PARP inhibition and pharmacological ascorbate demonstrate synergy in castration-resistant prostate cancer

Prostate cancer (PCa) is the second leading cause of cancer death for men in the United States. While organ-confined disease has reasonable expectation of cure, metastatic PCa is universally fatal upon recurrence during hormone therapy, a stage termed castration-resistant prostate cancer (CRPC). Until such time as molecularly defined subtypes can be identified and targeted using precision medicine, it is necessary to investigate new therapies that may apply to the CRPC population as a whole. The administration of ascorbate, more commonly known as ascorbic acid or Vitamin C, has proved lethal to and highly selective for a variety of cancer cell types. There are several mechanisms currently under investigation to explain how ascorbate exerts anti-cancer effects. A simplified model depicts ascorbate as a pro-drug for reactive oxygen species (ROS), which accumulate intracellularly and generate DNA damage. It was therefore hypothesized that poly(ADP-ribose) polymerase (PARP) inhibitors, by inhibiting DNA damage repair, would augment the toxicity of ascorbate.

Results

Two distinct CRPC models were found to be sensitive to physiologically relevant doses of ascorbate. Moreover, additional studies indicate that ascorbate inhibits CRPC growth in vitro via multiple mechanisms including disruption of cellular energy dynamics and accumulation of DNA damage. Combination studies were performed in CRPC models with ascorbate in conjunction with escalating doses of three different PARP inhibitors (niraparib, olaparib, and talazoparib). The addition of ascorbate augmented the toxicity of all three PARP inhibitors and proved synergistic with olaparib in both CRPC models. Finally, the combination of olaparib and ascorbate was tested in vivo in both castrated and non-castrated models. In both cohorts, the combination treatment significantly delayed tumor growth compared to monotherapy or untreated control.

Conclusions

These data indicate that pharmacological ascorbate is an effective monotherapy at physiological concentrations and kills CRPC cells. Ascorbate-induced tumor cell death was associated with disruption of cellular energy dynamics and accumulation of DNA damage. The addition of PARP inhibition increased the extent of DNA damage and proved effective at slowing CRPC growth both in vitro and in vivo. These findings nominate ascorbate and PARPi as a novel therapeutic regimen that has the potential to improve CRPC patient outcomes.

Mutant p53 elicits context-dependent pro-tumorigenic phenotypes

The tumor suppressor gene TP53 is the most frequently mutated gene in numerous cancer types, including prostate cancer (PCa). Specifically, missense mutations in TP53 are selectively enriched in PCa, and cluster to particular "hot spots" in the p53 DNA binding domain with mutation at the R273 residue occurring most frequently. While this residue is similarly mutated to R273C-p53 or R273H-p53 in all cancer types examined, in PCa selective enrichment of R273C-p53 is observed. Importantly, examination of clinical datasets indicated that TP53 heterozygosity can either be maintained or loss of heterozygosity (LOH) occurs. Thus, to mimic tumor-associated mutant p53, R273C-p53 and R273H-p53 isogenic PCa models were developed in the presence or absence of wild-type p53. In the absence of wild-type p53, both R273C-p53 and R273H-p53 exhibited similar loss of DNA binding, transcriptional profiles, and loss of canonical tumor suppressor functions associated with wild-type p53. In the presence of wild-type p53 expression, both R273C-p53 and R273H-p53 supported canonical p53 target gene expression yet elicited distinct cistromic and transcriptional profiles when compared to each other. Moreover, heterozygous modeling of R273C-p53 or R273H-p53 expression resulted in distinct phenotypic outcomes in vitro and in vivo. Thus, mutant p53 acts in a context-dependent manner to elicit pro-tumorigenic transcriptional profiles, providing critical insight into mutant p53-mediated prostate cancer progression.

Significance of distinct specifically enriched missense TP53 mutations in prostate cancer

377

Background: The most common TP53 alterations are missense mutations occurring in the DNA binding domain. The majority of missense p53 mutants (mut-p53) demonstrate oncogenic gain-of-function (GOF) abilities, irrespective of wild-type p53 presence, and thus contribute to a more aggressive disease. In prostate cancer (PCa), characterized by comparatively low overall mutational burden, TP53 is frequently mutated in both primary and advanced disease. Despite significant progress made in the field, detailed mechanisms of GOF in PCa remain undefined due to differing features of p53 mutants. Methods: Analysis of available datasets was performed to assess TP53 mutational status in PCa patient samples and its correlation with the clinical outcome. Using hormone therapy sensitive and CRPC cells, a panel of cell lines was generated to model the two most frequently occurring mutations in the presence or absence of wild-type TP53, as occurs clinically. CHIP-seq, gene expression arrays, and in vitro and in vivo biological assays were performed to interrogate the significance of mut-p53 in PCa. Results: In PCa, missense mutations are significantly associated with decreased progression-free and overall survival. In PCa patient samples these mutations most commonly occur at the R273 residue, demonstrating specific enrichment when compared to other cancers, with R273C alteration being the most frequent. Using our cell panel, CHIP-seq data revealed an expansion of the p53 cistrome upon expression of R273C and R273H mutants in a manner distinct from p53 stabilization in the presence of wt-p53. Moreover, analysis of the TP53 missense mutant-sensitive transcriptomes demonstrated differential gene expression between these mutants, related to the expression of wild-type TP53 in those cells. Finally, R273C and R273H p53 mutants elicited context dependent effects on canonical p53 functions, thereby modulating distinct downstream biological outcomes. Conclusions: These data expand our knowledge of the underlying mechanisms by which distinct gain-of-function p53 mutants affect prostate cancer, and can lead to identification of novel therapeutic targets to improve clinical outcomes in PCa patients harboring these mutations.

USP22 Functions as an Oncogenic Driver in Prostate Cancer by Regulating Cell Proliferation and DNA Repair

Emerging evidence indicates the deubiquitinase USP22 regulates transcriptional activation and modification of target substrates to promote pro-oncogenic phenotypes. Here, in vivo characterization of tumor-associated USP22 upregulation and unbiased interrogation of USP22-regulated functions in vitro demonstrated critical roles for USP22 in prostate cancer. Specifically, clinical datasets validated that USP22 expression is elevated in prostate cancer, and a novel murine model demonstrated a hyperproliferative phenotype with prostate-specific USP22 overexpression. Accordingly, upon overexpression or depletion of USP22, enrichment of cell-cycle and DNA repair pathways was observed in the USP22-sensitive transcriptome and ubiquitylome using prostate cancer models of clinical relevance. Depletion of USP22 sensitized cells to genotoxic insult, and the role of USP22 in response to genotoxic insult was further confirmed using mouse adult fibroblasts from the novel murine model of USP22 expression. As it was hypothesized that USP22 deubiquitylates target substrates to promote protumorigenic phenotypes, analysis of the USP22-sensitive ubiquitylome identified the nucleotide excision repair protein, XPC, as a critical mediator of the USP22-mediated response to genotoxic insult. Thus, XPC undergoes deubiquitylation as a result of USP22 function and promotes USP22-mediated survival to DNA damage. Combined, these findings reveal unexpected functions of USP22 as a driver of protumorigenic phenotypes and have significant implications for the role of USP22 in therapeutic outcomes. SIGNIFICANCE: The studies herein present a novel mouse model of tumor-associated USP22 overexpression and implicate USP22 in modulation of cellular survival and DNA repair, in part through regulation of XPC.

Synergistic effects of the PARP inhibitor olaparib and pharmacological ascorbate in castration-resistant prostate cancer

326

Background: The administration of ascorbate has proved lethal to and highly selective for a variety of cancer cell types; however, despite an increasingly impressive body of evidence, there has not been a robust effort to translate the observed in vitro and in vivo outcomes to the clinic. This is partially due to the fact that the mechanism by which ascorbate exerts its anti-cancer effect is still under investigation. A simplified model depicts ascorbate as a pro-drug for reactive oxygen species (ROS), which accumulate intracellularly and generate DNA damage. It was therefore hypothesized that poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi), by inhibiting DNA damage repair, would augment the toxicity of ascorbate. Methods: In vitro and in vivo models systems queried for anti-tumor effects of PARP inhibitors and ascorbate. Results: Two distinct castration-resistant prostate cancer (CRPC) models were sensitive to ascorbate at physiologically attainable concentrations. These in vitro models were then subjected to treatment with three different PARP inhibitors (olaparib, niraparib, and talazoparib) alone and in combination with ascorbate. The addition of a sub-lethal dose of ascorbate significantly increased cell death across a range of doses for all three PARP inhibitors. A combination index was generated for olaparib and ascorbate in both CRPC models; the results suggest a strongly synergistic relationship between olaparib and ascorbate. Use of a CRPC in vivo model demonstrated that the combination of olaparib and ascorbate significantly increased tumor doubling time compared to vehicle controls and monotherapy. This in vivo efficacy was even more profound in an additional model using castrated mice to mimic the effect of hormone therapy. Additional mechanistic studies are in progress to further investigate the potential for ascorbate and olaparib combination therapy. Conclusions: Ultimately, these data suggest the combination of ascorbate and PARP inhibitors could be an effective treatment for CRPC.

PARP-1 regulates DNA repair factor availability

PARP-1 holds major functions on chromatin, DNA damage repair and transcriptional regulation, both of which are relevant in the context of cancer. Here, unbiased transcriptional profiling revealed the downstream transcriptional profile of PARP-1 enzymatic activity. Further investigation of the PARP-1-regulated transcriptome and secondary strategies for assessing PARP-1 activity in patient tissues revealed that PARP-1 activity was unexpectedly enriched as a function of disease progression and was associated with poor outcome independent of DNA double-strand breaks, suggesting that enhanced PARP-1 activity may promote aggressive phenotypes. Mechanistic investigation revealed that active PARP-1 served to enhance E2F1 transcription factor activity, and specifically promoted E2F1-mediated induction of DNA repair factors involved in homologous recombination (HR). Conversely, PARP-1 inhibition reduced HR factor availability and thus acted to induce or enhance "BRCA-ness". These observations bring new understanding of PARP-1 function in cancer and have significant ramifications on predicting PARP-1 inhibitor function in the clinical setting.

MAPK Reliance via Acquired CDK4/6 Inhibitor Resistance in Cancer

Purpose: Loss of cell-cycle control is a hallmark of cancer, which can be targeted with agents, including cyclin-dependent kinase-4/6 (CDK4/6) kinase inhibitors that impinge upon the G1-S cell-cycle checkpoint via maintaining activity of the retinoblastoma tumor suppressor (RB). This class of drugs is under clinical investigation for various solid tumor types and has recently been FDA-approved for treatment of breast cancer. However, development of therapeutic resistance is not uncommon.Experimental Design: In this study, palbociclib (a CDK4/6 inhibitor) resistance was established in models of early stage, RB-positive cancer.Results: This study demonstrates that acquired palbociclib resistance renders cancer cells broadly resistant to CDK4/6 inhibitors. Acquired resistance was associated with aggressive in vitro and in vivo phenotypes, including proliferation, migration, and invasion. Integration of RNA sequencing analysis and phosphoproteomics profiling revealed rewiring of the kinome, with a strong enrichment for enhanced MAPK signaling across all resistance models, which resulted in aggressive in vitro and in vivo phenotypes and prometastatic signaling. However, CDK4/6 inhibitor-resistant models were sensitized to MEK inhibitors, revealing reliance on active MAPK signaling to promote tumor cell growth and invasion.Conclusions: In sum, these studies identify MAPK reliance in acquired CDK4/6 inhibitor resistance that promotes aggressive disease, while nominating MEK inhibition as putative novel therapeutic strategy to treat or prevent CDK4/6 inhibitor resistance in cancer. Clin Cancer Res; 24(17); 4201-14. ©2018 AACR.

Effect of bypass kinase pathways on acquired CDK4/6 inhibitor resistance

379

Background: Cyclin Dependent Kinase-4/6 (CDK4/6) kinase inhibitors have shown clinical benefit in treatment of solid tumor types, including breast cancer. However, resistance is common, and the underpinning mechanisms of action are not well understood. Given the dependence of CDK4/6 inhibitors on retinoblastoma tumor suppressor (RB) function for activity, this class of agents may be particularly effective in tumor types for which RB loss is infrequent or occurs late in tumor progression. Methods: Here, models of acquired palbociclib resistance were generated in early stage, RB positive cancers, wherein it was shown acquired palbociclib resistance resulted in cross-resistance to other CDK4/6 inhibitors under clinical testing. Results: Cells showing acquired resistance exhibited aggressive in vitro and in vivo phenotypes without genetic loss of RB or RB pathway members, including enhanced proliferative capacity, migratory potential, and characteristics of epithelial to mesenchymal transition. Further analyses through integration of RNA sequencing and phospho-proteomics identified activation of the MAPK signaling pathway as a mediator of CDK4/6 inhibitor resistance, capable of bypassing CDK4/6 activity. However, this altered kinase dependence resulted in sensitization to MEK inhibitors, suggestive of new clinical opportunities in CDK4/6 resistant tumors. Conclusions: In sum, the studies herein not only identify activation of the MAPK pathway as capable of bypassing the CDK4/6 requirement and promoting aggressive tumor characteristics, but nominate MEK inhibitors as potential mechanisms to treat or prevent CDK4/6 inhibitor resistance.

Synergy of radiotherapy and PD-1 blockade in Kras-mutant lung cancer

Radiation therapy (RT), a critical modality in the treatment of lung cancer, induces direct tumor cell death and augments tumor-specific immunity. However, despite initial tumor control, most patients suffer from locoregional relapse and/or metastatic disease following RT. The use of immunotherapy in non-small-cell lung cancer (NSCLC) could potentially change this outcome by enhancing the effects of RT. Here, we report significant (up to 70% volume reduction of the target lesion) and durable (up to 12 weeks) tumor regressions in conditional Kras-driven genetically engineered mouse models (GEMMs) of NSCLC treated with radiotherapy and a programmed cell death 1 antibody (αPD-1). However, while αPD-1 therapy was beneficial when combined with RT in radiation-naive tumors, αPD-1 therapy had no antineoplastic efficacy in RT-relapsed tumors and further induced T cell inhibitory markers in this setting. Furthermore, there was differential efficacy of αPD-1 plus RT among Kras-driven GEMMs, with additional loss of the tumor suppressor serine/threonine kinase 11/liver kinase B1 (Stk11/Lkb1) resulting in no synergistic efficacy. Taken together, our data provide evidence for a close interaction among RT, T cells, and the PD-1/PD-L1 axis and underscore the rationale for clinical combinatorial therapy with immune modulators and radiotherapy.

Assessment of Knee Joint Injuries with Low Field Strength Magnetic Resonance Imaging

BACKGROUND

Magnetic Resonance Imaging is an appropriate screening tool before therapeutic arthroscopy, making diagnostic arthroscopy unnecessary in most patients. This study aims to evaluate the MRI findings in knee injuries and diagnostic value of low Strength MRI for assessing Meniscal and cruciate ligament tear.

METHODS

A cross sectional study was conducted on patients undergoing "Magnetic Resonance Imaging of the Knee" for injuries of the knee and excluded patients undergoing MRI for other causes, poor diagnostic quality MRI and post operative MRI. All patients were interviewed for mechanism of injury and followed up for arthroscopic findings. Statistical analysis was doe using IBM SPSS 20.0.

RESULTS

A total of 81 MRIs was included in the study. Arthroscopic finding of only 32 patients could be followed up. Anterior cruciate ligament (ACL) tear was the most common internal ligament tear accounting for 34(42%) of cases followed by medial meniscus tear in 33(40.7%). Twisting 14( 42.4%)was the most common mechanism involved in medial meniscus tear while combined mechanism of injury was most common mechanism for ACL tear 16( 47.05%). The sensitivity of MRI for diagnosis of ACL tear and medial meniscus tear was 96.3% and 94.7% respectively. Specificity for ACL tear was however only 80% and that for medial meniscus tear was 100%.

CONCLUSIONS

The diagnostic value of MRI for diagnosing internal derangement of knee was high even with a low Tesla (0.3 T) MRI thus emphasizing the role of MRI as a non-invasive alternative to diagnostic arthroscopy.