Journal
MOLECULAR AND CELLULAR BIOLOGY
Volume 20, Issue 4, Pages 1436-1447Publisher
AMER SOC MICROBIOLOGY
DOI: 10.1128/MCB.20.4.1436-1447.2000
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Funding
- NIAMS NIH HHS [P30 AR042689, P30 AR42689] Funding Source: Medline
- NIDCR NIH HHS [P01 DE12467, P01 DE012467] Funding Source: Medline
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Normal human cells exhibit a limited replicative Life span in culture, eventually arresting growth by a process termed senescence. Progressive telomere shortening appears to trigger senescence in normal human fibroblasts and retinal pigment epithelial cells, as ectopic expression of the telomerase catalytic subunit, hTERT, immortalizes these cell types directly, Telomerase expression alone is insufficient to enable certain other cell types to evade senescence, hoc-ever. Such cells, including keratinocytes and mammary epithelial cells, appear to require loss of the pRB/p16(INK4a) cell cycle control mechanism in addition to hTERT expression to achieve immortality, To investigate the relationships among telomerase activity, cell cycle control, senescence, and differentiation, we expressed hTERT in two epithelial cell types, keratinocytes and mesothelial cells, and determined the effect on proliferation potential and on the function of cell-type-specific growth control and differentiation systems, Ectopic hTERT expression immortalized normal mesothelial cells and a premalignant, p16(INK4a)-negative keratinocyte line. In contrast when four keratinocyte strains cultured from normal tissue were transduced to express hTERT, they were incompletely rescued from senescence. After reaching the population doubling limit of their parent cell strains, hTERT(+) keratinocytes entered a slow growth phase of indefinite length, from which rare, rapidly dividing immortal cells emerged. These immortal cell lines frequently had sustained deletions of the CDK2NA/INK4A locus or otherwise were deficient in p16(INK4a) expression. They nevertheless typically retained other keratinocyte growth controls and differentiated normally in culture and in xenografts. Thus, keratinocyte replicative potential is limited by a p16(INK4n)-dependent mechanism, the activation of which can occur independent of telomere length, Abrogation of this mechanism together with telomerase expression immortalizes keratinocytes without affecting other major growth control or differentiation systems.
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