Radiation-induced bystander and other non-targeted effects: novel intervention points in cancer therapy?

A major problem in the search for new cancer drug targets is that the drugs are often toxic to normal tissues and require high doses to kill tumor cells. Therefore cellular targets which appear to involve low dose responses to cancer therapy are especially interesting since they could selectively target normal tissues which are not targeted by the treatment and thus may be responsible for unpleasant side effects or may be amenable to exploitation in order to improve the therapeutic ratio. One such target, which is the subject of this review, is radiation-induced bystander effects [RIBE], which result in the observation of radiation like responses in cells which have not been irradiated. RIBE is a novel phenomenon which indicates that at low doses, cell signaling is more important than direct DNA damage. Historically, DNA has always been considered to be the target for radiation therapy. The growing realization that signaling is important opens up several important therapeutic strategies which will be discussed in this review. RIBE appears to be the result of a generalized stress response in tissues or cells which is expressed at the level of the tissue, organ or organism rather than at the level of the individual cell. The signals may be produced by all exposed cells, but the response may require a quorum of cells in order to be expressed. The major response involving low LET (x- or gamma-ray) radiation exposure discussed in the existing literature is a death response. This has many characteristics of apoptosis but may be detected in cell lines without p53 expression, although the death response is suppressed in many tumor cell lines. While a death response in unirradiated normal cells around a tumor might appear to be adverse, it can in fact be protective and remove damaged cells from the population. If harnessed correctly, it could lead to the development of new drugs aimed not at tissue destruction but at enabling homeostatic mechanisms to control tumor expansion. In this scenario, the level of harmful or beneficial response will be related to the background damage, carried by the cell population, and the genetic programme determining response to damage. This focus may be important when attempting to predict the consequences of mixed therapies involving radiation and other cytotoxic agents. In this review, our current knowledge of the mechanisms underlying the induction of bystander effects by ionizing radiation is reviewed, and the question of how bystander effects may be harnessed to produce a new generation of anti-cancer drugs aimed at stabilization of tissue homeostasis rather than tissue destruction is considered.