Activation of extracellular signal-regulated kinase (ERK) in G2 phase delays mitotic entry through p21CIP1

GRK2-mediated AKT activation controls cell cycle progression and G2 checkpoint in a p53-dependent manner

Cell cycle checkpoints, activated by stressful events, halt the cell cycle progression, and prevent the transmission of damaged DNA. These checkpoints prompt cell repair but also trigger cell death if damage persists. Decision-making between these responses is multifactorial and context-dependent, with the tumor suppressor p53 playing a central role. In many tumor cells, p53 alterations lead to G1/S checkpoint loss and the weakening of the G2 checkpoint, rendering cell viability dependent on the strength of the latter through mechanisms not fully characterized. Cells with a strong pro-survival drive can evade cell death despite substantial DNA lesions. Deciphering the integration of survival pathways with p53-dependent and -independent mechanisms governing the G2/M transition is crucial for understanding G2 arrest functionality and predicting tumor cell response to chemotherapy. The serine/threonine kinase GRK2 emerges as a signaling node in cell cycle modulation. In cycling cells, but not in G2 checkpoint-arrested cells, GRK2 protein levels decline during G2/M transition through a process triggered by CDK2-dependent phosphorylation of GRK2 at the S670 residue and Mdm2 ubiquitination. We report now that this downmodulation in G2 prevents the unscheduled activation of the PI3K/AKT pathway, allowing cells to progress into mitosis. Conversely, higher GRK2 levels lead to tyrosine phosphorylation by the kinase c-Abl, promoting the direct association of GRK2 with the p85 regulatory subunit of PI3K and AKT activation in a GRK2 catalytic-independent manner. Hyperactivation of AKT is conditioned by p53's scaffolding function, triggering FOXO3a phosphorylation, impaired Cyclin B1 accumulation, and CDK1 activation, causing a G2/M transition delay. Upon G2 checkpoint activation, GRK2 potentiates early arrest independently of p53 through AKT activation. However, its ability to overcome the G2 checkpoint in viable conditions depends on p53. Our results suggest that integrating the GRK2/PI3K/AKT axis with non-canonical functions of p53 might confer a survival advantage to tumor cells.

Selective inhibition of HDAC6 by N-acylhydrazone derivative reduces the proliferation and induces senescence in carcinoma hepatocellular cells

Hepatocellular carcinoma (HCC) is a significant contributor to cancer-related deaths globally. Systemic therapy is the only treatment option for HCC at an advanced stage, with limited therapeutic response. In this study, we evaluated the antitumor potential of four N-acylhydrazone (NAH) derivatives, namely LASSBio-1909, 1911, 1935, and 1936, on HCC cell lines. We have previously demonstrated that the aforementioned NAH derivatives selectively inhibit histone deacetylase 6 (HDAC6) in lung cancer cells, but their effects on HCC cells have not been explored. Thus, the present study aimed to evaluate the effects of NAH derivatives on the proliferative behavior of HCC cells. LASSBio-1911 was the most cytotoxic compound against HCC cells, however its effects were minimal on normal cells. Our results showed that LASSBio-1911 inhibited HDAC6 in HCC cells leading to cell cycle arrest and decreased cell proliferation. There was also an increase in the frequency of cells in mitosis onset, which was associated with disturbing mitotic spindle formation. These events were accompanied by elevated levels of CDKN1A mRNA, accumulation of CCNB1 protein, and sustained ERK1 phosphorylation. Furthermore, LASSBio-1911 induced DNA damage, resulting in senescence and/or apoptosis. Our findings indicate that selective inhibition of HDAC6 may provide an effective therapeutic strategy for the treatment of advanced HCC, including tumor subtypes with integrated viral genome. Further, in vivo studies are required to validate the antitumor effect of LASSBio-1911 on liver cancer.

Relating individual cell division events to single-cell ERK and Akt activity time courses

Biochemical correlates of stochastic single-cell fates have been elusive, even for the well-studied mammalian cell cycle. We monitored single-cell dynamics of the ERK and Akt pathways, critical cell cycle progression hubs and anti-cancer drug targets, and paired them to division events in the same single cells using the non-transformed MCF10A epithelial line. Following growth factor treatment, in cells that divide both ERK and Akt activities are significantly higher within the S-G2 time window (~ 8.5-40 h). Such differences were much smaller in the pre-S-phase, restriction point window which is traditionally associated with ERK and Akt activity dependence, suggesting unappreciated roles for ERK and Akt in S through G2. Simple metrics of central tendency in this time window are associated with subsequent cell division fates. ERK activity was more strongly associated with division fates than Akt activity, suggesting Akt activity dynamics may contribute less to the decision driving cell division in this context. We also find that ERK and Akt activities are less correlated with each other in cells that divide. Network reconstruction experiments demonstrated that this correlation behavior was likely not due to crosstalk, as ERK and Akt do not interact in this context, in contrast to other transformed cell types. Overall, our findings support roles for ERK and Akt activity throughout the cell cycle as opposed to just before the restriction point, and suggest ERK activity dynamics may be more important than Akt activity dynamics for driving cell division in this non-transformed context.

microRNA-Mediated Encoding and Decoding of Time-Dependent Signals in Tumorigenesis

microRNAs, pivotal post-transcriptional regulators of gene expression, in the past decades have caught the attention of researchers for their involvement in different biological processes, ranging from cell development to cancer. Although lots of effort has been devoted to elucidate the topological features and the equilibrium properties of microRNA-mediated motifs, little is known about how the information encoded in frequency, amplitude, duration, and other features of their regulatory signals can affect the resulting gene expression patterns. Here, we review the current knowledge about microRNA-mediated gene regulatory networks characterized by time-dependent input signals, such as pulses, transient inputs, and oscillations. First, we identify the general characteristic of the main motifs underlying temporal patterns. Then, we analyze their impact on two commonly studied oncogenic networks, showing how their dysfunction can lead to tumorigenesis.

Regulation of Cell Cycle Progression by Growth Factor-Induced Cell Signaling

The cell cycle is the series of events that take place in a cell, which drives it to divide and produce two new daughter cells. The typical cell cycle in eukaryotes is composed of the following phases: G1, S, G2, and M phase. Cell cycle progression is mediated by cyclin-dependent kinases (Cdks) and their regulatory cyclin subunits. However, the driving force of cell cycle progression is growth factor-initiated signaling pathways that control the activity of various Cdk–cyclin complexes. While the mechanism underlying the role of growth factor signaling in G1 phase of cell cycle progression has been largely revealed due to early extensive research, little is known regarding the function and mechanism of growth factor signaling in regulating other phases of the cell cycle, including S, G2, and M phase. In this review, we briefly discuss the process of cell cycle progression through various phases, and we focus on the role of signaling pathways activated by growth factors and their receptor (mostly receptor tyrosine kinases) in regulating cell cycle progression through various phases.

Cell-intrinsic mechanisms of drug tolerance to systemic therapies in cancer

In cancer patients with metastatic disease, the rate of complete tumor response to systemic therapies is low, and residual lesions persist in the majority of patients due to early molecular adaptation in cancer cells. A growing body of evidence suggests that a subpopulation of drug-tolerant « persister » cells - a reversible phenotype characterized by reduced drug sensitivity and decreased cell proliferation - maintains residual disease and may serve as a reservoir for resistant phenotypes. The survival of these residual tumor cells can be caused by reactivation of specific signaling pathways, phenotypic plasticity (i.e., transdifferentiation), epigenetic or metabolic reprogramming, downregulation of apoptosis as well as transcriptional remodeling. In this review, we discuss the molecular mechanisms that enable adaptive survival in drug-tolerant cells. We describe the main characteristics and dynamic nature of this persistent state, and highlight the current therapeutic strategies that may be used to interfere with the establishment of drug-tolerant cells, as an alternative to improve objective response to systemic therapies and delay the emergence of resistance to improve long-term survival.

Spatiotemporal control of pathway sensors and cross-pathway feedback regulate a differentiation MAPK pathway in yeast

Mitogen-activated protein kinase (MAPK) pathways control cell differentiation and the response to stress. In Saccharomyces cerevisiae, the MAPK pathway that controls filamentous growth (fMAPK) shares components with the pathway that regulates the response to osmotic stress (HOG). Here, we show that the two pathways exhibit different patterns of activity throughout the cell cycle. The different patterns resulted from different expression profiles of genes encoding mucin sensors that regulate the pathways. Cross-pathway regulation from the fMAPK pathway stimulated the HOG pathway, presumably to modulate fMAPK pathway activity. We also show that the shared tetraspan protein Sho1p, which has a dynamic localization pattern throughout the cell cycle, induced the fMAPK pathway at the mother-bud neck. A Sho1p-interacting protein, Hof1p, which also localizes to the mother-bud neck and regulates cytokinesis, also regulated the fMAPK pathway. Therefore, spatial and temporal regulation of pathway sensors, and cross-pathway regulation, control a MAPK pathway that regulates cell differentiation in yeast.

Dual specificity phosphatase 5 regulates perfusion recovery in experimental peripheral artery disease

Peripheral artery disease (PAD) is caused by atherosclerotic occlusions of vessels outside the heart, particularly those of the lower extremities. Angiogenesis is one critical physiological response to vessel occlusion in PAD, but our understanding of the molecular mechanisms involved in angiogenesis is incomplete. Dual specificity phosphatase 5 (DUSP5) has been shown to play a key role in embryonic vascular development, but its role in post-ischemic angiogenesis is not known. We induced hind limb ischemia in mice and found robust upregulation of Dusp5 expression in ischemic hind limbs. Moreover, in vivo knockdown of Dusp5 resulted in impaired perfusion recovery in ischemic limbs and was associated with increased limb necrosis. In vitro studies showed upregulation of DUSP5 in human endothelial cells exposed to ischemia, and knockdown of DUSP5 in these ischemic endothelial cells resulted in impaired endothelial cell proliferation and angiogenesis, but did not alter apoptosis. Finally, we show that these effects of DUSP5 on post-ischemic angiogenesis are a result of DUSP5-dependent decrease in ERK1/2 phosphorylation and p21 protein expression. Thus, we have identified a role of DUSP5 in post-ischemic angiogenesis and implicated a DUSP5-ERK-p21 pathway that may serve as a therapeutic target for the modulation of post-ischemic angiogenesis in PAD.

HBV-related hepatocarcinogenesis: the role of signalling pathways and innovative ex vivo research models

BACKGROUND

Hepatitis B virus (HBV) is the leading cause of liver cancer, but the mechanisms by which HBV causes liver cancer are poorly understood and chemotherapeutic strategies to cure liver cancer are not available. A better understanding of how HBV requisitions cellular components in the liver will identify novel therapeutic targets for HBV associated hepatocellular carcinoma (HCC).

MAIN BODY

The development of HCC involves deregulation in several cellular signalling pathways including Wnt/FZD/β-catenin, PI3K/Akt/mTOR, IRS1/IGF, and Ras/Raf/MAPK. HBV is known to dysregulate several hepatocyte pathways and cell cycle regulation resulting in HCC development. A number of these HBV induced changes are also mediated through the Wnt/FZD/β-catenin pathway. The lack of a suitable human liver model for the study of HBV has hampered research into understanding pathogenesis of HBV. Primary human hepatocytes provide one option; however, these cells are prone to losing their hepatic functionality and their ability to support HBV replication. Another approach involves induced-pluripotent stem (iPS) cell-derived hepatocytes. However, iPS technology relies on retroviruses or lentiviruses for effective gene delivery and pose the risk of activating a range of oncogenes. Liver organoids developed from patient-derived liver tissues provide a significant advance in HCC research. Liver organoids retain the characteristics of their original tissue, undergo unlimited expansion, can be differentiated into mature hepatocytes and are susceptible to natural infection with HBV.

CONCLUSION

By utilizing new ex vivo techniques like liver organoids it will become possible to develop improved and personalized therapeutic approaches that will improve HCC outcomes and potentially lead to a cure for HBV.

ErbB Receptors and Cancer

The ErbB receptor family, also known as the EGF receptor family or type I receptor family, includes the epidermal growth factor (EGF) receptor (EGFR) or ErbB1/Her1, ErbB2/Her2, ErbB3/Her3, and ErbB4/Her4. Among all RTKs, EGFR was the first RTK identified and the first one linked to cancer. Thus, EGFR has also been the most intensively studied among all RTKs. ErbB receptors are activated after homodimerization or heterodimerization. The ErbB family is unique among the various groups of receptor tyrosine kinases (RTKs) in that ErbB3 has impaired kinase activity, while ErbB2 does not have a direct ligand. Therefore, heterodimerization is an important mechanism that allows the activation of all ErbB receptors in response to ligand stimulation. The activated ErbB receptors bind to many signaling proteins and stimulate the activation of many signaling pathways. The specificity and potency of intracellular signaling pathways are determined by positive and negative regulators, the specific composition of activating ligand(s), receptor dimer components, and the diverse range of proteins that associate with the tyrosine phosphorylated C-terminal domain of the ErbB receptors. ErbB receptors are overexpressed or mutated in many cancers, especially in breast cancer, ovarian cancer, and non-small cell lung cancer. The overexpression and overactivation of ErbB receptors are correlated with poor prognosis, drug resistance, cancer metastasis, and lower survival rate. ErbB receptors, especially EGFR and ErbB2 have been the primary choices as targets for developing cancer therapies.

Orchestrating cellular signaling pathways-the cellular "conductor" protein tyrosine phosphatase interacting protein 51 (PTPIP51)

The protein tyrosine phosphatase interacting protein 51 (PTPIP51) is thought to regulate crucial cellular functions such as mitosis, apoptosis, migration, differentiation and communication between organelles as a scaffold protein. These diverse functions are modulated by the tyrosine/serine phosphorylation status of PTPIP51. This review interconnects the insights obtained about the action of PTPIP51 in mitogen-activated protein kinase signaling, nuclear factor kB signaling, calcium homeostasis and chromosomal segregation and identifies important signaling hubs. The interference of PTPIP51 in such multiprotein complexes and their PTPIP51-modulated cross-talk makes PTPIP51 an ideal target for novel drugs such as the small molecule LDC-3. Graphical Abstract ᅟ.

Intercellular propagation of extracellular signal-regulated kinase activation revealed by in vivo imaging of mouse skin

Extracellular signal-regulated kinase (ERK) is a key effector of many growth signalling pathways. In this study, we visualise epidermal ERK activity in living mice using an ERK FRET biosensor. Under steady-state conditions, the epidermis occasionally revealed bursts of ERK activation patterns where ERK activity radially propagated from cell to cell. The frequency of this spatial propagation of radial ERK activity distribution (SPREAD) correlated with the rate of epidermal cell division. SPREADs and proliferation were stimulated by 12-O-tetradecanoylphorbol 13-acetate (TPA) in a manner dependent on EGF receptors and their cognate ligands. At the wounded skin, ERK activation propagated as trigger wave in parallel to the wound edge, suggesting that ERK activation propagation can be superimposed. Furthermore, by visualising the cell cycle, we found that SPREADs were associated with G2/M cell cycle progression. Our results provide new insights into how cell proliferation and transient ERK activity are synchronised in a living tissue.

DOI:http://dx.doi.org/10.7554/eLife.05178.001

Our skin is our largest organ; it provides a barrier that protects the underlying tissues and internal organs from the external environment and acts as one of our first lines of defense against infection. Both of these roles subject the skin to wear and tear and so it must constantly create new skin cells to replace those lost or damaged. However, if this renewal process goes awry it can lead to excessive cell growth or skin cancer. To avoid this, cells tightly regulate the pathways that stimulate skin renewal.

Skin renewal involves growth signals activating an enzyme called ERK. When and where the ERK enzyme is activated is normally tightly regulated, and many kinds of cancer have been linked to ERK becoming active at the wrong time or in the wrong place. Despite the importance of ERK in skin cells, a number of technical challenges have made it difficult to study how these signals are passed from cell to cell.

Hiratsuka et al. have now examined genetically altered mice that produce a fluorescent sensor molecule that makes it possible to see ERK activity in living skin cells. The skin of anesthetized mice was observed under a microscope, and time-lapse videos revealed occasional ‘firework-like’ bursts of ERK activity. At first the ERK enzyme was active in a small cluster of skin cells, then ERK activity was seen in the surrounding cells—appearing to spread outwards over the course of several minutes—before the activity stopped. Hiratsuka et al. named this pattern of activity a ‘Spatial Propagation of Radial ERK Activity Distribution’, or SPREAD for short.

By studying SPREADs in the skin on the ears and the back of these mice, Hiratsuka et al. learned that these bursts of ERK activity coincided with skin cell growth; the bursts happened more frequently in the areas where the skin cells were dividing. Applying a chemical that stimulates cell division to the skin of the mice triggered more bursts of ERK activity; whereas fewer bursts were observed if Hiratsuka et al. used other chemicals to block the activity of some of the signaling proteins that work upstream of ERK.

Further experiments suggested that SPREADs encourage cells to progress through the cycle of events that leads a cell to divide; blocking these bursts caused the cell to pause at the stage just before it would normally divide. Hiratsuka et al. also observed similar patterns of ERK activity moving out like waves from the edges of skin wounds. Further research using similar methods will reveal how growth signals are triggered and propagated in healthy and diseased tissues, not only in the skin but also other organs such as the liver, intestine, and muscles.

DOI:http://dx.doi.org/10.7554/eLife.05178.002

The activation of G protein-coupled receptor 30 (GPR30) inhibits proliferation of estrogen receptor-negative breast cancer cells in vitro and in vivo

There is an urgent clinical need for safe and effective treatment agents and therapy targets for estrogen receptor negative (ER−) breast cancer. G protein-coupled receptor 30 (GPR30), which mediates non-genomic signaling of estrogen to regulate cell growth, is highly expressed in ER− breast cancer cells. We here showed that activation of GPR30 by the receptor-specific agonist G-1 inhibited the growth of ER− breast cancer cells in vitro. Treatment of ER− breast cancer cells with G-1 resulted in G2/M-phase arrest, downregulation of G2-checkpoint regulator cyclin B, and induction of mitochondrial-related apoptosis. The G-1 treatment increased expression of p53 and its phosphorylation levels at Serine 15, promoted its nuclear translocation, and inhibited its ubiquitylation, which mediated the growth arrest effects on cell proliferation. Further, the G-1 induced sustained activation and nuclear translocation of ERK1/2, which was mediated by GPR30/epidermal growth factor receptor (EGFR) signals, also mediated its inhibition effects of G-1. With extensive use of siRNA-knockdown experiments and inhibitors, we found that upregulation of p21 by the cross-talk of GPR30/EGFR and p53 was also involved in G-1-induced cell growth arrest. In vivo experiments showed that G-1 treatment significantly suppressed the growth of SkBr3 xenograft tumors and increased the survival rate, associated with proliferation suppression and upregulation of p53, p21 while downregulation of cyclin B. The discovery of multiple signal pathways mediated the suppression effects of G-1 makes it a promising candidate drug and lays the foundation for future development of GPR30-based therapies for ER− breast cancer treatment.

Mutated in colorectal cancer (MCC) is a novel oncogene in B lymphocytes

Background

Identification of novel genetic risk factors is imperative for a better understanding of B lymphomagenesis and for the development of novel therapeutic strategies. TRAF3, a critical regulator of B cell survival, was recently recognized as a tumor suppressor gene in B lymphocytes. The present study aimed to identify novel oncogenes involved in malignant transformation of TRAF3-deficient B cells.

Methods

We used microarray analysis to identify genes differentially expressed in TRAF3^−/− mouse splenic B lymphomas. We employed lentiviral vector-mediated knockdown or overexpression to manipulate gene expression in human multiple myeloma (MM) cell lines. We analyzed cell apoptosis and proliferation using flow cytometry, and performed biochemical studies to investigate signaling mechanisms. To delineate protein-protein interactions, we applied affinity purification followed by mass spectrometry-based sequencing.

Results

We identified mutated in colorectal cancer (MCC) as a gene strikingly up-regulated in TRAF3-deficient mouse B lymphomas and human MM cell lines. Aberrant up-regulation of MCC also occurs in a variety of primary human B cell malignancies, including non-Hodgkin lymphoma (NHL) and MM. In contrast, MCC expression was not detected in normal or premalignant TRAF3^−/− B cells even after treatment with B cell stimuli, suggesting that aberrant up-regulation of MCC is specifically associated with malignant transformation of B cells. In elucidating the functional roles of MCC in malignant B cells, we found that lentiviral shRNA vector-mediated knockdown of MCC induced apoptosis and inhibited proliferation in human MM cells. Experiments of knockdown and overexpression of MCC allowed us to identify several downstream targets of MCC in human MM cells, including phospho-ERK, c-Myc, p27, cyclin B1, Mcl-1, caspases 8 and 3. Furthermore, we identified 365 proteins (including 326 novel MCC-interactors) in the MCC interactome, among which PARP1 and PHB2 were two hubs of MCC signaling pathways in human MM cells.

Conclusions

Our results indicate that in sharp contrast to its tumor suppressive role in colorectal cancer, MCC functions as an oncogene in B cells. Our findings suggest that MCC may serve as a diagnostic marker and therapeutic target in B cell malignancies, including NHL and MM.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-014-0056-6) contains supplementary material, which is available to authorized users.

Intercellular redistribution of cAMP underlies selective suppression of cancer cell growth by connexin26

Connexins (Cx), which constitute gap junction intercellular channels in vertebrates, have been shown to suppress transformed cell growth and tumorigenesis, but the mechanism(s) still remain largely speculative. Here, we define the molecular basis by which Cx26, but less frequently Cx43 or Cx32, selectively confer growth suppression on cancer cells. Functional intercellular coupling is shown to be required, producing partial blocks of the cell cycle due to prolonged activation of several mitogenic kinases. PKA is both necessary and sufficient for the Cx26 induced growth inhibition in low serum and the absence of anchorage. Activation of PKA was not associated with elevated cAMP levels, but appeared to result from a redistribution of cAMP throughout the cell population, eliminating the cell cycle oscillations in cAMP required for efficient cell cycle progression. Cx43 and Cx32 fail to mediate this redistribution as, unlike Cx26, these channels are closed during the G2/M phase of the cell cycle when cAMP levels peak. Comparisons of tumor cell lines indicate that this is a general pattern, with growth suppression by connexins occurring whenever cAMP oscillates with the cell cycle, and the gap junction remain open throughout the cell cycle. Thus, gap junctional coupling, in the absence of any external signals, provides a general means to limit the mitotic rate of cell populations.

Cloning, in silico characterization and induction of TiKpp2 MAP kinase in Tilletia indica under the influence of host factor(s) from wheat spikes

In order to understand the molecular mechanism(s) associated with floret specificity, morphogenetic and disease development of Karnal bunt (KB) pathogen in wheat spikes, host factor(s) was isolated from KB prone susceptible stage of wheat spikes. An orthologue of Kpp2 gene involved in pheromone response and fungal development was isolated from Tilletia indica for analyzing its role in fungal development. The maximum expression of TiKpp2 gene was observed at 14th day and decreased thereafter. To investigate whether the fungus alters the expression levels of same kinase upon interaction with plants, T. indica cultures were treated with 1% of host factor(s). Such treatment induced the expression of TiKpp2 gene in time dependent manner. Host factor(s) treatment tends to increase the myelination in fungal cultures by lowering the sporidial production. Increase in myelination led to impose more pathogenicity levels in the host and prolific multiplication of pathogen inside host causing more damage to developing grains. In silico characterization and protein-protein interaction studies further suggests that isolated gene showed similarity with Ustilago maydis Kpp2 and induction of TiKpp2 might further activate a downstream transcription factor Prf1. The results of present study clearly suggest that host factor(s) derived from wheat spikes provide certain signal(s) which activate TiKpp2 gene during morphogenetic development of T. indica and affect the fungal growth and pathogenicity. In turn it also provides a plausible explanation for floret specificity of KB fungus in wheat.

Transcript and protein profiling identifies signaling, growth arrest, apoptosis, and NF-κB survival signatures following GNRH receptor activation

GNRH significantly inhibits proliferation of a proportion of cancer cell lines by activating GNRH receptor (GNRHR)-G protein signaling. Therefore, manipulation of GNRHR signaling may have an under-utilized role in treating certain breast and ovarian cancers. However, the precise signaling pathways necessary for the effect and the features of cellular responses remain poorly defined. We used transcriptomic and proteomic profiling approaches to characterize the effects of GNRHR activation in sensitive cells (HEK293-GNRHR, SCL60) in vitro and in vivo, compared to unresponsive HEK293. Analyses of gene expression demonstrated a dynamic response to the GNRH superagonist Triptorelin. Early and mid-phase changes (0.5-1.0 h) comprised mainly transcription factors. Later changes (8-24 h) included a GNRH target gene, CGA, and up- or downregulation of transcripts encoding signaling and cell division machinery. Pathway analysis identified altered MAPK and cell cycle pathways, consistent with occurrence of G(2)/M arrest and apoptosis. Nuclear factor kappa B (NF-κB) pathway gene transcripts were differentially expressed between control and Triptorelin-treated SCL60 cultures. Reverse-phase protein and phospho-proteomic array analyses profiled responses in cultured cells and SCL60 xenografts in vivo during Triptorelin anti-proliferation. Increased phosphorylated NF-κB (p65) occurred in SCL60 in vitro, and p-NF-κB and IκBε were higher in treated xenografts than controls after 4 days Triptorelin. NF-κB inhibition enhanced the anti-proliferative effect of Triptorelin in SCL60 cultures. This study reveals details of pathways interacting with intense GNRHR signaling, identifies potential anti-proliferative target genes, and implicates the NF-κB survival pathway as a node for enhancing GNRH agonist-induced anti-proliferation.

Mid-gestation ovine cardiomyocytes are vulnerable to mitotic suppression by thyroid hormone

Circulating fetal 3,3',5-tri-iodo-l-thyronine (T(3) ) is maintained at very low levels until a dramatic prepartum surge. 3,3',5-Tri-iodo-l-thyronine inhibits serum-stimulated proliferation in near-term ovine cardiomyocytes, but it is not known whether midgestation myocytes are also inhibited. Because early cessation of cardiomyocyte mitosis would result in an underendowed heart, we hypothesized that 0.67 gestation (100 of 145 days gestation) ovine cardiomyocytes would be insensitive to suppressive growth effects of T(3) . These younger cardiomyocytes were grown with T(3) in 10% serum-enriched media for 24 hours. Physiological (0.37, 0.75, and 1.5 nmol/L) concentrations of T(3) dramatically suppressed mitotic activity in cardiomyocytes (P < .001). 3,3',5-Tri-iodo-l-thyronine stimulated phosphorylation of extracellular signal-regulated kinase and AKT (also known as Protein Kinase B [PKB]) signaling pathways. Nevertheless, the protein content of the cell cycle suppressor, p21, increased 2-fold (P < .05), and promoter, cyclin D1, decreased by 50%. Contrary to our hypothesis, elevated levels of T(3) powerfully inhibit proliferation of midgestation fetal cardiomyocytes. Thus, midgestation maternal hyperthyroidism might lead to an underendowed fetal myocardium.

Activation of GPR30 inhibits the growth of prostate cancer cells through sustained activation of Erk1/2, c-jun/c-fos-dependent upregulation of p21, and induction of G(2) cell-cycle arrest

G-protein-coupled receptor-30 (GPR30) shows estrogen-binding affinity and mediates non-genomic signaling of estrogen to regulate cell growth. We here showed for the first time, in contrast to the reported promoting action of GPR30 on the growth of breast and ovarian cancer cells, that activation of GPR30 by the receptor-specific, non-estrogenic ligand G-1 inhibited the growth of androgen-dependent and androgen-independent prostate cancer (PCa) cells in vitro and PC-3 xenografts in vivo. However, G-1 elicited no growth or histological changes in the prostates of intact mice and did not inhibit growth in quiescent BPH-1, an immortalized benign prostatic epithelial cell line. Treatment of PC-3 cells with G-1 induced cell-cycle arrest at the G(2) phase and reduced the expression of G(2)-checkpoint regulators (cyclin-A2, cyclin-B1, cdc25c, and cdc2) and phosphorylation of their common transcriptional regulator NF-YA in PC-3 cells. With extensive use of siRNA-knockdown experiments and the MEK inhibitor PD98059 in this study, we dissected the mechanism underlying G-1-induced inhibition of PC-3 cell growth, which was mediated through GPR30, followed by sustained activation of Erk1/2 and a c-jun/c-fos-dependent upregulation of p21, resulting in the arrest of PC-3 growth at the G(2) phase. The discovery of this signaling pathway lays the foundation for future development of GPR30-based therapies for PCa.

MAPK pathway activation delays G2/M progression by destabilizing Cdc25B

Activation of the mitogen-activated protein kinase (MAPK) pathway by growth factors or phorbol esters during G(2) phase delays entry into mitosis; however, the role of the MAPK pathway during G(2)/M progression remains controversial. Here, we demonstrate that activation of the MAPK pathway with either epidermal growth factor or 12-O-tetradecanoylphorbol-13-acetate induces a G(2) phase delay independent of known G(2) phase checkpoint pathways but was specifically dependent on MAPK/extracellular signal-regulated kinase kinase (MEK1). Activation of MAPK signaling also blocked exit from a G(2) phase checkpoint arrest. Both the G(2) phase delay and blocked exit from the G(2) checkpoint arrest were mediated by the MEK1-dependent destabilization of the critical G(2)/M regulator cdc25B. Reintroduction of cdc25B overcame the MEK1-dependent G(2) phase delay. Thus, we have demonstrated a new function for MEK1 that controls G(2)/M progression by regulating the stability of cdc25B. This represents a novel mechanism by which factors that activate MAPK signaling can influence the timing of entry into mitosis, particularly exit from a G(2) phase checkpoint arrest.

Dysregulation of hepatocyte cell cycle and cell viability by hepatitis B virus

BACKGROUND/AIMS

Dysregulation of the cell cycle is frequently associated with tumor development. Hepatitis B virus (HBV) is associated with a significant risk of developing hepatocellular carcinoma but the effects of HBV on cell cycle regulation are not completely understood.

METHODS

We have used a recombinant adeno-HBV model system to investigate the effect of infection with HBV and the replication defective lamivudine resistant mutant rtM204I mutant on hepatocyte cell cycle and cell viability.

RESULTS

Huh7 cells synchronised at the G1/S phase of the cell cycle were arrested at the G2/M following infection with rAdHBV-wt and rAdHBV-M204I. This was accompanied by increased levels of p21(cip1), p-cdc2, cyclins D, A and B. Cell viability was reduced and cleaved caspase 3 levels were increased in HBV- and rtM204I-infected cells. rAdHBV-M204I-infected Huh7 cells also demonstrated significant up-regulation of phospho-ERK, phospho-Akt, p53 and phospho-Mdm2 compared to mock-infected cells. These changes were comparable to those following infection of Huh7 cells with rAdHBV-wt.

CONCLUSION

Our results suggest that HBV, regardless of phenotype, produces cell cycle arrest and reduced hepatocyte viability. Perturbations in these cellular processes are likely to underlie HBV-associated liver oncogenic transformation and may help explain the ongoing risk of developing hepatocellular carcinoma in individuals in whom the lamivudine resistant rtM204I mutant emerges.

Phosphorylated extracellular signal-regulated protein kinases 1 and 2 phosphorylate Sp1 on serine 59 and regulate cellular senescence via transcription of p21Sdi1/Cip1/Waf1

Expression of p21(Sdi1) downstream of p53 is essential for induction of cellular senescence, although cancer cell senescence can also occur in the p53 null condition. We report herein that senescence-associated phosphorylated extracellular signal-regulated protein kinases 1 and 2 (SA-pErk1/2) enhanced p21(Sdi1) transcription by phosphorylating Sp1 on Ser(59) downstream of protein kinase C (PKC) alpha. Reactive oxygen species (ROS), which was increased in cellular senescence, significantly activated both PKCalpha and PKCbetaI. However, PKCalpha, but not PKCbetaI, regulated ROS generation and cell proliferation in senescent cells along with activation of cdk2, proven by siRNAs. PKCalpha-siRNA also reduced SA-pErk1/2 expression in old human diploid fibroblast cells, accompanied with changes of senescence phenotypes to young cell-like. Regulation of SA-pErk1/2 was also confirmed by using catalytically active PKCalpha and its DN-mutant construct. These findings strongly suggest a new pathway to regulate senescence phenotypes by ROS via Sp1 phosphorylation between PKCalpha and SA-pErk1/2: employing GST-Sp1 mutants and MEK inhibitor analyses, we found that SA-pErk1/2 regulated Sp1 phosphorylation on the Ser(59) residue in vivo, but not threonine, in cellular senescence, which regulated transcription of p21(Sdi1) expression. In summary, PKCalpha, which was activated in senescent cells by ROS strongly activated Erk1/2, and the SA-pErk1/2 in turn phosphorylated Sp1 on Ser(59). Sp1-enhanced transcription of p21(Sdi1) resulted in regulation of cellular senescence in primary human diploid fibroblast cells.

Cytosolic accumulation of gammaH2AX is associated with tropomyosin-related kinase A-induced cell death in U2OS cells

Tropomyosin-related kinase A (TrkA) plays an important role in cell survival, differentiation, and apoptosis in various neuronal and nonneuronal cell types. Here we show that TrkA overexpression by the Tet-On system mimics NGF-mediated activation pathways in the absence of nerve growth factor (NGF) stimulation in U2OS cells. In addition, p53 upregulation upon DNA damage was inhibited by TrkA, and p21 was upregulated by TrkA in a p53-independent manner. TrkA overexpression caused cell death by interrupting cell cycle progression, and TrkA-induced cell death was diminished in the presence of its specific inhibitor GW441756. Interestingly, TrkA-mediated cell death was strongly related to gammaH2AX production and poly (ADP-ribose) polymerase cleavage in the absence of DNA damage inducer. In this study, we also reveal that gammaH2AX production by TrkA is blocked by TrkA kinase inhibitors K-252a and GW441756, and it is also significantly inhibited by JNK inhibitor SP600125. Moreover, reduction of cell viability by TrkA was strongly suppressed by SP600125 treatment, suggesting a critical role of JNK in TrkA-induced cell death. We also found that gammaH2AX and TrkA were colocalized in cytosol in the absence of DNA damage, and the nuclear localization of gammaH2AX induced by DNA damage was partly altered to cytosol by TrkA overexpression. Our results suggest that the abnormal cytosolic accumulation of gammaH2AX is implicated in TrkA-induced cell death in the absence of DNA damage.

The ERK-RSK1 activation by growth factors at G2 phase delays cell cycle progression and reduces mitotic aberrations

Growth factors accelerate G0 to S progression in the cell cycle, however, the roles of growth factors in other cell cycle phases are largely unknown. Here, we show that treatment of HeLa cells with hepatocyte growth factor (HGF) at G2 phase induced the G2/M transition delay as evidenced by FACS analysis as well as by mitotic index and time-lapse analyses. Growth factors such as epidermal growth factor (EGF) and fibroblast growth factor (FGF) also induced G2/M transition delay like HGF. HGF treatment at G2 phase causes a delayed activation of cyclin B1-associated kinase and a diminished nuclear translocation of cyclin B1. Either U0126, a MAPK kinase (MEK) inhibitor, or kinase-dead mutant of ribosomal S6 kinase (RSK) abolished the delay. Additionally, knockdown of RSK1, but not RSK2, with siRNA abrogated the delay, indicating that the extracellular-regulated protein kinase (ERK)-RSK1 mediates the HGF-induced delay. We further found that the delay in G2/M transition of cells expressing oncogenic HGF receptor, M1268T, was abolished by RSK1 knockdown. Intriguingly, we observed that HGF induced chromosomal segregation defects, and depletion of RSK1, but not RSK2, aggravated these chromosomal aberrations. Taken together, the ERK-RSK1 activation by growth factors delays G2/M transition and this might be required to maintain genomic integrity during growth factor stimulation.

S-Phase-specific activation of PKC alpha induces senescence in non-small cell lung cancer cells

Protein kinase C (PKC) has been widely implicated in positive and negative control of cell proliferation. We have recently shown that treatment of non-small cell lung cancer (NSCLC) cells with phorbol 12-myristate 13-acetate (PMA) during G1 phase inhibits the progression into S phase, an effect mediated by PKC delta-induced up-regulation of the cell cycle inhibitor p21 Cip1. However, PMA treatment in asynchronously growing NSCLC cells leads to accumulation of cells in G2/M. Studies in post-G1 phases revealed that PMA induced an irreversible G2/M cell cycle arrest in NSCLC cells and conferred morphological and biochemical features of senescence, including elevated SA-beta-Gal activity and reduced telomerase activity. Remarkably, this effect was phase-specific, as it occurred only when PKC was activated in S, but not in G1, phase. Mechanistic analysis revealed a crucial role for the classical PKC alpha isozyme as mediator of the G2/M arrest and senescence, as well as for inducing p21(Cip1) an obligatory event for conferring the senescence phenotype. In addition to the unappreciated role of PKC isozymes, and specifically PKC alpha, in senescence, our data introduce the paradigm that discrete PKCs trigger distinctive responses when activated in different phases of the cell cycle via a common mechanism that involves p21 Cip1 up-regulation.

ERK is a novel regulatory kinase for poly(A) polymerase

Poly(A) polymerase (PAP), which adds poly(A) tails to the 3′ end of mRNA, can be phosphorylated at several sites in the C-terminal domain. Phosphorylation often mediates regulation by extracellular stimuli, suggesting PAP may be regulated by such stimuli. In this study, we found that phosphorylation of PAP was increased upon growth stimulation and that the mitogen-activated protein kinase ERK was responsible for the increase in phosphorylation. We identified serine 537 of PAP as a unique phosphorylation site by ERK. PAP phosphorylation of serine 537 by ERK increased its nonspecific polyadenylation activity in vitro. This PAP activity was also activated by stimulation of ERK with phorbol-12-myristate-13-acetate in vivo. These data suggest that ERK is a novel regulatory kinase for PAP and further, that PAP activity could be regulated by extracellular stimuli through an ERK-dependent signaling pathway(s).

Modulation of MAPK pathways and cell cycle by replicating hepatitis B virus: factors contributing to hepatocarcinogenesis

BACKGROUND/AIMS

Chronic infection with the hepatitis B virus (HBV) is strongly associated with the development of hepatocellular carcinoma but the mechanism by which this occurs is unknown. Numerous studies have focused on the HBV X protein showing that it activates signal transduction pathways while few have investigated these changes in HBV-replicating hepatocytes.

METHODS

We utilized the recombinant adenovirus system to deliver a replication competent HBV genome into Huh7 and primary marmoset hepatocytes (PMH) to examine the effects of active viral replication on the regulation of Ras-ERK signal transduction and related pathways.

RESULTS

Huh7 cells and PMHs replicating HBV demonstrated significant upregulation in phosphorylated ERK, Akt, c-myc together with increased p53, cyclin B1 and p21(cip1) expression and cell cycle progression to G2 phase in the absence of increased cell proliferation. Phosphorylation of the key cell survival kinase, Akt, was significantly increased, resulting in increased serine phosphorylation of the downstream target, GSK3-beta.

CONCLUSIONS

These results demonstrated simultaneous activation of the MAP Kinase and Akt pathways in HBV-replicating hepatocytes that resulted in dysregulation in the control of cell cycle progression and which help explain the early pathogenic mechanisms that underlie malignant transformation associated with chronic hepatitis B infection.

Isothiocyanate E-4IB induces MAPK activation, delayed cell cycle transition and apoptosis

INTRODUCTION

Epidemiologic studies point towards a significant correlation between the dietary intake of isothiocyanate-containing foods and the reduced risk for cancer.

METHODS AND RESULTS

In the current investigation, we examined the consequence of activating of signalling pathways during the release the cells from the block at G(1)/S boundary by synthetic isothiocyanate E-4IB. Using synchronized leukaemic HL60 cells, we show that activation of mitogen-activated protein kinases ERK1/2, c-Jun N-terminal kinase and p38 signalling pathways by E-4IB are coupled with delayed transition through the cell cycle and rapid cell cycle arrest resulted in diminished mitochondrial membrane potential culminating in apoptosis. These events were accompanied by histone deacetylase inhibition, increase of double strand DNA breaks detected by histone H2AX phosphorylation and up-regulation of cell cycle regulatory protein p21 and phosphorylation of CDC25C phosphatase.

CONCLUSION

These findings suggest that the activation of mitogen-activated protein kinases signalling pathways, followed by the induction cell cycle arrest and apoptosis, might be responsible for anticancer activities of E-4IB.