Interaction of secreted insulin-like growth factor-I (IGF-I) with cell surface receptors is the dominant mechanism of IGF-I's autocrine actions.
In a prior report we presented evidence that insulin-like growth factor-I (IGF-I) can act in an autocrine fashion by demonstrating that FRTL-5 cells transfected with hIGF-IA fusion genes express and secrete biologically active IGF-I that renders the stimulation of DNA synthesis in FRTL-5 cells independent of their requirement for exogenous IGFs or insulin. To determine if IGF-I's autocrine actions require secretion or can be mediated by interactions with intracellular receptors, we have created a new line of FRTL-5 cells that express a mutant IGF-IA precursor containing the endoplasmic reticulum retention amino acid sequence, Lys-Asp-Glu-Leu (KDEL), at its carboxyl terminus. The mutant IGF-IA/KDEL precursor expressed by stably transfected FRTL-5 cells was shown to be retained intracellularly and to have biological activity comparable with mature IGF-I, as judged by the activity of partially purified IGF-IA/KDEL in wild type FRTL-5 cells. Expression of IGF-IA/KDEL in FRTL-5 cells, however, neither augmented TSH-stimulated DNA synthesis nor stimulated IGF-binding protein-5 expression, as does IGF-IA expression in transfected FRTL-5 cells and the addition of exogenous IGF-I to wild type FRTL-5 cells. IGF-IA/KDEL expression, however, desensitized FRTL-5 cells to the actions of exogenous IGF-I despite having only minimal effects on cell surface type I receptor number, suggesting that intracellular IGF-I is capable of significant biological actions. The failure of IGF-IA/KDEL to replicate the actions of secreted IGF-I, taken together with the findings that a monoclonal antibody against IGF-I blocked IGF-I's actions in IGF-I-secreting transfected FRTL-5 cells, provides evidence that IGF-I secretion and interaction with cell surface type I IGF receptors is the dominant mechanism of IGF-I's autocrine actions.
Dai
Z
Z
Department of Pediatrics, University of North Carolina, Chapel Hill 27599-7220.
Stiles
A D
AD
Moats-Staats
B
B
Van Wyk
J J
JJ
D'Ercole
A J
AJ
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D002455:Cell Division; D002478:Cells, Cultured; D003001:Cloning, Molecular; D015870:Gene Expression; D066298:In Vitro Techniques; D018975:Insulin-Like Growth Factor Binding Protein 5; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008969:Molecular Sequence Data; D009838:Oligodeoxyribonucleotides / Q000737:chemistry; D011498:Protein Precursors / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017451:Receptors, Somatomedin; D013329:Structure-Activity Relationship; D014162:Transfection
D002352:Carrier Proteins; D018975:Insulin-Like Growth Factor Binding Protein 5; D009838:Oligodeoxyribonucleotides; D011498:Protein Precursors; D012333:RNA, Messenger; D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D007334:Insulin-Like Growth Factor I