Dual mechanisms for the inhibition of E2F binding to RB by cyclin-dependent kinase-mediated RB phosphorylation

A Signal-Finding Study of Abemaciclib in Heavily Pretreated Patients with Metastatic Castration-Resistant Prostate Cancer: Results from CYCLONE 1

PURPOSE

Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors radically changed the treatment paradigm for breast cancer. Similar to estrogen receptor in breast cancer, androgen receptor signaling activates cyclin D-CDK4/6, driving proliferation and resistance to hormonal manipulation in prostate cancer. This study was designed to detect signals of clinical activity for abemaciclib in treatment-refractory metastatic castration-resistant prostate cancer (mCRPC).

PATIENTS AND METHODS

Eligible patients had progressive mCRPC, measurable disease, and previously received ≥1 novel hormonal agent(s) and 2 lines of taxane chemotherapy. Abemaciclib 200 mg twice daily was administered on a continuous dosing schedule. Primary endpoint was objective response rate (ORR) without concurrent bone progression. This study was designed to detect a minimum ORR of 12.5%.

RESULTS

At trial entry, 40 (90.9%) of 44 patients had objective radiographic disease progression, 4 (9.1%) had prostate-specific antigen (PSA)-only progression, and 20 (46.5%) had visceral metastases (of these, 60% had liver metastases). Efficacy analyses are as follows: ORR without concurrent bone progression: 6.8%; disease control rate: 45.5%; median time to PSA progression: 6.5 months [95% confidence interval (CI), 3.2-NA]; median radiographic PFS; 2.7 months (95% CI, 1.9-3.7); and median OS, 8.4 months (95% CI, 5.6-12.7). Most frequent grade ≥3 treatment-emergent adverse events (AE) were neutropenia (25.0%), anemia, and fatigue (11.4% each). No grade 4 or 5 AEs were related to abemaciclib.

CONCLUSIONS

Abemaciclib monotherapy was well tolerated and showed clinical activity in this heavily pretreated population, nearly half with visceral metastases. This study is considered preliminary proof-of-concept and designates CDK4/6 as a valid therapeutic target in prostate cancer.

The G1/S transition is promoted by Rb degradation via the E3 ligase UBR5

Mammalian cells make the decision to divide at the G1/S transition in response to diverse signals impinging on the retinoblastoma protein Rb, a cell cycle inhibitor and tumor suppressor. Rb is inhibited by two parallel pathways. In the canonical pathway, Cyclin D-Cdk4/6 kinase complexes phosphorylate and inactivate Rb. In the second, recently discovered pathway, Rb's concentration decreases during G1 to promote cells progressing through the G1/S transition. However, the mechanisms underlying this second pathway are unknown. Here, we found that Rb's concentration drop in G1 and recovery in S/G2 is controlled by phosphorylation-dependent protein degradation. In early G1 phase, un- and hypo-phosphorylated Rb is targeted by the E3 ligase UBR5. UBR5 knockout cells have higher Rb concentrations in early G1, exhibit a lower G1/S transition rate, and are more sensitive to Cdk4/6 inhibition. This last observation suggests that UBR5 inhibition can strengthen the efficacy of Cdk4/6 inhibitor-based cancer therapies.

Cell cycle plasticity underlies fractional resistance to palbociclib in ER+/HER2- breast tumor cells

The CDK4/6 inhibitor palbociclib blocks cell cycle progression in Estrogen receptor-positive, human epidermal growth factor 2 receptor-negative (ER+/HER2-) breast tumor cells. Despite the drug's success in improving patient outcomes, a small percentage of tumor cells continues to divide in the presence of palbociclib-a phenomenon we refer to as fractional resistance. It is critical to understand the cellular mechanisms underlying fractional resistance because the precise percentage of resistant cells in patient tissue is a strong predictor of clinical outcomes. Here, we hypothesize that fractional resistance arises from cell-to-cell differences in core cell cycle regulators that allow a subset of cells to escape CDK4/6 inhibitor therapy. We used multiplex, single-cell imaging to identify fractionally resistant cells in both cultured and primary breast tumor samples resected from patients. Resistant cells showed premature accumulation of multiple G1 regulators including E2F1, retinoblastoma protein, and CDK2, as well as enhanced sensitivity to pharmacological inhibition of CDK2 activity. Using trajectory inference approaches, we show how plasticity among cell cycle regulators gives rise to alternate cell cycle "paths" that allow individual tumor cells to escape palbociclib treatment. Understanding drivers of cell cycle plasticity, and how to eliminate resistant cell cycle paths, could lead to improved cancer therapies targeting fractionally resistant cells to improve patient outcomes.

Static electric field (SEF) exposure promotes the proliferation of B lymphocytes

With the rapid development of ultra-high voltage direct current (UHV DC) transmission technology, the intensity of electric fields in the surrounding environment of UHV DC transmission lines significantly increased, which raised public concerns about the potential health effects of electric fields. Previous studies have shown that the exposure of electromagnetic field was associated with cancer. B lymphocytes can produce autoantibodies and tumor growth factors through proliferation, which contributes to the development of cancer. Therefore, this study explored the effect and mechanism of static electric field (SEF) generated by DC transmission lines on the proliferation levels of B lymphocytes. Male mice were exposed to SEF. After the exposure of 7 and 14 days, the proliferation levels of B lymphocytes in the spleens of mice were measured, respectively. To validate biological effect discovered in animal experiments and elucidate the mechanism of the effect from the perspective of signaling pathways, lymphocytes were exposed to SEF. After the exposure of 24, 48 or 72 h, the proliferation levels of B lymphocytes, the expression levels of key proteins and cell cycle were determined. This study found that SEF exposure activated NF-κB pathway by stimulating ERK1/2 pathway and promoted B lymphocytes to enter S phase from G0/G1 phase. Meanwhile, SEF exposure also promoted B lymphocytes to enter G2 phase. Namely, SEF exposure significantly promoted the proliferation of B lymphocytes. This discovery provided theoretical and practical support for the prevention or application of negative or positive effects caused by SEF exposure and provided directions for future research.

Bromocriptine monotherapy overcomes prostate cancer chemoresistance in preclinical models

Chemoresistance is a major obstacle in the clinical management of metastatic, castration-resistant prostate cancer (PCa). It is imperative to develop novel strategies to overcome chemoresistance and improve clinical outcomes in patients who have failed chemotherapy. Using a two-tier phenotypic screening platform, we identified bromocriptine mesylate as a potent and selective inhibitor of chemoresistant PCa cells. Bromocriptine effectively induced cell cycle arrest and activated apoptosis in chemoresistant PCa cells but not in chemoresponsive PCa cells. RNA-seq analyses revealed that bromocriptine affected a subset of genes implicated in the regulation of the cell cycle, DNA repair, and cell death. Interestingly, approximately one-third (50/157) of the differentially expressed genes affected by bromocriptine overlapped with known p53-p21- retinoblastoma protein (RB) target genes. At the protein level, bromocriptine increased the expression of dopamine D2 receptor (DRD2) and affected several classical and non-classical dopamine receptor signal pathways in chemoresistant PCa cells, including adenosine monophosphate-activated protein kinase (AMPK), p38 mitogen-activated protein kinase (p38 MAPK), nuclear factor kappa B  (NF-κB), enhancer of zeste homolog 2 (EZH2), and survivin. As a monotherapy, bromocriptine treatment at 15 mg/kg, three times per week, via the intraperitoneal route significantly inhibited the skeletal growth of chemoresistant C4-2B-TaxR xenografts in athymic nude mice. In summary, these results provided the first preclinical evidence that bromocriptine is a selective and effective inhibitor of chemoresistant PCa. Due to its favorable clinical safety profiles, bromocriptine could be rapidly tested in PCa patients and repurposed as a novel subtype-specific treatment to overcome chemoresistance.

Cell growth and the cell cycle: New insights about persistent questions

Before a cell divides into two daughter cells, it typically doubles not only its DNA, but also its mass. Numerous studies in cells ranging from yeast to mammals have shown that cellular growth, stimulated by nutrients and/or growth factor signaling, is a prerequisite for cell cycle progression in most types of cells. The textbook view of growth-regulated cell cycles is that growth signaling activates the transcription of G1 Cyclin genes to induce cell proliferation, and also stimulates anabolic metabolism and cell growth in parallel. However, genetic knockout tests in model organisms indicate that this is not the whole story, and new studies show that additional, "smarter" mechanisms help to coordinate the cell cycle with growth itself. Here we summarize recent advances in this field, and discuss current models in which growth signaling regulates cell proliferation by targeting core cell cycle regulators via non-transcriptional mechanisms.

Genetic risk factors in melanoma etiopathogenesis and the role of genetic counseling: A concise review

Melanoma is a highly aggressive cancer originating from melanocytes. Its etiopathogenesis is strongly related to genetic, epigenetic, and environmental factors. Melanomas encountered in clinical practice are predominantly sporadic, whereas hereditary melanomas account for approximately 10% of the cases. Hereditary melanomas mainly develop due to mutations in the CDKN2A gene, which encodes two tumor suppressor proteins involved in the cell cycle regulation. CDKN2A, along with CDK4, TERT, and POT1 genes, is a high-risk gene for melanoma. Among the genes that carry a moderate risk are MC1R and MITF, whose protein products are involved in melanin synthesis. The environment also contributes to the development of melanoma. Patients at risk of melanoma should be offered genetic counseling to discuss genetic testing options and the importance of skin UV protection, avoidance of sun exposure, and regular preventive dermatological examinations. Although cancer screening cannot prevent the development of the disease, it allows for early diagnosis when the survival rate is the highest.

Understanding Retinoblastoma Post-Translational Regulation for the Design of Targeted Cancer Therapies

Simple Summary

Rb1 is a regulator of cell cycle progression and genomic stability. This review focuses on post-translational modifications, their effect on Rb1 interactors, and their role in intracellular signaling in the context of cancer development. Finally, we highlight potential approaches to harness these post-translational modifications to design novel effective anticancer therapies.

Abstract

The retinoblastoma protein (Rb1) is a prototypical tumor suppressor protein whose role was described more than 40 years ago. Together with p107 (also known as RBL1) and p130 (also known as RBL2), the Rb1 belongs to a family of structurally and functionally similar proteins that inhibits cell cycle progression. Given the central role of Rb1 in regulating proliferation, its expression or function is altered in most types of cancer. One of the mechanisms underlying Rb-mediated cell cycle inhibition is the binding and repression of E2F transcription factors, and these processes are dependent on Rb1 phosphorylation status. However, recent work shows that Rb1 is a convergent point of many pathways and thus the regulation of its function through post-translational modifications is more complex than initially expected. Moreover, depending on the context, downstream signaling can be both E2F-dependent and -independent. This review seeks to summarize the most recent research on Rb1 function and regulation and discuss potential avenues for the design of novel cancer therapies.

Hypophosphorylated pRb knock-in mice exhibit hallmarks of aging and vitamin C-preventable diabetes

Despite extensive analysis of pRB phosphorylation in vitro, how this modification influences development and homeostasis in vivo is unclear. Here, we show that homozygous Rb^∆K4 and Rb^∆K7 knock‐in mice, in which either four or all seven phosphorylation sites in the C‐terminal region of pRb, respectively, have been abolished by Ser/Thr‐to‐Ala substitutions, undergo normal embryogenesis and early development, notwithstanding suppressed phosphorylation of additional upstream sites. Whereas Rb^∆K4 mice exhibit telomere attrition but no other abnormalities, Rb^∆K7 mice are smaller and display additional hallmarks of premature aging including infertility, kyphosis, and diabetes, indicating an accumulative effect of blocking pRb phosphorylation. Diabetes in Rb^∆K7 mice is insulin‐sensitive and associated with failure of quiescent pancreatic β‐cells to re‐enter the cell cycle in response to mitogens, resulting in induction of DNA damage response (DDR), senescence‐associated secretory phenotype (SASP), and reduced pancreatic islet mass and circulating insulin level. Pre‐treatment with the epigenetic regulator vitamin C reduces DDR, increases cell cycle re‐entry, improves islet morphology, and attenuates diabetes. These results have direct implications for cell cycle regulation, CDK‐inhibitor therapeutics, diabetes, and longevity.

Monitoring Cellular Proliferation, Migration, and Apoptosis Associated with Atherosclerosis Plaques In Vitro

Bromodeoxyuridine/5-bromo-2'-deoxyuridine (BrdU) is a nucleoside analog of thymidine and its incorporation into DNA during replication within S-phase of the cell cycle is used to quantify cell proliferation. Quantification of incorporated BrdU is considered the most direct measure of cell proliferation, and here we describe BrdU incorporation into cultured vascular smooth muscle cells (VSMCs) and endothelial cells in vitro. Incorporation of fluorescent-labeled ethynyldeoxyuridine/5-ethynyl-2'-deoxyuridine (EdU) is a novel alternative to BrdU assays and presents significant advantages. This method of detection of EdU based on a simple "click" chemical reaction, which covalently bonds EdU to a fluorescent dye is also outlined in this chapter with a protocol for quantitative analysis of EdU incorporation using a Fiji-based macro. We also describe how proliferation can be assessed by quantification of classical proliferative markers such as phopsho-Ser807/811 retinoblastoma (Rb), proliferating cell nuclear antigen (PCNA) and cyclin D1 by Western blotting. As these markers are involved in different aspects of the cell cycle regulation, examining their expression levels can not only reveal the relative population of proliferating cells but can also improve our understanding of the mechanism of action of a given treatment or intervention. The scratch wound assay is a simple and cost-effective technique to quantify cell migration. A protocol which involves creating a wound in a cell cultured monolayer and measuring the distance migrated by the cells after a predefined time period is also described. Gap creation can also be achieved via physical cell exclusion where cells are seeded in distinct reservoirs of a cell culture insert which reveal a gap upon removal. Cell migration may then be quantified by monitoring the rate of gap closure. The presence of cleaved caspase-3 is a marker of programmed cell death (apoptosis). To detect cleaved caspase-3 in vitro, immunocytochemistry and fluorescence can be performed as outlined in this chapter.

Treatment of Retinoblastoma 1-Intact Hepatocellular Carcinoma With Cyclin-Dependent Kinase 4/6 Inhibitor Combination Therapy

BACKGROUND AND AIMS

Synthetic cyclin-dependent kinase (CDK) 4/6 inhibitors exert antitumor effects by forcing RB1 in unphosphorylated status, causing not only cell cycle arrest but also cellular senescence, apoptosis, and increased immunogenicity. These agents currently have an indication in advanced breast cancers and are in clinical trials for many other solid tumors. HCC is one of promising targets of CDK4/6 inhibitors. RB family dysfunction is often associated with the initiation of HCC; however, this is revivable, as RB family members are not frequently mutated or deleted in this malignancy.

APPROACH AND RESULTS

Loss of all Rb family members in transformation related protein 53 (Trp53)^-/- mouse liver resulted in liver tumor reminiscent of human HCC, and re-expression of RB1 sensitized these tumors to a CDK4/6 inhibitor, palbociclib. Introduction of an unphosphorylatable form of RB1 (RB7LP) into multiple liver tumor cell lines induced effects similar to palbociclib. By screening for compounds that enhance the efficacy of RB7LP, we identified an I kappa B kinase (IKK)β inhibitor Bay 11-7082. Consistently, RB7LP expression and treatment with palbociclib enhanced IKKα/β phosphorylation and NF-κB activation. Combination therapy using palbociclib with Bay 11-7082 was significantly more effective in hepatoblastoma and HCC treatment than single administration. Moreover, blockade of IKK-NF-κB or AKT pathway enhanced effects of palbociclib on RB1-intact KRAS Kirsten rat sarcoma viral oncogene homolog mutated lung and colon cancers.

CONCLUSIONS

In conclusion, CDK4/6 inhibitors have a potential to treat a wide variety of RB1-intact cancers including HCC when combined with an appropriate kinase inhibitor.

CEP78 functions downstream of CEP350 to control biogenesis of primary cilia by negatively regulating CP110 levels

CEP78 is a centrosomal protein implicated in ciliogenesis and ciliary length control, and mutations in the CEP78 gene cause retinal cone-rod dystrophy associated with hearing loss. However, the mechanism by which CEP78 affects cilia formation is unknown. Based on a recently discovered disease-causing CEP78 p.L150S mutation, we identified the disease-relevant interactome of CEP78. We confirmed that CEP78 interacts with the EDD1-DYRK2-DDB1^VPRBP E3 ubiquitin ligase complex, which is involved in CP110 ubiquitination and degradation, and identified a novel interaction between CEP78 and CEP350 that is weakened by the CEP78^L150S mutation. We show that CEP350 promotes centrosomal recruitment and stability of CEP78, which in turn leads to centrosomal recruitment of EDD1. Consistently, cells lacking CEP78 display significantly increased cellular and centrosomal levels of CP110, and depletion of CP110 in CEP78-deficient cells restored ciliation frequency to normal. We propose that CEP78 functions downstream of CEP350 to promote ciliogenesis by negatively regulating CP110 levels via an EDD1-dependent mechanism.

Hyperoxia Inhibits Proliferation of Retinal Endothelial Cells in a Myc-Dependent Manner

Oxygen supplementation is necessary to prevent mortality in severely premature infants. However, the supraphysiological concentration of oxygen utilized in these infants simultaneously creates retinovascular growth attenuation and vasoobliteration that induces the retinopathy of prematurity. Here, we report that hyperoxia regulates the cell cycle and retinal endothelial cell proliferation in a previously unknown Myc-dependent manner, which contributes to oxygen-induced retinopathy.

RB/E2F1 as a Master Regulator of Cancer Cell Metabolism in Advanced Disease

Loss of the retinoblastoma (RB) tumor suppressor protein is a critical step in reprogramming biological networks that drive cancer progression, although mechanistic insight has been largely limited to the impact of RB loss on cell-cycle regulation. Here, isogenic modeling of RB loss identified disease stage-specific rewiring of E2F1 function, providing the first-in-field mapping of the E2F1 cistrome and transcriptome after RB loss across disease progression. Biochemical and functional assessment using both in vitro and in vivo models identified an unexpected, prominent role for E2F1 in regulation of redox metabolism after RB loss, driving an increase in the synthesis of the antioxidant glutathione, specific to advanced disease. These E2F1-dependent events resulted in protection from reactive oxygen species in response to therapeutic intervention. On balance, these findings reveal novel pathways through which RB loss promotes cancer progression and highlight potentially new nodes of intervention for treating RB-deficient cancers. SIGNIFICANCE: This study identifies stage-specific consequences of RB loss across cancer progression that have a direct impact on tumor response to clinically utilized therapeutics. The study herein is the first to investigate the effect of RB loss on global metabolic regulation and link RB/E2F1 to redox control in multiple advanced diseases.This article is highlighted in the In This Issue feature, p. 2113.

Lamin B1 acetylation slows the G1 to S cell cycle transition through inhibition of DNA repair

The integrity and regulation of the nuclear lamina is essential for nuclear organization and chromatin stability, with its dysregulation being linked to laminopathy diseases and cancer. Although numerous posttranslational modifications have been identified on lamins, few have been ascribed a regulatory function. Here, we establish that lamin B1 (LMNB1) acetylation at K134 is a molecular toggle that controls nuclear periphery stability, cell cycle progression, and DNA repair. LMNB1 acetylation prevents lamina disruption during herpesvirus type 1 (HSV-1) infection, thereby inhibiting virus production. We also demonstrate the broad impact of this site on laminar processes in uninfected cells. LMNB1 acetylation negatively regulates canonical nonhomologous end joining by impairing the recruitment of 53BP1 to damaged DNA. This defect causes a delay in DNA damage resolution and a persistent activation of the G1/S checkpoint. Altogether, we reveal LMNB1 acetylation as a mechanism for controlling DNA repair pathway choice and stabilizing the nuclear periphery.

CVB3 VP1 interacts with MAT1 to inhibit cell proliferation by interfering with Cdk-activating kinase complex activity in CVB3-induced acute pancreatitis

Coxsackievirus B3 (CVB3) belongs to the genus Enterovirus of the family Picornaviridae and can cause acute acinar pancreatitis in adults. However, the molecular mechanisms of pathogenesis underlying CVB3-induced acute pancreatitis have remained unclear. In this study, we discovered that CVB3 capsid protein VP1 inhibited pancreatic cell proliferation and exerted strong cytopathic effects on HPAC cells. Through yeast two-hybrid, co-immunoprecipitation, and confocal microscopy, we show that Menage a trois 1 (MAT1), a subunit of the Cdk-Activating Kinase (CAK) complex involved in cell proliferation and transcription, is a novel interaction protein with CVB3 VP1. Moreover, CVB3 VP1 inhibited MAT1 accumulation and localization, thus interfering with its interaction with CDK7. Furthermore, CVB3 VP1 could suppress CAK complex enzymic phosphorylation activity towards RNA Pol II and CDK4/6, direct substrates of CAK. VP1 also suppresses phosphorylation of retinoblastoma protein (pRb), an indirect CAK substrate, especially at phospho-pRb Ser780 and phospho-pRb Ser807/811 residues, which are associated with cell proliferation. Finally, we present evidence using deletion mutants that the C-terminal domain (VP1-D8, 768-859aa) is the minimal VP1 region required for its interaction with MAT1, and furthermore, VP1-D8 alone was sufficient to arrest cells in G1/S phase as observed during CVB3 infection. Taken together, we demonstrate that CVB3 VP1 can inhibit CAK complex assembly and activity through direct interaction with MAT1, to block MAT1-mediated CAK-CDK4/6-Rb signaling, and ultimately suppress cell proliferation in pancreatic cells. These findings substantially extend our basic understanding of CVB3-mediated pancreatitis, providing strong candidates for strategic therapeutic targeting.

Overlapping targets exist between the Par-4 and miR-200c axis which regulate EMT and proliferation of pancreatic cancer cells

The last decade has witnessed a substantial expansion in the field of microRNA (miRNA) biology, providing crucial insights into the role of miRNAs in disease pathology, predominantly in cancer progression and its metastatic spread. The discovery of tumor-suppressing miRNAs represents a potential approach for developing novel therapeutics. In this context, through miRNA microarray analysis, we examined the consequences of Prostate apoptosis response-4 (Par-4), a well-established tumor-suppressor, stimulation on expression of different miRNAs in Panc-1 cells. The results strikingly indicated elevated miR-200c levels in these cells upon Par-4 overexpression. Intriguingly, the Reverse Phase Protein Array (RPPA) analysis revealed differentially expressed proteins (DEPs), which overlap between miR200c- and Par-4-transfected cells, highlighting the cross-talks between these pathways. Notably, Phospho-p44/42 MAPK; Bim; Bcl-xL; Rb Phospho-Ser807, Ser811; Akt Phospho-Ser473; Smad1/5 Phospho-Ser463/Ser465 and Zyxin scored the most significant DEPs among the two data sets. Furthermore, the GFP-Par-4-transfected cells depicted an impeded expression of critical mesenchymal markers viz. TGF-β1, TGF-β2, ZEB-1, and Twist-1, concomitant with augmented miR-200c and E-cadherin levels. Strikingly, while Par-4 overexpression halted ZEB-1 at the transcriptional level; contrarily, silencing of endogenous Par-4 by siRNA robustly augmented the Epithelial-mesenchymal transition (EMT) markers, along with declining miR-200c levels. The pharmacological Par-4-inducer, NGD16, triggered Par-4 expression which corresponded with increased miR-200c resulting in the ZEB-1 downregulation. Noteworthily, tumor samples obtained from the syngenic mouse pancreatic cancer model revealed elevated miR-200c levels in the NGD16-treated mice that positively correlated with the Par-4 and E-cadherin levels in vivo; while a negative correlation was evident with ZEB-1 and Vimentin.

•

Prostate apoptosis response-4 (Par-4) stimulation elevates the endogenous miR-200c levels

•

Par-4- mediated miR-200c induction modulates the ZEB-1/miR-200c axis

•

Pharmacological Par-4 inducer, NGD16, boosts the miR-200c and E-cadherin levels in vivo.

•

Overlapping targets between miR 200c and Par-4 signaling axis highlight the cross-talks between these pathways.

Cx43 and AKAP95 regulate G1/S conversion by competitively binding to cyclin E1/E2 in lung cancer cells

Background

This study aimed to overexpress or silence connexin 43 (Cx43) and A‐kinase anchoring protein 95 (AKAP95) in human A549 cells to explore their effects on cyclins and on G1/S conversion when the interrelationship of Cx43, AKAP95, and cyclin E1/E2 changes.

Methods

The study mainly used Western blot analysis and Co‐immuno precipitation to detect the target protein in Cx43/AKAP95 over expressed human A549 cells, and the relationship of proteins Cx43, AKAP95 and Cyclin E during G1‐S phase was explored with qualitative and quantitative analysis.

Results

The overexpression of Cx43 inhibited the expression of cyclin D1 and E1 by accelerating their degradation and reduced the Cdk2 activity that blocked the DNA transcription activity. However, the overexpression of AKAP95 increased the expression of cyclin D1 and E1 and inhibited their degradation, and enhanced the Cdk2 activity that promoted the DNA transcription activity. Cx43 and AKAP95 competitively bound to cyclin E1/E2, and the competitive binding affected the Cdk2 activity, Rb phosphorylation, DNA transcription activity, and G1/S conversion.

Conclusions

This study showed that the expression of ERK1/2, PKA, and PKB increased when BEAS‐2B cells were treated with PDGF‐BB, suggesting that ERK1/2, PKA, and PKB might be involved in the binding of AKAP95 with cyclin E, or the separation of AKAP95 from Cx43 from cyclin E1/E2. The specific mechanism underlying this process still needs further exploration.

Pan-cancer molecular analysis of the RB tumor suppressor pathway

The retinoblastoma tumor suppressor gene (RB1) plays a critical role in coordinating multiple pathways that impact cancer initiation, disease progression, and therapeutic responses. Here we probed molecular features associated with the RB-pathway across 31 tumor-types. While the RB-pathway has been purported to exhibit multiple mutually exclusive genetic events, only RB1 alteration is mutually exclusive with deregulation of CDK4/6 activity. An ER+ breast cancer model with targeted RB1 deletion was used to identify signatures of CDK4/6 activity and RB-dependency (CDK4/6-RB integrated signature). This signature was prognostic in tumor-types with gene expression features indicative of slower growth. Single copy loss on chromosome 13q encompassing the RB1 locus is prevalent in many cancers, yielding reduced expression of multiple genes in cis, and is inversely related to the CDK4/6-RB integrated signature supporting a cause-effect relationship. Genes that are positively and inversely correlated with the CDK4/6-RB integrated signature define new tumor-specific pathways associated with RB-pathway activity.

Unveiling the transcriptome alteration of POMC neuron in diet-induced obesity

Loss of neuron homeostasis in the arcuate nucleus (ARC) is responsible for diet-induced-obesity (DIO). We previously reported that loss of Rb1 gene compromised the homeostasis of anorexigenic POMC neurons in ARC and induced obesity in mice. To evaluate the development of DIO, we propose to analyze the transcriptomic alteration of POMC neurons in mice following high fat diet (HFD) feeding. We isolated these neurons from established DIO mice and performed transcriptomic profiling using RNA-seq. In total, 1066 genes (628 upregulated and 438 downregulated) were identified as differentially expressed genes (DEGs). Pathway enrichment analysis with these DEGs further revealed that "cell cycle," "apoptosis," "chemokine signaling," and "sphingolipid metabolism" pathways were correlated with DIO development. Moreover, we validated that the pRb protein, a key regulator of "cell cycle pathway," was inactivated by phosphorylation in POMC neurons by HFD feeding. Importantly, the reversal of deregulated cell cycle by stereotaxic delivering of the unphosphorylated pRbΔP in ARC significantly meliorated the DIO. Collectively, our study provides insights into the mechanisms related to the loss of homeostasis of POMC neurons in DIO, and suggests pRb phosphorylation as a potential intervention target to treat DIO.

CRISPR/Cas9 Editing of the Polyomavirus Tumor Antigens Inhibits Merkel Cell Carcinoma Growth In Vitro

Merkel cell carcinoma (MCC) is an aggressive type of skin cancer whose main causative agent is Merkel cell polyomavirus (MCPyV). MCPyV is integrated into the genome of the tumor cells in most MCCs. Virus-positive tumor cells constitutively express two viral oncoproteins that promote cell growth: the small (sT) and the large (LT) tumor antigens (TAs). Despite the success of immunotherapies in patients with MCC, not all individuals respond to these treatments. Therefore, new therapeutic options continue to be investigated. Herein, we used CRISPR/Cas9 to target the viral oncogenes in two virus-positive MCC cell lines: MS-1 and WAGA. Frameshift mutations introduced in the target sequence upon repair of the Cas9-induced DNA break resulted in decreased LT protein levels, which subsequently impaired cell proliferation, caused cell cycle arrest, and led to increased apoptosis. Importantly, a virus-negative non-MCC cell line (HEK293T) remained unaffected, as well as those cells expressing a non-targeting single-guide RNA (sgRNA). Thus, we presumed that the noted effects were not due to the off-target activity of the TAs-targeting sgRNAs. Additionally, WAGA cells had altered levels of cellular proteins involved in cell cycle regulation, supporting the observed cell cycle. Taken together, our findings provide evidence for the development of a CRISPR/Cas9-based therapeutic option for virus-positive MCC.

Retinoblastoma Inactivation Induces a Protumoral Microenvironment via Enhanced CCL2 Secretion

Cancer cell-intrinsic properties caused by oncogenic mutations have been well characterized; however, how specific oncogenes and tumor suppressors impact the tumor microenvironment (TME) is not well understood. Here, we present a novel non-cell-autonomous function of the retinoblastoma (RB) tumor suppressor in controlling the TME. RB inactivation stimulated tumor growth and neoangiogenesis in a syngeneic and orthotropic murine soft-tissue sarcoma model, which was associated with recruitment of tumor-associated macrophages (TAM) and immunosuppressive cells such as Gr1⁺CD11b⁺ myeloid-derived suppressor cells (MDSC) or Foxp3⁺ regulatory T cells (Treg). Gene expression profiling and analysis of genetically engineered mouse models revealed that RB inactivation increased secretion of the chemoattractant CCL2. Furthermore, activation of the CCL2-CCR2 axis in the TME promoted tumor angiogenesis and recruitment of TAMs and MDSCs into the TME in several tumor types including sarcoma and breast cancer. Loss of RB increased fatty acid oxidation (FAO) by activating AMP-activated protein kinase that led to inactivation of acetyl-CoA carboxylase, which suppresses FAO. This promoted mitochondrial superoxide production and JNK activation, which enhanced CCL2 expression. These findings indicate that the CCL2-CCR2 axis could be an effective therapeutic target in RB-deficient tumors. SIGNIFICANCE: These findings demonstrate the cell-nonautonomous role of the tumor suppressor retinoblastoma in the tumor microenvironment, linking retinoblastoma loss to immunosuppression.

CEP128 Localizes to the Subdistal Appendages of the Mother Centriole and Regulates TGF-β/BMP Signaling at the Primary Cilium

The centrosome is the main microtubule-organizing center in animal cells and comprises a mother and daughter centriole surrounded by pericentriolar material. During formation of primary cilia, the mother centriole transforms into a basal body that templates the ciliary axoneme. Ciliogenesis depends on mother centriole-specific distal appendages, whereas the role of subdistal appendages in ciliary function is unclear. Here, we identify CEP128 as a centriole subdistal appendage protein required for regulating ciliary signaling. Loss of CEP128 did not grossly affect centrosomal or ciliary structure but caused impaired transforming growth factor-β/bone morphogenetic protein (TGF-β/BMP) signaling in zebrafish and at the primary cilium in cultured mammalian cells. This phenotype is likely the result of defective vesicle trafficking at the cilium as ciliary localization of RAB11 was impaired upon loss of CEP128, and quantitative phosphoproteomics revealed that CEP128 loss affects TGF-β1-induced phosphorylation of multiple proteins that regulate cilium-associated vesicle trafficking.

Proteogenomic Analysis of Human Colon Cancer Reveals New Therapeutic Opportunities

We performed the first proteogenomic study on a prospectively collected colon cancer cohort. Comparative proteomic and phosphoproteomic analysis of paired tumor and normal adjacent tissues produced a catalog of colon cancer-associated proteins and phosphosites, including known and putative new biomarkers, drug targets, and cancer/testis antigens. Proteogenomic integration not only prioritized genomically inferred targets, such as copy-number drivers and mutation-derived neoantigens, but also yielded novel findings. Phosphoproteomics data associated Rb phosphorylation with increased proliferation and decreased apoptosis in colon cancer, which explains why this classical tumor suppressor is amplified in colon tumors and suggests a rationale for targeting Rb phosphorylation in colon cancer. Proteomics identified an association between decreased CD8 T cell infiltration and increased glycolysis in microsatellite instability-high (MSI-H) tumors, suggesting glycolysis as a potential target to overcome the resistance of MSI-H tumors to immune checkpoint blockade. Proteogenomics presents new avenues for biological discoveries and therapeutic development.

SMAD3 directly regulates cell cycle genes to maintain arrest in granulosa cells of mouse primordial follicles

Primordial follicles, consisting of granulosa cell (GC)-enveloped oocytes are maintained in a state of developmental arrest until activated to grow. The mechanism that operates to maintain this arrested state in GCs is currently unknown. Here, we show the TGFβ-activated transcription factor SMAD3 is expressed in primordial GC nuclei alongside the cell cycle proteins, cyclin D2 (CCND2) and P27. Using neonatal C57/Bl6 mouse ovaries densely populated with primordial follicles, CCND2 protein co-localised and was detected in complex with P27 by immunofluorescence and co-immunoprecipitation, respectively. In the same tissue, SMAD3 co-precipitated with DNA sequences upstream of Ccnd2 and Myc transcription start sites implicating both as direct SMAD3 targets. In older ovaries follicle growth was associated with nuclear exclusion of SMAD3 and reduced P27 and CCND2 in GCs, alongside elevated Myc expression. Brief (2 H) exposure of neonatal ovaries to TGFβ1 (10 ng/ml) in vitro led to immediate dissociation of SMAD3 from the Ccnd2 and Myc promoters. This coincided with elevated Myc and phospho-S6, an indicator of mTOR signalling, followed by a small increase in mean primordial GC number after 48 H. These findings highlight a concentration-dependent role for TGFβ signalling in the maintenance and activation of primordial follicles, through SMAD-dependent and independent signalling pathways, respectively.

E2F is required for STAT3-mediated upregulation of cyclin B1 and Cdc2 expressions and contributes to G2-M phase transition

Activation of transcription factor STAT3 is involved in cell proliferation, differentiation, and cell survival. Constitutive activation of STAT3 pathway has been associated with the oncogenesis of various types of cancers. It has been reported that STAT3 plays a key role in the G1 to S phase cell cycle transition induced by the cytokine receptor subunit gp130, through the upregulation of cyclins D1, D2, D3, A, and Cdc25A and the concomitant downregulation of p21 and p27. However, its role in mediating G2-M phase transition has not been studied. The cyclin B1/Cdc2 complex is widely accepted as the trigger of mitosis in all organisms and is believed to be necessary for progression through S phase and keep active during the G2-M transition and progression. In the present study, we found that activation of STAT3 stimulates cyclin B1 and Cdc2 expressions. Deletion and site-directed mutations on cyclin B1 and Cdc2 promoters indicated that E2F element mediates the upregulation of these two promoters in a STAT3-dependent manner. The findings reported here demonstrated that STAT3 participates in modulating G2-M phase checkpoint by regulating gene expressions of cyclin B1 and Cdc2 via E2F.

Involvement of the STAT5-cyclin D/CDK4-pRb pathway in β-cell proliferation stimulated by prolactin during pregnancy

During pregnancy, maternal pancreatic β-cells undergo a compensatory expansion in response to the state of insulin resistance, where prolactin (PRL) plays a major role. Retinoblastoma protein (Rb) has been shown to critically regulate islet proliferation and function. The aim of the study was to explore the role of Rb in β-cell mass expansion during pregnancy. Expression of pocket protein family and E2Fs were examined in mouse islets during pregnancy and in insulinoma cells (INS-1) stimulated by PRL. PRL-stimulated INS-1 cells were used to explore the signaling pathway that regulates Rb downstream of the PRL receptor. Pancreas-specific Rb-knockout (Rb-KO) mice were assessed to evaluate the in vivo function of Rb in β-cell proliferation during pregnancy. During pregnancy, expression of Rb, phospho-Rb (p-Rb), p107, and E2F1 increased, while p130 decreased in maternal islets. With PRL stimulation, induction of Rb expression occurred mainly in the nucleus, while p-Rb was predominantly in the cytoplasm. Inhibition of STAT5 significantly restrained the expression of CDK4, Rb, p-Rb, and E2F1 in PRL-stimulated INS-1 cells with attenuation in cell cycle progression. Reduction of Rb phosphorylation by CDK4 inhibition blocked PRL-mediated proliferation of INS-1 cells. On the other hand, knockdown of Rb using siRNA led to an induction in E2F1 leading to cell cycle progression from G₁ to S and G₂/M phase, similar to the effects of PRL-mediated induction of p-Rb that led to cell proliferation. With Rb knockdown, PRL did not lead to further increase in cell cycle progression. Similarly, while Rb-KO pregnant mice displayed better glucose tolerance and higher insulin secretion, they had similar β-cell mass and proliferation to wild-type pregnant controls, supporting the essential role of Rb suppression in augmenting β-cell proliferation during pregnancy. Rb-E2F1 regulation plays a pivotal role in PRL-stimulated β-cell proliferation. PRL promotes Rb phosphorylation and E2F1 upregulation via STAT5-cyclin D/CDK4 pathway during pregnancy.

Microtubules play an essential role in the survival of primary acute lymphoblastic leukemia cells advancing through G1 phase

We recently reported that primary acute lymphoblastic leukemia (ALL) cells are susceptible to the microtubule depolymerizing agent vincristine (VCR) in G1 phase. This finding prompted testing another G1 phase-active compound, palbociclib (PCB), a highly selective inhibitor of cyclin-dependent kinases 4/6 (CDK4/6), alone and in combination with VCR. PCB used alone caused G1 arrest in ALL cells with no effect on cell viability, and similar results were obtained for the retinoblastoma (RB)-proficient T98G glioblastoma cell line. In contrast, HeLa cells failed to arrest in the presence of PCB, consistent with their lack of dependence on the CDK4/6-RB pathway. When ALL cells were pretreated with PCB, they became refractory to death in G1 phase induced by VCR treatment, whereas HeLa cells retained VCR sensitivity after PCB pretreatment. Immunofluorescence microscopy showed that PCB did not disrupt the microtubule network nor prevent VCR from doing so. Furthermore, ALL cells pretreated with PCB retained susceptibility to the Bcl-2/Bcl-xL inhibitor ABT-263, indicating that downstream apoptotic signaling was unaffected. When released from PCB-enforced arrest, ALL cells reinitiated cycling and regained sensitivity to VCR. ALL cells treated with cycloheximide also arrested in G1 phase and became insensitive to VCR, independently reinforcing conclusions derived from PCB-imposed arrest. Thus, primary ALL cells advancing through G1 phase are strictly dependent on functional microtubules for survival whereas microtubules are dispensable for G1-arrested cells. These findings provide novel insight into interphase microtubule function and, from a therapy standpoint, strongly caution against combining microtubule targeting agents and CDK4/6 inhibitors for ALL.

The Strange Case of CDK4/6 Inhibitors: Mechanisms, Resistance, and Combination Strategies

CDK4/6 inhibitors have emerged as a powerful class of agents with clinical activity in a number of malignancies. Targeting the cell cycle represents a core attack on a defining feature of cancer. However, the mechanisms through which selective CDK4/6 targeted agents act has few parallels in the current pharmaceutical armamentarium against cancer. Notably, CDK4/6 inhibitors act downstream of most mitogenic signaling cascades, which have implications both related to clinical efficacy and resistance. Core knowledge of cell cycle processes has provided insights into mechanisms of intrinsic resistance to CDK4/6 inhibitors; however, the basis of acquired resistance versus durable response is only beginning to emerge. This review focuses on the mechanism of action and biomarkers to direct the precision use of CDK4/6 inhibitors and rationally-developed combination therapies.

Short Telomeres Induce p53 and Autophagy and Modulate Age-Associated Changes in Cardiac Progenitor Cell Fate

Aging severely limits myocardial repair and regeneration. Delineating the impact of age-associated factors such as short telomeres is critical to enhance the regenerative potential of cardiac progenitor cells (CPCs). We hypothesized that short telomeres activate p53 and induce autophagy to elicit the age-associated change in CPC fate. We isolated CPCs and compared mouse strains with different telomere lengths for phenotypic characteristics of aging. Wild mouse strain Mus musculus castaneus (CAST) possessing short telomeres exhibits early cardiac aging with cardiac dysfunction, hypertrophy, fibrosis, and senescence, as compared with common lab strains FVB and C57 bearing longer telomeres. CAST CPCs with short telomeres demonstrate altered cell fate as characterized by cell cycle arrest, senescence, basal commitment, and loss of quiescence. Elongation of telomeres using a modified mRNA for telomerase restores youthful properties to CAST CPCs. Short telomeres induce autophagy in CPCs, a catabolic protein degradation process, as evidenced by reduced p62 and increased accumulation of autophagic puncta. Pharmacological inhibition of autophagosome formation reverses the cell fate to a more youthful phenotype. Mechanistically, cell fate changes induced by short telomeres are partially p53 dependent, as p53 inhibition rescues senescence and commitment observed in CAST CPCs, coincident with attenuation of autophagy. In conclusion, short telomeres activate p53 and autophagy to tip the equilibrium away from quiescence and proliferation toward differentiation and senescence, leading to exhaustion of CPCs. This study provides the mechanistic basis underlying age-associated cell fate changes that will enable identification of molecular strategies to prevent senescence of CPCs. Stem Cells 2018;36:868-880.

Preparation of Primary Acute Lymphoblastic Leukemia Cells in Different Cell Cycle Phases by Centrifugal Elutriation

The ability to synchronize cells has been central to advancing our understanding of cell cycle regulation. Common techniques employed include serum deprivation; chemicals which arrest cells at different cell cycle phases; or the use of mitotic shake-off which exploits their reduced adherence. However, all of these have disadvantages. For example, serum starvation works well for normal cells but less well for tumor cells with compromised cell cycle checkpoints due to oncogene activation or tumor suppressor loss. Similarly, chemically-treated cell populations can harbor drug-induced damage and show stress-related alterations. A technique which circumvents these problems is counterflow centrifugal elutriation (CCE), where cells are subjected to two opposing forces, centrifugal force and fluid velocity, which results in the separation of cells on the basis of size and density. Since cells advancing through the cycle typically enlarge, CCE can be used to separate cells into different cell cycle phases. Here we apply this technique to primary acute lymphoblastic leukemia cells. Under optimal conditions, an essentially pure population of cells in G1 phase and a highly enriched population of cells in G2/M phases can be obtained in excellent yield. These cell populations are ideally suited for studying cell cycle-dependent mechanisms of action of anticancer drugs and for other applications. We also show how modifications to the standard procedure can result in suboptimal performance and discuss the limitations of the technique. The detailed methodology presented should facilitate application and exploration of the technique to other types of cells.

MicroRNA-140 mediates RB tumor suppressor function to control stem cell-like activity through interleukin-6

We established an in vitro cell culture system to determine novel activities of the retinoblastoma (Rb) protein during tumor progression. Rb depletion in p53-null mouse-derived soft tissue sarcoma cells induced a spherogenic phenotype. Cells retrieved from Rb-depleted spheres exhibited slower proliferation and less efficient BrdU incorporation, however, much higher spherogenic activity and aggressive behavior. We discovered six miRNAs, including mmu-miR-18a, -25, -29b, -140, -337, and -1839, whose expression levels correlated tightly with the Rb status and spherogenic activity. Among these, mmu-miR-140 appeared to be positively controlled by Rb and to antagonize the effect of Rb depletion on spherogenesis and tumorigenesis. Furthermore, among genes potentially targeted by mmu-miR-140, Il-6 was upregulated by Rb depletion and downregulated by mmu-mir-140 overexpression. Altogether, we demonstrate the possibility that mmu-mir-140 mediates the Rb function to downregulate Il-6 by targeting its 3′-untranslated region. Finally, we detected the same relationship among RB, hsa-miR-140 and IL-6 in a human breast cancer cell line MCF-7. Because IL-6 is a critical modulator of malignant features of cancer cells and the RB pathway is impaired in the majority of cancers, hsa-miR-140 might be a promising therapeutic tool that disrupts linkage between tumor suppressor inactivation and pro-inflammatory cytokine response.

Tumor microenvironment conditions alter Akt and Na+/H+ exchanger NHE1 expression in endothelial cells more than hypoxia alone: implications for endothelial cell function in cancer

BACKGROUND

Chronic angiogenesis is a hallmark of most tumors and takes place in a hostile tumor microenvironment (TME) characterized by hypoxia, low nutrient and glucose levels, elevated lactate and low pH. Despite this, most studies addressing angiogenic signaling use hypoxia as a proxy for tumor conditions. Here, we compared the effects of hypoxia and TME conditions on regulation of the Na⁺/H⁺ exchanger NHE1, Ser/Thr kinases Akt1-3, and downstream effectors in endothelial cells.

METHODS

Human umbilical vein endothelial cells (HUVEC) and Ea.hy926 endothelial cells were exposed to simulated TME (1% hypoxia, low serum, glucose, pH, high lactate) or 1% hypoxia for 24 or 48 h, with or without NHE1 inhibition or siRNA-mediated knockdown. mRNA and protein levels of NHE1, Akt1-3, and downstream effectors were assessed by qPCR and Western blotting, vascular endothelial growth factor (VEGF) release by ELISA, and motility by scratch assay.

RESULTS

Within 24 h, HIF-1α level and VEGF mRNA level were increased robustly by TME and modestly by hypoxia alone. The NHE1 mRNA level was decreased by both hypoxia and TME, and NHE1 protein was reduced by TME in Ea.hy926 cells. Akt1-3 mRNA was detected in HUVEC and Ea.hy926 cells, Akt1 most abundantly. Akt1 protein expression was reduced by TME yet unaffected by hypoxia, while Akt phosphorylation was increased by TME. The Akt loss was partly reversed by MCF-7 human breast cancer cell conditioned medium, suggesting that in vivo, the cancer cell secretome may compensate for adverse effects of TME on endothelial cells. TME, yet not hypoxia, reduced p70S6 kinase activity and ribosomal protein S6 phosphorylation and increased eIF2α phosphorylation, consistent with inhibition of protein translation. Finally, TME reduced Retinoblastoma protein phosphorylation and induced poly-ADP-ribose polymerase (PARP) cleavage consistent with inhibition of proliferation and induction of apoptosis. NHE1 knockdown, mimicking the effect of TME on NHE1 expression, reduced Ea.hy926 migration. TME effects on HIF-1α, VEGF, Akt, translation, proliferation or apoptosis markers were unaffected by NHE1 knockdown/inhibition.

CONCLUSIONS

NHE1 and Akt are downregulated by TME conditions, more potently than by hypoxia alone. This inhibits endothelial cell migration and growth in a manner likely modulated by the cancer cell secretome.

BRG1 and BRM loss selectively impacts RB and P53, respectively: BRG1 and BRM have differential functions in vivo

The SWI/SNF complex is an important regulator of gene expression that functions by interacting with a diverse array of cellular proteins. The catalytic subunits of SWI/SNF, BRG1 and BRM, are frequently lost alone or concomitantly in a range of different cancer types. This loss abrogates SWI/SNF complex function as well as the functions of proteins that are required for SWI/SNF function, such as RB1 and TP53. Yet while both proteins are known to be dependent on SWI/SNF, we found that BRG1, but not BRM, is functionally linked to RB1, such that loss of BRG1 can directly or indirectly inactivate the RB1 pathway. This newly discovered dependence of RB1 on BRG1 is important because it explains why BRG1 loss can blunt the growth-inhibitory effect of tyrosine kinase inhibitors (TKIs). We also observed that selection for Trp53 mutations occurred in Brm-positive tumors but did not occur in Brm-negative tumors. Hence, these data indicate that, during cancer development, Trp53 is functionally dependent on Brm but not Brg1. Our findings show for the first time the key differences in Brm- and Brg1-specific SWI/SNF complexes and help explain why concomitant loss of Brg1 and Brm frequently occurs in cancer, as well as how their loss impacts cancer development.

SIAH ubiquitin ligases regulate breast cancer cell migration and invasion independent of the oxygen status

Seven-in-absentia homolog (SIAH) proteins are evolutionary conserved RING type E3 ubiquitin ligases responsible for the degradation of key molecules regulating DNA damage response, hypoxic adaptation, apoptosis, angiogenesis, and cell proliferation. Many studies suggest a tumorigenic role for SIAH2. In breast cancer patients SIAH2 expression levels correlate with cancer aggressiveness and overall patient survival. In addition, SIAH inhibition reduced metastasis in melanoma. The role of SIAH1 in breast cancer is still ambiguous; both tumorigenic and tumor suppressive functions have been reported. Other studies categorized SIAH ligases as either pro- or antimigratory, while the significance for metastasis is largely unknown. Here, we re-evaluated the effects of SIAH1 and SIAH2 depletion in breast cancer cell lines, focusing on migration and invasion. We successfully knocked down SIAH1 and SIAH2 in several breast cancer cell lines. In luminal type MCF7 cells, this led to stabilization of the SIAH substrate Prolyl Hydroxylase Domain protein 3 (PHD3) and reduced Hypoxia-Inducible Factor 1α (HIF1α) protein levels. Both the knockdown of SIAH1 or SIAH2 led to increased apoptosis and reduced proliferation, with comparable effects. These results point to a tumor promoting role for SIAH1 in breast cancer similar to SIAH2. In addition, depletion of SIAH1 or SIAH2 also led to decreased cell migration and invasion in breast cancer cells. SIAH knockdown also controlled microtubule dynamics by markedly decreasing the protein levels of stathmin, most likely via p27(Kip1). Collectively, these results suggest that both SIAH ligases promote a migratory cancer cell phenotype and could contribute to metastasis in breast cancer.

Comprehensive functional analysis of the tousled-like kinase 2 frequently amplified in aggressive luminal breast cancers

More aggressive and therapy-resistant oestrogen receptor (ER)-positive breast cancers remain a great clinical challenge. Here our integrative genomic analysis identifies tousled-like kinase 2 (TLK2) as a candidate kinase target frequently amplified in ∼10.5% of ER-positive breast tumours. The resulting overexpression of TLK2 is more significant in aggressive and advanced tumours, and correlates with worse clinical outcome regardless of endocrine therapy. Ectopic expression of TLK2 leads to enhanced aggressiveness in breast cancer cells, which may involve the EGFR/SRC/FAK signalling. Conversely, TLK2 inhibition selectively inhibits the growth of TLK2-high breast cancer cells, downregulates ERα, BCL2 and SKP2, impairs G1/S cell cycle progression, induces apoptosis and significantly improves progression-free survival in vivo. We identify two potential TLK2 inhibitors that could serve as backbones for future drug development. Together, amplification of the cell cycle kinase TLK2 presents an attractive genomic target for aggressive ER-positive breast cancers.

Luminal B oestrogen receptor positive breast cancers are generally aggressive tumors with poor outcomes. Here, the authors show that the kinase TLK2 is amplified and overexpressed in these tumors and correlates with reduced survival, TLK2 inhibition induces apoptosis in vitro and improves survival in mice.

Regulation of the Embryonic Cell Cycle During Mammalian Preimplantation Development

The preimplantation development stage of mammalian embryogenesis consists of a series of highly conserved, regulated, and predictable cell divisions. This process is essential to allow the rapid expansion and differentiation of a single-cell zygote into a multicellular blastocyst containing cells of multiple developmental lineages. This period of development, also known as the germinal stage, encompasses several important developmental transitions, which are accompanied by dramatic changes in cell cycle profiles and dynamics. These changes are driven primarily by differences in the establishment and enforcement of cell cycle checkpoints, which must be bypassed to facilitate the completion of essential cell cycle events. Much of the current knowledge in this area has been amassed through the study of knockout models in mice. These mouse models are powerful experimental tools, which have allowed us to dissect the relative dependence of the early embryonic cell cycles on various aspects of the cell cycle machinery and highlight the extent of functional redundancy between members of the same gene family. This chapter will explore the ways in which the cell cycle machinery, their accessory proteins, and their stimuli operate during mammalian preimplantation using mouse models as a reference and how this allows for the usually well-defined stages of the cell cycle to be shaped and transformed during this unique and critical stage of development.

A Strategy for Direct Chemical Activation of the Retinoblastoma Protein

The retinoblastoma (Rb) tumor suppressor protein negatively regulates cell proliferation by binding and inhibiting E2F transcription factors. Rb inactivation occurs in cancer cells upon cyclin-dependent kinase (Cdk) phosphorylation, which induces E2F release and activation of cell cycle genes. We present a strategy for activating phosphorylated Rb with molecules that bind Rb directly and enhance affinity for E2F. We developed a fluorescence polarization assay that can detect the effect of exogenous compounds on modulating affinity of Rb for the E2F transactivation domain. We found that a peptide capable of disrupting the compact inactive Rb conformation increases affinity of the repressive Rb-E2F complex. Our results demonstrate the feasibility of discovering novel molecules that target the cell cycle and proliferation through directly targeting Rb rather than upstream kinase activity.

Differentiation resistance through altered retinoblastoma protein function in acute lymphoblastic leukemia: in silico modeling of the deregulations in the G1/S restriction point pathway

BACKGROUND

As in many cancer types, the G1/S restriction point (RP) is deregulated in Acute Lymphoblastic Leukemia (ALL). Hyper-phosphorylated retinoblastoma protein (hyper-pRb) is found in high levels in ALL cells. Nevertheless, the ALL lymphocyte proliferation rate for the average patient is surprisingly low compared to its normal counterpart of the same maturation level. Additionally, as stated in literature, ALL cells possibly reside at or beyond the RP which is located in the late-G1 phase. This state may favor their differentiation resistant phenotype. A major phenomenon contributing to this fact is thought to be the observed limited redundancy in the phosphorylation of retinoblastoma protein (pRb) by the various Cyclin Dependent Kinases (Cdks). The latter may result in partial loss of pRb functions despite hyper-phosphorylation.

RESULTS

To test this hypothesis, an in silico model aiming at simulating the biochemical regulation of the RP in ALL is introduced. By exploiting experimental findings derived from leukemic cells and following a semi-quantitative calibration procedure, the model has been shown to satisfactorily reproduce such a behavior for the RP pathway. At the same time, the calibrated model has been proved to be in agreement with the observed variation in the ALL cell cycle duration.

CONCLUSIONS

The proposed model aims to contribute to a better understanding of the complex phenomena governing the leukemic cell cycle. At the same time it constitutes a significant first step in the creation of a personalized proliferation rate predictor that can be used in the context of multiscale cancer modeling. Such an approach is expected to play an important role in the refinement and the advancement of mechanistic modeling of ALL in the context of the emergent and promising scientific domains of In Silico Oncology and more generally In Silico Medicine.

Plumericin inhibits proliferation of vascular smooth muscle cells by blocking STAT3 signaling via S-glutathionylation

The etiology of atherosclerosis and restenosis involves aberrant inflammation and proliferation, rendering compounds with both anti-inflammatory and anti-mitogenic properties as promising candidates for combatting vascular diseases. A recent study identified the iridoid plumericin as a new scaffold inhibitor of the pro-inflammatory NF-κB pathway in endothelial cells. We here examined the impact of plumericin on the proliferation of primary vascular smooth muscle cells (VSMC). Plumericin inhibited serum-stimulated proliferation of rat VSMC. It arrested VSMC in the G1/G0-phase of the cell cycle accompanied by abrogated cyclin D1 expression and hindered Ser 807/811-phosphorylation of retinoblastoma protein. Transient depletion of glutathione by the electrophilic plumericin led to S-glutathionylation as well as hampered Tyr705-phosphorylation and activation of the transcription factor signal transducer and activator of transcription 3 (Stat3). Exogenous addition of glutathione markedly prevented this inhibitory effect of plumericin on Stat3. It also overcame downregulation of cyclin D1 expression and the reduction of biomass increase upon serum exposure. This study revealed an anti-proliferative property of plumericin towards VSMC which depends on plumericin's thiol reactivity and S-glutathionylation of Stat3. Hence, plumericin, by targeting at least two culprits of vascular dysfunction -inflammation and smooth muscle cell proliferation -might become a promising electrophilic lead compound for vascular disease therapy.

Metabolic Reprogramming of Pancreatic Cancer Mediated by CDK4/6 Inhibition Elicits Unique Vulnerabilities

Due to loss of p16ink4a in pancreatic ductal adenocarcinoma (PDA), pharmacological suppression of CDK4/6 could represent a potent target for treatment. In PDA models, CDK4/6 inhibition had a variable effect on cell cycle but yielded accumulation of ATP and mitochondria. Pharmacological CDK4/6 inhibitors induce cyclin D1 protein levels; however, RB activation was required and sufficient for mitochondrial accumulation. CDK4/6 inhibition stimulated glycolytic and oxidative metabolism and was associated with an increase in mTORC1 activity. MTOR and MEK inhibitors potently cooperate with CDK4/6 inhibition in eliciting cell-cycle exit. However, MTOR inhibition fully suppressed metabolism and yielded apoptosis and suppression of tumor growth in xenograft models. The metabolic state mediated by CDK4/6 inhibition increases mitochondrial number and reactive oxygen species (ROS). Concordantly, the suppression of ROS scavenging or BCL2 antagonists cooperated with CDK4/6 inhibition. Together, these data define the impact of therapeutics on PDA metabolism and provide strategies for converting cytostatic response to tumor cell killing.

Phosphorylation of the Retinoblastoma protein (Rb) on serine-807 is required for association with Bax

The recent finding that the Retinoblastoma protein (Rb) is able to regulate apoptosis in a non-transcriptional manner directly at the mitochondria by interaction with the pro-apoptotic protein Bax prompted this investigation of the complex formed between Rb and Bax. Because the function of Rb in the cellular processes of proliferation, apoptosis, senescence and differentiation is regulated by phosphorylation we endeavored to elucidate the phosphorylation status of Rb with respect to its association with Bax and its role in apoptosis. In this study we found that Rb phosphorylated on at least 4 C-terminal phosphorylation sites (S608, S795, S807/S811, and T821) is present at the mitochondria under non-stressed cellular conditions. An in vitro binding assay showed that Bax binds to Rb phosphorylated at S807/S811 in 3 cancer cell types. Physiologically relevant association between Bax and Rb phosphorylated on S807/S811 was demonstrated by reciprocal co-immunoprecipitation experiments using antibodies specific for Rb phosphorylated on S807/S811 and Bax. Mutant Rb proteins expressed in Rb-null C33A cells showed that phosphorylation of S807 of Rb promotes association with Bax and that mimicking phosphorylation at S807 of Rb can block the induction of apoptosis due to PNUTS downregulation. Finally using siRNA to activate phosphatase activity in MCF7 cells, Rb is dephosphorylated at several sites including S807/S811, dissociates from Bax and apoptosis is triggered. These studies show that phosphorylation of Rb regulates its association with Bax and its role in apoptosis.

Aberrant Retinoblastoma (RB)-E2F Transcriptional Regulation Defines Molecular Phenotypes of Osteosarcoma

We previously identified two distinct molecular subtypes of osteosarcoma through gene expression profiling. These subtypes are associated with distinct tumor behavior and clinical outcomes. Here, we describe mechanisms that give rise to these molecular subtypes. Using bioinformatic analyses, we identified a significant association between deregulation of the retinoblastoma (RB)-E2F pathway and the molecular subtype with worse clinical outcomes. Xenotransplantation models recapitulated the corresponding behavior for each osteosarcoma subtype; thus, we used cell lines to validate the role of the RB-E2F pathway in regulating the prognostic gene signature. Ectopic RB resets the patterns of E2F regulated gene expression in cells derived from tumors with worse clinical outcomes (molecular phenotype 2) to those comparable with those observed in cells derived from tumors with less aggressive outcomes (molecular phenotype 1), providing a functional association between RB-E2F dysfunction and altered gene expression in osteosarcoma. DNA methyltransferase and histone deacetylase inhibitors similarly reset the transcriptional state of the molecular phenotype 2 cells from a state associated with RB deficiency to one seen with RB sufficiency. Our data indicate that deregulation of RB-E2F pathway alters the epigenetic landscape and biological behavior of osteosarcoma.

Anticancer properties of extracts from Opuntia humifusa against human cervical carcinoma cells

In this study, we found that the total polyphenol and ascorbic acid levels in the fruit of Opuntia humifusa are higher than those in other parts of the plant. We further hypothesized that antioxidants in O. humifusa might affect the growth or survival of cancer cells. Hexane extracts of seeds and ethyl acetate extracts of fruits and stems significantly suppressed the proliferation of HeLa cervical carcinoma cells, but did not affect the proliferation of normal human BJ fibroblasts. Additionally, the extracts of O. humifusa induced G1 phase arrest in HeLa cells. The O. humifusa extracts reduced the levels of G1 phase-associated cyclin D1, cyclin-dependent kinase 4 (Cdk4), and phosphorylated retinoblastoma proteins. Moreover, p21(WAF1/Cip1) and p53 expression significantly increased after treatment. We examined the effects of ethyl acetate extracts of O. humifusa fruit (OHF) on HeLa cells xenograft tumor growth. OHF treatment significantly reduced tumor volume and this decrease was correlated with decreased Cdk4 and cyclin D1 expression. Furthermore, flavonoids, trans Taxifolin, and dihydrokaempferol, were isolated from OHF. Thus, this extract may be a promising candidate for treating human cervical carcinoma.

Cyclin D activates the Rb tumor suppressor by mono-phosphorylation

The widely accepted model of G₁ cell cycle progression proposes that cyclin D:Cdk4/6 inactivates the Rb tumor suppressor during early G₁ phase by progressive multi-phosphorylation, termed hypo-phosphorylation, to release E2F transcription factors. However, this model remains unproven biochemically and the biologically active form(s) of Rb remains unknown. In this study, we find that Rb is exclusively mono-phosphorylated in early G₁ phase by cyclin D:Cdk4/6. Mono-phosphorylated Rb is composed of 14 independent isoforms that are all targeted by the E1a oncoprotein, but show preferential E2F binding patterns. At the late G₁ Restriction Point, cyclin E:Cdk2 inactivates Rb by quantum hyper-phosphorylation. Cells undergoing a DNA damage response activate cyclin D:Cdk4/6 to generate mono-phosphorylated Rb that regulates global transcription, whereas cells undergoing differentiation utilize un-phosphorylated Rb. These observations fundamentally change our understanding of G₁ cell cycle progression and show that mono-phosphorylated Rb, generated by cyclin D:Cdk4/6, is the only Rb isoform in early G₁ phase.

DOI:http://dx.doi.org/10.7554/eLife.02872.001

Cells go through a tightly controlled, multi-step procedure before they divide. This cell division program—the cell cycle—is necessary for preventing unrestrained cellular growth, which may lead to cancer. Proteins called cyclins control the progression through each of the phases of the cell cycle, with different cyclins working during different phases.

During the G1 phase of the cell cycle, cells grow in size and produce the proteins that are required to copy DNA. Once a cell passes a checkpoint called the 'restriction point' at the end of the G1 phase, it is committed to dividing. It is therefore particularly important to keep events during G1 phase in check.

The Retinoblastoma tumor suppresor protein (Rb) is a key player in regulating the G1 phase. Rb sequesters transcription factors that are essential for the cell cycle to progress. Previously, it was thought that a complex called cyclin D added more and more phosphates to the Rb protein during the G1 phase. This process predicted a slow release of transcription factors, which attach to DNA and start the process of DNA replication. While many studies have presented data that is consistent with this model, direct biochemical evidence of these events is lacking.

Narasimha, Kaulich, Shapiro et al. now present biochemical analyses of Rb proteins that show—completely unexpectedly—that the cyclin D complex adds just one phosphate group to Rb during the G1 phase, although this group can be added to one of fourteen different sites. The resulting 'mono-phosphorylated' Rb varieties can each sequester different transcription factors and stop them working.

At the restriction point, many more phosphate groups are then rapidly added, and the Rb protein is inactivated by a different cyclin. This cyclin—called Cyclin E—then drives cells into the next phase of the cell cycle. Establishing how cyclin E is activated is a priority for future research.

DOI:http://dx.doi.org/10.7554/eLife.02872.002

Flavopiridol synergizes with sorafenib to induce cytotoxicity and potentiate antitumorigenic activity in EGFR/HER-2 and mutant RAS/RAF breast cancer model systems

Oncogenic receptor tyrosine kinase (RTK) signaling through the Ras-Raf-Mek-Erk (Ras-MAPK) pathway is implicated in a wide array of carcinomas, including those of the breast. The cyclin-dependent kinases (CDKs) are implicated in regulating proliferative and survival signaling downstream of this pathway. Here, we show that CDK inhibitors exhibit an order of magnitude greater cytotoxic potency than a suite of inhibitors targeting RTK and Ras-MAPK signaling in cell lines representative of clinically recognized breast cancer (BC) subtypes. Drug combination studies show that the pan-CDK inhibitor, flavopiridol (FPD), synergistically potentiated cytotoxicity induced by the Raf inhibitor, sorafenib (SFN). This synergy was most pronounced at sub-EC50 SFN concentrations in MDA-MB-231 (KRAS-G13D and BRAF-G464V mutations), MDA-MB-468 [epidermal growth factor receptor (EGFR) overexpression], and SKBR3 [ErbB2/EGFR2 (HER-2) overexpression] cells but not in hormone-dependent MCF-7 and T47D cells. Potentiation of SFN cytotoxicity by FPD correlated with enhanced apoptosis, suppression of retinoblastoma (Rb) signaling, and reduced Mcl-1 expression. SFN and FPD were also tested in an MDA-MB-231 mammary fat pad engraftment model of tumorigenesis. Mice treated with both drugs exhibited reduced primary tumor growth rates and metastatic tumor load in the lungs compared to treatment with either drug alone, and this correlated with greater reductions in Rb signaling and Mcl-1 expression in resected tumors. These findings support the development of CDK and Raf co-targeting strategies in EGFR/HER-2-overexpressing or RAS/RAF mutant BCs.