Differential Effects of a Phorbol Ester on Carboxypeptidase E in Cultured Astrocytes and AtT‐20 Cells, a Neuroendocrine Cell Line

Abstract: Cultured astrocytes have been shown to secrete various neuropeptides and the neuropeptide processing enzyme, carboxypeptidase E (CPE). The secretion of CPE enzymatic activity from astrocytes has been shown previously to be increased approximately twofold by treatment with tetradecanoylphorbol 13‐acetate (TPA), a phorbol ester. In this study, metabolic labeling with [³⁵S]Met was utilized to examine the effect of TPA on the biosynthesis of CPE protein in cultured astrocytes and in AtT‐20 cells, a pituitary‐derived cell line. Treatment of astrocytes with 0.1 μg/ml TPA for 24 h caused an 80% increase in the level of radiolabeled CPE in both the media and the cells, indicating that the synthesis of CPE was stimulated by the TPA. AtT‐20 cells also secreted more radiolabeled CPE in response to TPA, but this increase was offset by a proportional decrease in the cellular level of radiolabeled CPE, and synthesis of CPE was not stimulated in this cell line. Northern blot analysis demonstrated that 0.1 μg/ml TPA elevated CPE mRNA by approximately 50% in cultured astrocytes but not in AtT‐20 cells. Quantitative in situ hybridization studies demonstrated that the TPA‐induced increase in CPE mRNA expression was largely due to increases in the number of cells expressing CPE mRNA, although for astrocytes from some brain regions 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 signaling.