Fibroblast growth factor messenger ribonucleic acid expression in ahuman astrocytoma cell line:regulation by serum and celldensity

Abstract

A human astrocytoma cell line, U87-MG, synthesizes a growth factor which is structurally related to basicfibroblast growth factor (bFGF) by several criteria:1) it binds to heparin-Sepharose and elutes at 2 mNaCI; 2) it cross-reacts with N-terminal specific anti-bFGF antibodies; 3) it is a potent mitogen for rabbitfetal chondrocytes. Northern blotting analysis of total RNA reveals that the cells express high levels oftwo bFGF mRNA transcripts of 7 and 3.7 kilobase insize. The levels of both transcripts rise rapidly(within 3 h) after addition of serum to serum-deprived cultures, reach a maximum within 6-12 h andremain elevated for at least 24 h. Basic FGF mRNAexpression was low in confluent cultures but wasincreased after replating at sparse density. Transcript levels began to increase 4 h after plating, reaching a maximum (7-fold above confluent cells)within 24 h. The rise in bFGF mRNA levels waspreceded by a rapid rise in c-myc expression whichpeaked 8 h after plating and then declined. The levelof bFGF mRNA expression began to decline by 48h, before any detectable increase in cell number, and returned to control levels when the cells reachedconfluence after 10 days. The level of transforminggrowth factorβmRNA, which is also expressed inthese cells, was not affected by cell density. Theeffects of density on bFGF mRNA Ifevels were notduplicated by culturing low density cells in conditioned medium from confluent cultures or in mediumcontaining 10 ng/ml bFGF. These findings indicate that bFGF expression is tightly regulated by cell density, and may be directlycontrolled by cell-cell contact or by factors presentin the extracellular matrix. The transient peak ofexpression shortly after plating suggests that bFGFexpression is closely associated with factors regulating cell cycle progression, and may help to explainthe low levels of expression detected in differentiated normal tissues. © 1988 by The Endocrine Society.