Modulatory effect of dexamethasone on ornithine decarboxylase activity and gene expression: A possible post-transcriptional regulation by a neutral metalloprotease

Publisher Website
Google Scholar
Abstract
The intracellular effect of dexamethasone (DXME) on the activity and gene expression of ornithine decarboxylase (ODC) was studied in Syrian hamster embryo cells (SHE). The ODC activity (expressed as nmoles decarboxylated ornithine mg−1 protein h−1) was 4·61 ± 0·14 in untreated cells, whereas it increased to 14·38 ± 0·26 after 5h treatment with 1·6 × 10−7M TPA. In contrast, DXME (2·5 × 10−5M) reduced the ODC activity by 50 per cent to 2·35 ± 0·22. In cells co‐treated for 5 h with TPA and DXME, ODC acitivity decreased to the level of the untreated cells. However, when DXME was added 3 h after TPA treatment for 2 h, in the continuous presence of TPA, the ODC activity unexpectedly increased further to 16·44 ± 1·05. The modulation of ODC activity correlated partly with the level of ODC mRNA. Thus when cells were treated with TPA, and ODC mRNA increased threefold, whereas it decreased by 30 per cent when the cells were exposed to DXME. In TPA–DXME co‐treated cells, as in TPA pretreated cells followed by DXME for 2 h, a decrease (31·25 per cent and 12·5 per cent respectively) was observed in ODC mRNA. In turnover studies, DXME was found to increase the stability of ODC; the discrepancy between ODC activity and ODC mRNA levels could result from an inhibitory effect of the corticoid on proteolysis of ODC. Studies of lysosomal protease showed that the activities of cathepsins L, B and H decreased following TPA treatment. DXME also inhibited cathepsin L and B activities, but stimulated cathepsin H. Analysis of neutral cytosolic protease activity by gelatin and casein zymograms showed that TPA strongly stimulated the activity of a 70 kDa gelatinase. DXME was able to inhibit the induction of such cytosolic proteases under all the treatment conditions. When the cytosolic protein was incubated in vitro, at 37°C, in the presence of 2 mM CaCl2, the ODC activity decreased by 50 per cent after 30 min incubation. Further decrease was achieved when p‐amino‐phenylmercuric acetate, a protease activator, was added. Proteolytic activity was not inhibited after the addition of 10 μg ml−1 of TIMP‐1. In contrast, the addition of EDTA restored the ODC activity completely. We postulate that modification of the 70 kDa cytosolic metalloprotease activity could interact with the post‐transcriptional regulation of ODC.