MODULATION OF TRANSCRIPTION OF THE RAT FIBRONECTIN GENE BY CELL-DENSITY

The fibronectin (FN) gene is under complex regulatory control in vitro and in vivo. Sequences from the rat FN gene directed efficient expres sion of a lacZ reporter gene product, beta-galactosidase, in NIH/3T3 m ouse fibroblasts. Stable transfectants were generated to facilitate st udies of gene regulation by cell growth state. The expression of FN-la cZ constructs increased approximately twofold when cultures attained c onfluence, relative to total protein. The magnitude of this increase c orrelates well with that observed for FN mRNA levels and protein synth esis rate. Fragments containing 4.9, 0.9, or 0.3 kbp upstream of the t ranscription start site are equally responsive to cell density and/or cell contact. Deletion of a cAMP-responsive element enhanced the respo nse, suggesting a negative role for this sequence motif and demonstrat ing that the FN gene is regulated by cell density at the transcription al level. The effect oi high cell density is apparently different from decreased growth rate, as incubation with low serum did not result in increased expression of the lacZ reporter. Finally, conditioned mediu m from dense cells did not enhance reporter gene expression in sparse cells, suggesting that the density signal is not transmitted via a sol uble factor. (C) 1996 Wiley Liss, Inc.