Platelet-activating factor enhances production of insulin-like growth factor binding proteins in a human adenocarcinoma cell line (HEC-1A)

We have recently demonstrated the existence of an autocrine growth loop driven by platelet-activating factor (PAF) in the human endometrial adenocarcinoma cell line HEC-1A. To investigate a possible cooperation between PAF and the insulin-like growth factor (IGF) system in this cell line, the effect of PAF on insulin-like growth factor binding protein (IGFBP) production as well as binding and biological activities of IGF-I, IGF-II, and the analog Des(1-3)IGF-I have been evaluated. Analysis of self- and cross-displacement curves of [I-125]IGF-I binding to HEC-1A cells indicates the presence of a single class of binding sites, with affinity constants of 1-4 nM for IGF-I and IGF-II and 70 nM for Des(1-3)IGF-I, which binds to IGFBPs with lower affinity. Insulin does not apparently bind to this binding site, Moreover, the addition of increasing concentrations of IGF-I leads to a paradoxical increase of binding, These results indicate a similarity of this binding site to IGFBPs. The presence of IGFBPs has been demonstrated by Western ligand blot analysis of HEC-1A conditioned medium which shows the presence of two bands of 32-34 and 40-45 kDa. By Western immunoblotting analysis, the two bands were respectively identified as IGFBP-2 and IGFBP-3. Incubation with PAF (1 mu M) highly increases the release of the two IGFBPs from the cells, Such an effect is inhibited by the PAF receptor antagonist 1.659,989, by the PKC inhibitor sangivamycin, and by the tyrosine kinase inhibitor genistein. PAF also induces a time-dependent increase of mRNA expression for IGFBP-3, suggesting an effect on synthesis of this protein, IGF-I and IGF-II (0.1-100 nM) are almost ineffective in inducing [H-3]thymidine incorporation, whereas a slight proliferative effect is observed with Des(1-3)IGF-I which also increases PAF synthesis. These data demonstrate a modulatory action of PAF on IGFBP secretion in HEC-1A cells and indicate that the IGF system plays a minor, if any, modulatory role on proliferation of this cell line.