KINETICS OF I-125 PDGF BINDING AND DOWN-REGULATION OF PDGF RECEPTOR IN HUMAN ARTERIAL SMOOTH-MUSCLE CELL STRAINS DURING CELLULAR SENESCENCEIN-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 demonstrat e changes in the receptor systems for growth factors during cellular s enescence. We investigated the kinetics of I-125-PDGF(-BB) binding and down-regulation of the PDGF receptor in three human arterial SMC stra ins during cellular aging. The number of specific I-125-PDGF binding s ites 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 n umber of receptors per cell-surface area decreased with increasing in vitro age. The apparent Kd for the I-125-PDGF binding decreased with i n vitro senescence. The internalization and degradation of I-125-PDGF per receptor were significantly reduced in senescent SMCs and the amou nt of I-125-PDGF that escaped degradation and was recycled back to the cell surface was significantly greater in senescent SMCs than young c ells. Furthermore, down-regulation of the PDGF receptor was significan tly greater in senescent SMCs than young cells. Immunoblot studies dem onstrated that changes in beta-subunit of the PDGF receptor accounted for those in the studies using I-125-PDGF and that tyrosine phosphoryl ation of the PDGF receptor was significantly greater in young SMCs tha n aged cells. Our results suggest that age-related changes in the rece ptor systems for PDCF may be important contributors to the failure of DNA synthesis in senescent SMCs.(C) 1995 Wiley-Liss. Inc.