The tamoxifen-induced suppression of telomerase activity in the human hepatoblastoma cell line HepG2: a result of post-translational regulation.
OBJECTIVE
Patients with advanced hepatocellular carcinoma (HCC) have shown to benefit from tamoxifen treatment. The mechanisms of tamoxifen action in HCC, however, are not yet clearly understood. Results from studies on the human hepatoblastoma cell line HepG2 provide evidence that estrogen-receptor-alpha-independent antiproliferative actions of tamoxifen in HCC are mediated by the suppression of telomerase activity [5].


METHODS
We investigate the pathway of the tamoxifen-induced down-regulation of telomerase activity, using HepG2 cells incubated over 24 h or 48 h in the presence of 20 microM tamoxifen.


RESULTS
The transcriptional levels of the three telomerase core components-human telomerase RNA (hTR), human telomerase reverse transcriptase (hTERT) (all variants), and telomerase-associated protein (TP1)-did not change during tamoxifen treatment, as revealed by RT-PCR analysis. Furthermore, the hTERT splice pattern was not shifted from the active full-length variant (+alpha/+beta) to the inactive deletion variants (-alpha; -beta; -alpha/-beta) and the level of the 120 kDa hTERT full-length protein remained constant, as shown by Western blot analysis. Protein kinase C (PKC) activity has been suggested to be crucial for post-translational up-regulation of telomerase activity. In HepG2 cells, we observed a tamoxifen-induced suppression of the total protein kinase C (PKC) activity (cytosolic and membrane-bound). Inhibition of PKC with bisindolylmaleimide I resulted in a reduction of telomerase activity, as revealed by TRAP-assay. Alpha-tocopherol (vitamin E) diminished the effects of tamoxifen on PKC-activity as well as on telomerase activity.


CONCLUSIONS
We conclude that the tamoxifen-induced decrease of telomerase activity in HepG2 cells is mediated post-translationally via suppression of PKC-activity.
Brandt
Sebastian
S
Department of Physiology, University of Bonn, Nussallee 11, Bonn, Germany. sebastian.brandt@uni-bonn.de
Heller
Hartmut
H
Schuster
Klaus-Dieter
KD
Grote
Jürgen
J
D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D047628:Estrogen Receptor alpha / Q000502:physiology; D018197:Hepatoblastoma / Q000473:pathology*; D006801:Humans; D008113:Liver Neoplasms / Q000473:pathology*; D011493:Protein Kinase C / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D013629:Tamoxifen / Q000494:pharmacology*; D019098:Telomerase / Q000037:antagonists & inhibitors / Q000187:drug effects* / Q000378:metabolism*; D014407:Tumor Cells, Cultured; D015854:Up-Regulation
D018931:Antineoplastic Agents, Hormonal; D004791:Enzyme Inhibitors; D047628:Estrogen Receptor alpha; D013629:Tamoxifen; D011493:Protein Kinase C; D019098:Telomerase