Studies from this laboratory have provided evidence that in addition to DNA damage, ionizing radiation can act upon cellular membranes to initiate apoptotic death. Specifically, it has been demonstrated that acid sphingomyelinase (ASMase)-mediated ceramide generation signals radiation-induced apoptosis. Although supported by biochemical, cell biological, and genetic data, the prevalence of this apoptotic mechanism remains unknown. The purpose of current studies is to establish the biochemical regulation of ceramide generation in response to radiation, and the tissues in which this response prevails. New model systems and reagents have been developed to study the tissue distribution of this response and the potential for its modulation by pharmacologic intervention. These pathophysiologic studies may have direct relevance to human disease with immediate potential for clinical translation.
Endothelial cells appear particularly sensitive to ASMase-mediated apoptosis in vitro and in vivo. Disruption of the ASMase gene but not the p53 gene inhibits microvascular endothelial apoptosis in a variety of irradiated tissues, and treatment with basic fibroblast growth factor (bFGF) produces a similar anti-apoptotic effect via inhibition of ASMase.
Further, microvascular endothelial apoptosis constitutes a primary and critical event in the pathogenesis of radiation damage to several normal tissues, including the small intestines, lungs, and the central nervous system. Blocking endothelial apoptosis pharmacologically by intravenous bFGF or genetically by ASMase deletion, prevented the evolution of tissue damage, organ failure, and death from radiation enteritis or pneumonitis. These tissue responses in rodents serve as readout systems in pharmacologic approaches to modulate the level of radiation-induced ceramide and its pro-apoptotic function, and in the design of signaling-based apoptosis therapy for tumors and tissues that use ceramide as mediator of radiation damage.
In addition to the investigations described above, we have a prgram on the role of DNA damage-induced apoptosis. Our data strongly support the concept that DNA damage activates the enzyme ceramide synthase via the de novo synthesis of the sphingolipid and ceramide within the mitochondria. Ceramide synthase activation is negatively regulated by the Ataxia Telangiectasia-Mutated (ATM) gene. In human prostate cell lines LNCaP and CRW22Rv1, which are radioresistant, TPA (phorbol ester) reduces ATM levels and serves as a radiosensitizer. These are new and heretofore unknown mechanisms that could provide a basis for modulating the radiation response in resistant human prostate tumor clones. Targeted disruption of ATM in prostate cancer cells might lead to radiation-induced apoptosis via ceramide synthase signaling. Further identification of molecular targets within this pathway and development of specific reagents or small molecules aimed at ATM inactivation would appear warranted in the treatment of prostate cancer.
Zvi Fuks has been elected a member of the Institute of Medicine. The Institute of Medicine (IOM) is a branch of the National Academies and was established to honor professional achievement in the health sciences and to serve as a national resource for recommendations on issues related to medicine, biomedical sciences, and health. Membership in the IOM is considered one of the highest honors in the fields of health and medicine.