Abstract 948: Silibinin selectively inhibits growth and progression of urethane-induced lung tumors in wild-type but not inducible nitric oxide synthase (-/-) mice: potential usefulness of micro-computed tomography scanning in lung cancer chemoprevention studies

Abstract

Abstract Lung cancer is the second most common cancer but accounts for highest mortality in both men and women in the United States and Europe. The overall 5 year survival rate is still 16% underscoring the importance of new approaches to prevention and treatment of this deadly malignancy. Since sustained and excessive nitric oxide (NO) generation is positively correlated with lung cancer development and progression, recent studies have also focused on the involvement of inducible NO synthase (iNOS) and its inhibition by natural and synthetic agents for both prevention and therapy of lung cancer. Here we evaluated the role of iNOS in lung tumor occurrence and progression as well as the angiopreventive efficacy of silibinin employing B6/129J wild-type (WT) and iNOS (−/−) mice. Mice were injected with 1 mg/g body weight (i.p.) urethane once weekly for seven consecutive weeks, and thereafter were divided into two groups and gavaged with vehicle (control) or silibinin (742 mg/kg body weight) 5 days/week for 18 weeks. Using micro-computed tomography (micro-CT), we monitored the growth and progression of lung tumors from 8 through 20 weeks in WT mice. At 25 weeks, mice were sacrificed and their lung tumor multiplicity and diameters recorded. Quantification of micro-CT data showed that silibinin treatment significantly decreases urethane-induced tumor number and size in WT mice. This correlated with the decreases in tumor number and size directly measured at the termination of the study. Genetic ablation of iNOS decreased urethane-induced tumor multiplicity by 89% (P<0.001) compared to WT mice, but unlike its strong chemopreventive efficacy in WT mice, silibinin treatment failed to decrease tumor number or size in iNOS (-/-) mice. Tumors in WT control mice expressed more iNOS than those in silibinin-treated mice. Further analyses of tumors from WT mice showed that compared to controls silibinin treatment significantly decreased newly formed microvessels as measured by nestin immunostaining, activation of STAT3 and NF-κB as determined by pSTAT3 (ser727) and p65NF-κB (ser276) levels, and proliferation as measured by PCNA expression. Whereas a decrease in lung tumor number in iNOS (-/-) mice compared to WT confirms previous studies that iNOS is a critical regulator of lung tumorigenesis; this study identified an essential role of iNOS in lung tumor angiogenesis. The lack of effect of silibinin in iNOS null mice suggests that silibinin exerts its angiopreventive effects at least in part through its inhibition of iNOS expression in lung tumors. Together, our results suggest iNOS as a potential target for treating lung cancer and support the value of micro-CT as a non-invasive imaging modality for evaluating the efficacy of lung cancer chemopreventive agents. (RO1 CA113876) Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 948.