The BRCA Tumor Suppressor Network in Chromosome Damage Repair by Homologous Recombination

Mutations in the BRCA1 and BRCA2 genes predispose afflicted individuals to breast, ovarian, and other cancers. The BRCA-encoded products form complexes with other tumor suppressor proteins and with the recombinase enzyme RAD51 to mediate chromosome damage repair by homologous recombination and also to protect stressed DNA replication forks against spurious nucleolytic attrition. Understanding how the BRCA tumor suppressor network executes its biological functions would provide the foundation for developing targeted cancer therapeutics, but progress in this area has been greatly hampered by the challenge of obtaining purified BRCA complexes for mechanistic studies. In this article, we review how recent effort begins to overcome this technical challenge, leading to functional and structural insights into the biochemical attributes of these complexes and the multifaceted roles that they fulfill in genome maintenance. We also highlight the major mechanistic questions that remain.

Tumor suppressor MCPH1 regulates gene expression profiles related to malignant conversion and chromosomal assembly

Strong inherited predisposition to breast cancer is estimated to cause about 5–10% of all breast cancer cases. As the known susceptibility genes, such as BRCA1 and BRCA2, explain only a fraction of this, additional predisposing genes and related biological mechanisms are actively being searched for. We have recently identified a recurrent MCPH1 germline mutation, p.Arg304ValfsTer3, as a breast cancer susceptibility allele. MCPH1 encodes a multifunctional protein involved in maintenance of genomic integrity and it is also somatically altered in various cancer types, including breast cancer. Additionally, biallelic MCPH1 mutations are causative for microcephaly and at cellular level premature chromosome condensation. To study the molecular mechanisms leading to cancer predisposition and malignant conversion, here we have modeled the effect of MCPH1 p.Arg304ValfsTer3 mutation using gene‐edited MCF10A breast epithelial cells. As a complementary approach, we also sought for additional potential cancer driver mutations in MCPH1 p.Arg304ValfsTer3 carrier breast tumors. We show that mutated MCPH1 de‐regulates transcriptional programs related to invasion and metastasis and leads to downregulation of histone genes. These global transcriptional changes are mirrored by significantly increased migration and invasion potential of the cells as well as abnormal chromosomal condensation both before and after mitosis. These findings provide novel molecular insights to MCPH1 tumor suppressor functions and establish a role in regulation of transcriptional programs related to malignant conversion and chromosomal assembly. The MCPH1 p.Arg304ValfsTer3 carrier breast tumors showed recurrent tumor suppressor gene TP53 mutations, which were also significantly over‐represented in breast tumors with somatically inactivated MCPH1.

What's new?

Even though several breast cancer susceptibility genes have been identified, additional molecular mechanisms behind predisposition and the promotion of malignant conversion remain obscure. Here, the authors show that a previously‐identified breast cancer predisposing allele in tumor suppressor MCPH1 deregulates transcriptional programs related to invasion and metastasis and leads to down‐regulation of histone genes. These global transcriptional changes are mirrored by increased cell migration and invasion potential and abnormal chromosomal condensation. The findings provide novel molecular insights into MCPH1 tumor suppressor functions and establish a role in the regulation of transcriptional programs related to malignant conversion and chromosomal assembly.

Specific allelic variants of SNPs in the MDM2 and MDMX genes are associated with earlier tumor onset and progression in Caucasian breast cancer patients

Background

Genetic factors play a substantial role in breast cancer etiology. Genes encoding proteins that have key functions in the DNA damage response, such as p53 and its inhibitors MDM2 and MDMX, are most likely candidates to harbor allelic variants that influence breast cancer susceptibility. The aim of our study was to comprehensively analyze the impact of SNPs in the TP53, MDM2, and MDMX genes in conjunction with TP53 mutational status regarding the onset and progression of breast cancer.

Methods

In specimen from 815 breast cancer patients, five SNPs within the selected genes were analyzed: TP53 – Arg72Pro (rs1042522), MDM2 – SNP285 (rs2279744), SNP309 (rs117039649); MDMX – SNP31826 (rs1563828), and SNP34091 (rs4245739). Classification of the tumors was evaluated by histomorphology. Subtyping according hormone receptor status, HER2-status and proliferation rate enabled provision of the clinico-pathological surrogate of intrinsic subtypes.

Results

The homozygous C-allele of MDM2 SNP285 was significantly associated with a younger age-at-diagnosis of 44.2 years, in contrast to G/G- and G/C-patients (62.4, 62.7 yrs., respectively; p = 0.0007; log-Rank-test). In contrast, there was no difference regarding the age-at-diagnosis for patients with the respective genotypes of MDM2 SNP309 (p = 0.799; log-Rank-test). In patients with estrogen receptor (ER)-positive and TP53-mutated tumors, however, the T/T-genotype of the MDM2 SNP309 was significantly associated with an earlier average age-at-diagnosis compared with T/G+G/G-patients (53.5 vs. 68.2 yrs; p = 0.002; log-Rank-test). In the triple-negative subgroup, the G/G-patients had an average age-at-diagnosis of 51 years compared with 63 years for SNP309T carriers (p = 0.004; log-Rank-test) indicating a susceptibility of the G/G genotype for the development of triple negative breast cancer. Patients with the A/A-genotype of MDMX SNP31826 with ER-negative tumors were diagnosed 11 years earlier compared with patients and ER-positive tumors (53.2 vs. 64.4 yrs; p = 0.025, log-Rank-test). Furthermore, in luminal B-like patients (HER2-independent) the C/C-genotype of MDMX SNP34091 was significantly correlated with a decreased event-free survival compared with the A/A-genotype (p < 0.001; log-Rank-test).

Conclusions

We showed that SNPs in the MDM2 and MDMX genes affect at least in part the onset and progression of breast cancer dependent on the ER-status. Our findings provide further evidence for the distinct etiological pathways in ER-negative and ER-positive breast cancers.

Dual Role of the Alternative Reading Frame ARF Protein in Cancer

The CDKN2a/ARF locus expresses two partially overlapping transcripts that encode two distinct proteins, namely p14ARF (p19Arf in mouse) and p16INK4a, which present no sequence identity. Initial data obtained in mice showed that both proteins are potent tumor suppressors. In line with a tumor-suppressive role, ARF-deficient mice develop lymphomas, sarcomas, and adenocarcinomas, with a median survival rate of one year of age. In humans, the importance of ARF inactivation in cancer is less clear whereas a more obvious role has been documented for p16INK4a. Indeed, many alterations in human tumors result in the elimination of the entire locus, while the majority of point mutations affect p16INK4a. Nevertheless, specific mutations of p14ARF have been described in different types of human cancers such as colorectal and gastric carcinomas, melanoma and glioblastoma. The activity of the tumor suppressor ARF has been shown to rely on both p53-dependent and independent functions. However, novel data collected in the last years has challenged the traditional and established role of this protein as a tumor suppressor. In particular, tumors retaining ARF expression evolve to metastatic and invasive phenotypes and in humans are associated with a poor prognosis. In this review, the recent evidence and the molecular mechanisms of a novel role played by ARF will be presented and discussed, both in pathological and physiological contexts.

The Enigmatic Protein Kinase C-eta

Protein kinase C (PKC), a multi-gene family, plays critical roles in signal transduction and cell regulation. Protein kinase C-eta (PKCη) is a unique member of the PKC family since its regulation is distinct from other PKC isozymes. PKCη was shown to regulate cell proliferation, differentiation and cell death. It was also shown to contribute to chemoresistance in several cancers. PKCη has been associated with several cancers, including renal cell carcinoma, glioblastoma, breast cancer, non-small cell lung cancer, and acute myeloid leukemia. However, mice lacking PKCη were more susceptible to tumor formation in a two-stage carcinogenesis model, and it is downregulated in hepatocellular carcinoma. Thus, the role of PKCη in cancer remains controversial. The purpose of this review article is to discuss how PKCη regulates various cellular processes that may contribute to its contrasting roles in cancer.

Species-specific mechanisms of tumor suppression are fundamental drivers of vertebrate speciation: critical implications for the 'war on cancer'

We recently reported our detection of an anthropoid primate-specific, 'kill switch' tumor suppression system that reached its greatest expression in humans, but that is fully functional only during the first twenty-five years of life, corresponding to the primitive human lifespan that has characterized the majority of our species' existence. This tumor suppression system is based upon the kill switch being triggered in cells in which p53 has been inactivated; such kill switch consisting of a rapid, catastrophic increase in ROS caused by the induction of irreversible uncompetitive inhibition of glucose-6- phosphate dehydrogenase (G6PD), which requires high concentrations of both inhibitor (DHEA) and G6P substrate. While high concentrations of intracellular DHEA are readily available in primates from the importation and subsequent de-sulfation of circulating DHEAS into p53-affected cells, both an anthropoid primate-specific sequence motif (GAAT) in the glucose-6-phosphatase (G6PC) promoter, and primate-specific inactivation of de novo synthesis of vitamin C by deletion of gulonolactone oxidase (GLO) were required to enable accumulation of G6P to levels sufficient to enable irreversible uncompetitive inhibition of G6PD. Malignant transformation acts as a counterforce opposing vertebrate speciation, particularly increases in body size and lifespan that enable optimized exploitation of particular niches. Unique mechanisms of tumor suppression that evolved to enable niche exploitation distinguish vertebrate species, and prevent one vertebrate species from serving as a valid model system for another. This here-to-fore unrecognized element of speciation undermines decades of cancer research data, using murine species, which presumed universal mechanisms of tumor suppression, independent of species. Despite this setback, the potential for pharmacological reconstitution of the kill switch tumor suppression system that distinguishes our species suggests that 'normalization' of human cancer risk, from its current 40% to the 4% of virtually all other large, long-lived species, represents a realistic near-term goal.

BAP1 suppresses tumor development by inducing ferroptosis upon SLC7A11 repression

The tumor suppressor BRCA1-associated protein 1 (BAP1) is a deubiquitinase that removes histone 2A ubiquitination. How BAP1 suppresses tumor development remains elusive. Our recent study identified the cystine transporter solute carrier family 7 member 11 (SLC7A11) as a critical BAP1 target, and showed that BAP1 promotes ferroptosis (a non-apoptotic cell death) through repressing SLC7A11 expression, resulting in tumor suppression.

p53 Represses the Mevalonate Pathway to Mediate Tumor Suppression

There are still gaps in our understanding of the complex processes by which p53 suppresses tumorigenesis. Here we describe a novel role for p53 in suppressing the mevalonate pathway, which is responsible for biosynthesis of cholesterol and nonsterol isoprenoids. p53 blocks activation of SREBP-2, the master transcriptional regulator of this pathway, by transcriptionally inducing the ABCA1 cholesterol transporter gene. A mouse model of liver cancer reveals that downregulation of mevalonate pathway gene expression by p53 occurs in premalignant hepatocytes, when p53 is needed to actively suppress tumorigenesis. Furthermore, pharmacological or RNAi inhibition of the mevalonate pathway restricts the development of murine hepatocellular carcinomas driven by p53 loss. Like p53 loss, ablation of ABCA1 promotes murine liver tumorigenesis and is associated with increased SREBP-2 maturation. Our findings demonstrate that repression of the mevalonate pathway is a crucial component of p53-mediated liver tumor suppression and outline the mechanism by which this occurs.

Fabrication and evaluation of a γ-PGA-based self-assembly transferrin receptor-targeting anticancer drug carrier

Background

cis-Dichlorodiamineplatinum (CDDP) was one of the most common used drugs in clinic for cancer treatment. However, CDDP caused a variety of side effects. The abundant carboxyl groups on the surface of poly glutamic acid (PGA) could be modified with various kinds of targeted ligands. PGA delivery system loaded CDDP for cancer therapies possesses potential to overcome the side effects.

Materials and methods

In this study, we constructed a safe and efficient anticancer drug delivery system PGA–Asp–maleimide–cisplatin–peptide complex (PAMCP), which was loaded with CDDP and conjugated with the transferrin receptor (TFR)-targeting peptide through a maleimide functional linker. The size of PAMCP was identified by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Fluorescence microscopy and flow cytometry methods were used to detect the cell targeting ability in vitro. The MTT assay was used to detect targeted toxicity in vitro. The in vivo acute toxicity was tested in Kun Ming (KM) mice. The tumor suppression activity and drug distribution was analyzed in nude mice bearing with HeLa tumor cells.

Results

The nano-size was 110±28 nm detected with TEM and 89±18 nm detected with DLS, respectively. Fluorescence microscopy and flow cytometry methods indicated that PAMCP possessed excellent cell targeting ability in vitro. The MTT assay suggested that PAMCP was excellent for targeted toxicity. The acute in vivo toxicity study revealed that the body mass index and serum index in the PAMCP-treated group were superior to those in the CDDP-treated group (P<0.001), and no obvious differences were detected on comparing with the PBS- or PGA–Asp–maleimide–P8 (PAMP)-treated groups. PAMCP reduced the toxicity of CDDP, suppressed tumor cell growth, and achieved efficient anti-tumor effects in vivo. After PAMCP treatment, the toxicity of CDDP was reduced and tumor growth was more remarkably inhibited compared with the free CDDP treatment group (P<0.01). Much stronger (5–10 folds) fluorescence intensity in tumor tissue was detected compared with the irrelevant-peptide group for drug distribution analysis detected with the frozen section approach.

Conclusion

Our data highlighted that PAMCP reduced the side effects of CDDP and exhibited stronger anti-tumor effects. Therefore, PAMCP presented the potential to be a safe and effective anticancer pharmaceutical formulation for future clinical applications.

Treatment with Lactobacillus Retards the Tumor Growth of Head and Neck Squamous Cell Carcinoma Cells Inoculated in Mice

Bacteria have been used for more than a century to treat solid tumors. Because solid tumors generate an anaerobic environment, we evaluated the anti-tumor effect of the obligate anaerobe strain KK378, derived from Lactobacillus casei (L. casei), using mice bearing head and neck cancer. Wild-type L. casei is a nonpathogenic bacterium that is commonly used in foods. Moreover, patients with head and neck squamous cell carcinoma often have multiple cancers and cervical lymph node metastasis that can be directly sensed beneath the skin. To establish the animal model bearing head and neck cancer, we inoculated each of human squamous cell carcinoma cell lines, SAS, HSQ89, and HSC2, on the back skin of BALB/cSlc-nu/nu mice. After tumor formation, L. casei KK378 was administered directly into the tumor, and tumor size and serum cytokine levels were analyzed. Mice injected with 10⁸ cfu of L. casei KK378 showed reduction in tumor growth compared with PBS control; especially, the SAS tumor was significantly reduced (p = 0.008). Administered L. casei KK378 was detected in tumor tissues but not in normal tissues (liver, kidney, and lung) of SAS tumor-bearing mice, which was associated with increased blood cytokines (TNF-α, IFN-γ, IL-5, IL-10, and IL-12). Among these cytokines, the serum levels of IFN-γ and TNF-α were significantly increased (p < 0.05). In conclusion, L. casei KK378 infection may suppress tumor growth by inducing the host immune response. Direct injection of Lactobacillus into the tumor could be a potential strategy to treat head and neck squamous cell carcinoma.

Targeted p53 on Small-Molecules-Induced Ferroptosis in Cancers

Ferroptosis is a type of programmed cell death characterized by the accumulation of lipid reactive oxygen species (L-ROS) driven by the oxidative degeneration of lipids in an iron-dependent manner. The mechanism by which lipid oxidative degradation drives ROS-ferroptosis involves metabolic dysfunctions that result in impaired intracellular metabolic processes and ROS production. Recent studies have found that p53 acts as a positive regulator of ferroptosis by promoting ROS production. p53 directly regulates the metabolic versatility of cells by favoring mitochondrial respiration, leading to ROS-mediated ferroptosis. In mild stress, p53 protects cell survival via eliminating ROS; additionally, in human colorectal cancer, p53 antagonizes ferroptosis by formation of the DPP4–p53 complex. In short, the mechanisms of p53-mediated ROS production underlying cellular response are poorly understood. In the context of recent research results, the indistinct roles of p53 on ROS-mediated ferroptosis are scrutinized to understand the mechanism underlying p53-mediated tumor suppression.

Detection of a novel, primate-specific 'kill switch' tumor suppression mechanism that may fundamentally control cancer risk in humans: an unexpected twist in the basic biology of TP53

The activation of TP53 is well known to exert tumor suppressive effects. We have detected a primate-specific adrenal androgen-mediated tumor suppression system in which circulating DHEAS is converted to DHEA specifically in cells in which TP53 has been inactivated DHEA is an uncompetitive inhibitor of glucose-6-phosphate dehydrogenase (G6PD), an enzyme indispensable for maintaining reactive oxygen species within limits survivable by the cell. Uncompetitive inhibition is otherwise unknown in natural systems because it becomes irreversible in the presence of high concentrations of substrate and inhibitor. In addition to primate-specific circulating DHEAS, a unique, primate-specific sequence motif that disables an activating regulatory site in the glucose-6-phosphatase (G6PC) promoter was also required to enable function of this previously unrecognized tumor suppression system. In human somatic cells, loss of TP53 thus triggers activation of DHEAS transport proteins and steroid sulfatase, which converts circulating DHEAS into intracellular DHEA, and hexokinase which increases glucose-6-phosphate substrate concentration. The triggering of these enzymes in the TP53-affected cell combines with the primate-specific G6PC promoter sequence motif that enables G6P substrate accumulation, driving uncompetitive inhibition of G6PD to irreversibility and ROS-mediated cell death. By this catastrophic 'kill switch' mechanism, TP53 mutations are effectively prevented from initiating tumorigenesis in the somatic cells of humans, the primate with the highest peak levels of circulating DHEAS. TP53 mutations in human tumors therefore represent fossils of kill switch failure resulting from an age-related decline in circulating DHEAS, a potentially reversible artifact of hominid evolution.

The BRCT Domains of the BRCA1 and BARD1 Tumor Suppressors Differentially Regulate Homology-Directed Repair and Stalled Fork Protection

The BRCA1 tumor suppressor preserves genome integrity through both homology-directed repair (HDR) and stalled fork protection (SFP). In vivo, BRCA1 exists as a heterodimer with the BARD1 tumor suppressor, and both proteins harbor a phosphate-binding BRCT domain. Here, we compare mice with mutations that ablate BRCT phospho-recognition by Bard1 (Bard1^S563F and Bard1^K607A) or Brca1 (Brca1^S1598F). Brca1^S1598F abrogates both HDR and SFP, suggesting that both pathways are likely impaired in most BRCA1 mutant tumors. Although not affecting HDR, the Bard1 mutations ablate poly(ADP-ribose)-dependent recruitment of BRCA1/BARD1 to stalled replication forks, resulting in fork degradation and chromosome instability. Nonetheless, Bard1^S563F/S563F and Bard1^K607A/K607A mice, unlike Brca1^{S1598F/S1598F} mice, are not tumor prone, indicating that HDR alone is sufficient to suppress tumor formation in the absence of SFP. Nevertheless, because SFP, unlike HDR, is also impaired in heterozygous Brca1/Bard1 mutant cells, SFP and HDR may contribute to distinct stages of tumorigenesis in BRCA1/BARD1 mutation carriers.

Downregulation of Autophagy-Related Proteins 1, 5, and 16 in Testicular Germ Cell Tumors Parallels Lowered LC3B and Elevated p62 Levels, Suggesting Reduced Basal Autophagy

Autophagy is a cellular “self-digestion” process known to be essential for various physiological and pathological pathways, including cancer, where its role appears to be context-dependent. In this work, we aimed to investigate the level of autophagy by evaluating the expression of key autophagy-related proteins (ATGs) in testicular germ cell tumors (TGCT) for which autophagy has been rarely investigated. We decided to use an immunohistochemical (IHC) staining approach employing a tissue microarray (TMA). Software-based evaluation of the integrated optical densities (IODs) of these proteins indicated a significant downregulation of ATG1, ATG5, and ATG16L1. Accordingly, reduced levels of microtubule-associated proteins 1A/1B light chain 3B (LC3B) were found to parallel increases in sequestosome-1 (SQSTM1 or p62), a protein normally degraded via autophagy, suggesting an in vivo reduction in autophagy with TGCT. Thus, our work provides evidence for a tumor suppressive function of autophagy in the development of TGCT and supports the concept of a context-dependent role of autophagy in tumorigenesis which is tumor type-dependent.

p53 tumor suppressor and iron homeostasis

Iron is an essential nutrient for all living organisms and plays a vital role in many fundamental biochemical processes, such as oxygen transport, energy metabolism, and DNA synthesis. Due to its capability to produce free radicals, iron has deleterious effects and thus, its level needs to be tightly controlled in the body. Deregulation of iron metabolism is known to cause diseases, including anemia by iron deficiency and hereditary hemochromatosis by iron overload. Interestingly, dysregulated iron metabolism occurs frequently in tumor cells and contributes to tumorigenesis. In this review, we will discuss the role of p53 tumor suppressor in iron homeostasis.

Neat-en-ing up our understanding of p53 pathways in tumor suppression

Although the p53 transcription factor has a well-established role in tumor suppression, little is known about how the non-coding targets of p53 mediate its tumor suppression function. Analysis of ncRNAs regulated by p53 revealed Neat1 as a direct p53 target gene. Neat1 has physiological roles in the development and differentiation of the mammary gland and corpus luteum, but its roles in cancer have been conflicting. To unequivocally understand Neat1 function in cancer, we used Neat1 null mice. Interestingly, we found that Neat1 deficiency promotes transformation both in oncogene-expressing fibroblasts and in a mouse model for pancreatic cancer. Specifically, Neat1 loss in the pancreas results in the enhanced development of preneoplastic lesions associated with dampened expression of differentiation genes. While the exact mechanisms underlying tumor suppression are unknown, there are several described mechanisms that may be responsible for Neat1-mediated tumor suppression. Collectively, these findings suggest that Neat1 enforces differentiation to suppress pancreatic cancer.

Crosstalk between PKCα and PI3K/AKT Signaling Is Tumor Suppressive in the Endometrium

Protein kinase C (PKC) isozymes are commonly recognized as oncoproteins based on their activation by tumor-promoting phorbol esters. However, accumulating evidence indicates that PKCs can be inhibitory in some cancers, with recent findings propelling a shift in focus to understanding tumor suppressive functions of these enzymes. Here, we report that PKCα acts as a tumor suppressor in PI3K/AKT-driven endometrial cancer. Transcriptional suppression of PKCα is observed in human endometrial tumors in association with aggressive disease and poor prognosis. In murine models, loss of PKCα is rate limiting for endometrial tumor initiation. PKCα tumor suppression involves PP2A-family-dependent inactivation of AKT, which can occur even in the context of genetic hyperactivation of PI3K/AKT signaling by coincident mutations in PTEN, PIK3CA, and/or PIK3R1. Together, our data point to PKCα as a crucial tumor suppressor in the endometrium, with deregulation of a PKCα→PP2A/PP2A-like phosphatase signaling axis contributing to robust AKT activation and enhanced endometrial tumorigenesis.

Alterations of methionine metabolism in hepatocarcinogenesis: the emergent role of glycine N-methyltransferase in liver injury

The methionine and folate cycles play a fundamental role in cell physiology and their alteration is involved in liver injury and hepatocarcinogenesis. Glycine N-methyltransferase is implicated in methyl group supply, DNA methylation, and nucleotide biosynthesis. It regulates the cellular S-adenosylmethionine/S-adenosylhomocysteine ratio and S-adenosylmethionine-dependent methyl transfer reactions. Glycine N-methyltransferase is absent in fast-growing hepatocellular carcinomas and present at a low level in slower growing HCC ones. The mechanism of tumor suppression by glycine N-methyltransferase is not completely known. Glycine N-methyltransferase inhibits hepatocellular carcinoma growth through interaction with Dep domain-containing mechanistic target of rapamycin (mTor)-interacting protein, a binding protein overexpressed in hepatocellular carcinoma. The interaction of the phosphatase and tensin homolog inhibitor, phosphatidylinositol 3,4,5-trisphosphate-dependent rac exchanger, with glycine N-methyltransferase enhances proteasomal degradation of this exchanger by the E3 ubiquitin ligase HectH. Glycine N-methyltransferase also regulates genes related to detoxification and antioxidation pathways. It supports pyrimidine and purine syntheses and minimizes uracil incorporation into DNA as consequence of folate depletion. However, recent evidence indicates that glycine N-methyltransferase targeted into nucleus still exerts strong anti-proliferative effects independent of its catalytic activity, while its restriction to cytoplasm prevents these effects. Our current knowledge suggest that glycine N-methyltransferase plays a fundamental, even if not yet completely known, role in cellular physiology and highlights the need to further investigate this role in normal and cancer cells.

Atorvastatin Inhibits the HIF1α-PPAR Axis, Which Is Essential for Maintaining the Function of Human Induced Pluripotent Stem Cells

We herein report a novel mechanism of action of statin preparations using a new drug discovery method. Milk fat globule-EGF factor 8 protein (MFG-E8) was identified from the secretory component of mouse embryonic fibroblast (MEF) as a cell adhesion-promoting factor effective for screening active cellular agents of human induced pluripotent stem cells (hiPSCs) in vitro using electrochemical impedance. Our analyses showed that atorvastatin did not cause death in myocardial cells differentiated from hiPSCs but reduced the pluripotent cell survival in vitro when using serum- and albumin-free media, and inhibited the ability to form teratomas in mice. This result could have been already the cytopathic effect of atorvastatin, and complete elimination of hiPSCs was confirmed in the xenotransplantation assay. The administration of atorvastatin to hiPSCs caused the expression of hypoxia inducible factor (HIF)1α mRNA to be unchanged at 6 hr and downregulated at 24 hr. In addition, the inhibition of the survival of hiPSCs was confirmed by HIF1α-peroxisome proliferator-activated receptor (PPAR) axis inhibition. These results suggest that the addition of atorvastatin to hiPSC cultures reduces the survival of pluripotent cells by suppressing the HIF1α-PPAR axis. In summary, the HIF1α-PPAR axis has an important role in maintaining the survival of pluripotent hiPSCs.

The Guardian of the Genome Revisited: p53 Downregulates Genes Required for Telomere Maintenance, DNA Repair, and Centromere Structure

The p53 protein has been extensively studied for its capacity to prevent proliferation of cells with a damaged genome. Surprisingly, however, our recent analysis of mice expressing a hyperactive mutant p53 that lacks the C-terminal domain revealed that increased p53 activity may alter genome maintenance. We showed that p53 downregulates genes essential for telomere metabolism, DNA repair, and centromere structure and that a sustained p53 activity leads to phenotypic traits associated with dyskeratosis congenita and Fanconi anemia. This downregulation is largely conserved in human cells, which suggests that our findings could be relevant to better understand processes involved in bone marrow failure as well as aging and tumor suppression.

E6AP promotes prostate cancer by reducing p27 expression

Prostate cancer (PC) is the most common cancer in men. Elevated levels of E3 ligase, E6-Associated Protein (E6AP) were previously linked to PC, consistent with increased protein expression in a subset of PC patients. In cancers, irregular E3 ligase activity drives proteasomal degradation of tumor suppressor proteins. Accordingly, E3 ligase inhibitors define a rational therapy to restore tumor suppression. The relevant tumor suppressors targeted by E6AP in PC are yet to be fully identified. In this study we show that p27, a key cell cycle regulator, is a target of E6AP in PC. Down regulation of E6AP increases p27 expression and enhances its nuclear accumulation in PC. We demonstrate that E6AP regulates p27 expression by inhibiting its transcription in an E2F1-dependent manner. Concomitant knockdown of E6AP and p27 partially restores PC cell growth, supporting the contribution of p27 to the overall effect of E6AP on prostate tumorigenesis. Overall, we unravelled the E6AP-p27 axis as a new promoter of PC, exposing an attractive target for therapy through the restoration of tumor suppression.

Oncogene-induced senescence: a potential breakpoint mechanism against malignant transformation in plasma cell disorders

Oncogene-induced senescence (OIS) is a cellular tumor-suppressive mechanism present in several premalignant conditions. Here, we analyze the possible impact of OIS on malignant transformation in plasma cell disorders. Tumor samples from 125 patients with different disease stages were analyzed immunohistochemically for expression of senescence markers. Protein expression of cyclin-dependent kinase inhibitor p21^Cip1/Waf1 was significantly higher in smoldering multiple myeloma (SMM) compared to monoclonal gammopathy of undetermined significance (MGUS) (p = .02) or symptomatic multiple myeloma (MM) (p = .005). SMM plasma cells expressing p21^Cip1/Waf1 were negative for Ki67, consistent with senescence. While p27^Kip1 was highly expressed in healthy controls, MGUS and SMM, expression decreased significantly in MM (p = .02). SMM plasma cells displayed a mutually exclusive expression of p21^Cip1/Waf1/p27^Kip1 suggesting compensatory mechanisms of senescence. In conclusion, we found markers of cellular senescence differentially expressed in SMM compared to MGUS and MM supporting the hypothesis of OIS as a breakpoint mechanism against malignant transformation in plasma cell disorders.

A Novel Target of p53, TCF21, Can Respond to Hypoxia by MAPK Pathway Inactivation in Uterine Corpus Endometrial Carcinoma

Uterine corpus endometrial carcinoma (UCEC) is a common malignancy in the female reproductive system, associated with high morbidity and mortality. Despite the high prevalence of UCEC, molecular understanding of uterine endothelium tumorigenesis remains poorly understood. In this study, we reported that transcription factor 21 (TCF21) inhibits cancer cell proliferation and invasion following overexpression, in vitro and in vivo. Moreover, in response to hypoxia, TCF21 is highly expressed in UCEC cells carrying wild-type p53, and is transcriptional target of p53. We observed that TCF21 interferes with the MAP kinase pathway, which is supported by a substantially reduced level of phosphorylated mitogen-activated protein kinase 1 (MAPK1 or ERK) in cells expressing a higher level of TCF21. Furthermore, we identified the specific region of TCF21 that is responsible for its interaction with mitogen-activated protein kinase 1 (MEK) and a subsequently reduced activity of ERK. Using molecular docking and mutagenesis analysis, we validated a special domain of TCF21, which can reduce the activity of the MAPK pathway and inhibit uterine endothelium tumor cell growth in vitro. Overall, our study determined that TCF21, a hypoxia-driven p53 target, functions as a tumor suppressor in UCEC and presents as a therapeutic target for tumor treatment.

CBX7 suppresses cell proliferation, migration, and invasion through the inhibition of PTEN/Akt signaling in pancreatic cancer

Chromobox protein homolog 7 (CBX7), one of the polycomb group (PcG) proteins, is a transcriptional repressor involved in the regulation of cell proliferation and senescence. In the present study, we showed that CBX7 negatively regulates the proliferation, viability, chemoresistance, and migration of pancreatic cancer cells. Overexpression of CBX7 significantly inhibited the proliferation of pancreatic cancer cells in vitro and in vivo. Depletion of CBX7 facilitated their growth. CBX7 also impaired the viability and chemoresistance of pancreatic cancer cells. Transwell assays showed that CBX7 reduces the migratory capacity of pancreatic cancer cells. Of note, CBX7 reduced PTEN/Akt signaling in pancreatic cancer cells by increasing PTEN transcription, suggesting involvement of PTEN/Akt pathway in the tumor suppressive activity of CBX7. In addition, immunohistochemical analysis the CBX7 and PTEN expression in 74 surgically resected pancreatic ductal adenocarcinoma (PDAC) specimens revealed that CBX7 expression is significantly downregulated in pancreatic ductal adenocarcinoma, compared to normal pancreatic tissues. Reduced expression of CBX7 and PTEN was associated with increased malignancy grade in pancreatic adenocarcinoma, whereas maintenance of CBX7 and PTEN expression showed a trend toward a longer survival. These findings suggest CBX7 is an important tumor suppressor that negatively modulates PTEN/Akt signaling during pancreatic tumorigenesis.

The Yin and Yang of YY1 in tumor growth and suppression

Yin Yang-1 (YY1) is a zinc finger protein and member of the GLI-Kruppel family that can activate or inactivate gene expression depending on interacting partners, promoter context and chromatin structure, and may be involved in the transcriptional control of ∼10% of the total mammalian gene set. A growing body of literature indicates that YY1 is overexpressed in multiple cancer types and that increased YY1 levels correlate with poor clinical outcomes in many cancers. However, the role of YY1 in the promotion or suppression of tumor growth remains controversial and its regulatory effects may be tumor cell type dependent at least in experimental systems. The molecular mechanisms responsible for the apparently conflicting roles of YY1 are not yet fully elucidated. This review highlights recent advances in our understanding of regulatory insights involving YY1 function in a range of cancer types. For example, YY1's roles in tumor growth involve stabilization of hypoxia-inducible factor HIF-1α in a p53 independent manner, negative regulation of miR-9 transcription, control of MYCT1 transcription, a novel miR-193a-5p-YY1-APC axis, intracellular ROS and mitochondrial superoxide generation, p53 reduction and EGFR activation, control of genes associated with mitochondrial energy metabolism and miRNA regulatory networks involving miR-7, miR-9, miR-34a, miR-186, miR-381, miR-584-3p and miR-635. On the other hand, tumor suppressor roles of YY1 appear to involve YY1 stimulation of tumor suppressor BRCA1, increased Bax transcription and apoptosis involving cytochrome c release and caspase-3/-7 cleavage, induction of heme oxygenase-1, inhibition of pRb phosphorylation and p21 binding to cyclin D1 and cdk4, reduced expression of long noncoding RNA of SOX2 overlapping transcript, and MUC4/ErbB2/p38/MEF2C-dependent downregulation of MMP-10. YY1 expression is associated with that of cancer stem cell markers SOX2, BMI1 and OCT4 across many cancers suggesting multidynamic regulatory control and groups of cancers with distinct molecular signatures. Greater understanding of the mechanistic roles of YY1 will in turn lead to the development of more specific approaches to modulate YY1 expression and activity with therapeutic potential.