α-Fetoprotein (AFP) is present in high levels in fetal fluids, certain neoplasias, and regenerating liver. Although AFP’s physiological role remains an enigma, we have recently demonstrated mitogenic activity for AFP. Using a primary monolayer culture system, we have further investigated the proliferative activity of purified AFP. Porcine granulosa cells from small ovarian follicles were attached for 2 days in Ham’s F-12-Dulbecco’s Modified Eagle’s Medium (1:1) and 5% fetal calf serum, followed by 6 days of culture in medium containing 0.25% plasma-derived serum plus 25 μg/ml low density lipoprotein with or without growth factors and/or purified human AFP. In this system AFP alone does not stimulate proliferation. However, when combined with epidermal growth factor (EGF) and insulin-like growth factor-I (IGF-I; 10 ng/ml each), AFP (5 μg/ml) significantly (P < 0.01) enhanced growth factor-mediated proliferation 4.5-fold over that of medium controls. Equivalent doses of purified human serum albumin or transferrin demonstrated no effect. The effects of AFP were dose dependent, with significant (P < 0.05) enhancement of proliferation (2.7-fold over controls) observed with as little as 0.313 μg/ml AFP. Increased proliferation was noticed as early as 24 h after the addition of AFP and by 48 h AFP, EGF, and IGF-I had significantly (P < 0.05) increased proliferation over that seen in medium controls, cells treated with EGF plus IGF-I, or cells treated with 10% fetal calf serum plus EGF, and this trend continued linearly over 5 days of culture. AFP (5 μg/ml) significantly increased the proliferative response observed with increasing doses of EGF, IGF-I, or EGF plus IGF-I, but did not appear to alter the dose-response curves. AFP dose-dependently (1.25-5 μg/ml) and significantly (P < 0.05) increased proliferation of porcine granulosa cells in response to platelet-derived growth factor (PDGF) and EGF (25 and 10 ng/ml, respectively), but not to PDGF alone. In contrast, AFP produced no further proliferation of porcine thecal cells in response to PDGF plus EGF. Binding of EGF, IGF-I, or PDGF to purified AFP could not be demonstrated. These results demonstrate that physiological levels of AFP, although not mitogenic alone, can significantly enhance the mitogenic activity of EGF plus IGF-I/PDGF and may function to modulate growth factor-mediated cell proliferation during development and neoplasia.
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