EVIDENCE FOR AN AGE-RELATED DYSFUNCTION IN THE ANTIPROLIFERATIVE RESPONSE TO TRANSFORMING GROWTH-FACTOR-BETA IN VASCULAR SMOOTH-MUSCLE CELLS

Previous studies have indicated that aged animals show an increased intimal hyperplasia after arterial injury. The present studies examined the hypothesis that the increased serum-free proliferation of aged smooth muscle cells (SMC), in vitro, was due to a loss of an antiproliferative signal, such as transforming growth factor-beta1 (TGF-beta1). Northern blot analysis of the mRNA derived from old (>19 mo) or young (3-4 mo) rat aortic SMC indicated that both groups had an equivalent level of the 2.5 kB TGF-beta1 message. Metabolic labeling with S-35-methionine and immunoprecipitation for TGF-beta1 confirmed the de novo synthesis of TGF-beta1 in rat SMC. Old and young SMC supernatants showed equal levels of active or latent (acid-activated) TGF-beta activity. Despite the similarities in the production of TGFbeta1, old SMC were refractory to inhibition by TGF-beta1, whereas young SMC were markedly inhibited (80%) by low levels of TGF-beta1 (IC50 < 5 pg/ml). Binding studies at 4-degrees-C indicated that old SMC exhibited reduced binding capacity for I-125-TGF-beta1. Cross-linking studies confirmed that old SMC showed reduced binding of I-125-TGF-beta1 to membrane sites corresponding to the high molecular weight type III receptor, as well as the 85-kDa type II and 65-kDa type I receptor. However, at 37-degrees-C, old SMC degraded I-125 TGF-beta1 more rapidly than young SMC. Combined, this data suggests that SMC derived from older animals are capable of normal production of TGF-beta1 but fail to respond to the autocrine growth inhibitory effects of this agent, thereby leading to enhanced proliferation.