Tumor extracelluar hyroxyapatite: a potential biomarker for imaging ovarian cancer

Abstract

<h3>Objectives:</h3> Identifying new strategies to improve response to poly ADP-ribose polymerase inhibitors (PARPi) in women with homologous recombination (HR)-proficient ovarian cancer is a key clinical challenge. The nuclear orphan receptor NR4A1 has pro-tumor effects in ovarian cancer cells through transcriptional regulation of key genes promoting growth, survival and HR DNA repair. NR4A1 is also expressed in immune cells in the tumor microenvironment and may play a role in inhibiting re-polarization of pro-tumor M2-like tumor-associated macrophages (TAMs) to a more effective anti-tumor M1-like TAM phenotype. Our group has shown that the NF-kappaB (NF-ĸB) pathway is a key regulator of TAM phenotype, with upregulated NF-ĸB signaling promoting M1-like functions. Our goal therefore was to determine whether inhibition of NR4A1 can improve responses to PARPi both by increased tumor DNA damage and apoptosis, and by promoting an anti-tumor M1 phenotype in TAMs by modulating NF-ĸB activity. <h3>Methods:</h3> Cultured HR-proficient ID8 mouse ovarian cancer cells, mouse ovarian TAMs and mouse bone marrow-derived macrophages were treated with the established NR4A1 antagonist C-DIM-pPHOH (C-DIM) and/or the PARPi olaparib (OLA) for 24-72 hours. C57BL/6 mice with established intra-peritoneal ID8 tumors were treated with vehicle, C-DIM, OLA or combined C-DIM/OLA for 3 weeks. Standard techniques were used to determine cell growth (Sulforhodamine B assay), HR efficiency (<i>BRCA1</i> and RAD51 foci formation), NF-ĸB activity (nuclear p65, luciferase reporter activity), and expression of markers of DNA damage (pH2AX), apoptosis (cleaved PARP), proliferation (PCNA), and macrophage M1 (CCL3, iNOS) and M2 (CD206, arginase-1) polarization. Statistical significance (p<0.05) between group means was determined by Student's t test for <i>in vitro</i> studies and Mann-Whitney test for mouse studies. <h3>Results:</h3> In ID8 cells, co-treatment with C-DIM and OLA significantly reduced cell growth and HR efficiency, and increased DNA-damage-induced apoptosis, compared to OLA treatment alone. NR4A1 was highly expressed in macrophages, and C-DIM increased NF-ĸB activity and induced M1 polarization alone and when combined with olaparib. Similar effects of combined treatment were observed <i>in vivo.</i> Compared to mice treated with OLA alone, there was significantly reduced ascites volume and omental tumor mass in C-DIM/OLA mice at the time of sacrifice. Combined treatment also reduced proliferation and increased DNA damage and apoptosis in harvested tumors, and increased M1 polarization of TAMs in ascites fluid, compared to OLA mice. <h3>Conclusions:</h3> NR4A1 inhibition sensitizes ovarian tumors to PARPi though direct effects on tumors by targeting the HR pathway, and by modulating TAM function towards an anti-tumor phenotype. These findings have important implications for expanding the use of PARPi in women with HR-proficient ovarian tumors, which would benefit a substantial number of women with this devastating disease.