Regulation of basic fibroblast growth factor mRNA expression in rat C6 glioma cells

Multiple basic fibroblast growth factor (bFGF) mRNAs are transcribed in rat brain at 6.0, 3.7, 2.5, 1.8, 1.6, 1.4, and 1.0 kb. These seven transcripts are also seen in Rat-1 fibroblasts and ras-transformed Rat-1 fibroblasts in culture. However, only a single bFGF transcript at 6.0 kb is detectable in the rat astrocytoma cell line, C6, and this mRNA is identical to that seen in a primary culture of cortical astrocytes. C6 glioma cells also transcribe message for FGF receptor 1 (FR1), suggesting possible autocrine growth by these cells. Growth factor activity in a C6 cell lysate was characterized by heparin affinity chromatography and Western blot analysis using an anti-bFGF antibody. Proteins of 18, 21.5, and 22 kDa were detected in C6 cells, indicating that the 6.0-kb mRNA is translated into the three characteristic bFGF proteins. Rat-1 fibroblasts also synthesize bFGF proteins of identical molecular weight. The small transcripts detected in brain probably represent bFGF or FGF-related mRNAs in cell types other than glia, such as fibroblasts, endothelial cells, or neurons. In cultured C6 cells, bFGF protein levels are highest in confluent, quiescent cells, whereas mRNA levels are low. Addition of serum, phorbol ester, or cycloheximide to both C6 cells and fibroblasts induces the level bFGF mRNA transcripts 10-fold after 1-4 h. This rapid induction after cell activation indicates that bFGF is an early response gene. Therefore, even though there are abundant intracellular stores of the factor, the transcriptional activation seen after mitogenic activation of cells implies that de novo bFGF mRNA synthesis is an important part of the mitogenic response.