The importance of
telomerase, the
enzyme that maintains telomere length, has been reported in many
malignancies in general and in
multiple myeloma (MM) in particular.
Proteasome inhibitors are clinically used to combat effectively MM. Since the mechanism of action of
proteasome inhibitors has not been fully described we sought to clarify its potential effect on
telomerase activity (TA) in MM cells. Previously we showed that the first generation
proteasome inhibitor bortezomib (Brt) inhibits TA in MM cells by both transcriptional and post-translational mechanisms and has a potential clinical significance. In the current study we focused around the anti-
telomerase activity of the new generation of
proteasome inhibitors,
epoxomicin (EP) and
MG-132 in order to clarify whether
telomerase inhibition represents a class effect. We have exposed MM cell lines, ARP-1, CAG, RPMI 8226 and U266 to EP or MG and the following parameters were assessed: viability; TA, hTERT expression, the binding of hTERT (human
telomerase reverse transcriptase)
transcription factors and post-translational modifications.
Epoxomicin and
MG-132 differentially downregulated the proliferation and TA in all MM cell lines. The downregulation of TA and the expression of hTERT were faster in CAG than in ARP-1 cells.
Epoxomicin was more potent than
MG-132 and therefore further mechanistic studies were performed using this compound. The inhibition of TA was mainly transcriptionally regulated. The binding of three positive regulator
transcription factors: SP1, c-Myc and NF-κB to the hTERT promoter was decreased by EP in CAG cells as well as their total cellular expression. In ARP-1 cells the SP1 and c-MYC binding and
protein levels were similarly affected by EP while NF-κB was not affected. Interestingly, the
transcription factor WT-1 (Wilms' tumor-1) exhibited an increased binding to the hTERT promoter while its total cellular amount remained unchanged. Our results combined with our previous study of
bortezomib define
telomerase as a general target for
proteasome inhibitors. The inhibitory effect of TA is exerted by several regulatory levels, transcriptional and post translational. SP1, C-Myc and NF-κB were involved in mediating these effects. A novel finding of this study is the role of WT-1 in the regulation of
telomerase which appears as a negative regulator of hTERT expression. The results of this study may contribute to future development of
telomerase inhibition as a therapeutic modality in MM.