Production of tumor growth‐inhibiting protein by the neonatal mouse brain: Dependence on intracellular glutamine metabolism

We previously reported that the mouse brain at the neonatal stage but not at the adult stage secreted a carcinostatic factor of 62,000 Da, termed NBCF, which inhibited clonal growth and DNA synthesis of malignant cells preferentially over those of normal cells. In the present study, NBCF production by the neonatal mouse brain in culture was investigated. Addition of L‐glutamine to the culture medium markedly promoted NBCF production in a concentration‐dependent manner. The production seemed to be specific to glutamine, since no promotive effect was exerted by L‐glutamic acid, its analogue DL‐α‐aminoadipic acid, L‐aspargine, or L‐aspartic acid or by other amino acids or vitamins. NBCF production was markedly reduced in culture medium either devoid of L‐glutamine or containing 6‐diazo‐5‐oxo‐L‐norleucine, a glutamine antagonist, or L‐metionine sulfoximine, an inhibitor of glutamine synthetase. Thus NBCF production was promoted by extracellular supply, intracellular synthesis, and utilization of L‐glutamine but was not affected by its deamidated form or homologue amino acids. On the other hand, NBCF production was completly repressed by addition of cycloheximide to the culture medium. The repressive effect was also exerted by actinomycin D although not completely, whereas cytosine β‐D‐arabinofuranoside did not repress NBCF production. These results indicated that NBCF production by cultivation was independent of DNA replication but dependend mostly on a transcription stage and its following stages and partly on a translation stage from the preexisting mRNA to the protein.