Basic FGF in astroglial, microglial, and neuronal cultures: characterization of binding sites and modulation of release by lymphokines and trophic factors.

The present study characterizes whether basic fibroblast growth factor (bFGF) is present and released from astroglia, microglia, and hippocampal neurons in vitro. For cell content, bFGF-like immunoreactivity (IR) of cell extracts was measured, whereas release was determined by assessing the levels of bFGF-like IR in media. In addition, the effects of lymphokines and trophic factors that are known to be released from these cells on bFGF release were examined. For all three cell types, bFGF-like IR in extracts of cell lysates was detectable. In addition, media content was highest in astroglial cultures and lowest in neuronal cultures. Although bFGF-like IR of neuronal and microglial media appeared to increase with time in culture, this was likely due to significant astroglial proliferation. Thus, notable levels of bFGF are released by astroglia in vitro. In astroglia, bFGF release was enhanced by interleukin-1 (IL-1), IL-6, and epidermal growth factor (EGF), but not by other lymphokines or NGF. In contrast, bFGF in microglial media was reduced by IL-3, EGF, and NGF, but slightly augmented by gamma-interferon (IFN); other lymphokines were ineffective. In addition, no effects were seen in the neuronal cultures. It is likely that the bFGF found in glial media interacts with bFGF receptors since in both glial and neuronal cell types, a single class of low-capacity (Bmax), high-affinity (Kd) bFGF binding sites was evident. The possibility that endogenous bFGF acts as an autocrine factor for astroglia was further supported by experiments that tested the mitogenic effects of exogenous bFGF on glial cells. bFGF significantly enhanced 3H-thymidine uptake into astroglial, but not microglial, cells in vitro. Thus, the present study demonstrates that a complex regulation of glial bFGF release by astroglia and microglia occurs in vitro. Moreover, the results are consistent with an autocrine role for bFGF in astroglial cultures.