EPIDERMAL GROWTH-FACTOR (EGF) RECEPTOR DENSITY CONTROLS MITOGENIC ACTIVATION OF NORMAL RAT-KIDNEY (NRK) CELLS BY EGF

Normal rat kidney (NRK) fibroblasts are immortalized ce:ls that are st rictly dependent on externally added growth factors for proliferation. When cultured in the presence of epidermal growth factor (ECF) as the only growth stimulating hormone, these cells have a normal phenotype and undergo density-dependent growth inhibition. It has been postulate d that this density-arrest results from a decrease of ECF receptor lev els below a threshold level which makes these cells unresponsive to st imulation by ECF. In the present study, we show that NRK cells, made q uiescent by serum-deprivation at submaximum density, are mitogenically still responsive to ECF, but show enhanced mitogenic stimulation afte r 8 hr pre-treatment with either transforming growth factor beta (TGF beta) or retinoic acid (RA), while prostaglandin F-2 alpha (PGF(2 alph a)) and bradykinin (BK) enhance the mitogenic stimulation by EGF only slightly under these conditions. Addition of TGF beta or RA results in an increase of both (125)(sic)-EGF-binding capacity and EGF receptor mRNA levels. Using flow cytometric analysis, we show that pre-treatmen t with TGF beta or RA increases the percentage of cells entering the c ell cycle as a function of time. Furthermore, pre-treatment of the cel ls with TGF beta or RA increases the rate of mitogen-activated protein kinase (MAPK) phosphorylation by ECF. PCF2 alpha and BK also increase ECF receptor levels, but only with delayed kinetics. These results sh ow that already in serum-deprived quiescent NRK cells, EGF receptor le vels limit ECF-induced mitogenic stimulation. This observation provide s further evidence for the regulating role of the ECF receptor in dens ity-dependent growth control of NRK cells. (C) 1998 Wiley-Liss, Inc.