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Biomedical subjects

J W Shay

Publications and source records attributed to J W Shay.

At least 127 records · Page 7Linked to original sources
PubMed1996

Telomerase activity in human germline and embryonic tissues and cells.

Telomerase is a ribonucleoprotein that synthesizes telomere repeats onto chromosome ends and is involved in maintaining telomere length in germline tissues and in immortal and cancer cells. In the present study, the temporal regulation of expression of telomerase activity was examined in human germline and somatic tissues and cells during development. Telomerase activity was detected in fetal, newborn, and adult testes and ovaries, but not in mature spermatozoa or oocytes. Blastocysts expressed high levels of telomerase activity as did most human somatic tissues at 16-20 weeks of development with the exception of human brain tissue. This activity could no longer be detected in the somatic tissues examined from the neonatal period onward. Neither placenta nor cultured fetal amniocytes contained detectable telomerase activity. Fetal tissues explanted into primary cell culture showed a dramatic decline in telomerase activity which became undetectable after the first passage in vitro. Elucidation of the regulatory pathways involved in the repression of telomerase activity during development may lead to the ability to manipulate telomerase levels and explore the consequences both for cellular aging and for the survival of cancer cells.

Base Sequence↗
PubMed1996

Inhibition of human telomerase activity by peptide nucleic acids.

We report the inhibition of human telomerase activity by peptide nucleic acids (PNAs). PNAs recognize the RNA component of human telomerase (hTR) and inhibit activity of the enzyme with IC50 values in the picomolar to nanomolar range. Inhibition depends on targeting exact functional boundaries of the hTR template and is 10- to 50-fold more efficient than inhibition by analogous phosphorothioate (PS) oligomers. In contrast to high selectivity of inhibition by PNAs, PS oligomers inhibit telomerase in a non-sequence-selective fashion. These results demonstrate that PNAs can control the enzymatic activity of ribonucleoproteins and possess important advantages relative to PS oligomers in both the affinity and the specificity of their recognition. These observations should facilitate the development of effective inhibitors of telomerase activity and affinity probes of telomerase structure.

Base Sequence↗
PubMed1996

Refining the telomere-telomerase hypothesis of aging and cancer.

While there has been substantial experimental evidence in support of the telomere-telomerase hypothesis of aging and cancer, it has been suggested that the theory has been oversimplified to the exclusion of alternative mechanisms in the progression of cancer. This review strives to present an overview of some of the areas that have not been well explained and to indicate where multiple interpretations of the data are possible.

Aging↗
PubMed1996

Age-dependent alterations of c-fos and growth regulation in human fibroblasts expressing the HPV16 E6 protein.

Normal human cells in culture become senescent after a limited number of population doublings. Senescent cells display characteristic changes in gene expression, among which is a repression of the ability to induce the c-fos gene. We have proposed a two-stage model for cellular senescence in which the mortality stage 1 (M1) mechanism can be overcome by agents that bind both the product of the retinoblastoma susceptibility gene (pRB)-like pocket proteins and p53. In this study we determined whether the repression of c-fos at M1 was downstream of the p53 or pRB-like "arms" of the M1 mechanism. We examined c-fos expression during the entire lifespan of normal human fibroblasts carrying E6 (which binds p53), E7 (which binds pRB), or both E6 and E7 of human papilloma virus type 16. The results indicate a dramatic change in cellular physiology at M1. Before M1, c-fos inducibility is controlled by an E6-independent mechanism that is blocked by E7. After M1, c-fos inducibility becomes dependent on E6 whereas E7 has no effect. In addition, a novel oscillation of c-fos expression with an approximately 2-h periodicity appears in E6-expressing fibroblasts post-M1. Accompanying this shift at M1 is a dramatic change in the ability to divide in low serum. Before M1, E6-expressing fibroblasts growth arrest in 0.3% serum, although they continue dividing under those conditions post-M1. These results demonstrate the unique physiology of fibroblasts during the extended lifespan between M1 and M2 and suggest that p53 might participate in the process that represses the c-fos gene at the onset of cellular senescence.

Aging↗
PubMed1996

Telomerase activity in human cancer.

The chromosome ends are specialized nucleoprotein structures called telomeres, which are essential for stable chromosome maintenance. In tumor-derived cell lines telomeres are maintained by the ribonucleoprotein enzyme telomerase. Telomerase activity is repressed in almost all normal human somatic cells. Due to the end replication problem, progressive telomere shortening occurs in normal somatic cells, leading to a limited replicative capacity and eventually resulting in cellular senescence. In the presence of viral oncogenes or some somatic mutations that block cellular senescence, cells continue to divide and telomere erosion continues. This continuing telomere erosion ultimately leads to the activation of telomerase, a necessary event for the sustained growth of most human tumors.

Aging↗
PubMed1996

Detection of telomerase activity in malignant and nonmalignant skin conditions.

Telomeres are the end regions of linear chromosomes, and in normal somatic cells the lengths of telomeres shorten with successive cell divisions. Telomerase, a ribonucleoprotein enzyme, maintains the length of telomeres in immortal and germline cells. Although present in human fetal tissues, shortly after birth telomerase activity is not detectable except in germline cells, hematopoietic cells, and most human primary tumors. In the present study we show telomerase activity to be present in 73 of 77 basal cell carcinomas, 15 of 18 nonmetastatic cutaneous squamous cell carcinomas, and 6 of 7 cutaneous melanomas, contrasting with extremely low levels detected in sun-protected skin. Sun-damaged skin, psoriatic lesional skin, and skin from lesions of poison ivy dermatitis, however, have increased levels of telomerase activity compared to sun-protected skin, although less than that detected in tumor tissue. Because telomerase activity can be found in inflammatory lesions of the skin, this indicates that telomerase activity does not always correlate with the malignant phenotype. In addition, we show that telomerase activity is localized to the epidermis of newborn foreskin, which suggests that telomerase is expressed constitutively by cells in the epidermis. Finding higher levels of telomerase activity in sun-exposed skin compared to nonexposed skin suggests that environmental factors may modulate telomerase activity.

Adult↗
PubMed1996

Regulation of telomerase activity in immortal cell lines.

Telomerase is a ribonucleoprotein whose activity has been detected in germ line cells, immortal cells, and most cancer cells. Except in stem cells, which have a low level of telomerase activity, its activity is absent from normal somatic tissues. Understanding the regulation of telomerase activity is critical for the development of potential tools for the diagnosis and treatment of cancer. Using the telomeric repeat amplification protocol, we found that immortal, telomerase-positive, pseudodiploid human cells (HT1080 and HL60 cells) sorted by flow repressed in quiescent cells. This was true whether quiescence was induced by contact inhibition (NIH 3T3 mouse cells), growth factor removal (bromodeoxyuridine-blocked mouse myoblasts), reexpression of cellular senescence (the reversibly immortalized IDH4 cells), or irreversible cell differentiation (HL60 promyelocytic leukemia cells and C2C12 mouse myoblasts). Taken together, these results indicate that telomerase is active throughout the cell in dividing, immortal cells but that its activity is repressed in cells that exit the cell cycle. This suggests that quiescent stem cells that have the potential to express telomerase may remain unaffected by potential antitelomerase cancer therapies.

3T3 Cells↗
PubMed1996

Telomeres and telomerase in human leukemias.

There is increasing evidence supporting the hypothesis that telomere shortening both in vitro and in vivo, is the clock that counts cell divisions and determines the onset of cellular senescence. Cells that overcome the normal senescence mechanisms do so by stabilizing telomere length, probably due to the activity of telomerase, a ribonucleoprotein enzyme that synthesizes telomeric repeats. Most human primary tumors contain telomerase, while the cells of most normal tissues lack this activity. A hypothesis gaining prominence is that the activation of telomerase is necessary for the sustained growth of most solid tumors. Since normal hematopoietic stem cells and some of their progeny already express telomerase activity, it is important to consider whether or not telomere shortening and telomerase activity play any role in cancer progression in various forms of leukemia. This review includes a discussion of the utility of telomere length and/or telomerase activity measurements in the diagnosis and prognosis of leukemia as well as the potential value of antitelomerase therapy for the leukemias.

Base Sequence↗
PubMed1996

Telomerase activity in human acute myelogenous leukemia: inhibition of telomerase activity by differentiation-inducing agents.

The terminal regions of human chromosomes, the telomeres, shorten with each cell division in most normal somatic cells. Telomerase, a ribonucleoprotein that synthesizes telomeric DNA onto chromosomal ends, is activated in germline cells and almost all tumor cells. Telomerase activity maintains the stability of telomere length, resulting in indefinite cellular proliferation (immortality). In the present study, telomerase activity was analyzed in leukemic mononuclear blood cells obtained from 56 patients with acute myelogenous leukemia (AML) with known cytogenetic alterations. Heterogenous levels of telomerase activity were observed and generally correlated with cytogenetic status. Patients with 11q abnormalities and -5/-7 (unfavorable cytogenetics) tended to have high telomerase activity compared with cells obtained from AML patients with other types of cytogenetics. Additional studies with a larger cohort of patients will determine whether these differences are statistically significant. Chemotherapy agents that result in differentiation of leukemic cells also resulted in inhibition of telomerase activity. Knowledge of telomerase activity in patients with AML, before and throughout therapy, may have clinical utility for following disease progression and may predict early cancer relapse.

Adult↗
PubMed1995

Telomerase activity in human brain tumours.

Malignant gliomas are invasive into surrounding brain and are refractory to therapy. Telomerase stabilises telomere length and may immortalise cells to allow unlimited proliferation. Our analysis of telomerase activity in 90 human gliomas showed that 19 of 19 oligodendrogliomas and 38 of 51 glioblastoma multiformes have detectable telomerase activity. The absence of telomerase activity in anaplastic astrocytomas (2/20 positive) and in one-quarter (13/51) of the glioblastomas suggests that these tumours follow different pathways of neoplastic progression. Thus we have found that a distinct subgroup of brain tumour consists of transformed yet pre-immortal cells.

Adolescent↗
PubMed1995

Activation of telomerase in human lymphocytes and hematopoietic progenitor cells.

This is the first report describing up-regulation of telomerase activity in human normal cells. Telomerase, a ribonucleoprotein enzyme, has been thought to be involved in maintaining telomere length stability in germline and most cancer cells, but not in normal cells. However, in the present study, we demonstrate that telomerase activity is detectable at low levels in normal human T and B cells, increases by in vitro mitogenic stimulation, increases in hematopoietic progenitor cells upon their proliferation and differentiation, and decreases with aging. Understanding the regulation of telomerase activity in normal cells may provide important insights not only into the mechanisms of normal cellular senescence but also into the mechanisms of telomerase activity deregulation as part of cancer development.

Adolescent↗
PubMed1995

Telomerase activity in gastric cancer.

Although many genetic alterations have been reported in gastric cancer, it is not known whether all gastric tumors are capable of indefinite proliferative potential, e.g., immortality. The expression of telomerase and stabilization of telomeres are concomitant with the attainment of immortality in tumor cells; thus, the measurement of telomerase activity in clinically obtained tumor samples may provide important information useful both as a diagnostic marker to detect immortal cancer cells in clinical materials and as a prognostic indicator of patient outcome. Telomerase activity was analyzed in 66 primary gastric cancers with the use of a PCR-based assay. The majority of tumors (85%) displayed telomerase activity, but telomerase was undetectable in 10 tumors (15%), 8 of which were early stage tumors. Most of the tumors with telomerase activity were large and of advanced stages, including metastases. Survival rate of patients of tumors with detectable telomerase activity was significantly shorter than that of those without telomerase activity. Alterations of telomere length (reduced/elongated terminal restriction fragments) were detected in 14 of 66 (21%) gastric cancers, and all 14 had telomerase activity. Cellular DNA contents revealed that all 22 aneuploid tumors had detectable telomerase activity. The present results indicate that telomerase activation may be required as a critical step in the multigenetic process of tumorigenesis, and that telomerase is frequently but not always activated as a late event in gastric cancer progression.

Base Sequence↗
PubMed1995

Telomerase activity in human liver tissues: comparison between chronic liver disease and hepatocellular carcinomas.

Telomerase activity was examined in 105 frozen samples from human normal liver tissues, chronic liver disease, and hepatocellular carcinoma (HCC). Telomerase activity was positive in 28 of 33 HCC tissues regardless of tumor stage or size. Telomerase was expressed in 15 of 18 differentiated HCC nodules smaller than 3 cm. HCC tissues from all eight hepatitis B virus-positive patients were telomerase positive, while telomerase activity was not detected in normal liver tissues (0 of 4). Weak telomerase activity was only detected in 1 of 22 nontumor liver tissues from HCC patients. Interestingly, in 19 of 38 hepatitis tissues and 6 of 8 cirrhotic liver tissues from apparently cancer-free patients, very weak telomerase activity was detected. These results indicate that the expression of telomerase may play a crucial role in hepatocarcinogenesis.

Base Sequence↗
PubMed1995

Telomerase activity in small-cell and non-small-cell lung cancers.

BACKGROUND: Telomerase is an enzyme that adds hexameric TTAGGG nucleotide repeats onto the ends of vertebrate chromosomal DNAs (i.e., telomeres) to compensate for losses that occur with each round of DNA replication. Somatic cells do not have telomerase activity and stop dividing when the telomeric ends of at least some chromosomes have been shortened to a critical length. It has been suggested that immortalized cells (including some, but probably not all, cancer cells) continue to proliferate indefinitely because they express telomerase. PURPOSE: To investigate whether expression of telomerase is a prerequisite for the development of naturally occurring human cancers, we assayed the levels of telomerase activity in specimens of human lung tumor and adjacent normal tissue. METHODS: Using a polymerase chain reaction-based assay, we examined telomerase activity in 136 primary lung cancer tissues and 68 adjacent noncancerous tissues obtained by surgical resection. We also studied telomerase activity in four primary and 23 metastatic lesions obtained through biopsy, (two patients) or autopsy (10 patients). Relative telomerase activity levels were estimated by serial dilutions of extracts prepared from the specimens. Telomerase activity was also assayed in extracts of cells present in pleural fluids from three patients with adenocarcinoma of the lung. RESULTS: Among surgically resected samples, telomerase activity was detected in 109 (80.1%) of 136 primary lung cancer tissues and in three (4.4%) of 68 normal adjacent tissues. All 11 surgically resected specimens of primary small-cell lung cancer (from 11 patients) revealed high levels of telomerase activity, whereas the activity ranged from undetectable to high levels in the 125 surgically resected specimens of primary non-small-cell lung cancer tissue (from 125 patients). Generally, high levels of telomerase activity were observed in metastatic lesions and tumors with altered telomere length. A few primary and, surprisingly, some metastatic tumors did not appear to have detectable telomerase activity. Telomerase activity was, however, detected in cells present in all tested pleural fluids obtained (from three patients with adenocarcinoma of the lung). CONCLUSION: The subset of non-small-cell lung cancers that exhibits only low or undetectable levels of telomerase activity may contain primarily mortal cancer cells. Cancers that exhibit high levels of telomerase activity, such as all of the small-cell lung cancers examined in this study, are likely to consist mainly of immortal cells. IMPLICATIONS: Telomerase activity may be useful both as a diagnostic marker to detect the existence of immortal lung cancer cells in clinical materials and as a target for therapeutic intervention.

Base Sequence↗
PubMed1995

Telomere shortening is associated with cell division in vitro and in vivo.

In humans, the amount of terminal (TTAGGG)n, telomeric DNA decreases during aging of various somatic cell types in vitro and in vivo. While the factors accounting for telomere shortening have not been thoroughly established, the inability of the DNA replication machinery to completely copy chromosomal termini (the "end replication problem") and the absence in somatic cells of telomerase, the enzyme that synthesizes telomeric DNA de novo, is a likely mechanism. One prediction of this hypothesis is that telomere shortening should be dependent on cell division. Thus we analyzed telomere length in actively dividing and quiescent cells in vitro and in vivo. In circular outgrowths of cultured human diploid fibroblasts (HDF), cells at the outer periphery had a significantly lower mean terminal restriction fragment (TRF) length (P = 0.011) and telomeric signal intensity (P = 0.024) than cells at the center. Also, the rate of telomere shortening over time for HDFs held quiescent was not statistically significant (m = -12 bp/day, P = 0.16) while that for serially passaged cells was significant (m = -34 bp/day, P = 0.017). To examine the rate of telomere shortening for quiescent cells in vivo, we measured mean TRF length in brain tissue from adult donors ranging in age from 32-75 years. No significant decrease was observed as a function of donor age (P = 0.087), in contrast to the shortening of telomere length that occurs during in vivo aging of mitotically active cells (P = 0.0001). These observations show that telomere shortening is largely, if not entirely, dependent on cell division and support the end replication problem as a mechanism for this process and the use of telomere length as a biomarker for replicative capacity.

Adult↗