EPIGENETIC REGULATION OF NOTCH AND C-MYB IN T-ACUTE LYMPHOBLASTIC LEUKEMIA

Histone deacetylases (HDACs) are enzymes involved in the remodeling of chromatin. In recent years, inhibition of HDACs has emerged as a potential strategy to reverse aberrant epigenetic changes associated with cancer. In fact, HDAC inhibitors promote apoptosis, induce cell cycle arrest and differentiation of tumor cells, by mechanisms which remain in part unknown. In our studies we observed that treatment of T-Acute Lymphoblastic Leukemia (T-ALL) cell lines with the pan-HDAC inhibitor Trichostatin A (TSA), caused marked reduction of Notch1, Notch3 and c-Myb protein levels. The mRNA expression levels of the two Notch receptors did not change, on the contrary c-Myb transcript decreased. This result suggests that Notch1 and Notch3 could be regulated post-transcriptionally and/or post-translationally following TSA treatment. Blockage of HDAC activity also decreased the expression of Notch target transcripts such as pTα, CR2 and DTX-1, indicating a general down-regulation of Notch signaling. Moreover, inhibition of HDACs exerts strong pro-apoptotic effects in all cell lines tested (n=3). These findings are confirmed in a panel of primary T-ALL cells from xenografts (n=7) and treated in vitro with TSA, albeit heterogeneous responses were observed. To identify HDAC family member(s) responsible for these effects, we are currently exploiting class specific-HDACi as well as shRNA approaches. At the same time, we are investigating whether increased protein degradation may account for Notch reduction. following TSA treatment. To this end, we treated cells with proteasome and lysosome inhibitors, after TSA treatment. Protein levels of the two Notch receptors were rescued using the lysosome inhibitor chloroquine, suggesting involvement of the endocytic pathway, whereas proteasome inhibitors had minimal effects. Prospectively, HDAC inhibitors could represent a novel therapeutic approach for poor prognosis T-ALL patients, alone or in combination with conventional chemotherapy