Modeling DNA double-strand break repair kinetics as an epiregulated cell-community-wide (epicellcom) response to radiation stress

The LNT model for cancer induction is not supported by radiobiological data

The hallmarks of cancer have been the focus of much research and have influenced the development of risk models for radiation-induced cancer. However, natural defenses against cancer, which constitute the hallmarks of cancer prevention, have largely been neglected in developing cancer risk models. These natural defenses are enhanced by low doses and dose rates of ionizing radiation, which has aided in the continuation of human life over many generations. Our natural defenses operate at the molecular, cellular, tissue, and whole-body levels and include epigenetically regulated (epiregulated) DNA damage repair and antioxidant production, selective p53-independent apoptosis of aberrant cells (e.g. neoplastically transformed and tumor cells), suppression of cancer-promoting inflammation, and anticancer immunity (both innate and adaptive components). This publication reviews the scientific bases for the indicated cancer-preventing natural defenses and evaluates their implication for assessing cancer risk after exposure to low radiation doses and dose rates. Based on the extensive radiobiological evidence reviewed, it is concluded that the linear-no-threshold (LNT) model (which ignores natural defenses against cancer), as it relates to cancer risk from ionizing radiation, is highly implausible. Plausible models include dose-threshold and hormetic models. More research is needed to establish when a given model (threshold, hormetic, or other) applies to a given low-dose-radiation exposure scenario.

Radiation-hormesis phenotypes, the related mechanisms and implications for disease prevention and therapy

Humans are continuously exposed to ionizing radiation throughout life from natural sources that include cosmic, solar, and terrestrial. Much harsher natural radiation and chemical environments existed during our planet's early years. Mammals survived the harsher environments via evolutionarily-conserved gifts ̶ a continuously evolving system of stress-induced natural protective measures (i.e., activated natural protection [ANP]). The current protective system is differentially activated by stochastic (i.e., variable) low-radiation-dose thresholds and when optimally activated in mammals includes antioxidants, DNA damage repair, p53-related apoptosis of severely-damaged cells, reactive-oxygen-species (ROS)/reactive-nitrogen-species (RNS)- and cytokine-regulated auxiliary apoptosis that selectively removes aberrant cells (e.g., precancerous cells), suppression of disease promoting inflammation, and immunity against cancer cells. The intercellular-signaling-based protective system is regulated at least in part via epigenetic reprogramming of adaptive-response genes. When the system is optimally activated, it protects against cancer and some other diseases, thereby leading to hormetic phenotypes (e.g., reduced disease incidence to below the baseline level; reduced pain from inflammation-related problems). Here, some expressed radiation hormesis phenotypes and related mechanisms are discussed along with their implications for disease prevention and therapy.

Dynamics between cancer cell subpopulations reveals a model coordinating with both hierarchical and stochastic concepts

Tumors are often heterogeneous in which tumor cells of different phenotypes have distinct properties. For scientific and clinical interests, it is of fundamental importance to understand their properties and the dynamic variations among different phenotypes, specifically under radio- and/or chemo-therapy. Currently there are two controversial models describing tumor heterogeneity, the cancer stem cell (CSC) model and the stochastic model. To clarify the controversy, we measured probabilities of different division types and transitions of cells via in situ immunofluorescence. Based on the experiment data, we constructed a model that combines the CSC with the stochastic concepts, showing the existence of both distinctive CSC subpopulations and the stochastic transitions from NSCCs to CSCs. The results showed that the dynamic variations between CSCs and non-stem cancer cells (NSCCs) can be simulated with the model. Further studies also showed that the model can be used to describe the dynamics of the two subpopulations after radiation treatment. More importantly, analysis demonstrated that the experimental detectable equilibrium CSC proportion can be achieved only when the stochastic transitions from NSCCs to CSCs occur, indicating that tumor heterogeneity may exist in a model coordinating with both the CSC and the stochastic concepts. The mathematic model based on experimental parameters may contribute to a better understanding of the tumor heterogeneity, and provide references on the dynamics of CSC subpopulation during radiotherapy.

First generation stochastic gene episilencing (step1) model and applications to in vitro carcinogen exposure

A novel first-generation stochastic gene episilencing (STEP1) model is introduced for quantitatively characterizing the probability of in vitro epigenetically silencing (episilencing) specific tumor-suppressor-microRNA (miRNA) genes by carcinogen exposure. Although the focus is mainly on in-vitro exposure of human cells to ionizing radiation, the mathematical formulations presented are general and can be applied to other carcinogens. With the STEP1 model, a fraction fj of the surviving target cells can have their tumor-suppressor-miRNA gene of type j silenced while the remaining fraction, 1 - fj , of the surviving cells do not undergo gene episilencing. Suppressor gene episilencing is assumed to arise as a Poisson process characterized with and exponential distribution of episilencing doses with mean dj . In addition to providing mathematical functions for evaluating the single-target-gene episilencing probability, functions are also provided for the multi-target-gene episilencing probability for simultaneously silencing of multiple tumor-suppressor-miRNA genes. Functional relationships are first developed for moderate doses where adaptive responses are unlikely and are then modified for low doses where adaptation can occur. Results apply to a specific follow-up time t after carcinogen exposure that exceeds the maximum time for the occurrence of an induced episilencing event.