Ex vivo Culture and Lentiviral Transduction of Benign Prostatic Hyperplasia (BPH) Samples

To assess oncogenic potential, classical transformation assays are based on cell line models. However, cell line based models do not reflect the complexity of human tissues. We thus developed an inducible expression system for gene expression in ex vivo human tissues, which maintain native tissue architecture, such as epithelia and stroma. To validate the system, we transduced and expressed known tumor suppressors (p53, p33ING1b), oncoproteins (RasV12, p47ING3), or controls (empty vector, YFP) in ex vivo prostate tissues, then assessed proliferation by immunohistochemistry of markers (H3S10^phos). Herein, we describe how to generate lentiviral vectors and particules, successfully transduce human prostate tissues, induce exogenous gene expression, and assess cellular proliferation.

Human ex vivo prostate tissue model system identifies ING3 as an oncoprotein

BACKGROUND

Although the founding members of the INhibitor of Growth (ING) family of histone mark readers, ING1 and ING2, were defined as tumour suppressors in animal models, the role of other ING proteins in cellular proliferation and cancer progression is unclear.

METHODS

We transduced ex vivo benign prostate hyperplasia tissues with inducible lentiviral particles to express ING proteins. Proliferation was assessed by H3S10phos immunohistochemistry (IHC). The expression of ING3 was assessed by IHC on a human prostate cancer tissue microarray (TMA). Gene expression was measured by DNA microarray and validated by real-time qPCR.

RESULTS

We found that ING3 stimulates cellular proliferation in ex vivo tissues, suggesting that ING3 could be oncogenic. Indeed, ING3 overexpression transformed normal human dermal fibroblasts. We observed elevated levels of ING3 in prostate cancer samples, which correlated with poorer patient survival. Consistent with an oncogenic role, gene-silencing experiments revealed that ING3 is required for the proliferation of breast, ovarian, and prostate cancer cells. Finally, ING3 controls the expression of an intricate network of cell cycle genes by associating with chromatin modifiers and the H3K4me3 mark at transcriptional start sites.

CONCLUSIONS

Our investigations create a shift in the prevailing view that ING proteins are tumour suppressors and redefine ING3 as an oncoprotein.

ING3 promotes prostate cancer growth by activating the androgen receptor

BACKGROUND

The androgen receptor (AR) is a major driver of prostate cancer, and increased AR levels and co-activators of the receptor promote the development of prostate cancer. INhibitor of Growth (ING) proteins target lysine acetyltransferase or lysine deacetylase complexes to the histone H3K4Me3 mark of active transcription, to affect chromatin structure and gene expression. ING3 is a stoichiometric member of the TIP60 lysine acetyltransferase complex implicated in prostate cancer development.

METHODS

Biopsies of 265 patients with prostate cancer were stained for ING3, pan-cytokeratin, and DNA. LNCaP and C4-2 androgen-responsive cells were used for in vitro assays including immunoprecipitation, western blotting, Luciferase reporter assay and quantitative polymerase chain reaction. Cell viability and migration assays were performed in prostate cancer cell lines using scrambled siRNA or siRNA targeting ING3.

RESULTS

We find that ING3 levels and AR activity positively correlate in prostate cancer. ING3 potentiates androgen effects, increasing expression of androgen-regulated genes and androgen response element-driven reporters to promote growth and anchorage-independent growth. Conversely, ING3 knockdown inhibits prostate cancer cell growth and invasion. ING3 activates the AR by serving as a scaffold to increase interaction between TIP60 and the AR in the cytoplasm, enhancing receptor acetylation and translocation to the nucleus. Activation is independent of ING3's ability to target the TIP60 complex to H3K4Me3, identifying a previously unknown chromatin-independent cytoplasmic activity for ING3. In agreement with in vitro observations, analysis of The Cancer Genome Atlas (TCGA) data (n = 498) and a prostate cancer tissue microarray (n = 256) show that ING3 levels are higher in aggressive prostate cancers, with high levels of ING3 predicting shorter patient survival in a low AR subgroup. Including ING3 levels with currently used indicators such as the Gleason score provides more accurate prognosis in primary prostate cancer.

CONCLUSIONS

In contrast to the majority of previous reports suggesting tumor suppressive functions in other cancers, our observations identify a clear oncogenic role for ING3, which acts as a co-activator of AR in prostate cancer. Data from TCGA and our previous and current tissue microarrays suggest that ING3 levels correlate with AR levels and that in patients with low levels of the receptor, ING3 level could serve as a useful prognostic biomarker.

Abstract 2940: Identifying the minimal region of the ING1 tumor suppressor capable of efficiently killing cancer cells

ING1b is a type II tumor suppressor and a stoichiometric member of HDAC-containing protein complexes. ING1b overexpression promotes apoptosis, and decreased levels of ING1b are frequently observed in human tumors and cancer cell lines. Previously, we reported the identification of the regions of ING1b protein required for its pro-apoptotic function. Using deletion mutant analysis and transient expression in HEK293 cells we have determined that ING1b-derived peptides comprised of the NLS/NTS domain and the third alpha helix (A3H) of ING1b including the N-terminal portion of Lamin Interaction Domain (LID) are able to induce apoptosis at levels comparable to those of the full length ING1b, despite their lacking the PHD domain through which they affect the histone epigenetic code. Here, we report that adenoviral delivery of the A3H-NLS/NTS peptide led to a significant decrease in cell survival and induced apoptosis in a majority of tested cancer cell lines. The list of sensitive cancer cell lines was broad and included lines derived from both primary and metastatic tumor sources. As shown using an Annexin V binding assay, decreased survival of cells infected with adenovirus expressing A3H-NLS/NTS peptide was due to a large number of infected cells undergoing apoptosis. In fact, infecting triple negative tumorigenic MDA-MB-468 breast cancer line with 30 MOI of the Ad-A3H-NLS/NTS virus triggered rapid apoptosis in nearly 70% of infected cells. Importantly, these effects were time and dose dependent. Also, using a cell line carrying an inducible p53-expression system, we established that the ability of Ad-A3H-NLS/NTS to induce apoptosis is p53-independent. Next, we evaluated the cell death inducing properties of Ad-A3H-NLS/NTS using 10 breast cancer cell lines that were chosen based on their sensitivity to common HDAC inhibitors. While the overall sensitivity to Ad-A3H-NLS/NTS induced cell death varied among these lines, we observed no obvious correlation between the sensitivity to Ad-A3H-NLS/NTS and sensitivity/resistance to HDAC inhibitors. These findings suggest that Ad-A3H-NLS/NTS may have a broad application, including in those cancer cells lines that are normally resistant to conventional chemotherapy. Furthermore, combined application of Ad-A3H-NLS/NTS with either Trichostatin A (TSA) or Panobinostat (LBH-589) resulted in a strong additive, and possibly synergistic killing effect. Using the virus with either TSA or LBH-589 increased efficacy by nearly 3.5-fold as compared to the treatment with either TSA or LBH-589 along. Currently, the synergy between the Ad-A3H-NLS/NTS and common chemotherapeutics is being closely evaluated. Our long-term goal is to develop ING1b-based therapeutics that can be used as an adjuvant therapy in combination with existing cancer treatments.

Citation Format: Oleksandr Boyko, Karl T. Riabowol. Identifying the minimal region of the ING1 tumor suppressor capable of efficiently killing cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2940. doi:10.1158/1538-7445.AM2015-2940

Abstract 5125: Defining the minimal ING1b-derived peptide that is pro-apoptotic

The Inhibitor of Growth 1b (ING1b) is a type II tumor suppressor and a core, stoichiometric member of HDAC-containing chromatin-modifying complexes. ING1b contributes to regulation of gene expression, DNA damage repair and stress signalling, cell growth and senescence, tumorigenesis and apoptosis. Mislocalization of ING1b and decreased levels of ING1b are commonly found in human tumors and cancer cell lines, suggesting the possibility of using ING1b levels as a prognostic marker in clinics. Consistent with a pro-apoptotic function, ING1b protein positively regulates p21 and bax gene expression, and ING1b overexpression promotes apoptosis in p53-dependent and p53-independent manners. The inactivation of apoptosis pathways that is frequently observed in cancer cells causes a dramatic decrease in the efficiency of many cancer treatments. The inactivation of ING1b and suppression of apoptosis in tumors and a greater sensitivity of cancer vs. normal cells to elevated levels of ING1b suggest that modulation of ING1b expression in tumors may serve as a viable approach for cancer therapy. Our aim is to expand understanding of the molecular mechanism(s) by which ING1b promotes apoptosis in cancer cells. We strive to define ING1b regions that are necessary for its apoptotic function in order to design minimal recombinant peptides with potent apoptosis-inducing properties. We report that peptides containing the third alpha helix (A3H) and NLS/NTS domains of the ING1b protein are able to induce apoptosis at levels comparable with those seen for full length ING1b protein. Cells overexpressing full length ING1b protein or the A3H-NLS/NTS peptide show similar changes in cell morphology characteristic of apoptosis, exhibit increased levels of PARP cleavage, and display similar levels of apoptosis as determined by FACS analysis using an Annexin V assay. While the A3H region is necessary but not sufficient, the NLS/NTS domain is required, and partially sufficient, for induction of apoptosis. The A3H-NLS peptide exhibited strong nucleolar localization characteristic of full length ING1b. Consistently, overexpression of ING1b deletion mutants lacking the A3H-NLS/NTS sequence failed to induce apoptosis. Substituting ING1b's NLS/NTS domain with the short conventional NLS and NTS sequences dramatically reduced pro-apoptotic properties of recombinant peptides, thus indicating a novel molecular function of NLS/NTS domain in ING1b-mediated apoptosis. Currently, we assess the efficacy of A3H-NLS peptide for controlling cancer cell growth and progression in various breast cancer cell lines via adenovirus-mediated gene delivery. Our long-term goal is to better define apoptosis pathways and to develop ING1b-based therapeutics for the selective induction of apoptosis in cancer cells.

Citation Format: Oleksandr Boyko, Karl T. Riabowol. Defining the minimal ING1b-derived peptide that is pro-apoptotic. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5125. doi:10.1158/1538-7445.AM2014-5125

Down-regulation of the inhibitor of growth 1 (ING1) tumor suppressor sensitizes p53-deficient glioblastoma cells to cisplatin-induced cell death

Impaired tumor suppressor functions, such as deficient p53, are characteristic for glioblastoma multiforme (GBM) and can cause resistance to DNA-damaging agents like cisplatin. We have recently shown that the INhibitor of Growth 1 (ING1) tumor suppressor is down-regulated in malignant gliomas and that the decrease of ING1 expression correlates with histological grade of malignancy, suggesting a role for ING1 in the pathogenesis and progression of malignant gliomas. Based on this background, the purpose of our current study was to examine the potential impact of ING1 protein levels on DNA-damage response in GBM. Using LN229 GBM cells, which express ING1 proteins and harbor mutant TP53, we are the first to show that DNA damage by cisplatin or ionizing radiation differentially induced the two major ING1 splicing isoforms. The p47 ING1a isoform, that promotes deacetylation of histones, thus formation of heterochromatic regions of DNA, which are less susceptible to DNA damage, was preferentially induced by >50-fold. This might represent a response to protect DNA from damage. Also, ING1 knockdown by siRNA accelerated transit of cells through G1 phase, consistent with ING1 serving a tumor suppressor function, and caused cells to enter apoptosis more rapidly in response to cisplatin. Our results indicate that malignant gliomas may down-regulate ING1 to allow more efficient tumor growth and progression. Also, ING1 down-regulation may sensitize GBM cells with deficient p53 to treatment with cisplatin.

Acceleration of Telomere Loss by Chemotherapy Is Greater in Older Patients with Locally Advanced Head and Neck Cancer

PURPOSE

Chronic viral infection and combinations of chemotherapeutic drugs have been reported to accelerate telomere erosion. Here, we asked if chemoradiotherapy, using the single agent cisplatin, would accelerate telomere loss in head and neck cancer patients, and whether loss was linked to smoking status, age, gender, or stage of disease at diagnosis.

EXPERIMENTAL DESIGN

Blood samples were collected from 20 patients with squamous cell cancer of the head and neck before, during, and after chemoradiotherapy. Following DNA isolation from peripheral blood mononuclear cells, telomere length was measured by terminal restriction fragment analysis.

RESULTS

Chemoradiotherapy increased the rate of telomere erosion>100-fold. Telomere length before treatment in chemoradiotherapy patients was similar to age-matched controls. Although smokers began with significantly shorter telomeres, smoking status did not affect chemoradiotherapy-induced attrition, nor did gender or stage of disease. We also make the novel observation that a significantly greater telomere loss occurred in response to treatment in older patients, with those younger than 55 years losing an average of 400 bp of telomeric DNA compared with the 880 bp lost by those over 55 years.

CONCLUSIONS

The lack of telomere length difference before treatment suggests that shortened telomeres may not be a risk factor for development of head and neck cancer in the age range we examined. Chemoradiotherapy caused a severe telomere length reduction in all patients. The significant difference seen in the elderly (P=0.018) suggests that chemoradiotherapy may have more severe effects on the replicative capacity of blood cells in older patients.

Paternal age is positively linked to telomere length of children

Telomere length is linked to age-associated diseases, with shorter telomeres in blood associated with an increased probability of mortality from infection or heart disease. Little is known about how human telomere length is regulated despite convincing data from twins that telomere length is largely heritable, uniform in various tissues during development until birth and variable between individuals. As sperm cells show increasing telomere length with age, we investigated whether age of fathers at conception correlated with telomere length of their offspring. Telomere length in blood from 125 random subjects was shown to be positively associated with paternal age (+22 bp yr -1, 95% confidence interval 5.2-38.3, P = 0.010), and paternal age was calculated to affect telomere length by up to 20% of average telomere length per generation. Males lose telomeric sequence faster than females (31 bp yr -1, 17.6-43.8, P < 0.0001 vs. 14 bp yr -1, 3.5-24.8, P < 0.01) and the rate of telomere loss slows throughout the human lifespan. These data indicate that paternal age plays a role in the vertical transmission of telomere length and may contribute significantly to the variability of telomere length seen in the human population, particularly if effects are cumulative through generations.

Murine Models of Life Span Extension

Mice are excellent experimental models for genetic research and are being used to investigate the genetic component of organismal aging. Several mutant mice are known to possess defects in the growth hormone/insulin-like growth factor 1 (GH/IGF-1) neurohormonal pathway and exhibit dwarfism together with extended life span. Their phenotypes resemble those of mice subjected to caloric restriction. Targeted mutations that affect components of this pathway, including the GH receptor, p66Shc, and the IGF-1 receptor (IGF-1R), also extend life span; mutations that affect IGF-1R or downstream components of the pathway decouple longevity effects from dwarfism. These effects on life span may result from an increased capacity to resist oxidative damage.

A novel transcriptional inhibitory element differentially regulates the cyclin D1 gene in senescent cells

Senescent human diploid fibroblasts are unable to initiate DNA synthesis following mitogenic stimulation and adopt a unique gene expression profile distinct from young or quiescent cells. In this study, a novel transcriptional regulatory element was identified in the 5'-untranslated region of the cyclin D1 gene. We show that this element differentially suppresses cyclin D1 expression in young versus senescent fibroblasts. Electrophoretic mobility shift assays revealed abundant complexes forming with young cell nuclear extracts compared with senescent cell nuclear extracts. Binding was maintained in young quiescent cells, showing that loss of this activity was specific to senescent cells and not an effect of cell cycle arrest. Site-directed mutagenesis within this cyclin D1 inhibitory element (DIE) abolished binding activity and selectively increased cyclin D1 promoter activity in young but not in senescent cells. Sequences with homology to the DIE were found in the 5'-untranslated regions of other genes known to be up-regulated during cellular aging, suggesting that protein(s) that bind the DIE might be responsible for the coordinate increase in transcription of many genes during cellular aging. This study provides evidence that loss of transcriptional repressor activity contributes to the up-regulation of cyclin D1, and possibly additional age-regulated genes, during cellular senescence.

Accelerated replicative senescence of the peripheral immune system induced by HIV infection

OBJECTIVES

HIV induces rapid turnover of T lymphocytes but whether this leads to replicative senescence of CD4+ and CD8+ cells and contributes to AIDS symptoms is unclear. The aim of this study was to address this question by analyzing telomere length in blood cell populations as a measure of replicative history in a significant number of patients infected with HIV.

DESIGN

Total peripheral blood mononuclear cells (PBMCs), CD4+ or CD8+ cells were isolated from blood collected from a total of 73 HIV patients and 27 controls. Samples were isolated to measure telomere length, telomerase activity and proliferative ability, and analyses were carried out in a blind experimental protocol.

METHODS

PBMCs isolated on Ficoll-Hypaque gradients were washed and prepared for additional fractionation into CD4+ and CD8+ cells using antibody-bound magnetic beads. Total PBMCs, CD4+ and CD8+ cells were used for cell cycle analysis, for telomerase activity assays and were measured for telomere length using the terminal restriction fragment assay.

RESULTS

Telomere analyses in this study show a clear (P < 0.0001) inverse relationship between telomere length and progression of immunosuppression, with HIV infection resulting in a five-fold or greater acceleration of aging of the circulating PBMC component of the immune system. Patients who are 37 years old showed telomere lengths similar to uninfected 75-year-olds. Telomere loss correlated well with progression of AIDS and with reduced proliferative ability of patient PBMCs but was unrelated to telomerase activity. Mean telomere length was shorter in both CD4+ and CD8+ cells, with three-fold higher rates of telomere loss for CD8+ lymphocytes.

CONCLUSIONS

These data provide strong support for the occurrence of accelerated replicative aging of the peripheral immune system, possibly resulting in a loss of T cells leading to AIDS symptoms.

A role for serine proteases in mediating phorbol ester-induced differentiation of HL-60 cells

Treatment of human HL-60 promyelocytic leukemia cells with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) results in increases in proteolytic activity and maturation toward the monocytic lineage. To investigate the potential roles that different classes of proteases play in the monocytic differentiation of HL-60 cells, cells were treated with phorbol ester in the presence of various serine and cysteine protease inhibitors. The serine protease inhibitors 4-(2-aminoethyl)-benzenesulfonyl fluoride (AEBSF), N-alpha-tosyl-phenylalanine chloromethyl ketone (TPCK), and N-alpha-tosyl-lysine chloromethyl ketone (TLCK) repressed a number of phenotypic markers of monocytic differentiation including surface expression of the CD11b integrin, cell aggregate formation, cell cycle exit, and cell death. CD11b was not detected at the cell surface by FACS analysis up to 24 h after induction of differentiation; however, both CD11b mRNA and protein were present. Downregulation of c-myc mRNA and upregulation of c-fos and egr-1 mRNA and protein, which normally occur during TPA-induced differentiation, were not affected by inclusion of the protease inhibitors. These data indicate that serine proteases specifically mediate many of the phenotypic aspects of TPA-induced monocytic differentiation but are not involved with the induction or repression of differentiation-sensitive transcription factors and suggest that serine protease activity is required for intracellular processing of CD11b.

Decreased expression and activity of the immediate-early growth response (Egr-1) gene product during cellular senescence

Human diploid fibroblasts (HDFs) undergo a limited number of population doublings in culture before reaching the end of their proliferative life span, an event termed in vitro cellular senescence. Considerable evidence suggests that altered expression of key genes involved in the mitogenic response may be responsible for the inability of senescent cells to proliferate. Here we examined the expression and activity of the early growth response-1 (egr-1) gene, an "immediate-early" gene that is believed to link extracellular mitogenic signals to cell-cycle progression. We found that egr-1 was strongly downregulated in senescent HDFs at the level of mRNA, protein, and DNA binding activity. Decreased DNA binding activity of Egr-1 in vitro corresponded to decreased transcriptional activation in vivo. To further understand the mechanism of egr-1 downregulation, we examined the potential role of the serum response elements (SREs) present in the egr-1 promoter. Electrophoretic mobility shift studies using young and old cell nuclear extracts showed a marked decrease in serum response factor (SRF) binding activity to the SRE in old compared to young cells. Loss of SRF binding activity has been correlated with the loss of expression of another growth-related immediate-early gene (c-fos). These results suggest a common mechanism for the downregulation of c-fos, egr-1, and other SRE-dependent, mitogen-responsive genes during cellular senescence.

Increased expression of cyclin D2 during multiple states of growth arrest in primary and established cells

Cyclin D2 is a member of the family of D-type cyclins that is implicated in cell cycle regulation, differentiation, and oncogenic transformation. To better understand the role of this cyclin in the control of cell proliferation, cyclin D2 expression was monitored under various growth conditions in primary human and established murine fibroblasts. In different states of cellular growth arrest initiated by contact inhibition, serum starvation, or cellular senescence, marked increases (5- to 20-fold) were seen in the expression levels of cyclin D2 mRNA and protein. Indirect immunofluorescence studies showed that cyclin D2 protein localized to the nucleus in G0, suggesting a nuclear function for cyclin D2 in quiescent cells. Cyclin D2 was also found to be associated with the cyclin-dependent kinases CDK2 and CDK4 but not CDK6 during growth arrest. Cyclin D2-CDK2 complexes increased in amounts but were inactive as histone H1 kinases in quiescent cells. Transient transfection and needle microinjection of cyclin D2 expression constructs demonstrated that overexpression of cyclin D2 protein efficiently inhibited cell cycle progression and DNA synthesis. These data suggest that in addition to a role in promoting cell cycle progression through phosphorylation of retinoblastoma family proteins in some cell systems, cyclin D2 may contribute to the induction and/or maintenance of a nonproliferative state, possibly through sequestration of the CDK2 catalytic subunit.

Circular YAC vectors containing a small mammalian origin sequence can associate with the nuclear matrix

Three different mammalian origins of DNA replication, 343, S3, and X24, have been cloned into a 15.8 kb circular yeast vector pYACneo. Subsequent transfection into HeLa cells resulted in the isolation of several stably maintained clones. Two cell lines, C343e2 and CS3e1, were found to have sequences maintained as episomes in long-term culture with a stability per generation of approximately 80%. Both episomes also contain matrix attachment region (MAR) sequences which mediate the binding of DNA to the nuclear skeleton and are thought to play a role in DNA replication. Using high salt extraction of the nucleus and fluorescent in situ hybridization, we were able to demonstrate an association of the 343 episome with the nuclear matrix, most probably through functional MAR sequences that allow an association with the nuclear matrix and associated regions containing essential replication proteins. The presence of functional MARs in small episomal sequences may facilitate the replication and maintenance of transfected DNA as an episome and improve their utility as small episomal constructs, potential microchromosomes.

Immunoreactivity for the Fas ligand in the mammalian enteric nervous system

The Fas ligand induces apoptosis in activated immunocytes that express the Fas receptor. Fas-ligand transcripts have been found previously in murine intestine but the intestinal tissues that express Fas ligand have not been identified. We used immunohistochemistry to examine the expression of the Fas ligand in the enteric nervous system of rats, mice, guinea-pigs, ferrets and humans. Fas-ligand immunoreactivity was detectable in enteric nerve fibres and neurons in all species tested, representing 25%-50% of the neurons in rats, mice and guinea-pigs. An antigen of approximately 48 kDa was detected by Western blot analysis with Fas-ligand antiserum in the dissected enteric plexuses of duodenum from a C3H/HeJ mouse. In gld mice that harbour a Fas-ligand mutation, Fas-ligand immunoreactivity was slightly more intense in neurons and fibres and was also apparent in submucosal lymphocytes. In the myenteric plexuses of guinea-pig ileum and human colon, Fas-ligand immunoreactivity was not contained in neurons exhibiting nicotinamide-adenine dinucleotide phosphate-diaphorase activity. In the submucosal plexus of guinea-pig ileum, labelled neurons included some neuropeptide-Y-containing neurons but none with vasoactive intestinal polypeptide. We conclude that the Fas ligand is expressed by a large subset of enteric neurons and may provide the basis for cytotoxic neuroimmune interactions in the intestines.

Selective inhibition of telomerase activity during terminal differentiation of immortal cell lines

Telomerase, the enzyme that maintains the ends of linear eukaryotic chromosomes, is active in human germ cells and in a majority of tumor tissues and immortalized cell lines. In contrast, most mature somatic cells and tissues contain low or undetectable telomerase activity, implying a stringent negative regulatory control mechanism. We report here that telomerase activity is dramatically inhibited during the terminal differentiation of HL-60 human promyelocytic leukemia cells to monocytic and granulocytic lineages. A loss of telomerase activity was seen in response to three different inducers of differentiation, was independent of differentiation-induced apoptosis, and occurred in the presence of unaltered expression of the RNA component of telomerase. Reduction in telomerase activity was also observed during the differentiation of murine F9 teratocarcinoma and C2C12 myoblast cells. In contrast, induced differentiation of murine p19 embryonal carcinoma and Neuro 2a neuroblastoma cells did not result in a loss of telomerase activity. These results are therefore consistent with the absence of telomerase activity in human somatic cells and the presence of telomerase activity in many somatic murine cells and tissues.

Regulation of gene expression and transcription factor binding activity during cellular aging

Human diploid fibroblasts (HDFs) undergo a limited number of population doublings in vitro and are widely used as a model of cellular aging. Despite growing evidence that cellular aging occurs as a result of altered gene expression, little is known about the activity of transcription factors that regulate gene expression in aging cells. Here we survey the relevant literature regarding altered gene expression and the role of transcription factors during cellular aging, focusing upon the serum response factor (SRF). SRF is hyperphosphorylated in senescent HDFs and fails to bind to the serum-response element in the c-fos promoter. Differential phosphorylation during replicative aging may contribute, at least in part, to the altered activity of SRF and possibly other transcription factors and to subsequent changes in the expression of serum-regulated genes in senescent HDFs.

Differential effects of transforming growth factor-beta 1 on the expression of matrix metalloproteinases and tissue inhibitors of metalloproteinases in young and old human fibroblasts

The balance between the activities of matrix metalloproteinases (MMPs) and the tissue inhibitors of metalloproteinases (TIMPs) is an important control point in tissue remodeling. Previous studies have demonstrated elevated expression of the MMPs collagenase and stromelysin-1 by aged human diploid fibroblasts compared to early-passage cultures. We show here that aging cells display an altered response to transforming growth factor-beta 1 (TGF beta 1) that selectively affects MMP mRNA expression. In both young and old cells, phorbol myristoyl-13 acetate (PMA) induced the expression of transcripts of collagenase, stromelysin-1, gelatinase-B, TIMP-1, and TIMP-3. In young cells, TGF beta 1 reciprocally modulated PMA-induced MMP and TIMP gene expression leading to reduced levels of transcripts for the MMPs and augmented accumulation of TIMP-1 and TIMP-3 mRNAs. However, repressing effects of TGF beta 1 on collagenase, stromelysin-1, and gelatinase-B RNA expression were not apparent in old cells, though induction of the TIMP genes was unimpaired. By electrophoretic mobility shift analysis the nuclear transcription factors AP1 and serum response factor (SRF) showed reduced levels of DNA binding activities in old fibroblasts compared to young cells. A probe for the TGF beta-inhibitory element (TIE) gave equivalent levels of complexes with nuclear extracts from both types of cells, though of different mobilities. We conclude that the effects of TGF beta 1 on MMP and TIMP gene expression involve different cellular intermediaries, and suggest that altered composition or modification of TIE binding factors in aging cells may underlie the failure of TGF beta 1-mediated transcription repression. This mechanism may contribute to elevated constitutive expression of MMPs in old cells and to the connective tissue deterioration that accompanies the aging process.

Monitoring mRNA expression by polymerase chain reaction: the "primer-dropping" method

We have developed a method to monitor mRNA expression that is based upon the reverse transcriptase-polymerase chain reaction (RT-PCR) and includes multiple sets of primer pairs in coamplification reactions. To observe relative changes in mRNA steady-state levels, each target in a multiplex reaction was amplified to within a predetermined range by using PCR cycle numbers specific for each target. Optimal PCR cycle numbers for target templates were determined by preliminary titration experiments performed using the "primer-dropping" method. By employing this method, the overall amplification reaction was limited, permitting the PCR products to remain within the exponential range of the amplification curve and yet be detectable on ethidium bromide-stained gels. We demonstrated the utility of this method by monitoring the expression kinetics of cyclins A, B1, D1, and E, and of the immediate-early genes c-fos, c-myc, and beta-actin. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was included in the multiplex reactions as an endogenous internal standard to control for variations in product abundances due to differences in individual RT and PCR reaction efficiencies. Changes in gene expression of less than twofold to greater than 75-fold were readily distinguished.

Loss of serum response element-binding activity and hyperphosphorylation of serum response factor during cellular aging

Human diploid fibroblasts undergo a limited number of population doublings in vitro and are used widely as a model of cellular aging. Despite growing evidence that cellular aging occurs as a consequence of altered gene expression, little is known about the activity of transcription factors in aging cells. Here, we report a dramatic reduction in the ability of proteins extracted from the nuclei of near-senescent fibroblasts to bind the serum response element which is necessary for serum-induced transcription of the c-fos gene. In contrast, the activities of proteins binding to the RNA polymerase core element, TATA, as well as to the cyclic AMP response element were maintained during cellular aging. While no major differences in the expression of the serum response factor (SRF) that binds the serum response element were seen between early-passage and late-passage cells, hyperphosphorylation of SRF was observed in near-senescent cells. Furthermore, removal of phosphatase inhibitors during the isolation of endogenous nuclear proteins restored the ability of SRF isolated from old cells to bind the SRE. These data, therefore, indicate that hyperphosphorylation of SRF plays a role in altering the ability of this protein to bind to DNA and regulate gene expression in senescent cells.

Transcription factor activity during cellular aging of human diploid fibroblasts

Human diploid fibroblasts display a limited proliferative life span in vitro, which is directly correlated to the age of the donor from which the cells were explanted. In an effort to identify molecular events that may underlie the loss of proliferative potential in aging fibroblasts, we have determined, at the protein level, the abundance of several cell-cycle-regulated proteins and the activity of the two major members of the activator protein-1 (AP-1) DNA binding complex. We find that cyclin A and p34cdc2 expression is decreased by two- to four-fold in old fibroblasts, but that Fos expression and binding activity are reduced by as much as 95% in old, as opposed to young cells, despite equivalent amounts of p105Rb and Jun proteins being expressed. We have further determined that the composition of the protein complex which binds a consensus (-TGACTCA-) AP-1 site changes dramatically during in vitro aging. Since we have shown previously that AP-1 activity is required for progression through the cell cycle, we propose that the quantitative and qualitative changes seen in AP-1 may play a direct role in the gradual loss of proliferative ability seen as cells approach senescence.

Testing the in vivo role of protein kinase C and c-fos in neurite outgrowth by microinjection of antibodies into PC12 cells

To define the molecular bases of growth factor-induced signal transduction pathways, antibodies known to block the activity of either protein kinase C (PKC) or the fos protein were introduced into PC12 cells by microinjection. The antibody against PKC significantly inhibited neurite outgrowth when scored 24 h after microinjection and exposure to nerve growth factor (NGF). Microinjection of antibodies to fos significantly increased the percentage of neurite-bearing cells after exposure to either NGF or basic fibroblast growth factor (bFGF) but inhibited the stimulation of DNA synthesis by serum, suggesting that in PC12 cells, fos is involved in cellular proliferation. Thus, activation of PKC is involved in the induction of neurite outgrowth by NGF, but expression of the fos protein, which is induced by both NGF and bFGF, is not necessary and inhibits neurite outgrowth.

Multiple sequence elements of a single functional class are required for cyclic AMP responsiveness of the mouse c-fos promoter

Agents that elevate the intracellular concentration of cyclic AMP (cAMP) rapidly and transiently induce expression of the c-fos proto-oncogene in BALB/c 3T3 cells. We show that the mouse c-fos promoter-enhancer region contains multiple elements that contribute to cAMP responsiveness of the promoter in transient expression assays. The most potent element was found to correspond to a previously mapped basal promoter element and protein-binding site located 65 base pairs upstream of the transcriptional initiation site. This element and two less potent sites contained a match to the cAMP response element (CRE) core sequence defined in several mammalian genes. The relative potencies of these elements corresponded with their relative affinities for cellular factors that bound to the CRE in vitro. Mutation of all three elements failed to abolish completely cAMP responsiveness of the c-fos promoter in the transient expression assay. However, we present evidence that this residual responsiveness may have been due to sequences present in vector DNA. Finally, we show, by using a new microinjection competition assay, that a double-stranded oligonucleotide carrying the major c-fos CRE is sufficient to block induction of the endogenous c-fos gene by cAMP. Therefore, induction of the endogenous gene requires positively acting cellular factors that interact with a single functional class of regulatory sites in the c-fos gene. Unrelated regulatory elements, such as the serum response element and putative AP-2 sites, are not by themselves sufficient to mediate the cAMP response.

The catalytic subunit of cAMP-dependent protein kinase induces expression of genes containing cAMP-responsive enhancer elements

Transcriptional regulation of eukaryotic genes by cyclic AMP requires a cAMP-dependent protein kinase (A kinase). Two hypotheses have been proposed to explain how the holoenzyme of the A kinase induces transcription. The regulatory subunits of the A kinase, which bind cAMP and DNA, and have amino-acid homology with the Escherichia coli catabolite activator protein could directly stimulate gene expression. Alternatively, phosphorylation by the catalytic subunits could induce transcription by activating proteins involved in gene transcription. To distinguish between these models, we microinjected purified preparations of the catalytic and regulatory subunits of A kinase into tissue culture cells and monitored expression of a stably integrated fusion gene containing a cAMP-responsive human promoter fused to a bacterial reporter gene, or of the endogenous c-fos gene. The catalytic subunit stimulated expression of these genes, whereas the regulatory subunit did not. These results indicate that the catalytic subunit of A kinase is sufficient to induce expression of two cAMP-responsive genes, without increasing levels of cAMP.

Identification of microinjected cells using biotinylated antibodies and Strep-avidin-conjugated horseradish peroxidase

Results from experiments using needle microinjection of cells are often compromised by an inability to readily demonstrate which cells within a population have been injected. The technique described here allows the unambiguous identification of cells that have been successfully microinjected. Sequential incubation of fixed cells with biotinylated anti-immunoglobulin antibodies, followed by horseradish peroxidase (HRP)-conjugated Strep-avidin and HRP substrate, provides a sensitive assay for identification of cells containing trace amounts of immunoglobulins. This allows direct correlation to the presence of injected molecules of effects on cell morphology, the ability to enter into DNA synthesis, or expression of specific genes. By a variety of criteria, nonspecific immunoglobulins do not adversely affect cellular processes when injected by themselves or in the presence of other proteins known to have biological effects when injected, such as cAMP-dependent protein kinase and the ras oncogene protein.

Heat shock is lethal to fibroblasts microinjected with antibodies against hsp70

Synthesis of a small group of highly conserved proteins in response to elevated temperature and other agents that induce stress is a universal feature of prokaryotic and eukaryotic cells. Although correlative evidence suggests that these proteins play a role in enhancing survival during and after stress, there is no direct evidence to support this in mammalian cells. To assess the role of the most highly conserved heat shock protein (hsp) family during heat shock, affinity-purified monoclonal antibodies to hsp70 were introduced into fibroblasts by needle microinjection. In addition to impairing the heat-induced translocation of hsp70 proteins into the nucleus after mild heat shock treatment, injected cells were unable to survive a brief incubation at 45 degrees C. Cells injected with control antibodies survived a similar heat shock. These results indicate that functional hsp70 is required for survival of these cells during and after thermal stress.

Microinjection of fos-specific antibodies blocks DNA synthesis in fibroblast cells

Transcription of the protooncogene c-fos is increased greater than 10-fold within minutes of treatment of fibroblasts with serum or purified growth factors. Recent experiments with mouse 3T3 cell lines containing inducible fos antisense RNA constructs have shown that induced fos antisense RNA transcripts cause either a marked inhibition of growth in continuously proliferating cells or, conversely, a minimal effect except during the transition from a quiescent (G0) state into the cell cycle. Since intracellular production of large amounts of antisense RNA does not completely block gene expression, we microinjected affinity-purified antibodies raised against fos to determine whether and when during the cell cycle c-fos expression was required for cell proliferation. Using this independent method, we found that microinjected fos antibodies efficiently blocked serum-stimulated DNA synthesis when injected up to 6 to 8 h after serum stimulation of quiescent REF-52 fibroblasts. Furthermore, when fos antibodies were injected into asynchronously growing cells, a consistently greater number of cells was prevented from synthesizing DNA than when cells were injected with nonspecific immunoglobulins. Thus, whereas the activity of c-fos may be necessary for transition of fibroblasts from G0 to G1 of the cell cycle, its function is also required during the early G1 portion of the cell cycle to allow subsequent DNA synthesis.

Amplification of inter-Alu extrachromosomal DNA during cellular ageing: retraction and explanation

We reported previously the age-dependent appearance of extrachromosomal circular DNA bands hybridizing to a human DNA fragment ('inter-Alu'), isolated from a genomic cluster of AluI repeats. Such bands appeared or increased at late passage in four out of six human fibroblast strains (six out of nine cell expansions); moreover, all DNAs (9/9) obtained from peripheral lymphocytes of aged donors, but none (0/8) from young donors, revealed a non-genomic inter-Alu band at congruent to 4.8 kilobases (kb). Subsequent data extended these numbers to 16/24 aged donors compared with 0/18 young donors. These results were interpreted as evidence of age-dependent DNA rearrangement in normal human cells. We now report that the 'extra' bands were of microbial origin, although clearly occurring in an age-dependent manner.

Extrachromosomal circular DNA and aging cells

A DNA sequence situated in the human genome between Alu-repeat clusters ("Inter-Alu" DNA) is progressively amplified in extrachromosomal DNA, including covalently closed DNA circles, during serial passage of diploid fibroblasts. A single size-class of Inter-Alu circles is also amplified in lymphocytes from 16 of 24 old donors and yet is not detected in cells from 18 young donors.