REGULATION OF PLATELET-DERIVED GROWTH-FACTOR RECEPTOR EXPRESSION BY CELL CONTEXT OVERRIDES REGULATION BY CYTOKINES

Immunocytochemical data has indicated that platelet-derived growth fac tor receptor beta-subunit (PDGFR beta) expression by connective tissue cells is up-regulated in many disease states. To investigate potentia l causes of this up-regulation, we have evaluated conditions that regu late PDGF receptor transcript levels in cultured diploid human fibrobl ast model systems. We found combinations of soluble mediators and cell ''context,'' which can regulate receptor transcripts (and receptor pr otein) over a 50-fold range, with cell context factors being far more potent regulators than soluble mediators. For cells grown under standa rd monolayer conditions on plastic, levels of both PDGFR beta and PDGF R alpha increase 10-fold as culture density increases. Cells grown in suspension or in three-dimensional gels express 10- to 20-fold higher transcript levels than cells plated on plastic at comparable density a nd serum concentration. The soluble mediators tested, in eluding 14 cy tokines and conditioned medium from activated lymphocytes, have only m odest effects on transcript levels. Lymph decreases PDGFR beta transcr ipt expression 4-fold, suggesting that a component of interstitial flu id contributes to maintenance of the low basal level of expression in normal tissues. The mitogenic responsiveness of cells cultured at diff erent densities parallels the level of PDGFR beta expression. Blocking anti-PDGF receptor antibodies decrease receptor availability and mito genic responsiveness in parallel. In both cases, the striking overlap between the PDGF-BB binding and mitogenesis dose-response curves sugge sts that the level of PDGF receptor expression can limit responsivenes s to PDGF. Overall, these results suggest that the up-regulation of PD GF receptor expression seen under pathological conditions may be due t o disruption of the cell's normal environment/context/cell shape/cell attachment and that this could serve to ensure that a proliferative re sponse to PDGF would occur only under conditions in which there had be en significant tissue damage. (C) 1996 Wiley-Liss, Inc.