DIFFERENTIAL-EFFECTS OF A PHORBOL ESTER ON CARBOXYPEPTIDASE-E IN CULTURED ASTROCYTES AND ATT-20 CELLS, A NEUROENDOCRINE CELL-LINE

Cultured astrocytes have been shown to secrete various neuropeptides a nd the neuropeptide processing enzyme, carboxypeptidase E (CPE). The s ecretion of CPE enzymatic activity from astrocytes has been shown prev iously to be increased approximately twofold by treatment with tetrade canoylphorbol 13-acetate (TPA), a phorbol ester. In this study, metabo lic labeling with [S-35]Met was utilized to examine the effect of TPA on the biosynthesis of CPE protein in cultured astrocytes and in AtT-2 0 cells, a pituitary-derived cell line. Treatment of astrocytes with 0 .1 mug/ml TPA for 24 h caused an 80% increase in the level of radiolab eled CPE in both the media and the cells, indicating that the synthesi s of CPE was stimulated by the TPA. AtT-20 cells also secreted more ra diolabeled CPE in response to TPA, but this increase was offset by a p roportional decrease in the cellular level of radiolabeled CPE, and sy nthesis of CPE was not stimulated in this cell line. Northern blot ana lysis demonstrated that 0.1 mug/ml TPA elevated CPE mRNA by approximat ely 50% in cultured astrocytes but not in AtT-20 cells. Quantitative i n situ hybridization studies demonstrated that the TPA-induced increas e in CPE mRNA expression was largely due to increases in the number of cells expressing CPE mRNA, although for astrocytes from some brain re gions the average level of CPE mRNA per cell was also elevated by TPA. These results suggest that astrocytes can be induced to express CPE, which is consistent with a role for astrocytes in intercellular signal ing.