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1.  JNK1 stress signaling is hyper-activated in high breast density and the tumor stroma 
Cell Cycle  2013;13(4):580-599.
Mammography is an important screening modality for the early detection of DCIS and breast cancer lesions. More specifically, high mammographic density is associated with an increased risk of breast cancer. However, the biological processes underlying this phenomenon remain largely unknown. Here, we re-interrogated genome-wide transcriptional profiling data obtained from low-density (LD) mammary fibroblasts (n = 6 patients) and high-density (HD) mammary fibroblasts (n = 7 patients) derived from a series of 13 female patients. We used these raw data to generate a “breast density” gene signature consisting of >1250 transcripts that were significantly increased in HD fibroblasts, relative to LD fibroblasts. We then focused on the genes that were increased by ≥ 1.5-fold (P < 0.05) and performed gene set enrichment analysis (GSEA), using the molecular signatures database (MSigDB). Our results indicate that HD fibroblasts show the upregulation and/or hyper-activation of several key cellular processes, including the stress response, inflammation, stemness, and signal transduction. The transcriptional profiles of HD fibroblasts also showed striking similarities to human tumors, including head and neck, liver, thyroid, lung, and breast cancers. This may reflect functional similarities between cancer-associated fibroblasts (CAFs) and HD fibroblasts. This is consistent with the idea that the presence of HD fibroblasts may be a hallmark of a pre-cancerous phenotype. In these biological processes, GSEA predicts that several key signaling pathways may be involved, including JNK1, iNOS, Rho GTPase(s), FGF-R, EGF-R, and PDGF-R-mediated signal transduction, thereby creating a pro-inflammatory, pro-proliferative, cytokine, and chemokine-rich microenvironment. HD fibroblasts also showed significant overlap with gene profiles derived from smooth muscle cells under stress (JNK1) and activated/infected macrophages (iNOS). Thus, HD fibroblasts may behave like activated myofibroblasts and macrophages, to create and maintain a fibrotic and inflammatory microenvironment. Finally, comparisons between the HD fibroblast gene signature and breast cancer tumor stroma revealed that JNK1 stress signaling is the single most significant biological process that is shared between these 2 data sets (with P values between 5.40E-09 and 1.02E-14), and is specifically associated with tumor recurrence. These results implicate “stromal JNK1 signaling” in the pathogenesis of human breast cancers and the transition to malignancy. Augmented TGF-β signaling also emerged as a common feature linking high breast density with tumor stroma and breast cancer recurrence (P = 5.23E-05). Similarities between the HD fibroblast gene signature, wound healing, and the cancer-associated fibroblast phenotype were also noted. Thus, this unbiased informatics analysis of high breast density provides a novel framework for additional experimental exploration and new hypothesis-driven breast cancer research, with a focus on cancer prevention and personalized medicine.
PMCID: PMC3988118  PMID: 24434780
EGF; FGF; JNK; PDGF; SAPK; TGF-beta; breast cancer; cancer-associated fibroblasts; fibrosis; gene signature; inflammation; mammographic density; mammography; microenvironment; stress signaling; tumor stroma; wound healing
2.  JNK signaling is needed to tolerate chromosomal instability 
Cell Cycle  2013;13(4):622-631.
Chromosomal instability (CIN), as a common feature of tumors, represents a potential therapeutic target if ways can be found to specifically cause apoptosis in unstably dividing cells. We have previously shown that if signaling through the JNK pathway is reduced, apoptosis is triggered in models of chromosomal instability induced by loss of the spindle checkpoint. Here we identify components upstream and downstream of JNK that are able to mediate this effect, and test the involvement of p53 and DNA damage in causing apoptosis when JNK signaling is reduced in CIN cells. We show that cell cycle progression timing has a strong effect on the apoptosis seen when JNK signaling is reduced in genetically unstable cells: a shortened G2 phase enhances the apoptosis, while lengthening G2 rescues the JNK-deficient CIN cell death phenotype. Our findings suggest that chromosomal instability represents a significant stress to dividing cells, and that without JNK signaling, cells undergo apoptosis because they lack a timely and effective response to DNA damage.
PMCID: PMC3988119  PMID: 24335260
chromosomal instability; JNK; Drosophila; apoptosis; Mad2
3.  Exploring the activated adipogenic niche 
Cell Cycle  2014;13(2):184-190.
Adult adipose tissue contains a large supply of progenitors that can renew fat cells for homeostatic tissue maintenance and adaptive growth or regeneration in response to external challenges. However, the in vivo mechanisms that control adipocyte progenitor behavior are poorly characterized. We recently demonstrated that recruitment of adipocyte progenitors by macrophages is a central feature of adipose tissue remodeling under various adipogenic conditions. Catabolic remodeling of white adipose tissue by β3-adrenergic receptor stimulation requires anti-inflammatory M2-polarized macrophages to clear dying adipocytes and to recruit new brown adipocytes from progenitors. In this Extra Views article, we discuss in greater detail the cellular elements of adipogenic niches and report a strategy to isolate and characterize the subpopulations of macrophages and adipocyte progenitors that actively participate in adrenergic tissue remodeling. Further characterization of these subpopulations may facilitate identification of new cellular targets to improve metabolic and immune function of adipose tissue.
PMCID: PMC3906235  PMID: 24394850
adipocyte progenitors; macrophages; proliferation; adipogenesis; adipose tissue remodeling
4.  c-MYC–miRNA circuitry 
Cell Cycle  2014;13(2):191-198.
MYC (c-Myc) deregulation has been frequently associated with aggressive lymphomas and adverse clinical outcome in B-cell malignancies. MYC has been implicated in controlling the expression of miRNAs, and MYC-regulated miRNAs affect virtually all aspects of the hallmarks of MYC-driven lymphomas. Increasing evidence has indicated that there is significant cross-talk between MYC and miRNAs, with MYC regulating expression of a number of miRNAs, resulting in widespread repression of miRNA and, at the same time, MYC being subjected to regulation by miRNAs, leading to sustained MYC activity and the corresponding MYC downstream pathways. Thus, these combined effects of MYC overexpression and downregulation of miRNAs play a central regulatory role in the MYC oncogenic pathways and MYC-driven lymphomagenesis. Here, we provide biological insight on the function of MYC-regulated miRNAs, the mechanisms of MYC-induced miRNA repression, and the complicated feedback circuitry underlying lymphoma progression, as well as potential therapeutic targets in aggressive B-cell lymphomas.
PMCID: PMC3906236  PMID: 24394940
miRNA; c-MYC; aggressive B-cell lymphomas; epigenetic; methylation
5.  Identification of ribonucleotide reductase M2 as a potential target for pro-senescence therapy in epithelial ovarian cancer 
Cell Cycle  2013;13(2):199-207.
Epithelial ovarian cancer (EOC) is the leading cause of gynecological-related cancer deaths in the United States. There is, therefore, an urgent need to develop novel therapeutic strategies for this devastating disease. Cellular senescence is a state of stable cell growth arrest that acts as an important tumor suppression mechanism. Ribonucleotide reductase M2 (RRM2) plays a key role in regulating the senescence-associated cell growth arrest by controlling biogenesis of 2'-deoxyribonucleoside 5′-triphosphates (dNTPs). The role of RRM2 in EOC remains poorly understood. Here we show that RRM2 is expressed at higher levels in EOCs compared with either normal ovarian surface epithelium (P < 0.001) or fallopian tube epithelium (P < 0.001). RRM2 expression significantly correlates with the expression of Ki67, a marker of cell proliferation (P < 0.001). Moreover, RRM2 expression positively correlates with tumor grade and stage, and high RRM2 expression independently predicts a shorter overall survival in EOC patients (P < 0.001). To delineate the functional role of RRM2 in EOC, we knocked down RRM2 expression in a panel of EOC cell lines. Knockdown of RRM2 expression inhibits the growth of human EOC cells. Mechanistically, RRM2 knockdown triggers cellular senescence in these cells. Notably, this correlates with the induction of the DNA damage response, a known mediator of cellular senescence. These data suggest that targeting RRM2 in EOCs by suppressing its activity is a novel pro-senescence therapeutic strategy that has the potential to improve survival of EOC patients.
PMCID: PMC3906237  PMID: 24200970
epithelial ovarian cancer; cellular senescence; ribonucleotide reductase M2 (RRM2); DNA damage response; cell proliferation
6.  Activities of multiple cancer-related pathways are associated with BRAF mutation and predict the resistance to BRAF/MEK inhibitors in melanoma cells 
Cell Cycle  2013;13(2):208-219.
Drug resistance is a major obstacle in the targeted therapy of melanoma using BRAF/MEK inhibitors. This study was to identify BRAF V600E-associated oncogenic pathways that predict resistance of BRAF-mutated melanoma to BRAF/MEK inhibitors. We took in silico approaches to analyze the activities of 24 cancer-related pathways in melanoma cells and identify those whose activation was associated with BRAF V600E and used the support vector machine (SVM) algorithm to predict the resistance of BRAF-mutated melanoma cells to BRAF/MEK inhibitors. We then experimentally confirmed the in silico findings. In a microarray gene expression dataset of 63 melanoma cell lines, we found that activation of multiple oncogenic pathways preferentially occurred in BRAF-mutated melanoma cells. This finding was reproduced in 5 additional independent melanoma datasets. Further analysis of 46 melanoma cell lines that harbored BRAF mutation showed that 7 pathways, including TNFα, EGFR, IFNα, hypoxia, IFNγ, STAT3, and MYC, were significantly differently expressed in AZD6244-resistant compared with responsive melanoma cells. A SVM classifier built on this 7-pathway activation pattern correctly predicted the response of 10 BRAF-mutated melanoma cell lines to the MEK inhibitor AZD6244 in our experiments. We experimentally showed that TNFα, EGFR, IFNα, and IFNγ pathway activities were also upregulated in melanoma cell A375 compared with its sub-line DRO, while DRO was much more sensitive to AZD6244 than A375. In conclusion, we have identified specific oncogenic pathways preferentially activated in BRAF-mutated melanoma cells and a pathway pattern that predicts resistance of BRAF-mutated melanoma to BRAF/MEK inhibitors, providing novel clinical implications for melanoma therapy.
PMCID: PMC3906238  PMID: 24200969
melanoma; BRAF mutation; MEK inhibitor; BRAF V600E inhibitor; drug resistance; oncogenic pathway
7.  iRFP is a sensitive marker for cell number and tumor growth in high-throughput systems 
Cell Cycle  2013;13(2):220-226.
GFP and luciferase are used extensively as markers both in vitro and in vivo although both have limitations. The utility of GFP fluorescence is restricted by high background signal and poor tissue penetrance. Luciferase throughput is limited in vitro by the requirement for cell lysis, while in vivo, luciferase readout is complicated by the need for substrate injection and the dependence on endogenous ATP. Here we show that near-infrared fluorescent protein in combination with widely available near-infrared scanners overcomes these obstacles and allows for the accurate determination of cell number in vitro and tumor growth in vivo in a high-throughput manner and at negligible per-well costs. This system represents a significant advance in tracking cell proliferation in tissue culture as well as in animals, with widespread applications in cell biology.
PMCID: PMC3906239  PMID: 24200967
near-infrared fluorescence; iRFP cell number quantification; in vivo; cancer
8.  miR-888 is an expressed prostatic secretions-derived microRNA that promotes prostate cell growth and migration 
Cell Cycle  2013;13(2):227-239.
microRNAs (miRNAs) are a growing class of small non-coding RNAs that exhibit widespread dysregulation in prostate cancer. We profiled miRNA expression in syngeneic human prostate cancer cell lines that differed in their metastatic potential in order to determine their role in aggressive prostate cancer. miR-888 was the most differentially expressed miRNA observed in human metastatic PC3-ML cells relative to non-invasive PC3-N cells, and its levels were higher in primary prostate tumors from cancer patients, particularly those with seminal vesicle invasion. We also examined a novel miRNA-based biomarker source called expressed prostatic secretions in urine (EPS urine) for miR-888 expression and found that its levels were preferentially elevated in prostate cancer patients with high-grade disease. These expression studies indicated a correlation for miR-888 in disease progression. We next tested how miR-888 regulated cancer-related pathways in vitro using human prostate cancer cell lines. Overexpression of miR-888 increased proliferation and migration, and conversely inhibition of miR-888 activity blocked these processes. miR-888 also increased colony formation in PC3-N and LNCaP cells, supporting an oncogenic role for this miRNA in the prostate. Our data indicates that miR-888 functions to promote prostate cancer progression and can suppress protein levels of the tumor suppressor genes RBL1 and SMAD4. This miRNA holds promise as a diagnostic tool using an innovative prostatic fluid source as well as a therapeutic target for aggressive prostate cancer.
PMCID: PMC3906240  PMID: 24200968
non-coding RNA; microRNA; miRNA; miR-888; prostate; prostate cancer; expressed prostatic secretions urine; EPS urine
9.  DDB2 association with PCNA is required for its degradation after UV-induced DNA damage 
Cell Cycle  2013;13(2):240-248.
DDB2 is a protein playing an essential role in the lesion recognition step of the global genome sub-pathway of nucleotide excision repair (GG-NER) process. Among the proteins involved in the DNA damage response, p21CDKN1A (p21) has been reported to participate in NER, but also to be removed by proteolytic degradation, thanks to its association with PCNA. DDB2 is involved in the CUL4-DDB1 complex mediating p21 degradation; however, the direct interaction between DDB2, p21 and PCNA has been never investigated. Here, we show that DDB2 co-localizes with PCNA and p21 at local UV-induced DNA-damage sites, and these proteins co-immunoprecipitate in the same complex. In addition, we provide evidence that p21 is not able to bind directly DDB2, but, to this end, the presence of PCNA is required. Direct physical association of recombinant DDB2 protein with PCNA is mediated by a conserved PIP-box present in the N-terminal region of DDB2. Mutation of the PIP-box resulted in the loss of protein interaction. Interestingly, the same mutation, or depletion of PCNA by RNA interference, greatly impaired DDB2 degradation induced by UV irradiation. These results indicate that DDB2 is a PCNA-binding protein, and that this association is required for DDB2 proteolytic degradation.
PMCID: PMC3906241  PMID: 24200966
DDB2; PCNA; UV damage; nucleotide excision repair; p21CDKN1A
10.  Effects of cyclic AMP response element binding protein–Zhangfei (CREBZF) on the unfolded protein response and cell growth are exerted through the tumor suppressor p53 
Cell Cycle  2013;13(2):279-292.
Zhangfei/CREBZF, a basic region-leucine zipper (bLZip) transcription factor, is a potent suppressor of growth and the unfolded protein response (UPR) in some cancer cell lines, including the canine osteosarcoma cell line, D-17. However, the effects of Zhangfei are not universal, and it has no obvious effects on untransformed cells and some cancer cell lines, suggesting that Zhangfei may act through an intermediary that is either not induced or is defective in cells that it does not affect. Here we identify the tumor suppressor protein p53 as this intermediary. We show the following: in cells ectopically expressing Zhangfei, the protein stabilizes p53 and co-localizes with it in cellular nuclei; the bLZip domain of Zhangfei is required for its profound effects on cell growth and interaction with p53. Suppression of p53 by siRNA at least partially inhibits the effects of Zhangfei on the UPR and cell growth. The effects of Zhangfei on D-17 cells is mirrored by its effects on the p53-expressing human osteosarcoma cell line U2OS, while Zhangfei has no effect on the p53-null osteosarcoma cell line MG63. In U2OS cells, Zhangfei displaces the E3 ubiquitin ligase mouse double minute homolog 2 (Mdm2) from its association with p53, suggesting a mechanism for the effects of Zhangfei on p53.
PMCID: PMC3906244  PMID: 24200963
cell cycle; protein domains; p53; osteosarcoma; protein translocation; Zhangfei/CREBZF; unfolded protein response; Mdm2; basic-leucine zipper domain
11.  Different functions of HOPS isoforms in the cell 
Cell Cycle  2013;13(2):293-302.
Hepatocyte odd protein shuttling (HOPS) moves between nucleus and cytoplasm. HOPS overexpression leads to cell cycle arrest in G0/G1, and HOPS knockdown causes centrosome alterations, with subsequent abnormal cell division. Recently, we demonstrated that HOPS acts as a functional bridge in NPM-p19Arf interactions. Here we show that HOPS is present in 3 different isoforms that play distinct intracellular functions. Although HOPS is a transmembrane ubiquitin, an isoform with intermediate molecular weight is cleaved from the membrane and released into the cytosol, to act as the shuttling protein. We identified a signal peptide peptidase structure in N-terminal membrane-bound HOPS that allows the regulated intramembrane proteolysis (RIP) system to control the relative amounts of the released, shuttling isoform capable of binding NPM. These results argue for distinct, isoform-specific functions of HOPS in the nucleolus, nucleus, and cytoplasm and provide insight into the dynamics of HOPS association with NPM, whose mutation and subsequent delocalization is found in 30% of acute myeloid leukemia patients.
PMCID: PMC3906245  PMID: 24240191
hepatocyte odd protein shuttling; nucleophosmin; transmembrane and ubiquitin-like domain containing 1; tumor suppressor gene/regulated intramembrane proteolysis (RIP) system; intramembrane-cleaving proteases (iCliPs)/shuttling protein
12.  Arterial territory-specific phosphorylated retinoblastoma protein species and CDK2 promote differences in the vascular smooth muscle cell response to mitogens 
Cell Cycle  2013;13(2):315-323.
Despite recent advances in medical procedures, cardiovascular disease remains a clinical challenge and the leading cause of mortality in the western world. The condition causes progressive smooth muscle cell (SMC) dedifferentiation, proliferation, and migration that contribute to vascular restenosis. The incidence of disease of the internal mammary artery (IMA), however, is much lower than in nearly all other arteries. The etiology of this IMA disease resistance is not well understood. Here, using paired primary IMA and coronary artery SMCs, serum stimulation, siRNA knockdowns, and verifications in porcine vessels in vivo, we investigate the molecular mechanisms that could account for this increased disease resistance of internal mammary SMCs. We show that the residue-specific phosphorylation profile of the retinoblastoma tumor suppressor protein (Rb) appears to differ significantly between IMA and coronary artery SMCs in cultured human cells. We also report that the differential profile of Rb phosphorylation may follow as a consequence of differences in the content of cyclin-dependent kinase 2 (CDK2) and the CDK4 phosphorylation inhibitor p15. Finally, we present evidence that siRNA-mediated CDK2 knockdown alters the profile of Rb phosphorylation in coronary artery SMCs, as well as the proliferative response of these cells to mitogenic stimulation. The intrinsic functional and protein composition specificity of the SMCs population in the coronary artery may contribute to the increased prevalence of restenosis and atherosclerosis in the coronary arteries as compared with the internal mammary arteries.
PMCID: PMC3906247  PMID: 24240190
CDK2; cell proliferation; cell migration; coronary artery; internal mammary artery; smooth muscle cells; retinoblastoma protein phosphorylation
13.  USP3 counteracts RNF168 via deubiquitinating H2A and γH2AX at lysine 13 and 15 
Cell Cycle  2013;13(1):106-114.
Histone ubiquitination plays a vital role in DNA damage response (DDR), which is important for maintaining genomic integrity in eukaryotic cells. In DDR, ubiquitination of histone H2A and γH2AX by the concerted action of ubiquitin (Ub) ligases, RNF168 and RNF8, generates a cascade of ubiquitination signaling. However, little is known about deubiquitinating enzymes (DUBs) that may catalyze the removal of Ub from these histones. This study demonstrated that USP3, an apparent DUB for mono-ubiquitinated H2A, is indeed the enzyme for deubiquitinating Ub conjugates of γH2AX and H2A from lysine sites, where the ubiquitination is initiated by RNF168. Here, we showed that ectopic expression of USP3 led to the deubiquitination of both H2A and γH2AX in response to UV-induced DNA damage. Moreover, ectopic USP3 expression abrogated FK2 antibody-reactive Ub-conjugate foci, which co-localize with damage-induced γH2AX foci. In addition, USP3 overexpression impaired the accumulation of downstream repair factors BRCA1 and 53BP1 at the damage sites in response to both UV and γ-irradiation. We further identified that the USP3 removes Ub at lysine 13 and 15 of H2A and γH2AX, as well as lysine 118 and 119 of H2AX in response to DNA damage. Taken together, the results suggested that USP3 is a negative regulator of ubiquitination signaling, counteracting RNF168- and RNF8-mediated ubiquitination.
PMCID: PMC3925719  PMID: 24196443
53BP1; BRCA1; DNA repair; RNF168; USP3; deubiquitinating enzyme; histone modification; ubiquitin ligase; γH2AX
14.  Src kinase modulates the apoptotic p53 pathway by altering HIPK2 localization 
Cell Cycle  2013;13(1):115-125.
Non-receptor tyrosine kinase Src is a master regulator of cell proliferation. Hyperactive Src is a potent oncogene and a driver of cellular transformation and carcinogenesis. Homeodomain-interacting protein kinase 2 (HIPK2) is a tumor suppressor mediating growth suppression and apoptosis upon genotoxic stress through phosphorylation of p53 at Ser46. Here we show that Src phosphorylates HIPK2 and changes its subcellular localization. Using mass spectrometry we identified 9 Src-mediated Tyr-phosphorylation sites within HIPK2, 5 of them positioned in the kinase domain. By means of a phosphorylation-specific antibody we confirm that Src mediates phosphorylation of HIPK2 at Tyr354. We demonstrate that ectopic expression of Src increases the half-life of HIPK2 by interfering with Siah-1-mediated HIPK2 degradation. Moreover, we find that hyperactive Src binds HIPK2 and redistributes HIPK2 from the cell nucleus to the cytoplasm, where both kinases partially colocalize. Accordingly, we find that hyperactive Src decreases chemotherapeutic drug-induced p53 Ser46 phosphorylation and apoptosis activation. Together, our results suggest that Src kinase suppresses the apoptotic p53 pathway by phosphorylating HIPK2 and relocalizing the kinase to the cytoplasm.
PMCID: PMC3925721  PMID: 24196445
Src; HIPK2; p53; chemotherapeutic drug; apoptosis
15.  Inhibition of endocytic vesicle fusion by Plk1-mediated phosphorylation of vimentin during mitosis 
Cell Cycle  2013;13(1):126-137.
Endocytic vesicle fusion is inhibited during mitosis, but the molecular pathways that mediate the inhibition remain unclear. Here we uncovered an essential role of Polo-like kinase 1 (Plk1) in this mechanism. Phosphoproteomic analysis revealed that Plk1 phosphorylates the intermediate filament protein vimentin on Ser459, which is dispensable for its filament formation but is necessary for the inhibition of endocytic vesicle fusion in mitosis. Furthermore, this mechanism is required for integrin trafficking toward the cleavage furrow during cytokinesis. Our results thus identify a novel mechanism for fusion inhibition in mitosis and implicate its role in vesicle trafficking after anaphase onset.
PMCID: PMC3925722  PMID: 24196446
endosome; mitosis; Plk1; vesicle fusion; vimentin
16.  Macromitophagy, neutral lipids synthesis, and peroxisomal fatty acid oxidation protect yeast from “liponecrosis”, a previously unknown form of programmed cell death 
Cell Cycle  2013;13(1):138-147.
We identified a form of cell death called “liponecrosis.” It can be elicited by an exposure of the yeast Saccharomyces cerevisiae to exogenous palmitoleic acid (POA). Our data imply that liponecrosis is: (1) a programmed, regulated form of cell death rather than an accidental, unregulated cellular process and (2) an age-related form of cell death. Cells committed to liponecrotic death: (1) do not exhibit features characteristic of apoptotic cell death; (2) do not display plasma membrane rupture, a hallmark of programmed necrotic cell death; (3) akin to cells committed to necrotic cell death, exhibit an increased permeability of the plasma membrane for propidium iodide; (4) do not display excessive cytoplasmic vacuolization, a hallmark of autophagic cell death; (5) akin to cells committed to autophagic death, exhibit a non-selective en masse degradation of cellular organelles and require the cytosolic serine/threonine protein kinase Atg1p for executing the death program; and (6) display a hallmark feature that has not been reported for any of the currently known cell death modalities—namely, an excessive accumulation of lipid droplets where non-esterified fatty acids (including POA) are deposited in the form of neutral lipids. We therefore concluded that liponecrotic cell death subroutine differs from the currently known subroutines of programmed cell death. Our data suggest a hypothesis that liponecrosis is a cell death module dynamically integrated into a so-called programmed cell death network, which also includes the apoptotic, necrotic, and autophagic modules of programmed cell death. Based on our findings, we propose a mechanism underlying liponecrosis.
PMCID: PMC3925724  PMID: 24196447
mechanisms of programmed cell death; yeast; autophagy; autophagic cell death; mitophagy; apoptosis; necrosis; fatty acids; peroxisomes; lipid droplets
17.  Src and CXCR4 are involved in the invasiveness of breast cancer cells with acquired resistance to lapatinib 
Cell Cycle  2013;13(1):148-156.
Lapatinib is a dual EGFR and ErbB-2 tyrosine kinase inhibitor that has significantly improved the clinical outcome of ErbB-2-overexpressing breast cancer patients. However, patients inexorably develop mechanisms of resistance that limit the efficacy of the drug. In order to identify potential targets for therapeutic intervention in lapatinib-resistant patients, we isolated, from ErbB-2-overexpressing SK-Br-3 breast cancer cells, the SK-Br-3 Lap-R-resistant subclone, which is able to routinely grow in 1 µM lapatinib. Resistant cells have a more aggressive phenotype compared with parental cells, as they show a higher ability to invade through a matrigel-coated membrane. Lapatinib-resistant cells have an increased Src kinase activity and persistent levels of activation of ERK1/2 and AKT compared with parental cells. Treatment with the Src inhibitor saracatinib in combination with lapatinib reduces AKT and ERK1/2 phosphorylation and restores the sensitivity of resistant cells to lapatinib. SK-Br-3 Lap-R cells also show levels of expression of CXCR4 that are higher compared with parental cells and are not affected by Src inhibition. Treatment with saracatinib or a specific CXCR4 antibody reduces the invasive ability of SK-Br-3 Lap-R cells, with the two drugs showing cooperative effects. Finally, blockade of Src signaling significantly increases TRAIL-induced cell death in SK-Br-3 Lap-R cells. Taken together, our results demonstrate that breast cancer cells with acquired resistance to lapatinib have a more aggressive phenotype compared with their parental counterpart, and that Src signaling and CXCR4 play an important role in this phenomenon, thus representing potential targets for therapeutic intervention in lapatinib-resistant breast cancer patients.
PMCID: PMC3925726  PMID: 24200972
ErbB-2; breast cancer; lapatinib; resistance; Src kinase; saracatinib; CXCR4
18.  GSI promotes vincristine-induced apoptosis by enhancing multi-polar spindle formation 
Cell Cycle  2013;13(1):157-166.
Gamma secretase inhibitors (GSI), cell-permeable small-molecule inhibitors of gamma secretase activity, had been originally developed for the treatment of Alzheimer disease. In recent years, it has been exploited in cancer research to inhibit Notch signaling that is aberrantly activated in various cancers. We previously found that GSI could synergize with anti-microtubule agent, vincristine (VCR) in a Notch-independent manner. Here, we delineate the underlying cell cycle-related mechanism using HeLa cells, which have strong mitotic checkpoints. GSI enhanced VCR-induced cell death, although GSI alone did not affect cell viability at all. GSI augmented VCR-induced mitotic arrest in a dose-dependent manner, which was preceded by apoptotic cell death, as shown by an increase in Annexin V-positive and caspase-positive cell population. Furthermore, GSI amplified multi-polar spindle formation triggered by VCR. Altogether, we show the evidence that GSI enhances VCR-induced apoptosis in HeLa cells via multi-polar mitotic spindle formation, independent of Notch signaling. These data suggest that one or more GS substrates, yet to be identified, in a post-GS processed form, may play a role in maintaining functional centrosomes/mitotic spindles. More significantly, the synergistic effect of GSI in combination with VCR could be exploited in clinical setting to improve the efficacy of VCR.
PMCID: PMC3925728  PMID: 24200971
gamma secretase inhibitor; Vincristine; mitotic arrest; apoptosis; spindle assembly checkpoint; multi-polar spindle; centrosome; Notch
19.  Incomplete and transitory decrease of glycolysis 
Cell Cycle  2013;13(1):16-22.
During vessel sprouting, a migratory endothelial tip cell guides the sprout, while proliferating stalk cells elongate the branch. Tip and stalk cell phenotypes are not genetically predetermined fates, but are dynamically interchangeable to ensure that the fittest endothelial cell (EC) leads the vessel sprout. ECs increase glycolysis when forming new blood vessels. Genetic deficiency of the glycolytic activator PFKFB3 in ECs reduces vascular sprouting by impairing migration of tip cells and proliferation of stalk cells. PFKFB3-driven glycolysis promotes the tip cell phenotype during vessel sprouting, since PFKFB3 overexpression overrules the pro-stalk activity of Notch signaling. Furthermore, PFKFB3-deficient ECs cannot compete with wild-type neighbors to form new blood vessels in chimeric mosaic mice. In addition, pharmacological PFKFB3 blockade reduces pathological angiogenesis with modest systemic effects, likely because it decreases glycolysis only partially and transiently.
PMCID: PMC3925729  PMID: 24335389
endothelial cell; angiogenesis; glycolysis; metabolism; vessel sprouting
20.  The tail that wags the dog 
Cell Cycle  2013;13(1):23-31.
DNA polymerase δ (Pol δ) is a key enzyme in eukaryotic DNA replication. Human Pol δ is a heterotetramer whose p12 subunit is degraded in response to DNA damage, leading to the in vivo conversion of Pol δ4 to Pol δ3. Two E3 ubiquitin ligases, RNF8 and CRL4Cdt2, participate in the DNA damage-induced degradation of p12. We discuss how these E3 ligases integrate the formation of Pol δ3 and ubiquitinated PCNA for DNA repair processes. CRL4Cdt2 partially degrades p12 during normal cell cycle progression, thereby generating Pol δ3 during S phase. This novel finding extends the current view of the role of Pol δ3 in DNA repair and leads to the hypothesis that it participates in DNA replication. The coordinated regulation of licensing factors and Pol δ3 by CRL4Cdt2 now opens new avenues for control of DNA replication. A parallel study of Pol δ4 and Pol δ3 in Okazaki fragment processing provides evidence for a role of Pol δ3 in DNA replication. We discuss several new perspectives of the role of the 2 forms of Pol δ in DNA replication and repair, as well the significance of the integration of p12 regulation in DNA repair and cell cycle progression.
PMCID: PMC3925730  PMID: 24300032
CRL4Cdt2; DNA damage; DNA polymerase δ; DNA replication; RNF8; cell cycle; cell cycle progression; p12 subunit
21.  EGFR signaling and autophagy dependence for growth, survival, and therapy resistance 
Cell Cycle  2013;13(1):42-51.
The epidermal growth factor receptor (EGFR) is amplified or mutated in various human epithelial tumors. Its expression and activation leads to cell proliferation, differentiation, and survival. Consistently, EGFR amplification or expression of EGFR variant 3 (EGFRvIII) is associated with resistance to conventional cancer therapy through activation of pro-survival signaling and DNA-repair mechanisms. EGFR targeting has successfully been exploited as strategy to increase treatment efficacy. Nevertheless, these targeting strategies have only been proven effective in a limited percentage of human tumors.
Recent knowledge indicates that EGFR deregulated tumors display differences in autophagy and dependence on autophagy for growth and survival and the use of autophagy to increase resistance to EGFR-targeting drugs. In this review the dependency on autophagy and its role in mediating resistance to EGFR-targeting agents will be discussed. Considering the current knowledge, autophagy inhibition could provide a novel strategy to enhance therapy efficacy in treatment of EGFR deregulated tumors.
PMCID: PMC3925733  PMID: 24335351
EGFR; EGFRvIII; mutations; autophagy; cancer treatment; treatment resistance; hypoxia; starvation; metabolic stress
22.  E1a promotes c-Myc-dependent replicative stress 
Cell Cycle  2013;13(1):52-61.
The E1a gene from adenovirus is known to be a potent inducer of chemo/radiosensitivity in a wide range of tumors. However, the molecular bases of its radiosensitizer properties are still poorly understood. In an attempt to study this effect, U87MG cells, derived from a radio-resistant tumor as glioblastoma, where infected with lentivirus carrying E1a gene developing an acute sensitivity to ionizing radiation. The induction of radiosensitivity correlated with a marked G2/M phase accumulation and a potent apoptotic response. Our findings demonstrate that c-Myc plays a pivotal role in E1a-associated radiosensitivity through the induction of a replicative stress situation, as our data support by genetic approaches, based in interference and overexpression in U87MG cells. In fact, we present evidence showing that Chk1 is a novel transcriptional target of E1a gene through the effect exerted by this adenoviral protein onto c-Myc. Moreover, c-Myc upregulation also explains the marked phosphorylation of H2AX associated to E1a expression in the absence of DNA damage. Indeed, all these observations were applicable to other experimental models, such as T98G, LN-405 and A172, rendering the same pattern in terms of radiosensitivity, cell cycle distribution, upregulation of Chk1, c-Myc, and phosphorylation pattern of H2AX. In summary, our data propose a novel mechanism to explain how E1a mediates radiosensitivity through the signaling axis E1a→c-Myc→ replicative stress situation. This novel mechanism of E1a-mediated radiosensitivity could be the key to open new possibilities in the current therapy of glioblastoma.
PMCID: PMC3925735  PMID: 24196438
replicative stress; E1a; Chk1; c-Myc; radiosensitivity; glioblastoma
23.  Myotube-derived exosomal miRNAs downregulate Sirtuin1 in myoblasts during muscle cell differentiation 
Cell Cycle  2013;13(1):78-89.
It has recently been established that exosomes can mediate intercellular cross-talk under normal and pathological conditions through the transfer of specific miRNAs. As muscle cells secrete exosomes, we addressed the question of whether skeletal muscle (SkM) exosomes contained specific miRNAs, and whether they could act as “endocrine signals” during myogenesis. We compared the miRNA repertoires found in exosomes released from C2C12 myoblasts and myotubes and found that 171 and 182 miRNAs were exported into exosomes from myoblasts and myotubes, respectively. Interestingly, some miRNAs were expressed at higher levels in exosomes than in their donor cells and vice versa, indicating a selectivity in the incorporation of miRNAs into exosomes. Moreover miRNAs from C2C12 exosomes were regulated during myogenesis. The predicted target genes of regulated exosomal miRNAs are mainly involved in the control of important signaling pathways for muscle cell differentiation (e.g., Wnt signaling pathway). We demonstrated that exosomes from myotubes can transfer small RNAs (C. elegans miRNAs and siRNA) into myoblasts. Moreover, we present evidence that exosome miRNAs secreted by myotubes are functionally able to silence Sirt1 in myoblasts. As Sirt1 regulates muscle gene expression and differentiation, our results show that myotube–exosome miRNAs could contribute to the commitment of myoblasts in the process of differentiation. Until now, myokines in muscle cell secretome provided a conceptual basis for communication between muscles. Here, we show that miRNA exosomal transfer would be a powerful means by which gene expression is orchestrated to regulate SkM metabolic homeostasis.
PMCID: PMC3925739  PMID: 24196440
exosomes; skeletal muscle; differentiation; microRNAs; Sirtuin 1
24.  A critical role of glucose-6-phosphate dehydrogenase in TAp73-mediated cell proliferation 
Cell Cycle  2013;12(24):3720-3726.
The pentose phosphate pathway (PPP) provides ribose and NADPH that support biosynthesis and antioxidant defense. Our recent findings suggest that the p53-related protein TAp73 enhances the PPP flux. TAp73 stimulates the expression of glucose-6-phophate dehydrogenase (G6PD), the rate-limiting enzymes of the PPP. Through this regulation, TAp73 promotes the accumulation of macromolecules and increases cellular capability to withstand oxidative stresses. TAp73 also regulates other metabolic enzymes, and the relative importance of these targets in TAp73-mediated cell growth is not well understood. Here we show that, like in other cell lines, TAp73 is required for supporting proliferation and maintaining the expression of G6PD in the human lung cancer H1299 cells. Restoration of G6PD expression almost fully rescues the defects in cell growth caused by TAp73 knockdown, suggesting that G6PD is the major proliferative target of TAp73 in these cells. G6PD expression is elevated in various tumors, correlating with the upregulation of TAp73. These results indicate that TAp73 may function as an oncogene, and that G6PD is likely a focal point of regulation in oncogenic growth.
PMCID: PMC3905063  PMID: 24270845
p73; TAp73; ROS; G6PD; pentose phosphate pathway; metabolism; cell proliferation
25.  On the origin of the stereoselective affinity of Nutlin-3 geometrical isomers for the MDM2 protein 
Cell Cycle  2013;12(24):3727-3735.
The stereoselective affinity of small-molecule binding to proteins is typically broadly explained in terms of the thermodynamics of the final bound complex. Using Brownian dynamics simulations, we show that the preferential binding of the MDM2 protein to the geometrical isomers of Nutlin-3, an effective anticancer lead that works by inhibiting the interaction between the proteins p53 and MDM2, can be explained by kinetic arguments related to the formation of the MDM2:Nutlin-3 encounter complex. This is a diffusively bound state that forms prior to the final bound complex. We find that the MDM2 protein stereoselectivity for the Nutlin-3a enantiomer stems largely from the destabilization of the encounter complex of its mirror image enantiomer Nutlin-3b, by the K70 residue that is located away from the binding site. On the other hand, the trans-Nutlin-3a diastereoisomer exhibits a shorter residence time in the vicinity of MDM2 compared with Nutlin-3a due to destabilization of its encounter complex by the collective interaction of pairs of charged residues on either side of the binding site: Glu25 and Lys51 on one side, and Lys94 and Arg97 on the other side. This destabilization is largely due to the electrostatic potential of the trans-Nutlin-3a isomer being largely positive over extended continuous regions around its structure, which are otherwise well-identified into positive and negative regions in the case of the Nutlin-3a isomer. Such rich insight into the binding processes underlying biological selectivity complements the static view derived from the traditional thermodynamic analysis of the final bound complex. This approach, based on an explicit consideration of the dynamics of molecular association, suggests new avenues for kinetics-based anticancer drug development and discovery.
PMCID: PMC3905064  PMID: 24270847
Brownian dynamics simulation; MDM2 protein; nutlin stereoselectivity; stereoselectivity; encounter complex; kinetics-based drug discovery; residence time

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