KINETICS OF I-125 PDGF BINDING AND DOWN-REGULATION OF PDGF RECEPTOR IN HUMAN ARTERIAL SMOOTH-MUSCLE CELL STRAINS DURING CELLULAR SENESCENCE IN-VITRO

Platelet-derived growth factor (PDGF) is one of the major mitogens in serum to stimulate replication of human smooth muscle cells (SMCs) in culture. Previous studies using human fibroblasts failed to demonstrate changes in the receptor systems for growth factors during cellular senescence. We investigated the kinetics of I-125-PDGF(-BB) binding and down-regulation of the PDGF receptor in three human arterial SMC strains during cellular aging. The number of specific I-125-PDGF binding sites per cell increased slightly at a population doubling level (PDL) of 60%-80% of life span and then decreased at the PDL above 90%. The number of receptors per cell-surface area decreased with increasing in vitro age. The apparent Kd for the I-125-PDGF binding decreased with in vitro senescence. The internalization and degradation of I-125-PDGF per receptor were significantly reduced in senescent SMCs and the amount of I-125-PDGF that escaped degradation and was recycled back to the cell surface was significantly greater in senescent SMCs than young cells. Furthermore, down-regulation of the PDGF receptor was significantly greater in senescent SMCs than young cells. Immunoblot studies demonstrated that changes in beta-subunit of the PDGF receptor accounted for those in the studies using I-125-PDGF and that tyrosine phosphorylation of the PDGF receptor was significantly greater in young SMCs than aged cells. Our results suggest that age-related changes in the receptor systems for PDCF may be important contributors to the failure of DNA synthesis in senescent SMCs.(C) 1995 Wiley-Liss. Inc.