Elevated HDAC activity and altered histone phospho-acetylation confer acquired radio-resistant phenotype to breast cancer cells

Abstract

Background: Poor-responsiveness of tumors to radiotherapy is a major clinical problem. Owing to the dynamic nature of the epigenome, the identification and targeting of potential epigenetic modifiers may be helpful to curb radio-resistance. This requires a detailed exploration of the epigenetic changes that occur during the acquirement of radio-resistance. Such an understanding can be applied for effective utilization of treatment adjuncts to enhance the efficacy of radiotherapy and reduce the incidence of tumor recurrence. Results: This study explored the epigenetic alterations that occur during the acquirement of radio-resistance. Sequential irradiation of MCF7 breast cancer cell line up to 20 Gy generated a radio-resistant model. Micrococcal nuclease digestion demonstrated the presence of compact chromatin architecture coupled with decreased levels of histone PTMs H3K9ac, H3K27 ac, and H3S10pK14ac in the G 0 /G 1 and mitotic cell cycle phases of the radio-resistant cells. Further investigation revealed that the radio-resistant population possessed high HDAC and low HAT activity, thus making them suitable candidates for HDAC inhibitor-based radio-sensitization. Treatment of radio-resistant cells with HDAC inhibitor valproic acid led to the retention of γH2AX and decreased H3S10p after irradiation. Additionally, an analysis of 38 human patient samples obtained from 8 different tumor types showed variable tumor HDAC activity, thus demonstrating inter-tumoral epigenetic heterogeneity in a patient population. Conclusion: The study revealed that an imbalance of HAT and HDAC activities led to the loss of site-specific histone acetylation and chromatin compaction as breast cancer cells acquired radio-resistance. Due to variation in the tumor HDAC activity among patients, our report suggests performing a prior assessment of the tumor epigenome to maximize the benefit of HDAC inhibitor-based radio-sensitization.

Dynamic Alterations of Histone H3 Phospho-acetylation Correlate With Radio Sensitivity of Mitotic Cells During DNA Damage

“…Most remarkably, radiated mitotic cells adopted a more de-condensed chromatin con guration after cell cycle progression, which correlated with their reduced cell survival. This observation is in complete concordance with our recent report that suggests increased heterochromatinization upon acquirement of radio-resistance (44). Hence, these data strongly indicate that global chromatin con guration plays an essential role in determining cell fate towards radiation induced DNA damage.…”

“…Therefore, histone PTM alterations that occur in response to DNA damage could be a cell cycle phase speci c event (39)(40)(41). Earlier studies from our group reported a dynamic kinetics of H3S10ph, observed only during the DDR of the relatively radioresistant G 0 /G 1 phase (42,43) Our recent report also indicates an association of altered histone phosphoacetylation with breast cancer radio-resistance (44). These reports suggest that a distinct interplay of H3 phospho-acetylation could regulate cell cycle phase speci c radio resistance.…”

“…The interest in H3S10ph and its nearby acetyl mark(s) originates from our previous reports about association of H3 phospho-acetylation with G 0 /G 1 phase speci c DDR and radio-resistance (42)(43)(44).…”

A Paradigm Revolution or Just Better Resolution—Will Newly Emerging Superresolution Techniques Identify Chromatin Architecture as a Key Factor in Radiation-Induced DNA Damage and Repair Regulation?

“…After exposure to high doses of sparsely ionizing radiation or even low doses of densely ionizing radiation, there is a serious risk that numerous and possibly clustered DSBs will not be repaired in a timely manner, leading to separation of broken DNA ends, misrejoining of these ends, and formation of often lethal chromosomal aberrations. These events probably explain why fast repair mechanisms have evolved and are preferred by organisms with large genomes [5]. However, a fast rate of repair may be at the expense of repair accuracy, resulting in smaller mutations, some of which may be carcinogenic and thus no less dangerous than larger mutations.…”

A three layered histone epigenetics in breast cancer metastasis

“…Preclinical study in lung carcinoma cells showed that HDAC inhibitor SAHA could sensitize these cells to radiation with minimum effects to normal cells that would enhance the effect of radiotherapy of the cancer cells [194]. Another preclinical study in breast cancer cells MCF7, proved that a proper balance between HDACs and HATs is necessary to maintain the histone acetylation thereby the compaction of the chromatin to resist the development of resistant of cancer cells to radiation therapy [195]. So, an early diagnosis of tumor HDAC activity would increase the efficiency of HDAC/radiotherapy strategy of treating cancer cell [196].…”