Abstract
We have investigated the autocrine regulation of insulin-like growth factor-II (IGF-II) signaling by the insulin-like growth factor-I receptor (IGF-IR) and the insulin-like growth factor-II/mannose 6-phosphate receptor (IGF-IIR) in MCF-7 breast cancer cells, employing retroviruses encoding both IGF-I, IGF-II, and IGF-I and II mutants with reductions in affinity for either the IGF-IR or the IGF-IIR. These studies revealed reciprocal roles for IGF-IR and IGF-IIR affinity in the regulation of autocrine IGF-II activity. IGF-IR affinity was required for serum-free proliferation but also for efficient IGF-II secretion. In contrast, cellular proliferation, receptor tyrosine kinase-dependent signaling, and extracellular IGF-II protein accumulation were all reduced in the presence of IGF-IIR affinity. Inhibition of IGF-II signaling appeared to be the sole consequence of IGF-IIR affinity, as no cellular responses attributable to selective IGF-IIR binding by a reduced IGF-IR affinity IGF-II mutant could be detected. By operating as an IGF-II antagonist, the IGF-IIR has tumor suppressor-like properties, a suggestion consistent with reports of loss of heterozygosity at the IGF-IIR locus in a variety of human malignancies.
