Vascular smooth muscle cell (SMC) growth is under the influence of various growth factors. We demonstrate that platelet-derived growth factor (PDGF) stimulates DNA synthesis of cultured bovine aortic SMCs by 2.5- to 3.5-fold. PDGF also exhibits additivity with insulin and insulin-like growth factor I (IGF-I) for DNA synthesis and cellular proliferation. Insulin (2 x 10(-6) M), IGF-I (1 x 10(-8) M), and PDGF (1 x 10(-9) M) cause a 60-80% increase in cell numbers over basal, but PDGF with insulin or IGF causes a 40-150% increase over basal. No additivity between insulin and IGF-I is evident. PDGF also induces commitment to DNA synthesis earlier than insulin or IGF-I. After exposure to PDGF for 4 h, SMCs incorporate 3H-thymidine to 60% of maximum (with PDGF alone) levels (achieved after exposure of 12 h or longer). Insulin and IGF-I exposure for 4 h, on the other hand, achieves 3H-thymidine incorporation that is only a 20-30% of maximum (with insulin or IGF-I alone). Insulin, IGF-I, and PDGF increase mRNA levels of the protooncogene c-myc. This induction begins within 30 min of exposure to these growth factors which causes a 4- to 6-fold increase in c-myc mRNA levels. Additivity is also observed between PDGF with insulin or IGF-I, but not between insulin or IGF-I, in c-myc induction. C-myc mRNA levels remain elevated as long as the hormones are present, although there's a tendency for the mRNA levels to fall off with insulin and IGF-I.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin and insulin-like growth factor I (IGF-I) are structurally related polypeptides that stimulate DNA synthesis and cellular proliferation, probably through a common pathway. Human arterial smooth muscle cells in culture demonstrated the presence of high-affinity receptors for both these hormones. Insulin and IGF-I both exhibited cross-reactivity to each other's receptors but with an affinity that is 100-fold less than for the homologous receptor. To examine more closely the receptor responsible for producing the growth effects, we used the polyclonal antibody against the insulin receptor, B2, and a monoclonal antibody to the IGF-I receptor, αIR3 . We studied the growth effects of insulin and IGF-I as measured by stimulation of c-myc, DNA synthesis, and cellular proliferation in the presence and absence of these antibodies. F(ab') fragments of the anti-insulin-receptor antibody at a concentration of 10 μg/ml were capable of displacing >90% of the bound insulin, thus establishing an effective insulin-receptor blockade. Under such blockade, insulin and IGF-I were both capable of doubling the amount of DNA synthesis and cell number in cultured human arterial smooth muscle cells. However, in the presence of a 1:2500 dilution of the monoclonal antibody αlR3 , which caused a 90% displacement of IGF-I bound to its receptor, both the insulin and IGF-I effects on stimulating DNA synthesis or cellular proliferation were inhibited by >90%. These findings demonstrate that the IGF-I receptor is the common pathway for the growth effects of both insulin and IGF-I. Induction of the protooncogene c-myc is recognized as an early response to many mitogenic stimuli. In human smooth muscle cells, insulin (1 × 10−7M) and IGF-I (1 × 10−9M) both cause a 5- to 10-fold increase in c-myc mRNA levels. Induction of c-myc could not be assessed under conditions of selective IGF-I-receptor blockade with the antibody αIR3, because, surprisingly, the antibody itself stimulated c-myc mRNA levels. This induction of c-myc with αIR3, which does not increase DNA synthesis or cellular proliferation, suggests that c-myc induction does not entirely correlate with the growth effects of these hormones.

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