Proliferative lifespan checkpoints: cell-type specificity and influence on tumour biology

Molecular Docking and Molecular Dynamics Studies Reveal the Anticancer Potential of Medicinal-Plant-Derived Lignans as MDM2-P53 Interaction Inhibitors

The interaction between the tumor suppressor protein p53 and its negative regulator, the MDM2 oncogenic protein, has gained significant attention in cancer drug discovery. In this study, 120 lignans reported from Ferula sinkiangensis and Justicia procumbens were assessed for docking simulations on the active pocket of the MDM2 crystal structure bound to Nutlin-3a. The docking analysis identified nine compounds with higher docking scores than the co-crystallized reference. Subsequent AMDET profiling revealed satisfactory pharmacokinetic and safety parameters for these natural products. Three compounds, namely, justin A, 6-hydroxy justicidin A, and 6'-hydroxy justicidin B, were selected for further investigation due to their strong binding affinities of -7.526 kcal/mol, -7.438 kcal/mol, and -7.240 kcal/mol, respectively, which surpassed the binding affinity of the reference inhibitor Nutlin-3a (-6.830 kcal/mol). To assess the stability and reliability of the binding of the candidate hits, a molecular dynamics simulation was performed over a duration of 100 ns. Remarkably, the thorough analysis demonstrated that all the hits exhibited stable molecular dynamics profiles. Based on their effective binding to MDM2, favorable pharmacokinetic properties, and molecular dynamics behavior, these compounds represent a promising starting point for further refinement. Nevertheless, it is essential to synthesize the suggested compounds and evaluate their activity through in vitro and in vivo experiments.

BRCA1, BRCA2, TP53, PIK3CA, PTEN and AKT1 genes mutations in Burkina Faso breast cancer patients: prevalence, spectrum and novel variant

BRCA1 and BRCA2 are the two most commonly mutated tumor suppressor genes associated with hereditary breast cancer (BC). Also, mutations in TP53, PIK3CA, PTEN and AKT1 were observed at a high frequency in BC with their mutation spectrum exhibiting a subgroup particularity with enormous clinical significance in the prevention, classification and treatment of cancers. Unfortunately, the mutation spectrum of these genes is still unknown in most Sub-Saharan African population. Therefore, using samples from 133 unselected BC patients, we aimed to assess the contribution of these mutations by direct Sanger sequencing. The analysis revealed pathogenic germline variants on BRCA1 exon 11 (c.3331C > T, 0.75%) and BRCA2 exon 11 (c.5635G > T, c.6211delA; 1.5%). Five other pathogenic variants were identified in 61 of the 133 subjects (45.86%), with 39.09% for PIK3CA, 12.78% for TP53. Interestingly, a variant in PIK3CA found in high frequency in our population was different from the one usually found in other populations (c.1634A > C, 38.34%), and four patients carried mutations linked to Cowen Syndrome 5 c.[1634A > C;1658_1659delGTinsC]. A novel variant (c.312G > T) was found in TP53 gene at 12.78%. Overall, mutation carriers were found more in Her2 negative and in patients that underwent surgery and chemotherapy. No pathogenic variant was found in PTEN and AKT1. Our population displayed a high frequency of PIK3CA mutations with an unusual distribution and spectrum as well as a relatively low prevalence of BRCA mutations. Our results provided novel data on an unstudied population and may help in prevention, and the establishment of suitable therapeutic approaches for our population.

Recent advances on the intervention sites targeting USP7-MDM2-p53 in cancer therapy

The ubiquitin-specific protease 7 (USP7)-murine double minute 2 (MDM2)-p53 network plays an important role in the regulation of p53, a tumor suppressor which plays critical roles in regulating cell growth, proliferation, cell cycle progression, apoptosis and immune response. The overexpression of USP7 and MDM2 in human cancers contributes to cancer initiation and progression, and their inhibition reactivates p53 signalings and causes cell cycle arrest and apoptosis. Herein, the current state of pharmacological characterization, potential applications in cancer treatment and mechanism of action of small molecules used to target and inhibit MDM2 and USP7 proteins are highlighted, along with the outcomes in clinical and preclinical settings. Moreover, challenges and advantages of these strategies, as well as perspectives in USP7-MDM2-p53 field are analyzed in detail. The investigation and application of MDM2 and USP7 inhibitors will deepen our understanding of the function of USP7-MDM2-p53 network, and feed in the development of effective and safe cancer therapies where USP7-MDM2-p53 network is implicated.

Downregulation of PTEN mediates bleomycin-induced premature senescence in lung cancer cells by suppressing autophagy

Objective

Bleomycin is an important chemotherapeutic drug that activates premature senescence to decrease the tumorigenic process. We aimed to investigate the role of phosphatase and tensin homolog deleted on chromosome ten (PTEN) in bleomycin-induced premature senescence in lung cancer cells.

Methods

Human lung cancer A549 cells were incubated in the presence of different concentrations of bleomycin for 5 days. A lentivirus vector was used to silence the PTEN gene, followed by stimulation with bleomycin (1 µg/mL). Changes were evaluated by senescence-associated β-galactosidase staining, reverse transcription-polymerase chain reaction, and western blot.

Results

Treatment with bleomycin induced premature senescence. PTEN expression was decreased and key downstream molecules in the phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway were gradually activated following bleomycin treatment. Silencing PTEN reduced autophagy and accelerated senescence of A549 cells. Autophagy levels were also increased and senescence markers were reduced after inhibiting mTOR.

Conclusions

Downregulation of PTEN mediates bleomycin-induced premature senescence in lung cancer cells by suppressing autophagy via the PI3K/Akt/mTOR pathway. These findings provide new insights into the potential role of PTEN as a molecular target for cancer chemotherapy.

AMPK-mediated downregulation of connexin43 and premature senescence of mesangial cells under high-glucose conditions

Diabetic nephropathy is associated with premature senescence. Our previous study showed that glomerular mesangial cells (GMCs) appeared to take on senescent phenotypes under high-glucose conditions in conjunction with the downregulation of connexin43 (Cx43). In this study, we investigated whether AMPK-mediated Cx43 expression and premature senescence in diabetic nephropathy are associated with mTOR activation. From in vivo and in vitro studies, we found decreased expression of Cx43 and p-AMPK but increased expression of p21 both in the glomeruli of diabetic nephropathy and in primary GMCs cultured in high glucose. Activating AMPK or inhibiting mTOR prevented the downregulation of Cx43 and reversed GMC senescence. Dominant-negative AMPK expression both reduced Cx43 expression and induced GMC senescence. Furthermore, AMPK regulated Cx43 expression and GMC senescence mainly through the inhibition of mTOR, although other pathways cannot be ruled out. This study demonstrated that AMPK signaling pathways play an important role in the regulation of the Cx43 expression that accompanies GMC senescence under high-glucose conditions.

Genetic alterations in poorly differentiated and undifferentiated thyroid carcinomas

Thyroid gland presents a wide spectrum of tumours derived from follicular cells that range from well differentiated, papillary and follicular carcinoma (PTC and FTC, respectively), usually carrying a good prognosis, to the clinically aggressive, poorly differentiated (PDTC) and undifferentiated thyroid carcinoma (UTC).It is usually accepted that PDTC and UTC occur either de novo or progress from a pre-existing well differentiated carcinoma through a multistep process of genetic and epigenetic changes that lead to clonal expansion and neoplastic development. Mutations and epigenetic alterations in PDTC and UTC are far from being totally clarified. Assuming that PDTC and UTC may derive from well differentiated thyroid carcinomas (WDTC), it is expected that some PDTC and UTC would harbour genetic alterations that are typical of PTC and FTC. This is the case for some molecular markers (BRAF and NRAS) that are present in WDTC, PDTC and UTC. Other genes, namely P53, are almost exclusively detected in less differentiated and undifferentiated thyroid tumours, supporting a diagnosis of PDTC or, much more often, UTC. Thyroid-specific rearrangements RET/PTC and PAX8/PPARγ, on the other hand, are rarely found in PDTC and UTC, suggesting that these genetic alterations do not predispose cells to dedifferentiation. In the present review we have summarized the molecular changes associated with the two most aggressive types of thyroid cancer.

High-throughput protein expression analysis using tissue microarray technology of a large well-characterised series identifies biologically distinct classes of breast cancer confirming recent cDNA expression analyses

Recent studies on gene molecular profiling using cDNA microarray in a relatively small series of breast cancer have identified biologically distinct groups with apparent clinical and prognostic relevance. The validation of such new taxonomies should be confirmed on larger series of cases prior to acceptance in clinical practice. The development of tissue microarray (TMA) technology provides methodology for high-throughput concomitant analyses of multiple proteins on large numbers of archival tumour samples. In our study, we have used immunohistochemistry techniques applied to TMA preparations of 1,076 cases of invasive breast cancer to study the combined protein expression profiles of a large panel of well-characterized commercially available biomarkers related to epithelial cell lineage, differentiation, hormone and growth factor receptors and gene products known to be altered in some forms of breast cancer. Using hierarchical clustering methodology, 5 groups with distinct patterns of protein expression were identified. A sixth group of only 4 cases was also identified but deemed too small for further detailed assessment. Further analysis of these clusters was performed using multiple layer perceptron (MLP)-artificial neural network (ANN) with a back propagation algorithm to identify key biomarkers driving the membership of each group. We have identified 2 large groups by their expression of luminal epithelial cell phenotypic characteristics, hormone receptors positivity, absence of basal epithelial phenotype characteristics and lack of c-erbB-2 protein overexpression. Two additional groups were characterized by high c-erbB-2 positivity and negative or weak hormone receptors expression but showed differences in MUC1 and E-cadherin expression. The final group was characterized by strong basal epithelial characteristics, p53 positivity, absent hormone receptors and weak to low luminal epithelial cytokeratin expression. In addition, we have identified significant differences between clusters identified in this series with respect to established prognostic factors including tumour grade, size and histologic tumour type as well as differences in patient outcomes. The different protein expression profiles identified in our study confirm the biologic heterogeneity of breast cancer and demonstrate the clinical relevance of classification in this manner. These observations could form the basis of revision of existing traditional classification systems for breast cancer.

Truncated N-terminal mutants of SV40 large T antigen as minimal immortalizing agents for CNS cells

Immortalized central nervous system (CNS) cell lines are useful as in vitro models for innumerable purposes such as elucidating biochemical pathways, studies of effects of drugs, and ultimately, such cells may also be useful for neural transplantation. The SV40 large T (LT) oncoprotein, commonly used for immortalization, interacts with several cell cycle regulatory factors, including binding and inactivating p53 and retinoblastoma family cell-cycle regulators. In an attempt to define the minimal requirements of SV40 T antigen for immortalizing cells of CNS origin, we constructed T155c, encoding the N-terminal 155 amino acids of LT. The p53 binding region is known to reside in the C-terminal region of LT. An additional series of mutants was produced to further narrow the molecular targets for immortalization, and plasmid vectors were constructed for each. In a p53 temperature sensitive cell line model, T64-7B, expression of T155c and all constructs having mutations outside of the first 82 amino acids were capable of overriding cell-cycle block at the non-permissive growth temperature. Several cell lines were produced from fetal rat mesencephalic and cerebral cortical cultures using the T155c construct. The E107K construct contained a mutation in the Rb binding region, but was nonetheless capable of overcoming cell cycle block in T64-7B cell and immortalizing primary cultured cells. Cells immortalized with T155c were often highly dependent on the presence of bFGF for growth. Telomerase activity, telomere length, growth rates, and integrity of the p53 gene in cells immortalized with T155c did not change over 100 population doublings in culture, indicating that cells immortalized with T155c were generally stable during long periods of continuous culture.

Divergent patterns of telomere maintenance mechanisms among human sarcomas: sharply contrasting prevalence of the alternative lengthening of telomeres mechanism in Ewing's sarcomas and osteosarcomas

Two types of telomere maintenance mechanisms (TMMs) have been described in human tumors: telomerase activation and alternative lengthening of telomeres (ALT). Although the vast majority of epithelial tumors rely on telomerase activation, many mesenchymal tumors rely on ALT for telomere maintenance, but within this tumor group, the TMMs used by translocation-associated sarcomas have not been systematically studied. We studied telomere lengths and telomerase expression and activity in 30 uncultured tumor samples and in 10 cell lines of Ewing's sarcoma, a prototypical translocation-associated sarcoma, and compared the data to an identical analysis of 60 osteosarcomas, the most common type of sarcoma lacking a specific translocation. Telomerase activity was demonstrated in 21 Ewing's sarcoma tumor samples (70%) and in 9 of 10 Ewing's sarcoma cell lines. Evidence of ALT, indicated by the presence of long and heterogeneous telomeres, was observed only in the cell line without telomerase activity and in none of the 30 Ewing's sarcoma tumor samples. The 9 Ewing's sarcoma patients whose tumors lacked detectable telomerase activity did not differ significantly from the remaining patients in age, stage, EWSR1-FLI1 fusion type, prevalence of TP53 point mutations, or overall survival. The low prevalence of ALT in Ewing's sarcoma contrasted sharply with our data on TMMs in 60 osteosarcomas, which showed ALT in 38 of 60 cases (P<0.0001). The present study, together with emerging published data on other sarcoma types, suggests that a predominance of telomerase activation in the absence of ALT may characterize sarcomas with specific chromosomal translocations (such as Ewing's sarcoma), whereas a high prevalence of ALT appears typical of sarcomas with nonspecific complex karyotypes (such as osteosarcoma).

Drop of connexin 43 in replicative senescence of human fibroblasts HEL-299 as a possible biomarker of senescence

The expression of connexin 43 (cx43) and cell-cell communication were studied in replicative senescence of cultured HEL-299 fibroblasts. A progressive decrease in fluorescent dye transfer was detected by a scrape-loading technique in aging fibroblasts. This change was accounted for by a marked decrease in the amount of cx43 in aging cells, as detected by western blot analysis (cell extracts) and indirect fluorescence (cells in culture). However, semiquantitative RT-PCR assays of cx43 mRNA did not reveal appreciable changes, which suggests several possible explanations for the mechanism(s) underlying the decrease of cx43 in aging cells. These findings support the idea that the reduced expression of cx43 might be a biomarker of cell senescence.

Sequential extension of proliferative lifespan in human fibroblasts induced by over-expression of CDK4 or 6 and loss of p53 function

Replicative senescence is thought to be a significant barrier to human tumorigenesis, which in human fibroblasts, and many other cell types, can be overcome experimentally by combined loss of function of p53 and Rb 'pathways'. To avoid the confounding pleiotropic effects of HPVE7 frequently used in such studies, here we have employed retroviral vectors over-expressing CDK4 or CDK6 as a more representative model of naturally-occurring mutations targeting the Rb pathway. We show that these can extend fibroblast lifespan by approximately 10 population doublings, ending in a viable senescence-like state which contrasts with the apoptotic end-stage seen with E7. Compared with 'normal' senescence, this growth arrest was, in most cases, not accompanied by any further increase in p21(Waf1) levels but with up to a 19-fold increase in p16(Ink4a). Surprisingly however, this could not explain arrest, since expression of mutant CDK4 and/or CDK6, incapable of binding p16(Ink4a), did not confer any greater lifespan extension than the wild-type CDKs. Subsequent abrogation of p53 function by a second vector, encoding HPVE6, downregulated p21(Waf1) and conferred a second lifespan extension, ending in a crisis-like state, consistent with full escape from senescence. These data: (i) point to a back-up 'senescence' mechanism distinct from induction of p21(Waf1) or p16(Ink4a); and (ii) provide an in vitro model of clonal evolution through successive dysfunction of Rb and p53 pathways in a relevant human cell context.

p53 mutation in breast cancer. Correlation with cell kinetics and cell of origin

AIM

Several studies have investigated the expression of the cytokeratins (CKs), vimentin, the epithelial growth factor receptor (EGFR), the oestrogen receptor (ER), and the progesterone receptor (PgR), in breast cancer, but no study has directly compared p53 mutations with these phenotypic and differentiation markers in the same case. The present study was designed to provide some of this information.

METHODS

The expression of the p53 and bcl-2 proteins was evaluated by immunohistochemistry in relation to phenotypic characteristics and cellular kinetic parameters (mitotic index and apoptotic index) in 37 cases of ductal carcinoma in situ (DCIS) and 27 cases of infiltrating ductal carcinoma (IDC) of the breast. In addition, p53 gene mutation was examined by polymerase chain reaction single strand conformation polymorphism analysis (SSCP).

RESULTS

Thirteen cases (eight DCIS and five IDC) showed expression of CK8, CK14, CK18, vimentin, and EGFR, consistent with a stem cell phenotype, whereas 44 cases (27 DCIS and 17 IDC) showed expression of CK8 and CK1, weak or negative expression of CK18, but were negative for vimentin and EGFR, consistent with a luminal cell phenotype. DCIS and IDC cases with a stem cell phenotype were ER/PgR negative and intermediately or poorly differentiated. In contrast, the cases with luminal cell phenotype were ER/PgR positive and well or intermediately differentiated. In addition, intermediately or poorly differentiated cases with a stem cell phenotype showed higher proliferative activity (per cent of MIB-l positive cells) than did intermediately or well differentiated cases with a luminal cell phenotype. Both DCIS and IDC cases with a stem cell phenotype were p53 positive and bcl-2 negative by immunohistochemistry. In IDC, p53 expression was associated with a reduction of both mitotic index and apoptotic index compared with DCIS. Most of the tumours showing a more differentiated phenotype (luminal) were p53 negative and bcl-2 positive. In these cases, cell kinetic parameters increased from DCIS to IDC. These data suggest the existence of subsets of DCIS and IDC that, because of their phenotypic characteristics, could be derived from subpopulations of normal breast cells having different control mechanisms of cell proliferation and neoplastic progression.

CONCLUSIONS

These results are compatible with the hypothesis that the phenotype of the cell of origin constrains both tumour phenotype and the choice of genetic events; however, the occurrence of p53 mutants by chance during neoplastic transformation cannot be excluded.

Regulation of the cell cycle in normal and pathological glia

Precise regulation of the glial cell cycle is essential during nervous system development and in response to injury, whereas disruption of cell cycle control is associated with malignant glial tumors and other nervous system diseases. The Ras signaling pathway plays a central role in regulating the mammalian cell cycle, and uncontrolled Ras signaling has been implicated in a wide range of human cancers, including malignant gliomas. Recent studies in glia have demonstrated that activation of Ras can either induce or inhibit proliferation through complex interactions among downstream signaling pathways impinging on cell cycle regulatory proteins. Studies in Schwann cells have begun to delineate the pathways by which Ras regulates the cell cycle in normal and pathological glia, and have identified promising targets for therapeutic intervention in the treatment of PNS and CNS malignant glial tumors.

Senescence represses the nuclear localization of the serum response factor and differentiation regulates its nuclear localization with lineage specificity

The differentiation of cultured 3T3T mesenchymal stem cells into adipocytes represses growth factor responsiveness by limiting the nuclear localization of the serum response factor (SRF) that binds to and activates the promoters of growth control genes that contain the serum response elements (SRE), such as junB and c-fos. The regulation of SRF nuclear localization by adipocyte differentiation is specific, because we show that adipocyte differentiation does not repress the nuclear localization of six other transacting factors. To determine if repression of growth factor responsiveness that occurs during senescence also represses the nuclear localization of SRF, we studied normal human WI-38 fibroblasts at low versus high population doublings. The results show that SRF localizes to the nucleus of proliferative cells whereas in senescent cells SRF can not be detected in the nucleus. This result is apparent in both immunofluorescence assays and in western blot analysis. We next evaluated the cellular distribution of SRF in selected human tissues to determine whether the loss of proliferative potential in vivo could have a different effect on SRF nuclear localization. We found that in cells of the small bowel mucosa, differentiation modulates SRF nuclear localization in an opposite manner. Minimal SRF expression and nuclear localization is evident in undifferentiated cells at the base of crypts whereas increased SRF expression and nuclear localization is evident in differentiated cells at the surface tip of the villus. These results together establish that regulation of SRF expression and nuclear localization is important in senescence and differentiation in a lineage specific manner.

Radiation-induced senescence-like growth arrest requires TP53 function but not telomere shortening

Suzuki, K., Mori, I., Nakayama, Y., Miyakoda, M., Kodama, S. and Watanabe, M. Radiation-Induced Senescence-like Growth Arrest Requires TP53 Function but not Telomere Shortening. Normal human diploid cells irradiated with X rays showed permanent cell cycle arrest and exhibited senescence-like phenotypes including the expression of senescence-associated beta-galactosidase (SA-beta-gal). X irradiation caused persistent phosphorylation of TP53 at Ser 15 and accumulation of the TP53 protein, followed by the induction of CDKN1A (also known as p21(Waf1/Cip1)) and CDKN2A (also known as p16), preceded the expression of SA-beta-gal. NCI-H1299 human lung carcinoma cells, in which no TP53 protein was expressed, were irradiated with X rays with or without the exogenous expression of TP53 gene. Although induction of TP53 protein alone could induce SA-beta-gal expression, the frequency of SA-beta-gal-positive cells was significantly increased when TP53-induced H1299 cells were exposed to X rays. The mean terminal restriction fragment length in normal human cells was approximately 12 kb and did not change in SA-beta-gal-positive cells. These results indicate that ionizing radiation induces senescence-like growth arrest that is dependent on TP53 function but independent of telomere shortening. Our findings suggest that cells harboring irreparable DNA damage are programmed to undergo premature senescence to maintain the integrity of the genome.

Differential gene expression in mouse mammary adenocarcinomas in the presence and absence of wild type p53

The tumor suppressor p53 transcriptionally regulates a large number of target genes that may affect cell growth and cell death pathways. To better understand the role of p53 loss in tumorigenesis, we have developed a mouse mammary cancer model, the Wnt-1 TG/p53 model. Wnt-1 transgenic females that are p53-/- develop mammary adenocarcinomas that arise sooner, grow faster, appear more anaplastic, and have higher levels of chromosomal instability than their Wnt-1 transgenic p53+/+ counterparts. In this study, we used several assays to determine whether the presence or absence of p53 affects gene expression patterns in the mammary adenocarcinomas. Most of the differentially expressed genes are increased in p53+/+ tumors and many of these represent known target genes of p53 (p21WAF/C1P1, cyclin G1, alpha smooth muscle actin, and cytokeratin 19). Some of these genes (cytokeratin 19, alpha smooth muscle actin, and kappa casein) represent mammary gland differentiation markers which may contribute to the inhibited tumor progression and are consistent with the more differentiated histopathology observed in the p53+/+ tumors. Several differentially expressed genes are growth regulatory in function (p21, c-kit, and cyclin B1) and their altered expression levels correlate well with the differing growth properties of the p53+/+ and p53-/- tumors. Thus, while tumors can arise and progress in the presence of functioning wild type p53, p53 may directly or indirectly regulate expression of an array of genes that facilitate differentiation and inhibit proliferation, contributing to a more differentiated, slow growing, and genomically stable phenotype.

Cell-cycle control in cell-biomaterial interactions: expression of p53 and Ki67 in human umbilical vein endothelial cells in direct contact and extract testing of biomaterials

Current biocompatibility testing involves the demonstration of cell proliferation, which is usually interpreted as a sign of positive biocompatibility when the materials sustain cell proliferation. As the field of biomaterials research is rapidly moving toward tissue-engineered devices and hybrid organs, control of cell function has become a main topic. Cell function, which involves specific differentiation pathways, cannot be separated from cell-cycle control. The study of cell-cycle control is an important extension of routine proliferation assays and has extensive roots in developmental and tumor biology. We studied the expression of the tumour suppressor gene p53 and the proliferation-associated antigen Ki67 of endothelial cells in response to biomaterial contact. Cells were seeded in six- or 24-well plates, in which one or three 12-mm-diameter biomaterial disks were laid down. After 48- and 72-h incubation periods, cells were processed for flow cytometry, immunofluorescence, or Western blotting. The following materials were used: titanium, NiCr alloy, and CoCr alloy. Cells were also exposed to 24-h (ISO-norm) extracts in 25-cm(2) culture flasks (600, 000 cells) for 24 and 48 h. For extract testing, serially diluted Ni-ion suspensions were also used. Human umbilical vein endothelial cells adhered to metal surfaces and started forming a monolayer within 3 days. Ki67 expression was positive in more than 60% after 2 days and decreased markedly after 3 days of adhesion. During this time cells developed focal contacts and produced a fibronectin matrix. p53 expression could be demonstrated with Western blotting and flow cytometry, but not with immunofluorescence. Differences due to both culturing time and material were found in expression patterns with both methods. Inverse correlations between Ki67 and p53 expression were detected, which are probably based on culture kinetics. The results indicate that expression of p53 and also Ki67 is clearly influenced by biomaterials in direct contact testing, despite the absence of obvious morphological differences. The p53 marker can be used for defining cell function in more detail, although the correlation with specific physiological function has still to be clarified.

Thymic involution in aging

The size of the naive T-cell pool is governed by output from the thymus and not by replication. This pool contributes cells to the activated/memory T-cell pool whose size can be increased through cell multiplication; both pools together constitute the peripheral T-cell pool. Aging is associated with involution of the thymus leading to a reduction in its contribution to the naive T-cell pool; however, despite this diminished thymic output, there is no significant decline in the total number of T cells in the peripheral T-cell pool. There are, however, considerable shifts in the ratios of both pools of cells, with an increase in the number of activated/memory T cells and the accumulation in older individuals of cells that fail to respond to stimuli as efficiently as T cells from younger individuals. Aging is also associated with a greater susceptibility to some infections and some cancers. An understanding of the causal mechanism of thymic involution could lead to the design of a rational therapy to reverse the loss of thymic tissue, renew thymic function, increase thymic output, and potentially improve immune function in aged individuals.

Posttranslational modifications of p53 in replicative senescence overlapping but distinct from those induced by DNA damage

Replicative senescence in human fibroblasts is absolutely dependent on the function of the phosphoprotein p53 and correlates with activation of p53-dependent transcription. However, no evidence for posttranslational modification of p53 in senescence has been presented, raising the possibility that changes in transcriptional activity result from upregulation of a coactivator. Using a series of antibodies with phosphorylation-sensitive epitopes, we now show that senescence is associated with major changes at putative regulatory sites in the N and C termini of p53 consistent with increased phosphorylation at serine-15, threonine-18, and serine-376 and decreased phosphorylation at serine-392. Ionizing and UV radiation generated overlapping but distinct profiles of response, with increased serine-15 phosphorylation being the only common change. These results support a direct role for p53 in signaling replicative senescence and are consistent with the generation by telomere erosion of a signal which shares some but not all of the features of DNA double-strand breaks.

The in vitro life-span of human periodontal ligament fibroblasts

The in vitro life-span of human periodontal ligament fibroblasts (PDLF) was studied on clones from periodontium of teeth extracted due to periodontitis and dental caries (69 clones/192 individuals, aged 20-80 years) and from periodontium of teeth extracted for orthodontic reasons (23 clones/26 individuals, aged 15-19 years). In the primary cultures the ratio of the number of cells expressing senescence-associated beta-galactosidase (SA-beta-Gal) to the total number of cells is significantly larger in PDLF (92 clones; 11.1+/-4.9%) than in human gingival fibroblasts (GF) (10 clones; 0.5+/-0.1 %). The finite population doubling numbers (PD) of PDLF are not age-matched and the mean PD of PDLF (7.1+/-2.9) is significantly smaller than GF (28.5+/-3.2), IMR-90 (human lung fibroblasts, 5 clones; 44.3 +/- 2.2), and human osteoblasts (5 clones; 19.7+/-1.4). Comparing the ratio of the number of SA-beta-Gal positive cells to the total number of cells in primary culture, and the finite PD in PDLF cultures: 1) the ratio of 15-19 years old donor group is significantly smaller than in the other donor groups (20-29, 30-39, 40-49, 50-59 and 60-80 years old), and 2) there were no statistically significant differences among the 20-29, 30-39, 40-49 and 50-59 year old donor groups, and the 30-39, 40-49, 50-59 and 60-80 year old donor groups. These findings suggest that the in vitro life-span of PDLF is shorter than other fibroblasts in the connective tissues and that PDLF may undergo senescence in adult clones without relation to donor's age. There may be more aged fibroblasts in periodontium than in other tissues, such as gingiva and lung.

Impairment of osteocalcin production in senescent periodontal ligament fibroblasts

Osteocalcin production of senescent periodontal ligament fibroblasts (PDLF) with the expression of senescence-associated beta-galactosidase (SA-beta-Gal) was investigated on clones from 50-80 years old donors (n=20) with teeth extracted due to periodontitis and dental caries, and from 15-19 year old donors (n=20) with normal teeth extracted for orthodontic reasons. Immunohistochemically, the nonsenescent PDLF in all cultures in passage 2 showed strong reactivity with anti-osteocalcin. The reactive intensity of PDLF (passage 2, PD 3.0) was significantly stronger in 50-80 year old donor group than in 15-19 year old donor group, suggesting that osteocalcin production of PDLF cultured in early passage is larger in cells from adult population than in cells from adolescent population. In PDLF cultures in passage 2 from 50-80 year old donor, two types of senescent cells were found: one with strong reactivity to anti-osteocalcin and the other with little detectable reactivity. The culture consisted of senescent PDLF (passage 8, PD 14.8) did not include cells which have a detectable reactivity with anti-osteocalcin immunohistochemically and the reactive intensity was significantly weaker in the senescent culture than in the culture in passage 2 by ELISA. This suggests that the production potential of osteocalcin is impaired in PDLF with aging in culture. Further, the reactive intensity with anti-osteocalcin of PDLF in passage 2 deprived of serum for 48 h was 6% of that of cells cultured with serum and the reaction increased after serum stimulation, suggesting that the osteocalcin production in PDLF in early passage is implicated in mitogenic stimulation.

Telomere length and telomerase activity in malignant lymphomas at diagnosis and relapse

Telomere length maintenance, in the vast majority of cases executed by telomerase, is a prerequisite for long-term proliferation. Most malignant tumours, including lymphomas, are telomerase-positive and this activity is a potential target for future therapeutic interventions since inhibition of telomerase has been shown to result in telomere shortening and cell death in vitro. One prerequisite for the suitability of anti-telomerase drugs in treating cancer is that tumours exhibit shortened telomeres compared to telomerase-positive stem cells. A scenario is envisioned where the tumour burden is reduced using conventional therapy whereafter remaining tumour cells are treated with telomerase inhibitors. In evaluating the realism of such an approach it is essential to know the effects on telomere status by traditional therapeutic regimens. We have studied the telomere lengths in 47 diagnostic lymphomas and a significant telomere shortening was observed compared to benign lymphoid tissues. In addition, telomere length and telomerase activity were studied in consecutive samples from patients with relapsing non-Hodgkin's lymphomas. Shortened, unchanged and elongated telomere lengths were observed in the relapse samples. The telomere length alterations found in the relapsing lymphomas appeared to be independent of telomerase and rather represented clonal selection random at the telomere length level. These data indicate that anti-telomerase therapy would be suitable in only a fraction of malignant lymphomas.

Cancer and the cell cycle

The purpose of this short review is to provide an overview of mammalian somatic cell cycle events and their controls. Cell cycle-related studies have been under way for only 5% of this millennium, yet since then nearly 20,000 references have appeared. This vast literature cannot be detailed here, nor can fundamental information obtained with other organisms such as yeast and Xenopus, or topics such as the abbreviated cell cycle in early embryonic cells. (General references include Murray and Hunt [1993] The cell cycle, an introduction. New York: Oxford University Press, and Denhardt [1999] In: The molecular basis of cell cycle and growth control. p 225-304. New York: John Wiley & Sons, Inc.) J. Cell Biochem. Suppls. 32/33:166-172, 1999.

Contrasting effects of activating mutations of GalphaS and the thyrotropin receptor on proliferation and differentiation of thyroid follicular cells

The cyclic AMP pathway is a major regulator of thyrocyte function and proliferation and, predictably, its inappropriate activation is associated with a sub-set of human thyroid tumours. Activating mutations are, however, more common in the thyrotropin receptor (TSHR) than in its downstream transducer, Galphas. To investigate whether this reflects an inherent difference in their oncogenic potency, we compared the effects of retrovirally-transduced mutant (A623I) TSHR or (Q227L) Galphas (GSP), using the rat thyroid cell line FRTL5 and primary human thyrocytes. In FRTL5, expression of GSP or mutant (m) TSHR induced a 2 - 3-fold increase in basal levels of cAMP. This was associated with TSH-independent proliferation (assessed by both cell number and DNA synthesis) and function (as shown by increased expression of thyroglobulin (Tg) and the sodium/iodide symporter). In primary cultures, expression of mTSHR, but not GSP, consistently induced formation of colonies with epithelial morphology and thyroglobulin expression, capable of 10 - 15 population doublings (PD) compared to less than three in controls. Thus, while mTSHR and GSP exert similar effects in FRTL5, use of primary cultures reveals a major difference in their ability to induce sustained proliferation in normal human thyrocytes, and provides the first direct evidence that mTSHR is sufficient to initiate thyroid tumorigenesis.

Differential display analysis can reveal patterns of gene expression in immortalised hepatoma cells which are similar to those observed in young adult but not old adult liver cells

We used the differential display technique to examine whether there were any patterns of gene expression which were characteristic of both young adult rat liver and of immortalised rat hepatoma cell lines, but not of old adult rat liver. No genes were detected which appeared to be clearly expressed in young liver and immortalised cell lines, but not in old liver. However, 14 genes were detected in old liver which were down-regulated in young liver and the hepatoma cell lines. This observation lends support to the idea that immortalisation of malignant cells may involve, at least in some aspects, a reversal of the ageing process in these cells and that the genes involved have a recessive action.

Control of replicative life span in human cells: barriers to clonal expansion intermediate between M1 senescence and M2 crisis

The accumulation of genetic abnormalities in a developing tumor is driven, at least in part, by the need to overcome inherent restraints on the replicative life span of human cells, two of which-senescence (M1) and crisis (M2)-have been well characterized. Here we describe additional barriers to clonal expansion (Mint) intermediate between M1 and M2, revealed by abrogation of tumor-suppressor gene (TSG) pathways by individual human papillomavirus type 16 (HPV16) proteins. In human fibroblasts, abrogation of p53 function by HPVE6 allowed escape from M1, followed up to 20 population doublings (PD) later by a second viable proliferation arrest state, MintE6, closely resembling M1. This occurred despite abrogation of p21(WAF1) induction but was associated with and potentially mediated by a further approximately 3-fold increase in p16(INK4a) expression compared to its level at M1. Expression of HPVE7, which targets pRb (and p21(WAF1)), also permitted clonal expansion, but this was limited predominantly by increasing cell death, resulting in a MintE7 phenotype similar to M2 but occurring after fewer PD. This was associated with, and at least partly due to, an increase in nuclear p53 content and activity, not seen in younger cells expressing E7. In a different cell type, thyroid epithelium, E7 also allowed clonal expansion terminating in a similar state to MintE7 in fibroblasts. In contrast, however, there was no evidence for a p53-regulated pathway; E6 was without effect, and the increases in p21(WAF1) expression at M1 and MintE7 were p53 independent. These data provide a model for clonal evolution by successive TSG inactivation and suggest that cell type diversity in life span regulation may determine the pattern of gene mutation in the corresponding tumors.

Telomerase activity is sufficient to allow transformed cells to escape from crisis

The introduction of simian virus 40 large T antigen (SVLT) into human primary cells enables them to proliferate beyond their normal replicative life span. In most cases, this temporary escape from senescence eventually ends in a second proliferative block known as "crisis," during which the cells cease growing or die. Rare immortalization events in which cells escape crisis are frequently correlated with the presence of telomerase activity. We tested the hypothesis that telomerase activation is the critical step in the immortalization process by studying the effects of telomerase activity in two mortal SVLT-Rasval12-transformed human pancreatic cell lines, TRM-6 and betalox5. The telomerase catalytic subunit, hTRT, was introduced into late-passage cells via retroviral gene transfer. Telomerase activity was successfully induced in infected cells, as demonstrated by a telomerase repeat amplification protocol assay. In each of nine independent infections, telomerase-positive cells formed rapidly dividing cell lines while control cells entered crisis. Telomere lengths initially increased, but telomeres were then maintained at their new lengths for at least 20 population doublings. These results demonstrate that telomerase activity is sufficient to enable transformed cells to escape crisis and that telomere elongation in these cells occurs in a tightly regulated manner.

A reassessment of the telomere hypothesis of senescence

According to the telomere hypothesis of senescence, the telomeric shortening that accompanies the replication of normal somatic cells acts as the mitotic clock that eventually results in their permanent exit from the cell cycle. Although evidence consistent with the telomere hypothesis continues to accumulate, on the basis of recent findings it is suggested that instead of a single clock mechanism there are multiple inducers of senescence.

Genes involved in the control of cellular proliferative potential

Evidence that control of cellular proliferative potential may be linked to telomere length, along with data indicating that other factors may also be involved, will be reviewed. According to the telomere hypothesis of senescence, the sequential loss of telomeric repeat DNA that occurs during the replication of normal somatic cells eventually dictates the onset of the permanently nonreplicative state known as senescence. Many immortalized cells express telomerase, a ribonucleoprotein enzyme that replaces the telomeric DNA that would otherwise be lost due to replication. However, some immortalized human cells may avoid telomeric shortening without using telomerase. The mechanism involved is currently unknown, but other eukaryotes are able to replace telomeric DNA through (1) recombination and copy switching or (2) retrotransposition. Human fibroblasts that lose p53 function proliferate a limited number of times beyond the population-doubling level at which their normal counterparts become senescent. Lack of functional retinoblastoma (Rb) protein (or equivalent events, such as loss of p16INK4 function, resulting in abrogation of Rb regulatory activity) also permits a temporary extension of proliferative potential. The p53 and pRb effects are additive, indicating that they exert their control on proliferative potential separately. The temporary life span extension associated with loss of p53 and/or Rb pathway function is accompanied by continued telomere shortening. The proliferation arrest that eventually ensues in p53-minus cells or in p53-minus/Rb-minus cells may be regarded as terminal proliferation arrest states serving as a backup to senescence. p53-minus/Rb-minus cells cannot proliferate further unless they acquire the ability to prevent telomeric shortening. Somatic cell hybridization and microcell-mediated chromosome transfer experiments indicate that immortalization involves the loss of function of other, as yet unidentified, genes; some of these may normally repress telomerase expression in somatic cells.

Resistance to 5-fluorouracil

1. Primary and secondary resistance to the widely used antimetabolite 5-fluorouracil (5-FU) are common phenomena in cancer chemotherapy. Because 5-FU still remains the agent of choice in the treatment of, for example, colorectal cancer, circumvention of resistance is of vital importance. 2. Resistance to fluoropyrimidines is a multifactorial event, which includes transport mechanisms, metabolism, molecular mechanisms, protection from apoptosis, and resistance via cell cycle kinetics. To date, the prediction of primary resistance to 5-FU in the clinic is limited to few studies focusing mainly on the key enzyme thymidylate synthase. To gain a deeper insight into the key events responsible for 5-FU resistance in vivo, the evaluation of additional parameters such as other (fluoro)pyrimidine converting enzymes, the mutational status of regulators of apoptosis, and tumour angiogenesis is currently under investigation. 3. Most studies on the circumvention of fluoropyrimidine resistance refer to preclinical investigations and were rarely confirmed in clinical trials. Although our understanding of resistance to 5-FU leaves many open questions, the fundamental insights accomplished during the last years provide a rational understanding to exceed the bounds of the actual therapeutic schedules.

Telomerase activity in relation to p53 status and clinico-pathological parameters in breast cancer

Cell cycle deregulation can occur at different levels in cancer. In human breast cancer it includes overexpression of cyclins D1 and E, down-regulation of cyclin-dependent kinase inhibitors and inactivation of the retinoblastoma and p53 tumor suppressor proteins. Telomerase activity is strongly associated with an immortal phenotype and expression of telomerase is linked to the cell cycle. We have recently demonstrated a connection between specific cell cycle defects within the pRB pathway and levels of telomerase activity in breast cancer. In the present study, 106 tumors were investigated for p53 gene and protein status. By single strand conformation polymorphism (SSCP) analysis, 15% showed mutations within exons 5-8 and by immunohistochemistry (IHC), 29% were p53 positive. Tumors with a telomerase activity above median (i.e., telomerase(high)) were significantly associated with p53 protein accumulation (p = 0.004), but not with p53 gene mutations. The strongest telomerase expression was found in tumors with p53 protein accumulation. Morphologic grade, estrogen and progesterone receptor expression differed significantly between the telomerase(high) and telomerase(low) groups (p < 0.0001, p = 0.016 and p = 0.046, respectively), but no difference was observed for stage or nodal status. Telomerase(high) tumors were significantly associated with a poor prognosis for node-negative (N0) patients (p = 0.008), but not for node-positive (N+) patients, whereas the opposite was demonstrated for tumors with p53 accumulation. The survival data indicated that telomerase expression has biological importance particularly for N0 tumors, suggesting that telomerase(low) tumors constitute a group of "pre-immortalized" tumors with a good prognosis.

Dissociation of telomere dynamics from telomerase activity in human thyroid cancer cells

Prevention of telomere erosion through acquisition of telomerase activity is thought to be an essential mechanism in most human cancer cells for avoidance of cellular senescence and crisis. It has been generally assumed that once telomerase has been activated, no further telomere shortening should ensue. We show here, however, that a much more complex pattern of telomere dynamics can exist in telomerase-positive immortal cancer cells. Using a panel of subclones derived from a human thyroid cancer cell line, K1E7, we found that some clones show persistent decline in mean telomere restriction fragment (TRF) length by up to 2 kb over 450 population doublings (pd), despite sustained high telomerase activity (as assessed by the in vitro "TRAP" assay). TRF length subsequently stabilized at around 5 kb, but with no corresponding increase in telomerase activity. One clone showed an even more unexpected biphasic time course, with the mean TRF length initially increasing by 1.5 kb over 90 pd, before "plateauing" and then returning over a similar period to its original value, again without any correlation to TRAP activity. Such dissociations between telomere dynamics and telomerase activity support the existence of additional controls on telomere length in the intact cell. Our observations are consistent with current negative-feedback models of telomere length regulation by telomere binding proteins and these cell lines should prove useful experimental tools for their further evaluation.

Reinitiation of DNA synthesis and cell division in senescent human fibroblasts by microinjection of anti-p53 antibodies

In human fibroblasts, growth arrest at the end of the normal proliferative life span (induction of senescence) is dependent on the activity of the tumor suppressor protein p53. In contrast, once senescence has been established, it is generally accepted that reinitiation of DNA synthesis requires loss of multiple suppressor pathways, for example, by expression of Simian virus 40 (SV40) large T antigen, and that even this will not induce complete cell cycle traverse. Here we have used microinjection of monoclonal antibodies to the N terminus of p53, PAb1801 and DO-1, to reinvestigate the effect of blocking p53 function in senescent human fibroblasts. Unexpectedly, we found that both antibodies induce senescent cells to reenter S phase almost as efficiently as SV40, accompanied by a reversion to the "young" morphology. Furthermore, this is followed by completion of the cell division cycle, as shown by the appearance of mitoses, and by a four- to fivefold increase in cell number 9 days after injection. Immunofluorescence analysis showed that expression of the p53-inducible cyclin/kinase inhibitor p21sdi1/WAF1 was greatly diminished by targeting p53 with either PAb1801 or DO-1 but remained high and, moreover, still p53 dependent in cells expressing SV40 T antigen. As previously observed for induction, the maintenance of fibroblast senescence therefore appears to be critically dependent on functional p53. We suggest that the previous failure to observe this by using SV40 T-antigen mutants to target p53 was most probably due to incomplete abrogation of p53 function.

Ras versus cyclin-dependent kinase inhibitors

In the past year, complex interactions between Ras and the cell cycle have been identified. In primary cells, activated Ras induces a cell-cycle arrest via the induction of cyclin-dependent kinase inhibitors (CDKIs). Oncogenic changes that cooperate with Ras act by neutralising CDKIs by various mechanisms. In the absence of a negative growth signal from Ras, such as in most immortalised cell lines, Ras acts positively on the cell cycle. Insights have been made into the mechanisms by which Ras abrogates remaining cell-cycle controls.

Mammalian telomerase: catalytic subunit and knockout mice

For the second time this year random cDNA sequencing, in combination with data from unicellular eukaryotes, has made a significant contribution to the analysis of human telomerase. Two groups have reported mammalian homologues of the Tetrahymena p80 telomerase-associated protein, in both cases the key breakthrough being mammalian cDNA clones with database matches to Tetrahymena p80. This has now been joined by the sequence of a candidate for the human telomerase catalytic subunit. The discovery that its message abundance closely follows telomerase activity could make a major impact on the utility of telomerase as a diagnostic marker for human malignancy. In addition, Blasco et al . report the phenotype of a transgenic mouse deleted for the mTR gene, which encodes the essential RNA component of telomerase. Interestingly tumour formation is unaffected in these mice, strengthening the argument that telomerase expression in mouse tumourigenesis is an innocent bystander rather than a necessary event. However, fundamental differences between the genomic organisation of mouse and human telomeres mean that the mouse is not a straightforward model to critically test the role of telomere loss and telomerase in human malignancy.