Vascular endothelial growth factor (VEGF) is an angiogenic hormone that is produced by and supports the growth of many types of malignancies. The present study shows that insulin-like growth factor 1 (IGF-I), a mitogen that promotes the propagation of cancers through autocrine and paracrine mechanisms, increases the expression of mRNA for VEGF and production of VEGF protein by COLO 205 colon carcinoma cells. IGF-I also induces expression of VEGF mRNA in SW620, LSLiM6, and HCT15 colon carcinoma cells showing that this is a common response to IGF-I. Whereas IGF-I induced VEGF mRNA in each cell line examined (2.3-12-fold), it induced proliferation only in COLO 205 and LSLiM6 cells. Thus, the proliferative response induced by IGF-I and its ability to induce VEGF occur through distinguishable mechanisms. IGF-I increases the cellular content of VEGF mRNA by increasing the rate of transcription (5-fold after 4 h) and also by increasing the half-life of VEGF mRNA (0.6 +/- 0.07 h in control cells to 2.0 +/- 0.37 h in IGF-I-treated cells). Monoclonal antibody (alphaIR3) directed against the type 1 IGF receptor significantly attenuated the ability of IGF-I to promote expression of VEGF mRNA. Interestingly, by itself alphaIR3 acted as a weak agonist and induced a modest increase in the cellular content of VEGF mRNA. alphaIR3 also promoted tyrosine phosphorylation of the beta subunit of the IGF-I receptor, and the magnitude of this response was comparable with that induced by IGF-I. These observations point to a nonlinear relationship between activation of the IGF-I receptor and induction of VEGF mRNA. Thus, in addition to its direct, growth stimulatory effect on transformed cells, IGF-I induces the expression of VEGF which can promote the progression of cancer by regulating the development of new blood vessels.