Differential effects of cyclic AMP on induction of nitric oxide synthase in 3T3-L1 cells and brown adipocytes

The aim of this study was to determine whether cyclic AMP (cAMP) pathways alter the nitric oxide (NO) production mediated by inducible NO synthase (iNOS) in adipocytes. The treatment of 3T3-L1 cells, a model of white adipocytes, with the combination of lipopolysaccharide (L), tumor necrosis factor-α (T), and interferon-γ (I) synergistically induced iNOS, leading to the production of NO. Enhancers of intracellular cAMP (dibutyryl cAMP, forskolin, and IBMX) inhibited the NO production elicited by LTI, whereas H89, a specific inhibitor of PKA, stimulated the NO production in 3T3-L1 cells. In rat brown adipocyte cell line, the combined treatment with LT synergistically elicited the NO production, and the cAMP analogues further enhanced it. Forskolin inhibited the NO production in 3T3-L1 cells, but enhanced it in brown adipocytes, in a dose-dependent manner. The changes in NO production paralleled the change in iNOS mRNA and protein level in both cell types. The activation of NF-κB by LTI/LT was blocked in 3T3-L1 cells, but enhanced in brown adipocytes, by the co-treatment with cAMP analogues. The protein level of 1-κBα, a NF-κB stabilizer, changed reciprocally to that of NF-κB activity in each cell type. These results suggest that cAMP regulates iNOS expression in adipocytes through modulating NF-κB activity. The differential regulation of iNOS in 3T3-L1 cells from that in the brown adipocytes indicates that intracellular signal pathways activated by cAMP are different between the cell types.