Cell-density-dependent modulation of the rat insulin-like-growth-factor-binding protein 2 and its gene

The steady-state level of the rat insulin-like-growth-factor-binding protein 2 (IGFBP-2) and insulin-like-growth-factor-II (IGF-II) mRNA increased approximately 20-fold when BRL-3A cells were cultured at increasingly higher cell densities. This increase could not be accounted for by paracrine or autocrine factors, or by the addition of insulin, IGF-I, transforming growth factor beta (TGF-beta), cAMP or IGFBP-2 to the culture medium. A reporter gene assay carrying the promoter domain of the IGFBP-2 gene indicated that the promoter-dependent IGFBP-2 transcription is tenfold higher in high-density cells. The increase in the IGFBP-2 message was accompanied by an increase in the level of protein in the medium. When confluent BRL-3A cells were reseeded at low cell density, the IGFBP-2 mRNA disappeared at a rate significantly faster than in normal conditions. A protein synthesis inhibitor, cycloheximide, was able to prevent the decay of the message observed after the switch from high to low densities. In summary, these findings suggest a regulatory link between cell density and IGFBP-2.