Differential response of malignant BALB/c mammary epithelial cells to the multiplication-stimulating activity of insulin

Investigations were done to distinguish normal from malignant BALB/c mammary epithelial cells by growth differences. In a high concentration (15%) of serum, normal epithelial cells (from mice at midterm of first pregnancy) and malignant BALB/c mammary epithelial cells in primary culture divided at the same rate (doubling time = 24 hr) and exhibited an extremely short longevity (three doublings). Insulin did not affect growth rate or longevity. Growth rate of normal and malignant cellss was serum-dependent. At serum concentrations limiting for normal cell multiplication, malignant cells showed no proliferative advantage. Insulin complemented but did not replace limiting serum for multiplication of both normal and malignant cells. When allowed to become confluent before three doublings, normal and malignant cell exhibited a contact-mediated (or density-mediated) growth inhibition. Both normal and malignant cells were arrested in the G1 phase of the cell cycle at the same density. Near physiologic doses of insulin stimulated DNA synthesis and mitosis in malignant cells but not in normal cells. Approximately 6% of malignant cells were stimulated to divide in the presence of insulin. Maximum DNA synthesis occurred at 24 hours and maximum mitotic activity at 30 hours after addition of insulin. The data suggested that malignant mammary epithelial cells are more sensitive to insulin as an overgrowth-stimulating factor than are normal cells.