Elevated cyclin G2 expression intersects with DNA damage checkpoint signaling and is required for a potent G2/M checkpoint arrest response to doxorubicin

Exosomal derived miR-1246 from hydroquinone-transformed cells drives S phase accumulation arrest by targeting cyclin G2 in TK6 cells

BACKGROUND

Hydroquinone (HQ), a major metabolite of benzene and known hematotoxic carcinogen. MicroRNA 1246 (miR-1246), an oncogene, regulates target genes in carcinogenesis including leukemia. This study investigates the impact of exosomal derived miR-1246 from HQ-transformed (HQ19) cells on cell-to-cell communication in recipient TK6 cells.

METHODS

RNA sequencing was used to identify differentially expressed exosomal miRNAs in HQ19 cells and its phosphate buffered solution control cells (PBS19), which were then confirmed using qRT-PCR. The impact of exosomal miR-1246 derived from HQ-transformed cells on cell cycle distribution was investigated in recipient TK6 cells.

RESULTS

RNA sequencing analysis revealed that 34 exosomal miRNAs were upregulated and 158 miRNAs were downregulated in HQ19 cells compared with PBS19 cells. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses predicted that their targets are enriched in cancer development-related pathways, such as MAPK signaling, microRNAs in cancer, apoptosis, PI3K-Akt signaling, cell cycle, Ras signaling, and Chronic myeloid leukemia. Eleven miRNAs were confirmed to have differential expression through qRT-PCR, with 6 upregulated (miR-140-3p, miR-551b-3p, miR-7-5p, miR-1290, miR-92a-3p, and miR-1246) and 5 downregulated (miR-183-5p, miR-26a-5p, miR-30c-5p, miR-205-5p, and miR-99b-3p). Among these, miR-1246 exhibited the highest expression level. HQ exposure resulted in a concentration-dependent increase in miR-1246 levels and decrease Cyclin G2 (CCNG2) levels in TK6 cells. Similarly, exosomes from HQ19 exhibited similar effects as HQ exposure. Dual luciferase reporter gene assays indicated that miR-1246 could band to CCNG2. After HQ exposure, exosomal miR-1246 induced cell cycle arrest at the S phase, elevating the expression of genes like pRb, E2F1, and Cyclin D1 associated with S phase checkpoint. However, silencing miR-1246 caused G2/M-phase arrest.

CONCLUSION

HQ-transformed cells' exosomal miR-1246 targets CCNG2, regulating TK6 cell cycle arrest, highlighting its potential as a biomarker for HQ-induced malignant transformation.

Exploring the ATR-CHK1 pathway in the response of doxorubicin-induced DNA damages in acute lymphoblastic leukemia cells

Doxorubicin (Dox) is one of the most commonly used anthracyclines for the treatment of solid and hematological tumors such as B-/T cell acute lymphoblastic leukemia (ALL). Dox compromises topoisomerase II enzyme functionality, thus inducing structural damages during DNA replication and causes direct damages intercalating into DNA double helix. Eukaryotic cells respond to DNA damages by activating the ATM-CHK2 and/or ATR-CHK1 pathway, whose function is to regulate cell cycle progression, to promote damage repair, and to control apoptosis. We evaluated the efficacy of a new drug schedule combining Dox and specific ATR (VE-821) or CHK1 (prexasertib, PX) inhibitors in the treatment of human B-/T cell precursor ALL cell lines and primary ALL leukemic cells. We found that ALL cell lines respond to Dox activating the G2/M cell cycle checkpoint. Exposure of Dox-pretreated ALL cell lines to VE-821 or PX enhanced Dox cytotoxic effect. This phenomenon was associated with the abrogation of the G2/M cell cycle checkpoint with changes in the expression pCDK1 and cyclin B1, and cell entry in mitosis, followed by the induction of apoptosis. Indeed, the inhibition of the G2/M checkpoint led to a significant increment of normal and aberrant mitotic cells, including those showing tripolar spindles, metaphases with lagging chromosomes, and massive chromosomes fragmentation. In conclusion, we found that the ATR-CHK1 pathway is involved in the response to Dox-induced DNA damages and we demonstrated that our new in vitro drug schedule that combines Dox followed by ATR/CHK1 inhibitors can increase Dox cytotoxicity against ALL cells, while using lower drug doses. • Doxorubicin activates the G2/M cell cycle checkpoint in acute lymphoblastic leukemia (ALL) cells. • ALL cells respond to doxorubicin-induced DNA damages by activating the ATR-CHK1 pathway. • The inhibition of the ATR-CHK1 pathway synergizes with doxorubicin in the induction of cytotoxicity in ALL cells. • The inhibition of ATR-CHK1 pathway induces aberrant chromosome segregation and mitotic spindle defects in doxorubicin-pretreated ALL cells.

Genome-wide analysis for nanofiber induced global gene expression profile: A study in MC3T3-E1 cells by RNA-Seq

Nanofibers are one of the attractive biomaterials that can provide unique environments to direct cell behaviors. However, how nanofiber structure affects the global gene expression of laden cells remains unclear. Herein, high-throughput mRNA sequencing (RNA-seq) is applied to analyze the transcriptome of the MC3T3-E1 cells (a model osteoblast cell line) cultured on electrospun nanofibers. The cell-adhesive poly(L-lactide) nanofibers and membranes are developed by the mussel-inspired coating of gelatin-dopamine conjugate under H₂O₂-mediated oxidation. The MC3T3-E1 cells cultured on nanofibers exhibit elongated morphology and increased proliferation compared with those on membranes. The differences in global gene expression profiles are determined by RNA-seq, in which 905 differentially expressed genes (DEGs) are identified. Significantly, the DEGs related to cytoskeleton, promotion of cell cycle progression, cell adhesion, and cell proliferation, are higher expressed in the cells on nanofibers, while the DEGs involved in cell-cycle arrest and osteoblast mineralization are up-regulated in the cells on membranes. This study elucidates the roles of nanofiber structure in affecting gene expression of laden cells at the whole transcriptome level, and it will lay the foundation for understanding nanofiber-guided cell behaviors.

Short-term transcriptomic changes in the mouse neural tube induced by an acute alcohol exposure

Alcohol exposure during the formation and closure of the neural tube, or neurulation (embryonic day [E] 8-10 in mice; ∼4th week of human pregnancy), perturbs development of midline brain structures and significantly disrupts gene expression in the rostroventral neural tube (RVNT). Previously, alcohol exposure during neurulation was found to alter gene pathways related to cell proliferation, p53 signaling, ribosome biogenesis, immune signaling, organogenesis, and cell migration 6 or 24 h after administration. Our current study expands upon this work by investigating short-term gene expression changes in the RVNT following a single binge-like alcohol exposure during neurulation. Female C57BL/6J mice were administered a single dose of 2.9 g/kg alcohol or vehicle on E9.0 to target mid-neurulation. The RVNTs of stage-matched embryos were collected 2 or 4 h after exposure and processed for RNA-seq. Functional profiling was performed with g:Profiler, as well as with the CiliaCarta and DisGeNet databases. Two hours following E9.0 alcohol exposure, 650 genes in the RVNT were differentially expressed. Functional enrichment analysis revealed that pathways related to cellular metabolism, gene expression, cell cycle, organogenesis, and Hedgehog signaling were down-regulated, and pathways related to cellular stress response, p53 signaling, and hypoxia were up-regulated by alcohol. Four hours after alcohol exposure, 225 genes were differentially expressed. Biological processes related to metabolism, RNA binding, ribosome biogenesis, and methylation were down-regulated, while protein localization and binding, autophagy, and intracellular signaling pathways were up-regulated. Two hours after alcohol exposure, the differentially expressed genes were associated with disease terms related to eye and craniofacial development and anoxia. These data provide further information regarding the biological functions targeted by alcohol exposure during neurulation in regions of the neural tube that give rise to alcohol-sensitive midline brain structures. Disruption of these gene pathways contributes to the craniofacial and brain malformations associated with prenatal alcohol exposure.

Chenodeoxycholic Acid (CDCA) Promoted Intestinal Epithelial Cell Proliferation by Regulating Cell Cycle Progression and Mitochondrial Biogenesis in IPEC-J2 Cells

Chenodeoxycholic acid (CDCA), a primary bile acid (BA), has been demonstrated to play an important role as a signaling molecule in various physiological functions. However, the role of CDCA in regulating intestinal epithelial cell (IEC) function remains largely unknown. Herein, porcine intestinal epithelial cells (IPEC-J2) were used as an in vitro model to investigate the effects of CDCA on IEC proliferation and explore the underlying mechanisms. IPEC-J2 cells were treated with CDCA, and flow cytometry and transcriptome analysis were adopted to investigate the effects and potential molecular mechanisms of CDCA on the proliferation of IECs. Our results indicated that adding 50 μmol/L of CDCA in the media significantly increased the proliferation of IPEC-J2 cells. In addition, CDCA treatment also hindered cell apoptosis, increased the proportion of G0/G1 phase cells in the cell cycle progression, reduced intracellular ROS, and MDA levels, and increased mitochondrial membrane potential, antioxidation enzyme activity (T-AOC and CAT), and intracellular ATP level (p < 0.05). RNA-seq results showed that CDCA significantly upregulated the expression of genes related to cell cycle progression (Cyclin-dependent kinase 1 (CDK1), cyclin G2 (CCNG2), cell-cycle progression gene 1 (CCPG1), Bcl-2 interacting protein 5 (BNIP5), etc.) and downregulated the expression of genes related to mitochondrial biogenesis (ND1, ND2, COX3, ATP6, etc.). Further KEGG pathway enrichment analysis showed that CDCA significantly enriched the signaling pathways of DNA replication, cell cycle, and p53. Collectively, this study demonstrated that CDCA could promote IPEC-J2 proliferation by regulating cell cycle progression and mitochondrial function. These findings provide a new strategy for promoting the intestinal health of pigs by regulating intestinal BA metabolism.

miR-17-5p drives G2/M-phase accumulation by directly targeting CCNG2 and is related to recurrence of head and neck squamous cell carcinoma

Background

The human miR-17-92 polycistron is the first reported and most well-studied onco-miRNA with a cluster of seven miRNAs. miR-17-5p, a member of the miR-17-92 family, plays an important role in tumor cell proliferation, apoptosis, migration and invasion. However, few studies have shown the role of miR-17-5p in the cell cycle of head and neck squamous cell carcinoma (HNSCC).

Methods

RT-qPCR was used to detect miR-17-5p expression levels in 64 HNSCC tissues and 5 cell lines. The relationship between the expression of miR-17-5p in the tissues and the clinical characteristics of the patients was analyzed. HNSCC cells were transfected with an miR-17-5p mimic or inhibitor to evaluate cell cycle distribution by flow cytometry. Cell cycle distribution of cells transfected with target gene was evaluated using flow cytometry. Dual-luciferase reporter assay was used to detect the regulatory effect of miR-17-5p on target gene expression.

Results

In the present study, we found that miR-17-5p expression in HNSCC tissues and cell lines was remarkably increased, and miR-17-5p is related to recurrence in HNSCC patients. Silencing miR-17-5p blocked HNSCC cells in G2/M phase, whereas its overexpression propelled cell cycle progression. More importantly, we verified that miR-17-5p negatively regulated CCNG2 mRNA and protein expression by directly targeting its 3’UTR.

Conclusion

These findings suggest that miR-17-5p might act as a tumor promoter and prognostic factor for recurrence in HNSCC patients.

Understanding doxorubicin associated calcium remodeling during triple-negative breast cancer treatment: an in silico study

Aim:

Triple-negative breast cancer (TNBC) is the most malignant subtype of breast cancer with high heterogeneity, rapid progression, and paucity of treatment options. The most effective chemotherapeutic drug used to treat TNBC is doxorubicin (Doxo) which is an anthracycline antibiotic. However, Doxo treatment alters cytosolic calcium dynamics leading to drug-resistance condition. The aim of this study is to capture the alterations in the activity of various calcium channels and pumps during Doxo treatment and their consequences on cytosolic calcium dynamics that ultimately result in drug resistance.

Methods:

In the present study, a mathematical model is proposed to capture the complex dynamical landscape of intracellular calcium during Doxo treatment. This study provides an insight into Doxo remodeling of calcium dynamics and associated drug-resistance effect. The model was first analyzed analytically and then explored through numerical simulation using techniques like global sensitivity analysis, parameter recalibration, etc.

Results:

The model is used to predict the potential combination therapy for Doxo that can overcome Doxo associated drug resistance. The results show targeting the dysregulated Ca²⁺ channels and pumps might provide efficient chemotherapy in TNBC. It was also observed that the indispensability of calcium influx rate is paramount in the Doxo drug resistance. Finally, three drugs were identified from existing literature that could be used as a combination therapy along with Doxo.

Conclusions:

The investigation highlights the importance of integrating the calcium signaling of various calcium regulating compounds for their effective anti-tumor effects deliverance along with chemotherapeutic agents. The results from this study might provide a new direction to the experimental biologists to explore different combination therapies with Doxo to enhance its anti-tumor effect.

Cyclin G2 Is Involved in the Proliferation of Placental Trophoblast Cells and Their Interactions with Endothelial Cells

Background

Remodeling of maternal spiral arteries after embryo implantation relies on well-regulated trophoblast functions. Although cyclin G2 (CCNG2) is thought to be involved in placental development and function, its role in trophoblasts and the mechanisms underlying placental development and function remain unclear. The present study investigated the potential role of CCNG2 in trophoblast cell proliferation and their interactions with endothelial cells.

Material/Methods

CCNG2 levels were modified by stable infection of HTR8/SVneo cells with lentiviruses overexpressing and silencing CCNG2. Cell proliferation was measured using CCK-8 assays. Network formation assays were performed using trophoblasts alone and co-cultured trophoblasts and endothelial cells to measure angiogenesis of trophoblasts and trophoblast-endothelial interactions. Levels of angiogenic factors (VEGF and sFlt-1) in the supernatant were measured by ELISA, and the expression of cell cycle regulatory (cyclin D1) and invasive (MMP2, MMP3, MMP9) markers implicated in artery remodeling were measured by western blotting.

Results

Ectopic expression of CCNG2 blocked the proliferation of HTR8/SVneo cells, as well as their abilities to form networks and integrate into human umbilical vein endothelial cells, whereas CCNG2 inhibition had the opposite effects. CCNG2 upregulation significantly reduced the expression of VEGF, cyclin D1, MMP2, MMP3, and MMP9, but enhanced the expression of sFlt-1. In contrast, CCNG2 downregulation had the opposite effects.

Conclusions

CCNG2 plays a critical role in trophoblast proliferation and trophoblast-endothelial cell interactions by significant affecting cell cycle, angiogenic, and invasive markers. CCNG2 may thus be a novel marker for the treatment of placental disorders.

β-radiating radionuclides in cancer treatment, novel insight into promising approach

Targeted radionuclide therapy, known as molecular radiotherapy is a novel therapeutic module in cancer medicine. β-radiating radionuclides have definite impact on target cells via interference in cell cycle and particular signalings that can lead to tumor regression with minimal off-target effects on the surrounding tissues. Radionuclides play a remarkable role not only in apoptosis induction and cell cycle arrest, but also in the amelioration of other characteristics of cancer cells. Recently, application of novel β-radiating radionuclides in cancer therapy has been emerged as a promising therapeutic modality. Several investigations are ongoing to understand the underlying molecular mechanisms of β-radiating elements in cancer medicine. Based on the radiation dose, exposure time and type of the β-radiating element, different results could be achieved in cancer cells. It has been shown that β-radiating radioisotopes block cancer cell proliferation by inducing apoptosis and cell cycle arrest. However, physical characteristics of the β-radiating element (half-life, tissue penetration range, and maximum energy) and treatment protocol determine whether tumor cells undergo cell cycle arrest, apoptosis or both and to which extent. In this review, we highlighted novel therapeutic effects of β-radiating radionuclides on cancer cells, particularly apoptosis induction and cell cycle arrest.

Heterogenous Nuclear Ribonucleoprotein H1 Promotes Colorectal Cancer Progression through the Stabilization of mRNA of Sphingosine-1-Phosphate Lyase 1

The oncogenic properties of heterogeneous nuclear ribonucleoprotein H1 (hnRNP H1) have been reported, although the tumor-promoting mechanism remains unclear. We herein report the mechanism underlying colorectal cancer cell progression mediated by hnRNP H1. The growth of colorectal cancer cells was suppressed by hnRNP H1 downregulation. A terminal deoxynucleotidyl transferase dUTP nick-end labeling assay revealed the anti-apoptotic effect of hnRNP H1 in colorectal cancer cells. An RNA immunoprecipitation assay revealed that hnRNP H1 bound to sphingosine-1-phosphate lyase 1 (SGPL1). Reverse transcription-polymerase chain reaction revealed the high expression of hnRNP H1 mRNA in colorectal cancer cells and Spearman's rank correlation coefficient showed a strong positive correlation between hnRNP H1 mRNA and SGPL1 mRNA. An siRNA of hnRNP H1 decreased SGPL1 mRNA expression in colorectal cancer cells, but not in non-tumorous cells. These findings suggested that hnRNP H1 increased SGPL1 mRNA expression specifically in cancer cells through direct binding. Targeted knockdown of hnRNP H1 or SGPL1 with siRNAs upregulated p53 phosphorylation and p53-associated molecules, resulting in cell growth inhibition, while hnRNP H1 upregulated the mRNA of SGPL1 and inhibited p53 activation, thereby promoting tumor cell growth. This is a novel mechanism underlying colorectal cancer cell progression mediated by hnRNP H1-SGPL1 mRNA stabilization.

Cyclin G2 regulates canonical Wnt signalling via interaction with Dapper1 to attenuate tubulointerstitial fibrosis in diabetic nephropathy

Cyclin G2 (CCNG2) is an atypical cyclin that inhibits cell cycle progression and is often dysregulated in human cancers. Cyclin G2 in the occurrence and development of diabetic nephropathy (DN), one of the most severe diabetic complications, has not been fully identified. In this study, we investigated the function and regulatory mechanism of cyclin G2 in DN. In vivo studies revealed that a deficiency of cyclin G2 significantly increased albuminuria and promoted tubulointerstitial fibrosis in established DN. Cyclin G2 regulated the expression of fibrosis‐related proteins via the canonical Wnt signalling pathway in renal tubular epithelial cells. Moreover, the binding of cyclin G2 to Dapper1 (Dpr1/DACT1), a protein involved in Wnt signalling, decreased the phosphorylation of Dpr1 at Ser762 by casein kinase 1 (CK1) and suppressed the Wnt signalling pathway. These findings reveal that cyclin G2 can protect against renal injury and fibrosis associated with DN and, thus, is a new target for the prevention and treatment of diabetic complications.

Atypical cyclins: the extended family portrait

Regulation of cell division is orchestrated by cyclins, which bind and activate their catalytic workmates, the cyclin-dependent kinases (CDKs). Cyclins have been traditionally defined by an oscillating (cyclic) pattern of expression and by the presence of a characteristic "cyclin box" that determines binding to the CDKs. Noteworthy, the Human Genome Sequence Project unveiled the existence of several other proteins containing the "cyclin box" domain. These potential "cyclins" have been named new, orphan or atypical, creating a conundrum in cyclins nomenclature. Moreover, although many years have passed after their discovery, the scarcity of information regarding these possible members of the family has hampered the establishment of criteria for systematization. Here, we discuss the criteria that define cyclins and we propose a classification and nomenclature update based on structural features, interactors, and phylogenetic information. The application of these criteria allows to systematically define, for the first time, the subfamily of atypical cyclins and enables the use of a common nomenclature for this extended family.

Preparation of a size selective nanocomposite through temperature assisted co-assembly of gelatin and pluronic F127 for passive targeting of doxorubicin

The study demonstrates the importance of the weight ratio of F127 and gelatin in forming size selective nanoconjugate through a thermal relaxation approach and its potential as an efficient drug delivery system of doxorubicin with reduced side effects.

Assessment of doxorubicin-induced remodeling of Ca2+ signaling and associated Ca2+ regulating proteins in MDA-MB-231 breast cancer cells

Triple-negative breast cancers (TNBC) are often associated with high relapse rates, despite treatment with chemotherapy agents such as doxorubicin. A better understanding of the signaling and molecular changes associated with doxorubicin may provide novel insights into strategies to enhance treatment efficacy. Calcium signaling is involved in many pathways influencing the efficacy of chemotherapy agents such as proliferation and cell death. However, there are a limited number of studies exploring the effect of doxorubicin on calcium signaling in TNBC. In this study, MDA-MB-231 triple-negative, basal breast cancer cells stably expressing the genetically-encoded calcium indicator GCaMP6m (GCaMP6m-MDA-MB-231) were used to define alterations in calcium signaling. The effects of doxorubicin in GCaMP6m-MDA-MB-231 cells were determined using live cell imaging and fluorescence microscopy. Changes in mRNA levels of specific calcium regulating proteins as a result of doxorubicin treatment were also assessed using real time qPCR. Doxorubicin (1 μM) produced alterations in intracellular calcium signaling, including enhancing the sensitivity of MDA-MB-231 cells to ATP stimulation and prolonging the recovery time after store-operated calcium entry. Upregulation in mRNA levels of ORAI1, TRPC1, SERCA1, IP₃R2 and PMCA2 with doxorubicin 1 μM treatment was also observed. Doxorubicin treatment is associated with specific remodeling in calcium signaling in MDA-MB-231 cells, with associated changes in mRNA levels of specific calcium-regulating proteins.

An epigenetic screening determines BET proteins as targets to suppress self-renewal and tumorigenicity in canine mammary cancer cells

Targeting self-renewal and tumorigenicity has been proposed as a potential strategy against cancer stem cells (CSCs). Epigenetic proteins are key modulators of gene expression and cancer development contributing to regulation and maintenance of self-renewal and tumorigenicity. Here, we have screened a small-molecule epigenetic inhibitor library using 3D in vitro models in order to determine potential epigenetic targets associated with self-renewal and tumorigenicity in Canine Mammary Cancer (CMC) cells. We identified inhibition of BET proteins as a promising strategy to inhibit CMC colonies and tumorspheres formation. Low doses of (+)-JQ1 were able to downregulate important genes associated to self-renewal pathways such as WNT, NOTCH, Hedgehog, PI3K/AKT/mTOR, EGF receptor and FGF receptor in CMC tumorspheres. In addition, we observed downregulation of ZEB2, a transcription factor important for the maintenance of self-renewal in canine mammary cancer cells. Furthermore, low doses of (+)-JQ1 were not cytotoxic in CMC cells cultured in 2D in vitro models but induced G2/M cell cycle arrest accompanied by upregulation of G2/M checkpoint-associated genes including BTG2 and CCNG2. Our work indicates the BET inhibition as a new strategy for canine mammary cancers by modulating the self-renewal phenotype in tumorigenic cells such as CSCs.

A novel DNA-binding motif in prostate tumor overexpressed-1 (PTOV1) required for the expression of ALDH1A1 and CCNG2 in cancer cells

PTOV1 is a transcription and translation regulator and a promoter of cancer progression. Its overexpression in prostate cancer induces transcription of drug resistance and self-renewal genes, and docetaxel resistance. Here we studied PTOV1 ability to directly activate the transcription of ALDH1A1 and CCNG2 by binding to specific promoter sequences. Chromatin immunoprecipitation and electrophoretic mobility shift assays identified a DNA-binding motif inside the PTOV-A domain with similarities to known AT-hooks that specifically interacts with ALDH1A1 and CCNG2 promoters. Mutation of this AT-hook-like sequence significantly decreased the expression of ALDH1A1 and CCNG2 promoted by PTOV1. Immunohistochemistry revealed the association of PTOV1 with mitotic chromosomes in high grade prostate, colon, bladder, and breast carcinomas. Overexpression of PTOV1, ALDH1A1, and CCNG2 significantly correlated with poor prognosis in prostate carcinomas and with shorter relapse-free survival in colon carcinoma. The previously described interaction with translation complexes and its direct binding to ALDH1A1 and CCNG2 promoters found here reveal the PTOV1 capacity to modulate the expression of critical genes at multiple levels in aggressive cancers. Remarkably, the AT-hook motifs in PTOV1 open possibilities for selective targeting its nuclear and/or cytoplasmic activities.

Cyclin G2 Suppresses Glomerulosclerosis by Regulating Canonical Wnt Signalling

Recent data has shown that cyclin G2 (CCNG2) is an atypical cyclin that inhibits cell cycle progression and is often dysregulated in human cancers. The involvement of cyclin G2 in the occurrence and development of diabetic nephropathy (DN) has not been determined. In the present study, we conducted cyclin G2 knockout studies to determine whether this protein regulates glomerulosclerosis in DN mice. We found that cyclin G2 regulated the expression of renal glomerulosclerosis-related proteins via the canonical Wnt signalling pathway in glomerular mesangial cells. A cyclin G2 deficiency resulted in more severe renal injury in DN mice. These findings provided new insight into the pathogenesis of DN, revealing that cyclin G2 has a protective role in glomerulosclerosis and is a potential new target for the prevention and treatment of DN.

siRNA Knockdown of RRM2 Effectively Suppressed Pancreatic Tumor Growth Alone or Synergistically with Doxorubicin

Pancreatic cancer is currently one of the deadliest of the solid malignancies, whose incidence and death rates are increasing consistently during the past 30 years. Ribonucleotide reductase (RR) is a rate-limiting enzyme that catalyzes the formation of deoxyribonucleotides from ribonucleotides, which are essential for DNA synthesis and replication. In this study, 23 small interfering RNAs (siRNAs) against RRM2, the second subunit of RR, were designed and screened, and one of them (termed siRRM2), with high potency and good RNase-resistant capability, was selected. Transfection of siRRM2 into PANC-1, a pancreatic cell line, dramatically repressed the formation of cell colonies by inducing remarkable cell-cycle arrest at S-phase. When combining with doxorubicin (DOX), siRRM2 improved the efficacy 4 times more than applying DOX alone, suggesting a synergistic effect of siRRM2 and DOX. Moreover, the combined application of siRRM2-loaded lipid nanoparticle and DOX significantly suppressed the tumor growth on the PANC-1 xenografted murine model. The inhibition efficiency revealed by tumor weight at the endpoint of the treatment reached more than 40%. Hence, siRRM2 effectively suppressed pancreatic tumor growth alone or synergistically with DOX. This study provides a feasible target gene, a drug-viable siRNA, and a promising therapeutic potential for the treatment of pancreatic cancer.

Cyclin G and the Polycomb Repressive complexes PRC1 and PR-DUB cooperate for developmental stability

In Drosophila, ubiquitous expression of a short Cyclin G isoform generates extreme developmental noise estimated by fluctuating asymmetry (FA), providing a model to tackle developmental stability. This transcriptional cyclin interacts with chromatin regulators of the Enhancer of Trithorax and Polycomb (ETP) and Polycomb families. This led us to investigate the importance of these interactions in developmental stability. Deregulation of Cyclin G highlights an organ intrinsic control of developmental noise, linked to the ETP-interacting domain, and enhanced by mutations in genes encoding members of the Polycomb Repressive complexes PRC1 and PR-DUB. Deep-sequencing of wing imaginal discs deregulating CycG reveals that high developmental noise correlates with up-regulation of genes involved in translation and down-regulation of genes involved in energy production. Most Cyclin G direct transcriptional targets are also direct targets of PRC1 and RNAPolII in the developing wing. Altogether, our results suggest that Cyclin G, PRC1 and PR-DUB cooperate for developmental stability.

DNA damage response-initiated cytokine secretion in bone marrow stromal cells promotes chemoresistance of myeloma cells

Acquisition of chemoresistance accounts for a major cause of chemotherapy failure for multiple myeloma (MM). Bone marrow stromal cells (BMSCs) are considered to play a pivotal role in modulating drug resistance of MM cells. However, the underlying mechanism whereby BMSCs, particularly damaged stromal cells, affects chemoresistance has not been identified yet. Here, we show exposure to doxorubicin doxorubicin (Dox) induced dramatic ATM (ataxia-telangiectasia-mutated)-dependent DNA damage response (DDR) and increased secretion of interleukin (IL)-6 in HS-5 cell line and primary BMSCs derived from healthy donors. Specifically, IL-6-containing conditioned media (CM) derived from Dox-pretreated stromal cells displayed significant protective effect on Dox-induced apoptosis of MM cells. Also, treatment of BMSCs with ATM kinase inhibitor markedly reduced IL-6 secretion and concurrently, partially reversed CM-mediated chemoresistance in myeloma cells. These data indicate that DNA-damaging drug triggers an ATM-dependent DDR in BMSCs, leading to increased cytokine secretion and resistance of myeloma cells to chemotherapy-induced apoptosis.

ERβ Sensitizes NSCLC to Chemotherapy by Regulating DNA Damage Response

The expression of wild-type estrogen receptor β (ESR2/ERβ1) correlates with clinical outcome in patients with non-small cell lung cancer (NSCLC). However, the molecular mechanism that accounts for this association is currently poorly understood. ERβ1 was previously linked to chemotherapy response in patients with breast cancer and in breast cancer cells. The effect of the receptor in NSCLC cells after chemotherapy treatment, a common remedy for advanced NSCLC, has not been studied. Here, upregulation of ERβ1 increases the sensitivity of NSCLC cells to treatment with doxorubicin and etoposide. This effect was primarily observed in p53-defecient NSCLC cells. In these cells, ERβ1 either enhanced G₂-M cell-cycle arrest by activating the checkpoint kinase 1 (Chk1) and altering downstream signaling or induced apoptosis. The expression of p63 target genes that control G₂-M checkpoint activation was altered by ERβ1 suggesting an ERβ1-p63 transcriptional cooperation in lung cancer cells that affects DNA damage response (DDR). These results suggest involvement of ERβ1 in the mechanism that regulates DNA damage response in NSCLC cells and support the potential predictive and therapeutic value of the receptor in clinical management of the disease.Implications: This study demonstrating the impact of ERβ1 on chemosensitivity of NSCLC cells suggests the predictive value of the receptor for successful response of tumors to chemotherapy and the potential benefit of chemotherapy-treated patients from the use of ER ligands. Mol Cancer Res; 16(2); 233-42. ©2017 AACR.

The Role of DCT in HPV16 Infection of HaCaTs

Persistent infection with high-risk human papillomavirus (HPV) genotype is a major factor leading to many human cancers. Mechanisms of HPV entry into host cells and genome trafficking towards the nucleus are incompletely understood. Dopachrome tautomerase (DCT) was identified as a cellular gene required for HPV infection in HeLa cells on a siRNA screen study. Here, we confirm that DCT knockdown significantly decreases HPV infection in the human keratinocyte HaCaT cells as was observed in HeLas. We investigated the effects of DCT knockdown and found that DCT depletion caused increased reactive oxygen species (ROS) levels, DNA damage and altered cell cycle in HaCaT cells. We observed increased viral DNA localization at the endoplasmic reticulum but an overall decrease in infection in DCT knockdown cells. This observation suggests that viral DNA might be retained in the ER due to altered cell cycle, and viral particles are incapable of further movement towards the nucleus in DCT knockdown cells.

Comprehensive phenotypic analysis of knockout mice deficient in cyclin G1 and cyclin G2

Cyclin G1 (CycG1) and Cyclin G2 (CycG2) play similar roles during the DNA damage response (DDR), but their detailed roles remain elusive. To investigate their distinct roles, we generated knockout mice deficient in CycG1 (G1KO) or CycG2 (G2KO), as well as double knockout mice (DKO) deficient in both proteins. All knockouts developed normally and were fertile. Generation of mouse embryonic fibroblasts (MEFs) from these mice revealed that G2KO MEFs, but not G1KO or DKO MEFs, were resistant to DNA damage insults caused by camptothecin and ionizing radiation (IR) and underwent cell cycle arrest. CycG2, but not CycG1, co-localized with γH2AX foci in the nucleus after γ-IR, and γH2AX-mediated DNA repair and dephosphorylation of CHK2 were delayed in G2KO MEFs. H2AX associated with CycG1, CycG2, and protein phosphatase 2A (PP2A), suggesting that γH2AX affects the function of PP2A via direct interaction with its B'γ subunit. Furthermore, expression of CycG2, but not CycG1, was abnormal in various cancer cell lines. Kaplan-Meier curves based on TCGA data disclosed that head and neck cancer patients with reduced CycG2 expression have poorer clinical prognoses. Taken together, our data suggest that reduced CycG2 expression could be useful as a novel prognostic marker of cancer.

Cyclin G2 promotes cell cycle arrest in breast cancer cells responding to fulvestrant and metformin and correlates with patient survival

Definition of cell cycle control proteins that modify tumor cell resistance to estrogen (E2) signaling antagonists could inform clinical choice for estrogen receptor positive (ER+) breast cancer (BC) therapy. Cyclin G2 (CycG2) is upregulated during cell cycle arrest responses to cellular stresses and growth inhibitory signals and its gene, CCNG2, is directly repressed by E2-bound ER complexes. Our previous studies showed that blockade of HER2, PI3K and mTOR signaling upregulates CycG2 expression in HER2+ BC cells, and that CycG2 overexpression induces cell cycle arrest. Moreover, insulin and insulin-like growth factor-1 (IGF-1) receptor signaling strongly represses CycG2. Here we show that blockade of ER-signaling in MCF7 and T47D BC cell lines enhances the expression and nuclear localization of CycG2. Knockdown of CycG2 attenuated the cell cycle arrest response of E2-depleted and fulvestrant treated MCF7 cells. These muted responses were accompanied by sustained inhibitory phosphorylation of retinoblastoma (RB) protein, expression of cyclin D1, phospho-activation of ERK1/2 and MEK1/2 and expression of cRaf. Our work indicates that CycG2 can form complexes with CDK10, a CDK linked to modulation of RAF/MEK/MAPK signaling and tamoxifen resistance. We determined that metformin upregulates CycG2 and potentiates fulvestrant-induced CycG2 expression and cell cycle arrest. CycG2 knockdown blunts the enhanced anti-proliferative effect of metformin on fulvestrant treated cells. Meta-analysis of BC tumor microarrays indicates that CCNG2 expression is low in aggressive, poor-prognosis BC and that high CCNG2 expression correlates with longer periods of patient survival. Together these findings indicate that CycG2 contributes to signaling networks that limit BC.

MicroRNA-1246 promotes growth and metastasis of colorectal cancer cells involving CCNG2 reduction

Colorectal cancer (CRC) is the third most common cancer type and the fourth leading cause of cancer‑associated mortality worldwide. MicroRNA (miR)‑1246 is involved in differentiation, invasion, metastasis and chemoresistance of certain types of tumor cells. CCNG2 encodes an unconventional cyclin homolog, cyclin G2 (CycG2), associated with growth inhibition, which correlated significantly with lymph node metastasis, clinical stage, histological grade and poor overall survival in numerous cancer types. To investigate the regulation of miR‑1246 on CycG2 expression, and their effects on proliferation and metastasis of CRC, HCT‑116 and LOVO cells were transfected with pre‑miR‑1246 anti‑miR‑1246 and their negative controls. It was demonstrated that the expression of miR‑1246 was significantly increased in CRC tissues and cell lines, which was the opposite of CycG2. miR‑1246 negatively regulated the expression of CycG2 in HCT‑116 and LOVO CRC cells. CCNG2 is a direct target of miR‑1246 in CRC cells. Overexpression of miR‑1246 induced cell proliferation, migration and invasion, while knockdown of miR‑1246 inhibited proliferation, migration and invasion in the CRC cells. Upregulation of miR‑1246 mediated the malignant progression of CRC and is partly attributed to the downregulation of the expression of CycG2. Consequently, these findings provided a molecular basis for the role of miR‑1246/CCNG2 in the progression of human CRC and suggested a novel target for the treatment of CRC.

Cyclin G Functions as a Positive Regulator of Growth and Metabolism in Drosophila

In multicellular organisms, growth and proliferation is adjusted to nutritional conditions by a complex signaling network. The Insulin receptor/target of rapamycin (InR/TOR) signaling cascade plays a pivotal role in nutrient dependent growth regulation in Drosophila and mammals alike. Here we identify Cyclin G (CycG) as a regulator of growth and metabolism in Drosophila. CycG mutants have a reduced body size and weight and show signs of starvation accompanied by a disturbed fat metabolism. InR/TOR signaling activity is impaired in cycG mutants, combined with a reduced phosphorylation status of the kinase Akt1 and the downstream factors S6-kinase and eukaryotic translation initiation factor 4E binding protein (4E-BP). Moreover, the expression and accumulation of Drosophila insulin like peptides (dILPs) is disturbed in cycG mutant brains. Using a reporter assay, we show that the activity of one of the first effectors of InR signaling, Phosphoinositide 3-kinase (PI3K92E), is unaffected in cycG mutants. However, the metabolic defects and weight loss in cycG mutants were rescued by overexpression of Akt1 specifically in the fat body and by mutants in widerborst (wdb), the B'-subunit of the phosphatase PP2A, known to downregulate Akt1 by dephosphorylation. Together, our data suggest that CycG acts at the level of Akt1 to regulate growth and metabolism via PP2A in Drosophila.

Size and growth of an organism are adjusted to nutritional conditions by a complex regulatory network involving the Insulin receptor and TOR signaling cascades. Drosophila melanogaster has been used in the past as a genetically tractable model to unravel the complex circuitry by genetic means. We have identified CycG as an important player in the regulation of TOR signaling. CycG mutants are underweight in the midst of food and show typical signs of TOR defects. We provide evidence that CycG acts at the level of Akt1 kinase that links the Insulin receptor and TOR signaling cascades. Molecular and genetic data point to an interplay of CycG and phosphatase PP2A, a well established negative regulator of Akt1 activity. Moreover, CycG may influence PP2A-Akt1 binding. We propose that CycG, by impeding PP2A-Akt1 interaction, acts as a positive regulator of growth in Drosophila.

Vitamin K2 and cotylenin A synergistically induce monocytic differentiation and growth arrest along with the suppression of c-MYC expression and induction of cyclin G2 expression in human leukemia HL-60 cells

Although all-trans retinoic acid (ATRA) is a standard and effective drug used for differentiation therapy in acute promyelocytic leukemia, ATRA-resistant leukemia cells ultimately emerge during this treatment. Therefore, the development of new drugs or effective combination therapy is urgently needed. We demonstrate that the combined treatment of vitamin K2 and cotylenin A synergistically induced monocytic differentiation in HL-60 cells. This combined treatment also synergistically induced NBT-reducing activity and non-specific esterase-positive cells as well as morphological changes to monocyte/macrophage-like cells. Vitamin K2 and cotylenin A cooperatively inhibited the proliferation of HL-60 cells in short-term and long-term cultures. This treatment also induced growth arrest at the G1 phase. Although 5 µg/ml cotylenin A or 5 µM vitamin K2 alone reduced c-MYC gene expression in HL-60 cells to approximately 45% or 80% that of control cells, respectively, the combined treatment almost completely suppressed c-MYC gene expression. We also demonstrated that the combined treatment of vitamin K2 and cotylenin A synergistically induced the expression of cyclin G2, which had a positive effect on the promotion and maintenance of cell cycle arrest. These results suggest that the combination of vitamin K2 and cotylenin A has therapeutic value in the treatment of acute myeloid leukemia.

Differential Molecular Stress Responses to Low Compared to High Doses of Ionizing Radiation in Normal Human Fibroblasts

Understanding the mechanisms producing low dose ionizing radiation specific biological effects represents one of the major challenges of radiation biology. Although experimental evidence does suggest that various molecular stress response pathways may be involved in the production of low dose effects, much of the detail of those mechanisms remains elusive. We hypothesized that the regulation of various stress response pathways upon irradiation may differ from one another in complex dose-response manners, causing the specific and subtle low dose radiation effects. In the present study, the transcription level of 22 genes involved in stress responses were analyzed using RT-qPCR in normal human fibroblasts exposed to a range of gamma-doses from 1 to 200 cGy. Using the alkali comet assay, we also measured the level of DNA damages in dose-response and time-course experiments. We found non-linear dose responses for the repair of DNA damage after exposure to gamma-radiation. Alterations in gene expression were also not linear with dose for several of the genes examined and did not follow a single pattern. Rather, several patterns could be seen. Our results suggest a complex interplay of various stress response pathways triggered by low radiation doses, with various low dose thresholds for different genes.

ELAS1-mediated inhibition of the cyclin G1-B'γ interaction promotes cancer cell apoptosis via stabilization and activation of p53

Radiation therapy (RT) is useful for selectively killing cancer cells. However, because high levels of ionizing radiation (IR) are toxic to normal cells, RT cannot be applied repeatedly to cancer patients. Therefore, novel chemicals that enhance the efficacy of chemoradiotherapy (CRT) would be valuable. Here, we report that ELAS1, a peptide corresponding to the protein phosphatase 2A (PP2A) association domain of cyclin G1 (CycG1), can enhance the efficacy of CRT. ELAS1 interacts with the PP2A B'γ-subunit and competitively inhibits association with CycG1, thereby preventing the PP2A holoenzyme from dephosphorylating target proteins, Mdm2 (pT218) and p53 (pS46), following DNA double-strand break (DSB) insults. Doxycycline (Dox)-induced overexpression of Myc-ELAS1 caused γ-irradiation to induce apoptosis in human osteosarcoma (U2OS) cells, at 1/10th the effective dosage of γ-irradiation required for apoptosis in Myc-vector-expressing cells; ELAS1 peptide incorporation into U2OS cells also showed similar apoptotic effects. Moreover, administration of DSB-inducing chemicals, camptothecin (CPT) or irinotecan, to Myc-ELAS1-expressing U2OS cells also induced efficient apoptosis with only 1/100th (CPT) or 1/5th (irinotecan) of the amounts of drugs required for this effect in Myc-vector-expressing cells. Taken together, ELAS1 may be important for the design of ELAS1-mimetic compounds to improve CRT efficacy.

Iodine-131 induces apoptosis in HTori-3 human thyrocyte cell line and G2/M phase arrest in a p53-independent pathway

Iodine‑131 is known to destroy residual thyroid tissue following surgical resection of differentiated thyroid carcinoma and is widely used to treat hyperthyroidism. However, the mechanism by which iodine‑131 induces apoptosis and cell cycle arrest in the human thyrocyte cell line, Htori‑3, remains to be elucidated. In the present study, the cytotoxic effect of iodine‑131 on the HTori‑3 cell line and the underlying mechanism of iodine‑131‑induced cell apoptosis were investigated. Cell viability was analyzed using an MTT assay, while cell apoptosis and cell cycle arrest were determined using flow cytometry. Reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR) and western blot analyses were performed to determine the changes in the expression levels of p53, B‑cell lymphoma 2 (Bcl‑2), Fas and growth arrest and DNA damage‑inducible 45 (GADD45), following iodine‑131 treatment. The results demonstrated that iodine‑131 may inhibit HTori‑3 cell growth via cell apoptosis and G2/M phase arrest in a time‑ and dose‑dependent manner. The iodine‑131 dose required for 50% growth inhibition of HTori‑3 cell viability 48 h after treatment was 27.75±2.22 MBq/ml. Upregulation of Fas and downregulation of Bcl‑2 expression levels were observed following iodine‑131 treatment. The results of RT‑qPCR revealed an increase in the GADD45 mRNA expression following HTori‑3 cell exposure to iodine‑131. Notably, the mRNA and protein expression levels of p53 were not altered following iodine‑131 treatment. In conclusion, iodine‑131 may induce apoptosis in HTori‑3 cells by downregulating the expression of Bcl‑2 and upregulating the expression of Fas. In addition, iodine‑131 may upregulate GADD45 mRNA expression in HTori‑3 cells, resulting in G2/M phase arrest in a p53‑independent pathway.

TiO2 nanoparticles induce DNA double strand breaks and cell cycle arrest in human alveolar cells

TiO2 nanoparticles (NPs) have the second highest global annual production (∼3000 tons) among the metal-containing NPs. These NPs are used as photocatalysts for bacterial disinfection, and in various other consumer products including sunscreen, food packaging, therapeutics, biosensors, surface cleaning agents, and others. Humans are exposed to these NPs during synthesis (laboratory), manufacture (industry), and use (consumer products, devices, medicines, etc.), as well as through environmental exposures (disposal). Hence, there is great concern regarding the health effects caused by exposure to NPs and, in particular, to TiO2 NPs. In the present study, the genotoxic potential of TiO2 NPs in A549 cells was examined, focusing on their potential to induce ROS, different types of DNA damage, and cell cycle arrest. We show that TiO2 NPs can induce DNA damage and a corresponding increase in micronucleus frequency, as evident from the comet and cytokinesis-block micronucleus assays. We demonstrate that DNA damage may be attributed to increased oxidative stress and ROS generation. Furthermore, genomic and proteomic analyses showed increased expression of ATM, P53, and CdC-2 and decreased expression of ATR, H2AX, and Cyclin B1 in A549 cells, suggesting induction of DNA double strand breaks. The occurrence of double strand breaks was correlated with cell cycle arrest in G2/M phase. Overall, the results indicate the potential for genotoxicity following exposure to these TiO2 NPs, suggesting that use should be carefully monitored.

Macrophage mitochondrial and stress response to ingestion of Cryptococcus neoformans

Human infection with Cryptococcus neoformans, a common fungal pathogen, follows deposition of yeast spores in the lung alveoli. The subsequent host-pathogen interaction can result in eradication, latency, or extrapulmonary dissemination. Successful control of C. neoformans infection is dependent on host macrophages, but macrophages display little ability to kill C. neoformans in vitro. Recently, we reported that ingestion of C. neoformans by mouse macrophages induces early cell cycle progression followed by mitotic arrest, an event that almost certainly reflects host cell damage. The goal of the present work was to understand macrophage pathways affected by C. neoformans toxicity. Infection of macrophages by C. neoformans was associated with alterations in protein translation rate and activation of several stress pathways, such as hypoxia-inducing factor-1-α, receptor-interacting protein 1, and apoptosis-inducing factor. Concomitantly we observed mitochondrial depolarization in infected macrophages, an observation that was replicated in vivo. We also observed differences in the stress pathways activated, depending on macrophage cell type, consistent with the nonspecific nature of C. neoformans virulence known to infect phylogenetically distant hosts. Our results indicate that C. neoformans infection impairs multiple host cellular functions and undermines the health of these critical phagocytic cells, which can potentially interfere with their ability to clear this fungal pathogen.

Strigolactone analogues induce apoptosis through activation of p38 and the stress response pathway in cancer cell lines and in conditionally reprogrammed primary prostate cancer cells

Strigolactones are a novel class of plant hormones produced in roots and regulate shoot and root development. We have previously shown that synthetic strigolactone analogues potently inhibit growth of breast cancer cells and breast cancer stem cells. Here we show that strigolactone analogues inhibit the growth and survival of an array of cancer-derived cell lines representing solid and non-solid cancer cells including: prostate, colon, lung, melanoma, osteosarcoma and leukemic cell lines, while normal cells were minimally affected. Treatment of cancer cells with strigolactone analogues was hallmarked by activation of the stress-related MAPKs: p38 and JNK and induction of stress-related genes; cell cycle arrest and apoptosis evident by increased percentages of cells in the sub-G1 fraction and Annexin V staining. In addition, we tested the response of patient-matched conditionally reprogrammed primary prostate normal and cancer cells. The tumor cells exhibited significantly higher sensitivity to the two most potent SL analogues with increased apoptosis confirmed by PARP1 cleavage compared to their normal counterpart cells. Thus, Strigolactone analogues are promising candidates for anticancer therapy by their ability to specifically induce cell cycle arrest, cellular stress and apoptosis in tumor cells with minimal effects on growth and survival of normal cells.

Depletion of C3orf1/TIMMDC1 inhibits migration and proliferation in 95D lung carcinoma cells

In our previous study, we identified an association of high expression of c3orf1, also known as TIMMDC1 (translocase of inner mitochondrial membrane domain-containing protein 1), with metastatic characteristics in lung carcinoma cells. To investigate the preliminary function and mechanism of this mitochondrial protein, we depleted C3orf1 expression by introducing siRNA into 95D lung carcinoma cells. We demonstrated that C3orf1 depletion significantly suppressed 95D cell growth and migration. We confirmed C3orf1 localization in the inner mitochondrial membrane and showed that mitochondrial viability, membrane potential, and ATPase activity were remarkably reduced upon depletion of C3orf1. Microarray data indicated that genes involved in regulation of cell death, migration, and cell-cycle arrest were significantly altered after C3orf1 depletion for 48 h. The expression of genes involved in focal adhesion, ECM-receptor interaction, and p53-signaling pathways were notably altered. Furthermore, cell-cycle arrest genes such as CCNG2 and PTEN as well as genes involved in cell migration inhibition, such as TIMP3 and COL3A1, were upregulated after C3orf1 depletion in 95D cells. Concurrently, expression of the migration-promoting gene NUPR1 was markedly reduced, as confirmed by real-time PCR. We conclude that C3orf1 is critical for mitochondrial function, migration, and proliferation in 95D lung carcinoma cells. Depletion of C3orf1 inhibited cell migration and cell proliferation in association with upregulation of genes involved in cell-cycle arrest and cell migration inhibition. These results suggest that C3orf1 (TIMMDC1) may be a viable treatment target for lung carcinoma, and that further study of the role of this protein in lung carcinoma pathogenesis is justified.

EZH2 silencing with RNA interference induces G2/M arrest in human lung cancer cells in vitro

Nonsmall-cell lung cancer has a high mortality rate and poor prognosis. In the present study, we silenced EZH2 and explored the consequent cell cycle changes. The expression of cell-cycle-related proteins, including p53, p21, Cdc2, and cyclin B1, was detected with western blotting, and the cell cycle distribution was determined with flow cytometry. Inhibition of EZH2 expression changed the cell cycle distribution, in particular inducing G₂/M arrest. Expression of Cdc2 and cyclin B1 was significantly decreased in A549 and HTB-56 cells after EZH2-siRNA treatment. In addition, p53 expression was increased by 21% and 18%, and p21 expression was increased by 31% and 23%, in A549 and HTB-56 cells, respectively, in the presence of EZH2-siRNA. This study clearly demonstrates that modulation of EZH2 expression with siRNA affects the cell cycle and the expression levels of p53 and p21, thereby changing cyclin B1 and Cdc2 expression and inducing G₂/M arrest. These results may explain the observed antitumor activity of EZH2 silencing. Such explorations of the molecular mechanism of EZH2 will help us develop novel approaches to the diagnosis, treatment, and prevention of nonsmall-cell lung cancer.

Down-regulation of cyclin G2 by insulin, IGF-I (insulin-like growth factor 1) and X10 (AspB10 insulin): role in mitogenesis

The mechanisms whereby insulin analogues may cause enhanced mitogenicity through activation of either the IR (insulin receptor) or the IGF-IR (insulin-like growth factor 1 receptor) are incompletely understood. We demonstrate that in L6 myoblasts expressing only IGF-IRs as well as in the same cells overexpressing the IR, IGF-I (insulin-like growth factor 1), insulin and X10 (AspB10 insulin) down-regulate the mRNA expression level of the cell cycle inhibitor cyclin G2, as measured by qRT-PCR (quantitative reverse transcription-PCR), and induce cell growth measured by [6-(3)H]thymidine incorporation into DNA. Western blotting showed a marked down-regulation of cyclin G2 at the protein level in both cell lines. Overexpression of cyclin G2 in the two cell lines diminished the mitogenic effect of all three ligands. The use of specific inhibitors indicated that both the MAPK (mitogen-activated protein kinase) and the PI3K (phosphoinositide 3-kinase) pathways mediate the down-regulation of Ccng2. The down-regulation of CCNG2 by the three ligands was also observed in other cell lines: MCF-7, HMEC, Saos-2, R(-)/IR and INS-1. These results indicate that regulation of cyclin G2 is a key mechanism whereby insulin, insulin analogues and IGF-I stimulate cell proliferation.

B-myb is an essential regulator of hematopoietic stem cell and myeloid progenitor cell development

The B-myb (MYBL2) gene is a member of the MYB family of transcription factors and is involved in cell cycle regulation, DNA replication, and maintenance of genomic integrity. However, its function during adult development and hematopoiesis is unknown. We show here that conditional inactivation of B-myb in vivo results in depletion of the hematopoietic stem cell (HSC) pool, leading to profound reductions in mature lymphoid, erythroid, and myeloid cells. This defect is autonomous to the bone marrow and is first evident in stem cells, which accumulate in the S and G2/M phases. B-myb inactivation also causes defects in the myeloid progenitor compartment, consisting of depletion of common myeloid progenitors but relative sparing of granulocyte-macrophage progenitors. Microarray studies indicate that B-myb-null LSK(+) cells differentially express genes that direct myeloid lineage development and commitment, suggesting that B-myb is a key player in controlling cell fate. Collectively, these studies demonstrate that B-myb is essential for HSC and progenitor maintenance and survival during hematopoiesis.

Cell cycle regulation of purine synthesis by phosphoribosyl pyrophosphate and inorganic phosphate

Cells must increase synthesis of purine nucleotides/deoxynucleotides before or during S-phase. We found that rates of purine synthesis via the de novo and salvage pathways increased 5.0- and 3.3-fold respectively, as cells progressed from mid-G1-phase to early S-phase. The increased purine synthesis could be attributed to a 3.2-fold increase in intracellular PRPP (5-phosphoribosyl-α-1-pyrophosphate), a rate-limiting substrate for de novo and salvage purine synthesis. PRPP can be produced by the oxidative and non-oxidative pentose phosphate pathways, and we found a 3.1-fold increase in flow through the non-oxidative pathway, with no change in oxidative pathway activity. Non-oxidative pentose phosphate pathway enzymes showed no change in activity, but PRPP synthetase is regulated by phosphate, and we found that phosphate uptake and total intracellular phosphate concentration increased significantly between mid-G1-phase and early S-phase. Over the same time period, PRPP synthetase activity increased 2.5-fold when assayed in the absence of added phosphate, making enzyme activity dependent on cellular phosphate at the time of extraction. We conclude that purine synthesis increases as cells progress from G1- to S-phase, and that the increase is from heightened PRPP synthetase activity due to increased intracellular phosphate.

Recruitment of cyclin G2 to promyelocytic leukemia nuclear bodies promotes dephosphorylation of γH2AX following treatment with ionizing radiation

Cyclin G2 (CycG2) and Cyclin G1 (CycG1), two members of the Cyclin G subfamily, share high amino acid homology in their Cyclin G boxes. Functionally, they play a common role as association partners of the B'γ subunit of protein phosphatase 2A (PP2A) and regulate PP2A function, and their expression is increased following DNA damage. However, whether or not CycG1 and CycG2 have distinct roles during the cellular DNA damage response has remained unclear. Here, we report that CycG2, but not CycG1, co-localized with promyelocytic leukemia (PML) and γH2AX, forming foci following ionizing radiation (IR), suggesting that CycG2 is recruited to sites of DNA repair and that CycG1 and CycG2 have distinct functions. PML failed to localize to nuclear foci when CycG2 was depleted, and vice versa. This suggests that PML and CycG2 mutually influence each other's functions following IR. Furthermore, we generated CycG2-knockout (Ccng2 (-/-) ) mice to investigate the functions of CycG2. These mice were born healthy and developed normally. However, CycG2-deficient mouse embryonic fibroblasts displayed an abnormal response to IR. Dephosphorylation of γH2AX and checkpoint kinase 2 following IR was delayed in Ccng2 (-/-) cells, suggesting that DNA damage repair may be perturbed in the absence of CycG2. Although knockdown of B'γ in wild-type cells also delayed dephosphorylation of γH2AX, knockdown of B'γ in Ccng2 (-/-) cells prolonged this delay, suggesting that CycG2 cooperates with B'γ to dephosphorylate γH2AX. Taken together, we conclude that CycG2 is localized at DNA repair foci following DNA damage, and that CycG2 regulates the dephosphorylation of several factors necessary for DNA repair.

Cyclins and cell cycle control in cancer and disease

Cyclin D1 overexpression is found in more than 50% of human breast cancers and causes mammary cancer in transgenic mice. Dysregulation of cyclin D1 gene expression or function contributes to the loss of normal cell cycle control during tumorigenesis. Recent studies have demonstrated that cyclin D1 conducts additional specific functions to regulate gene expression in the context of local chromatin, promote cellular migration, and promote chromosomal instability. It is anticipated that these additional functions contribute to the pathology associated with dysregulated cyclin D1 abundance. This article discusses evidence that examines the functional roles that cyclin D1 may play in cancer with an emphasis on other cyclin family members that also may contribute to cancer and disease in a similar fashion.

Tumor suppressor p53 plays a key role in induction of both tristetraprolin and let-7 in human cancer cells

Tristetraprolin (TTP) and let-7 microRNA exhibit suppressive effects on cell growth through down-regulation of oncogenes. Both TTP and let-7 are often repressed in human cancers, thereby promoting oncogenesis by derepressing their target genes. However, the precise mechanism of this repression is unknown. We here demonstrate that p53 stimulated by the DNA-damaging agent doxorubicin (DOX) induced the expression of TTP in cancer cells. TTP in turn increased let-7 levels through down-regulation of Lin28a. Correspondingly, cancer cells with mutations or inhibition of p53 failed to induce the expression of both TTP and let-7 on treatment with DOX. Down-regulation of TTP by small interfering RNAs attenuated the inhibitory effect of DOX on let-7 expression and cell growth. Therefore, TTP provides an important link between p53 activation induced by DNA damage and let-7 biogenesis. These novel findings provide a mechanism for the widespread decrease in TTP and let-7 and chemoresistance observed in human cancers.