L'inibizione di TERT compromette la proliferazione cellulare attraverso un meccanismo indipendente dalla lunghezza dei telomeri e può essere sfruttata come un efficiente approccio antitumorale

Telomerase is a ribonucleoprotein complex containing a catalytic protein with telomere-specific reverse transcriptase (TERT) activity, which synthesizes telomeric sequences de novo utilizing an internal RNA template. Maintenance of telomere length by telomerase is critical for overcoming replicative senescence and acquiring unlimited replicative potential. Telomerase activity is undetectable in normal somatic cells, but its reactivation/upregulation is a universal event taking place in the vast majority of human tumours. Several studies indicated that TERT, beyond maintaining telomeres, can contribute to tumour development through regulation of many telomere-length independent processes, thus suggesting that telomerase inhibition can be exploited as an efficient therapeutic approach in tumours, regardless of telomere length. In accordance with this, we had previously demonstrated that short-term TERT inhibition by BIBR1532 (BIBR), an inhibitor of TERT catalytic activity, in in vitro B-cell malignancy models induced cell cycle arrest and apoptosis and activated DNA damage response (DDR) via telomere-length independent mechanism. The aim of this study consisted in evaluating the effects of short-term Tert inhibition as a potential anticancer strategy in in vivo zebrafish model. In addition, we investigated the molecular mechanism(s) through which TERT inhibition impairs cell cycle progression in in vitro B-cell malignancy models, with the goal to unravel the cellular pathways being (de)regulated by telomerase during oncogenic transformation. We found that short-term Tert inhibition by BIBR in wild-type (wt) zebrafish embryos reduced cell proliferation, induced an accumulation of cells in the S-phase and ultimately led to apoptosis associated with the activation of DDR; all these effects were unrelated to telomere shortening/dysfunction. BIBR treatment showed no effects in tert mutant embryos. Moreover, short-term TERT inhibition negatively impacted the proliferation and viability of human malignant B cells xenografted in zebrafish embryos. Additionally, xenografted tumour cells, treated with BIBR, displayed a significantly higher apoptotic rate compared to untreated control cells. The in vitro results in B-cell malignancy models showed that short-term BIBR treatment altered NF-κB signalling, by reducing p65 nuclear levels, leading to down-regulation of a subset of NF-κB target genes, including MYC, IκBα, BCL2 and Survivin. In addition, we found that the NF-κB inhibitor PDTC, similarly to TERT inhibition, downregulated MYC expression and arrested cell cycle progression with an accumulation of cells in the S-phase supporting the involvement of the NF-κB signalling in the modulatory role played by TERT in the cell cycle. Moreover, MYC down-regulation, induced by TERT inhibition, was associated with increased expression and nuclear localization of p21, thus favouring its cell cycle inhibitory functions. Overall, our results highlight the potential use of a therapeutic approach based on TERT inhibition in the treatment of B-cell malignancies beyond telomere length. Telomerase inhibitors can be adopted alone or in combination with the existing chemotherapeutic regimen to target both NF-κB p65 and MYC-driven tumours, including post-transplant lymphoproliferative disorders and Burkitt’s lymphomas.