Trastuzumab-Resistant HER2+ Breast Cancer Cells Retain Sensitivity to Poly (ADP-Ribose) Polymerase (PARP) Inhibition.

HER2-targeted therapies, such as trastuzumab, have increased the survival rates of HER2+ breast cancer patients. However, despite these therapies, many tumors eventually develop resistance to these therapies. Our lab previously reported an unexpected sensitivity of HER2+ breast cancer cells to poly (ADP-ribose) polymerase inhibitors (PARPi), agents that target homologous recombination (HR)-deficient tumors, independent of a DNA repair deficiency. In this study, we investigated whether HER2+ trastuzumab-resistant (TR) breast cancer cells were susceptible to PARPi and the mechanism behind PARPi induced cytotoxicity. We demonstrate that the PARPi ABT-888 (veliparib) decreased cell survival in vitro and tumor growth in vivo of HER2+ TR breast cancer cells. PARP-1 siRNA confirmed that cytotoxicity was due, in part, to PARP-1 inhibition. Furthermore, PARP-1 silencing had variable effects on the expression of several NF-κB-regulated genes. In particular, silencing PARP-1 inhibited NF-κB activity and reduced p65 binding at the IL8 promoter, which resulted in a decrease in IL8 mRNA and protein expression. Our results provide insight in the potential mechanism by which PARPi induces cytotoxicity in HER2+ breast cancer cells and support the testing of PARPi in patients with HER2+ breast cancer resistant to trastuzumab. Mol Cancer Ther; 17(5); 921-30. ©2018 AACR.

Let-7 Status Is Crucial for PARP1 Expression in HER2-Overexpressing Breast Tumors.

HER2+ breast tumors have been shown to express elevated levels of PARP1 protein. Yet, the mechanism by which PARP1 is upregulated in HER2+ breast cancer is unknown. Here, knockdown of HER2 (ERBB2) in HER2+ breast cancer cells resulted in a reduction in PARP1 protein. Conversely, ectopic overexpression of HER2 in a non-HER2-overexpressing cell line resulted in increased PARP1 protein levels. Alterations in HER2 expression had no significant effect on PARP1 transcript levels. Instead, HER2 mRNA status was inversely correlated with let-7a miRNA levels in breast cancer cells. Ectopic expression of let-7a miRNA resulted in downregulation of PARP1 protein, whereas expression of the let-7a anti-miRNA increased PARP1 protein. Furthermore, luciferase assays demonstrate that let-7a regulates PARP1 via its 3'UTR. Importantly, let-7a was significantly lower in human HER2+ breast tumors compared with HER2- breast tumors and inversely correlated with PARP1 protein levels. Finally, HER2+ breast cancer cells exhibited similar cytotoxicity to ectopic let-7a expression as the PARP inhibitor veliparib (ABT-888). Collectively, these results reveal that increased PARP1 expression in HER2+ breast cancers is regulated by the let-7a miRNA, and that let-7a is a potential strategy to suppress PARP1 activity.Implications: This study reports the novel findings that HER2 increases PARP1 protein via suppression of the let-7a miRNA, which regulates the PARP1 3'-UTR. Moreover, HER2 status correlates with high PARP1 and low let-7a in breast cancer clinical specimens. Mol Cancer Res; 15(3); 340-7. ©2016 AACR.

PARP1 and phospho-p65 protein expression is increased in human HER2-positive breast cancers.

Previous studies have shown that basal breast cancers, which may have an inherent "BRCAness" phenotype and sensitivity to inhibitors of poly (ADP-Ribose) polymerase (PARP), express elevated levels of PARP1. Our lab recently reported that HER2+ breast cancers also exhibit sensitivity to PARP inhibitors (PARPi) by attenuating the NF-κB pathway. In this study, we assessed PARP1 and phospho-p65, a marker of activated NF-κB levels in human breast cancer tissues. PARP1 and PARP2 copy number, mRNA, and protein expression was assessed by interrogating the PAM-50 defined breast cancer patient set from the TCGA using cBioPortal. PARP1 and phospho-p65 immunohistochemistry and correlation to clinical parameters was conducted using 307 primary breast cancer specimens (132 basal, 82 luminal, 93 HER2+) through univariate and multivariate analyses. In the PAM50 breast cancer data set, PARP1 and 2 expression was altered in 24/58 (41 %) HER2+, 32/81 (40 %) basal, and 75/324 (23 %) luminal A/B breast cancer patients. This correlated with a statistically significant increase in PARP1 protein levels in HER2+ and basal but not luminal breast cancers (p = 0.003, p = 0.027, p = 0.289, respectively). No change in PARP2 protein level was observed. Interestingly, using breast cancer specimens from 307 patients, HER2 positivity correlated with elevated PARP1 expression (p < 0.0001) and was three times more likely than HER2 negative breast cancers to exhibit high PARP1 levels. No significant differences were noted between race, ER status, or PR status for PARP1 expression. Additionally, we found a significant correlation between HER2 status and phospho-p65 expression (p < 0.0001). Lastly, a direct correlation between PARP1 and phospho-p65 (p < 0.0001) was noted. These results indicate a potential connection between HER2, PARP1, and phospho-p65. Furthermore, these data suggest that the PARPi sensitivity we previously observed in HER2+ breast cancer cells may be due to elevated PARP1 expression.

An ex vivo assay of XRT-induced Rad51 foci formation predicts response to PARP-inhibition in ovarian cancer.

OBJECTIVE: BRCA-positive ovarian cancer patients derive benefit PARP inhibitors. Approximately 50% of ovarian cancer tumors have homologous recombination (HR) deficiencies and are therefore "BRCA-like," possibly rendering them sensitive to PARP inhibition. However, no predictive assay exists to identify these patients. We sought to determine if irradiation-induced Rad51 foci formation, a known marker of HR, correlated to PARP inhibitor response in an ovarian cancer model. METHODS: Ovarian cancer cell lines were exposed to PARP-inhibitor ABT-888 to determine effect on growth. Rad51 protein expression prior to irradiation was determined via Western blot. Cultured cells and patient-derived xenograft tumors (PDX) were irradiated and probed for Rad51 foci. In vivo PDX tumors were treated with ABT-888 and carboplatin; these results were correlated with the ex vivo ionizing radiation assay. RESULTS: Three of seven cell lines were sensitive to ABT-888. Sensitive lines had the lowest Rad51 foci formation rate after irradiation, indicating functional HR deficiency. Approximately 50% of the PDX samples had decreased Rad51 foci formation. Total Rad51 protein levels were consistently low, suggesting that DNA damage induction is required to characterize HR status. The ex vivo IR assay accurately predicted which PDX models were sensitive to PARP inhibition in vitro and in vivo. ABT-888 alone reduced orthotopic tumor growth by 51% in A2780ip2 cell line, predicted to respond by the ex vivo assay. Three PDX models' response also correlated with the assay. CONCLUSIONS: The ex vivo IR assay correlates with response to PARP inhibition. Analysis of total Rad51 protein is not a reliable substitute.

Discussion of PARP inhibitors in cancer therapy.

Poly (ADP-ribose) polymerases (PARP) are a family of enzymes that play a very important role in preserving the integrity of the genome. Recently, PARP inhibitors have been shown to enhance the therapeutic ratio in cancer patients due to their specific targeting of homologous recombination repair-defective tumors, through a synthetic lethal interaction. Researchers are also presently investigating novel strategies for the treatment of sporadic cancers by combining PARP inhibitors with other DNA-damaging agents. This review will focus on recently patented PARP inhibitors and literature that supports the reported claims presented in these patents. The patents reviewed were categorized into two groups: PARP inhibitors as a single-agent or in combination with other agents for the treatment of various types of cancer. These compounds are currently in clinical trials and, if successful, can greatly impact therapeutic index in cancer therapy.

A general framework of persistence strategies for biological systems helps explain domains of life.

The nature and cause of the division of organisms in superkingdoms is not fully understood. Assuming that environment shapes physiology, here we construct a novel theoretical framework that helps identify general patterns of organism persistence. This framework is based on Jacob von Uexküll's organism-centric view of the environment and James G. Miller's view of organisms as matter-energy-information processing molecular machines. Three concepts describe an organism's environmental niche: scope, umwelt, and gap. Scope denotes the entirety of environmental events and conditions to which the organism is exposed during its lifetime. Umwelt encompasses an organism's perception of these events. The gap is the organism's blind spot, the scope that is not covered by umwelt. These concepts bring organisms of different complexity to a common ecological denominator. Ecological and physiological data suggest organisms persist using three strategies: flexibility, robustness, and economy. All organisms use umwelt information to flexibly adapt to environmental change. They implement robustness against environmental perturbations within the gap generally through redundancy and reliability of internal constituents. Both flexibility and robustness improve survival. However, they also incur metabolic matter-energy processing costs, which otherwise could have been used for growth and reproduction. Lineages evolve unique tradeoff solutions among strategies in the space of what we call "a persistence triangle." Protein domain architecture and other evidence support the preferential use of flexibility and robustness properties. Archaea and Bacteria gravitate toward the triangle's economy vertex, with Archaea biased toward robustness. Eukarya trade economy for survivability. Protista occupy a saddle manifold separating akaryotes from multicellular organisms. Plants and the more flexible Fungi share an economic stratum, and Metazoa are locked in a positive feedback loop toward flexibility.