Enhancement of prolactin (PRL)-stimulated mitogenesis of Nb2 rat lymphoma cell cultures by insulin-like growth factor I (IGF-I)

IGF-I stimulated the mitogenesis of Nb2 cells in the presence of suboptimal mitogenic concentrations of prolactin (PRL, 0.01 or 0.1 ng/ml). In the presence of 1 ng/ml or 10 ng/ml, concentrations of PRL that produced a maximal or near maximal mitogenic response in Nb2 cells, the addition of insulin-like growth factor-I (IGF-I) at 5 or 10 ng/ml did not further enhance mitogenesis. This effect was selective for IGF-I since IGF-II, epidermal growth factor (EGF), insulin, transforming growth factor-β (TGF-β) or platelet-derived growth factor (PDGF) did not stimulate mitogenesis in the presence or absence of PRL. That the ability of IGF-I to stimulate mitogenesis of these cells required PRL was suggested by the ability of PRL antiserum to block the IGF-I effect. Monoclonal antibody to IGF-I reduced the mitogenic response to one identical to PRL alone. In the presence of a suboptimal mitogenic concentration of PRL, IGF-I was an equally effective comitogen when added at 0 time or at 6 h after PRL stimulation, suggestive of an effect of IGF-I in late G 1 phase of the cell cycle. Effects of IGF-I on ornithine decarboxylase, a G 1 enzymatic marker, were compatible with this interpretation. In order to be assured that IGF-I exerted its action through a receptor-mediated process, we evaluated the presence of IGF-I receptors on Nb2 cells. Receptors were present which bound IGF-I>IGF-II⪢ insulin. The IC 50 was 35 nM for IGF-I, 280 nM for IGF-II and 875 nM for insulin. The K d for IGF-I was 5.45 pM and the B max was 1.32 pmol/2 × 10 6 cells. These observations suggest that IGF-I may facilitate DNA synthesis and mitogenesis in PRL-dependent Nb2 cells in the same way that IGF-I and EGF serve as progression factors for BALB/c 3T3 mouse fibroblasts [Leof, Van Wyk, O'Keefe & Pledger (1983), Exp. Cell Res., 147, 202–208; Leof, Wharton, Van Wyk & Pledger (1982), Exp. Cell Res., 141, 107–115; Campisi & Pardee (1984), Molec. cell. Biol., 4, 1807–1814]. That PRL is required as a necessary component for IGF-I mitogenic action is demonstrated by the lack of any effect of IGF-I in the absence of PRL.