Stromal cells derived from benign prostatic hyperplasia synthesize and secrete measurable levels of insulin-like growth factor (IGF). Seventy-two-hour conditioned medium obtained from these cells contains IGF-II at levels ranging from 125-177 ng/mL.10(6) cells. IGF-I is almost undetectable. RT-PCR analysis has demonstrated that the genes for both the type I IGF receptor (IGF-IR) and the type II IGF receptor (IGF-IIR) are expressed by benign stromal cells in vitro. Competition binding analysis for IGF-I and IGF-II confirmed the existence of binding sites for both ligands with respective Kd and binding capacities of 4.9 x 10(-9) mol/L and 6.6 x 10(5) sites/cell and 4.7 x 10(-9) mol/L and 3.8 x 10(6) sites/cell. Under serum-free conditions, IGF-I and IGF-II at 500 ng/mL induce 80% and 113% increases in stromal cell density, respectively, over a 96-h period. Incubation with the IGF-IR-neutralizing antibody alpha IR3 (50 micrograms/mL) reduces the rate of stromal cell proliferation by approximately 60-80% even in the presence of stimulatory concentrations of IGFs. Camptothecin-induced apoptosis is inhibited by the addition of IGF-I and -II (500 ng/mL). alpha IR3 suppresses these survival signals and itself induces cell death in the prostatic stroma. The data suggest that IGF-IR is a pivotal molecule in prostatic stromal cell maintenance, and that specific antagonism may offer a novel means of controlling the fibromuscular expansion characteristic of benign prostatic hyperplasia.