Nitric Oxide Regulates Tumor Cell Cross-Talk with Stromal Cells in the Tumor Microenvironment of the Liver

Ningling Kang Decker^*, Soha S. Abdelmoneim^*, Usman Yaqoob^*, Helen Hendrickson^*, Joe Hormes^*, Mike Bentley, Henry Pitot, Raul Urrutia^*, Greg J. Gores^* and Vijay H. Shah^*

From the Department of Physiology,^* Gastroenterology Research Unit, Cancer Cell Biology Program, Fiterman Center for Digestive Diseases, Mayo Clinic, Rochester, Minnesota; the Department of Biological Sciences, Minnesota State University, Mankato, Minnesota; and the Cancer Center, University of Wisconsin, Madison, Wisconsin

Tumor progression is regulated through paracrine interactionsbetween tumor cells and stromal cells in the microenvironment,including endothelial cells and myofibroblasts. Nitric oxide(NO) is a key molecule in the regulation of tumor-microenvironmentinteractions, although its precise role is incompletely defined.By using complementary in vitro and in vivo approaches, we studiedthe effect of endothelial NO synthase (eNOS)-derived NO on livertumor growth and metastasis in relation to adjacent stromalmyofibroblasts and matrix because liver tumors maintain a rich,vascular stromal network enriched with phenotypically heterogeneousmyofibroblasts. Mice with an eNOS deficiency developed livertumors more frequently in response to carcinogens compared withcontrol animals. In a surgical model of pancreatic cancer livermetastasis, eNOS overexpression in the tumor microenvironmentattenuated both the number and size of tumor implants. NO promotedanoikis of tumor cells in vitro and limited their invasive capacity.Because tumor cell anoikis and invasion are both regulated bymyofibroblast-derived matrix, we explored the effect of NO ontumor cell protease expression. Both microarray and Westernblot analysis revealed eNOS-dependent down-regulation of thematrix protease cathepsin B within tumor cells, and silencingof cathepsin B attenuated tumor cell invasive capacity in asimilar manner to that observed with eNOS overexpression. Thus,a NO gradient within the tumor microenvironment influences tumorprogression through orchestrated molecular interactions betweentumor cells and stroma.