Antitumour effect of the mitochondrial complex III inhibitor Atovaquone in combination with anti-PD-L1 therapy in mouse cancer models

Immune checkpoint blockade (ICB) provides effective and durable responses for several tumour types by unleashing an immune response directed against cancer cells. However, a substantial number of patients treated with ICB develop relapse or do not respond, which has been partly attributed to the immune-suppressive effect of tumour hypoxia. We have previously demonstrated that the mitochondrial complex III inhibitor atovaquone alleviates tumour hypoxia both in human xenografts and in cancer patients by decreasing oxygen consumption and consequently increasing oxygen availability in the tumour. Here, we show that atovaquone alleviates hypoxia and synergises with the ICB antibody anti-PD-L1, significantly improving the rates of tumour eradication in the syngeneic CT26 model of colorectal cancer. The synergistic effect between atovaquone and anti-PD-L1 relied on CD8+ T cells, resulted in the establishment of a tumour-specific memory immune response, and was not associated with any toxicity. We also tested atovaquone in combination with anti-PD-L1 in the LLC (lung) and MC38 (colorectal) cancer syngeneic models but, despite causing a considerable reduction in tumour hypoxia, atovaquone did not add any therapeutic benefit to ICB in these models. These results suggest that atovaquone has the potential to improve the outcomes of patients treated with ICB, but predictive biomarkers are required to identify individuals likely to benefit from this intervention.

Small-Molecule Polθ Inhibitors Provide Safe and Effective Tumor Radiosensitization in Preclinical Models

PURPOSE

DNA polymerase theta (Polθ, encoded by the POLQ gene) is a DNA repair enzyme critical for microhomology mediated end joining (MMEJ). Polθ has limited expression in normal tissues but is frequently overexpressed in cancer cells and, therefore, represents an ideal target for tumor-specific radiosensitization. In this study we evaluate whether targeting Polθ with novel small-molecule inhibitors is a feasible strategy to improve the efficacy of radiotherapy.

EXPERIMENTAL DESIGN

We characterized the response to Polθ inhibition in combination with ionizing radiation in different cancer cell models in vitro and in vivo.

RESULTS

Here, we show that ART558 and ART899, two novel and specific allosteric inhibitors of the Polθ DNA polymerase domain, potently radiosensitize tumor cells, particularly when combined with fractionated radiation. Importantly, noncancerous cells were not radiosensitized by Polθ inhibition. Mechanistically, we show that the radiosensitization caused by Polθ inhibition is most effective in replicating cells and is due to impaired DNA damage repair. We also show that radiosensitization is still effective under hypoxia, suggesting that these inhibitors may help overcome hypoxia-induced radioresistance. In addition, we describe for the first time ART899 and characterize it as a potent and specific Polθ inhibitor with improved metabolic stability. In vivo, the combination of Polθ inhibition using ART899 with fractionated radiation is well tolerated and results in a significant reduction in tumor growth compared with radiation alone.

CONCLUSIONS

These results pave the way for future clinical trials of Polθ inhibitors in combination with radiotherapy.

Automated 96-well format high throughput colony formation assay for siRNA library screen

The colony formation assay is the gold-standard technique to assess cell viability after treatment with cytotoxic reagents, ionizing radiation, and cytotoxic combinatorial treatments. This protocol describes a high-throughput automated and high-content imaging approach to screen siRNA molecular libraries in HeLa cervical cancer cells in 96-well format. We detail reverse transfection of cells with siRNAs, followed by ionizing radiation, fixing, and staining of the plates for automated colony counting. This protocol can be used across a broad range of cell types. For complete details on the use and execution of this protocol, please refer to Tiwana et al. (2015).

Abstract 5697: Targeting PARP inhibitor resistance with Polθ inhibitors

To target DNA repair vulnerabilities in cancer, we discovered nanomolar potent, selective, low molecular weight (MW), allosteric inhibitors of the polymerase function of DNA polymerase Polθ, including ART558. ART558 inhibits the major Polθ-mediated DNA repair process, Theta-Mediated End Joining (TMEJ), without targeting Non-Homologous End Joining. Moreover, we show that exposure to ART558 can elicit DNA damage and synthetic lethality in BRCA1- or BRCA2-mutant tumor cells and enhances the effects of a PARP inhibitor. Genetic perturbation screening revealed that defects in the 53BP1/Shieldin complex, which are a cause of PARP inhibitor resistance, result in in vitro and in vivo sensitivity to Polθ polymerase inhibitors. Mechanistically, ART558 increases biomarkers of single-stranded DNA and synthetic lethality in 53BP1-defective cells. The inhibition of DNA nucleases that promote end-resection, such as Exo1 or Blm-Dna2 reversed these effects, implicating these in the synthetic lethal mechanism-of-action. Taken together, these observations describe a drug class that elicits BRCA-gene synthetic lethality and PARP inhibitor synergy, as well as targeting a biomarker-defined mechanism of PARPi-resistance.

Citation Format: Diana Zatreanu, Helen Robinson, Omar Alkhatib, Marie Boursier, Harry Finch, Lerin Geo, Diego Grande, Vera Grinkevich, Robert Heald, Sophie Langdon, Jayesh Majithiya, Claire McWhirter, Niall Martin, Shaun Moore, Joana Neves, Eeson Rajendra, Marco Ranzani, Theresia Schaedler, Martin Stockley, Kimberley Wiggins, Rachel Brough, Sandhya Sridhar, Aditi Gulati, Nan Shao, Luted Badder, Daniela Novo, Eleanor Knight, Rebecca Marlow, Syed Haider, Elsa Callen, Graeme Hewitt, Joost Schimmel, Remko Prevo, Christina Alli, Amanda Ferdinand, Cameron Bell, Peter Blencowe, Chris Bot, Mathew Calder, Mark Charles, Jayne Curry, Tennyson Ekwuru, Andre Nussenzweig, Marcel Tijsterman, Andrew N. Tutt, Simon Boulton, Geoff Higgins, Stephen J. Pettitt, Graeme C. Smith, Christopher J. Lord. Targeting PARP inhibitor resistance with Polθ inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5697.

Abstract P056: Polθ inhibitors elicit BRCA-gene synthetic lethality and target PARP inhibitor resistance

To target DNA repair vulnerabilities in cancer, we discovered nanomolar potent, selective, low molecular weight, allosteric inhibitors of the polymerase function of DNA polymerase Theta (Polθ), including ART558. ART558 inhibits the major Polθ-mediated DNA repair process, Theta-Mediated End Joining (TMEJ), without targeting Non-Homologous End Joining. Moreover, we show that exposure to ART558 can elicit DNA damage and synthetic lethality in BRCA1- or BRCA2-mutant tumour cells and enhances the effects of a PARP inhibitor. Genetic perturbation screening revealed that defects in the 53BP1/Shieldin complex, which are a cause of PARP inhibitor resistance, result in in vitro and in vivo sensitivity to Polθ polymerase inhibitors. Mechanistically, ART558 increases biomarkers of single-stranded DNA and synthetic lethality in 53BP1-defective cells. The inhibition of DNA nucleases that promote end-resection reversed these effects, suggesting that resection via Exo1 or Blm-Dna2 being a cause, at least in part, of the ART558 sensitivity phenotype. Taken together, these observations describe a drug class that elicits BRCA-gene synthetic lethality and PARP inhibitor synergy, as well as targeting a biomarker-defined mechanism of PARPi-resistance.

Citation Format: Diana A. Zatreanu, Helen M. R. Robinson, Omar Alkhatib, Marie Boursier, Harry Finch, Lerin Geo, Diego Grande, Vera Grinkevich, Robert Heald, Sophie Langdon, Jayesh Majithiya, Claire McWhirter, Niall M. B. Martin, Shaun Moore, Joana Neves, Eeson Rajendra, Marco Ranzani, Theresia Schaedler, Martin Stockley, Kimberley Wiggins, Rachel Brough, Sandhya Sridhar, Aditi Gulati, Nan Shao, Luned M Badder, Daniela Novo, Eleanor G. Knight, Rebecca Marlow, Syed Haider, Elsa Callen, Graeme Hewitt, Joost Schimmel, Remko Prevo, Christina Alli, Amanda Ferdinand, Cameron Bell, Peter Blencowe, Mathew Calder, Mark Charles, Jayne Curry, Tennyson Ekwuru, Katherine Ewings, Andre Nussenzweig, Marcel Tijsterman, Andrew Tutt, Simon J. Boulton, Geoff S. Higgins, Steve Pettitt, Graeme C. M. Smith, Christopher J. Lord. Polθ inhibitors elicit BRCA-gene synthetic lethality and target PARP inhibitor resistance [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P056.

Polθ inhibitors elicit BRCA-gene synthetic lethality and target PARP inhibitor resistance

To identify approaches to target DNA repair vulnerabilities in cancer, we discovered nanomolar potent, selective, low molecular weight (MW), allosteric inhibitors of the polymerase function of DNA polymerase Polθ, including ART558. ART558 inhibits the major Polθ-mediated DNA repair process, Theta-Mediated End Joining, without targeting Non-Homologous End Joining. In addition, ART558 elicits DNA damage and synthetic lethality in BRCA1- or BRCA2-mutant tumour cells and enhances the effects of a PARP inhibitor. Genetic perturbation screening revealed that defects in the 53BP1/Shieldin complex, which cause PARP inhibitor resistance, result in in vitro and in vivo sensitivity to small molecule Polθ polymerase inhibitors. Mechanistically, ART558 increases biomarkers of single-stranded DNA and synthetic lethality in 53BP1-defective cells whilst the inhibition of DNA nucleases that promote end-resection reversed these effects, implicating these in the synthetic lethal mechanism-of-action. Taken together, these observations describe a drug class that elicits BRCA-gene synthetic lethality and PARP inhibitor synergy, as well as targeting a biomarker-defined mechanism of PARPi-resistance.

Mitochondrial Inhibitor Atovaquone Increases Tumor Oxygenation and Inhibits Hypoxic Gene Expression in Patients with Non-Small Cell Lung Cancer

PURPOSE

Tumor hypoxia fuels an aggressive tumor phenotype and confers resistance to anticancer treatments. We conducted a clinical trial to determine whether the antimalarial drug atovaquone, a known mitochondrial inhibitor, reduces hypoxia in non-small cell lung cancer (NSCLC).

PATIENTS AND METHODS

Patients with NSCLC scheduled for surgery were recruited sequentially into two cohorts: cohort 1 received oral atovaquone at the standard clinical dose of 750 mg twice daily, while cohort 2 did not. Primary imaging endpoint was change in tumor hypoxic volume (HV) measured by hypoxia PET-CT. Intercohort comparison of hypoxia gene expression signatures using RNA sequencing from resected tumors was performed.

RESULTS

Thirty patients were evaluable for hypoxia PET-CT analysis, 15 per cohort. Median treatment duration was 12 days. Eleven (73.3%) atovaquone-treated patients had meaningful HV reduction, with median change -28% [95% confidence interval (CI), -58.2 to -4.4]. In contrast, median change in untreated patients was +15.5% (95% CI, -6.5 to 35.5). Linear regression estimated the expected mean HV was 55% (95% CI, 24%-74%) lower in cohort 1 compared with cohort 2 (P = 0.004), adjusting for cohort, tumor volume, and baseline HV. A key pharmacodynamics endpoint was reduction in hypoxia-regulated genes, which were significantly downregulated in atovaquone-treated tumors. Data from multiple additional measures of tumor hypoxia and perfusion are presented. No atovaquone-related adverse events were reported.

CONCLUSIONS

This is the first clinical evidence that targeting tumor mitochondrial metabolism can reduce hypoxia and produce relevant antitumor effects at the mRNA level. Repurposing atovaquone for this purpose may improve treatment outcomes for NSCLC.

Targeting TOPK sensitises tumour cells to radiation-induced damage by enhancing replication stress

T-LAK-originated protein kinase (TOPK) overexpression is a feature of multiple cancers, yet is absent from most phenotypically normal tissues. As such, TOPK expression profiling and the development of TOPK-targeting pharmaceutical agents have raised hopes for its future potential in the development of targeted therapeutics. Results presented in this paper confirm the value of TOPK as a potential target for the treatment of solid tumours, and demonstrate the efficacy of a TOPK inhibitor (OTS964) when used in combination with radiation treatment. Using H460 and Calu-6 lung cancer xenograft models, we show that pharmaceutical inhibition of TOPK potentiates the efficacy of fractionated irradiation. Furthermore, we provide in vitro evidence that TOPK plays a hitherto unknown role during S phase, showing that TOPK depletion increases fork stalling and collapse under conditions of replication stress and exogenous DNA damage. Transient knockdown of TOPK was shown to impair recovery from fork stalling and to increase the formation of replication-associated single-stranded DNA foci in H460 lung cancer cells. We also show that TOPK interacts directly with CHK1 and Cdc25c, two key players in the checkpoint signalling pathway activated after replication fork collapse. This study thus provides novel insights into the mechanism by which TOPK activity supports the survival of cancer cells, facilitating checkpoint signalling in response to replication stress and DNA damage.

The anti-malarial drug atovaquone potentiates platinum-mediated cancer cell death by increasing oxidative stress

Platinum chemotherapies are highly effective cytotoxic agents but often induce resistance when used as monotherapies. Combinatorial strategies limit this risk and provide effective treatment options for many cancers. Here, we repurpose atovaquone (ATQ), a well-tolerated & FDA-approved anti-malarial agent by demonstrating that it potentiates cancer cell death of a subset of platinums. We show that ATQ in combination with carboplatin or cisplatin induces striking and repeatable concentration- and time-dependent cell death sensitization in vitro across a variety of cancer cell lines. ATQ induces mitochondrial reactive oxygen species (mROS), depleting intracellular glutathione (GSH) pools in a concentration-dependent manner. The superoxide dismutase mimetic MnTBAP rescues ATQ-induced mROS production and pre-loading cells with the GSH prodrug N-acetyl cysteine (NAC) abrogates the sensitization. Together, these findings implicate ATQ-induced oxidative stress as key mediator of the sensitizing effect. At physiologically achievable concentrations, ATQ and carboplatin furthermore synergistically delay the growth of three-dimensional avascular spheroids. Clinically, ATQ is a safe and specific inhibitor of the electron transport chain (ETC) and is concurrently being repurposed as a candidate tumor hypoxia modifier. Together, these findings suggest that ATQ is deserving of further study as a candidate platinum sensitizing agent.

External Beam Radiation Therapy and Enadenotucirev: Inhibition of the DDR and Mechanisms of Radiation-Mediated Virus Increase

Ionising radiation causes cell death through the induction of DNA damage, particularly double-stranded DNA (dsDNA) breaks. Evidence suggests that adenoviruses inhibit proteins involved in the DNA damage response (DDR) to prevent recognition of double-stranded viral DNA genomes as cellular dsDNA breaks. We hypothesise that combining adenovirus treatment with radiotherapy has the potential for enhancing tumour-specific cytotoxicity through inhibition of the DDR and augmentation of virus production. We show that EnAd, an Ad3/Ad11p chimeric oncolytic adenovirus currently being trialled in colorectal and other cancers, targets the DDR pathway at a number of junctures. Infection is associated with a decrease in irradiation-induced 53BP1 and Rad51 foci formation, and in total DNA ligase IV levels. We also demonstrate a radiation-associated increase in EnAd production in vitro and in a pilot in vivo experiment. Given the current limitations of in vitro techniques in assessing for synergy between these treatments, we adapted the plaque assay to allow monitoring of viral plaque size and growth and utilised the xCELLigence cell adhesion assay to measure cytotoxicity. Our study provides further evidence on the interaction between adenovirus and radiation in vitro and in vivo and suggests these have at least an additive, and possibly a synergistic, impact on cytotoxicity.

Increased EZH2 expression in prostate cancer is associated with metastatic recurrence following external beam radiotherapy

BACKGROUND

Enhancer of zeste 2 (EZH2) promotes prostate cancer progression. We hypothesized that increased EZH2 expression is associated with postradiotherapy metastatic disease recurrence, and may promote radioresistance.

METHODS

EZH2 expression was investigated using immunohistochemistry in diagnostic prostate biopsies of 113 prostate cancer patients treated with radiotherapy with curative intent. Associations between EZH2 expression in malignant and benign tissue in prostate biopsy cores and outcomes were investigated using univariate and multivariate Cox regression analyses. LNCaP and PC3 cell radiosensitivity was investigated using colony formation and γH2AX assays following UNC1999 chemical probe-mediated EZH2 inhibition.

RESULTS

While there was no significant association between EZH2 expression and biochemical recurrence following radiotherapy, univariate analysis revealed that prostate cancer cytoplasmic and total EZH2 expression were significantly associated with metastasis development postradiotherapy (P = 0.034 and P = 0.003, respectively). On multivariate analysis, the prostate cancer total EZH2 expression score remained statistically significant (P = 0.003), while cytoplasmic EZH2 expression did not reach statistical significance (P = 0.053). No association was observed between normal adjacent prostate EZH2 expression and biochemical recurrence or metastasis. LNCaP and PC3 cell treatment with UNC1999 reduced histone H3 lysine 27 tri-methylation levels. Irradiation of LNCaP or PC3 cells with a single 2 Gy fraction with UNC1999-mediated EZH2 inhibition resulted in a statistically significant, though modest, reduction in cell colony number for both cell lines. Increased γH2AX foci were observed 24 hours after ionizing irradiation in LNCaP cells, but not in PC3, following UNC1999-mediated EZH2 inhibition vs controls.

CONCLUSIONS

Taken together, these results reveal that high pretreatment EZH2 expression in prostate cancer in diagnostic biopsies is associated with an increased risk of postradiotherapy metastatic disease recurrence, but EZH2 function may only at most play a modest role in promoting prostate cancer cell radioresistance.

Buparlisib with thoracic radiotherapy and its effect on tumour hypoxia: A phase I study in patients with advanced non-small cell lung carcinoma

BACKGROUND

Pre-clinically, phosphoinositide 3-kinase (PI3K) inhibition radiosensitises tumours by increasing intrinsic radiosensitivity and by reducing tumour hypoxia. We assessed whether buparlisib, a class 1 PI3K inhibitor, can be safely combined with radiotherapy in patients with non-small cell lung carcinoma (NSCLC) and investigated its effect on tumour hypoxia.

METHODS

This was a 3 + 3 dose escalation and dose expansion phase I trial in patients with advanced NSCLC. Buparlisib dose levels were 50 mg, 80 mg and 100 mg once daily orally for 2 weeks, with palliative thoracic radiotherapy (20 Gy in 5 fractions) delivered during week 2. Tumour hypoxic volume (HV) was measured using 18F-fluoromisonidazole positron-emission tomography-computed tomography at baseline and following 1 week of buparlisib.

RESULTS

Twenty-one patients were recruited with 9 patients evaluable for maximum tolerated dose (MTD) analysis. No dose-limiting toxicity was reported; therefore, 100 mg was declared the MTD, and 10 patients received this dose in the expansion phase. Ninety-four percent of treatment-related adverse events were ≤grade 2 with fatigue (67%), nausea (24%) and decreased appetite (19%) most common per patient. One serious adverse event (grade 3 hypoalbuminaemia) was possibly related to buparlisib. No unexpected radiotherapy toxicity was reported. Ten (67%) of 15 patients evaluable for imaging analysis were responders with 20% median reduction in HV at the MTD.

CONCLUSION

This is the first clinical trial to combine a PI3K inhibitor with radiotherapy in NSCLC and investigate the effects of PI3K inhibition on tumour hypoxia. This combination was well tolerated and PI3K inhibition reduced hypoxia, warranting investigation into whether this novel class of radiosensitisers can improve radiotherapy outcomes.

T-LAK cell-originated protein kinase (TOPK): an emerging target for cancer-specific therapeutics

'Targeted' or 'biological' cancer treatments rely on differential gene expression between normal tissue and cancer, and genetic changes that render tumour cells especially sensitive to the agent being applied. Problems exist with the application of many agents as a result of damage to local tissues, tumour evolution and treatment resistance, or through systemic toxicity. Hence, there is a therapeutic need to uncover specific clinical targets which enhance the efficacy of cancer treatment whilst minimising the risk to healthy tissues. T-LAK cell-originated protein kinase (TOPK) is a MAPKK-like kinase which plays a role in cell cycle regulation and mitotic progression. As a consequence, TOPK expression is minimal in differentiated cells, although its overexpression is a pathophysiological feature of many tumours. Hence, TOPK has garnered interest as a cancer-specific biomarker and biochemical target with the potential to enhance cancer therapy whilst causing minimal harm to normal tissues. Small molecule inhibitors of TOPK have produced encouraging results as a stand-alone treatment in vitro and in vivo, and are expected to advance into clinical trials in the near future. In this review, we present the current literature pertaining to TOPK as a potential clinical target and describe the progress made in uncovering its role in tumour development. Firstly, we describe the functional role of TOPK as a pro-oncogenic kinase, followed by a discussion of its potential as a target for the treatment of cancers with high-TOPK expression. Next, we provide an overview of the current preclinical progress in TOPK inhibitor discovery and development, with respect to future adaptation for clinical use.

Nucleoporin 54 contributes to homologous recombination repair and post-replicative DNA integrity

The nuclear pore complex (NPC) machinery is emerging as an important determinant in the maintenance of genome integrity and sensitivity to DNA double-strand break (DSB)-inducing agents, such as ionising radiation (IR). In this study, using a high-throughput siRNA screen, we identified the central channel NPC protein Nup54, and concomitantly its molecular partners Nup62 and Nup58, as novel factors implicated in radiosensitivity. Nup54 depletion caused an increase in cell death by mitotic catastrophe after IR, and specifically enhanced both the duration of the G2 arrest and the radiosensitivity of cells that contained replicated DNA at the time of IR exposure. Nup54-depleted cells also exhibited increased formation of chromosome aberrations arisen from replicated DNA. Interestingly, we found that Nup54 is epistatic with the homologous recombination (HR) factor Rad51. Moreover, using specific DNA damage repair reporters, we observed a decreased HR repair activity upon Nup54 knockdown. In agreement with a role in HR repair, we also demonstrated a decreased formation of HR-linked DNA synthesis foci and sister chromatid exchanges after IR in cells depleted of Nup54. Our study reveals a novel role for Nup54 in the response to IR and the maintenance of HR-mediated genome integrity.

CDK1 inhibition sensitizes normal cells to DNA damage in a cell cycle dependent manner

Cyclin-dependent kinase 1 (CDK1) orchestrates the transition from the G2 phase into mitosis and as cancer cells often display enhanced CDK1 activity, it has been proposed as a tumor specific anti-cancer target. Here we show that the effects of CDK1 inhibition are not restricted to tumor cells but can also reduce viability in non-cancer cells and sensitize them to radiation in a cell cycle dependent manner. Radiosensitization by the specific CDK1 inhibitor, RO-3306, was determined by colony formation assays in three tumor lines (HeLa, T24, SQ20B) and three non-cancer lines (HFL1, MRC-5, RPE). Initial results showed that CDK1 inhibition radiosensitized tumor cells, but did not sensitize normal fibroblasts and epithelial cells in colony formation assays despite effective inhibition of CDK1 signaling. Further investigation showed that normal cells were less sensitive to CDK1 inhibition because they remained predominantly in G1 for a prolonged period when plated in colony formation assays. In contrast, inhibiting CDK1 a day after plating, when the cells were going through G2/M phase, reduced their clonogenic survival both with and without radiation. Our finding that inhibition of CDK1 can damage normal cells in a cell cycle dependent manner indicates that targeting CDK1 in cancer patients may lead to toxicity in normal proliferating cells. Furthermore, our finding that cell cycle progression becomes easily stalled in non-cancer cells under normal culture conditions has general implications for testing anti-cancer agents in these cells.

Abstract LB-B07: Photoreceptor phosphodiesterase 6H expression regulates HCT116 proliferation and cell cycle progression

Phosphodiesterase 6 (PDE6) protein complex, which is responsible for the turnover of the second messenger cyclic guanosine monophosphate (cGMP) and is involved in visual signal transduction of light stimuli, is highly expressed in certain cancer cell lines. Despite having been discovered in the outer segment of rods and cones of the retina, PDE6 protein is also detected in brain, adrenal glands, lungs, and gonads. Genes encoding the catalytic subunits of the enzyme are known to be upregulated in certain breast, non-small cell lung cancer and medulloblastoma cells. PDE6H, which encodes the cone-specific inhibitory gamma subunit of PDE6 protein, is part of the gene signature of achromatopsia and congenital stationary night blindness while its relation to cancer remains unclear. We recently identified PDE6H as a cell cycle controller in HCT116 colorectal cancer cell line, as part of an siRNA screen in which DNA content measurement via flow cytometry was undertaken to monitor cell cycle distribution. The screen was conducted with a custom library of metabolic genes commonly amplified in primary tumours, as well as those known to be involved in the key regulatory points of metabolic pathways.

We have shown that PDE6H knockdown and pharmacological inhibition of PDE6 protein hindered the activity of the protein complex, causing an accumulation of cGMP in various colorectal and breast cancer cell lines. This inhibition resulted in a G1 cell cycle arrest and disruption of G1/S checkpoint mechanism. PDE6H knockdown inhibited cell proliferation and cellular oxidative phosphorylation. It also altered mitochondrial morphology and induced apoptosis. Our results suggest that the gamma subunit is important for the function of photoreceptor phosphodiesterase PDE6 complex and PDE6H inhibition can be explored as an anti-proliferative strategy.

Citation Format: Ceren Yalaz, Syed Haider, Jianzhou Chen, Remko Prevo, Mick Woodcock, Adrian L. Harris, Ruth J. Muschel. Photoreceptor phosphodiesterase 6H expression regulates HCT116 proliferation and cell cycle progression [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr LB-B07.

TOPK modulates tumour-specific radiosensitivity and correlates with recurrence after prostate radiotherapy

BACKGROUND

Tumour-specific radiosensitising treatments may enhance the efficacy of radiotherapy without exacerbating side effects. In this study we determined the radiation response following depletion or inhibition of TOPK, a mitogen-activated protein kinase kinase family Ser/Thr protein kinase that is upregulated in many cancers.

METHODS

Radiation response was studied in a wide range of cancer cell lines and normal cells using colony formation assays. The effect on cell cycle progression was assessed and the relationship between TOPK expression and therapeutic efficacy was studied in a cohort of 128 prostate cancer patients treated with radical radiotherapy.

RESULTS

TOPK knockdown did not alter radiation response in normal tissues, but significantly enhanced radiosensitivity in cancer cells. This result was recapitulated in TOPK knockout cells and with the TOPK inhibitor, OTS964. TOPK depletion altered the G1/S transition and G2/M arrest in response to radiation. Furthermore, TOPK depletion increased chromosomal aberrations, multinucleation and apoptotic cell death after irradiation. These results suggest a possible role for TOPK in the radiation-induced DNA damage checkpoints. These findings have clinical relevance, as elevated TOPK protein expression was associated with poorer clinical outcomes in prostate cancer patients treated with radical radiotherapy.

CONCLUSIONS

This study demonstrates that TOPK disruption may cause tumour-specific radiosensitisation in multiple different tumour types.

Depletion of signal recognition particle 72kDa increases radiosensitivity

The identification of genetic determinants that underpin tumor radioresistance can help the development of targeted radiosensitizers or aid personalization of radiotherapy treatment. Here we identify signal recognition particle 72kDa (SRP72) as a novel gene involved in radioresistance. Knockdown of SRP72 resulted in significant radiosensitization of HeLa (cervical), PSN-1 (pancreatic), and T24 (bladder), BT-549 (breast) and MCF7 (breast) tumor lines as measured by colony formation assays. SRP72 depletion also resulted in the radiosensitization of normal lung fibroblast cell lines (HFL1 and MRC-5), demonstrating that the effect is not restricted to tumor cells. Increased radiosensitivity was not due to impaired DNA damage signaling or repair as assessed by γ-H2AX foci formation. Instead SRP72 depletion was associated with elevated levels of apoptosis after irradiation, as measured by caspase 3/7 activity, PARP-cleavage and Annexin-V staining, and with an induction of the unfolded protein response. Together, our results show that SRP72 is a novel gene involved in radioresistance.

Dysregulated mitophagy and mitochondrial organization in optic atrophy due to OPA1 mutations

Objective:

To investigate mitophagy in 5 patients with severe dominantly inherited optic atrophy (DOA), caused by depletion of OPA1 (a protein that is essential for mitochondrial fusion), compared with healthy controls.

Methods:

Patients with severe DOA (DOA plus) had peripheral neuropathy, cognitive regression, and epilepsy in addition to loss of vision. We quantified mitophagy in dermal fibroblasts, using 2 high throughput imaging systems, by visualizing colocalization of mitochondrial fragments with engulfing autophagosomes.

Results:

Fibroblasts from 3 biallelic OPA1(−/−) patients with severe DOA had increased mitochondrial fragmentation and mitochondrial DNA (mtDNA)–depleted cells due to decreased levels of OPA1 protein. Similarly, in siRNA-treated control fibroblasts, profound OPA1 knockdown caused mitochondrial fragmentation, loss of mtDNA, impaired mitochondrial function, and mitochondrial mislocalization. Compared to controls, basal mitophagy (abundance of autophagosomes colocalizing with mitochondria) was increased in (1) biallelic patients, (2) monoallelic patients with DOA plus, and (3) OPA1 siRNA–treated control cultures. Mitophagic flux was also increased. Genetic knockdown of the mitophagy protein ATG7 confirmed this by eliminating differences between patient and control fibroblasts.

Conclusions:

We demonstrated increased mitophagy and excessive mitochondrial fragmentation in primary human cultures associated with DOA plus due to biallelic OPA1 mutations. We previously found that increased mitophagy (mitochondrial recycling) was associated with visual loss in another mitochondrial optic neuropathy, Leber hereditary optic neuropathy (LHON). Combined with our LHON findings, this implicates excessive mitochondrial fragmentation, dysregulated mitophagy, and impaired response to energetic stress in the pathogenesis of mitochondrial optic neuropathies, potentially linked with mitochondrial mislocalization and mtDNA depletion.

Identification of vitamin B1 metabolism as a tumor-specific radiosensitizing pathway using a high-throughput colony formation screen

Colony formation is the gold standard assay for determining reproductive cell death after radiation treatment, since effects on proliferation often do not reflect survival. We have developed a high-throughput radiosensitivity screening method based on clonogenicity and screened a siRNA library against kinases. Thiamine pyrophosphokinase-1 (TPK1), a key component of Vitamin B1/thiamine metabolism, was identified as a target for radiosensitization. TPK1 knockdown caused significant radiosensitization in cancer but not normal tissue cell lines. Other means of blocking this pathway, knockdown of thiamine transporter-1 (THTR1) or treatment with the thiamine analogue pyrithiamine hydrobromide (PyrH) caused significant tumor specific radiosensitization. There was persistent DNA damage in cells irradiated after TPK1 and THTR1 knockdown or PyrH treatment. Thus this screen allowed the identification of thiamine metabolism as a novel radiosensitization target that affects DNA repair. Short-term modulation of thiamine metabolism could be a clinically exploitable strategy to achieve tumor specific radiosensitization.

Targeting ATR in DNA damage response and cancer therapeutics

The ataxia telangiectasia and Rad3-related (ATR) plays an important role in maintaining genome integrity during DNA replication through the phosphorylation and activation of Chk1 and regulation of the DNA damage response. Preclinical studies have shown that disruption of ATR pathway can exacerbate the levels of replication stress in oncogene-driven murine tumors to promote cell killing. Additionally, inhibition of ATR can sensitise tumor cells to radiation or chemotherapy. Accumulating evidence suggests that targeting ATR can selectively sensitize cancer cells but not normal cells to DNA damage. Furthermore, in hypoxic conditions, ATR blockade results in overloading replication stress and DNA damage response causing cell death. Despite the attractiveness of ATR inhibition in the treatment of cancer, specific ATR inhibitors have remained elusive. In the last two years however, selective ATR inhibitors suitable for in vitro and - most recently - in vivo studies have been identified. In this article, we will review the literature on ATR function, its role in DDR and the potential of ATR inhibition to enhance the efficacy of radiation and chemotherapy.

Targeting ATR in vivo using the novel inhibitor VE-822 results in selective sensitization of pancreatic tumors to radiation

Combined radiochemotherapy is the currently used therapy for locally advanced pancreatic ductal adenocarcinoma (PDAC), but normal tissue toxicity limits its application. Here we test the hypothesis that inhibition of ATR (ATM-Rad3-related) could increase the sensitivity of the cancer cells to radiation or chemotherapy without affecting normal cells. We tested VE-822, an ATR inhibitor, for in vitro and in vivo radiosensitization. Chk1 phosphorylation was used to indicate ATR activity, γH2AX and 53BP1 foci as evidence of DNA damage and Rad51 foci for homologous recombination activity. Sensitivity to radiation (XRT) and gemcitabine was measured with clonogenic assays in vitro and tumor growth delay in vivo. Murine intestinal damage was evaluated after abdominal XRT. VE-822 inhibited ATR in vitro and in vivo. VE-822 decreased maintenance of cell-cycle checkpoints, increased persistent DNA damage and decreased homologous recombination in irradiated cancer cells. VE-822 decreased survival of pancreatic cancer cells but not normal cells in response to XRT or gemcitabine. VE-822 markedly prolonged growth delay of pancreatic cancer xenografts after XRT and gemcitabine-based chemoradiation without augmenting normal cell or tissue toxicity. These findings support ATR inhibition as a promising new approach to improve the therapeutic ration of radiochemotherapy for patients with PDAC.

The novel ATR inhibitor VE-821 increases sensitivity of pancreatic cancer cells to radiation and chemotherapy

DNA damaging agents such as radiotherapy and gemcitabine are frequently used for the treatment of pancreatic cancer. However, these treatments typically provide only modest benefit. Improving the low survival rate for pancreatic cancer patients therefore remains a major challenge in oncology. Inhibition of the key DNA damage response kinase ATR has been suggested as an attractive approach for sensitization of tumor cells to DNA damaging agents, but specific ATR inhibitors have remained elusive. Here we investigated the sensitization potential of the first highly selective and potent ATR inhibitor, VE-821, in vitro. VE-821 inhibited radiation- and gemcitabine-induced phosphorylation of Chk1, confirming inhibition of ATR signaling. Consistently, VE-821 significantly enhanced the sensitivity of PSN-1, MiaPaCa-2 and primary PancM pancreatic cancer cells to radiation and gemcitabine under both normoxic and hypoxic conditions. ATR inhibition by VE-821 led to inhibition of radiation-induced G 2/M arrest in cancer cells. Reduced cancer cell radiosurvival following treatment with VE-821 was also accompanied by increased DNA damage and inhibition of homologous recombination repair, as evidenced by persistence of γH2AX and 53BP1 foci and inhibition of Rad51 foci, respectively. These findings support ATR inhibition as a novel approach to improve the efficacy and therapeutic index of standard cancer treatments across a large proportion of pancreatic cancer patients.

Homologous recombination mediates S-phase-dependent radioresistance in cells deficient in DNA polymerase eta

DNA polymerase eta (pol η) is the only DNA polymerase causally linked to carcinogenesis in humans. Inherited deficiency of pol η in the variant form of xeroderma pigmentosum (XPV) predisposes to UV-light-induced skin cancer. Pol η-deficient cells demonstrate increased sensitivity to cisplatin and oxaliplatin chemotherapy. We have found that XP30R0 fibroblasts derived from a patient with XPV are more resistant to cell kill by ionising radiation (IR) than the same cells complemented with wild-type pol η. This phenomenon has been confirmed in Burkitt's lymphoma cells, which either expressed wild-type pol η or harboured a pol η deletion. Pol η deficiency was associated with accumulation of cells in S-phase, which persisted after IR. Cells deficient in pol η demonstrated increased homologous recombination (HR)-directed repair of double strand breaks created by IR. Depletion of the HR protein, X-ray repair cross-complementing protein 3 (XRCC3), abrogated the radioresistance observed in pol η-deficient cells as compared with pol η-complemented cells. These findings suggest that HR mediates S-phase-dependent radioresistance associated with pol η deficiency. We propose that pol η protein levels in tumours may potentially be used to identify patients who require treatment with chemo-radiotherapy rather than radiotherapy alone for adequate tumour control.

NVP-BEZ235 and NVP-BGT226, dual phosphatidylinositol 3-kinase/mammalian target of rapamycin inhibitors, enhance tumor and endothelial cell radiosensitivity

BACKGROUND

The phosphatidylinositol 3-kinase (PI3K)/Akt pathway is activated in tumor cells and promotes tumor cell survival after radiation-induced DNA damage. Because the pathway may not be completely inhibited after blockade of PI3K itself, due to feedback through mammalian target of rapamycin (mTOR), more effective inhibition might be expected by targeting both PI3K and mTOR inhibition.

MATERIALS AND METHODS

We investigated the effect of two dual PI3K/mTOR (both mTORC1 and mTORC2) inhibitors, NVP-BEZ235 and NVP-BGT226, on SQ20B laryngeal and FaDu hypopharyngeal cancer cells characterised by EGFR overexpression, on T24 bladder tumor cell lines with H-Ras mutation and on endothelial cells. Analysis of target protein phosphorylation, clonogenic survival, number of residual γH2AX foci, cell cycle and apoptosis after radiation was performed in both tumor and endothelial cells. In vitro angiogenesis assays were conducted as well.

RESULTS

Both compounds effectively inhibited phosphorylation of Akt, mTOR and S6 target proteins and reduced clonogenic survival in irradiated tumor cells. Persistence of DNA damage, as evidenced by increased number of γH2AX foci, was detected after irradiation in the presence of PI3K/mTOR inhibition, together with enhanced G2 cell cycle delay. Treatment with one of the inhibitors, NVP-BEZ235, also resulted in decreased clonogenicity after irradiation of tumor cells under hypoxic conditions. In addition, NVP-BEZ235 blocked VEGF- and IR-induced Akt phosphorylation and increased radiation killing in human umbilical venous endothelial cells (HUVEC) and human dermal microvascular dermal cells (HDMVC). NVP-BEZ235 inhibited VEGF-induced cell migration and capillary tube formation in vitro and enhanced the antivascular effect of irradiation. Treatment with NVP-BEZ235 moderately increased apoptosis in SQ20B and HUVEC cells but not in FaDu cells, and increased necrosis in both tumor and endothelial all cells tumor.

CONCLUSIONS

The results of this study demonstrate that PI3K/mTOR inhibitors can enhance radiation-induced killing in tumor and endothelial cells and may be of benefit when combined with radiotherapy.

Abstract 5501: Evaluation of the first potent and highly selective inhibitor of ATR kinase: An approach to selectively sensitize cancer cells to ionizing radiation and hypoxia

DNA damaging agents, such as ionizing radiation (IR), have provided a basis for the treatment of solid tumors for decades, yet they provide only modest benefit for most patients. This reflects, in part, the efficient repair of DNA damage via a complex signaling and repair network known as the DNA damage response (DDR). The ATR, ATM and DNA-PK phosphoinositol 3-kinase-like serine/threonine protein kinases (PIKKs) are the key regulators of the DDR to IR-induced double-strand DNA breaks. This acts to detect the DNA lesions, enforce checkpoints to arrest cell cycle progression, and stimulate repair. Various components of the DDR have been shown to be commonly defective in cancer cells. These cells are widely hypothesized to become dependent on their remaining DDR pathways for survival from DNA damage. The tumor microenvironment has been reported to induce a DDR and in particular, ATR-mediated signaling in response to severe hypoxia.

While inhibitors of a number of DDR enzymes, including ATM, DNA-PK, Chk1 and PARP, have been reported, there have been no reports of drug-like ATR inhibitors. Using the first potent and selective ATR inhibitor (VE-821), we now characterize the effects of ATR inhibition on the cellular response to IR and hypoxia. VE-821 sensitizes many cancer cell lines to IR in both clonogenic and short-term (MTS) assays. For example, VE-821 reduced the surviving fraction of MiaPaca-2 cells treated with 6 Gy IR by 3-fold. Consistent with this, ATR inhibition ablated both RAD51 focus formation and phosphorylation of Chk1 (but not Chk2) after IR treatment, delayed the IR-induced G2/M arrest analyzed by flow cytometry and slowed down DNA replication rates measured using DNA fiber technology. Furthermore, we provide evidence that a basis for the sensitization of cancer cell lines to IR with VE-821 is a synthetic lethal interaction between loss of ATM signaling (a frequent event in cancer resulting from loss of function of proteins such as ATM or p53) and ATR inhibition when cells are treated with IR. In keeping with these results, normal cells (with functional ATM) do not appear sensitized to IR by ATR inhibition. In this case a compensatory DDR is activated, which in turn leads to checkpoint arrest and a strong survival response. Importantly, ATR inhibition also sensitized cancer cells to IR under hypoxic conditions, and to anoxia alone.

These studies show for the first time that a selective ATR inhibitor can preferentially sensitize cancer cells to ionizing radiation under both normoxic and hypoxic conditions and exploit a synthetic lethal interaction between ATM and ATR signaling. This underpins the broad potential of ATR inhibition as a highly promising new strategy to improve the efficacy of DNA damaging therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5501. doi:10.1158/1538-7445.AM2011-5501

Overexpression of POLQ confers a poor prognosis in early breast cancer patients

Depletion of POLQ (DNA polymerase theta) has recently been shown to render tumour cells more sensitive to radiotherapy whilst having little or no effect on normal tissues. This finding led us to investigate whether tumours that overexpress POLQ are associated with an adverse outcome. We therefore correlated the clinical outcomes of two retrospective series of patients with early breast cancer with the expression levels of POLQ, as determined by microarray gene expression analysis. We found that a significant number of tumours overexpressed POLQ and that overexpression was correlated with ER negative disease (p=0.047) and high tumour grade (p=0.004), both of which are associated with poor clinical outcomes. POLQ overexpression was associated with poor relapse free survival rates on both univariate (HR 5.80; 95% CI, 2.220 to 15.159; p<0.001) and multivariate analysis (HR 8.086; 95% CI 2.340 to 27.948 p=0.001). Analysis of other published clinical series confirmed that POLQ overexpression is associated with adverse clinical outcomes. The poor prognosis associated with POLQ is independent of other clinical or pathological features. The mechanism that causes this adverse outcome remains to be elucidated but may in part arise from resistance to adjuvant treatment. These findings, combined with the limited normal tissue expression of POLQ, make it a very appealing target for possible clinical exploitation.

A small interfering RNA screen of genes involved in DNA repair identifies tumor-specific radiosensitization by POLQ knockdown

The effectiveness of radiotherapy treatment could be significantly improved if tumor cells could be rendered more sensitive to ionizing radiation (IR) without altering the sensitivity of normal tissues. However, many of the key therapeutically exploitable mechanisms that determine intrinsic tumor radiosensitivity are largely unknown. We have conducted a small interfering RNA (siRNA) screen of 200 genes involved in DNA damage repair aimed at identifying genes whose knockdown increased tumor radiosensitivity. Parallel siRNA screens were conducted in irradiated and unirradiated tumor cells (SQ20B) and irradiated normal tissue cells (MRC5). Using gammaH2AX foci at 24 hours after IR, we identified several genes, such as BRCA2, Lig IV, and XRCC5, whose knockdown is known to cause increased cell radiosensitivity, thereby validating the primary screening end point. In addition, we identified POLQ (DNA polymerase ) as a potential tumor-specific target. Subsequent investigations showed that POLQ knockdown resulted in radiosensitization of a panel of tumor cell lines from different primary sites while having little or no effect on normal tissue cell lines. These findings raise the possibility that POLQ inhibition might be used clinically to cause tumor-specific radiosensitization.

Class I PI3 kinase inhibition by the pyridinylfuranopyrimidine inhibitor PI-103 enhances tumor radiosensitivity

Cell signaling initiated at the epidermal growth factor receptor (EGFR), RAS oncoproteins, or PI3K contributes to a common pathway that promotes tumor survival after radiation-induced DNA damage. Inhibition of signaling at the level of EGFR, RAS, and PI3K has been tested, but clinical applicability has been shown only at the level of the EGFR or by inhibiting RAS indirectly with prenyltransferase inhibitors. Inhibition of PI3K with LY294002 or wortmannin lacks specificity and has shown unacceptable toxicity in preclinical studies. We previously showed that inhibiting class I PI3K expression with siRNA resulted in enhanced radiation killing of tumor cells. Here, we tested the possibility of achieving specific tumor cell radiosensitization with a pharmacologic inhibitor of class I PI3K, the pyridinylfuranopyrimidine inhibitor PI-103. Our results show that inhibiting PI3K activity reduces phosphorylation of AKT at serine 473. Reduced survival is seen in cells with AKT activation and seems preferential for tumor cells over cells in which AKT activity is not elevated. Reduced survival is accompanied by persistence of DNA damage as evidenced by persistence of gamma H2AX and Rad 51 foci after irradiation in the presence of the inhibitor. Reduced survival does not result from cell cycle redistribution during the PI-103 treatment intervals tested, although combining PI-103 treatment with radiation enhances the G(2)-M delay observed after irradiation. These results indicate that pharmacologic inhibitors with enhanced specificity for class I PI3K may be of benefit when combined with radiotherapy.

CCL21 expression pattern of human secondary lymphoid organ stroma is conserved in inflammatory lesions with lymphoid neogenesis

CCL21 is a homeostatic lymphoid chemokine instrumental in the recruitment and organization of T cells and dendritic cells into lymphoid T areas. In human secondary lymphoid organs (SLOs), CCL21 is produced by cells distributed throughout the T zone, whereas high endothelial venules (HEVs) lack CCL21 mRNA. A critical question remains whether the development of ectopic lymphoid tissue (ELT) in chronic inflammation recapitulates the features of SLOs. Thus, we systematically investigated in situ the cellular sources of CCL21 in SLOs and ELTs in several human diseases characterized by lymphoid neogenesis. By in situ hybridization and the use of combinatorial cell markers, we show that CCL21-producing vessels in inflamed tissues systematically display typical markers of lymphatic vessels, whereas, as in SLOs, ectopic HEVs do not synthesize detectable levels of CCL21. We also provide first-time evidence that a common pattern of CCL21 expression by CD45-negative myofibroblast-like cells localized in extra-HEV position and organized in a fibroblastic reticular network similarly characterizes human SLOs and organized ELTs. Altogether, our results demonstrate that in humans the pattern of CCL21 production in SLOs is maintained during inflammation and that the phenotypic and functional properties of stromal cells, found in SLO T-cell areas, are reproduced at ectopic sites.

Inflammation-induced uptake and degradation of the lymphatic endothelial hyaluronan receptor LYVE-1

The hyaluronan receptor LYVE-1 is selectively expressed in the endothelium of lymphatic capillaries, where it has been proposed to function in hyaluronan clearance and hyaluronan-mediated leukocyte adhesion. However, recent studies suggest that hyaluronan homeostasis is unperturbed in LYVE-1(-/-) mice and that lymphatic adhesion/transmigration may be largely mediated by ICAM-1 and VCAM-1 rather than LYVE-1. Here we have explored the possibility that LYVE-1 functions during inflammation and report that the receptor is down-regulated by pro-inflammatory cytokines. Using cultured primary lymphatic endothelial cells, we show that surface expression of LYVE-1 is rapidly and reversibly lost after exposure to tumor necrosis factor-alpha (TNFalpha) and TNFbeta via internalization and degradation of the receptor in lysosomes, coupled with a shutdown in gene expression. Curiously, internalization does not result in significant uptake of hyaluronan, a process that is largely insensitive to the novel LYVE-1 adhesion blocking monoclonal antibody 3A, and proceeds almost equally in resting and inflammation-activated lymphatic endothelial cells. Finally, we show that TNF can induce down-modulation of LYVE-1 in ex vivo murine dermal tissue explants and present evidence that the process occurs in vivo, in the context of murine allergen-induced skin inflammation. These findings suggest that LYVE-1 can function independently of hyaluronan and have implications for the use of LYVE-1 as a histological marker for lymphangiogenesis in human pathology.

Normal lymphatic development and function in mice deficient for the lymphatic hyaluronan receptor LYVE-1

The hyaluronan receptor LYVE-1 is expressed abundantly on the surfaces of lymphatic vessels and lymph node sinus endothelial cells from early development, where it has been suggested to function both in cell adhesion/transmigration and as a scavenger for hyaluronan turnover. To investigate the physiological role(s) of LYVE-1, we generated mice in which the gene for the receptor was inactivated by replacement with a beta-galactosidase reporter. LYVE-1(-/-) mice displayed an apparently normal phenotype, with no obvious alteration in lymphatic vessel ultrastructure or function and no apparent change in secondary lymphoid tissue structure or cellularity. In addition, the levels of hyaluronan in tissue and blood were unchanged. LYVE-1(-/-) mice also displayed normal trafficking of cutaneous CD11c(+) dendritic cells to draining lymph nodes via afferent lymphatics and normal resolution of oxazolone-induced skin inflammation. Finally, LYVE-1(-/-) mice supported normal growth of transplanted B16F10 melanomas and Lewis lung carcinomas. These results indicate that LYVE-1 is not obligatory for normal lymphatic development and function and suggest either the existence of compensatory receptors or a role more specific than that previously envisaged.

A quantitative analysis of lymphatic vessels in human breast cancer, based on LYVE-1 immunoreactivity

This study was undertaken to determine the highly sensitive method for detecting tumour lymphatic vessels in all the fields of each slide (LV), lymphatic microvessel density (LMVD) and lymphatic vessel invasion (LVI) and to compare them with other prognostic parameters using immunohistochemical staining with polyclonal (PCAB) and monoclonal antibodies (MCAB) to the lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1), and the pan-endothelial marker factorVIII in a series of 67 human breast cancers. In all LYVE-1-stained sections, LV (some of which contained red blood cells) were frequently found localised in extralobular stroma, dermis, connective tissue stroma and adjacent to artery and vein, but were rare within the intralobular stroma or the tumour body (3/67 cases) or areas of widespread invasion. In contrast small blood vessels were observed in intra- and extralobular stroma in the factor VIII-stained sections. Quantitation of vessel numbers revealed that LYVE-1/PCAB detected a significantly larger number of LV than either H&E or LYVE-1/MCAB (P<0.0001). LYVE-1/PCAB detected LVI in 25/67 cases (37.3%) and their presence was significantly associated with both lymph node metastasis (χ²=4.698, P=0.0248) and unfavourable overall survival (OS) (P=0.0453), while not relapse- free survival (RFS) (P=0.2948). LMVD had no influence for RFS and OS (P=0.4879, P=0.1463, respectively). Our study demonstrates that immunohistochemistry with LYVE-1/PCAB is a highly sensitive method for detecting tumour LV/LVI in breast cancer and LVI is a useful prognostic indicator for lymphatic tumour dissemination.

Rapid plasma membrane-endosomal trafficking of the lymph node sinus and high endothelial venule scavenger receptor/homing receptor stabilin-1 (FEEL-1/CLEVER-1)

The sinusoidal endothelia of liver, spleen, and lymph node are major sites for uptake and recycling of waste macromolecules through promiscuous binding to a disparate family of scavenger receptors. Among the most complex is stabilin-1, a large multidomain protein containing tandem fasciclin domains, epidermal growth factor-like repeats, and a C-type lectin-like hyaluronan-binding Link module, which functions as an endocytic receptor for acetylated low density lipoprotein and advanced glycation end products. Intriguingly, stabilin-1 has also been reported to mediate both homing of leukocytes across lymph node high endothelial venules and adhesion of metastatic tumor cells to peritumoral lymphatic vessels. Currently, however, it is not clear how stabilin-1 mediates these distinct functions. To address the issue, we have investigated the tissue and subcellular localization of stabilin-1 in detail and assessed the functional status of its Link module. We show that stabilin-1 is almost entirely intracellular in lymph node high endothelial venules, lymphatic sinus endothelium, and cultured endothelial cells but that a finite population, detectable only by fluorescent antibody or fluorescein-labeled (Fl)-acetylated low density lipoprotein uptake, cycles rapidly between the plasma membrane and EEA-1+ve (early endosome antigen 1) early endosomes. In addition, we show using full-length stabilin-1 cDNA and a stabilin-1/CD44 chimera in HeLa cells that intracellular targeting is influenced by the transmembrane domain/cytoplasmic tail, which contains a putative dileucine (DXXLL) Golgi to endosomal sorting signal. Finally, we provide evidence that the stabilin-1 Link domain binds neither hyaluronan nor other glycosaminoglycans. These properties support a role for stabilin-1 as a rapidly recycling scavenger receptor and argue against a role in cell adhesion or lymphocyte homing.

Dynamic populations of dendritic cell-specific ICAM-3 grabbing nonintegrin-positive immature dendritic cells and liver/lymph node-specific ICAM-3 grabbing nonintegrin-positive endothelial cells in the outer zones of the paracortex of human lymph nodes

In the paracortex of lymph nodes, cellular immune responses are generated against antigens captured in peripheral tissues by dendritic cells (DCs). DC-SIGN (dendritic cell-specific ICAM-3 grabbing nonintegrin), a C-type lectin exclusively expressed by DCs, functions as an antigen receptor as well as an adhesion receptor. A functional homologue of DC-SIGN, L-SIGN (liver/lymph node-SIGN, also called DC-SIGN-related), is expressed by liver sinus endothelial cells. In lymph nodes, both DC-SIGN and L-SIGN are expressed. In this study, we analyzed the distribution of these two SIGN molecules in detail in both normal and immunoreactive lymph nodes. DC-SIGN is expressed by mature DCs in paracortical areas and in addition by DCs with an immature phenotype in the outer zones of the paracortex. L-SIGN expression was also detected in the outer zones on sinus endothelial cells characterized by their expression of the lymphatic endothelial markers LYVE-1 and CLEVER-1. During both cellular and humoral immune responses changes in the amount of DC-SIGN+ immature and mature DCs and L-SIGN+ endothelial cells were observed, indicating that the influx or proliferation of these cells is dynamically regulated.

Distribution of lymphatic vessels in normal and arthritic human synovial tissues

OBJECTIVES

To investigate the distribution of lymphatic vessels in normal, rheumatoid arthritis (RA) and osteoarthritis (OA) synovium.

METHODS

Synovial tissues from 5 normal controls, 14 patients with RA, and 16 patients with OA were studied. Lymphatic vessels were identified by immunohistochemistry using antibodies directed against the lymphatic endothelial hyaluronan receptor (LYVE-1) and recognised blood vessel endothelial markers (factor VIII, CD34, CD31).

RESULTS

Lymphatic vessels were found in all zones of the normal, OA, and RA synovial membrane. Few lymphatic vessels were seen in the sublining zone in normal and OA synovium which did not show villous hypertrophy. However, in both RA synovium and OA synovium showing villous hypertrophy and a chronic inflammatory cell infiltrate, numerous lymphatic vessels were seen in all zones of the synovial membrane, including the sublining zone of the superficial subintima.

CONCLUSIONS

Lymphatic vessels are present in normal and arthritic synovial tissues and are more numerous and prominent where there is oedema and an increase in inflammatory cells in the subintima, particularly in RA. This may reflect increased transport of hyaluronan and leucocyte trafficking in inflamed synovial tissues.

Absence of lymphangiogenesis and intratumoural lymph vessels in human metastatic breast cancer

Early metastasis to lymph nodes is a frequent complication in human breast cancer. However, the extent to which this depends on lymphangiogenesis or on invasion of existing lymph vessels remains controversial. Although proliferating intratumoural lymphatics that promote nodal metastasis have been demonstrated in experimental breast tumours overexpressing VEGF-C, it has yet to be determined whether the same phenomena occur in spontaneous human breast cancers. To address this important issue, the present study investigated the lymphatics in primary human breast carcinoma (75 cases of invasive ductal and lobular breast cancer) by quantitative immunohistochemical staining for the lymphatic endothelial hyaluronan receptor LYVE-1, the blood vascular marker CD34, and the nuclear proliferation marker pKi67. None of the breast carcinomas was found to contain dividing lymph vessels, even in areas of active haemangiogenesis. Furthermore, the majority of non-dividing lymph vessels were confined to the tumour periphery where their incidence was low and unrelated to tumour size, grade or nodal status; rather, their density was inversely correlated with tumour aggressiveness as assessed by macrophage density (p = 0.009), and blood microvessel density (p = 0.05, Spearman Rank), as well as with distance from the tumour edge. Finally, a proportion of the peritumoural lymphatics contained tumour emboli associated with hyaluronan, indicating a possible role for LYVE-1/hyaluronan interactions in lymphatic invasion or metastasis. These results suggest that naturally occurring breast carcinomas invade and destroy lymph vessels rather than promoting their proliferation; that breast tumour lymphangiogenesis may not always occur at physiological VEGF-C levels; and that nodal metastasis can proceed via pre-existing lymphatics.

Intratumoral lymphangiogenesis and lymph node metastasis in head and neck cancer

How tumors access and spread via the lymphatics is not understood. Although it is clear that dissemination via the blood system involves hemangiogenesis, it is uncertain whether tumors also induce lymphangiogenesis or simply invade existing peritumoral vessels. To address the issue we quantitated tumor lymph vessels in archival specimens of head and neck cancer by immunostaining for the recently described lymphatic endothelial marker LYVE-1, the vascular endothelial marker CD34, and the pKi67 proliferation marker, correlating lymph vessel density and proliferation index with clinical and pathological variables. Discrete "hotspots" of intratumoral small proliferating lymphatics were observed in all carcinomas, and a high intratumoral lymph vessel density was associated with neck node metastases (n = 23; P = 0.027) and an infiltrating margin of tumor invasion (P = 0.046) in the oropharyngeal subgroup. Quantitation of the lymphangiogenic growth factor vascular endothelial growth factor C by real-time PCR and immunohistochemistry revealed higher levels of mRNA in tumor tissue than in normal samples (n = 8; P = 0.017), but no obvious correlation with intratumoral lymphatics. Our results provide new evidence that proliferating lymphatics can occur in human cancers and may in some cases contribute to lymph node metastasis.

LYVE-1, the lymphatic system and tumor lymphangiogenesis

Previous research into hyaluronan (HA) has focused on the role of this abundant tissue glycosaminoglycan in promoting cell migration through interactions with its transmembrane receptor CD44 on inflammatory leukocytes and tumor cells. The recent discovery of a new HA receptor, LYVE-1 (lymphatic vessel endothelial HA receptor), expressed predominantly in lymphatic vessels, highlights another aspect of HA biology: its continuous transit through the lymphatic system and its potential involvement in lymph node homing by CD44+ leukocytes and tumor cells. The functional role of LYVE-1 in lymphatic vessels and its application as a marker to study tumor lymphangiogenesis are important areas of investigation.

Mouse LYVE-1 is an endocytic receptor for hyaluronan in lymphatic endothelium

The glycosaminoglycan hyaluronan is a key substrate for cell migration in tissues during inflammation, wound healing, and neoplasia. Unlike other matrix components, hyaluronan (HA) is turned over rapidly, yet most degradation occurs not locally but within distant lymph nodes, through mechanisms that are not yet understood. While it is not clear which receptors are involved in binding and uptake of hyaluronan within the lymphatics, one likely candidate is the lymphatic endothelial hyaluronan receptor LYVE-1 recently described in our laboratory (Banerji, S., Ni, J., Wang, S., Clasper, S., Su, J., Tammi, R., Jones, M., and Jackson, D.G. (1999) J. Cell Biol. 144, 789-801). Here we present evidence that LYVE-1 is involved in the uptake of hyaluronan by lymphatic endothelial cells using a new murine LYVE-1 orthologue identified from the EST data base. We show that mouse LYVE-1 both binds and internalizes hyaluronan in transfected 293T fibroblasts in vitro and demonstrate using immunoelectron microscopy that it is distributed equally among the luminal and abluminal surfaces of lymphatic vessels in vivo. In addition, we show by means of specific antisera that expression of mouse LYVE-1 remains restricted to the lymphatics in homozygous knockout mice lacking a functional gene for CD44, the closest homologue of LYVE-1 and the only other Link superfamily HA receptor known to date. Together these results suggest a role for LYVE-1 in the transport of HA from tissue to lymph and imply that further novel hyaluronan receptors must exist that can compensate for the loss of CD44 function.

Adenoviral expression of vascular endothelial growth factor-C induces lymphangiogenesis in the skin

The growth of blood and lymphatic vasculature is mediated in part by secreted polypeptides of the vascular endothelial growth factor (VEGF) family. The prototype VEGF binds VEGF receptor (VEGFR)-1 and VEGFR-2 and is angiogenic, whereas VEGF-C, which binds to VEGFR-2 and VEGFR-3, is either angiogenic or lymphangiogenic in different assays. We used an adenoviral gene transfer approach to compare the effects of these growth factors in adult mice. Recombinant adenoviruses encoding human VEGF-C or VEGF were injected subcutaneously into C57Bl6 mice or into the ears of nude mice. Immunohistochemical analysis showed that VEGF-C upregulated VEGFR-2 and VEGFR-3 expression and VEGF upregulated VEGFR-2 expression at 4 days after injection. After 2 weeks, histochemical and immunohistochemical analysis, including staining for the lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1), the vascular endothelial marker platelet-endothelial cell adhesion molecule-1 (PECAM-1), and the proliferating cell nuclear antigen (PCNA) revealed that VEGF-C induced mainly lymphangiogenesis in contrast to VEGF, which induced only angiogenesis. These results have significant implications in the planning of gene therapy using these growth factors.

Vascular endothelial growth factor-C-mediated lymphangiogenesis promotes tumour metastasis

Metastasis is a frequent and lethal complication of cancer. Vascular endothelial growth factor-C (VEGF-C) is a recently described lymphangiogenic factor. Increased expression of VEGF-C in primary tumours correlates with dissemination of tumour cells to regional lymph nodes. However, a direct role for VEGF-C in tumour lymphangiogenesis and subsequent metastasis has yet to be demonstrated. Here we report the establishment of transgenic mice in which VEGF-C expression, driven by the rat insulin promoter (Rip), is targeted to beta-cells of the endocrine pancreas. In contrast to wild-type mice, which lack peri-insular lymphatics, RipVEGF-C transgenics develop an extensive network of lymphatics around the islets of Langerhans. These mice were crossed with Rip1Tag2 mice, which develop pancreatic beta-cell tumours that are neither lymphangiogenic nor metastatic. Double-transgenic mice formed tumours surrounded by well developed lymphatics, which frequently contained tumour cell masses of beta-cell origin. These mice frequently developed pancreatic lymph node metastases. Our findings demonstrate that VEGF-C-induced lymphangiogenesis mediates tumour cell dissemination and the formation of lymph node metastases.

VEGF-D promotes the metastatic spread of tumor cells via the lymphatics

Metastasis to local lymph nodes via the lymphatic vessels is a common step in the spread of solid tumors. To investigate the molecular mechanisms underlying the spread of cancer by the lymphatics, we examined the ability of vascular endothelial growth factor (VEGF)-D, a ligand for the lymphatic growth factor receptor VEGFR-3/Flt-4, to induce formation of lymphatics in a mouse tumor model. Staining with markers specific for lymphatic endothelium demonstrated that VEGF-D induced the formation of lymphatics within tumors. Moreover, expression of VEGF-D in tumor cells led to spread of the tumor to lymph nodes, whereas expression of VEGF, an angiogenic growth factor which activates VEGFR-2 but not VEGFR-3, did not. VEGF-D also promoted tumor angiogenesis and growth. Lymphatic spread induced by VEGF-D could be blocked with an antibody specific for VEGF-D. This study demonstrates that lymphatics can be established in solid tumors and implicates VEGF family members in determining the route of metastatic spread.

Induction of tumor lymphangiogenesis by VEGF-C promotes breast cancer metastasis

Metastasis of breast cancer occurs primarily through the lymphatic system, and the extent of lymph node involvement is a key prognostic factor for the disease. Whereas the significance of angiogenesis for tumor progression has been well documented, the ability of tumor cells to induce the growth of lymphatic vessels (lymphangiogenesis) and the presence of intratumoral lymphatic vessels have been controversial. Using a novel marker for lymphatic endothelium, LYVE-1, we demonstrate here the occurrence of intratumoral lymphangiogenesis within human breast cancers after orthotopic transplantation onto nude mice. Vascular endothelial growth factor (VEGF)-C overexpression in breast cancer cells potently increased intratumoral lymphangiogenesis, resulting in significantly enhanced metastasis to regional lymph nodes and to lungs. The degree of tumor lymphangiogenesis was highly correlated with the extent of lymph node and lung metastases. These results establish the occurrence and biological significance of intratumoral lymphangiogenesis in breast cancer and identify VEGF-C as a molecular link between tumor lymphangiogenesis and metastasis.