Cell death mechanisms in head and neck cancer cells in response to low and high-LET radiation

Autophagy is the main driver of radioresistance of HNSCC cells in mild hypoxia

Hypoxia poses a significant challenge to the effectiveness of radiotherapy in head and neck squamous cell carcinoma (HNSCC) patients, and it is imperative to discover novel approaches to overcome this. In this study, we investigated the underlying mechanisms contributing to x-ray radioresistance in HPV-negative HNSCC cells under mild hypoxic conditions (1% oxygen) and explored the potential for autophagy modulation as a promising therapeutic strategy. Our findings show that HNSCC cells exposed to mild hypoxic conditions exhibit increased radioresistance, which is largely mediated by the hypoxia-inducible factor (HIF) pathway. We demonstrate that siRNA knockdown of HIF-1α and HIF-1β leads to increased radiosensitivity in HNSCC cells under hypoxia. Hypoxia-induced radioresistance was not attributed to differences in DNA double strand break repair kinetics, as these remain largely unchanged under normoxic and hypoxic conditions. Rather, we identify autophagy as a critical protective mechanism in HNSCC cells following irradiation under mild hypoxia conditions. Targeting key autophagy genes, such as BECLIN1 and BNIP3/3L, using siRNA sensitizes these cells to irradiation. Whilst autophagy's role in hypoxic radioresistance remains controversial, this study highlights the importance of autophagy modulation as a potential therapeutic approach to enhance the effectiveness of radiotherapy in HNSCC.

The role of autophagy in hypoxia-induced radioresistance

Radiotherapy is a widely used treatment modality against cancer, and although survival rates are increasing, radioresistant properties of tumours remain a significant barrier for curative treatment. Tumour hypoxia is one of the main contributors to radioresistance and is common in most solid tumours. Hypoxia is responsible for many molecular changes within the cell which helps tumours to survive under such challenging conditions. These hypoxia-induced molecular changes are predominantly coordinated by the hypoxia inducible factor (HIF) and have been linked with the ability to confer resistance to radiation-induced cell death. To overcome this obstacle research has been directed towards autophagy, a cellular process involved in self degradation and recycling of macromolecules, as HIF plays a large role in its coordination under hypoxic conditions. The role that autophagy has following radiotherapy treatment is conflicted with evidence of both cytoprotective and cytotoxic effects. This literature review aims to explore the intricate relationship between radiotherapy, hypoxia, and autophagy in the context of cancer treatment. It provides valuable insights into the potential of targeting autophagy as a therapeutic strategy to improve the response of hypoxic tumours to radiotherapy.

Gold-Coated Superparamagnetic Iron Oxide Nanoparticles Functionalized to EGF and Ce6 Complexes for Breast Cancer Diagnoses and Therapy

Superparamagnetic iron oxide nanoparticles (SPIONs) have some limitations in the physiological environment, however, a modification on their surface, such as a core-shell structure with gold (SPIONs@Au), can enhance their applicability. In this study, SPIONs were synthesized by the chemical coprecipitation method, stabilized by sodium citrate, and followed by the gold-coating process. SPIONs@Au were functionalized with EGF-α-lipoic acid and chlorin e6 (Ce6)-cysteamine complexes, composing a Theranostic Nanoprobe (TP). The outcomes showed that the SPIONs@Au had changed in color to red and had an absorption band centered at 530 nm. The coating was verified in the TEM micrographs in bright and dark fields by EDS mapping, which indicated the presence of Au and Fe. The Ce6-cysteamine complex had a resonant band at 670 nm that enabled the diagnosis of biological samples using fluorescence analysis. In the measure of TNBC cell uptake, the maximum value of TP fluorescence intensity was obtained within 4 h of internalization. At 2 h, the incorporation of the TP in the cytoplasm as well as in the nuclei was observed, suggesting that it could be employed as a diagnostic marker. The PTT results showed significant percentages of apoptosis in the TNBC cell line, which confirms the efficacy of the TP.

Effectiveness of PARP inhibition in enhancing the radiosensitivity of 3D spheroids of head and neck squamous cell carcinoma

A critical risk factor for head and neck squamous cell carcinoma (HNSCC), particularly of the oropharynx, and the response to radiotherapy is human papillomavirus (HPV) type-16/18 infection. Specifically, HPV-positive HNSCC display increased radiosensitivity and improved outcomes, which has been linked with defective signalling and repair of DNA double-strand breaks (DSBs). This differential response to radiotherapy has been recapitulated in vitro using cell lines, although studies utilising appropriate 3D models that are more reflective of the original tumour are scarce. Furthermore, strategies to enhance the sensitivity of relatively radioresistant HPV-negative HNSCC to radiotherapy are still required. We have analysed the comparative response of in vitro 3D spheroid models of oropharyngeal squamous cell carcinoma to x-ray (photon) irradiation and provide further evidence that HPV-positive cells, in this case now grown as spheroids, show greater inherent radiosensitivity compared to HPV-negative spheroids due to defective DSB repair. We subsequently analysed these and an expanded number of spheroid models, with a particular focus on relatively radioresistant HPV-negative HNSCC, for impact of poly(ADP-ribose) polymerase (PARP) inhibitors (olaparib and talazoparib) in significantly inhibiting spheroid growth in response to photons but also proton beam therapy. We demonstrate that in general, PARP inhibition can further radiosensitise particularly HPV-negative HNSCC spheroids to photons and protons leading to significant growth suppression. The degree of enhanced radiosensitivity was observed to be dependent on the model and on the tumour site (oropharynx, larynx, salivary gland, or hypopharynx) from which the cells were derived. We also provide evidence suggesting that PARP inhibitor effectiveness relates to homologous recombination repair proficiency. Interestingly though, we observed significantly enhanced effectiveness of talazoparib versus olaparib specifically in response to proton irradiation. Nevertheless, our data generally support that PARP inhibition in combination with radiotherapy (photons and protons) should be considered further as an effective treatment for HNSCC, particularly for relatively radioresistant HPV-negative tumours.

Chromatin and the Cellular Response to Particle Radiation-Induced Oxidative and Clustered DNA Damage

Exposure to environmental ionizing radiation is prevalent, with greatest lifetime doses typically from high Linear Energy Transfer (high-LET) alpha particles via the radioactive decay of radon gas in indoor air. Particle radiation is highly genotoxic, inducing DNA damage including oxidative base lesions and DNA double strand breaks. Due to the ionization density of high-LET radiation, the consequent damage is highly clustered wherein ≥2 distinct DNA lesions occur within 1–2 helical turns of one another. These multiply-damaged sites are difficult for eukaryotic cells to resolve either quickly or accurately, resulting in the persistence of DNA damage and/or the accumulation of mutations at a greater rate per absorbed dose, relative to lower LET radiation types. The proximity of the same and different types of DNA lesions to one another is challenging for DNA repair processes, with diverse pathways often confounding or interplaying with one another in complex ways. In this context, understanding the state of the higher order chromatin compaction and arrangements is essential, as it influences the density of damage produced by high-LET radiation and regulates the recruitment and activity of DNA repair factors. This review will summarize the latest research exploring the processes by which clustered DNA damage sites are induced, detected, and repaired in the context of chromatin.

Key biological mechanisms involved in high-LET radiation therapies with a focus on DNA damage and repair

DNA damage and repair studies are at the core of the radiation biology field and represent also the fundamental principles informing radiation therapy (RT). DNA damage levels are a function of radiation dose, whereas the type of damage and biological effects such as DNA damage complexity, depend on radiation quality that is linear energy transfer (LET). Both levels and types of DNA damage determine cell fate, which can include necrosis, apoptosis, senescence or autophagy. Herein, we present an overview of current RT modalities in the light of DNA damage and repair with emphasis on medium to high-LET radiation. Proton radiation is discussed along with its new adaptation of FLASH RT. RT based on α-particles includes brachytherapy and nuclear-RT, that is proton-boron capture therapy (PBCT) and boron-neutron capture therapy (BNCT). We also discuss carbon ion therapy along with combinatorial immune-based therapies and high-LET RT. For each RT modality, we summarise relevant DNA damage studies. Finally, we provide an update of the role of DNA repair in high-LET RT and we explore the biological responses triggered by differential LET and dose.