| Abstract: | Ornithine decarboxylase (ODC, EC 4.1.1.17) activity is induced in the RAW264 macrophage-like cell line by bacterial lipopolysaccharide (LPS). As little as 0.1 ng/ml LPS promoted an increase in ODC activity, while maximal ODC activity (30-fold above control) was induced with 1.0 microgram/ml LPS. An increase in ODC activity was detectable within 90 min of LPS addition. The LPS-induced increase in ODC activity was prevented by inhibitors of protein and RNA synthesis. The induction of the enzyme by LPS was not dependent on prostaglandin production. However, PGE2 (1 microgram/ml) and 8-bromo-cyclic AMP (1 mM), neither of which had an effect on ODC activity when added alone, each acted synergistically to enhance the LPS induction of ODC activity. Enzyme induction was not associated with an alteration in Km for ornithine, which remained constant at 0.04 mM. The extent of the increase in ODC in response to LPS increased with increasing cellular density. This relationship was dependent not on absolute cell density of the monolayer but on the cell number in relation to medium volume, and this dependence could be extrapolated to the origin. Addition of conditioned media from LPS-stimulated but not unstimulated cultures enhanced the ODC increase in sparsely plated cultures in response to a maximal concentration of LPS. The addition of polymyxin B, a reagent that blocks the effects of LPS, including the increase in ODC activity, did not totally inhibit the conditioned medium stimulation. This data indicates that two signals, LPS and a LPS-induced mediator, are involved in the induction of ODC activity in RAW264 cells. |
