title: Nitric oxide synthase inhibition enhances the antitumor effect of radiation in the treatment of squamous carcinoma xenografts.
abstract: This study tests whether the nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine (L-NNA), combines favorably with ionizing radiation (IR) in controlling squamous carcinoma tumor growth. Animals bearing FaDu and A431 xenografts were treated with L-NNA in the drinking water. IR exposure was 10 Gy for tumor growth and survival studies and 4 Gy for ex vivo clonogenic assays. Cryosections were examined immunohistochemically for markers of apoptosis and hypoxia. Blood flow was assayed by fluorescent microscopy of tissue cryosections after i.v. injection of fluorospheres. Orally administered L-NNA for 24 hrs reduces tumor blood flow by 80% (p<0.01). Within 24 hrs L-NNA treatment stopped tumor growth for at least 10 days before tumor growth again ensued. The growth arrest was in part due to increased cell killing since a combination of L-NNA and a single 4 Gy IR caused 82% tumor cell killing measured by an ex vivo clonogenic assay compared to 49% by L-NNA or 29% by IR alone. A Kaplan-Meyer analysis of animal survival revealed a distinct survival advantage for the combined treatment. Combining L-NNA and IR was also found to be at least as effective as a single i.p. dose of cisplatin plus IR. In contrast to the in vivo studies, exposure of cells to L-NNA in vitro was without effect on clonogenicity with or without IR. Western and immunochemical analysis of expression of a number of proteins involved in NO signaling indicated that L-NNA treatment enhanced arginase-2 expression and that this may represent vasculature remodeling and escape from NOS inhibition. For tumors such as head and neck squamous carcinomas that show only modest responses to inhibitors of specific angiogenic pathways, targeting NO-dependent pro-survival and angiogenic mechanisms in both tumor and supporting stromal cells may present a potential new strategy for tumor control.
authors.affiliation: Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia, United States of America. rjcardnell@vcu.edu
authors.forename: Robert J G
authors.initials: RJ
authors.lastname: Cardnell
authors.forename: Ross B
authors.initials: RB
authors.lastname: Mikkelsen
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology* / Q000627:therapeutic use; D017209:Apoptosis / Q000187:drug effects / Q000528:radiation effects; D001810:Blood Volume / Q000187:drug effects / Q000528:radiation effects; D002294:Carcinoma, Squamous Cell / Q000188:drug therapy* / Q000473:pathology / Q000503:physiopathology / Q000532:radiotherapy*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects / Q000528:radiation effects; D002945:Cisplatin / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology* / Q000627:therapeutic use; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects / Q000528:radiation effects; D006801:Humans; D008297:Male; D051379:Mice; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D019335:Nitroarginine / Q000494:pharmacology* / Q000627:therapeutic use; D011829:Radiation Dosage; D016019:Survival Analysis; D023041:Xenograft Model Antitumor Assays
chemical_list: D000970:Antineoplastic Agents; D004791:Enzyme Inhibitors; D019335:Nitroarginine; D019001:Nitric Oxide Synthase; D002945:Cisplatin