HEPATOCYTE-CONDITIONED MEDIUM POTENTIATES INSULIN-LIKE GROWTH-FACTOR (IGF) 1 AND 2 STIMULATED DNA-SYNTHESIS OF CULTURED FAT-STORING CELLS

IGF-1 and IGF-2 stimulate dose-dependently DNA synthesis of nonconflue nt cultures of rat fat storing cells, a nonparenchymal type of liver c ells pathogenetically involved in the generation of liver fibrosis. Ma ximum stimulation of [H-3] thymidine incorporation of about 2.6-fold a bove control was reached with 100 ng/ml IGF-1 and 500 ng/ml IGF-2, res pectively. The DNA synthesis promoting action of both IGF-1 and IGF-2 was most efficiently potentiated by hepatocyte-conditioned medium rais ing the stimulatory effect up to 21-fold above control cultures. Lysat e of hepatocytes (up to 15 mug protein/ml) was not effective in potent iating the effect of IGF-1. IGF-1 is bound to free carrier protein(s) present in the medium of hepatocytes, but obviously absent in cell lys ate. Three molecular weight fractions in the ranges of 67 kd, 35 kd. a nd 25 kd could be identified in the medium, which potentiate the growt h-promoting effect of IGF-1. Applying Western ligand blot analysis, th ree molecular size classes of IGF-1 binding proteins in the conditione d media of rat hepatocytes were determined. The major binding protein had a M(r) of 28 34 kd, a minor portion was localized at M(r) 24 kd, w hereas trace binding affinities were found at M(r) of about 95 kd. It is suggested that IGF-1, IGF-2 and the complex array of' IGF-binding p roteins secreted by hepatocytes might be involved in the paracrine reg ulation of growth of fat storing cells.