Key words: vascular smooth muscle; nitric oxide synthase; Morris hepatoma; M-21 melanoma; tumor vessel maturationThe host tissue reaction to tumor invasion involves many nonmalignant cell types and stromal reactions (angiogenesis and grossly increased vascular permeability in the tumor; accelerated matrix synthesis and degradation; leukocyte infiltrate; secretion by nonmalignant cells of factors that affect the growth of malignant cells, etc.). 1,2 Solid tumors induce the formation of a blood supply from the neighboring vasculature that exhibits certain features relative to the two main vascular cell types. Angiogenesis driven by endothelial cell protease release, migration and mitosis is observed; the permeability of the microcirculation to macromolecules remains high, with such consequences as elevated tumor interstitial fluid pressure and flow and peritumoral edema. 3 In some tumors, the feeder vessels contain little vascular smooth muscle cells (VSMCs) and apparently no innervation. 4 Possibly correlated to these structural findings, hemodynamic studies show that the blood supply of primary tumors or metastases is remarkably resistant to the infusion of various pharmacological vasoconstrictor agents relatively to surrounding normal tissues, 5-9 suggesting that the tumor resistance vessels were in a state of maximal vasodilation or anergy. Other tumor blood vessels, however, exhibit differentiation, with recruitment of ␣-actin positive periendothelial cells (e.g., VSMCs ϩ pericytes) around tumor vessels. 10,11 Further, pericyte/VSMC presence indicates a maturation process in tumor neovessels, with a decreased dependence on vascular endothelial cell growth factor (VEGF), a greater histological stability relative to microvessels formed of endothelial cells alone and a greater resistance to anti-angiogenic therapy. 12,13 Whereas considerable efforts have been devoted to the analysis of the endothelial cell-tumor interactions, less is known about the behavior of VSMCs in tumor biology. To detect and model such an interaction, we have attempted reconstitution experiments where tumorigenic cell lines of rat or human origin were cultured in the presence of rat aortic rings as a source of differentiated and functional arterial VSMCs, to evaluate the consequences of coculture on the vascular tone by performing contractility assays.
MATERIAL AND METHODS
Cell linesThe rat Morris 7777 hepatocarcinoma cell line, subclone 7.6 14 was a gift from Professor L. Bélanger, CHUQ, Québec. This cell line is tumorigenic in syngeneic Buffalo rats. The human melanoma M-21 cell line was a gift of Dr. David Cheresh (Scripps Research Institute, San Diego, CA). It is tumorigenic in immunodeficient mice. 10,15 RBL (rat basophilic leukemia) cells were a gift from Dr. Julia Ember (Scripps Clinic Research Institute). Other cells lines were originally obtained from the ATTC and subcultured in our laboratories: the rat hepatoma H4-II-E-C3, the human hepatomas HEP-3B and HEP G2, and the human breast cancer line MDA-MB-231. All cell lines were pr...