Mechanisms of the induction of apoptosis mediated by radiation-induced cytokine release

Radioprotection and Radiomitigation: From the Bench to Clinical Practice

The development of protective agents against harmful radiations has been a subject of investigation for decades. However, effective (ideal) radioprotectors and radiomitigators remain an unsolved problem. Because ionizing radiation-induced cellular damage is primarily attributed to free radicals, radical scavengers are promising as potential radioprotectors. Early development of such agents focused on thiol synthetic compounds, e.g., amifostine (2-(3-aminopropylamino) ethylsulfanylphosphonic acid), approved as a radioprotector by the Food and Drug Administration (FDA, USA) but for limited clinical indications and not for nonclinical uses. To date, no new chemical entity has been approved by the FDA as a radiation countermeasure for acute radiation syndrome (ARS). All FDA-approved radiation countermeasures (filgrastim, a recombinant DNA form of the naturally occurring granulocyte colony-stimulating factor, G-CSF; pegfilgrastim, a PEGylated form of the recombinant human G-CSF; sargramostim, a recombinant granulocyte macrophage colony-stimulating factor, GM-CSF) are classified as radiomitigators. No radioprotector that can be administered prior to exposure has been approved for ARS. This differentiates radioprotectors (reduce direct damage caused by radiation) and radiomitigators (minimize toxicity even after radiation has been delivered). Molecules under development with the aim of reaching clinical practice and other nonclinical applications are discussed. Assays to evaluate the biological effects of ionizing radiations are also analyzed.

Changes in gene expression as one of the key mechanisms involved in radiation-induced bystander effect

The radiation-induced bystander effect (RIBE) refers to the manifestation of responses by non-targeted/non-hit cells or tissues situated in proximity to cells and tissues directly exposed to ionizing radiation (IR). The RIBE is elicited by agents and factors released by IR-hit cells. The growing body of data suggests that the underlying mechanisms of the RIBE are multifaceted depending both on the biological (characteristics of directly IR-exposed cells, bystander cells, intercellular milieu) and the physical (dose, rate and type of IR, time after exposure) factors/parameters. Although the exact identity of bystander signal(s) is yet to be identified, the published data indicate changes in gene expression for multiple types of RNA (mRNA, microRNA, mitochondrial RNA, long non-coding RNA, small nucleolar RNA) as being one of the major responses of cells and tissues in the context of the RIBE. Gene expression profiles demonstrate a high degree of variability between distinct bystander cell and tissue types. These alterations could independently, or in a signaling cascade, result in the manifestation of readily observable endpoints, including changes in viability and genomic instability. Here, the relevant publications on the gene candidates and signaling pathways involved in the RIBE are reviewed, and a framework for future studies, both in vitro and in vivo, on the genetic aspect of the RIBE is provided.

A Co-culture Method to Investigate the Crosstalk Between X-ray Irradiated Caco-2 Cells and PBMC

The protocol adopted in this work aims at unraveling how X-rays perturb the functioning of the intestinal barrier, focusing on the interplay between colorectal tumor cells and the immune system. Colorectal carcinoma is among the most common type of cancer, typically treated by surgery, chemotherapy, and radiotherapy. Advantages of radiotherapy in targeting the tumor are well known. However, even limited exposures of healthy tissues are of great concern, particularly regarding the effects on the intestinal barrier and the immune system. The adopted setup allows to study the interplay between two cell populations in a condition more similar to the physiological one, when compared to normal cell cultures. For this purpose, we resort to different techniques and we used an in vitro co-culture model, based on Caco-2 cells differentiated as a monolayer and PBMC, sharing the same culture medium. This protocol has been developed to focus on both macroscopic effects, i.e. cell viability and Trans-Epithelial Electrical Resistance (TEER), and, through western blot, molecular alterations, i.e. the activation of inflammatory pathway in immune cells and the tight junction protein expression in Caco-2 cells. Initial evaluation of radiation effects on Caco-2 cell viability was assessed via the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and Trypan blue assays, while TEER was measured at fixed time intervals through an ohmmeter specifically designed for co-culture systems. In this way, the effects due to radiation, the presence of Peripheral Blood Mononuclear Cells (PBMC), and eventually their synergistic effect, can be demonstrated. Through these complementary techniques, we observed a high radio-resistance of Caco-2 within the range of 2 - 10 Gy of X-rays and an increased Caco-2 monolayer permeability when PBMCs were added. In particular, PBMC presence was found to be associated with the variation in the tight junction scaffold proteins expression.

Prolonged exposure of resveratrol induces reactive superoxide species-independent apoptosis in murine prostate cells

Nitric oxide, a signaling molecule, inhibits mitochondrial respiration by binding with cytochrome c oxidase, resulting in elevated production of reactive superoxide species (reactive oxygen and nitrogen) in the mitochondria and increased susceptibility to cell death. Generation of mitochondrial superoxide species can be suppressed by natural compounds such as resveratrol, a dietary polyphenol found in the skin of red fruits. In various cancer cells, resveratrol shows anti-oxidant and cancer preventive properties. Since, the effect of resveratrol on reactive superoxide species-independent apoptosis in prostate cancer cells is not well illustrated; therefore, we investigated this phenomenon in TRAMP murine prostate cancer cells. To accomplish this, TRAMP cells were incubated with resveratrol, resveratrol + DETA-NONOate, DETA-NONOate (nitric oxide donor), resveratrol + L-NMMA, or L-NMMA (nitric oxide inhibitor) for 48 h, and reactive superoxide species in the mitochondria and culture supernatant were measured. In addition, the mitochondrial membrane potential, cell viability, expression of apoptotic markers (Bax and Bcl2), γ-H2A.x, p53, and caspase-3 was determined. We found that resveratrol suppressed reactive superoxide species such as reactive oxygen species in the mitochondria and nitric oxide in culture supernatant when compared to the DETA-NONOate treatment and disrupted the mitochondrial membrane potential. Resveratrol also reduced cell viability, altered the expression of apoptotic markers (Bax and Bcl2), and increased expression of γ-H2A.x (indicative marker of DNA fragmentation) and p53 (a critical DNA damage response protein). However, there was no appreciable modulation of the caspase-3. Therefore, our data suggest that resveratrol induces superoxide species-independent apoptosis and may act as a therapeutic agent against prostate cancer.

The Interplay between Radioresistant Caco-2 Cells and the Immune System Increases Epithelial Layer Permeability and Alters Signaling Protein Spectrum

Colorectal cancer is one of the most frequent type of cancer, with a higher incidence in the developed countries. Colorectal cancer is usually managed with both surgeries, chemotherapy and radiotherapy. Radiotherapy has the well-known advantage of targeting the tumor, minimizing normal tissue exposure. Nevertheless, during radiation treatment, exposure of healthy tissues is of great concern, in particular because of the effects on the intestinal barrier functions and on cells belonging to the immune system. The functional role of intestinal barrier in avoiding paracellular trafficking and controlling bacterial spread from gut it is well known and it is due to the presence of tight junction complexes. However, intestinal barrier is fundamental in participating to the interplay with immune system, especially considering the gut-associated lymphoid tissue. Until few years ago, radiotherapy was considered to bear only a depressive action on the immune system. However, it is now recognized that the release of pro-inflammatory signals and phenotypic changes in tumoral cells due to ionizing radiation could trigger the immune system against the tumor. In this work, we address how intestinal barrier functions are perturbed by X-ray doses in the range 0–10 Gy, focusing on the interplay between tumoral cells and the immune system. To this aim, we adopted a coculture model in which Caco-2 cells can be grown in presence/absence of peripheral blood mononuclear cells (PBMC). We focused our attention on changes in the proliferation, trans-epithelial electrical resistance (TEER), cytokine release, and proteins of the junctional complexes. Our results indicate a high radioresistance of Caco-2 in the investigated dose range, and an increased permeability of the tumoral cell layer due to the presence of PBMC. This is found to be correlated with activation of PBMC, inhibiting the apoptotic pathway, with the enhancement of cytokine release and with variation of tight junction scaffold protein expression levels, assumed to be related to IFN-γ- and TNF-α-mediated signaling.

Key mechanisms involved in ionizing radiation-induced systemic effects. A current review

Organisms respond to physical, chemical and biological threats by a potent inflammatory response, aimed at preserving tissue integrity and restoring tissue homeostasis and function. Systemic effects in an organism refer to an effect or phenomenon which originates at a specific point and can spread throughout the body affecting a group of organs or tissues. Ionizing radiation (IR)-induced systemic effects arise usually from a local exposure of an organ or part of the body. This stress induces a variety of responses in the irradiated cells/tissues, initiated by the DNA damage response and DNA repair (DDR/R), apoptosis or immune response, including inflammation. Activation of this IR-response (IRR) system, especially at the organism level, consists of several subsystems and exerts a variety of targeted and non-targeted effects. Based on the above, we believe that in order to understand this complex response system better one should follow a 'holistic' approach including all possible mechanisms and at all organization levels. In this review, we describe the current status of knowledge on the topic, as well as the key molecules and main mechanisms involved in the 'spreading' of the message throughout the body or cells. Last but not least, we discuss the danger-signal mediated systemic immune effects of radiotherapy for the clinical setup.