title: hTERT phosphorylation by PKC is essential for telomerase holoprotein integrity and enzyme activity in head neck cancer cells.
abstract: Telomerase activity is suppressed in normal somatic tissues but is activated in most cancer cells. We have previously found that all six telomerase subunit proteins, including hTERT and hsp90 are needed for full enzyme activity. Telomerase activity has been reported to be upregulated by protein kinase C (PKC), but the mechanism is not clear. In this study, we examined how PKC regulates telomerase activity in head and neck cancer cells. PKC inhibitor, bisindolylmaleimide I (BIS), inhibited telomerase activity but had no effect on the expressions of telomerase core subunits. RNA interference (RNAi) and in vitro phosphorylation studies revealed that PKC isoforms alpha, beta, delta, epsilon, zeta specifically involved in telomerase regulation, and the phosphorylation target was on hTERT. Treatment with the hsp-90 inhibitor novobiocin dissociated hsp90 and hTERT as revealed by immunoprecipitation and immunoblot analysis and reduced telomerase activity. Treatment with the PKC activator SC-10 restored the association of hsp90 and hTERT and reactivate telomerase, suggesting that hTERT phosphorylation by PKC is essential for telomerase holoenzyme integrity and function. Analysis on clinical normal and tumour tissues reveal that the expressions of PKC alpha, beta, delta, epsilon, zeta were higher in the tumour tissues, correlated with telomerase activity. Disruption of PKC phosphorylation by BIS significantly increased chemosensitivity to cisplatin. In conclusion, PKC isoenzymes alpha, beta, delta, epsilon, zeta regulate telomerase activity in head and neck cancer cells by phosphorylating hTERT. This phosphorylation is essential for telomerase holoenzyme assembly, leading to telomerase activation and oncogenesis. Manipulation of telomerase activity by PKC inhibitors is worth exploring as an adjuvant therapeutic approach.
authors.affiliation: Department of Radiation Oncology, Chang Gung Memorial Hospital, Taoyuan 333, Taiwan.
authors.forename: J T
authors.initials: JT
authors.lastname: Chang
authors.forename: Y-C
authors.initials: YC
authors.lastname: Lu
authors.forename: Y-J
authors.initials: YJ
authors.lastname: Chen
authors.forename: C-P
authors.initials: CP
authors.lastname: Tseng
authors.forename: Y-L
authors.initials: YL
authors.lastname: Chen
authors.forename: C-W
authors.initials: CW
authors.lastname: Fang
authors.forename: A-J
authors.initials: AJ
authors.lastname: Cheng
mesh_terms: D002471:Cell Transformation, Neoplastic; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004789:Enzyme Activation; D006258:Head and Neck Neoplasms / Q000201:enzymology* / Q000235:genetics; D006801:Humans; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*; D034622:RNA Interference; D019098:Telomerase / Q000378:metabolism*; D014407:Tumor Cells, Cultured; D015854:Up-Regulation
chemical_list: D004268:DNA-Binding Proteins; D011493:Protein Kinase C; D019098:Telomerase