Telomerase expression in human somatic cells does not induce changes associated with a transformed phenotype.

Expression of the human telomerase catalytic component, hTERT, in normal human somatic cells can reconstitute telomerase activity and extend their replicative lifespan. We report here that at twice the normal number of population doublings, telomerase-expressing human skin fibroblasts (BJ-hTERT) and retinal pigment epithelial cells (RPE-hTERT) retain normal growth control in response to serum deprivation, high cell density, G1 or G2 phase blockers and spindle inhibitors. In addition, we observed no cell growth in soft agar and detected no tumour formation in vivo. Thus, we find that telomerase expression in normal cells does not appear to induce changes associated with a malignant phenotype.

Cell-adhesion-dependent influences on genomic instability and carcinogenesis.

Adhesion-dependent cell signaling is known to be important in carcinogenesis. It is postulated that several types of adhesion molecules act as tumor suppressor genes by enforcing cell-substrate and cell-cell adhesion thereby preventing the migration of cells and their invasion into surrounding tissues. Recent evidence, however, suggests that disruption of adhesion systems can both initiate neoplastic transformation and contribute a rate-limiting step to progression. Adhesion may modulate neoplastic processes by altering pathways that control genomic stability. Analysis of the adhesion-controlled inactivation of the p53 protein and the concomitant relaxation of cell cycle checkpoint control could identify the critical contributions of adhesion-mediated influences to carcinogenesis.

Suppression of gene amplification in human cell hybrids.

Gene amplification, one example of genetic instability, is of prognostic and clinical importance in neoplasia. In tumorigenic cells, gene amplification occurs at a very high frequency, whereas in normal diploid fibroblasts the event is undetectable by the clonogenic assay. To investigate genetic control of gene amplification, amplification frequency was measured in hybrids of tumorigenic cells and normal diploid cells. The ability to amplify an endogenous gene behaved as a recessive genetic trait, and control of gene amplification potential segregated independently of tumorigenicity and immortality.

Know thy neighbor: stromal cells can contribute oncogenic signals.

Although the stroma within carcinogenic lesions is known to be supportive and responsive to tumors, new data increasingly show that the stroma also has a more active, oncogenic role in tumorigenesis. Stromal cells and their products can transform adjacent tissues in the absence of pre-existing tumor cells by inciting phenotypic and genomic changes in the epithelial cells. The oncogenic action of distinctive stromal components has been demonstrated through a variety of approaches, which provide clues about the cellular pathways involved.

UV radiation facilitates methotrexate resistance and amplification of the dihydrofolate reductase gene in cultured 3T6 mouse cells.

Pretreatment of 3T6 murine cells with the carcinogen UV radiation or N-acetoxy-N-acetylaminofluorene increased the number of methotrexate-resistant colonies. This carcinogen-induced enhancement was seen only at low toxicities. The enhancement was transient and was observed at its maximum when cells were subjected to methotrexate selection 12 to 24 h after treatment. The addition of a tumor-promoting agent, 12-O-tetradecanoylphorbol-13-acetate, during or after carcinogen treatment further enhanced this effect. A large proportion of the resistant colonies had an increase in the dihydrofolate reductase gene copy number and the relative proportions of colonies with amplified genes were similar, regardless of whether selected cells were untreated, treated with carcinogen, or treated with carcinogen plus promoter. We discuss some of the variables which both enhance the generation and improve the detection of methotrexate-resistant colonies, as well as certain implications of our results for the generation and mechanism of gene amplification.

Enhancement of methotrexate resistance and dihydrofolate reductase gene amplification by treatment of mouse 3T6 cells with hydroxyurea.

We investigated various parameters associated with the initial selection of mouse 3T6 cells for resistance to single concentrations of methotrexate and characterized resistant colonies for the presence of additional (amplified) copies of the dihydrofolate reductase gene. Our results indicate that the frequency of occurrence of dihydrofolate reductase gene amplification varies with the selecting concentration of methotrexate and is highly variable between clonally derived sublines of mouse 3T6 cells. Second, we increased the frequency of occurrence of cells with amplified dihydrofolate reductase genes by transiently inhibiting DNA synthesis with hydroxyurea before the selection of cells in single concentrations of methotrexate. This effect was dependent on the concentration of hydroxyurea, the time of exposure to the drug, and the time interval between the removal of hydroxyurea and the selection of cells in methotrexate.

The inhibition of ribosomal RNA synthesis and maturation in Novikoff hepatoma cells by 5-fluorouridine.

The inhibition of ribosomal RNA (rRNA) maturation by 5-fluorouridine (FUrd) in Novikoff hepatoma cells appears to depend upon the incorporation of the analog into the 45 S rRNA precursor. Precursor synthesized in the presence of FUrd is not processed into mature rRNA, but precursor synthesized in the absence of the analog is processed normally after the addition of the drug. The effect of FUrd on rRNA maturation is concentration dependent. At a concentration of 1 X 10(-4)M, the analog completely inhibits the formation of mature 18 S and 28 S rRNA; while at a concentration of 1 X 10(-7)M, the analog has no significant effect on rRNA maturation. These results suggest that some minimum degree of analog substitution is necessary to inhibit the maturation process. In addition to its inhibition of maturation, FUrd also inhibits the transcription of 45 S rRNA precursor. However, this effect of the drug is less complete and more time dependent that the effect on maturation. The inhibition of rRNA maturation by FUrd persists after removal of the analog from the culture medium. Cells that had been exposed to FUrd for 2 hr were unable to process 45 S rRNA precursor 20 hr after removal of the drug from the medium.

A novel coculture technique demonstrates that normal human prostatic fibroblasts contribute to tumor formation of LNCaP cells by retarding cell death.

The microenvironment influences the progression of an epithelial malignancy. To examine the effect of fibroblasts on epithelial cells by direct cell-cell contact in vitro, a coculture system was designed to assess cell death and proliferation of two cell populations when grown together. We used a green fluorescent dye to stain fibroblasts and distinguish them from unstained epithelial cells by a flow cytometer. We show that tumor cell death is 5-fold less when cocultured with normal human prostatic fibroblasts than when cultured alone. In contrast, proliferation of tumor cells was similar when cocultured with normal human prostatic fibroblasts or when grown alone. The reduction in tumor cell death during coculture appears to play a significant role in promoting tumor formation. Combination of prostatic fibroblasts with LNCaP xenografts formed large tumors at a high frequency with a low apoptotic index in vivo, whereas, LNCaP xenografts alone formed small infrequent tumors with a high apoptotic index. Therefore, prostatic fibroblasts promote tumor formation by retarding the apoptotic pathways in tumor cells.

Amplification potential in preneoplastic and neoplastic Syrian hamster embryo fibroblasts transformed by various carcinogens.

Using a well-defined in vitro model system for neoplastic progression, we have examined two basic characteristics in the acquisition of amplification potential. Since Syrian hamster embryo fibroblasts can be transformed by a variety of methods (spontaneously, chemically, virally, or by transfection with oncogenes), we determined whether the method of transformation affects the capability of a cell to amplify. In addition, since variants can be isolated from cell populations as they progress toward tumorigenicity, we can monitor changes in amplification potential during this multistep process. We find that the capability to amplify is independent of the method of transformation and that the acquisition of this ability occurs in a defined step in the transformation process. In this model system, acquisition of amplification ability occurred concomitantly with the loss of tumor suppression function.

Cellular adhesion regulates p53 protein levels in primary human keratinocytes.

To gain insight into p53 tissue-specific regulatory pathways and biological activities, we investigated mechanisms that may account for the elevated levels of p53 protein in human foreskin keratinocytes, relative to levels in dermal fibroblasts in vitro. Here, we report that the loss of cell anchorage resulted in an approximately 5-fold decrease in p53 levels in keratinocytes, which was reversible upon reattachment of cells to a substratum. In contrast, fibroblasts did not exhibit such adhesion-dependent regulation of p53 protein. Furthermore, p53 function was attenuated in keratinocytes relative to fibroblasts. These results link p53 to cell adhesion pathways and may provide a molecular basis for epigenetic differences in the maintenance of genomic stability among normal cell types.

Multiple mechanisms of N-(phosphonoacetyl)-L-aspartate drug resistance in SV40-infected precrisis human fibroblasts.

Normal and SV40-infected human fibroblasts were grown in the presence of the drug N-(phosphonoacetyl)-L-aspartate (PALA) and examined for evidence of genetic instability. Both cell populations were precrisis and showed a normal, diploid karyotype at early passage. In contrast to the normal IMR-90 cells, which showed growth arrest and did not form colonies in PALA, the SV40-infected IMR-90 cells formed colonies at a very high frequency and continued to cycle in the drug. The drug-resistant colonies senesced after continued growth in culture, indicating that this change in ability to amplify preceded immortalization. This is the first observation of mortal human cells overcoming the drug-induced growth arrest. Although all previously isolated PALA-resistant colonies demonstrated CAD gene amplification as the mechanism of the drug-resistant phenotype, these SV40-infected human cells also showed alternative mechanisms, including increases in gene copy number by aneuploidy and formation of an isochromosome 2p.

Separate pathways for p53 induction by ionizing radiation and N-(phosphonoacetyl)-L-aspartate.

The tumor suppressor gene product, p53, appears to be a significant participant in signaling pathways that mediate cellular responses to cytotoxic stresses. In particular, p53 appears to be a critical determinant of whether the cell lives or dies and how it progresses through the cell cycle after the cytotoxic exposure. Many of the molecular details for these signaling pathways remain to be elucidated, and whether all cytotoxic signals utilize the same pathway to increase p53 expression is not clear. Here, we demonstrate the existence of cell types in which the induction of p53 and associated G1 arrest by the antimetabolite, N-(phosphonoacetyl)-L-aspartate (PALA), is defective, whereas p53 induction and G1 arrest induced by ionizing radiation are intact. These observations demonstrate the existence of genetic defects that can alter p53 induction and associated cellular outcomes after some, but not all, cytotoxic insults and suggest distinct pathways of p53 induction by PALA and ionizing radiation.

Carcinoma-associated fibroblasts direct tumor progression of initiated human prostatic epithelium.

The present study demonstrates that fibroblasts associated with carcinomas stimulate tumor progression of initiated nontumorigenic epithelial cells both in an in vivo tissue recombination system and in an in vitro coculture system. Human prostatic carcinoma-associated fibroblasts grown with initiated human prostatic epithelial cells dramatically stimulated growth and altered histology of the epithelial population. This effect was not detected when normal prostatic fibroblasts were grown with the initiated epithelial cells under the same experimental conditions. In contrast, carcinoma-associated fibroblasts did not affect growth of normal human prostatic epithelial cells under identical conditions. From these data, we conclude that in this human prostate cancer model, carcinoma-associated fibroblasts stimulate progression of tumorigenesis. Thus, carcinoma-associated fibroblasts can direct tumor progression of an initiated prostate epithelial cell.

Defective G2 checkpoint function in cells from individuals with familial cancer syndromes.

The early events in the G2 checkpoint response to ionizing radiation (IR) were analyzed in diploid normal human fibroblasts (NHFs) and fibroblasts from patients with two heritable cancer syndromes. Exposure to gamma-radiation of asynchronously growing NHFs resulted in a rapid reduction in the number of cells in mitosis (G2 delay) and was accompanied by a quantitatively similar reduction in the p34CDC2/cyclin B in vitro histone H1 kinase activity as compared with sham-treated controls. This G2 delay was strong by 1 h following exposure to IR, maximal by 2 h, and was accompanied by an accumulation of tyrosine-phosphorylated p34CDC2 molecules. In contrast, fibroblasts from individuals with ataxia telangiectasia displayed significantly less reduction of the mitotic index or histone H1 kinase activity after IR. Low passage fibroblasts from individuals with Li-Fraumeni syndrome having one wild-type and one mutated p53 allele were similar to NHFs in their immediate G2 checkpoint response to IR, as were NHFs expressing the human papilloma virus type 16 E6 gene product (functionally inactivating p53) and low passage cells from p53-deficient mouse embryos. However, the p53-deficient fibroblasts were genomically unstable and became defective in their early G2 checkpoint response to IR. Furthermore, immortal Li-Fraumeni syndrome fibroblasts lacking wild-type p53 displayed an attenuated G2 checkpoint response. These results link the early events in G2 checkpoint response to IR in NHFs with a rapid inhibition of p34CDC2/cyclin B protein kinase activity and demonstrate that while not required for this immediate G2 delay, lack of p53 can lead to subsequent genetic alterations that result in defective G2 checkpoint function.

Human mammary epithelial cells exhibit a differential p53-mediated response following exposure to ionizing radiation or UV light.

The tumor suppressor protein, p53, plays a critical role as a transcriptional activator of downstream target genes involved in the cellular response to DNA damaging agents. We examined the cell cycle checkpoint response of human mammary epithelial cells (HMEC) and their isogenic fibroblast counterparts to ionizing (IR) and ultraviolet (UV) radiation, two genotoxic agents whose DNA damage response pathways involve p53. Using flow cytometric analysis, we found that both mortal and immortalized HMEC, which contain wild-type p53 sequence, do not exhibit a G1 arrest in response to IR, but show an intact G2 checkpoint. Supportive evidence from Western analyses revealed that there was neither an increase in p53 nor one of its downstream targets, p21WAF1, in HMEC exposed to IR. In contrast, isogenic mammary fibroblasts arrest at the G1 checkpoint and induce the p53 and p21WAF1 proteins following IR. By comparison, HMEC exposed to UV displayed an S phase arrest and induced the expression of p53 and p21WAF1. Our results show that the cellular response to DNA damage depends on both the type of damage introduced into the DNA and the specific cell type.

Minichromosome maintenance protein 2 expression in prostate: characterization and association with outcome after therapy for cancer.

The minichromosome maintenance (MCM) proteins are highly conserved proteins essential for initiating and regulating eukaryotic DNA replication. Recent studies have demonstrated the potential use of MCM proteins as markers of proliferation. We characterized the pattern of Mcm 2 staining in benign and malignant prostate tissues and examined the role of Mcm 2 expression in disease-free survival after surgery in men with localized prostate cancer. Tumors from 92 patients who underwent radical prostatectomy for prostate cancer (median follow-up of 54 months) were examined for Mcm 2 expression by immunohistochemistry using a monoclonal antibody. Prostate tissue from five men without histopathological evidence of prostate cancer was also stained for Mcm 2. Mcm 2 expression was quantified by calculating a labeling index, and patients were grouped according to degree of staining. An analysis of the association between Mcm 2 expression with traditional clinicopathological characteristics of prostate cancer was carried out. A Cox proportional hazards analysis was performed to determine whether Mcm 2 staining was a significant independent predictor of disease-free survival. Mcm 2 expression is low (<2%) and limited to the basal cell layer in nonmalignant prostate glands. Mcm 2 expression is consistently increased in malignant glands and is significantly associated with disease-free survival in univariate (P = 0.002) and multivariate (P = 0.01) analyses. Patients with high Mcm 2 expression exhibited shorter disease-free survival. Mcm 2 expression was not associated with any traditional clinical or pathological factors and therefore is an independent predictor of survival in these patients with prostate cancer. These data support evidence that Mcm 2 may serve as a novel proliferation marker in the prostate. Mcm 2 expression is an independent predictor of disease-free survival after definitive local therapy and has potential as a molecular marker for clinical outcome in prostate cancer.

Genetic and epigenetic influences in prostatic carcinogenesis (review).

Prostatic carcinogenesis is a multistep process with well-documented stages. Although prostate cancer is a major cause of mortality many small tumor foci never progress to form clinically significant disease, indicating that the disease process may be regulated at more than one level. Carcinogenesis is accompanied by increasing genetic damage to prostatic epithelial cells, however the pattern of genetic lesions is inconsistent. The differentiation of stromal cells surrounding tumors is more fibroblastic and less muscular than in normal prostate. The present communication reviews the roles of both genetic and, stromally derived, epigenetic effects on prostatic tumorigenesis.