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

Tumor growth-inhibitory glycoprotein secreted from the mouse brain at terminal stage of the ontogeny: molecular homogeneity and requirement of the retained protein conformation for exhibition of the cytotoxic action

The mouse brain at the neonatal stage but not at the adult or fetal stage, secretes a protein which inhibits growth and DNA synthesis of malignant cells preferentially over those of normal cells, and so is termed neonatal brain-derived carcinostatic factor (NBCF). In the present study, NBCF was prepared from conditioned medium cultured from neonatal brain, and was obtained by HPLC as a homogeneous protein (62 kDa, pI 9.1); physico-chemical and biological properties of NBCF differ from those of cerebral proteins known. A 1,2-cis-diol affinity column retained NBCF, which was thereafter eluted with D-sorbitol; NBCF treated with neuraminidase lost part of activity without emergence of new N-terminal amino acids, suggesting glycan moieties required for the activity exhibition. This is supported by repression of NBCF secretion by tunicamycin of doses as low as is not cytotoxic. NBCF did not hydrolyze target proteins; cytotoxic action of NBCF was not counteracted by a diversity of proteinase inhibitors. An ether extract of NBCF was not cytotoxic, whereas the ether-insoluble residuum retained part of the initial activity. NBCF was inactivated with immobilized trypsin or with dithiothreitol combined with guanidine more markedly than with either agent. Thus cytotoxicity exhibition of NBCF, mediated through actions other than proteolysis, is attributed to the proteinic principle but not to protein-bound lipophilic ligands, and requires retention of the protein conformation and intramolecularly buried SS bonds.

Cultured neurons release an inhibitor of astroglia proliferation (astrostatine)

Using in vitro techniques, we looked for a possible downregulation of rat astroglia proliferation by neuronal cells. We demonstrate that medium conditioned by 7-day-old rat cerebellar granule neurons or by 16-day-old rat embryo hippocampal neurons strongly inhibits the proliferation of cultured astroglial cells. Two neuronal cell lines, the PC12 rat pheocromocytoma and the neuro 2A (N2A) murine neuroblastoma also release such an activity. This release in N2A-conditioned medium (CM) occurs when the cells are at high density and show a low proliferation rate. This activity is present in media conditioned by neuronal cells, but not in media conditioned by normal astrocytes, by two glioma cell lines, or by one fibroblastic cell line. This proliferation inhibitor addresses normal astrocytes: the proliferation of two glioma cell lines, of a fibroblastic cell line, and of the two neuronal cell lines (PC12, N2A) is not inhibited by N2A CM. Moreover, this activity is directed against type 1 astrocytes, but not against type 2. Using three different assays, we demonstrate that DNA synthesis by astroglial cells is inhibited. N2A CM has no cytotoxic effect on astrocytes and does not modify their overall protein synthesis. Using affinity and gel filtration chromatography, we show that this activity is associated with a protein whose molecular weight ranges between 15 and 20 kDa. The possible relationship between this N2A cell-derived astroglia proliferation inhibitor and other types of potential glial proliferation inhibitors has been investigated. A brain glycoprotein immunologically related to epidermal growth factor receptor (EGFR) was reported to inhibit astroglial cell proliferation in vitro. Using polyclonal and monoclonal antibodies against EGFR, we were unable to immunoprecipitate the astrocyte proliferation inhibitor in N2A CM or to demonstrate by immunoblotting the presence of an EGFR-like immunoreactivity in the N2A CM or in the active chromatographic fractions of N2A CM. Transforming growth factor beta (TGF beta) is a well-known modulator of the proliferation of various cell types and was shown to be present in N2A CM. Using a polyclonal anti-TGF beta antibody that recognizes TGF beta on Western blots of N2A CM, we were unable to immunoprecipitate the astrocyte proliferation inhibitor of N2A CM. It seems thus far that the neuronal astroglia proliferation inhibitor is a new protein for which we propose the name astrostatine.