DLC1 suppresses NF-κB activity in prostate cancer cells due to its stabilizing effect on adherens junctions.

DLC1 (Deleted in Liver Cancer 1) gene encodes a RhoGTPase-activating protein (RhoGAP), which exerts most of its tumor suppressor functions through suppression of small Rho GTPases proteins RhoA, RhoB, RhoC and to some degree Cdc42, but not Rac. RhoGTPases are implicated in NF-κB activation in highly invasive prostate carcinoma (PCA), with consequences on cell proliferation, survival and metastatic capacity. Here we demonstrate that DLC1 transduction in two androgen-independent (AI) and highly metastatic PCA cell lines negatively regulates NF-κB activity in a GAP- and α-catenin-dependent manner. Expressed DLC1 protein suppresses the phosphorylation of NF-κB inhibitor, IκBα, causes its relocation from membrane ruffles into cytoplasm and attenuates its ubiquitination and subsequent degradation. DLC1-mediated NF-kB suppression and its effects are comparable to NF-κB inhibition using either shRNA knockdown or peptide inhibitor. Expression of transduced DLC1 suppressed the expression of NF-κB mediated genes. Such effects were found to be reliant on presence of calcium, indicating that the observed modifications are dependent on, and enabled by DLC-mediated stabilization of adherens junctions. These results expand the multitude of DLC1 interactions with other genes that modulate its oncosuppressive function, and may have potential therapeutic implications.

In vitro and in vivo effects of geranylgeranyltransferase I inhibitor P61A6 on non-small cell lung cancer cells.

BACKGROUND: Lung cancer is the leading cause of cancer-related mortality. Therapies against non-small cell lung cancer (NSCLC) are particularly needed, as this type of cancer is relatively insensitive to chemotherapy and radiation therapy. We recently identified GGTI compounds that are designed to block geranylgeranylation and membrane association of signaling proteins including the Rho family G-proteins. One of the GGTIs is P61A6 which inhibits proliferation of human cancer cells, causes cell cycle effects with G1 accumulation and exhibits tumor-suppressing effects with human pancreatic cancer xenografts. In this paper, we investigated effects of P61A6 on non-small cell lung cancer (NSCLC) cells in vitro and in vivo. METHODS: Three non-small cell lung cancer cell lines were used to test the ability of P61A6 to inhibit cell proliferation. Further characterization involved analyses of geranylgeranylation, membrane association and activation of RhoA, and anchorage-dependent and -independent growth, as well as cell cycle effects and examination of cell cycle regulators. We also generated stable cells expressing RhoA-F, which bypasses the geranylgeranylation requirement of wild type RhoA, and examined whether the proliferation inhibition by P61A6 is suppressed in these cells. Tumor xenografts of NSCLC cells growing in nude mice were also used to test P61A6's tumor-suppressing ability. RESULTS: P61A6 was shown to inhibit proliferation of NSCLC lines H358, H23 and H1507. Detailed analysis of P61A6 effects on H358 cells showed that P61A6 inhibited geranylgeranylation, membrane association of RhoA and caused G1 accumulation associated with decreased cyclin D1/2. The effects of P61A6 to inhibit proliferation could mainly be ascribed to RhoA, as expression of the RhoA-F geranylgeranylation bypass mutant rendered the cells resistant to inhibition by P61A6. We also found that P61A6 treatment of H358 tumor xenografts growing in nude mice reduced their growth as well as the membrane association of RhoA in the tumors. CONCLUSION: Thus, P61A6 inhibits proliferation of NSCLC cells and causes G1 accumulation associated with decreased cyclin D1/2. The result with the RhoA-F mutant suggests that the effect of P61A6 to inhibit proliferation is mainly through the inhibition of RhoA. P61A6 also shows efficacy to inhibit growth of xenograft tumor.

Role of DLC1 tumor suppressor gene and MYC oncogene in pathogenesis of human hepatocellular carcinoma: potential prospects for combined targeted therapeutics (review).

Hepatocellular carcinoma (HCC) is the third leading cause of cancer death, and its incidence is increasing worldwide in an alarming manner. The development of curative therapy for advanced and metastatic HCC is a high clinical priority. The HCC genome is complex and heterogeneous; therefore, the identification of recurrent genomic and related gene alterations is critical for developing clinical applications for diagnosis, prognosis and targeted therapy of the disease. This article focuses on recent research progress and our contribution in identifying and deciphering the role of defined genetic alterations in the pathogenesis of HCC. A significant number of genes that promote or suppress HCC cell growth have been identified at the sites of genomic reorganization. Notwithstanding the accumulation of multiple genetic alterations, highly recurrent changes on a single chromosome can alter the expression of oncogenes and tumor suppressor genes (TSGs) whose deregulation may be sufficient to drive the progression of normal hepatocytes to malignancy. A distinct and highly recurrent pattern of genomic imbalances in HCC includes the loss of DNA copy number (associated with loss of heterozygosity) of TSG-containing chromosome 8p and gain of DNA copy number or regional amplification of protooncogenes on chromosome 8q. Even though 8p is relatively small, it carries an unusually large number of TSGs, while, on the other side, several oncogenes are dispersed along 8q. Compelling evidence demonstrates that DLC1, a potent TSG on 8p, and MYC oncogene on 8q play a critical role in the pathogenesis of human HCC. Direct evidence for their role in the genesis of HCC has been obtained in a mosaic mouse model. Knockdown of DLC1 helps MYC in the induction of hepatoblast transformation in vitro, and in the development of HCC in vivo. Therapeutic interventions, which would simultaneously target signaling pathways governing both DLC1 and MYC functions in hepatocarcinogenesis, could result in progress in the treatment of liver cancer.

DLC1 interaction with α-catenin stabilizes adherens junctions and enhances DLC1 antioncogenic activity.

The DLC1 (for deleted in liver cancer 1) tumor suppressor gene encodes a RhoGAP protein that inactivates Rho GTPases, which are implicated in regulation of the cytoskeleton and adherens junctions (AJs), a cell-cell adhesion protein complex associated with the actin cytoskeleton. Malignant transformation and tumor progression to metastasis are often associated with changes in cytoskeletal organization and cell-cell adhesion. Here we have established in human cells that the AJ-associated protein α-catenin is a new binding partner of DLC1. Their binding was mediated by the N-terminal amino acids 340 to 435 of DLC1 and the N-terminal amino acids 117 to 161 of α-catenin. These proteins colocalized in the cytosol and in the plasma membrane, where together they associated with E-cadherin and ß-catenin, constitutive AJ proteins. Binding of DLC1 to α-catenin led to their accumulation at the plasma membrane and required DLC1 GAP activity. Knocking down α-catenin in DLC1-positive cells diminished DLC1 localization at the membrane. The DLC1-α-catenin complex reduced the Rho GTP level at the plasma membrane, increased E-cadherin's mobility, affected actin organization, and stabilized AJs. This process eventually contributed to a robust oncosuppressive effect of DLC1 in metastatic prostate carcinoma cells. Together, these results unravel a new mechanism through which DLC1 exerts its strong oncosuppressive function by positively influencing AJ stability.

DLC1 interaction with S100A10 mediates inhibition of in vitro cell invasion and tumorigenicity of lung cancer cells through a RhoGAP-independent mechanism.

The DLC1 gene encodes a Rho GTPase-activating protein (RhoGAP) that functions as a tumor suppressor in several common human cancers. The multidomain structure of DLC1 enables interaction with a number of other proteins. Here we report that the proinflammatory protein S100A10 (also known as p11), a key cell surface receptor for plasminogen which regulates pericellular proteolysis and tumor cell invasion, is a new binding partner of DLC1 in human cells. We determined that the 2 proteins colocalize in the cell cytoplasm and that their binding is mediated by central sequences in the central domain of DLC1 and the C-terminus of S100A10. Because the same S100A10 sequence also mediates binding to Annexin 2, we found that DLC1 competed with Annexin 2 for interaction with S100A10. DLC1 binding to S100A10 did not affect DLC1's RhoGAP activity, but it decreased the steady-state level of S100A10 expression in a dose-dependent manner by displacing it from Annexin 2 and making it accessible to ubiquitin-dependent degradation. This process attenuated plasminogen activation and resulted in inhibition of in vitro cell migration, invasion, colony formation, and anchorage-independent growth of aggressive lung cancer cells. These results suggest that a novel GAP-independent mechanism contributes to the tumor suppressive activity of DLC1, and highlight the importance and complexity of protein-protein interactions involving DLC1 in certain cancers.

Role of SV40 integration site at chromosomal interval 1q21.1 in immortalized CRL2504 cells.

We have applied a functional gene transfer strategy to show the importance of viral integration site in cellular immortalization. The large tumor antigen of SV40 is capable of extending the cellular life span by sequestering tumor suppressor proteins pRB and p53 in virus-transformed human cells. Although SV40 large T antigen is essential, it is not sufficient for cellular immortalization, suggesting that additional alterations in cellular genes are required to attain infinite proliferation. We show here that the disruption of human chromosomal interval at 1q21.1 by SV40 integration can be an essential step for cellular immortalization. The transfer of a 150-kb bacterial artificial chromosome (BAC) clone, RP364B14, corresponding to viral integration site in CRL2504 cells, reverted their immortal phenotype. Interestingly, the BAC transfer clones of CRL2504 cells displayed characteristics of either senescence as shown by beta-galactosidase activity or apoptosis as revealed by positive staining with M30 CytoDEATH antibody. The SV40 integration at 1q21.1, in the vicinity of epidermal differentiation complex (EDC) genes, resulted in the down-regulation of the filaggrin (FLG) gene that is part of the EDC. FLG gene expression was increased in BAC transfer senescent and apoptotic clones. Our results suggest that the disruption of native genomic sequence by SV40 may alter expression of genes involved in senescence and apoptosis by modulating chromatin structure. These studies imply that identification of genes located in the vicinity of viral integration sites in human cancers may be helpful in developing new diagnostic and therapeutic strategies.

Acquired genetic and functional alterations associated with transforming growth factor beta type I resistance in Hep3B human hepatocellular carcinoma cell line.

During the neoplastic process tumour cells frequently acquire resistance to the antiproliferative signals of transforming growth factor-beta (TGF-beta). Here we examined a human hepatocellular carcinoma cell line (Hep3B-TS) sensitive to TGF-beta signalling, and a derivative line (Hep3B-TR) rendered resistant to TGF-beta by stepwise exposure to TGF-beta(1). Comprehensive molecular cytogenetic analysis revealed that the acquisition of TGF-beta-resistance by Hep3B-TR cells was due to loss of TGF-beta receptor 2 (TGFbetaRII) gene. As demonstrated by spectral karyotyping and array-based comparative genomic hybridization, and in difference to Hep3B-TS cells, which have three rearranged and two normal copies of chromosome 3 that harbour the TGFbetaRII gene, Hep3B-TR cells have four rearranged and one apparently normal chromosome 3, which nonetheless underwent a critical microdeletion at the site of TGFbetaRII gene. Gene expression analysis using an oligonucleotide microarray of 21,397 genes showed that Hep3B-TR differentially expressed 307 genes, out of which 197 and 110 were up- and down-regulated, respectively, compared to Hep3B-TS. Six of differentially expressed genes were identified as downstream targets of the tumour necrosis factor (TNF) gene, suggesting that loss of TGFbetaRII triggered activation of the TNF pathway known to be regulated by TGF-beta(1) network. On the functional level, the TGF-beta-resistant Hep3B-TR cells displayed significantly enhanced capacity for anchorage independent growth and cell migration in vitro, and also increased tumorigenicity in vivo and in vitro and in vivo tumorigenicity compared with parental sensitive cells.

Recurrent and nonrandom DNA copy number and chromosome alterations in Myc transgenic mouse model for hepatocellular carcinogenesis: implications for human disease.

Mouse models for hepatocellular carcinoma (HCC) provide an experimental ground for dissecting the genetic and biological complexities of human liver cancer and contribute to our ability to gain insights into the relevance of candidate cancer genes. We examined, using spectral karyotyping (SKY) and array-based CGH (aCGH), seven cell lines derived from HCC spontaneously developed in transgenic Myc mice (Myc), and four cell lines established from tumors induced in nude mice by inoculation with the original Myc cells (nuMyc). All the cell lines exhibited gain of material from chromosomes 5, 6, 8, 10, 11, 15, and 19 and DNA copy-number loss from chromosomes 2, 4, 7, 9, 12, 14, and X. In addition, several recurrent chromosome reorganizations were found, including del(3), t(3;8), del(4), t(4;11), t(6;5), del(7), del(8), del(9), t(10;14), del(11), and del(16). Chromosome breakpoints underlying rearrangements clustered in the regions previously identified as important for the early stages of Myc-induced hepatocarcinogenesis. The results strongly suggest the importance of recurrent breakage and loss of chromosomes 4, 9, and 14 and gain of chromosomes 15 and 19 in mouse liver neoplasia. Genomic changes observed in Myc HCC cell lines are also recurrent in HCC developed in other transgenic mouse models, in mouse spontaneous HCC and derivative cell lines, and in preneoplastic liver lesions induced with chemical carcinogens. Overall, the present results document selective, nonrandom genomic changes involving chromosomal regions homologous to those implicated in human HCC.

DLC1 suppresses distant dissemination of human hepatocellular carcinoma cells in nude mice through reduction of RhoA GTPase activity, actin cytoskeletal disruption and down-regulation of genes involved in metastasis.

The process of cell dissemination from the primary tumors to distant sites is the most harmful event during cancer progression, and the leading cause of cancer death. We have previously demonstrated that restoration of DLC1 tumor suppressor gene expression in the DLC1-negative Focus and 7703K human hepatocellular carcinoma (HCC) cell lines induced caspase-3 mediated apoptosis, reduced cell growth in vitro and tumorigenicity in vivo and diminished the ability to migrate through Matrigel, a property suggestive of metastatic potential in vivo. We now show that subcutaneous tumors developing after inoculation of Focus and 7703K cells into nude mice disseminate cells to liver and lung, and this process is markedly suppressed by restoration of DLC1 expression. Inhibition of tumor cell dissemination was associated with lower levels of RhoA activity, an increase in rounded cells and a reduction in actin stress fibers and focal adhesion molecules that are of critical importance in cancer cell invasion and metastasis. In addition, DLC1 down-regulated the expression of osteopontin and matrix metalloproteinase-9, which are highly up-regulated in most primary HCC with associated metastases. These observations implicate the DLC1 gene in suppression of HCC cell dissemination and identify novel cellular and genetic alterations that contribute to prevention of metastasis, a life-threatening event in cancer progression.

Delayed kinetics of DNA double-strand break processing in normal and pathological aging.

Accumulation of DNA damage may play an essential role in both cellular senescence and organismal aging. The ability of cells to sense and repair DNA damage declines with age. However, the underlying molecular mechanism for this age-dependent decline is still elusive. To understand quantitative and qualitative changes in the DNA damage response during human aging, DNA damage-induced foci of phosphorylated histone H2AX (gamma-H2AX), which occurs specifically at sites of DNA double-strand breaks (DSBs) and eroded telomeres, were examined in human young and senescing fibroblasts, and in lymphocytes of peripheral blood. Here, we show that the incidence of endogenous gamma-H2AX foci increases with age. Fibroblasts taken from patients with Werner syndrome, a disorder associated with premature aging, genomic instability and increased incidence of cancer, exhibited considerably higher incidence of gamma-H2AX foci than those taken from normal donors of comparable age. Further increases in gamma-H2AX focal incidence occurred in culture as both normal and Werner syndrome fibroblasts progressed toward senescence. The rates of recruitment of DSB repair proteins to gamma-H2AX foci correlated inversely with age for both normal and Werner syndrome donors, perhaps due in part to the slower growth of gamma-H2AX foci in older donors. Because genomic stability may depend on the efficient processing of DSBs, and hence the rapid formation of gamma-H2AX foci and the rapid accumulation of DSB repair proteins on these foci at sites of nascent DSBs, our findings suggest that decreasing efficiency in these processes may contribute to genome instability associated with normal and pathological aging.

DLC-1:a Rho GTPase-activating protein and tumour suppressor.

The deleted in liver cancer 1 (DLC-1) gene encodes a GTPase activating protein that acts as a negative regulator of the Rho family of small GTPases. Rho proteins transduce signals that influence cell morphology and physiology, and their aberrant up-regulation is a key factor in the neoplastic process, including metastasis. Since its discovery, compelling evidence has accumulated that demonstrates a role for DLC-1 as a bona fide tumour suppressor gene in different types of human cancer. Loss of DLC-1 expression mediated by genetic and epigenetic mechanisms has been associated with the development of many human cancers, and restoration of DLC-1 expression inhibited the growth of tumour cells in vivo and in vitro. Two closely related genes, DLC-2 and DLC-3, may also be tumour suppressors. This review presents the current status of progress in understanding the biological functions of DLC-1 and its relatives and their roles in neoplasia.

Nonclassic functions of human topoisomerase I: genome-wide and pharmacologic analyses.

The biological functions of nuclear topoisomerase I (Top1) have been difficult to study because knocking out TOP1 is lethal in metazoans. To reveal the functions of human Top1, we have generated stable Top1 small interfering RNA (siRNA) cell lines from colon and breast carcinomas (HCT116-siTop1 and MCF-7-siTop1, respectively). In those clones, Top1 is reduced approximately 5-fold and Top2alpha compensates for Top1 deficiency. A prominent feature of the siTop1 cells is genomic instability, with chromosomal aberrations and histone gamma-H2AX foci associated with replication defects. siTop1 cells also show rDNA and nucleolar alterations and increased nuclear volume. Genome-wide transcription profiling revealed 55 genes with consistent changes in siTop1 cells. Among them, asparagine synthetase (ASNS) expression was reduced in siTop1 cells and in cells with transient Top1 down-regulation. Conversely, Top1 complementation increased ASNS, indicating a causal link between Top1 and ASNS expression. Correspondingly, pharmacologic profiling showed L-asparaginase hypersensitivity in the siTop1 cells. Resistance to camptothecin, indenoisoquinoline, aphidicolin, hydroxyurea, and staurosporine and hypersensitivity to etoposide and actinomycin D show that Top1, in addition to being the target of camptothecins, also regulates DNA replication, rDNA stability, and apoptosis. Overall, our studies show the pleiotropic nature of human Top1 activities. In addition to its classic DNA nicking-closing functions, Top1 plays critical nonclassic roles in genomic stability, gene-specific transcription, and response to various anticancer agents. The reported cell lines and approaches described in this article provide new tools to perform detailed functional analyses related to Top1 function.

Enrichment of a population of mammary gland cells that form mammospheres and have in vivo repopulating activity.

The identification of mammary gland stem cells (MGSC) or progenitors is important for the study of normal breast development and tumorigenesis. Based on their immunophenotype, we have isolated a population of mouse mammary gland cells that are capable of forming "mammospheres" in vitro. Importantly, mammospheres are enriched for cells that regenerate an entire mammary gland on implantation into a mammary fat pad. We also undertook cytogenetic analyses of mammosphere-forming cells after prolonged culture, which provided preliminary insight into the genomic stability of these cells. Our identification of new cell surface markers for enriching mammosphere-initiating cells, including endoglin and prion protein, will facilitate the elucidation of the cell biology of MGSC.

POTE paralogs are induced and differentially expressed in many cancers.

To identify new antigens that are targets for the immunotherapy of prostate and breast cancer, we used expressed sequence tag and genomic databases and discovered POTE, a new primate-specific gene family. Each POTE gene encodes a protein that contains three domains, although the proteins vary greatly in size. The NH2-terminal domain is novel and has properties of an extracellular domain but does not contain a signal sequence. The second and third domains are rich in ankyrin repeats and spectrin-like helices, respectively. The protein encoded by POTE-21, the first family member discovered, is localized on the plasma membrane of the cell. In humans, 13 highly homologous paralogs are dispersed among eight chromosomes. The expression of POTE genes in normal tissues is restricted to prostate, ovary, testis, and placenta. A survey of several cancer samples showed that POTE was expressed in 6 of 6 prostate, 12 of 13 breast, 5 of 5 colon, 5 of 6 lung, and 4 of 5 ovarian cancers. To determine the relative expression of each POTE paralog in cancer and normal samples, we employed a PCR-based cloning and analysis method. We found that POTE-2alpha, POTE-2beta, POTE-2gamma, and POTE-22 are predominantly expressed in cancers whereas POTE expression in normal tissues is somewhat more diverse. Because POTE is primate specific and is expressed in testis and many cancers but only in a few normal tissues, we conclude POTE is a new primate-specific member of the cancer-testis antigen family. It is likely that POTE has a unique role in primate biology.

Androgen inducibility of Fgf8 in Shionogi carcinoma 115 cells correlates with an adjacent t(5;19) translocation.

Fgf8 (fibroblast growth factor 8) was initially cloned from a mouse mammary tumor cell line derived from the androgen-dependent Shionogi carcinoma 115. The androgen-inducible expression of Fgf8 in this tumor controls its androgen-dependent phenotype, thus stimulating interest in this gene as a possible factor in human prostate cancer and other androgen-sensitive cancers. However, apart from Shionogi carcinoma 115, the androgen inducibility of Fgf8 is controversial. In the present study, having not detected androgen-inducible expression of Fgf8 in other mouse mammary cell lines or mouse prostate, we examined the Shionogi carcinoma 115-derived S115 cell line for mouse mammary tumor virus (MMTV) insertions or other nearby DNA rearrangements that might explain the androgen inducibility of Fgf8 in these cells. Southern blotting did not detect MMTV insertions near Fgf8 but did reveal a specific DNA rearrangement 3.7 kb upstream of Fgf8 in S115 cells and in other cells (SC115) independently derived from Shionogi carcinoma 115. Spectral karyotyping of S115 cells and sequencing of the cloned rearrangement junctions indicate that Fgf8 is involved in a t(5;19) translocation. The chromosome 5 sequence joined to Fgf8 is immediately adjacent to Smr2 (submaxillary gland androgen-regulated protein 2) and includes Muc10 (mucin 10), two genes that we show are testosterone inducible in S115 cells, suggesting that the androgen-dependent expression of Fgf8 in Shionogi carcinoma 115 and derivative cells results from this translocation. Together, these results suggest that androgen inducibility is not an inherent property of the Fgf8 gene, which has implications regarding this gene's proposed role in the etiology of hormone-responsive cancers.

Chromosomal aberrations in cell lines derived from thyroid tumors spontaneously developed in TRbetaPV/PV mice.

The etiology and genetic alterations of follicular thyroid carcinoma are not well understood. By targeting a mutation (PV) into the thyroid hormone receptor beta gene (TRbetaPV mouse), we created a knock-in mutant TRbeta(PV/PV) mouse that spontaneously develop follicular thyroid carcinoma with progression to metastasis similar to human follicular thyroid carcinoma. This mouse model provides a valuable tool to ascertain the nature and the extent of genomic rearrangements that occur during carcinogenesis of the thyroid. Spectral karyotyping analysis (SKY) of seven cell lines derived from thyroid tumors developed in TRbeta(PV/PV) mice showed that all of them had abnormal karyotypes, with chromosome number ranging from near-diploid (39-42 chromosomes) to hypotetraploid (63-79 chromosomes). These seven cell lines also exhibited a variety of structural chromosomal aberrations, including common recurrent translocations and deletions. This SKY analysis shows that the development and progression of follicular thyroid carcinoma in knock-in TRbeta(PV/PV) mutant mice comprise recurrent structural and numerical genomic changes, some of which mimic those described in human thyroid cancer.

The human AKNA gene expresses multiple transcripts and protein isoforms as a result of alternative promoter usage, splicing, and polyadenylation.

We previously showed that the human AKNA gene encodes an AT-hook transcription factor that regulates the expression of costimulatory cell surface molecules on lymphocytes. However, AKNA cDNA probes hybridize with multiple transcripts, suggesting either the existence of other homologous genes or a complex regulation operating on a single gene. Here we report evidence for the latter, as we find that AKNA is encoded by a single gene that spans a 61-kb locus of 24 exons on the fragile FRA9E region of human chromosome 9q32. This gene gives rise to at least nine distinct transcripts, most of which are expressed in a tissue-specific manner in lymphoid organs. Many of the AKNA transcripts originate from alternative splicing; others appear to derive from differential polyadenylation and promoter usage. The alternative AKNA transcripts are predicted to encode overlapping protein isoforms, some of which (p70 and p100) are readily detectable using a polyclonal anti-AKNA antisera that we generated. We also find that AKNA PEST-dependent cleavage into p50 polypeptides is targeted to mature B cells and appears to be required for CD40 upregulation. The unusual capacity of the AKNA gene to generate multiple transcripts and proteins may reflect its functional diversity, and it may also provide a fail-safe mechanism that preserves AKNA expression.

Functional identification of a BAC clone from 16q24 carrying a senescence gene SEN16 for breast cancer cells.

We have identified an 85 kb BAC clone, 346J21, that carries a cell senescence gene (SEN16), previously mapped to 16q24.3. Transfer and retention of 346J21 in breast cancer cell lines leads to growth arrest after 8-10 cell doublings, accompanied by the appearance of characteristic senescent cell morphology and senescence-associated acid beta-galactosidase activity. Loss of transferred BAC results in reversion to the immortal growth phenotype of the parental cancer cell lines. BAC 346J21 restores senescence in the human breast cancer cell lines, MCF.7 and MDA-MB468, and the rat mammary tumor cell line LA7, but not in the human glioblastoma cell line T98G. We postulate that inactivation of both copies of SEN16 is required for the immortalization of breast epithelial cells at an early stage of tumorigenesis. Positional mapping of 346J21 shows that SEN16 is distinct from other candidate tumor suppressor genes reported at 16q24.

Thirteen-exon-motif signature for vertebrate nuclear and mitochondrial type IB topoisomerases.

DNA topoisomerases contribute to various cellular activities that involve DNA. We previously identified a human nuclear gene that encodes a mitochondrial DNA topoisomerase. Here we show that genes for mitochondrial DNA topoisomerases (type IB) exist only in vertebrates. A 13-exon topoisomerase motif was identified as a characteristic of genes for both nuclear and mitochondrial type IB topoisomerases. The presence of this signature motif is thus an indicator of the coexistence of nuclear and mitochondrial type IB DNA topoisomerases. We hypothesize that the prototype topoisomerase IB with the 13-exon structure formed first, and then duplicated. One topoisomerase specialized for nuclear DNA and the other for mitochondrial DNA.

Senescing human cells and ageing mice accumulate DNA lesions with unrepairable double-strand breaks.

Humans and animals undergo ageing, and although their primary cells undergo cellular senescence in culture, the relationship between these two processes is unclear. Here we show that gamma-H2AX foci (gamma-foci), which reveal DNA double-strand breaks (DSBs), accumulate in senescing human cell cultures and in ageing mice. They colocalize with DSB repair factors, but not significantly with telomeres. These cryptogenic gamma-foci remain after repair of radiation-induced gamma-foci, suggesting that they may represent DNA lesions with unrepairable DSBs. Thus, we conclude that accumulation of unrepairable DSBs may have a causal role in mammalian ageing.