THE PROCESSING OF INSULIN-LIKE GROWTH-FACTOR-I (IGF-I) BY A CULTURED KIDNEY-CELL LINE IS ALTERED BY IGF-BINDING PROTEIN-3

The proximal renal tubule is a common site of peptide hormone metabolism, including that of insulin-like growth factor-I (IGF-I). To further explore the renal uptake and processing of IGF-I, a study was carried out with the proximal-like cultured opossum kidney (OK) cell line. [I-125]IGF-I associated with these cells in a specific manner. Association was competitively inhibited by IGF-I. Des(1-3)-IGF-I was equally effective, insulin had only a small effect, and the unrelated peptides, glucagon and GH, were without effect. Degradation was inhibited in similar manner. Comparisons of [I-125]IGF-I with [I-125]insulin revealed comparable cell association, but degradation of internalized IGF-I was several-fold slower. Furthermore, IGF-I degradation was less sensitive, by half, to the inhibitory effect of chloroquine. When OK cells were exposed to [I-125]IGF-I in the presence of IGF-binding protein-3 (IGFBP-3) cell association (binding and internalization) was reduced significantly. Of note, total cell degradation was reduced (P < 0.01), but the IGF-I that was internalized was degraded more rapidly than in control cells. Gel filtration and reverse phase HPLC revealed that the products of IGF-I degradation included large IGF-I-size intermediates in addition to trichloroacetic acid-soluble material. This product profile was not altered by IGFBP-3. Thus, as previously described for insulin, cultured OK cells possess specific IGF-I receptors and degrade internalized IGF-I. However, IGF-I processing differs from that of insulin, in that degradation is slower and relatively insensitive to competition by insulin. This study also shows that IGFBP-3 inhibits the binding and uptake of[I-l25]IGF-I by these kidney cells. However, once IGF-I is internalized, IGFBP-3 enhances degradation. Although the mechanism of this paradoxical action requires further study, analysis of the products of degradation suggests that the same enzymes are involved in IGF-I degradation regardless of whether IGFBP-3 is present.