Expression of cyclin E renders cyclin D-CDK4 dispensable for inactivation of the retinoblastoma tumor suppressor protein, activation of E2F, and G1-S phase progression

The CMG helicase and cancer: a tumor "engine" and weakness with missing mutations

The replicative Cdc45-MCM-GINS (CMG) helicase is a large protein complex that functions in the DNA melting and unwinding steps as a component of replisomes during DNA replication in mammalian cells. Although the CMG performs this important role in cell growth, the CMG is not a simple bystander in cell cycle events. Components of the CMG, specifically the MCM precursors, are also involved in maintaining genomic stability by regulating DNA replication fork speeds, facilitating recovery from replicative stresses, and preventing consequential DNA damage. Given these important functions, MCM/CMG complexes are highly regulated by growth factors such as TGF-ß1 and by signaling factors such as Myc, Cyclin E, and the retinoblastoma protein. Mismanagement of MCM/CMG complexes when these signaling mediators are deregulated, and in the absence of the tumor suppressor protein p53, leads to increased genomic instability and is a contributor to tumorigenic transformation and tumor heterogeneity. The goal of this review is to provide insight into the mechanisms and dynamics by which the CMG is regulated during its assembly and activation in mammalian genomes, and how errors in CMG regulation due to oncogenic changes promote tumorigenesis. Finally, and most importantly, we highlight the emerging understanding of the CMG helicase as an exploitable vulnerability and novel target for therapeutic intervention in cancer.

LncRNA STK4 antisense RNA 1 (STK4-AS1) promoted osteosarcoma by inhibiting p53 expression

BACKGROUND

LncRNA STK4 antisense RNA 1 (STK4-AS1) has been identified as a potential biomarker associated with multiple cancers. We proposed that STK4-AS1 plays a role in the proliferation of osteosarcoma by regulating the cell cycle.

METHODS

We compared the expression of STK4-AS1, p53, and p21 in osteosarcoma vs normal samples in clinical tissues and cell lines. We determined the effect of overexpression and knockdown of STK4-AS1 in p53 expressing osteosarcoma cells U2OS, p53 muted osteosarcoma cells MG63, and osteoblast cells hFOB on p53 and p21 expression and the cell viability. For U2OS and MG63, the cell cycle was analyzed and the expression of cyclin proteins was determined. We overexpressed p53 or p21 in STK4-AS1 overexpressed cells to explore the association of STK4-AS1 and p53 in U2OS.

RESULTS

The STK4-AS1 expression was higher and p53 and p21 expression were lower in osteosarcoma tissue and cells than in their non-cancer counterparts. The expression of STK4-AS1 was negatively correlated with the expression of p53 or p21. Knockdown of STK4-AS1 in U2OS decreased the cell viability, increased cells in the G0/G1 phase, decreased cells in the S and G2/M phase, decreased expression of cyclin A and B, increased p53 and p21, and had no effect on cyclin D and cyclin E, while overexpression of STK4-AS1 did the opposes. Overexpression of p53 or p21 recovered some changes caused by STK4-AS1 overexpression in U2OS. MG63 expressed no p53 and the expression of p21, cyclin A, and cyclin B, cell viability, and cell cycle were not affected by altered STK4-AS1 levels. In hFOB cells, the expression of p53 and p21 was decreased and the cell viability was increased when STK4-AS1 was overexpressed, but they were not affected when STK4-AS1 was knocked down.

CONCLUSION

LncRNA STK4-AS1 promoted the cell cycle of osteosarcoma cells by inhibiting p53 expression.

CDK4/6 initiates Rb inactivation and CDK2 activity coordinates cell-cycle commitment and G1/S transition

External signaling controls cell-cycle entry until cells irreversibly commit to the cell cycle to ensure faithful DNA replication. This process is tightly regulated by cyclin-dependent kinases (CDKs) and the retinoblastoma protein (Rb). Here, using live-cell sensors for CDK4/6 and CDK2 activities, we propose that CDK4/6 initiates Rb inactivation and CDK2 activation, which coordinates the timing of cell-cycle commitment and sequential G1/S transition. Our data show that CDK4/6 activation induces Rb inactivation and thereby E2F activation, driving a gradual increase in CDK2 activity. We found that rapid CDK4/6 inhibition can reverse cell-cycle entry until CDK2 activity reaches to high levels. This suggests that high CDK2 activity is required to initiate CDK2-Rb positive feedback and CDK4/6-indpendent cell-cycle progression. Since CDK2 activation also facilitates initiation of DNA replication, the timing of CDK2-Rb positive feedback is coupled with the G1/S transition. Our experiments, which acutely increased CDK2 activity by cyclin E1 overexpression, indicate that cells commit to the cell cycle before triggering DNA replication. Together, our data suggest that CDK4/6 inactivates Rb to begin E2F and CDK2 activation, and high CDK2 activity is necessary and sufficient to generate a bistable switch for Rb phosphorylation before DNA replication. These findings highlight how cells initiate the cell cycle and subsequently commit to the cell cycle before the G1/S transition.

Co-targeting CDK2 and CDK4/6 overcomes resistance to aromatase and CDK4/6 inhibitors in ER+ breast cancer

Resistance to aromatase inhibitor (AI) treatment and combined CDK4/6 inhibitor (CDK4/6i) and endocrine therapy (ET) are crucial clinical challenges in treating estrogen receptor-positive (ER+) breast cancer. Understanding the resistance mechanisms and identifying reliable predictive biomarkers and novel treatment combinations to overcome resistance are urgently needed. Herein, we show that upregulation of CDK6, p-CDK2, and/or cyclin E1 is associated with adaptation and resistance to AI-monotherapy and combined CDK4/6i and ET in ER+ advanced breast cancer. Importantly, co-targeting CDK2 and CDK4/6 with ET synergistically impairs cellular growth, induces cell cycle arrest and apoptosis, and delays progression in AI-resistant and combined CDK4/6i and fulvestrant-resistant cell models and in an AI-resistant autocrine breast tumor in a postmenopausal xenograft model. Analysis of CDK6, p-CDK2, and/or cyclin E1 expression as a combined biomarker in metastatic lesions of ER+ advanced breast cancer patients treated with AI-monotherapy or combined CDK4/6i and ET revealed a correlation between high biomarker expression and shorter progression-free survival (PFS), and the biomarker combination was an independent prognostic factor in both patients cohorts. Our study supports the clinical development of therapeutic strategies co-targeting ER, CDK4/6 and CDK2 following progression on AI-monotherapy or combined CDK4/6i and ET to improve survival of patients exhibiting high tumor levels of CDK6, p-CDK2, and/or cyclin E1.

Expression of Cell Cycle Markers and Proliferation Factors during Human Eye Embryogenesis and Tumorigenesis

The expression pattern of the markers p19, Ki-67, MSX1, MSX2, PDL1, pRB, and CYCLINA2 was quantitatively and semiquantitatively analyzed in histologic sections of the developing and postnatal human eye at week 8, in retinoblastoma, and in various uveal melanomas post hoc studies by double immunofluorescence. The p19 immunoreactivity characterized retinal and/or choroidal cells in healthy and tumor tissues: expression was lower in the postnatal retina than in the developing retina and retinoblastoma, whereas it was high in epithelioid melanomas. Ki67 expression was high in the developing eye, retinoblastoma, and choroidal melanomas. MSX1 and MSX2 expression was similar in the developing eye and retinoblastoma, whereas it was absent in the postnatal eye. Their different expression was evident between epithelioid and myxoid melanomas. Similarly, PDL1 was absent in epithelioid melanomas, whereas it was highly expressed in developing and tumor tissues. Expression of pRB and CYCA2 was characteristic of developing and tumorous eye samples but not of the healthy postnatal eye. The observed expression differences of the analyzed markers correlate with the origin and stage of cell differentiation of the tissue samples. The fine balance of expression could play a role in both human eye development and ocular tumorigenesis. Therefore, understanding their relationship and interplay could open new avenues for potential therapeutic interventions and a better understanding of the mechanisms underlying the developmental plasticity of the eye and the development of neoplasms.

Mitochondrial-cell cycle cross-talk drives endoreplication in heart disease

[Figure: see text].

Resistance Mechanisms to Combined CDK4/6 Inhibitors and Endocrine Therapy in ER+/HER2- Advanced Breast Cancer: Biomarkers and Potential Novel Treatment Strategies

The introduction of CDK4/6 inhibitors (CDK4/6i) in combination with endocrine therapy (ET) has revolutionized the treatment landscape for patients with estrogen receptor-positive (ER+) advanced breast cancer (ABC) and has become the new standard treatment. However, resistance to this combined therapy inevitably develops and represents a major clinical challenge in the management of ER+ ABC. Currently, elucidation of the resistance mechanisms, identification of predictive biomarkers, and development of novel effective combined targeted treatments to overcome the resistance are active areas of research. Given the heterogeneity of the resistance mechanisms towards combined CDK4/6i and ET, identification of a single universal predictive biomarker of resistance is unlikely. Novel approaches are being explored, including examination of multiple genetic alterations in circulating cell-free tumor DNA in liquid biopsies from ABC patients with disease progression on combined CDK4/6i and ET treatment. Here, we review the molecular basis of the main known resistance mechanisms towards combined CDK4/6i and ET and associated potential biomarkers. As inhibiting key molecules in the pathways driving resistance may play an important role in the selection of therapeutic strategies for patients who experience disease progression on combined CDK4/6i and ET, we also review preclinical and early phase clinical data on novel combination therapies for these patients.

Emodin regulates cell cycle of non-small lung cancer (NSCLC) cells through hyaluronan synthase 2 (HA2)-HA-CD44/receptor for hyaluronic acid-mediated motility (RHAMM) interaction-dependent signaling pathway

Background

Non-small cell lung cancers (NSCLC) account for most cases of lung cancer. More effort is needed to research new drug and combination therapies for this disease. An anthraquinone derivative, emodin shows anticancer potency. We hypothesis that emodin suppresses lung cancer cells through hyaluronan (HA) synthase 2-HA-CD44/receptor for hyaluronic acid-mediated motility (RHAMM) interaction-dependent signaling pathway mediated cell cycle regulation.

Methods

We tested the effect of emodin on viability, apoptosis, and HA secretion of 5 NSCLC cell lines. We used NSCLC cells A549 for two rounds of knockdown study: (1) knocking down either the synthases (HAS2 and HAS3) or the receptors (CD44 and RHAMM); (2) knocking down either HAS2 or HAS3. Then determined the effect of emodin on viability, HA secretion, cell cycle, and expression of cyclin proteins.

Results

Emodin suppressed viability and HA secretion of all 5 NSCLC cell lines except for HA secretion of H460. Emodin had a slight apoptosis induction effect on all cell lines and was not different among cell lines. The knockdown of either the synthases or the receptors blocked emodin effects on viability while the knockdown of HAS2 block emodin effects but not HAS3. Emodin increased cells in the G1/G0 phase, and decreased cells in the S and G2/M phase by down-regulating cyclin A and B and up-regulating cyclin C, D, and E. HAS2 knockdown blocked the effects of emodin on the cell cycle.

Conclusions

This study demonstrated that emodin regulates the cell cycle of NSCLC cells through the HAS2-HA-CD44/RHAMM interaction-dependent signaling pathway.

LncRNA RP1-85F18.6 affects osteoblast cells by regulating the cell cycle

A lncRNA RP1-85F18.6 was reported to affect cell growth by regulating the cell cycle. Here we tested whether it affects the proliferation of osteoblast cells by regulating the cell cycle. We determined the expression of RP1-85F18.6 in two osteoblast cell lines hFOB and HOB by qPCR. Then we knocked down or overexpressed RP1-85F18.6 in hFOB and tested the alteration of viability, cell cycle, and cell cycle regulatory proteins. Results showed that both hFOB and HOB expressed RP1-85F18.6. The knockdown of RP1-85F18.6 decreased the viability of hFOB, while the overexpression of it increased the viability. Higher expression of RP1-85F18.6 results in higher cell viability. The knockdown of RP1-85F18.6 caused an increase in the S phase cells and a decrease in the G2/M phase cells. The overexpression of RP1-85F18.6 caused a decrease in the S phase cells and an increase in the G2/M phase cells. The knockdown of RP1-85F18.6 decreased cyclin A, cdk1, E2F, cyclin B, p53, and p21, whereas the overexpression of RP1-85F18.6 increased cyclin A, cdk1, E2F, cyclin B, p53, and p21. This study demonstrated that RP1-85F18.6 is expressed in osteoblast cell lines hFOB and HOB. RP1-85F18.6 affects the proliferation of osteoblasts by regulating the cell cycle.

Integrating Old and New Paradigms of G1/S Control

The Cdk-Rb-E2F pathway integrates external and internal signals to control progression at the G1/S transition of the mammalian cell cycle. Alterations in this pathway are found in most human cancers, and specific cyclin-dependent kinase Cdk4/6 inhibitors are approved or in clinical trials for the treatment of diverse cancers. In the long-standing paradigm for G1/S control, Cdks inactivate the retinoblastoma tumor suppressor protein (Rb) through phosphorylation, which releases E2F transcription factors to drive cell-cycle progression from G1 to S. However, recent observations in the laboratory and clinic challenge central tenets of the current paradigm and demonstrate that our understanding of the Rb pathway and G1/S control is still incomplete. Here, we integrate these new findings with the previous paradigm to synthesize a current molecular and cellular view of the mammalian G1/S transition. A more complete and accurate understanding of G1/S control will lead to improved therapeutic strategies targeting the cell cycle in cancer.

Trichostatin A induces bladder cancer cell death via intrinsic apoptosis at the early phase and Sp1‑survivin downregulation at the late phase of treatment

Histone deacetylase (HDAC) inhibitors have been widely shown to result in cancer cell death. The present study investigated the mechanisms underlying the antitumor effects of the phytochemical trichostatin A (TSA), a classic pan-HDAC inhibitor, in 5,637 urinary bladder cancer cells. It was found that TSA caused cell cycle arrest at the G2/M and G1 phase accompanied by reduced expression of cyclin D1 and upregulated induction of p21. In addition, TSA induced morphological changes, reduced cell viability and apoptotic cell death in 5,637 cells through caspase-3 activation followed by PARP cleavage. The loss of mitochondrial membrane potential (MMP) indicated that TSA induced apoptosis in 5,637 cells through the intrinsic mitochondrial pathway. TSA significantly suppressed Akt activity at 12 h after treatment, suggesting that the apoptosis in the early phase was mediated by Akt inhibition. In addition, the protein level of transcription factor Sp1 was decreased at 24 h after TSA treatment, which likely led to the downregulation of survivin gene expression, and then contributed to the antitumor activity of TSA. Taken together, the present study delineated that TSA-induced growth inhibition and apoptosis in 5,637 cells was associated with pAKT inhibition and MMP loss at the early phase, followed by downregulation of Sp1 and survivin at the late phase of treatment.

Fangchinoline suppresses the proliferation, invasion and tumorigenesis of human osteosarcoma cells through the inhibition of PI3K and downstream signaling pathways

Osteosarcoma is the most common malignant bone tumor. Most patients diagnosed with osteosarcoma are less than 20 years of age. Osteosarcoma cells proliferate rapidly and invade other tissues. At present, neoadjuvant chemotherapy is the primary pharmacodynamic strategy to prevent the progression of osteosarcoma. However, adverse effects of this strategy limit its long-term application. Previous research has shown that fangchinoline exerts antitumor effects on several types of tumor cells; however, its effect on osteosarcoma cells remains unknown. The present study evaluated the effects of fangchinoline on the proliferation, apoptosis, migration and invasion of osteosarcoma cells in vitro and on their tumorigenesis in vivo and determined the possible underlying mechanism of action. Fangchinoline-treated MG63 and U20S cells showed significantly decreased proliferation and significantly increased apoptosis. Fangchinoline markedly suppressed the migration and invasion of the MG63 cells. Fangchinoline-treated MG63 cells showed significantly decreased expression of phosphoinositide 3-kinase (PI3K) and Akt^p-Thr308. Moreover, fangchinoline-treated MG63 cells showed downregulated expression of cyclin D1 and matrix metalloproteinase 2 and 9, which act downstream of PI3K, and upregulated expression of caspase-3 and caspase-8. Furthermore, fangchinoline suppressed the growth of subcutaneous osteosarcoma tumors in Balb/c mice subcutaneously injected with osteosarcoma cells. These findings suggest that fangchinoline inhibits the progression of osteosarcoma by suppressing the proliferation, migration and invasion and by accelerating the apoptosis of osteosarcoma cells. In addition, our results suggest that the mechanism underlying the antitumor effects of fangchinoline involve the inhibition of PI3K and its downstream signaling pathways.

Early and Late G1/S Cyclins and Cdks Act Complementarily to Enhance Authentic Human β-Cell Proliferation and Expansion

β-Cell regeneration is a key goal of diabetes research. Progression through the cell cycle is associated with retinoblastoma protein (pRb) inactivation via sequential phosphorylation by the "early" cyclins and cyclin-dependent kinases (cdks) (d-cyclins cdk4/6) and the "late" cyclins and cdks (cyclin A/E and cdk1/2). In β-cells, activation of either early or late G1/S cyclins and/or cdks is an efficient approach to induce cycle entry, but it is unknown whether the combined expression of early and late cyclins and cdks might have synergistic or additive effects. Thus, we explored whether a combination of both early and late cyclins and cdks might more effectively drive human β-cell cell cycle entry than either group alone. We also sought to determine whether authentic replication with the expansion of adult human β-cells could be demonstrated. Late cyclins and cdks do not traffic in response to the induction of replication by early cyclins and cdks in human β-cells but are capable of nuclear translocation when overexpressed. Early plus late cyclins and cdks, acting via pRb phosphorylation on distinct residues, complementarily induce greater proliferation in human β-cells than either group alone. Importantly, the combination of early and late cyclins and cdks clearly increased human β-cell numbers in vitro. These findings provide additional insight into human β-cell expansion. They also provide a novel tool for assessing β-cell expansion in vitro.

Cyclin d1 downregulation contributes to anticancer effect of isorhapontigenin on human bladder cancer cells

Isorhapontigenin (ISO) is a new derivative of stilbene compound that was isolated from the Chinese herb Gnetum Cleistostachyum and has been used for treatment of bladder cancers for centuries. In our current studies, we have explored the potential inhibitory effect and molecular mechanisms underlying isorhapontigenin anticancer effects on anchorage-independent growth of human bladder cancer cell lines. We found that isorhapontigenin showed a significant inhibitory effect on human bladder cancer cell growth and was accompanied with related cell cycle G(0)-G(1) arrest as well as downregulation of cyclin D1 expression at the transcriptional level in UMUC3 and RT112 cells. Further studies identified that isorhapontigenin downregulated cyclin D1 gene transcription via inhibition of specific protein 1 (SP1) transactivation. Moreover, ectopic expression of GFP-cyclin D1 rendered UMUC3 cells resistant to induction of cell-cycle G(0)-G(1) arrest and inhibition of cancer cell anchorage-independent growth by isorhapontigenin treatment. Together, our studies show that isorhapontigenin is an active compound that mediates Gnetum Cleistostachyum's induction of cell-cycle G(0)-G(1) arrest and inhibition of cancer cell anchorage-independent growth through downregulating SP1/cyclin D1 axis in bladder cancer cells. Our studies provide a novel insight into understanding the anticancer activity of the Chinese herb Gnetum Cleistostachyum and its isolate isorhapontigenin.

Role of p21 and cyclin E in normal and secalonic acid D-inhibited proliferation of human embryonic palatal mesenchymal cells

Secalonic acid D (SAD), a cleft palate-inducing teratogen, has been shown to inhibit proliferation/cell cycle progression in association with alteration in the levels of cell cycle regulators, p21 and cyclin E. These studies were conducted to test the hypotheses that p21 and cyclin E play an important functional role in normal human embryonic palatal mesenchymal (HEPM) cell cycle and that their up- and down-regulation, respectively, by SAD is functionally significant to its cell cycle block. Using small interfering RNA (siRNA) to silence p21 gene and transient transfection to overexpress cyclin E in control & SAD-treated HEPM cells, cell proliferation was assessed using a combination of cell numbers, thymidine uptake, CDK2 activity and Ki-67 expression. The results showed that silencing of p21 gene, although increased cell proliferation/numbers and CDK2 activity in normal HEPM cells, failed to counteract SAD-induced anti-proliferative effect despite inducing partial recovery of CDK2 activity. Similar effects were apparent with cyclin E overexpression. It is concluded that p21 and cyclin E are important for normal HEPM cell proliferation. However, SAD-induced deregulation of either protein, singly, may not be sufficient to induce anti-proliferative effect. Involvement of other cell cycle proteins such as cyclin D1 or of multiple proteins in SAD-induced cell cycle block needs to be examined.

The cancer/testis antigen CAGE with oncogenic potential stimulates cell proliferation by up-regulating cyclins D1 and E in an AP-1- and E2F-dependent manner

A cancer/testis antigen, CAGE, is widely expressed in various cancer tissues and cancer cell lines but not in normal tissues except the testis. In the present study, ectopic expression of CAGE in fibroblast cells resulted in foci formation, suggesting its cell-transforming ability. Using stable HeLa transfectant clones with the tetracycline-inducible CAGE gene, we found that CAGE overexpression stimulated both anchorage-dependent and -independent cell growth in vitro and promoted tumor growth in a xenograft mouse model. Cell cycle analysis showed that CAGE augments the levels of cyclin D1 and E, thereby activating cyclin-associated cyclin-dependent kinases and subsequently accelerating the G(1) to S progression. Moreover, increased cyclin D1 and E levels in CAGE-overexpressing cells were observed even in a growth arrested state, indicating a direct effect of CAGE on G(1) cyclin expression. CAGE-induced expression of cyclins D1 and E was found to be mediated by AP-1 and E2F-1 transcription factors, and among the AP-1 members, c-Jun and JunD appeared to participate in CAGE-mediated up-regulation of cyclin D1. CAGE overexpression also enhanced retinoblastoma phosphorylation and subsequent E2F-1 nuclear translocation. In contrast, small interfering RNA-mediated knockdown of CAGE suppressed the expression of G(1) cyclins, activation of AP-1 and E2F-1, and cell proliferation in both HeLa cervical cancer cells and Malme-3M melanoma cells. These results suggest that the cancer/testis antigen CAGE possesses oncogenic potential and promotes cell cycle progression by inducing AP-1- and E2F-dependent expression of cyclins D1 and E.

Towards a systems biology approach to mammalian cell cycle: modeling the entrance into S phase of quiescent fibroblasts after serum stimulation

Background

The cell cycle is a complex process that allows eukaryotic cells to replicate chromosomal DNA and partition it into two daughter cells. A relevant regulatory step is in the G₀/G₁ phase, a point called the restriction (R) point where intracellular and extracellular signals are monitored and integrated.

Subcellular localization of cell cycle proteins is increasingly recognized as a major factor that regulates cell cycle transitions. Nevertheless, current mathematical models of the G₁/S networks of mammalian cells do not consider this aspect. Hence, there is a need for a computational model that incorporates this regulatory aspect that has a relevant role in cancer, since altered localization of key cell cycle players, notably of inhibitors of cyclin-dependent kinases, has been reported to occur in neoplastic cells and to be linked to cancer aggressiveness.

Results

The network of the model components involved in the G₁ to S transition process was identified through a literature and web-based data mining and the corresponding wiring diagram of the G₁ to S transition drawn with Cell Designer notation. The model has been implemented in Mathematica using Ordinary Differential Equations. Time-courses of level and of sub-cellular localization of key cell cycle players in mouse fibroblasts re-entering the cell cycle after serum starvation/re-feeding have been used to constrain network design and parameter determination. The model allows to recapitulate events from growth factor stimulation to the onset of S phase. The R point estimated by simulation is consistent with the R point experimentally determined.

Conclusion

The major element of novelty of our model of the G₁ to S transition is the explicit modeling of cytoplasmic/nuclear shuttling of cyclins, cyclin-dependent kinases, their inhibitor and complexes. Sensitivity analysis of the network performance newly reveals that the biological effect brought about by Cki overexpression is strictly dependent on whether the Cki is promoting nuclear translocation of cyclin/Cdk containing complexes.

Cyclin D1 degradation is sufficient to induce G1 cell cycle arrest despite constitutive expression of cyclin E2 in ovarian cancer cells

D- and E-type cyclins mediate G(1)-S phase cell cycle progression through activation of specific cyclin-dependent kinases (cdk) that phosphorylate the retinoblastoma protein (pRb), thereby alleviating repression of E2F-DP transactivation of S-phase genes. Cyclin D1 is often overexpressed in a variety of cancers and is associated with tumorigenesis and metastasis. Loss of cyclin D can cause G(1) arrest in some cells, but in other cellular contexts, the downstream cyclin E protein can substitute for cyclin D and facilitate G(1)-S progression. The objective of this study was to determine if a flexible heteroarotinoid anticancer compound, SHetA2, regulates cell cycle proteins and cell cycle progression in ovarian cancer cells. SHetA2 induced cyclin D1 phosphorylation, ubiquitination, and proteasomal degradation, causing G(1) arrest in ovarian cancer cells despite continued cyclin E2 expression and independently of p53 and glycogen synthase kinase-3beta. Cyclin D1 loss inhibited pRb S780 phosphorylation by cyclin D1-cdk4/6 and released p21 from cyclin D1-cdk4/6-p21 protein complexes to form cyclin E2-cdk2-p21 complexes, which repressed phosphorylation of pRb S612 by cyclin E2-cdk2 and ultimately E2F-DP transcriptional activity. G(1) arrest was prevented by overexpression or preventing degradation of cyclin D1 but not by restoration of pRb S612 phosphorylation through p21 knockdown. In conclusion, we show that loss of cyclin D1 in ovarian cancer cells treated with SHetA2 is sufficient to induce G(1) cell cycle arrest and this strategy is not impeded by the presence of cyclin E2. Therefore, cyclin D1 is a sufficient therapeutic target in ovarian cancer cells.

Mammalian MCM loading in late-G(1) coincides with Rb hyperphosphorylation and the transition to post-transcriptional control of progression into S-phase

Background

Control of the onset of DNA synthesis in mammalian cells requires the coordinated assembly and activation of the pre-Replication Complex. In order to understand the regulatory events controlling preRC dynamics, we have investigated how the timing of preRC assembly relates temporally to other biochemical events governing progress into S-phase.

Methodology/Principal Finding

In murine and Chinese hamster (CHO) cells released from quiescence, the loading of the replicative MCM helicase onto chromatin occurs in the final 3–4 hrs of G₁. Cdc45 and PCNA, both of which are required for G₁-S transit, bind to chromatin at the G₁-S transition or even earlier in G₁, when MCMs load. An RNA polymerase II inhibitor (DRB) was added to synchronized murine keratinocytes to show that they are no longer dependent on new mRNA synthesis 3–4 hrs prior to S-phase entry, which is also true for CHO and human cells. Further, CHO cells can progress into S-phase on time, and complete S-phase, under conditions where new mRNA synthesis is significantly compromised, and such mRNA suppression causes no adverse effects on preRC dynamics prior to, or during, S-phase progression. Even more intriguing, hyperphosphorylation of Rb coincides with the start of MCM loading and, paradoxically, with the time in late-G₁ when de novo mRNA synthesis is no longer rate limiting for progression into S-phase.

Conclusions/Significance

MCM, Cdc45, and PCNA loading, and the subsequent transit through G₁-S, do not depend on concurrent new mRNA synthesis. These results indicate that mammalian cells pass through a distinct transition in late-G₁ at which time Rb becomes hyperphosphorylated and MCM loading commences, but that after this transition the control of MCM, Cdc45, and PCNA loading and the onset of DNA replication are regulated at the post-transcriptional level.

The rapid activation of N-Ras by alpha-thrombin in fibroblasts is mediated by the specific G-protein Galphai2-Gbeta1-Ggamma5 and occurs in lipid rafts

alpha-thrombin is a potent mitogen for fibroblasts and initiates a rapid signal transduction pathway leading to the activation of Ras and the stimulation of cell cycle progression. While the signaling events downstream of Ras have been studied in significant detail and appear well conserved across many species and cell types, the precise molecular events beginning with thrombin receptor activation and leading to the activation of Ras are not as well understood. In this study, we examined the immediate events in the rapid response to alpha-thrombin, in a single cell type, and found that an unexpected degree of specificity exists in the pathway linking alpha-thrombin to Ras activation. Specifically, although IIC9 cells express all three Ras isoforms, only N-Ras is rapidly activated by alpha-thrombin. Further, although several Galpha subunits associate with PAR1 and are released following stimulation, only Galpha(i2) couples to the rapid activation of Ras. Similarly, although IIC9 cells express many Gbeta and Ggamma subunits, only a subset associates with Galpha(i2), and of those, only a single Gbetagamma dimer, Gbeta(1)gamma(5), participates in the rapid activation of N-Ras. We then hypothesized that co-localization into membrane microdomains called lipid rafts, or caveolae, is at least partially responsible for this degree of specificity. Accordingly, we found that all components localize to lipid rafts and that disruption of caveolae abolishes the rapid activation of N-Ras by alpha-thrombin. We thus report the molecular elucidation of an extremely specific and rapid signal transduction pathway linking alpha-thrombin stimulation to the activation of Ras.

The extreme COOH terminus of the retinoblastoma tumor suppressor protein pRb is required for phosphorylation on Thr-373 and activation of E2F

The retinoblastoma protein pRb plays a pivotal role in G(1)- to S-phase cell cycle progression and is among the most frequently mutated gene products in human cancer. Although much focus has been placed on understanding how the A/B pocket and COOH-terminal domain of pRb cooperate to relieve transcriptional repression of E2F-responsive genes, comparatively little emphasis has been placed on the function of the NH(2)-terminal region of pRb and the interaction of the multiple domains of pRb in the full-length context. Using "reverse mutational analysis" of Rb(DeltaCDK) (a dominantly active repressive allele of Rb), we have previously shown that restoration of Thr-373 is sufficient to render Rb(DeltaCDK) sensitive to inactivation via cyclin-CDK phosphorylation. This suggests that the NH(2)-terminal region plays a more critical role in pRb regulation than previously thought. In the present study, we have expanded this analysis to include additional residues in the NH(2)-terminal region of pRb and further establish that the mechanism of pRb inactivation by Thr-373 phosphorylation is through the dissociation of E2F. Most surprisingly, we further have found that removal of the COOH-terminal domain of either RbDeltaCDK(+T373) or wild-type pRb yields a functional allele that cannot be inactivated by phosphorylation and is repressive of E2F activation and S-phase entry. Our data demonstrate a novel function for the NH(2)-terminal domain of pRb and the necessity for cooperation of multiple domains for proper pRb regulation.

A test of highly optimized tolerance reveals fragile cell-cycle mechanisms are molecular targets in clinical cancer trials

Robustness, a long-recognized property of living systems, allows function in the face of uncertainty while fragility, i.e., extreme sensitivity, can potentially lead to catastrophic failure following seemingly innocuous perturbations. Carlson and Doyle hypothesized that highly-evolved networks, e.g., those involved in cell-cycle regulation, can be resistant to some perturbations while highly sensitive to others. The “robust yet fragile” duality of networks has been termed Highly Optimized Tolerance (HOT) and has been the basis of new lines of inquiry in computational and experimental biology. In this study, we tested the working hypothesis that cell-cycle control architectures obey the HOT paradigm. Three cell-cycle models were analyzed using monte-carlo sensitivity analysis. Overall state sensitivity coefficients, which quantify the robustness or fragility of a given mechanism, were calculated using a monte-carlo strategy with three different numerical techniques along with multiple parameter perturbation strategies to control for possible numerical and sampling artifacts. Approximately 65% of the mechanisms in the G1/S restriction point were responsible for 95% of the sensitivity, conversely, the G2-DNA damage checkpoint showed a much stronger dependence on a few mechanisms; ∼32% or 13 of 40 mechanisms accounted for 95% of the sensitivity. Our analysis predicted that CDC25 and cyclin E mechanisms were strongly implicated in G1/S malfunctions, while fragility in the G2/M checkpoint was predicted to be associated with the regulation of the cyclin B-CDK1 complex. Analysis of a third model containing both G1/S and G2/M checkpoint logic, predicted in addition to mechanisms already mentioned, that translation and programmed proteolysis were also key fragile subsystems. Comparison of the predicted fragile mechanisms with literature and current preclinical and clinical trials suggested a strong correlation between efficacy and fragility. Thus, when taken together, these results support the working hypothesis that cell-cycle control architectures are HOT networks and establish the mathematical estimation and subsequent therapeutic exploitation of fragile mechanisms as a novel strategy for anti-cancer lead generation.

Centrosome duplication proceeds during mimosine-induced G1 cell cycle arrest

Centrosome duplication must remain coordinated with cell cycle progression to ensure the formation of a strictly bipolar mitotic spindle, but the mechanisms that regulate this coordination are poorly understood. Previous work has shown that prolonged S-phase is permissive for centrosome duplication, but prolonging either G2 or M-phase cannot support duplication. To examine whether G1 is permissive for centrosome duplication, we release serum-starved G0 cells into mimosine, which delays the cell cycle in G1. We find that in mimosine, centrosome duplication does occur, albeit slowly compared with cells that progress into S-phase; centrosome duplication in mimosine-treated cells also proceeds in the absence of a rise in Cdk2 kinase activity normally associated with the G1/S transition. CHO cells arrested with mimosine can also assemble more than four centrioles (termed "centrosome amplification"), but the extent of centrosome amplification during prolonged G1 is decreased compared to cells that enter S-phase and activate the Cdk2-cyclin complex. Together, our results suggest a model, which predicts that entry into S-phase and the rise in Cdk2 activity associated with this transition are not absolutely required to initiate centrosome duplication, but rather, serve to entrain the centrosome reproduction cycle with cell cycle progression.

CDK2 regulation through PI3K and CDK4 is necessary for cell cycle progression of primary rat hepatocytes

INTRODUCTION/OBJECTIVES

Cell cycle progression is driven by the coordinated regulation of cyclin-dependent kinases (CDKs). In response to mitogenic stimuli, CDK4 and CDK2 form complexes with cyclins D and E, respectively, and translocate to the nucleus in the late G(1) phase. It is an on-going discussion whether mammalian cells need both CDK4 and CDK2 kinase activities for induction of S phase.

METHODS AND RESULTS

In this study, we have explored the role of CDK4 activity during G(1) progression of primary rat hepatocytes. We found that CDK4 activity was restricted by either inhibiting growth factor induced cyclin D1-induction with the PI3K inhibitor LY294002, or by transient transfection with a dominant negative CDK4 mutant. In both cases, we observed reduced CDK2 nuclear translocation and reduced CDK2-Thr160 phosphorylation. Furthermore, reduced pRb hyperphosphorylation and reduced cellular proliferation were observed. Ectopic expression of cyclin D1 alone was not sufficient to induce CDK4 nuclear translocation, CDK2 activity or cell proliferation.

CONCLUSIONS

Thus, epidermal growth factor-induced CDK4 activity was necessary for CDK2 activation and for hepatocyte proliferation. These results also suggest that, in addition to regulating cyclin D1 expression, PI3K is involved in regulation of nuclear shuttling of cyclin-CDK complexes in G(1) phase.

Lycopene differentially induces quiescence and apoptosis in androgen-responsive and -independent prostate cancer cell lines

BACKGROUND & AIMS

Lycopene has been credited with a number of health benefits including a decrease in prostate cancer risk. Our study investigates the molecular mechanism underlying anti-cancer activity of lycopene-based products in androgen-responsive (LNCaP) and androgen-independent (PC3) cells.

METHODS

The effect of lycopene-based agents on prostate cancer growth and survival were examined using proliferation assays, bromodeoxyuridine incorporation and flow cytometric analysis of cellular DNA content. Biochemical effects of lycopene treatment were investigated by immunoblotting for changes in the absolute levels and phosphorylation states of cell cycle regulatory and signalling proteins.

RESULTS

LNCaP and PC3 cells treated with the lycopene-based agents undergo mitotic arrest, accumulating in G0/G1 phase. Immunoblot screening indicated that lycopene's antiproliferative effects are likely achieved through a block in G1/S transition mediated by decreased levels of cyclins D1 and E and cyclin dependent kinase 4 and suppressed Retinoblastoma phosphorylation. These responses correlated with decreased insulin-like growth factor-I receptor expression and activation, increased insulin-like growth factor binding protein 2 expression and decreased AKT activation. Exposure to lycopene at doses as low as 10 nM for 48 h induced a profound apoptotic response in LNCaP cells. In contrast PC3 cells were resistant to apoptosis at doses up to 1 microM.

CONCLUSIONS

Lycopene exposure can suppress phosphatidylinositol 3-kinase-dependent proliferative and survival signalling in androgen-responsive LNCaP and androgen-independent PC3 cells suggesting that the molecular mechanisms for the cytostatic and cytotoxic actions of lycopene involve induction of G0/G1 cell cycle arrest. This study supports further examination of lycopene as a potential agent for both the prevention and treatment of prostate cancer.

Cell cycle regulation by the Wee1 inhibitor PD0166285, pyrido [2,3-d] pyimidine, in the B16 mouse melanoma cell line

Background

Wee1 kinase plays a critical role in maintaining G2 arrest through its inhibitory phosphorylation of cdc2. In previous reports, a pyridopyrimidine molecule PD0166285 was identified to inhibit Wee1 activity at nanomolar concentrations. This G2 checkpoint abrogation by PD0166285 was demonstrated to kill cancer cells, there at a toxic highest dose of 0.5 μM in some cell lines for exposure periods of no longer than 6 hours. The deregulated cell cycle progression may have ultimately damaged the cancer cells. We herein report one of the mechanism by which PD0166285 leads to cell death in the B16 mouse melanoma cell line.

Methods

Tumor cell proliferation was determined by counting cell numbers. Cell cycle distribution was determined by flow cytometry. Morphogenesis analysis such as microtubule stabilization, Wee1 distribution, and cyclin B location were observed by immunofluorescence confocal microscopy. An immunoblot analysis of cdc2-Tyr15, cyclin D, E, p16, 21, 27, and Rb. A real-time PCR of the mRNA of cyclin D were completed.

Results

In our experiment, B16 cells also dramatically abrogated the G2 checkpoint and were found to arrest in the early G1 phase by treatment with 0.5 μM for 4 hours observed by flow cytometry. Cyclin D mRNA decreased within 4 hours observed by Real-time PCR. Rb was dephosphrylated for 24 hours. However, B16 cells did not undergo cell death after 0.5 μM treatment for 24 hours. Immnofluoscence microscopy showed that the cells become round and small in the morphogenesis. More interesting phenomena were that microtubule stabilization was blocked, and Wee1 distribution was restricted after treatment for 4 hours.

Conclusion

We analyzed the effect of Wee1 inhibitor PD0166285 described first by Wang in the G2 transition in the B16 melanoma cell line. The inhibitor PD0166285 abrogated G2/M checkpoint inducing early cell division. Moreover, we found that the treatment of cells with the inhibitor is related to microtubule stabilization and decrease in cyclin D transcription. These effects together suggest that Wee1 inhibitor may thus be a potentially useful anti-cancer therapy.

Inhibition of S/G2 phase CDK4 reduces mitotic fidelity

Cyclin-dependent kinase 4 (CDK4)/cyclin D has a key role in regulating progression through late G(1) into S phase of the cell cycle. CDK4-cyclin D complexes then persist through the latter phases of the cell cycle, although little is known about their potential roles. We have developed small molecule inhibitors that are highly selective for CDK4 and have used these to define a role for CDK4-cyclin D in G(2) phase. The addition of the CDK4 inhibitor or small interfering RNA knockdown of cyclin D3, the cyclin D partner, delayed progression through G(2) phase and mitosis. The G(2) phase delay was independent of ATM/ATR and p38 MAPK but associated with elevated Wee1. The mitotic delay was because of failure of chromosomes to migrate to the metaphase plate. However, cells eventually exited mitosis, with a resultant increase in cells with multiple or micronuclei. Inhibiting CDK4 delayed the expression of the chromosomal passenger proteins survivin and borealin, although this was unlikely to account for the mitotic phenotype. These data provide evidence for a novel function for CDK4-cyclin D3 activity in S and G(2) phase that is critical for G(2)/M progression and the fidelity of mitosis.

Geminin is bound to chromatin in G2/M phase to promote proper cytokinesis

Previous studies suggested that geminin plays a vital role in both origin assembly and DNA re-replication during S-phase; however, no data to support a role for geminin in G2/M cells have been described. Here it is shown that in G2/M-phase, geminin participates in the promotion of proper cytokinesis. This claim can be supported through a series of observations. First, geminin in G2/M is loaded onto chromatin after it is tyrosine phosphorylated. It is unlike S-phase geminin that resides in the nuclear soluble fraction, where it is exclusively S/T phosphorylated. Secondly, on chromatin, geminin gets S/T phosphorylated in late G1; this modification causes the release of geminin from the chromatin. Cyclins bind and phosphorylate geminin in a sequential, cell cycle-dependent manner. These modifications correlated well with geminin departure from the chromatin. This suggests that cyclin functions to either release geminin from chromatin or at least keep it at bay until late S-phase. Thirdly, depletion of geminin from a diploid mammary epithelial cell line (HME) causes cells to arrest in late G2/M-phase. Massive serine-10 phosphorylated histone H3 staining and survivin localization to mid-body were observed; this suggests that they could be arrested in either mitosis or at cytokinesis. Finally, while in the absence of geminin, cyclin B1, chk1 and cdc7 are all over expressed. This paper will demonstrate that only cdc7 is important in maintaining the cytokinesis arrest in the absence of geminin. Only double depletion of geminin and cdc7 induce apoptosis. Our results taken together show, for the first time, that phosphorylation-induction activates oscillation of geminin between both nuclear soluble and chromatin compartments. Chromatin-bound geminin species functions to initiate or maintain proper cytokineses. In the absence of geminin, cells arrest in cytokinesis; this defines a novel checkpoint, monitored by cdc7, rather than cyclin B1 or chk1.

A New IκB Kinase β Inhibitor Prevents Human Breast Cancer Progression through Negative Regulation of Cell Cycle Transition

Constitutive nuclear factor-kappaB (NF-kappaB) activity plays a crucial role in the development and progression of lymphoma, leukemia, and some epithelial cancers. Given the contribution of NF-kappaB in carcinogenesis, a novel approach that interferes with its activity might have therapeutic potential against cancers that respond poorly to conventional treatments. Here, we have shown that a new IkappaB kinase beta inhibitor, IMD-0354, suppressed the growth of human breast cancer cells, MDA-MB-231, HMC1-8, and MCF-7, by arresting cell cycle and inducing apoptosis. In an electrophoretic mobility shift assay and a reporter assay, IMD-0354 abolished the NF-kappaB activity in MDA-MB-231 cells in a dose-dependent manner. In the cells incubated with IMD-0354, cell cycle arrested at the G0-G1 phase and apoptotic cells were increased. The expression of some cell cycle regulatory molecules and antiapoptotic molecules was suppressed in cells treated with IMD-0354. On the other hand, cyclin-dependent kinase suppressor p27Kip1 was up-regulated by the addition of IMD-0354. Daily administration of IMD-0354 inhibited tumor expansion in immunodeficient mice into which MDA-MB-231 cells were transplanted. These results indicate that NF-kappaB may contribute to cell proliferation through up-regulation of cell cycle progression; accordingly, inhibition of NF-kappaB activity might have a therapeutic ability in the treatment of human breast cancers.

PDGF-C Controls proliferation and is down-regulated by retinoic acid in mouse embryonic palatal mesenchymal cells

BACKGROUND

Platelet-derived growth factor C (PDGF-C) was recently identified as a member of the PDGF ligand family. Some observation suggests that PDGF-C could play an important role in palatogenesis highlighted by the Pdgfc(-/-) mouse with cleft palate, which led us to examine the mechanism of PDGF-C signaling in palatogenesis. It is well known that retinoic acid (RA) is a teratogen that can effectively induce cleft palate in the mouse. Due to the critical roles of PDGF-C and RA in cleft palate, the link between cleft palate induced by RA and loss of PDGF-C was investigated.

METHODS

Retarded mesenchymal proliferation is an important cause for cleft palate. To clarify the mechanism of PDGF-C in palatogenesis, we evaluated the effects of PDGF-C and anti-PDGF-C neutralizing antibody on proliferation activity in mouse embryonic palatal mesenchymal (MEPM) cells.

RESULTS

Briefly, our results show PDGF-C promotes proliferation, anti-PDGF-C antibody inhibits it in MEPM cells, and RA downregulates the PDGF-C expression both at the mRNA and protein levels.

CONCLUSIONS

These demonstrate that PDGF-C is a potent mitogen for MEPM cells, implying that inactivated PDGF-C by gene-targeting or reduced PDGF-C by RA may both cause inhibition of proliferation in palatal shelves, which might account for the pathogenesis of cleft palate in Pdgfc(-/-) mouse or RA-treated mouse. In conclusion, our results suggest that PDGF-C signaling is a new mechanism of cleft palate induced by RA.

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The retinoblastoma protein--from bench to bedside

The retinoblastoma tumour suppressor protein (Rb) has come a long way since its initial discovery in 1986. Encoded by the first candidate tumour suppressor gene it has emerged a versatile and context-dependent modulator of cell behaviour. Its activity is managed by signalling networks sensing intra- and extracellular cues. These cues are relayed to hold or permit inactivation of Rb by phosphorylation. Loss or mutation of the retinoblastoma gene is rare in sporadic cancers but defects in the pathways that license inactivation of Rb are found in the majority of them, suggesting that loss of Rb control is central to tumour development and arguing that its reinstatement might reverse tumour formation. Furthermore, mouse models with engineered defects in the Rb-phosphorylating kinases provide evidence that moderation of Rb inactivation may be a strategy for the prevention of tumour formation. The rationale behind these arguments, their underlying molecular concepts and strategies towards therapeutic application will be discussed.

Signal transduction of phorbol 12-myristate 13-acetate (PMA)-induced growth inhibition of human monocytic leukemia THP-1 cells is reactive oxygen dependent

Human monocytic THP-1 cells can be induced to differentiate to macrophages when treated with phorbol 12-myristate 13-acetate (PMA). It is understood that before initiating cell differentiation, PMA treatment must first induce an inhibition of cell growth. Since the initial biochemical and molecular events that are associated with this growth inhibition have not been characterized, the present study was carried out to elucidate the molecular mechanisms associated with the PMA-induced growth arrest of THP-1 cells. Our results indicate that PMA inhibits THP-1 cells at G1-phase of the cell cycle, via a complex mechanism associated with the modulation of the expression of several cell cycle regulators, initiated by the cellular generation of reactive oxygen species (ROS). Both p21WAF1/CIP1 mRNA and protein were upregulated 24 h post PMA treatment as demonstrated by ribonuclease protection assay and Western blotting, respectively. Because these cells lack functional p53, this effect was independent of p53 activity. Electrophoretic mobility shift assay showed that the PMA-induced activation of the p21WAF1/CIP1 promoter was driven by the specific protein 1 (Sp1) transcription factor through Sp1-binding sites. Additionally, our study demonstrates that PMA-induces the upregulation of p21 through a protein kinase C (PKC)-mediated ROS-dependent signaling mechanism involving MAP kinase activation.

Mining for regulatory programs in the cancer transcriptome

DNA microarrays have been widely applied to cancer transcriptome analysis. The Oncomine database contains a large collection of such data, as well as hundreds of derived gene-expression signatures. We studied the regulatory mechanisms responsible for gene deregulation in these cancer signatures by searching for the coordinate regulation of genes with common transcription factor binding sites. We found that genes with binding sites for the archetypal cancer transcription factor, E2F, were disproportionately overexpressed in a wide variety of cancers, whereas genes with binding sites for other transcription factors, such as Myc-Max, c-Rel and ATF, were disproportionately overexpressed in specific cancer types. These results suggest that alterations in pathways activating these transcription factors may be responsible for the observed gene deregulation and cancer pathogenesis.

Patterns of Cyclin E, Retinoblastoma Protein, and p21Cip1/WAF1 Immunostaining in the Oncogenesis of Papillary Thyroid Carcinoma

Purpose: Uncontrolled cell proliferation, a hallmark of cancer, may result from an increased expression of cell cycle up-regulators, and/or from a reduced expression of cell cycle down-regulators. In the present study, we analyzed, by immunohistochemistry, the expression of a panel of three proteins: cyclin E and two cell cycle inhibitors, p21Cip1/WAF1 and retinoblastoma protein (pRb) product, in different stages of papillary thyroid carcinomas (PTC).

Experimental Design: We investigated immunostaining patterns of the proteins in question in 51 resected PTC in pathologic stages, ranging from pT1a to pT4, taking into consideration their relation to clinicohistopathologic factors.

Results: We observed a significant, progressive loss of expression of p21Cip1/WAF1 with advancing tumor grade. The differences reached values of significance between pT1a [papillary thyroid microcarcinomas (PMC)] and pT2 and between PMC and pT4 stages of PTC. pRb presented a similar immunostaining pattern to that of p21Cip1/WAF1 and the differences reached values of significance between pT1a and pT2, and between PMC and pT4 stages of PTC. The results of cyclin E immunostaining corresponded to our recently published result, and a negative correlation was observed between the immunostaining index of cyclin E and pRb.

Conclusions: The results of the present study suggest that cyclin E expression and suppression of pRb and p21Cip1/WAF1 may be characteristic patterns of immunostaining for PTC with a tendency to early metastasizing. If our results are confirmed in a larger study, the diagnostic panel, constructed of the antibodies against these proteins, may become a valuable tool in predicting the metastatic potential in PTC.