Eliminating hypoxic tumor cells improves response to PARP inhibitors in homologous recombination–deficient cancer models
Manal Mehibel, Yu Xu, Caiyun G. Li, Eui Jung Moon, Kaushik N. Thakkar, Anh N. Diep, Ryan K. Kim, Joshua D. Bloomstein, Yiren Xiao, Julien Bacal, Joshua C. Saldivar, Quynh-Thu Le, Karlene A. Cimprich, Erinn B. Rankin, Amato J. Giaccia
Manal Mehibel, Yu Xu, Caiyun G. Li, Eui Jung Moon, Kaushik N. Thakkar, Anh N. Diep, Ryan K. Kim, Joshua D. Bloomstein, Yiren Xiao, Julien Bacal, Joshua C. Saldivar, Quynh-Thu Le, Karlene A. Cimprich, Erinn B. Rankin, Amato J. Giaccia
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Research Article Cell biology Oncology

Eliminating hypoxic tumor cells improves response to PARP inhibitors in homologous recombination–deficient cancer models

Abstract

Hypoxia, a hallmark feature of the tumor microenvironment, causes resistance to conventional chemotherapy, but was recently reported to synergize with poly(ADP-ribose) polymerase inhibitors (PARPis) in homologous recombination–proficient (HR-proficient) cells through suppression of HR. While this synergistic killing occurs under severe hypoxia (<0.5% oxygen), our study shows that moderate hypoxia (2% oxygen) instead promotes PARPi resistance in both HR-proficient and -deficient cancer cells. Mechanistically, we identify reduced ROS-induced DNA damage as the cause for the observed resistance. To determine the contribution of hypoxia to PARPi resistance in tumors, we used the hypoxic cytotoxin tirapazamine to selectively kill hypoxic tumor cells. We found that the selective elimination of hypoxic tumor cells led to a substantial antitumor response when used with PARPi compared with that in tumors treated with PARPi alone, without enhancing normal tissue toxicity. Since human breast cancers with BRAC1/2 mutations have an increased hypoxia signature and hypoxia reduces the efficacy of PARPi, then eliminating hypoxic tumor cells should enhance the efficacy of PARPi therapy.

Authors

Manal Mehibel, Yu Xu, Caiyun G. Li, Eui Jung Moon, Kaushik N. Thakkar, Anh N. Diep, Ryan K. Kim, Joshua D. Bloomstein, Yiren Xiao, Julien Bacal, Joshua C. Saldivar, Quynh-Thu Le, Karlene A. Cimprich, Erinn B. Rankin, Amato J. Giaccia

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Figure 5

Hypoxia is also implicated in resistance to other inhibitors of the alt-NHEJ pathway.

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Hypoxia is also implicated in resistance to other inhibitors of the alt-...
(A) Clonogenic formation of SUM149, OVCAR8, and HT1080 treated with indicated doses of L67 for 7 days under normoxic (black lines) or hypoxic (red lines) culture conditions, followed by 7- to 10-day culture in the absence of inhibitor. Survival relative to vehicle-treated cells is plotted. Interaction P value, normoxia versus hypoxia, determined by 2-way ANOVA. n = 3. (B) Colony formation of SUM149 cells treated for 96 hours with L67 (3 μM) and olaparib (0.1 μM) or talazoparib (1 nM) under normoxia or hypoxia. Results expressed as survival ratio relative to vehicle-treated groups. P value calculated by t test, combination treatments versus single treatments. n = 4. (C) Colony formation of OVCAR8 cells treated for 96 hours with L67 (3 μM) and olaparib (1 μM) or talazoparib (10 nM) under normoxia or hypoxia. Results expressed as survival ratio relative to vehicle-treated groups. P value calculated by t test, combination treatments versus single treatments. n = 3. (D) Colony formation of HT1080 cells expressing the indicated CRISPR/CAS9 constructs to knockdown Polθ and treated for 7 days with the indicated doses of talazoparib under normoxic (left panel) or hypoxic (right panel) culture conditions, followed by 7- to 10-day culture in the absence of inhibitor. Survival relative to vehicle-treated cells is plotted. Interaction P value, normoxia versus hypoxia, determined by 2-way ANOVA. (E) qRT-PCR measuring PolQ mRNA levels in HT1080 cells. Measurements were normalized to 18S mRNA levels and expressed as fold change compared with CRISPR scrambled PolQ. n = 3. Data are represented as mean ± SEM (represented by error bars).