Interleukin-1β-mediated inhibition of arginase in RINm5F cells

Induction of nitric oxide synthase and generation of nitric oxide in pancreatic islet β-cells may mediate cytokine-induced dysfunction leading to insulin-dependent diabetes mellitus. Nitric oxide generation can be regulated by availability of arginine substrate which, in turn, may be affected by substrate utilization in competing pathways such as the arginase-catalysed formation of ornithine and urea. In this study we have investigated the activity of arginase in the rat insulinoma-derived cell line RINm5F and the effect on this of interleukin Iβ, the nitric oxide synthase reaction intermediate N(G)-hydroxy-L-arginine and the nitric oxide-generating compounds 3-morpholinosydnonimine and S-nitrosoglutathione. Cytosols from RINm5F cells treated with or without interleukin Iβ (0.1 nM, 18 h) were incubated (45 min, 37°C) with [U-¹⁴C]arginine. Radiolabelled products ([¹⁴C]citrulline from nitric oxide synthase, [¹⁴C]ornithine and [¹⁴C]urea from arginase) were separated by high-performance liquid chromatography or ion-exchange chromatography. Interleukin 1β increased citrulline production (from 0.01 ± 0.002 to 0.58 ± 0.03 pmol/μg cell protein), indicating induction of nitric oxide synthase, and significantly decreased production of both ornithine (from 4.60 ± 0.20 to 3.40 ± 0.20 pmol/μg) and urea (0.93 ± 0.05 to 0.69 ± 0.04 pmol/μg) (P < 0.001), indicating decreased activity of arginase. Arginase was significantly inhibited by N(G)-hydroxy-L-arginine (IC₅₀=50 μM), S-nitrosoglutathione (500 μM: 69 ± 7% of control) and 3-morpholinosydnonimine (1 mM: 57 ± 7% of control) (P < 0.05). We conclude that during cytokine-directed β-cell assault nitric oxide synthase-catalysed production of N(G)-hydroxy-L-arginine and nitric oxide may inhibit arginase thereby increasing the availability of arginine for nitric oxide production.