolytics and their targets.

Methods

Informational spectrum method (ISM) is engaged.

Results

We obtained the sequences of the peptides from the interacting proteins of CR3 and FOXO4 and evaluated their ability to disrupt the inter-molecular interactions between FOXO4 and DRI and CR3 and BDB, which are responsible for senescence (aging). Our results show that the peptides have different degrees of senolytic (anti-aging) activity, depending on their affinity for CR3 and BDB, or FOXO4 and DRI. We found that enhanced senescence 2 (ES2) has a higher affinity for CR3 and BDB than FOXO4 and DRI, and that the interaction between CR3 and BDB is crucial for aging. Therefore, ES2 and other CR3-based peptides are more potent senolytics than FOXO4-based peptides. Our findings are consistent with previous studies and reveal new insights into the mechanisms of senescence and senolytics. ES2 is considered the best senolytic candidate, as it is 3-7 times more effective than DRI. We verified that ES2 has a weaker interaction with FOXO4 than CR3. However, the performance of DRI has been noted to depend on its intramolecular interactions and stability. Hence, intramolecular analyses using the digital signal processing-based technique has become very vital and will follow.

Conclusion

CR3-based peptides are promising candidates for senolytic therapy. Senolytics are linear chains of amino acids that can target and eliminate senescent cells, which are cells that have stopped dividing and contribute to aging and age-related diseases. By using this proposed, novel computerized technique that is based on digital signal processing, senolytics can be easily analyzed and optimized for their effectiveness and safety. This provides a more rational approach to enhancing our longevity and well-being by offering interventions that can delay or reverse aging and insights that can advance the field of gerontology. This procedure also will compliment other approaches such as molecular stimulation, etc.

Protooncogene MYC drives human melanocyte melanogenesis and senescence

In spite of extensive research and advances on the molecular biology of melanoma, the process of melanocytic differentiation or its relationship with proliferation is poorly understood. The role of proto-oncogenes in normal melanocyte biology is also intriguing. Proto-oncogene MYC is overexpressed in 40% of melanomas. It has been suggested that MYC can mediate senescence bypass in malignant melanocytes, an important event in melanoma development, likely in cooperation with other oncogenic pathways. However, despite the apparent importance of MYC in melanoma, its functions in normal melanocytes are unknown. We have overexpressed MYC in freshly isolated human primary melanocytes and studied the effects on melanocytic proliferation and differentiation. MYC promoted a transient activation of melanocytes including cell cycle entry, DNA damage and cell migration. Subsequently, MYC induced melanogenesis, increased cellular size and complexity and senescence. Interestingly, we also found strong expression of MYC in regions of human nevi displaying high pigmentation and high expression of senescence marker p16. The results altogether show that MYC drives melanocytic differentiation and suggest that senescence is associated with differentiation. We discuss the implications into the mechanisms governing melanocytic differentiation and the development of melanoma.

Glutamatergic Signaling a Therapeutic Vulnerability in Melanoma

Like other cancers, melanomas are associated with the hyperactivation of two major cell signaling cascades, the MAPK and PI3K/AKT pathways. Both pathways are activated by numerous genes implicated in the development and progression of melanomas such as mutated BRAF, RAS, and NF1. Our lab was the first to identify yet another driver of melanoma, Metabotropic Glutamate Receptor 1 (protein: mGluR1, mouse gene: Grm1, human gene: GRM1), upstream of the MAPK and PI3K/AKT pathways. Binding of glutamate, the natural ligand of mGluR1, activates MAPK and PI3K/AKT pathways and sets in motion the deregulated cellular responses in cell growth, cell survival, and cell metastasis. In this review, we will assess the proposed modes of action that mediate the oncogenic properties of mGluR1 in melanoma and possible application of anti-glutamatergic signaling modulator(s) as therapeutic strategy for the treatment of melanomas.

Mechanisms and significance of therapy-induced and spontaneous senescence of cancer cells

In contrast to the well-recognized replicative and stress-induced premature senescence of normal somatic cells, mechanisms and clinical implications of senescence of cancer cells are still elusive and uncertain from patient-oriented perspective. Moreover, recent years provided multiple pieces of evidence that cancer cells may undergo senescence not only in response to chemotherapy or ionizing radiation (the so-called therapy-induced senescence) but also spontaneously, without any external insults. Since the molecular nature of the latter process is poorly recognized, the significance of spontaneously senescent cancer cells for tumor progression, therapy effectiveness, and patient survival is purely speculative. In this review, we summarize the most up-to-date research regarding therapy-induced and spontaneous senescence of cancer cells, by delineating the most important discoveries regarding the occurrence of these phenomena in vivo and in vitro. This review provides data collected from studies on various cancer cell models, and the narration is presented from the broader perspective of the most critical findings regarding the senescence of normal somatic cells.

Genome-wide screening identifies novel genes implicated in cellular sensitivity to BRAFV600E expression

The V600E mutation of BRAF (BRAF^V600E), which constitutively activates the ERK/MAPK signaling pathway, is frequently found in melanoma and other cancers. Like most other oncogenes, BRAF^V600E causes oncogenic stress to normal cells, leading to growth arrest (senescence) or apoptosis. Through genome-wide screening, we identified genes implicated in sensitivity of human skin melanocytes and fibroblasts to BRAF^V600E overexpression. Among the identified genes shared by the two cell types are proto-oncogenes ERK2, a component of the ERK/MAPK pathway, and VAV1, a guanine nucleotide exchange factor for Rho family GTPases that also activates the ERK/MAPK pathway. CDKN1A, which has been known to promote senescence of fibroblasts but not melanocytes, is implicated in sensitivity of the fibroblasts but not the melanocytes to BRAF^V600E overexpression. Disruptions of GPR4, a pH-sensing G-protein coupled receptor, and DBT, a subunit of the branched chain α-keto acid dehydrogenase that is required for the second and rate-limiting step of branched amino acid catabolism and implicated in maple syrup urine disease, are the most highly selected in the melanocytes upon BRAF^V600E overexpression. Disruption of DBT severely attenuates ERK/MAPK signaling, p53 activation, and apoptosis in melanocytes, at least in part due to accumulation of branched chain α-keto acids. The expression level of BRAF positively correlates with that of DBT in all cancer types and with that of GPR4 in most cancer types. Overexpression of DBT kills all four melanoma cell lines tested regardless of the presence of BRAF^V600E mutation. Our findings shed new lights on regulations of oncogenic stress signaling and may be informative for development of novel cancer treatment strategies.

Regulation of cellular senescence via the FOXO4-p53 axis

Forkhead box O (FOXO) and p53 proteins are transcription factors that regulate diverse signalling pathways to control cell cycle, apoptosis and metabolism. In the last decade both FOXO and p53 have been identified as key players in aging, and their misregulation is linked to numerous diseases including cancers. However, many of the underlying molecular mechanisms remain mysterious, including regulation of ageing by FOXOs and p53. Several activities appear to be shared between FOXOs and p53, including their central role in the regulation of cellular senescence. In this review, we will focus on the recent advances on the link between FOXOs and p53, with a particular focus on the FOXO4‐p53 axis and the role of FOXO4/p53 in cellular senescence. Moreover, we discuss potential strategies for targeting the FOXO4‐p53 interaction to modulate cellular senescence as a drug target in treatment of aging‐related diseases and morbidity.

Detection of Nucleotide Disbalance in Cells Undergoing Oncogene-Induced Senescence

DNA damage response has been characterized as an important mediator of senescence phenotypes induced by activated oncogenes in normal human cells. Depletion of intracellular deoxyribonucleotide pools has been recently recognized as one of the major causes for DNA damage in these cells. Cells undergoing oncogene-induced senescence display decreased expression of several rate-limiting enzymes involved in the biosynthesis of deoxyribonucleotides, including thymidylate synthase (TS) and ribonucleotide reductase (RR). Individual depletion of these enzymes leads to premature senescence. Reciprocally, ectopic expression of TS and RR or addition of deoxyribonucleosides resulted in suppression of senescence phenotypes in normal or tumor cells caused by overexpression of activated HRAS or depletion of C-MYC, respectively. Therefore, in the current chapter, we will describe a methodology for the quantitative measurement of nucleotide pools in senescent cells.

The Immortal Senescence

Activation of oncogenic signaling paradoxically results in the permanent withdrawal from cell cycle and induction of senescence (oncogene-induced senescence (OIS)). OIS is a fail-safe mechanism used by the cells to prevent uncontrolled tumor growth, and, as such, it is considered as the first barrier against cancer. In order to progress, tumor cells thus need to first overcome the senescent phenotype. Despite the increasing attention gained by OIS in the past 20 years, this field is still rather young due to continuous emergence of novel pathways and processes involved in OIS. Among the many factors contributing to incomplete understanding of OIS are the lack of unequivocal markers for senescence and the complexity of the phenotypes revealed by senescent cells in vivo and in vitro. OIS has been shown to play major roles at both the cellular and organismal levels in biological processes ranging from embryonic development to barrier to cancer progression. Here we will briefly outline major advances in methodologies that are being utilized for induction, identification, and characterization of molecular processes in cells undergoing oncogene-induced senescence. The full description of such methodologies is provided in the corresponding chapters of the book.

8-Cl-cAMP and PKA I-selective cAMP analogs effectively inhibit undifferentiated thyroid cancer cell growth

The main purpose of our work was to evaluate the effects of different cyclic adenosine monophosphate analogs on thyroid cancer-derived cell lines. In particular we studied 8-chloroadenosine-3',5'-cyclic monophosphate, the most powerful cyclic adenosine monophosphate analog, and the protein kinase A I-selective combination of 8-hexylaminoadenosine-3',5'cyclic monophosphate and 8-piperidinoadenosine-3',5'-cyclic monophosphate. The cyclic adenosine monophosphate/protein kinase A pathway plays a fundamental role in the regulation of thyroid cells growth. Site-selective cyclic adenosine monophosphate analogs are a class of cyclic adenosine monophosphate-derivate molecules that has been synthesized to modulate protein kinase A activity. Although the cyclic adenosine monophosphate/protein kinase A pathway plays a fundamental role in the regulation of thyroid cells proliferation, there are currently no studies exploring the role of cyclic adenosine monophosphate analogs in thyroid cancer. We evaluated the effects on cell proliferation, apoptosis activation and alterations of different intracellular pathways using 3-(4,5-dimetylthiazole-2-yl)-2,5-diphenyltetrazolium bromide assay, flow cytofluorimetry, western blotting, and kinase inhibitors. Our results show that both compounds have antiproliferative potential. Both treatments were able to modify protein kinase A RI/RII ratio, thus negatively influencing cancer cells growth. Moreover, the two treatments differentially modulated various signaling pathways that regulate cell proliferation and apoptosis. Both treatments demonstrated interesting characteristics that prompt further studies aiming to understand the intimate interaction between different intracellular pathways and possibly develop novel anticancer therapies for undifferentiated thyroid cancer.

SP600125 has a remarkable anticancer potential against undifferentiated thyroid cancer through selective action on ROCK and p53 pathways

Thyroid cancer is the most common endocrine malignancy with increasing incidence worldwide.

The majority of thyroid cancer cases are well differentiated with favorable outcome. However, undifferentiated thyroid cancers are one of the most lethal human malignancies because of their invasiveness, metastatization and refractoriness even to the most recently developed therapies.

In this study we show for the first time a significant hyperactivation of ROCK/HDAC6 pathway in thyroid cancer tissues, and its negative correlation with p53 DNA binding ability.

We demonstrate that a small compound, SP600125 (SP), is able to induce cell death selectively in undifferentiated thyroid cancer cell lines by specifically acting on the pathogenic pathways of cancer development. In detail, SP acts on the ROCK/HDAC6 pathway involved in dedifferentiation and invasiveness of undifferentiated human cancers, by restoring its physiological activity level. As main consequence, cancer cell migration is inhibited and, at the same time, cell death is induced through the mitotic catastrophe. Moreover, SP exerts a preferential action on the mutant p53 by increasing its DNA binding ability. In TP53-mutant cells that survive mitotic catastrophe this process results in p21 induction and eventually lead to premature senescence. In conclusion, SP has been proved to be able to simultaneously block cell replication and migration, the two main processes involved in cancer development and dissemination, making it an ideal candidate for developing new drugs against anaplastic thyroid cancer.

RAB7 counteracts PI3K-driven macropinocytosis activated at early stages of melanoma development

Derailed endolysosomal trafficking is emerging as a widespread feature of aggressive neoplasms. However, the oncogenic signals that alter membrane homeostasis and their specific contribution to cancer progression remain unclear. Understanding the upstream drivers and downstream regulators of aberrant vesicular trafficking is distinctly important in melanoma. This disease is notorious for its inter- and intra-tumoral heterogeneity. Nevertheless, melanomas uniformly overexpress a cluster of endolysosomal genes, being particularly addicted to the membrane traffic regulator RAB7. Still, the underlying mechanisms and temporal determinants of this dependency have yet to be defined. Here we addressed these questions by combining electron microscopy, real time imaging and mechanistic analyses of vesicular trafficking in normal and malignant human melanocytic cells. This strategy revealed Class I PI3K as the key trigger of a hyperactive influx of macropinosomes that melanoma cells counteract via RAB7-mediated lysosomal degradation. In addition, gain- and loss-of-function in vitro studies followed by histopathological validation in clinical biopsies and genetically-engineered mouse models, traced back the requirement of RAB7 to the suppression of premature cellular senescence traits elicited in melanocytes by PI3K-inducing oncogenes. Together, these results provide new insight into the regulators and modes of action of RAB7, broadening the impact of endosomal fitness on melanoma development.

Regulatory and functional connection of microphthalmia-associated transcription factor and anti-metastatic pigment epithelium derived factor in melanoma

Pigment epithelium-derived factor (PEDF), a member of the serine protease inhibitor superfamily, has potent anti-metastatic effects in cutaneous melanoma through its direct actions on endothelial and melanoma cells. Here we show that PEDF expression positively correlates with microphthalmia-associated transcription factor (MITF) in melanoma cell lines and human samples. High PEDF and MITF expression is characteristic of low aggressive melanomas classified according to molecular and pathological criteria, whereas both factors are decreased in senescent melanocytes and naevi. Importantly, MITF silencing down-regulates PEDF expression in melanoma cell lines and primary melanocytes, suggesting that the correlation in the expression reflects a causal relationship. In agreement, analysis of Chromatin immunoprecipitation coupled to high throughput sequencing (ChIP-seq) data sets revealed three MITF binding regions within the first intron of SERPINF1, and reporter assays demonstrated that the binding of MITF to these regions is sufficient to drive transcription. Finally, we demonstrate that exogenous PEDF expression efficiently halts in vitro migration and invasion, as well as in vivo dissemination of melanoma cells induced by MITF silencing. In summary, these results identify PEDF as a novel transcriptional target of MITF and support a relevant functional role for the MITF-PEDF axis in the biology of melanoma.

Interferon-γ induces senescence in normal human melanocytes

BACKGROUND

Interferon-γ (IFN-γ) plays an important role in the proceedings of vitiligo through recruiting lymphocytes to the lesional skin. However, the potential effects of IFN-γ on skin melanocytes and the subsequent contribution to the vitiligo pathogenesis are still unclear.

OBJECTIVE

To investigate the effects of IFN-γ on viability and cellular functions of melanocytes.

METHODS

Primary human melanocytes were treated with IFN-γ. Cell viability, apoptosis, cell cycle melanin content and intracellular reactive oxygen species (ROS) level were measured. mRNA expression was examined by real-time PCR. The release of interleukin 6 (IL-6) and heat shock protein 70 (HSP-70) was monitored by ELISA. β-galactosidase staining was utilized to evaluate melanocyte senescence.

RESULTS

Persistent IFN-γ treatment induced viability loss, apoptosis, cell cycle arrest and senescence in melanocytes. Melanocyte senescence was characterized as the changes in pigmentation and morphology, as well as the increase of β-galactosidase activity. Increase of p21Cip1/Waf1 protein was evident in melanocytes after IFN-γ treatment. IFN-γ induction of senescence was attenuated by siRNAs against p21, Janus kinase 2 (JAK2) or signal transducer and activator of transcription 1 (STAT1), but not by JAK1 siRNA nor by p53 inhibitor pifithrin-α. IFN-γ treatment increased the accumulation of intracellular ROS in melanocytes, while ROS scavenger N-acetyl cysteine (NAC) effectively inhibited IFN-γ induced p21 expression and melanocyte senescence. IL-6 and HSP-70 release was significantly induced by IFN-γ treatment, which was largely inhibited by NAC. The increase of IL-6 and HSP-70 release could also be observed in senescent melanocytes.

CONCLUSION

IFN-γ can induce senescence in melanocytes and consequently enhance their immuno-competency, leading to a vitiligo-prone milieu.

Recent progress in genetics of aging, senescence and longevity: focusing on cancer-related genes

It is widely believed that aging results from the accumulation of molecular damage, including damage of DNA and mitochondria and accumulation of molecular garbage both inside and outside of the cell. Recently, this paradigm is being replaced by the “hyperfunction theory”, which postulates that aging is caused by activation of signal transduction pathways such as TOR (Target of Rapamycin). These pathways consist of different enzymes, mostly kinases, but also phosphatases, deacetylases, GTPases, and some other molecules that cause overactivation of normal cellular functions. Overactivation of these sensory signal transduction pathways can cause cellular senescence, age-related diseases, including cancer, and shorten life span. Here we review some of the numerous very recent publications on the role of signal transduction molecules in aging and age-related diseases. As was emphasized by the author of the “hyperfunction model”, many (or actually all) of them also play roles in cancer. So these “participants” in pro-aging signaling pathways are actually very well acquainted to cancer researchers. A cancer-related journal such as Oncotarget is the perfect place for publication of such experimental studies, reviews and perspectives, as it can bridge the gap between cancer and aging researchers.

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.

Nectin like-5 overexpression correlates with the malignant phenotype in cutaneous melanoma

NECL-5 is involved in regulating cell–cell junctions, in cooperation with cadherins, integrins and platelet-derived growth factor receptor, that are essential for intercellular communication. Its role in malignant transformation was previously described. It has been reported that transformation of melanocytes is associated with altered expression of adhesion molecules suggesting the potential involment of NECL-5 in melanoma development and prognosis. To shed light on this issue, the expression and the role of NECL-5 in melanoma tissues was investigated by bioinformatic and molecular approaches. NECL-5 was up-regulated both at the mRNA and the protein levels in WM35, M14 and A375 cell lines compared with normal melanocytes. A subsequent analysis in primary and metastatic melanoma specimens confirmed “in vitro” findings. NECL-5 overexpression was observed in 53 of 59 (89.8%) and 12 of 12 (100%), primary melanoma and melanoma metastasis, respectively; while, low expression of NECL-5 was detected in 12 of 20 (60%) benign nevi. A significant correlation of NECL-5 overexpression was observed with most of known negative melanoma prognostic factors, including lymph-node involvement (P = 0.009) and thickness (P = 0.004). Intriguingly, by analyzing the large series of melanoma samples in the Xu dataset, we identified the transcription factor YY1 among genes positively correlated with NECL-5 (r = 0.5). The concordant computational and experimental data of the present study indicate that the extent of NECL-5 expression correlates with melanoma progression.

Ras-induced ROS upregulation affecting cell proliferation is connected with cell type-specific alterations of HSF1/SESN3/p21Cip1/WAF1 pathways

Oncogenes of the RAS family regulate many of the cell's activities, including proliferation, survival and differentiation. Activating mutations in these genes are common events for many types of cancer. One of the contradictory points concerning the biological significance of Ras activation is its dual effect (pro- or anti-proliferative) on cell reproduction. One of mechanisms by which Ras proteins influence cell growth is a regulation of intracellular level of reactive oxygen species (ROS), second messengers affecting variety of cellular processes including cell proliferation. Recently it was shown that repression of SESN1 and SESN3 genes, whose protein products control regeneration of peroxiredoxins, can play a critical role in Ras-induced ROS upregulation. In the present study we have found that Ras-induced repression of SESN3 expression and ROS upregulation is mediated via the modifications of transcriptional activity of HSF1. Interestingly, mutant Ras overexpression altered the activity of HSF1 in opposite directions in different cell contexts, in particular in human normal fibroblasts and HaCaT immortalized keratinocytes, but these opposite changes caused similar repression of SESN3 expression followed by elevation of ROS content and inhibition of cell proliferation in corresponding cell types. The inhibitory effect on cell proliferation was mediated by upregulation of p21(Cip1/WAF1). Thus, HSF1/SESN3/ROS/p21(Cip1/WAF1)-mediated deceleration of cell growth may contribute to cell defense systems protecting the organism from excessive proliferation of cells that overexpress activated Ras oncoproteins.

Ectopic NGAL expression can alter sensitivity of breast cancer cells to EGFR, Bcl-2, CaM-K inhibitors and the plant natural product berberine

Neutrophil gelatinase-associated lipocalin (NGAL, a.k.a Lnc2) is a member of the lipocalin family and has diverse roles. NGAL can stabilize matrix metalloproteinase-9 from autodegradation. NGAL is considered as a siderocalin that is important in the transport of iron. NGAL expression has also been associated with certain neoplasias and is implicated in the metastasis of breast cancer. In a previous study, we examined whether ectopic NGAL expression would alter the sensitivity of breast epithelial, breast and colorectal cancer cells to the effects of the chemotherapeutic drug doxorubicin. While abundant NGAL expression was detected in all the cells infected with a retrovirus encoding NGAL, this expression did not alter the sensitivity of these cells to doxorubicin as compared with empty vector-transduced cells. We were also interested in determining the effects of ectopic NGAL expression on the sensitivity to small-molecule inhibitors targeting key signaling molecules. Ectopic NGAL expression increased the sensitivity of MCF-7 breast cancer cells to EGFR, Bcl-2 and calmodulin kinase inhibitors as well as the natural plant product berberine. Furthermore, when suboptimal concentrations of certain inhibitors were combined with doxorubicin, a reduction in the doxorubicin IC 50 was frequently observed. An exception was observed when doxorubicin was combined with rapamycin, as doxorubicin suppressed the sensitivity of the NGAL-transduced MCF-7 cells to rapamycin when compared with the empty vector controls. In contrast, changes in the sensitivities of the NGAL-transduced HT-29 colorectal cancer cell line and the breast epithelial MCF-10A cell line were not detected compared with empty vector-transduced cells. Doxorubicin-resistant MCF-7/Dox (R) cells were examined in these experiments as a control drug-resistant line; it displayed increased sensitivity to EGFR and Bcl-2 inhibitors compared with empty vector transduced MCF-7 cells. These results indicate that NGAL expression can alter the sensitivity of certain cancer cells to small-molecule inhibitors, suggesting that patients whose tumors exhibit elevated NGAL expression or have become drug-resistant may display altered responses to certain small-molecule inhibitors.

Controversial aspects of oncogene-induced senescence

Oncogene-induced senescence (OIS) is a fail-safe mechanism that is developed to suppress cell proliferation caused by aberrant activation of oncoproteins in normal cells. Most of the available literature considers senescence to be caused by activated RAS or RAF proteins. In the current review, we will discuss some of the controversial aspects of RAS- or RAF-induced senescence in different types of normal cells: are tumor suppressors important for OIS? What is the role of DNA damage in OIS? Are there different types of OIS?

Mutations and deregulation of Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR cascades which alter therapy response

The Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR cascades are often activated by genetic alterations in upstream signaling molecules such as receptor tyrosine kinases (RTK). Certain components of these pathways, RAS, NF1, BRAF, MEK1, DUSP5, PP2A, PIK3CA, PIK3R1, PIK3R4, PIK3R5, IRS4, AKT, NFKB1, MTOR, PTEN, TSC1, and TSC2 may also be activated/inactivated by mutations or epigenetic silencing. Upstream mutations in one signaling pathway or even in downstream components of the same pathway can alter the sensitivity of the cells to certain small molecule inhibitors. These pathways have profound effects on proliferative, apoptotic and differentiation pathways. Dysregulation of components of these cascades can contribute to: resistance to other pathway inhibitors, chemotherapeutic drug resistance, premature aging as well as other diseases. This review will first describe these pathways and discuss how genetic mutations and epigenetic alterations can result in resistance to various inhibitors.

Narrowing the knowledge gaps for melanoma

Cutaneous melanoma originates from pigment producing melanocytes or their precursors and is considered the deadliest form of skin cancer. For the last 40 years, few treatment options were available for patients with late-stage melanoma. However, remarkable advances in the therapy field were made recently, leading to the approval of two new drugs, the mutant BRAF inhibitor vemurafenib and the immunostimulant ipilimumab. Although these drugs prolong patients' lives, neither drug cures the disease completely, emphasizing the need for improvements of current therapies. Our knowledge about the complex genetic and biological mechanisms leading to melanoma development has increased, but there are still gaps in our understanding of the early events of melanocyte transformation and disease progression. In this review, we present a summary of the main contributing factors leading to melanocyte transformation and discuss recent novel findings and technologies that will help answer some of the key biological melanoma questions and lay the groundwork for novel therapies.

Cooperative effects of Akt-1 and Raf-1 on the induction of cellular senescence in doxorubicin or tamoxifen treated breast cancer cells

Escape from cellular senescence induction is a potent mechanism for chemoresistance. Cellular senescence can be induced in breast cancer cell lines by the removal of estrogen signaling with tamoxifen or by the accumulation of DNA damage induced by the chemotherapeutic drug doxorubicin. Long term culturing of the hormone-sensitive breast cancer cell line MCF-7 in doxorubicin (MCF-7/DoxR) reduced the ability of doxorubicin, but not tamoxifen, to induce senescence. Two pathways that are often upregulated in chemo- and hormonal-resistance are the PI3K/PTEN/Akt/mTOR and Ras/Raf/MEK/ERK pathways. To determine if active Akt-1 and Raf-1 can influence drug-induced senescence, we stably introduced activated ΔAkt-1(CA) and ΔRaf-1(CA) into drug-sensitive and doxorubicin-resistant cells. Expression of a constitutively-active Raf-1 construct resulted in higher baseline senescence, indicating these cells possessed the ability to undergo oncogene-induced-senescence. Constitutive activation of the Akt pathway significantly decreased drug-induced senescence in response to doxorubicin but not tamoxifen in MCF-7 cells. However, constitutive Akt-1 activation in drug-resistant cells containing high levels of active ERK completely escaped cellular senescence induced by doxorubicin and tamoxifen. These results indicate that up regulation of the Ras/PI3K/PTEN/Akt/mTOR pathway in the presence of elevated Ras/Raf/MEK/ERK signaling together can contribute to drug-resistance by diminishing cell senescence in response to chemotherapy. Understanding how breast cancers containing certain oncogenic mutations escape cell senescence in response to chemotherapy and hormonal based therapies may provide insights into the design of more effective drug combinations for the treatment of breast cancer.

PP2A-B56α controls oncogene-induced senescence in normal and tumor human melanocytic cells

Oncoprotein C-MYC is overexpressed in human metastatic melanomas and melanoma-derived cells where it is required for suppression of oncogene-induced senescence (OIS). The genetic events that maintain high levels of C-MYC in melanoma cells and their role in OIS are unknown. Here, we report that C-MYC in cells from several randomly chosen melanoma lines was up-regulated at the protein level, and largely due to the increased protein stability. Of all known regulators of C-MYC stability, levels of B56α subunit of the PP2A tumor suppressor complex were substantially suppressed in all human melanoma cells compared to normal melanocytes. Accordingly, immuno-histochemical analysis revealed that the lowest and the highest amounts of PP2A-B56α were predominantly detected in metastatic melanoma tissues and in primary melanomas from patients with good clinical outcome, respectively. Importantly, PP2A-B56α overexpression suppressed C-MYC in melanoma cells and induced OIS, whereas depletion of PP2A-B56α in normal human melanocytes up-regulated C-MYC protein levels and suppressed BRAF^V600E- and, less efficiently, NRAS^Q61R-induced senescence. Our data reveal a mechanism of C-MYC overexpression in melanoma cells and identify a functional role for PP2A-B56α in OIS of melanocytic cells.