Dr. Zvi Fuks, former Chairman of the Department of Radiation Oncology at Memorial Sloan Kettering Cancer Center, recruited me in 1984, to open up the Radiation Oncology Laboratory and we have been working together since on high single dose (SD) radiation effects on endothelial cells. Our studies showed that radiation induces apoptotic cell death in endothelial cells both in vitro and in vivo and that this apoptotic pathway is not p53 dependent, but rather dependent on activation of the enzyme Acid Sphingomyelinase (ASMase). We also showed that angiogenic factors such as bFGF, which protect the endothelial cells from radiation-induced apoptosis, rescued mice from radiation-induced pneumonitis and radiation-induced GI syndrome. In 1999 in collaboration with Dr. Kolesnick we showed that SD radiation induced apoptosis in response to DNA damage via activation of the enzyme Ceramide Synthase (CerS) via the de novo synthesis of ceramide within the mitochondria. We showed that CerS activation was negatively regulated by the Ataxia Telangiectasia-Mutated (ATM) gene and that this pathway was activated in irradiated crypt stem cell clonogens (SCCs). Based on our data we suggested the hypothesis that endothelial apoptosis causes microvascular dysfunction, manifested as acute perfusion defects, which represses repair of DNA double strand breaks (DSBs) in crypt SCCs, determining the response to SD radiation in the generation of the GI syndrome. We further suggest that the genetic blueprint of DSB repair genes defines the SCC radiosensitivity, and that the radiation-induced epigenetic vascular component, controlled by the ceramide rheostat, down-modulates inherent repair function, determining the response phenotype. Based on this information we have begun development of a number of strategies to mitigate GI tract damage.
In addition since 1994, my program focuses on the role of DNA damage-induced apoptosis in human prostate cell lines LNCaP and CRW22Rv1, which are radioresistant, but TPA (phorbol ester), which reduces ATM levels serves as a radiosensitizer in these cells both in vitro and in vivo. Targeted disruption of ATM in prostate cancer cells leads to radiation-induced apoptosis via CerS signaling. Further identification of molecular targets within this pathway and development of specific reagents or small molecules aimed at ATM inactivation are the focus of ongoing studies in this program.
Recently we have shown that high dose (HD) chemotherapeutic drugs, like radiation, induces endothelial apoptosis, dependent on activation of the enzyme ASMase, and causes microvascular dysfunction in tumors, leading to tumor growth delay and tumor cures. We are currently testing whether the wave of microvascular dysfunction represses repair of DNA double strand breaks (DSBs) in these tumors and possible involvement of other mechanisms contributing to the tumor response. We are also determining whether HD chemotherapy generation of the GI syndrome might be due to the wave of microvascular dysfunction affecting crypt SCCs. Future studies will focus on the HD chemotherapy-induced endothelial apoptosis and microvascular dysfunction in metastatic models.