ivity of p53. Telomerase CYC202 Seliciclib expression often plays a critical role in cancer cell progression, including hematologic neoplasias . The rate limiting component of the telomerase holoenzyme is the catalytic subunit, human telomerase enzymatic reverse transcriptase, hTERT . HDACi induced hTERT regulation has been seen in many cell types , typcially in the form of hTERT derepression . This report is the first describing hTERT downregulation, with a 25 fold decrease in gene expression following HDAC inhibition in lymphoma cells The mechanistic reasons for this unique result are unclear and may have interesting cell type specific implications. The hTERT gene is a positive transcriptional target of Myc and is repressed by the Mxd proteins . Vorinostat induced Myc downregulation and Mxd1 upregulation in lymphoma cells can thus explain hTERT gene repression.
Increased telomerase expression can accompany disease progression, e.g., Regorafenib 755037-03-7 higher expression in chronic myelogenous leukemia blast crisis patients compared to those in the chronic phase . Notably, successful imatinib mesylate treatment of CML reduces telomerase activity , while high telomerase levels correlate with imatinib resistance . These observations suggest HDACi induced hTERT downregulation is a biologically significant event in vorinostat inhibition of lymphoma cell growth. MicroRNAs are key regulators of cell growth and differentiation due to messenger RNA downregulation . Their differential expression can be used to classify multiple Kretzner et al. Page 7 Cancer Res. Author manuscript, available in PMC 2012 June 1.
NIH PA Author Manuscript NIH PA Author Manuscript NIH PA Author Manuscript human tumor types, including subtypes of lymphomas . We show dose dependent downregulation of miR 17 5p, miR 17 3p, and miR 18 by vorinostat and TSA in L540 and DHL4 cells. These miRNAs are part of the miR 17 92 miRNA cluster, which is mycregulated and oncogenic in a Burkitt lymphoma mouse model, and is also implicated in other cancers . HDACi downregulation of these miRNAs is thus biologically significant and mechanistically plausible, given simultaneous repression of myc levels by HDACi. Three other non myc regulated miRNAs of significance in lymphomas and other hematologic cancers, miR 15b, miR 34a, and miR 155 exhibited responses to HDAC inhibition. MicroRNAs of the miR 15 and miR 16 family target the mRNA of Bcl 2 and their upregulation is thus associated with apoptosis .
We saw dose dependent downregulation of miR 15b in L540 and DHL 4 cell lines by vorinostat or TSA. miR 34a is a positive transcriptional target of p53 and was strongly upregulated in DHL 4 cells , however, its levels declined in L540 cells with HDACi treatment . miR 155 is generated from sequences within the non protein coding BIC RNA, and both RNAs are upregulated in some HL and DLBCL samples correlating with the activated B cell phenotype . miR 155 also has anti proliferative and pro apoptotic activities in melanoma cells and hematopoietic stem cells . We observed increases in miR 155 after HDACi treatment in L540 cells, although it was repressed in DHL 4 cells. Variable behavior of miR 34a and miR 155 may reflect the different lymphoma types represented by L540 and DHL 4 cells.
Differential effects on cells, of changes in the microRNA levels after treatment, as opposed to steady state overexpression, may contribute to differences in miR 155 activity between cell types. We have demonstrated the importance of myc downregulation in response to vorinostat alone and in the combined response to AKI,s and HDACi,s. In another hematopoietic malignancy model, reduced myc levels are critical for acute myeloid leukemia cell growth arrest by the HDACi valproic acid . Myc levels decline in many cell types undergoing differentiation, while those of Mxd genes rise . This counterbalance is consistent with a requirement for both Myc knockdown and Mxd1 over expression combined with Aki treatment, to mimic the synergistic effect of vo