Jerry W. Shay
University of Texas Southwestern Medical Center at Dallas, Department of Cell Biology, 5323 Harry Hines Boulevard, Dallas, Texas 75390-9039
(Tel: 214-648-3282 Fax: 214-648-8694 Email: Jerry.Shay@UTSouthwestern.edu)
There is substantial evidence in human cells that replicative senescence (Mortality Stage 1 or M1) is induced as a G1, p53-dependent cell cycle checkpoint growth arrest when one or a few short telomeres are recognized as damaged DNA. Senescence can be overcome by viral oncogenes (such as SV40 large T-antigen) that can inactivate both p53 and p16Ink4a /pRB checkpoint pathways. Such cells are not immortal but continue to divide and lose telomeric repeats. Terminal telomere shortening eventually causes a second growth arrest stage termed crisis or Mortality Stage 2 (M2). In contrast to mouse cells, escape from M2/crisis is a very rare event in human cells and is almost always accompanied by activation of telomerase, a cellular reverse transcriptase complex which synthesizes and extends telomeric repeats.
The expression of the catalytic subunit of human telomerase (hTERT), circumvents the induction of both senescence and crisis. hTERT immortalizes a variety of normal human cell types. Such "telomerized" cells have normal cell cycle controls, functional p53, p21Cip1, and p16Ink4a/pRB checkpoints, are contact inhibited, anchorage-dependent, require growth factors for proliferation, and possess a normal karyotype. In addition, hTERT does not affect the ability of cells to differentiate. A variety of reports now claim that inactivation of the p16 INK4a/pRB pathway is required in addition to telomere maintenance for the immortalization of cells such as skin keratinocytes and breast epithelial cells. The premature growth arrest of these cell types can be explained by an inadequate culture environment. Cultivating telomerase-expressing breast epithelial cells or keratinocytes on feeder layers avoids the growth arrest associated with increased p16INK4a. These results do not support a telomere-independent mechanism of replicative aging.
Human fibroblasts and breast epithelial cells that have several cellular pathways altered [large and small T-antigen (taking care of the p53/p16 cell cycle checkpoint pathways), hTERT (overcoming short telomeres), and expressing very high levels of H-ras V 12 (altering signal transduction pathways)], can progress to make tumors. In LFS breast epithelial cells that spontaneously immortalize (and have telomerase activity and are p53 and p16 deficient), high levels of H-ras V12 is sufficient to permit anchorage independent growth.
References:
(1) Bodnar, A.G., Ouellete, M., Frolkis, M., Holt, S.E., Chiu, C-P., Morin, G.B., Harley, C.B., Shay, J.W., Lichtsteiner, S., and Wright, W.E. Extension of lifespan by introduction of telomerase in normal human cells. Science, 279:349-352, 1998.
(2) Ouellette, M.M., McDaniel, L., Wright, W.E., Shay, J.W., and Shultz, R. The establishment of telomerase-immortalized cell lines representing human chromosome instability syndromes. Human Mol. Genetics, 9:403-411, 2000.
(3) Wright, W.E. and Shay, J.W. Telomere dynamics in cancer progression and prevention: fundamental differences in human and mouse telomere biology, Nature Medicine, 6:849-851, 2000.
(4) Wright, W.E. and Shay, J.W. Cellular senescence as a tumor-protection mechanism: the essential role of counting. Cur. Opin. Gen. Dev. 11:98-103, 2001.
(5) Shay, J.W. and Wright, W.E. When do telomeres matter? Science 291:839-840, 2001.
(6) Ramirez, R.D., Passons, C., Rohde, J., Morales, C.P., Herbert, B-S., Shay, J.W. and Wright, W.E. Putative telomere-independent mechanisms of replicative aging reflect inadequate growth conditions. Genes and Dev. 15: 2001.
Jerry W. Shay PhD 1966 BA - University of Texas
1968 MA - University of Texas
1972 PhD - University of Kansas
1972-1975 Postdoctoral - University of Colorado
1975- Assistant and Associate Professor, University of Texas Southwestern Medical Center at Dallas
1993- Professor and Ellison Medical Foundation
Senior Scholar - University of Texas
Southwestern Medical Center at Dallas
Speciality and Special Interest:
Role of telomeres and telomerase in aging and cancer