Direct demonstration of negative regulation of tumor growth and metastasis by host-inducible nitric oxide synthase

Inducible nitric oxide synthase (NOS) II expression can be induced in the tumor bed, predominantly in host cells that infiltrate and surround a tumor. However, the impact of this physiological NOS II expression in host cells on tumor growth and metastasis remains unclear because of a lack of appropriate experimental approaches. In the present study, three NOS II-null (NOS II^-/-) tumor cell lines, KX-dw1, KX-dw4, and KX-dw7, were established and verified using Southern, Northern, and Western blot analysis, and nitric oxide production assays. Cells from these lines were then s.c. and i.v. injected into NOS II^+/+ and NOS II^-/- C57BL/6 mice. NOS II protein expression and enzyme activity were clearly detected in the tumors that formed in NOS II^+/+ mice but not in those that formed in NOS II^-/- mice. Consistent with the absence of NOS II expression in the tumor stroma, KX-dw1, KX-dw4, and KX-dw7 cells grew much faster and produced many more experimental lung metastases in NOS II^-/- mice than in NOS II^+/+ mice. Therefore, physiological expression of NOS II in host cells directly inhibits tumor growth and metastasis.