Insulin and insulin-like growth factor II differentially regulate endocytic sorting and stability of insulin receptor isoform A.
The insulin receptor isoform A (IR-A) binds both insulin and insulin-like growth factor (IGF)-II, although the affinity for IGF-II is 3-10-fold lower than insulin depending on a cell and tissue context. Notably, in mouse embryonic fibroblasts lacking the IGF-IR and expressing solely the IR-A (R-/IR-A), IGF-II is a more potent mitogen than insulin. As receptor endocytosis and degradation provide spatial and temporal regulation of signaling events, we hypothesized that insulin and IGF-II could affect IR-A biological responses by differentially regulating IR-A trafficking. Using R-/IR-A cells, we discovered that insulin evoked significant IR-A internalization, a process modestly affected by IGF-II. However, the differential internalization was not due to IR-A ubiquitination. Notably, prolonged stimulation of R-/IR-A cells with insulin, but not with IGF-II, targeted the receptor to a degradative pathway. Similarly, the docking protein insulin receptor substrate 1 (IRS-1) was down-regulated after prolonged insulin but not IGF-II exposure. Similar results were also obtained in experiments using [NMeTyr(B26)]-insulin, an insulin analog with IR-A binding affinity similar to IGF-II. Finally, we discovered that IR-A was internalized through clathrin-dependent and -independent pathways, which differentially regulated the activation of downstream effectors. Collectively, our results suggest that a lower affinity of IGF-II for the IR-A promotes lower IR-A phosphorylation and activation of early downstream effectors vis à vis insulin but may protect IR-A and IRS-1 from down-regulation thereby evoking sustained and robust mitogenic stimuli.
Department of Urology and Endocrine Mechanisms and Hormone Action Program, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, USA.
Alaide
A
Morcavallo
Marco
M
Genua
Angela
A
Palummo
Emilia
E
Kletvikova
Jiri
J
Jiracek
Andrzej M
AM
Brzozowski
Renato V
RV
Iozzo
Antonino
A
Belfiore
Andrea
A
Morrione
D000818:Animals; D049109:Cell Proliferation / Q000187:drug effects; D002966:Clathrin / Q000378:metabolism; D015536:Down-Regulation / Q000187:drug effects; D004705:Endocytosis / Q000187:drug effects*; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins / Q000378:metabolism; D007335:Insulin-Like Growth Factor II / Q000494:pharmacology*; D008024:Ligands; D051379:Mice; D041681:NIH 3T3 Cells; D010766:Phosphorylation / Q000187:drug effects; D055550:Protein Stability / Q000187:drug effects; D021381:Protein Transport / Q000187:drug effects; D011972:Receptor, Insulin / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D047392:beta-Cyclodextrins / Q000494:pharmacology
D002966:Clathrin; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D008024:Ligands; D047392:beta-Cyclodextrins; C108732:methyl-beta-cyclodextrin; D007335:Insulin-Like Growth Factor II; D011972:Receptor, Insulin