Deficient processing and activity of type I insulin-like growth factor receptor in the furin-deficient LoVo-C5 cells

Endocrinology. 1998 Sep;139(9):3763-71. doi: 10.1210/endo.139.9.6184.


To investigate endoproteolytic processing of the type I insulin-like growth factor receptor (IGF-IR), we have examined its structure and activity in the furin-deficient LoVo-C5 cell line. Immunoprecipitation experiments using the monoclonal anti-IGF-IR antibody (alpha-IR3) showed that LoVo-C5 cells expressed a major high molecular mass receptor (200 kDa) corresponding to the unprocessed alpha/beta pro-receptor. A small amount of successfully cleaved alpha/beta heterodimers was also produced, indicating a residual endoproteolytic cleavage activity in these cells. In vitro, a soluble form of recombinant furin was able to cleave the pro-IGF-IR (200 kDa) into alpha-subunit (130 kDa) and beta-subunit (97 kDa). Measurement of IGF binding parameters in LoVo-C5 cells indicated a low number of typical type I IGF-binding sites (binding capacity, 5 x 10(3) sites/cell; Kd, 1.9 nM for IGF-I and 7.0 nM for IGF-II). These findings in LoVo-C5 contrast with those in HT29-D4 cells, which have active furin, and where IGF-IR (2.8 x 10(4) sites/cell) was fully processed. Moreover, the 200-kDa pro-IGF-IR of LoVo-C5 was unable to induce intracellular signaling, such as beta-subunit tyrosine autophosphorylation and insulin-related substrate-1 tyrosine phosphorylation. Flow immunocytometry analysis using alpha-IR3 antibody indicated that LoVo-C5 cells expressed 40% more receptors than HT29-D4 cells, suggesting that in LoVo-C5 cells only the small amount of mature type I IGF-IR binds IGFs with high affinity. To provide evidence for this idea, we showed that mild trypsin treatment of living LoVo-C5 cells partially restored alpha/beta cleavage of IGF-IR, and greatly enhanced (6-fold) the IGF-I binding capacity of LoVo-C5 cells, but did not restore IGF-IR signaling activity. Moreover, LoVo-C5 cells were totally unresponsive to IGF-I in terms of cell migration, in contrast to fully processed IGF-IR-HT29-D4 cells. Our data indicate that furin is involved in the endoproteolytic processing of the IGF-IR and suggest that this posttranslational event might be crucial for its ligand binding and signaling activities. However, our data do not exclude that other proprotein convertases could participate to IGF-IR maturation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • ADP Ribose Transferases*
  • Bacterial Toxins*
  • Cell Movement / drug effects
  • Drug Resistance
  • Exotoxins / pharmacology
  • Flow Cytometry
  • Furin
  • Humans
  • Insulin-Like Growth Factor I / metabolism*
  • Insulin-Like Growth Factor I / pharmacology
  • Phosphorylation
  • Protein Processing, Post-Translational*
  • Receptors, Somatomedin / metabolism*
  • Signal Transduction / physiology
  • Subtilisins / deficiency*
  • Trypsin / pharmacology
  • Tumor Cells, Cultured / drug effects
  • Tumor Cells, Cultured / metabolism
  • Tyrosine / metabolism
  • Virulence Factors*


  • Bacterial Toxins
  • Exotoxins
  • Receptors, Somatomedin
  • Virulence Factors
  • Tyrosine
  • Insulin-Like Growth Factor I
  • ADP Ribose Transferases
  • toxA protein, Pseudomonas aeruginosa
  • Subtilisins
  • Trypsin
  • Furin