Secretion and partial degradation of granulocyte-macrophage colony-stimulating factor (GM-CSF) of mouse L-P3 cells

Secretion of a granulocyte-macrophage colony-stimulating factor (GM-CSF) was accomplished by L-P3 cells in culture with a serum-free medium. Cell proliferation per se was not requisite for the production of GM-CSF; the cells continued secreting GM-CSF even after their growth had been suspended. The amount of GM-CSF accumulated in the conditioned medium was reasonably accounted by the daily rate of production, and the addition of a proteinase inhibitor such as leupeptin and pepstatin did not result in greater accumulation of GM-CSF in the culture. It is thus postulated that there is no significant proteolytic inactivation of the secreted GM-CSF in the culture. However, when partially purified GM-CSF preparation was chromatographed on a gel-filtration column in the presence of 0.1% Triton X-100, a derivative of the GM-CSF was yielded which had been diminished in the molecular weight and altered in the isoelectric point. On the other hand, when leupeptin was included in the solution during production and isolation of the factor, the yielded GM-CSF did not manifest such a detergent-induced transformation and maintained its isoelectric point at pH 3.5. It is thus assumed that, in the presence of the detergent, GM-CSF suffers deterioration by an endogenous proteinase and releases a sialoglycopeptide fragment without loosing its colony-stimulating activity.