Immortalization of normal human mammary epithelial cells in two steps by direct targeting of senescence barriers does not require gross genomic alterations

Telomerase reactivation and immortalization are critical for human carcinoma progression. However, little is known about the mechanisms controlling this crucial step, due in part to the paucity of experimentally tractable model systems that can examine human epithelial cell immortalization as it might occur in vivo. We achieved efficient non-clonal immortalization of normal human mammary epithelial cells (HMEC) by directly targeting the 2 main senescence barriers encountered by cultured HMEC. The stress-associated stasis barrier was bypassed using shRNA to p16^INK4; replicative senescence due to critically shortened telomeres was bypassed in post-stasis HMEC by c-MYC transduction. Thus, 2 pathologically relevant oncogenic agents are sufficient to immortally transform normal HMEC. The resultant non-clonal immortalized lines exhibited normal karyotypes. Most human carcinomas contain genomically unstable cells, with widespread instability first observed in vivo in pre-malignant stages; in vitro, instability is seen as finite cells with critically shortened telomeres approach replicative senescence. Our results support our hypotheses that: (1) telomere-dysfunction induced genomic instability in pre-malignant finite cells may generate the errors required for telomerase reactivation and immortalization, as well as many additional “passenger” errors carried forward into resulting carcinomas; (2) genomic instability during cancer progression is needed to generate errors that overcome tumor suppressive barriers, but not required per se; bypassing the senescence barriers by direct targeting eliminated a need for genomic errors to generate immortalization. Achieving efficient HMEC immortalization, in the absence of “passenger” genomic errors, should facilitate examination of telomerase regulation during human carcinoma progression, and exploration of agents that could prevent immortalization.     

Molecular changes accompanying senescence and immortalization of cultured human mammary epithelial cells

Limits on the proliferative potential of cultured normal human cells may be consequences of pathways that exist to suppress tumorigenicity. Human mammary epithelial cells (HMEC) employ several mechanisms to prevent unlimited growth. One mechanism may be activated by stress, and is associated with upregulated expression of p16(INK4a). In serum-free medium, some HMEC arise spontaneously which do not express p16. These "post-selection" HMEC are capable of long-term proliferation, but ultimately cease growth when their telomeres become very short. As they approach a growth plateau, termed agonescence, post-selection HMEC populations accumulate chromosome abnormalities. In contrast to the crisis exhibited by cells lacking functional p53, agonescent cells can be maintained as viable cultures. Although transduction of hTERT, the catalytic subunit of telomerase, into post-selection cells can, by itself, efficiently produce immortality and avoid agonescence, the errors that produce telomerase reactivation during carcinogenesis are not known. The block to endogenous telomerase reactivation in HMEC is extremely stringent. However, if one predisposing error is present, the probability greatly increases that additional error(s) required for immortalization may be generated by genomic instability encountered during agonescence. In p53(+) HMEC immortalized after chemical carcinogen exposure, the events involved in overcoming agonescence can be temporally separated from activation of telomerase. We have used the term "conversion" to describe the gradual process that leads to telomerase activation, telomere length stabilization, decreased p57 (KIP2) expression, and increased ability to grow uniformly well in the presence or absence of TGF beta. In the presence of active p53, conversion may represent a rate-limiting step in immortal transformation.     

The catalytic and the RNA subunits of human telomerase are required to immortalize equid primary fibroblasts

Many human primary somatic cells can be immortalized by inducing telomerase activity through the exogenous expression of the human telomerase catalytic subunit (hTERT). This approach has been extended to the immortalization of cell lines from several mammals. Here, we show that hTERT expression is not sufficient to immortalize primary fibroblasts from three equid species, namely donkey, Burchelli’s zebra and Grevy’s zebra. In vitro analysis of a reconstituted telomerase composed by hTERT and an equid RNA component of telomerase (TERC) revealed a low activity of this enzyme compared to human telomerase, suggesting a low compatibility of equid and human telomerase subunits. This conclusion was also strengthened by comparison of human and equid TERC sequences, which revealed nucleotide differences in key regions for TERC and TERT interaction. We then succeeded in immortalizing equid fibroblasts by expressing hTERT and hTERC concomitantly. Expression of both human telomerase subunits led to telomerase activity and telomere elongation, indicating that human telomerase is compatible with the other equid telomerase subunits and proteins involved in telomere metabolism. The immortalization procedure described herein could be extended to primary cells from other mammals. The availability of immortal cells from endangered species could be particularly useful for obtaining new information on the organization and function of their genomes, which is relevant for their preservation.     

Studies of the molecular mechanisms in the regulation of telomerase activity.

Telomerase, a specialized RNA-directed DNA polymerase that extends telomeres of eukaryotic chromosomes, is repressed in normal human somatic cells but is activated during development and upon neoplasia. Whereas activation is involved in immortalization of neoplastic cells, repression of telomerase permits consecutive shortening of telomeres in a chromosome replication-dependent fashion. This cell cycle-dependent, unidirectional catabolism of telomeres constitutes a mechanism for cells to record the extent of DNA loss and cell division number; when telomeres become critically short, the cells terminate chromosome replication and enter cellular senescence. Although neither the telomere signaling mechanisms nor the mechanisms whereby telomerase is repressed in normal cells and activated in neoplastic cells have been established, inhibition of telomerase has been shown to compromise the growth of cancer cells in culture; conversely, forced expression of the enzyme in senescent human cells extends their life span to one typical of young cells. Thus, to switch telomerase on and off has potentially important implications in anti-aging and anti-cancer therapy. There is abundant evidence that the regulation of telomerase is multifactorial in mammalian cells, involving telomerase gene expression, post-translational protein-protein interactions, and protein phosphorylation. Several proto-oncogenes and tumor suppressor genes have been implicated in the regulation of telomerase activity, both directly and indirectly; these include c-Myc, Bcl-2, p21(WAF1), Rb, p53, PKC, Akt/PKB, and protein phosphatase 2A. These findings are evidence for the complexity of telomerase control mechanisms and constitute a point of departure for piecing together an integrated picture of telomerase structure, function, and regulation in aging and tumor development-Liu, J.-P. Studies of the molecular mechanisms in the regulation of telomerase activity.     

Telomerase modulates expression of growth-controlling genes and enhances cell proliferation

Most somatic cells do not express sufficient amounts of telomerase to maintain a constant telomere length during cycles of chromosome replication. Consequently, there is a limit to the number of doublings somatic cells can undergo before telomere shortening triggers an irreversible state of cellular senescence. Ectopic expression of telomerase overcomes this limitation, and in conjunction with specific oncogenes can transform cells to a tumorigenic phenotype. However, recent studies have questioned whether the stabilization of chromosome ends entirely explains the ability of telomerase to promote tumorigenesis and have resulted in the hypothesis that telomerase has a second function that also supports cell division. Here we show that ectopic expression of telomerase in human mammary epithelial cells (HMECs) results in a diminished requirement for exogenous mitogens and that this correlates with telomerase-dependent induction of genes that promote cell growth. Furthermore, we show that inhibiting expression of one of these genes, the epidermal growth factor receptor (EGFR), reverses the enhanced proliferation caused by telomerase. We conclude that telomerase may affect proliferation of epithelial cells not only by stabilizing telomeres, but also by affecting the expression of growth-promoting genes.