Extra-telomeric function of telomerase and its regulation by the Akt pathway in prostate cancer.

摘要

Disease progression in prostate cancer is linked with development of apoptosis resistance, higher growth rate and metastatic properties of the cell. It is therefore critical to elucidate the cancer cell survival mechanisms to allow development of informative biomarkers and effective therapies for this condition.;The Akt pathway is a major survival pathway in human cancer and is central to development and progression of specific malignancies. Akt is known to regulate several different proteins including growth factors, cytokines and enzymes. One such enzyme that is known to be under its control is the telomerase holoenzyme. Akt enhances telomerase activity through phosphorylation of human telomerase reverse transcriptase subunit (hTERT) of the telomerase holoenzyme. Telomerase activity is a critical factor in the maintenance of chromosomal stability and cellular immortality. Recent studies have shown a direct correlation between telomerase activity and the stage of several cancers thus making it a potential target for anti-cancer therapy. Several anti-telomerase agents have been explored for their potential as anti-cancer agents. However, none have been approved for clinical trails till date. One of the major limitations of telomerase-directed therapies is the lengthy time lag observed between the start of the treatment and appearance of significant effects. Hence it is crucial to device drugs that target telomerase in a manner that will produce an immediate effect upon treatment.;Telomerase likely has roles beyond telomere maintenance including roles in tumor migration, invasion and protection from apoptosis, which could be explored to target telomerase for rapid induction of growth arrest. These effects are thought to be affected by the regulated levels of TERT in the cells. However the exact role of TERT in the biological behavior of the cells and the regulatory processes governing the role of hTERT remain under active research.;The current study identifies the extra-telomeric functions of TERT and investigates the contribution of the Akt pathway in the regulation of these functions in prostate cancer. We have observed that forced over-expression of hTERT in prostate cancer cells increases the capability to fight against various stress responses. PC-3 and LNCaP with aberrant TERT expression (PC-3HT and LNCaP-HT respectively) significantly stimulated growth in soft agar indicating that hTERT plays a role in promoting anchorage-independent growth of prostate cancer cells. Additionally, these cells also showed increased ability to recover from adriamycin treatment compared to the control cells. We also observed that high levels of TERT were able to promote greater survival of hormone-dependent LNCaP cells, under hormone-ablated conditions. Intriguingly, PC-3HT showed reduced expression of the ASK/JNK pathway suggesting that TERT may protect cells from undergoing endogenous stress in the cells. Interestingly, our data also revealed that these functions of TERT are independent of its regulation at the telomeres. We observed that conditional knockout of the TERT transgene reverses its effect on adriamycin treatment in PC-3 cells. These cells, although lacked elevated expression of TERT, had elongated telomeres like PC-3HT cells. To elucidate the mechanism by which TERT mediates its extra-telomeric effects on prostate cancer cells, we studied the contribution of the Akt pathway in regulating TERT functions. TERT mutants lacking the capability of Akt phosphorylation were developed and expressed in LNCaP and normal fibroblasts BJ cells. The mutant TERT was able to elongate and maintain telomeres like the wild-type TERT but was unable to increase anchorage-independent growth or resistance to adriamycin treatment indicating that Akt phosphorylation of TERT may be crucial for intact extra-telomeric functions of TERT. Further elucidating the mechanism by which Akt phosphorylation modulates biological activities of TERT could be critical in understanding the means by which telomere-independent functions of TERT could be targeted to achieve rapid induction of cancer cell death.