On a "chalone"-like factor for Ehrlich ascites mammary carcinoma

In an aqueous ultrafiltrate (10 000--50 000) prepared from Ehrlich ascites mammary carcinoma (EAC) cells, ascites fluid and bovine mammary gland, a new factor was obtained which is involved in the growth regulating system of EAC cells as shown by the following facts: 1) It reversibly inhibits the proliferation of EAC cells in a 24 h suspension culture, depending on their proliferative state. Thus, stationary cells from the plateau phase of growth in vivo are prevented from resuming growth in vitro, while cells taken from the active phase of in vivo growth are not inhibited under the same conditions. Likewise, stationary cells do not respond when incubated in serum before the addition of the factor (depending on serum concentrations and time). The dose-response curve levels off at higher factor concentrations so that the maximal inhibition is about 50--65% (explained with different sensitivities of the cells in the assay system). The time course of the inhibition as well as preliminary data from flow cytophotometry and labeling with tritiated thymidine indicate an interference with the progression of G1-phase cells into the S-phase. 2) The activity of the factor is counteracted by insulin and proinsulin, both known as growth factors. The insulin concentration needed is dependent on the factor concentration; an almost maximal inhibition can be prevented by physiological concentrations of insulin. The activity can be destroyed by heat and trypsin and differs also in other properties from that of polyamines. The factor could not be detected in lung, liver, spleen, kidney, heart and L 1210 ascites fluid of the mouse or in bovine malignant lymph nodes, thymus, kidney or liver. The factor was purified from a homogenate of bovine mammary gland by ultrafiltration and affinity chromatography on sepharose-bound wheat lectin. Polyacrylamide electrophoresis showed about 5 bands of which one contained the activity (in some experiments overlapping with a second one). In this way a 800fold purification (starting from the ultrafiltrate) could be obtained. The overall purification (relative to the centrifuged homogenate) can be calculated to be more than 100 000 fold.