The insulin-like growth factor (IGF)-I E-peptides modulate cell entry of the mature IGF-I protein.
Insulin-like growth factor (IGF)-I is a critical protein for cell development and growth. Alternative splicing of the igf1 gene gives rise to multiple isoforms. In rodents, proIGF-IA and proIGF-IB have different carboxy-terminal extensions called the E-peptides (EA and EB) and upon further posttranslational processing, produce the identical mature IGF-I protein. Rodent EB has been reported to have mitogenic and motogenic effects independent of IGF-I. However, effects of EA or EB on mature IGF-I, or whether proIGF-IA and proIGF-IB have different properties, have not been addressed. To determine whether the presence of EA or EB affected the distribution and stability of mature IGF-I protein, transient transfections of cDNAs encoding murine IGF-IA, IGF-IB, and mature IGF-I were performed in C2C12 cells, a skeletal muscle cell line. IGF-I secretion was measured by enzyme-linked immunosorbent assay of the media, and did not differ between expression of proIGF-IA, proIGF-IB, or mature IGF-I expression. Next, epitope-tagged constructs were transfected to determine cellular distribution of IGF-I, EA, and EB in the cells throughout the culture. IGF-I was detected in significantly fewer nontransfected cells in cultures transfected with mature IGF-I compared with transfection of proIGF-IA or proIGF-IB. These results demonstrate that EA and EB are not required for IGF-I secretion but that they increase cell entry of IGF-I from the media. This study provides evidence that the EA and EB may modulate IGF-I in addition to having independent activity.
Pfeffer
Lindsay A
LA
Department of Anatomy and Cell Biology, School of Dental Medicine, and Pennsylvania Muscle Institute, University of Pennsylvania, Philadelphia, PA 19104, USA.
Brisson
Becky K
BK
Lei
Hanqin
H
Barton
Elisabeth R
ER
D000818:Animals; D002460:Cell Line; D004705:Endocytosis / Q000502:physiology*; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D010446:Peptide Fragments / Q000235:genetics / Q000378:metabolism*; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism*; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism*; D014162:Transfection
D010446:Peptide Fragments; D020033:Protein Isoforms; D011498:Protein Precursors; D007334:Insulin-Like Growth Factor I