Abstract B34: Global histone modifications define early stress induced drug tolerance in cancer
Authors: Emran Abdullah Al , Menon Dinoop Ravindran , Soyer Peter , Gabrielli Brian , Sturm Rick , Herlyn Meenhard , Schaider Helmut 
Published: May 2016

Abstract
Background: Resistance to chemotherapy and targeted therapies is a major problem for cancer treatment. Acquired drug resistance is not only dependent on somatic mutations or drug efflux suggestive of alternative mechanisms like epigenetic changes and chromatin remodeling. Recent studies also suggest drug holidays after initial treatment with a drug re-sensitizing cancer cells to the same drug. We recently have identified early stress induced multi-drug resistant cancer cells termed induced drug tolerant cells (IDTCs).
Methods: Lung, breast, colon, liver and melanoma cancer cell lines (A549, SKBR3, HT-29, HEPG2, and WM1366) were exposed to chemotherapeutic drugs like Doxorubicin, Docetaxel,molecular inhibitors like Dabrafanib or Trametinib, or nutrient starvation (low glucose-1g/L) at different concentrations for twelve to twenty consecutive days for acquiring an IDTC state. Drug holiday was employed by withdrawing drug treatment for a period of seven days and maintained in normal growth medium. CD271 expression, which is a marker of IDTCs, was determined by immunofluorescence and FACS. Histone modifications were probed by immunofluorescence and common survival pathways were observed by western blot analysis. Genome wide expression analysis were performed by microarray with IDTCs of different cancer types compared to parental control
Results: Transcriptional active histone marks like H3K4me3 and repressive marks like- H3K9Me3 and H3K27me3 were altered (up-regulation of H3K9Me3 and down-regulation of H3K4Me3 and H3K27Me3) in IDTCs compared to parental cancer cells. These data were consistent in all of the five cancer types indicating that this effect is neither a consequence from a specific therapy nor for a particular cancer rather a common mechanism of initial drug resistance. Similar histone modification patterns were observed under conditions of nutrient starvation, if a drug holiday was provided, dynamic switching of histone modification was observed resembling the histone modifications of the parental population. Moreover microarray data suggest that, specific enzymes responsible for H3K4Me3, H3K27Me3 and H3K27Me3 were observed to show an altered expression profile in IDTCs. Such it can be concluded that multiple players maintain a particular histone modification and there is a dynamic switching of histone methylase and demethylase stabilizing a specific histone modification depending on the context. Along with that, different pro-survival pathways like ERK, AKT, MTOR and AMPK were upregulated in all five cancer types.
Conclusion: Global histone modifications render cancer cells to transform into IDTCs characterized by a distinct morphology, expression of IDTC markers like CD271, multidrug resistance and activation of pro-survival signaling. Histone modifications, chromatin remodelling, and DNA methylation act in cohort to maintain the euchromatin and heterochromatin state of IDTCs. We aim to further elucidate target genes of active and inactive histone modifiers in IDTCs crucial for the transition of early- to acquired permanent drug resistance.
Citation Format: Abdullah Al Emran, Dinoop Ravindran Menon, Peter Soyer, Brian Gabrielli, Rick Sturm, Meenhard Herlyn, Helmut Schaider. Global histone modifications define early stress induced drug tolerance in cancer. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Sep 24-27, 2015; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2016;76(2 Suppl):Abstract nr B34.