The p16INK4a-RB pathway: molecular link between cellular senescence and tumor suppression

Correlation of telomere length shortening with promoter methylation profile of p16/Rb and p53/p21 pathways in breast cancer

Unregulated cell growth, a major hallmark of cancer, is coupled with telomere shortening. Measurement of telomere length could provide important information on cell replication and proliferation state in cancer tissues. Telomere shortening and its potential correlation with downregulation of cell-cycle regulatory elements were studied by the examination of relative telomere length and methylation status of the TP53, P21 and P16 promoters in tissues from breast cancer patients. Telomere length was measured in 104 samples (52 tumors and paired adjacent normal breast tissues) by quantitative PCR. Methylation profile of selected genes was analyzed in all samples using a matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Our results demonstrated a significant shortening of tumor telomere regions compared with paired adjacent normal tissues (P<0.001). Similarly, telomere lengths were significantly shorter in advanced stage cases and in those with higher histological grades (P<0.05). Telomere shortening in cancer tissues was correlated with a different level of hypermethylation in the TP53, P21 and P16 promoters (r=-0.33, P=0.001; r=-0.70, P<0.0001 and r=-0.71, P<0.0001, respectively). The results suggested that inactivation of p16/Rb and/or p53/p21 pathways by hypermethylation may be linked to critical telomere shortening, leading to genome instability and ultimately to malignant transformation. Thus, telomere shortening and promoter hypermethylation of related genes both might serve as breast cancer biomarkers.

Genomic changes in chromosomes 10, 16, and X in malignant peripheral nerve sheath tumors identify a high-risk patient group

PURPOSE

The purpose of this study was to identify genetic aberrations contributing to clinical aggressiveness of malignant peripheral nerve sheath tumors (MPNSTs).

PATIENTS AND METHODS

Samples from 48 MPNSTs and 10 neurofibromas were collected from 51 patients with (n = 31) or without (n = 20) neurofibromatosis type 1 (NF1). Genome-wide DNA copy number changes were assessed by chromosomal and array-based comparative genomic hybridization (CGH) and examined for prognostic significance. For a subset of 20 samples, RNA microarray data were integrated with the genome data to identify potential target genes.

RESULTS

Forty-four (92%) MPNSTs displayed DNA copy number changes (median, 18 changes per tumor; range, 2 to 35 changes). Known frequent chromosomal gains at chromosome arms 8q (69%), 17q (67%), and 7p (52%) and losses from 9p (50%), 11q (48%), and 17p (44%) were confirmed. Additionally, gains at 16p or losses from 10q or Xq identified a high-risk group with only 11% 10-year disease-specific survival (P = .00005). Multivariate analyses including NF1 status, tumor location, size, grade, sex, complete remission, and initial metastatic status showed that the genomic high-risk group was the most significant predictor of poor survival. Several genes whose expression was affected by the DNA copy number aberrations were identified.

CONCLUSION

The presence of specific genetic aberrations was strongly associated with poor survival independent of known clinical risk factors. Conversely, within the total patient cohort with 34% 10-year disease-specific survival, a low-risk group was identified: without changes at chromosomes 10q, 16p, or Xq in their MPNSTs, the patients had 74% 10-year survival.

Nuclear functions of the HMG proteins

Although the three families of mammalian HMG proteins (HMGA, HMGB and HMGN) participate in many of the same nuclear processes, each family plays its own unique role in modulating chromatin structure and regulating genomic function. This review focuses on the similarities and differences in the mechanisms by which the different HMG families impact chromatin structure and influence cellular phenotype. The biological implications of having three architectural transcription factor families with complementary, but partially overlapping, nuclear functions are discussed.

Donor age-dependent acceleration of cellular aging by repeated ultraviolet A irradiation of human dermal fibroblasts derived from a single donor

The relationship between cellular aging and aging of entire organisms has been studied extensively.The findings are confusing, however, and no clear relationships have been demonstrated.The conflicting data may be due to individual differences among the donors of the studied cells.It is crucial to identify the changes in cellular properties that are the result of the aging process.Here, we used human dermal fibroblast cell lines established from a single donor at different ages to assess the influence of ultraviolet A (UVA) on cellular aging. These cell lines have the same genetic background and were obtained from a restricted body region. The results indicated that cellular aging was accelerated by UVA irradiation in a donor age-dependent manner. The ratio of lifespan shortening increased with donor age. Increased donor age not only decreased cell division, but also increased the growth arrest response to UVA irradiation. The characteristics of the cultured cells reflected the age-related changes in dermal fibroblasts.

Hepatitis B virus X protein overcomes stress-induced premature senescence by repressing p16(INK4a) expression via DNA methylation

Cellular senescence is an important tumor suppression process under diverse oncogenic conditions, entering a state of irreversible growth arrest to prevent damaged cells from undergoing aberrant proliferation. Developing a means of evading senescence thus seems to be a fundamental task that all cancer cells should solve early on. Here, we show that an oncogenic X protein of hepatitis B virus (HBx) overcomes cellular senescence provoked by a universal premature senescence inducer, H(2)O(2), in human hepatoma cells, as demonstrated by impaired induction of senescence-associated biomarkers, including morphological change, G(1) arrest, and beta-galactosidase activity, in the presence of HBx. HBx induced DNA hypermethylation of p16(INK4a) promoter and subsequently interfered action of transcription factors like Ets1 and Ets2 activated by H(2)O(2) through the p38(MAPK) pathway, resulting in inhibition of its transcription. Down-regulation of p16(INK4a) expression by HBx subsequently led to activation of G(1)-CDKs, phosphorylation of Rb, activation of E2F1, and finally evasion from G(1) arrest induced by H(2)O(2). Levels of another senescence regulator, p21(waf1), however, were not affected by HBx under our senescence-inducing conditions. In addition, the potentials of HBx to inactivate Rb and subsequently inhibit cellular senescence almost completely disappeared when levels of p16(INK4a) were recovered either by exogenous complementation or inhibition of the promoter hypermethylation. To our knowledge, our present study represents the first report that an oncogenic virus evades cellular senescence through epigenetic down-regulation of p16(INK4a) expression.

Clinical and immunohistochemical features associated with a response to bortezomib in patients with multiple myeloma

PURPOSE

Multiple myeloma is an incurable disease with heterogeneous clinical behavior. Bortezomib has offered some patients with relapsed and refractory disease an opportunity for prolonged survival. However, there remains a paucity of data in patients treated with bortezomib that accurately delineates and identifies such patients. This information is crucial to guide management.

EXPERIMENTAL DESIGN

In this study, we aimed to identify the patients most likely to respond to bortezomib salvage therapy. We analyzed the baseline clinical variables and profiled the baseline expression of a broad range of immunohistochemical markers of cell cycle activity, apoptosis, and angiogenesis in a large cohort of multiply relapsed myeloma patients recruited to one of two prospective multicentre trials assessing the efficacy of bortezomib salvage therapy.

RESULTS

Using the European Group for Bone Marrow Transplantation criteria, response (complete or partial) to bortezomib salvage therapy was associated with a previous history of complete response to alternative antimyeloma treatment. Patients who expressed cyclin D1 were more likely to achieve a response. In contrast, patients who expressed p16(INK4A), cytoplasmic p53, and the highest intensity of Bcl-2 staining had a poor response. Patients who achieved a response to bortezomib and those patients who expressed cyclin D1 at baseline showed a significant survival advantage. Patients who expressed FGFR3, a poor prognostic marker, responded equally well and had similar outcomes with bortezomib compared with FGFR3-negative patients.

CONCLUSIONS

Baseline clinical variables and selective immunohistochemical markers expressed by patients may be used effectively to identify patients that are most likely to achieve a meaningful clinical response to bortezomib salvage therapy.

RhoA regulates G1-S progression of gastric cancer cells by modulation of multiple INK4 family tumor suppressors

RhoA, a member of the Rho GTPase family, has been extensively studied in the regulation of cytoskeletal dynamics, gene transcription, cell cycle progression, and cell transformation. Overexpression of RhoA is found in many malignancies and elevated RhoA activity is associated with proliferation phenotypes of cancer cells. We reported previously that RhoA was hyperactivated in gastric cancer tissues and suppression of RhoA activity could partially reverse the proliferation phenotype of gastric cancer cells, but the underlying mechanism has yet to be elucidated. It has been reported that RhoA activation is crucial for the cell cycle G(1)-S procession through the regulation of Cip/Kip family tumor suppressors in benign cell lines. In this study, we found that selective suppression of RhoA or its effectors mammalian Diaphanous 1 and Rho kinase (ROCK) by small interfering RNA and a pharmacologic inhibitor effectively inhibited proliferation and cell cycle G(1)-S transition in gastric cancer lines. Down-regulation of RhoA-mammalian Diaphanous 1 pathway, but not RhoA-ROCK pathway, caused an increase in the expression of p21(Waf1/Cip1) and p27(Kip1), which are coupled with reduced expression and activity of CDK2 and a cytoplasmic mislocalization of p27(Kip1). Suppression of RhoA-ROCK pathway, on the other hand, resulted in an accumulation of p15(INK4b), p16(INK4a), p18(INK4c), and p19(INK4d), leading to reduced expression and activities of CDK4 and CDK6. Thus, RhoA may use two distinct effector pathways in regulating the G(1)-S progression of gastric cancer cells.

Simaomicin α, a polycyclic xanthone, induces G₁ arrest with suppression of retinoblastoma protein phosphorylation

Recent progress in cancer biology research has shown that abnormal proliferation in tumor cells can be attributed to aberrations in cell cycle regulation, especially in G₁ phase. During the course of searching for microbial metabolites that affect cell cycle distribution, we have found that simaomicin α, a polycyclic xanthone antibiotic, arrests the cell cycle at G₁ phase. Treatment of T-cell leukemia Jurkat cells with 3 nM simaomicin α induced an increase in the number of cells in G₁ and a decrease in those in G₂–M phase. Cell cycle aberrations induced by simaomicin α were also detected in colon adenocarcinoma HCT15 cells. Simaomicin α had antiproliferative activities in various tumor cell lines with 50% inhibitory concentration values in the range of 0.3–19 nM. Furthermore, simaomicin α induced an increase in cellular caspase-3 activity and DNA fragmentation, indicating that simaomicin α promotes apoptosis. The retinoblastoma protein phosphorylation status of simaomicin α-treated cell lysate was lower than that of control cells, suggesting that the target molecule of simaomicin α is in a pathway upstream of retinoblastoma protein phosphorylation. In the course of evaluating polycyclic xanthone antibiotics structurally related to simaomicin α, we also found that cervinomycin A1 stimulated accumulation of treated cells in G₁ phase. These results indicate that the polycyclic xanthones, including simaomicin α and cervinomycin A1, may be candidate cancer chemotherapeutic agents.

Senescence-associated secretory phenotypes reveal cell-nonautonomous functions of oncogenic RAS and the p53 tumor suppressor

Cellular senescence suppresses cancer by arresting cell proliferation, essentially permanently, in response to oncogenic stimuli, including genotoxic stress. We modified the use of antibody arrays to provide a quantitative assessment of factors secreted by senescent cells. We show that human cells induced to senesce by genotoxic stress secrete myriad factors associated with inflammation and malignancy. This senescence-associated secretory phenotype (SASP) developed slowly over several days and only after DNA damage of sufficient magnitude to induce senescence. Remarkably similar SASPs developed in normal fibroblasts, normal epithelial cells, and epithelial tumor cells after genotoxic stress in culture, and in epithelial tumor cells in vivo after treatment of prostate cancer patients with DNA-damaging chemotherapy. In cultured premalignant epithelial cells, SASPs induced an epithelial–mesenchyme transition and invasiveness, hallmarks of malignancy, by a paracrine mechanism that depended largely on the SASP factors interleukin (IL)-6 and IL-8. Strikingly, two manipulations markedly amplified, and accelerated development of, the SASPs: oncogenic RAS expression, which causes genotoxic stress and senescence in normal cells, and functional loss of the p53 tumor suppressor protein. Both loss of p53 and gain of oncogenic RAS also exacerbated the promalignant paracrine activities of the SASPs. Our findings define a central feature of genotoxic stress-induced senescence. Moreover, they suggest a cell-nonautonomous mechanism by which p53 can restrain, and oncogenic RAS can promote, the development of age-related cancer by altering the tissue microenvironment.

Cells with damaged DNA are at risk of becoming cancerous tumors. Although “cellular senescence” can suppress tumor formation from damaged cells by blocking the cell division that underlies cancer growth, it has also been implicated in promoting cancer and other age-related diseases. To understand how this might happen, we measured proteins that senescent human cells secrete into their local environment and found many factors associated with inflammation and cancer development. Different types of cells secrete a common set of proteins when they senesce. This senescence-associated secretory phenotype (SASP) occurs not only in cultured cells, but also in vivo in response to DNA-damaging chemotherapy. Normal cells that acquire a highly active mutant version of the RAS protein, which is known to contribute to tumor growth, undergo cellular senescence, and develop a very intense SASP, with higher levels of proteins secreted. Likewise, the SASP is more intense when cells lose the functions of the tumor suppressor p53. Senescent cells promote the growth and aggressiveness of nearby precancerous or cancer cells, and cells with a more intense SASP do so more efficiently. Our findings support the idea that cellular senescence can be both beneficial, in preventing damaged cells from dividing, and deleterious, by having effects on neighboring cells; this balance of effects is predicted by an evolutionary theory of aging.

By controlling how damaged cells modify their surrounding tissue environment, a tumor suppressor gene can restrain, and an oncogene can promote, the development of cancer.

Could removing arsenic from tobacco smoke significantly reduce smoker risks of lung cancer?

If a specific biological mechanism could be determined by which a carcinogen increases lung cancer risk, how might this knowledge be used to improve risk assessment? To explore this issue, we assume (perhaps incorrectly) that arsenic in cigarette smoke increases lung cancer risk by hypermethylating the promoter region of gene p16INK4a, leading to a more rapid entry of altered (initiated) cells into a clonal expansion phase. The potential impact on lung cancer of removing arsenic is then quantified using a three-stage version of a multistage clonal expansion (MSCE) model. This refines the usual two-stage clonal expansion (TSCE) model of carcinogenesis by resolving its intermediate or "initiated" cell compartment into two subcompartments, representing experimentally observed "patch" and "field" cells. This refinement allows p16 methylation effects to be represented as speeding transitions of cells from the patch state to the clonally expanding field state. Given these assumptions, removing arsenic might greatly reduce the number of nonsmall cell lung cancer cells (NSCLCs) produced in smokers, by up to two-thirds, depending on the fraction (between 0 and 1) of the smoking-induced increase in the patch-to-field transition rate prevented if arsenic were removed. At present, this fraction is unknown (and could be as low as zero), but the possibility that it could be high (close to 1) cannot be ruled out without further data.

Partial proteasome inhibition in human fibroblasts triggers accelerated M1 senescence or M2 crisis depending on p53 and Rb status

Proteasome-dependent degradation has been extensively investigated and has been shown to play a vital role in the maintenance of cellular homeostasis. Proteasome activity and expression are reduced during aging and replicative senescence. Its activation has been shown to confer lifespan extension in human diploid fibroblasts (HDFs), whereas partial proteasome inhibition triggers an irreversible premature senescent state in young HDFs. As p53 and Rb tumor suppressors regulate both replicative and premature senescence (RS and PS, respectively), in this study we investigated their implication in proteasome inhibition-mediated PS. By taking advantage of a variety of HDFs with defective p53 or/and Rb pathways, we reveal that proteasome activity inhibition to levels normally found in senescent human cells results in immediate growth arrest and/or moderate increase of apoptotic death. These effects are independent of the cellular genetic context. However, in the long term, proteasome inhibition-mediated PS can only be initiated and maintained in the presence of functional p53. More specifically, we demonstrate that following partial proteasome inhibition, senescence is dominant in HDFs with functional p53 and Rb molecules, crisis/death is induced in cells with high p53 levels and defective Rb pathway, whereas stress recovery and restoration of normal cycling occurs in cells that lack functional p53. These data reveal the continuous interplay between the integrity of proteasome function, senescence and cell survival.

Expression of p21cip1, p27kip1, and p16INk4a cyclin-dependent kinase inhibitors in papillary thyroid carcinoma: correlation with clinicopathological factors

In a variety of human malignancies, aberrant expression of proteins involved in the control of cell-cycle progression has been reported. In this study, p21cip1, p27kip1, and p16INk4a cyclin-dependent kinase inhibitors were analyzed to evaluate their usefulness in clinical management of papillary thyroid carcinoma (PTC). Archived material derived from 46 cases of PTC was analyzed immunohistochemically. Protein expression was ascertained on tissue microarrays, and results were correlated with clinicopathological features of the patients. Positive immunostaining was observed in 14 (30,4%) p21cip1, 26 (56,5%) p27kip1, and 14 (30,4%) p16INk4a cases. No significant correlation between p21cip1 or p27kip1 and clinical factors was found. In contrast, p16INk4a expression showed a significant correlation with initial extension of the disease. Therefore, 45.8% of patients with loco-regional extension were p16INk4a positive, whereas overexpression was only seen in 15.7% of cases with intrathyroid disease (p < 0.05). Moreover, all patients with simultaneous p16INk4a positivity and lack of p27kip1 staining (four patients) presented lymph node metastases. In contrast, only 12 (28.5%) of the remaining patients showed lymph node tumor involvement. In conclusion, p16INk4a expression suggests extrathyroid neck extension of PTC. This effect is enhanced when p27kip1 is negative. We think that their analysis by immunohistochemistry could be useful in the management of patients with PTC.

The biology of cancer stem cells

Cancers originally develop from normal cells that gain the ability to proliferate aberrantly and eventually turn malignant. These cancerous cells then grow clonally into tumors and eventually have the potential to metastasize. A central question in cancer biology is, which cells can be transformed to form tumors? Recent studies elucidated the presence of cancer stem cells that have the exclusive ability to regenerate tumors. These cancer stem cells share many characteristics with normal stem cells, including self-renewal and differentiation. With the growing evidence that cancer stem cells exist in a wide array of tumors, it is becoming increasingly important to understand the molecular mechanisms that regulate self-renewal and differentiation because corruption of genes involved in these pathways likely participates in tumor growth. This new paradigm of oncogenesis has been validated in a growing list of tumors. Studies of normal and cancer stem cells from the same tissue have shed light on the ontogeny of tumors. That signaling pathways such as Bmi1 and Wnt have similar effects in normal and cancer stem cell self-renewal suggests that common molecular pathways regulate both populations. Understanding the biology of cancer stem cells will contribute to the identification of molecular targets important for future therapies.

Nesprin-2 giant safeguards nuclear envelope architecture in LMNA S143F progeria cells

The S143F lamin A/C point mutation causes a phenotype combining features of myopathy and progeria. We demonstrate here that patient dermal fibroblast cells have dysmorphic nuclei containing numerous blebs and lobulations, which progressively accumulate as cells age in culture. The lamin A/C organization is altered, showing intranuclear and nuclear envelope (NE) aggregates and presenting often a honeycomb appearance. Immunofluorescence microscopy showed that nesprin-2 C-terminal isoforms and LAP2alpha were recovered in the cytoplasm, whereas LAP2beta and emerin were unevenly localized along the NE. In addition, the intranuclear organization of acetylated histones, histone H1 and the active form of RNA polymerase II were markedly different in patient cells. A subpopulation of mutant cells, however, expressing the 800 kDa nesprin-2 giant isoform, did not show an overt nuclear phenotype. Ectopic expression of p.S143F lamin A in fibroblasts recapitulates the patient cell phenotype, whereas no effects were observed in p.S143F LMNA keratinocytes, which highly express nesprin-2 giant. Overexpression of the mutant lamin A protein had a more severe impact on the NE of nesprin-2 giant deficient fibroblasts when compared with wild-type. In summary, our results suggest that the p.S143F lamin A mutation affects NE architecture and composition, chromatin organization, gene expression and transcription. Furthermore, our findings implicate a direct involvement of the nesprins in laminopathies and propose nesprin-2 giant as a structural reinforcer at the NE.

Identification and characterization of a novel protein ISOC2 that interacts with p16INK4a

p16(INK4a) is a multiple tumor suppressor, playing an important role in proliferation and tumorigenesis. To screen the p16(INK4a)-associated proteins, we performed a yeast two-hybrid assay and identified a novel protein isochorismatase domain containing 2 (ISOC2). ISOC2 conserves in different species, and encodes 205 and 210 amino acids in human and mouse, respectively. The expression of ISOC2 in mouse is universal but predominantly in uterus, stomach, and urinary tract system. Interaction between ISOC2 and p16(INK4a) was verified using in vitro pull-down assays and in vivo co-immunoprecipitation. Confocal microscopy studies using green and cyan fluorescent fusion proteins determined that ISOC2 co-localizes with p16(INK4a). Over-expressed ISOC2 is able to inhibit p16(INK4a) in dose-dependent manner. Our data indicated that ISOC2 is a novel functional protein, which is able to bind and co-localize with a tumor suppressor gene p16(INK4a). Over-expressed ISOC2 inhibits the expression of p16(INK4a), suggesting that this novel gene may play a role during the tumor development by interacting with p16(INK4a).

Telomere dysfunction and inactivation of the p16(INK4a)/Rb pathway in pyothorax-associated lymphoma

Previous studies have indicated that genome instability is involved in the lymphomagenesis of pyothorax-associated lymphoma (PAL), which develops in patients with a long-standing history of pyothorax. One of the well-known causes of genome instability is telomere dysfunction. In the present study, the condition of telomeres was analyzed in the cell lines and clinical samples from PAL. Telomere length (TL) in PAL cell lines was extremely short (<4.5 kbp). TL in tumor samples was broad in range, and shorter than that in the peripheral blood leukocytes from the matched patients. Three of five PAL cell lines showed frequent loss of telomere signals (telomere erosion); however, telomerase activity in PAL cell lines was similar to that in Burkitt lymphoma cell lines. Rb expression was detected in three PAL cell lines and four of 15 clinical samples, respectively. Rb protein expressed in three PAL cell lines was heavily phosphorylated, indicating that function of Rb protein was suppressed. p16(INK4a) expression was not detected in either cell lines or clinical samples. The promoter region in p16(INK4a) was heavily methylated in all cell lines as well as the clinical samples. Inactivation of the p16(INK4a)/Rb pathway may allow continuous cell division and critical telomere shortening, which induce genome instability, finally leading to malignant transformation. Taken together, telomere dysfunction and inactivation of the p16(INK4a)/Rb pathway might play a role for PAL development.

PI-3K/Akt signal pathway plays a crucial role in arsenite-induced cell proliferation of human keratinocytes through induction of cyclin D1

Exposure of arsenite can induce hyperproliferation of skin cells, which is believed to play important roles in arsenite-induced carcinogenesis by affecting both promotion and progression stages. However, the signal pathways and target genes activated by arsenite exposure responsible for the proliferation remain to be defined. In the present study, we found that: (1) exposure of human keratinocytic HaCat cells to arsenite caused an increase in cell proliferation, which was significantly inhibited by pretreatment of wortmannin, a specific chemical inhibitor of PI-3K/Akt signal pathway; (2) arsenite exposure was also able to activate PI-3K/Akt signal pathway, which thereby induced the elevation of cyclin D1 expression level in both HaCat cells and human primary keratinocytes based on that inhibition of PI-3K/Akt pathway by either pretreatment of wortmannin or the transfection of their dominant mutants, significantly inhibited cyclin D1 expression upon arsenite exposure; (3) PI-3K/Akt pathway is implicated in arsenite-induced proliferation of HaCat cells through the induction of cyclin D1 because either knockdown of cyclin D1 by its siRNA or inhibition of PI-3K/Akt signal pathway by their dominant mutants markedly impaired the proliferation of HaCat cells induced by arsenite exposure. Taken together, we provide the direct evidence that PI-3K/Akt pathway plays a role in the regulation of cell proliferation through the induction of cyclin D1 in human keratinocytes upon arsenite treatment. Given the importance of aberrant cell proliferation in cell transformation, we propose that the activation of PI-3K/Akt pathway and cyclin D1 induction may be the important mediators of human skin carcinogenic effect of arsenite.

Epigenetic reprogramming of liver cells in tamoxifen-induced rat hepatocarcinogenesis

Tamoxifen, a nonsteroidal anti-estrogen, is a potent genotoxic hepatocarcinogen in rats, with both tumor initiating and promoting properties. Recently it has been demonstrated that genotoxic carcinogens, in addition to exerting genotoxic effects, often cause epigenetic alterations and these induced epigenetic changes may play important mechanistic role in carcinogenesis. In the present study, we investigated the role of tamoxifen-induced epigenetic changes in hepatocarcinogenic process. The results of the study showed that exposure of female F344 rats to tamoxifen resulted in progressive loss of CpG methylation in regulatory sequences of long interspersed nucleotide elements (LINE-1) and prominent increase in expression of LINE-1 elements and c-myc proto-oncogene. The accumulation of tamoxifen-induced DNA lesions was accompanied by the decreased level of Rad51, Ku70, and DNA polymerase beta (Polbeta) proteins that play a crucial role in maintenance of genomic stability. Furthermore, feeding rats with tamoxifen-containing diet led to increased regenerative cell proliferation, as indicated by the increased level of Ki-67 and proliferating cell nuclear antigen (PCNA) proteins. These data indicate that exposure of animals to genotoxic hepatocarcinogen tamoxifen led to early phenotypical alterations in livers characterized by emergence of epigenetically reprogrammed cells with a specific cancer-related epigenetic phenotype prior to tumor formation.

Stem cells: tissue regeneration and cancer

Regenerative medicine is the promised paradigm of replacement and repair of damaged or senescent tissues. As the building blocks for organ development and tissue repair, stem cells have unique and wide-ranging capabilities, thus delineating their potential application to regenerative medicine. The recognition that consistent patterns of molecular mechanisms drive organ development and postnatal tissue regeneration has significant implications for a variety of pediatric diseases beyond replacement biology. The observation that organ-specific stem cells derive all of the differentiated cells within a given tissue has led to the acceptance of a stem cell hierarchy model for tissue development, maintenance, and repair. Extending the tissue stem cell hierarchical model to tissue carcinogenesis may revolutionize the manner in which we conceptualize cancer therapeutics. In this review, the clinical promise of these technologies and the emerging concept of "cancer stem cells" are examined. A basic understanding of stem cell biology is paramount to stay informed of this emerging technology and the accompanying research in this area with the potential for clinical application.

Colorectal mucinous adenocarcinoma: the clinicopathologic features and significance of p16 and p53 expression

PURPOSE

This study was designed to examine the clinicopathologic features and p53 and p16 expressions in colorectal mucinous adenocarcinoma and colorectal signet-ring cell carcinoma.

METHODS

The clinicopathologic features of 36 patients with colorectal mucinous adenocarcinoma and colorectal signet-ring cell carcinoma were analyzed and compared with 228 patients with colorectal adenocarcinomas. The p53 and p16 expressions in the colorectal mucinous adenocarcinoma and colorectal signet-ring cell carcinoma were studied by immunohistochemistry.

RESULTS

Colorectal mucinous adenocarcinoma and colorectal signet-ring cell carcinoma accounted for 14 percent of colorectal cancer. The median age at presentation was 67 years. Family history of colorectal cancer in their first-degree relatives was seen in 14 percent of these patients. Fifty-six percent of the carcinomas were located in the proximal colorectum, most commonly in the transverse colon. Two patients had ulcerative colitis. Compared with the usual colorectal adenocarcinoma, colorectal mucinous adenocarcinoma and colorectal signet-ring cell carcinoma was found more often in proximal colorectum (P = 0.002), larger (P = 0.05), and in advanced stages (P = 0.018). Forty-four percent (n = 16) of the colorectal mucinous adenocarcinoma and colorectal signet-ring cell carcinoma showed p53 expression. All the patients with colorectal mucinous adenocarcinoma and colorectal signet-ring cell carcinoma with a positive family history of colorectal adenocarcinoma had tumors that showed p53 expression (P = 0.012). Seventy-eight percent (n = 28) of the tumors showed p16 expression. The median survival of the patients with these tumors was 23 months. The survival of these patients with colorectal mucinous adenocarcinoma and colorectal signet-ring cell carcinoma was poorer if the lesions were of advanced stages (P = 0.023) or with family history of colorectal cancer (P = 0.0015). Also, patients with colorectal mucinous adenocarcinoma and colorectal signet-ring cell carcinoma that did not express p16 and p53 had better survival than other patients (P = 0.04).

CONCLUSIONS

Colorectal mucinous adenocarcinoma and colorectal signet-ring cell carcinoma had distinctive clinicopathologic features. Tumor staging, family history of colorectal cancer, and status of p53 and p16 expressions might predict prognosis in these patients.

DExD/H box RNA helicases: multifunctional proteins with important roles in transcriptional regulation

The DExD/H box family of proteins includes a large number of proteins that play important roles in RNA metabolism. Members of this family have been shown to act as RNA helicases or unwindases, using the energy from ATP hydrolysis to unwind RNA structures or dissociate RNA-protein complexes in cellular processes that require modulation of RNA structures. However, it is clear that several members of this family are multifunctional and, in addition to acting as RNA helicases in processes such as pre-mRNA processing, play important roles in transcriptional regulation. In this review I shall concentrate on RNA helicase A (Dhx9), DP103 (Ddx20), p68 (Ddx5) and p72 (Ddx17), proteins for which there is a strong body of evidence showing that they play important roles in transcription, often as coactivators or corepressors through their interaction with key components of the transcriptional machinery, such as CREB-binding protein, p300, RNA polymerase II and histone deacetylases.

Repair and regeneration: opportunities for carcinogenesis from tissue stem cells

This review will discuss the mechanisms of repair and regeneration in various tissue types and how dysregulation of these mechaisms may lead to cancer. Normal homeostasis involves a careful balance between cell loss and cell renewal. Stem and progenitor cells perform these biologic processes as the functional units of regeneration during both tissue homeostasis and repair. The concept of tissue stem cells capable of giving rise to all differentiated cells within a given tissue led to the concept of a cellulr hierarchy in tissues and in tumors. Thus, only a few cells may be necessary and sufficient for tissue repair or tumor regeneration. This is known as the hierarchical model of tumorigenesis. This report will compare this model with the stochastic model of tumorigenesis. Under normal circumstances, the processes of tissue regeneration or homeostasis are tightly regulated by several morphogen pathways to prevent excessive or inappropriate cell growth. This review presents the recent evidence that dysregulation of these processes may provide opportunities for carcinogenesis for the long-lived, highly proliferative tissue stem cell population. New findings of cancer initiating tissue stem cells identified in several solid and circulating cancers including breast, brain hematopoietic tumors will also be reviewed. Finally, this report reviews the cellular biology of cancer and its relevance to the development of more effective cancer treatment protocols.

Stabilization of the retinoblastoma protein by A-type nuclear lamins is required for INK4A-mediated cell cycle arrest

Mutations in the LMNA gene, which encodes all A-type lamins, including lamin A and lamin C, cause a variety of tissue-specific degenerative diseases termed laminopathies. Little is known about the pathogenesis of these disorders. Previous studies have indicated that A-type lamins interact with the retinoblastoma protein (pRB). Here we probe the functional consequences of this association and further examine links between nuclear structure and cell cycle control. Since pRB is required for cell cycle arrest by p16(ink4a), we tested the responsiveness of multiple lamin A/C-depleted cell lines to overexpression of this CDK inhibitor and tumor suppressor. We find that the loss of A-type lamin expression results in marked destabilization of pRB. This reduction in pRB renders cells resistant to p16(ink4a)-mediated G(1) arrest. Reintroduction of lamin A, lamin C, or pRB restores p16(ink4a)-responsiveness to Lmna(-/-) cells. An array of lamin A mutants, representing a variety of pathologies as well as lamin A processing mutants, was introduced into Lmna(-/-) cells. Of these, a mutant associated with mandibuloacral dysplasia (MAD R527H), as well as two lamin A processing mutants, but not other disease-associated mutants, failed to restore p16(ink4a) responsiveness. Although our findings do not rule out links between altered pRB function and laminopathies, they fail to support such an assertion. These findings do link lamin A/C to the functional activation of a critical tumor suppressor pathway and further the possibility that somatic mutations in LMNA contribute to tumor progression.

RPS6KA2, a putative tumour suppressor gene at 6q27 in sporadic epithelial ovarian cancer

We had previously defined by allele loss studies a minimal region at 6q27 (between D6S264 and D6S297) to contain a putative tumour suppressor gene. The p90 ribosomal S6 kinase-3 gene (p90 Rsk-3, RPS6KA2) maps in this interval. It is a serine-threonine kinase that signals downstream of the mitogen-activated protein kinase pathway. It is expressed in normal ovarian epithelium, whereas reduced or absent in tumours or cell lines. We show that RPS6KA2 is monoallelically expressed in the ovary suggesting that loss of a single expressed allele is sufficient to cause complete loss of expression in cancer cells. Further, we have identified two new isoforms of RPS6KA2 with an alternative start codon. Homozygous deletions were identified within the RPS6KA2 gene in two cell lines. Re-expression of RPS6KA2 in ovarian cancer cell lines suppressed colony formation. In UCI101 cells, the expression of RPS6KA2 reduced proliferation, caused G1 arrest, increased apoptosis, reduced levels of phosphorylated extracellular signal-regulated kinase and altered other cell cycle proteins. In contrast, small interfering RNA against RPS6KA2 showed the opposite effect in 41M cells. The above results suggest that RPS6KA2 is a putative tumour suppressor gene to explain allele loss at 6q27.

RNA regulation and cancer development

Cancer is viewed as a genetic disease. According to the currently accepted model of carcinogenesis, several consequential mutations in oncogenes or tumor suppressor genes are necessary for cancer development. In this model, mutated DNA sequence is transcribed to mRNA that is finally translated into functionally aberrant protein. mRNA is viewed solely as an intermediate between DNA (with 'coding' potential) and protein (with 'executive' function). However, recent findings suggest that (m)RNA is actively regulated by a variety of processes including nonsense-mediated decay, alternative splicing, RNA editing or RNA interference. Moreover, RNA molecules can regulate a variety of cellular functions through interactions with RNA, DNA as well as protein molecules. Although, the precise contribution of RNA molecules by themselves and RNA-regulated processes on cancer development is currently unknown, recent data suggest their important role in carcinogenesis. Here, we summarize recent knowledge on RNA-related processes and discuss their potential role in cancer development.

Valproic acid induces growth arrest, apoptosis, and senescence in medulloblastomas by increasing histone hyperacetylation and regulating expression of p21Cip1, CDK4, and CMYC

Valproic acid is a well-tolerated anticonvulsant that has been identified recently as a histone deacetylase inhibitor. To evaluate the antitumor efficacy and mechanisms of action of valproic acid in medulloblastoma and supratentorial primitive neuroectodermal tumor (sPNET), which are among the most common malignant brain tumors in children with poor prognosis, two medulloblastoma (DAOY and D283-MED) and one sPNET (PFSK) cell lines were treated with valproic acid and evaluated with a panel of in vitro and in vivo assays. Our results showed that valproic acid, at clinically safe concentrations (0.6 and 1 mmol/L), induced potent growth inhibition, cell cycle arrest, apoptosis, senescence, and differentiation and suppressed colony-forming efficiency and tumorigenicity in a time- and dose-dependent manner. The medulloblastoma cell lines were more responsive than the sPNET cell line and can be induced to irreversible suppression of proliferation and significantly reduced tumorigenicity by 0.6 and 1 mmol/L valproic acid. Daily i.p. injection of valproic acid (400 mg/kg) for 28 days significantly inhibited the in vivo growth of DAOY and D283-MED s.c. xenografts in severe combined immunodeficient mice. With Western hybridization and real-time reverse transcription-PCR, we further showed that the antitumor activities of valproic acid correlated with induction of histone (H3 and H4) hyperacetylation, activation of p21, and suppression of TP53, CDK4, and CMYC expression. In conclusion, valproic acid possesses potent in vitro and in vivo antimedulloblastoma activities that correlated with induction of histone hyperacetylation and regulation of pathways critical for maintaining growth inhibition and cell cycle arrest. Therefore, valproic acid may represent a novel therapeutic option in medulloblastoma treatment.

Cyclin D1 induction through IkappaB kinase beta/nuclear factor-kappaB pathway is responsible for arsenite-induced increased cell cycle G1-S phase transition in human keratinocytes

Environmental and occupational exposure to arsenite is associated with an increased risk of human cancers, including skin, urinary bladder, and respiratory tract cancers. Although much evidence suggests that alterations in cell cycle machinery are implicated in the carcinogenic effect of arsenite, the molecular mechanisms underlying the cell cycle alterations are largely unknown. In the present study, we observed that exposure of human keratinocyte HaCat cells to arsenite resulted in the promotion of cell cycle progression, especially G(1)-S transition. Further studies found that arsenite exposure was able to induce cyclin D1 expression. The induction of cyclin D1 by arsenite required nuclear factor-kappaB (NF-kappaB) activation, because the inhibition of IkappaB phosphorylation by overexpression of the dominant-negative mutant, IKKbeta-KM, impaired arsenite-induced cyclin D1 expression and G1-S transition. The requirement of IkappaB kinase beta (IKKbeta) for cyclin D1 induction was further confirmed by the findings that arsenite-induced cyclin D1 expression was totally blocked in IKKbeta knockout (IKKbeta(-/-)) mouse embryo fibroblasts. In addition, knockdown of cyclin D1 expression using cyclin D1-specific small interference RNA significantly blocked arsenite-induced cell cycle progression in HaCat cells. Taken together, our results show that arsenite-induced cell cycle from G(1) to S phase transition is through IKKbeta/NF-kappaB/cyclin D1-dependent pathway.

Phosphorylation of human oxoguanine DNA glycosylase (alpha-OGG1) modulates its function

Oxoguanine DNA glycosylase (OGG1) initiates the repair of 8-oxoguanine (8-oxoG), a major oxidative DNA base modification that has been directly implicated in cancer and aging. OGG1 functions in the base excision repair pathway, for which a molecular hand-off mechanism has been proposed. To date, only one functional and a few physical protein interactions have been reported for OGG1. Using the yeast two-hybrid system and a protein array membrane, we identified two novel protein interactions of OGG1, with two different protein kinases: Cdk4, a serine-threonine kinase, and c-Abl, a tyrosine kinase. We confirmed these interactions in vitro using recombinant proteins and in vivo by co-immunoprecipitation from whole cell extracts. OGG1 is phosphorylated in vitro by Cdk4, resulting in a 2.5-fold increase in the 8-oxoG/C incision activity of OGG1. C-Abl tyrosine phosphorylates OGG1 in vitro; however, this phosphorylation event does not affect OGG1 8-oxoG/C incision activity. These results provide the first evidence that a post-translational modification of OGG1 can affect its catalytic activity. The distinct functional outcomes from serine/threonine or tyrosine phosphorylation may indicate that activation of different signal transduction pathways modulate OGG1 activity in different ways.

Cyclins and cdks in development and cancer: a perspective

A fundamental aspect of cancer is dysregulated cell cycle control. Unlike normal cells that only proliferate when compelled to do so by developmental or other mitogenic signals in response to tissue growth needs, the proliferation of cancer cells proceeds essentially unchecked. This does not mean that cancer cell cycles are necessarily different from those found in normal cycling cells, but rather implies that cancer cells proliferate because they are no longer subject to proliferation-inhibitory influences arising from the stroma or from gene expression pattern changes consequent to 'terminal' differentiation, nor do they necessarily require extrinsic growth factors to recruit them into or maintain their proliferative state. Finally, cancer cells have also often avoided normal controls linked to cell cycle progression that halt proliferation in the presence of damaged DNA or other physiological insults. The result of these alterations is the inappropriate proliferation commonly associated with cancerous tumor formation. This review will summarize the current understanding of dysregulation of the G0/G1-to-S-phase transition in cancer cells, with particular emphasis on recent in vivo studies that suggest a need to rethink existing models of cell cycle control in development and tumorigenesis.

Nuclear Targeting and Cell Cycle Regulatory Function of Human BARD1

The BARD1 gene is mutated in a subset of breast and ovarian cancers, implicating BARD1 as a potential tumor suppressor. BARD1 gains a ubiquitin E3 ligase activity when heterodimerized with BRCA1, but the only known BRCA1-independent BARD1 function is a p53-dependent proapoptotic activity stimulated by nuclear export to the cytoplasm. We described previously the nuclear-cytoplasmic shuttling of BARD1, and in this study, we identify the transport sequences that target BARD1 to the nucleus and show that they are essential for BARD1 regulation of the cell cycle. We used deletion mapping and mutagenesis to define two active nuclear localization signals (NLSs) present in human BARD1 that are not conserved in rodent BARD1. Site-directed mutagenesis of the primary bipartite NLS abolished BARD1 nuclear import and caused its cytoplasmic accumulation. Using flow cytometry and 5-bromo-2-deoxyuridine incorporation assays, we discovered that transiently expressed BARD1 can elicit a p53-independent cell cycle arrest in G1 phase, and that this was abrogated by mutation of the BARD1 NLS but not by mutation of the nuclear export signal. Thus, BARD1 regulation of the cell cycle is a nuclear event and may be linked to its induced expression during mitosis and its possible involvement in the DNA damage checkpoint.