In the past few years, nuclear DNA damage-sensing mechanisms activated by ionizing radiation have been identified, including ATM/ATR and the DNA-dependent protein kinase. Less is known about sensing mechanisms for cytoplasmic ionization events and how these events influence nuclear processes. Several studies have demonstrated the importance of cytoplasmic signaling pathways in cytoprotection and mutagenesis. For cytoplasmic signaling, radiation-stimulated reactive oxygen species (ROS) and reactive nitrogen species (RNS) are essential activators of these pathways. This review summarizes recent studies on the chemistry of radiation-induced ROS/ RNS generation and emphasizes interactions between ROS and RNS and the relative roles of cellular ROS/ RNS generators as amplifiers of the initial ionization events. Cellular mechanisms for regulating ROS/RNS levels are discussed. The mechanisms by which cells sense ROS/RNS are examined in terms of how ROS/RNS modify protein structure and function, for example, interactions with metal-thiol clusters, protein tyrosine nitration, protein cysteine oxidation, S-thiolation and Snitrosylation. We propose that radiation-induced ROS are the initiators and that nitric oxide (NO ) or derivatives are the effectors activating these signal transduction pathways. In responding to cellular ionization events, the cell converts an oxidative signal to a nitrosative one because ROS are too reactive and unspecific in their reactions for regulatory purposes and the cell is equipped to precisely modulate NO levels.

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Bailie et al. [In Vitro Cell Dev. Biol. (1992) 28A, 621-624] reported that primary cultures of rat hepatocytes possess low affinity binding sites for nerve growth factor (NGF). NGF treatment of primary cultures of rat hepatocytes with a maximally effective concentration of NGF (20 ng/ml, 0.8 nM) caused acute phasic activation of Raf-1 and p42MAPkinase, and a smaller sustained activation of B-Raf. The transient increase in Raf-1 and p42MAPkinase activity returned to baseline within ~ 30 min. NGF treatment of hepatocytes did not induce expression of cyclin dependent kinase (cdk) inhibitor proteins, but instead stimulated cdk2 activity and increased [3H]thymidine incorporation into DNA. In contrast to hepatocytes, NGF treatment of PC12 pheochromocytoma cells caused large sustained activations of B-Raf and p42MAPkinase, and a lower phasic activation of Raf-1. The sustained activations of B-Raf and p42MAPkinase were for more than 5 h. Treatment of PC12 cells with NGF increased p21Cip1/WAF-1 expression, reduced cdk2 activity and inhibited DNA synthesis, the opposite to the effects of NGF treatment of hepatocytes. However when p42MAPkinase was chronically activated in hepatocytes, via infection with an inducible oestrogen receptor-Raf-1 fusion protein, expression of p21Cip-1/WAF1 and p16INK4a cdk inhibitor proteins increased, cdk2 activity decreased, and DNA synthesis decreased. Equally, treatment of hepatocytes with 50 mM ethanol elevated the basal activity of p42MAPkinase and temporally extended the ability of NGF treatment to activate p42MAPkinase. Ethanol and NGF co-treatment increased expression of p21Cip-1/WAF1 and p16INK4a cdk inhibitor proteins and decreased hepatocyte DNA synthesis. These data demonstrate that NGF can cause either acute/phasic or sustained activation of the MAP kinase cascade in different cell types. Acute activation of the MAP kinase cascade correlated with increased DNA synthesis. In contrast, sustained activation of the MAP kinase cascade correlated with increased expression of cdk inhibitor proteins, a reduction in cdk activity, and an inhibition of DNA synthesis. These data suggest a general mechanism exists where acute activation of the MAP kinase cascade promotes G1 progression/S phase entry and that chronic activation of the MAP kinase cascade inhibits this process.

Exposure of cells to ionizing radiation results in complex cellular responses resulting in cell death and altered proliferation states. The underlying cytotoxic, cytoprotective and cellular stress responses to radiation are mediated by existing signaling pathways, activation of which may be amplified by intrinsic cellular radical production systems. These signaling responses include the activation of plasma membrane receptors, the stimulation of cytoplasmic protein kinases, transcriptional activation, and altered cell cycle regulation. From the data presented, there is increasing evidence for the functional links between cellular signal transduction responses and DNA damage recognition and repair, cell survival, or cell death through apoptosis or reproductive mechanisms.

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