Numerous strategies have been proposed to specifically inhibit telomerase (human telomerase reverse transcriptase (hTERT)) but to date only a few are clinically relevant in anticancer therapy. Recently, we have shown that long-term treatment with all-trans retinoic acid (ATRA), a compound clinically approved for differentiation therapy of acute promyelocytic leukemia (APL), represses hTERT in differentiation-resistant APL cell lines leading to telomere shortening and death. This signaling requires the co-activation of the retinoic acid receptor a (RARa) and the retinoic X receptor (RXR). In contrast to differentiationtherapy, which is only successful in this subtype of leukemia, the telomerase-targeted pathway could also be of use in non-APL. Here, we demonstrate that repression of hTERT occurs in fresh blasts cells from patients with myeloid leukemias of various subtypes exposed ex vivo to ATRA or synthetic retinoids. These results support the idea that, by hTERT targeting, retinoids can induce telomere shortening and cell death and their integration in therapy protocols for myeloid leukemias refractory to maturation should be considered. Leukemia (2006)
IntroductionTelomerase is a ribonucleoprotein complex responsible for the maintenance of telomeres. Its activity is generally undetectable in normal somatic cells, while it is expressed in approximately 85% of the common cancers. Expression of the catalytic protein component of the telomerase complex, hTERT (human Telomerase Reverse Transcriptase) plays a fundamental role in cellular immortalization. Hematopoietic stem cells express telomerase, 1-3 but not at a sufficient level to fully maintain telomere length. It is acknowledged that a high level of telomerase activity is detected in acute myeloid leukemia (more than 10-fold compared to normal hematopoietic cells) and during progression of chronic myeloid leukemia (CML). [4][5][6][7][8] Telomere shortening has also been described associated with progression of myelodysplastic syndrome (MDS) to overt leukemia. 9,10 As most cancer cells are reliant on telomerase for their survival, it has become a promising target of anticancer therapy. Even though multiple strategies have been described to specifically inhibit telomerase in vitro, 11 most of them have not been tested yet in clinical trials or are not relevant because of their low efficacy and/or a high toxicity.Among myeloid leukemias, acute promyelocytic leukemia (APL) was found to be particularly sensitive to differentiation therapy using pharmacological doses of all-trans retinoic acid (ATRA), which induce remission of APL patients by stimulating differentiation of leukemic cells. [12][13][14] Recently, we identified a new maturation-independent pathway, by which ATRA exerts an antiproliferative activity through hTERT downregulation, leading to telomere shortening and cell death. [15][16][17] The significance and extension of this observation to malignancies resistant to retinoid-induced differentiation is currently unknown. Here, we questioned whether h...