Abstract
Exposure to genotoxic agents, such as irradiation produces DNA damage, the toxicity of which is augmented when the DNA repair is impaired. Poly (ADP-ribose) polymerase (PARP) inhibitors were found to be “synthetic lethal” in cells deficient in BRCA1 and BRCA2 that impair homologous recombination. However, since many tumors, including prostate cancer (PCa) rarely have on such mutations, there is considerable interest in finding alternative determinants of PARP inhibitor sensitivity. We evaluated the effectiveness of radiation in combination with the PARP inhibitor, rucaparib in PCa cells. The combination index for clonogenic survival following radiation and rucaparib treatments revealed synergistic interactions in a panel of PCa cell lines, being strongest for LNCaP and VCaP cells that express ETS gene fusion proteins. These findings correlated with synergistic interactions for senescence activation, as indicated by β--galactosidase staining. Absence of PTEN and presence of ETS gene fusion thus facilitated activation of senescence, which contributed to decreased clonogenic survival. Increased radiosensitivity in the presence of rucaparib was associated with persistent DNA breaks, as determined by χ-H2AX, p53BP1, and Rad51 foci. VCaP cells, which harbor the TMPRSS2-ERG gene fusion and PC3 cells that stably express a similar construct (fusion III) showed enhanced sensitivity towards rucaparib, which, in turn, increased the radiation response to a similar extent as the DNA-PKcs inhibitor NU7441. Rucaparib radiosensitized PCa cells, with a clear benefit of low dose-rate radiation (LDR) administered over a longer period of time that caused enhanced DNA damage. LDR mimicking brachytherapy, which is used successfully in the clinic, was most effective when combined with rucaparib by inducing persistent DNA damage and senescence, leading to decreased clonogenic survival. This combination was most effective in the presence of the TMPRSS2-ERG and in the absence of PTEN, indicating clinical potential for brachytherapy in patients with intermediate and high risk PCa.
Highlights
Prostate cancer (PCa) is the most frequently diagnosed tumor in men, accounting alone for 29% of incident cases [1]
Ionizing radiation and DNA-damaging agents significantly induce Poly (ADP-ribose) polymerase (PARP)-1 and levels of PARP are higher in tumors [9,10], PARP inhibitors (PARPi) could be used to sensitize to DNA-damaging chemo- or radio-therapy
Rucaparib (CO-338; formerly known as AG014699 and PF01367338), a PARPi represents a novel class of agents with documented antitumor activity against human cell lines and xenografts containing mutated or epigenetically silenced BRCA1/ 2 [4,27]
Summary
Prostate cancer (PCa) is the most frequently diagnosed tumor in men, accounting alone for 29% of incident cases [1]. Irradiation is an important treatment modality for PCa, with a clinical response achieved of ,85% It is used primarily for early-stage disease, as an adjuvant to surgery, and in combination with chemo-therapeutics that allow its use at lower doses, with higher efficiencies and with less cytotoxic effect to the adjacent normal tissues. (ADP-ribose) polymerase (PARP) is represented by a family of proteins that are expressed abundantly, are primarily localized in the nucleus, and are involved in many important cellular processes, such as the response to DNA damage, its repair, and when the damage is severe, cell death through apoptosis or necrosis [2]. PARP inhibitors (PARPi) represent a new class of agents that prevent the synthesis of poly-ADP ribose by impinging on the downstream DNA repair processes, and as a result the DNA damage persists [2]. Levels of PARP-1 are increased in advanced, castrate resistant PCa [9,10], its use in combination with radiation to enhance radiotherapy is appealing
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