BET bromodomain inhibitors synergize with ATR inhibitors in melanoma

Abstract

Metastatic malignant melanoma continues to be a challenging disease despite clinical translation of the comprehensive understanding of driver mutations and how melanoma cells evade immune attack. In Myc-driven lymphoma, efficacy of epigenetic inhibitors of the bromodomain and extra-terminal domain (BET) family of bromodomain proteins can be enhanced by combination therapy with inhibitors of the DNA damage response kinase ATR. Whether this combination is active in solid malignancies like melanoma, and how it relates to immune therapy, has not previously investigated. To test efficacy and molecular consequences of combination therapies cultured melanoma cells were used. To assess tumor responses to therapies in vivo we use patient-derived xenografts and B6 mice transplanted with B16F10 melanoma cells. Concomitant inhibition of BET proteins and ATR of cultured melanoma cells resulted in similar effects as recently shown in lymphoma, such as induction of apoptosis and p62, implicated in autophagy, senescence-associated secretory pathway and ER stress. In vivo, apoptosis and suppression of subcutaneous growth of patient-derived melanoma and B16F10 cells were observed. Our data suggest that ATRI/BETI combination therapies are effective in melanoma.

Pharmacological Inhibition of ATR Can Block Autophagy through an ATR-Independent Mechanism

“…Interestingly, cellular fractionation confirmed that both the cytoplasmic and nuclear expression of p62 increased ( Figure 1 D). To determine if p62 accumulation in these esophageal cell lines occurred via the same mechanism as previously reported, OE21 cells were treated with VX-970 in the absence of any other exogenous stress and qPCR carried out for the well-characterized target of ER stress; CHOP, as well as p62 ( Muralidharan et al., 2016 , 2017 ). Surprisingly and in contrast to the previous reports, neither the level of p62 nor CHOP mRNA was significantly altered in response to exposure to VX-970 ( Figure 1 E).…”

“…However, the reported induction of p62 observed in response to the ATR inhibitor, VE-821, was not linked to a blockage in autophagy as another commonly used marker of autophagy, light chain 3 (LC3), appeared unaffected ( Muralidharan et al., 2016 ). In a follow-on study focused on melanoma, inhibition of ATR was again demonstrated to lead to the induction of SASP/ER stress, which included an accumulation of p62 and the transcription factor C/EBP homologous protein (CHOP) ( Muralidharan et al., 2017 ). The accumulation of p62 has also been implicated in compromising DNA repair, genomic integrity, and in accelerated aging ( Eliopoulos et al., 2016 ; Hewitt et al., 2016 ; Kang et al., 2015 ; Kwon et al., 2012 ).…”

Bromodomain and extraterminal domain inhibition synergizes with WEE1‐inhibitor AZD1775 effect by impairing nonhomologous end joining and enhancing DNA damage in nonsmall cell lung cancer

“…Several papers also have reported that BET inhibitors synergize with cell cycle checkpoint modulators, such as CDK inhibitors or ATR inhibitors . Zhang et al .…”

“…Several papers also have reported that BET inhibitors synergize with cell cycle checkpoint modulators, such as CDK inhibitors 42 or ATR inhibitors. 31,43 Zhang et al 43 reported that BRD4 inhibitor synergized with an ATR inhibitor through reducing the phosphorylation of CHK1. CHK1 inhibitors would also synergize with BET inhibitors theoretically, as CHK1 inhibitors could drive mitotic entry.…”

“…JQ1 treatment has been shown to induce G1 arrest. 9,31,32 Unexpectedly, in 2/3 cell lines tested, JQ1 treatment in combination with AZD1775 resulted in a significant increase in the percentage of cells in G2-M phase as compared to the percentages after single treatments ( Figs. 4a-4c).…”