Loss of p53 function accelerates acquisition of telomerase activity in indefinite lifespan human mammary epithelial cell lines

Targeting senescence pathways to reverse drug resistance in cancer

Irreversible proliferation arrest (also called senescence) has emerged recently as a drug-responsive program able to influence the outcome of cancer chemotherapy. Since the drug amounts required for induction of proliferation arrest are much lower than those necessitated for induction of cell death, forcing cancer cells to undergo senescence may represent a less aggressive approach to control tumor progression. However, to achieve a long-standing control of proliferation, the ability of cancer cells to escape senescence and become drug resistant must be inhibited. Therefore, a clear understanding of the mechanisms that govern drug-induced senescence is critical and can lead to discovery of novel approaches to suppress drug resistance. The present review discusses the relevance of senescence in response to chemotherapy and the onset of drug resistance development. Particular emphasis is directed toward the utilization of findings from the field of research on aging, that can be applied to induction of senescence in cancer cells and reversal of their drug resistance phenotype. Proof of principle for this relationship is represented by the identification of inhibitors of aging associated proteases such as the proteasome and cathepsin L as novel and potent cancer drug resistance reversing agents.

Relief of p53-mediated telomerase suppression by p73

Reactivation of telomerase is a feature in many cancer cells. Telomerase activation inhibits telomere shortening, thereby preventing cell cycle arrest and apoptosis activated by shortened telomeres or chromosomal rearrangements. The tumor-suppressor gene product, p53, was previously shown to transcriptionally suppress the activation of the catalytic subunit of telomerase (hTERT). Here we have evaluated the role of p73 in hTERT regulation. We found that ectoptic expression of p73beta, in contrast to p73alpha or p53, in p53 null H1299 cells does not lead to suppression of hTERT transcription. However co-expression of p73alpha or p73beta together with p53 abolished p53-mediated hTERT suppression. This phenomenon was found to be dependent on the DNA binding ability of p73. We also show that p53-mediated suppression of hTERT transcription requires a minimum threshold level of p53, and p73 abrogates p53-mediated suppression by reducing p53 levels through the activation of HDM2. Moreover, p53-mediated hTERT suppression was not relieved by p73beta in cells depleted of HDM2 through small interfering RNA-mediated gene silencing. In addition, knockdown of HDM2 in MCF7 cells, which express moderately high levels of p73 and p53, resulted in the reduction of endogenous hTERT levels. Finally, knockdown of p73 in MCF7 cells resulted in increased p53 protein levels and a concomitant decrease in hTERT levels. Together, our data indicate a plausible way by which p73, through HDM2, can oppose p53 tumor suppressor function, thereby possibly contributing to tumorigenesis.

Accumulation and altered localization of telomere-associated protein TRF2 in immortally transformed and tumor-derived human breast cells

We have used cultured human mammary epithelial cells (HMEC) and breast tumor-derived lines to gain information on defects that occur during breast cancer progression. HMEC immortalized by a variety of agents (the chemical carcinogen benzo(a)pyrene, oncogenes c-myc and ZNF217, and/or dominant negative p53 genetic suppressor element GSE22) displayed marked upregulation (10-15 fold) of the telomere-binding protein, TRF2. Upregulation of TRF2 protein was apparently due to differences in post-transcriptional regulation, as mRNA levels remained comparable in finite lifespan and immortal HMEC. TRF2 protein was not upregulated by the oncogenic agents alone in the absence of immortalization, nor by expression of exogenously introduced hTERT genes. We found TRF2 levels to be at least twofold higher than in control cells in 11/15 breast tumor cell lines, suggesting that elevated TRF2 levels are a frequent occurrence during the transformation of breast tumor cells in vivo. The dispersed distribution of TRF2 throughout the nuclei in some immortalized and tumor-derived cells indicated that not all the TRF2 was associated with telomeres in these cells. The process responsible for accumulation of TRF2 in immortalized HMEC and breast tumor-derived cell lines may promote tumorigenesis by contributing to the cells' ability to maintain an indefinite lifespan.

p53-dependent down-regulation of telomerase is mediated by p21waf1

Inactivation of p53 and activation of telomerase occur in the majority of human cancers, raising the possibility of a link between these two pathways. Overexpression of wild-type p53 down-regulates the enzymatic activity of telomerase in various cancer cell lines through transcriptional repression of its catalytic subunit, human telomerase reverse transcriptase (hTERT). In this study, we re-evaluated the role of p53 in telomerase regulation using isogenic cell lines expressing physiological levels of p53. We demonstrate that endogenous wild-type p53 was able to down-regulate telomerase activity, hTERT mRNA levels, and promoter activity; however, the ability to repress hTERT expression was found to be cell type-specific. The integrity of the DNA-binding core domain, the N-terminal transactivation domain, and the C-terminal oligomerization domains of p53 was essential for hTERT promoter repression, whereas the proline-rich domain and the extreme C terminus were not required. Southwestern and chromatin immunoprecipitation experiments demonstrated lack of p53 binding to the hTERT promoter, raising the possibility of an indirect repressive mechanism. The down-regulation of hTERT promoter activity was abolished by a dominant-negative E2F1 mutant. Mutational analysis identified a specific E2F site responsible for p53-mediated repression. Knockdown of the key p53 transcriptional target, p21, was sufficient to eliminate the p53-dependent repression of hTERT. Inactivation of the Rb family using either viral oncoproteins or RNA interference attenuated the repression. Inhibition of histone deacetylases also interfered with the repression of hTERT by p53. Therefore, our results suggest that repression of hTERT by endogenous p53 is mediated by p21 and E2F.