MODULATORY EFFECT OF DEXAMETHASONE ON ORNITHINE DECARBOXYLASE ACTIVITY AND GENE-EXPRESSION - A POSSIBLE POSTTRANSCRIPTIONAL REGULATION BY ANEUTRAL METALLOPROTEASE

The intracellular effect of dexamethasone (DXME) on the activity and g ene expression of ornithine decarboxylase (ODC) was studied in Syrian hamster embryo cells (SHE). The ODC activity (expressed as nmoles deca rboxylated ornithine mg(-1) protein h(-1)) was 4.61 +/- 0.14 in untrea ted cells, whereas it increased to 14.38 +/- 0.26 after 5 h treatment with 1.6 x 10(-7) M TPA. In contrast, DXME (2.5 x 10(-5) M) reduced th e ODC activity by 50 per cent to 2.35 +/- 0.22. In cells co-treated fo r 5 h with TPA and DXME, ODC activity decreased to the level of the un treated cells. However, when DXME was added 3 h after TPA treatment fo r 2 h, in the continuous presence of TPA, the ODC activity unexpectedl y increased further to 16.44 +/- 105. The modulation of ODC activity c orrelated partly with the level of ODC mRNA. Thus when cells were trea ted with TPA, the ODC mRNA increased threefold, whereas it decreased b y 30 per cent when the cells were exposed to DXME. In TPA-DXME co-trea ted cells, as in TPA pretreated cells followed by DXME for 2 h, a decr ease (31.25 per cent and 12.5 per cent respectively) was observed in O DC mRNA. In turnover studies, DXME was found to increase the stability of ODC; the discrepancy between ODC activity and ODC mRNA levels coul d result from an inhibitory effect of the corticoid on proteolysis of ODC. Studies of lysosomal protease showed that the activities of cathe psins L, B and H decreased following TPA treatment. DXME also inhibite d cathepsin L and B activities, but stimulated cathepsin H. Analysis o f neutral cytosolic protease activity by gelatin and casein zymograms showed that TPA strongly stimulated the activity of a 70 kDa gelatinas e. DXME was able to inhibit the induction of such cytosolic proteases under all the treatment conditions. When the cytosolic protein was inc ubated in vitro, at 37 degrees C, in the presence of 2 mM CaCl2, the O DC 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 mu g ml(-1) of TIMP-1. In contrast, the addition of ED TA restored the ODC activity completely. We postulate that modificatio n of the 70 kDa cytosolic metalloprotease activity could interact with the posttranscriptional regulation of ODC.