Inhibition of differentiation of mouse myeloid leukemia cells by heat-stable calf serum components of very high molecular weight.

Mouse myeloid leukemia cells (Ml) were induced to differentiate into macrophages and granulocytes by various inducers including glucocorticoid. The tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) inhibited the induction of differentiation of Ml cells in medium containing calf serum, but enhanced the induction in medium containing fetal calf serum and several inducers. For elucidation of the factor(s) in serum affecting the response of Ml cells to tumour promoters, calf serum was fractionated by Sephadex G-200 gel filtration. Differentiation of Ml cells induced by dexamethasone was markedly inhibited by TPA and high mol. wt fractions of calf serum eluted in the void volume and low mol. wt fractions that co-migrated with bovine serum albumin. High mol. wt fractions alone inhibited the differentiation of Ml cells induced by dexamethasone, and also acted additively with TPA in inhibiting the differentiation. The inhibition by high mol. wt fractions was not related to cytotoxicity and was reversible. The differentiation of Ml cells induced by proteinous inducer or lipopolysaccharide was also inhibited by high mol. wt fractions. The inhibitory factor was heat stable (70 degrees C for 20 min or 90 degrees C for 10 min). These results suggest that the tumour promoter and calf serum components cooperate in inhibiting differentiation of mouse myeloid leukemia cells induced by various inducers.