Objectives. Nitric oxide (NO) is formed in many mammalian tissues, and a growing body of evidence suggests that NO is involved in cell growth and cell differentiation. Low concentrations of NO can stimulate cell growth; high concentrations result in cytostatic/cytotoxic effects. It has previously been shown that intravesical treatment with bacille Calmette-Guérin (BCG) for bladder cancer increases NO production in the human urinary bladder and that NO inhibits bladder cancer cell growth in vitro. In this study, we investigated nitric oxide synthase (NOS) activity in different bladder cancer cells and the role of the NO precursor l-arginine in cell proliferation.

Methods. NOS activity was assessed by citrulline assay in cultured normal human urothelial cells and bladder cancer cell lines T24 and MBT-2 before and after treatment with cytokines. We also measured cell growth at various l-arginine concentrations and after addition of the NOS inhibitor N^G-nitro-l-arginine (l-NNA) in unstimulated and cytokine-stimulated cells.

Results. Normal urothelial cells, as well as T24 and MBT-2 cells, showed calcium-dependent NOS activity under basal conditions. The bladder cancer cell lines also showed calcium-independent NOS activity in contrast to the normal cells. After cytokine treatment, both the normal cells and the cancer cell lines showed a marked increase in calcium-independent NOS activity. There was a dose-dependent stimulation of cell growth in the cancer cell lines after l-arginine addition, and this effect could be antagonized by l-NNA. Cytokine treatment inhibited cell growth, and this inhibition was partly reversed by l-NNA.

Conclusions. Normal urothelial cells and bladder cancer cell lines MBT-2 and T24 show NOS activity, and cytokine treatment induces calcium-independent NOS activity. Our results suggest that endogenous activity of the constitutively expressed form of NOS in unstimulated cells promotes cell proliferation, and NO production secondary to increased activity of the inducible form of NOS after cytokine treatment inhibits cell growth.