Erythropoietin and interleukin-1β modulate nitrite production in a Swiss 3T3 cell model of rheumatoid synovial fibroblasts

Erythropoietin (EPO), a haemopoietic growth factor and a primary regulator of erythropoiesis, is widely used to treat anaemia in various chronic complications of rheumatoid arthritis (RA). Fibro-blast-like cells, found in the pannus tissue of joints, are thought to contribute to the inflammatory pathology of RA. Thus for the current study we investigated the effects of recombinant human EPO (rHuEPO) on NO metabolism, using an interleukin-1β (IL-1β)-stimulated Swiss 3T3 fibroblast monolayer as a model for fibroblast activity in RA. The results show that, over 3 days, both alone and in combination with the pro-inflammatory cytokine IL-1β (10 ng/ml), rHuEPO (25 μ-units/ml) induced significant production of nitrite in cell culture supernatants. This is an indicator of NO production by nitric oxide synthase (NOS), which is a well-documented mediator of metalloproteinase-mediated tissue remodelling in RA. It therefore appears that, through modulation of NOS-dependent NO production, rHuEPO may influence remodelling of connective tissue in RA, independently of its established erythropoietic role.

Physical association of the small GTPase Rho with a 68-kDa phosphatidylinositol 4-phosphate 5-kinase in Swiss 3T3 cells.

Our previous work showed that post-translationally modified Rho in its GTP-bound state stimulated phosphatidylinositol 4-phosphate 5-kinase (PIP5K) activity in mouse fibroblast lysates. To investigate whether Rho physically interacts with PIP5K, we incubated immobilized Rho-GST with Swiss 3T3 cell lysates and tested for retained PIP5K activity. Rho-GST, but not Ras-GST or GST alone, bound significant PIP5K activity. The binding of PIP5K was independent of whether Rho was in a GTP- or GDP-bound state. An antibody against a 68-kDa human erythrocyte type I PIP5K recognized a single 68-kDa protein eluted from Rho-GST column. The Rho-associated PIP5K responded to phosphatidic acid differentially from the erythrocyte type I PIP5K, suggesting that it could be a distinct isoform not reported previously. Rho co-immunoprecipitated with the 68-kDa PIP5K from Swiss 3T3 lysates, demonstrating that endogenous Rho also interacts with PIP5K. ADP-ribosylation of Rho with C3 exoenzyme enhanced PIP5K binding by approximately eightfold, consistent with the ADP-ribosylated Rho functioning as a dominant negative inhibitor. These results demonstrate that Rho physically interacts with a 68-kDa PIP5K, although whether the association is direct or indirect is unknown.