Abstract LB-288: Restoration of peri-vascular nitric oxide gradient radio-sensitizes murine breast cancers via vascular normalization.

Background : Abnormal blood vessels in solid tumors hinder the delivery and efficacy of therapeutic agents. We and others have demonstrated that these physiological barriers can be overcome by normalization of the vascular ( Science 307: 58, 2005). We have also reported that nitric oxide (NO) produced by endothelial NO synthase (eNOS) in vascular endothelial cells mediates angiogenesis and vessel maturation while ectopic NO production by tumor cells impairs tissue NO distribution and interferes with these processes ( Nature Medicine 14:255, 2008). Inducible NO synthase (iNOS) is the most frequently expressed NOS in human tumors including breast cancers. Tumor hypoxia can also induce iNOS. Resulting heterogeneous NO distribution may contribute to the abnormality of tumor vasculature. Here, we determined the role of iNOS coming from tumor and/or stromal cells on the tumor vasculature and response to radiation therapy. Methods : Murine breast cancers (MCaIV and E0771) were grown orthotopically in the mammary fat pad in C3H and C57BL/6 mice, respectively. Perivascular cells were visualized using α-smooth muscle actin promoter driven green fluorescent protein or DsRed reporter mice. Tissue oxygenation was determined by a redox marker pimonizadole. To block iNOS in these tumors, we 1) silenced iNOS expression in tumor cells by shRNA, 2) used iNOS deficient mice to assess the role of iNOS in host stromal cells, and 3) used an iNOS selective inhibitor 1400W (10mg/kg/day). Fractionated radiation therapy (5 Gy/day × 3 days) was performed when mammary fat pad tumors reached 100 mm 3 . Results : Murine breast cancers express high levels of iNOS especially in vivo. The majority of iNOS expressing cells were tumor cells. Blood vessels have eNOS but not iNOS. Both genetic and pharmacological inhibition of iNOS resulted in an increased number of blood vessels (segments) and a decrease in the abnormal vessel dilation in the breast tumors. Furthermore, iNOS inhibition increased vessel maturation and suppressesd vascular hyperpermeability. Finally, iNOS silencing and inhibition decreased hypoxia and thus, improved tumor response to radiation therapy. In contrast, these parameters were not different between iNOS deficient and wild-type mice suggesting tumor cell-derived NO dominancy. Conclusion : In this study, we found that restoration of tissue distribution of NO in murine breast cancers to a normal pattern could normalize tumor vasculature both morphologically and functionally. The perivascular NO gradient significantly alleviated tissue hypoxia and improved efficacy of radiation therapy. Our data suggest that aberrant production of NO from tumor cells interfere with vessel maturation, and that restoration of tissue NO distribution by selective NOS modulation can improve tumor vascular function and facilitate concomitantly administered cytotoxic therapies. (Supported by NIH grant CA096915) Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-288. doi:10.1158/1538-7445.AM2011-LB-288