Effects of IL-6 or IGF-I on the cell proliferation of ILKM2, ILKM3, and NOP2 cell lines that differ in IL-6Rα expression levels. (A) Myeloma cells isolated from myeloma patients were viably cultured with IL-6 (2 ng/mL) and IGF-I (100 ng/mL). Myeloma cells from 3 myeloma patients were cultured with or without IL-6 and IGF-I in serum-containing complete media for 2 weeks, and the relative viable cell numbers were counted by flow cytometry with forward and side scatters at a constant flow rate for one minute. (B) In serum-free media, IL-6 (1 ng/mL) could support NOP2 growth, whereas the cell proliferation of ILKM2 and ILKM3 was promoted by IL-6 together with IGF-I (3 ng/mL). The growth curves for 8 days cultured with medium alone (♦), with IL-6 (▪), with IGF-I (▴), or with IL-6 and IGF-I (•) are indicated. Values of relative viable cell numbers are indicated by the means and SDs obtained from 3 independent experiments. Statistical analysis by Student t test indicated that viable cell numbers of NOP2 after day 2 between medium alone and medium with IL-6 were significantly different (P < .001). Differences of viable cell numbers of NOP2 after day 6 between IGF-I and IL-6 were also significant (P < .001). Viable cell numbers of ILKM2 and ILKM3 after day 2 between medium alone and medium with IL-6 and IGF-I were significantly different (P < .001). Differences of viable cell numbers of ILKM2 after day 2 and of ILKM3 after day 4 between IGF-I or IL-6 alone and IL-6 together with IGF-I were also significant (P < .001). (C) The cell surface expression of IL-6Rα of NOP2, ILKM2, and ILKM3 is shown by flow cytometry. Compared with ILKM2 and ILKM3, NOP2 exhibited a markedly increased expression of IL-6Rα.