Regulation of Dipeptidyl Aminopeptidase I and Angiotensin Converting Enzyme Activities in Cultured Murine Brain Cells by Cortisol and Thyroid Hormone

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Abstract
Cultures of dissociated brain cells from 15-day-old fetal mice were grown in the presence and absence of 20 or 50 nM triiodothyronine (T3), 30 or 300 nM cortisol, and 30 nM cortisol plus 50 nM T3 added to chemically defined media or in media supplemented with 15% serum from control and hypothyroid calves. The specific activities of five lysosomal enzymes—N-acetyl galactosaminidase, β-gluc-uronidase, β-galactosidase, cathepsin B, and dipeptidyl aminopeptidase I (DAP-I)—were higher in cells grown in calf serum than in cells grown in defined media. Of these enzymes, only DAP-I was elevated in activity when the cells were grown in hypothyroid calf serum instead of control calf serum. Elevation of DAP-I activity was reversed by addition of 20 nM T3 to hypothyroid calf serum. The enzymatic properties of DAP-I were similar whether the cells were grown in control or hypothyroid calf serum and were similar to those reported for human fibroblasts and the purified enzyme. When the cells were grown in defined media, cortisol decreased the activities of all lysosomal enzymes, with 300 nM cortisol being more effective than 30 nM cortisol. Addition of 50 nM T3 to 30 nM cortisol decreased DAP-I activity more than 30 nM cortisol alone, but 50 nMT3 alone in defined media did not alter DAP-I levels. The reduction of DAP-I activity in these cells by T3 required cortisol, unidentified components in serum, or both. All enzyme activities increased during the period when these cells normally make myelin membrane components. Significant amounts of all lysosomal enzymes were found in the defined media except cathepsin B and DAP-I. These cells also contained an angiotensin converting enzyme (ACE) activity that was chloride dependent, inhibited by 2 μM angiotensin converting enzyme inhibitor, and inactivated by 1 mM EDTA. ACE levels were increased in cells exposed to 50 nM T3, 30 nM cortisol, and 50 nM T3 plus 30 nM cortisol. Significant amounts of ACE activity were also excreted by these cells into defined media.