Effect of conditioned serum-free media from MCF-7 cells on the induction of DNA-synthesis by IGF-I and E2 in the three MCF-7 strains. Conditioned media (CM) were generated by incubating MCF-7S, ATCC and NKI cells in serum-free, estrogen-free medium for two days, after which the medium was collected and centrifuged to spin down the cell debris. Serum-starved cells were grown in 24-wells plates, and were incubated in normal serum-free medium (indicated as [-] in the figure legend) or in the conditioned serum-free media (indicated as CM MCF-7S, CM MCF-7ATCC, or CM MCF-7 NKI). The cells were given no mitogens (-), 20 ng/ml of IGF-I (I(20)), 2 ng/ml of IGF-I (I(2)), 2 ng/ml of IGF-I in combination with 1 nM E2 (I(2)E2), or 1 nM E2 (E2) and were harvested after 30 h. 3H-thymidine (2 μCi/ml) was added to the culture 6 h before harvesting. Indicated values are means of triplicate measurements. The values were normalized for the incorporation induced by 20 ng/ml of IGF-I (100%).