IGF-1 receptor signalling determines the mitogenic potency of insulin analogues in human smooth muscle cells and fibroblasts.
OBJECTIVE
Mitogenic activity of insulin and insulin analogues and the involvement of the IGF-1 receptor (IGF-1R) is still a controversial issue. We compared levels of the proteins IGF-1R and insulin receptor (InsR) in fibroblasts and smooth muscle cells from healthy donors and assessed the downstream signalling and growth-promoting activity of insulin and insulin analogues.


METHODS
DNA synthesis was monitored in human fibroblasts and coronary artery smooth muscle cells. Using small interfering RNAs, the levels of IGF-1 and InsR were reduced by 95 and 75%, respectively.


RESULTS
Enhanced mitogenic potency of insulin and insulin analogues was observed which correlated with increased levels of IGF-1R and/or IRS-1. A reduction in the IGF-1R level significantly blunted stimulation of Akt phosphorylation by IGF-1, AspB10 and glargine by 72, 58 and 40%, respectively. Akt phosphorylation in response to insulin remained unaffected. Silencing of InsR did not significantly alter Akt phosphorylation in response to IGF-1, AspB10 and glargine. IGF-1R knockdown reduced the stimulation of DNA synthesis in response to IGF-1 and glargine to a level identical to that produced by insulin.


CONCLUSIONS
These data show a prominent role of IGF-1R/Akt signalling in mediating the mitogenic effects of insulin analogues. Regular insulin stimulates DNA synthesis by exclusively activating InsR, whereas insulin analogues mainly signal through IGF-1R. It is suggested that inter-individual differences in the levels of proteins of the IGF-1R system may function as a critical determinant of the mitogenic potency of insulin analogues.
Eckardt
K
K
Institute of Clinical Biochemistry and Pathobiochemistry, German Diabetes Center, Auf'm Hennekamp 65, D-40225, Dusseldorf, Germany.
May
C
C
Koenen
M
M
Eckel
J
J
D000293:Adolescent; D000328:Adult; D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D002648:Child; D005260:Female; D005347:Fibroblasts / Q000187:drug effects* / Q000502:physiology; D006801:Humans; D007328:Insulin / Q000031:analogs & derivatives* / Q000494:pharmacology*; D061267:Insulin Aspart; D000069036:Insulin Glargine; D061268:Insulin Lispro; D049528:Insulin, Long-Acting; D008297:Male; D008875:Middle Aged; D008934:Mitogens / Q000494:pharmacology; D008938:Mitosis / Q000187:drug effects*; D032389:Myocytes, Smooth Muscle / Q000187:drug effects* / Q000502:physiology; D017526:Receptor, IGF Type 1 / Q000378:metabolism / Q000502:physiology*; D015398:Signal Transduction / Q000187:drug effects
D007328:Insulin; D061268:Insulin Lispro; D049528:Insulin, Long-Acting; D008934:Mitogens; C070690:insulin, Asp(B10)-; D000069036:Insulin Glargine; C479079:insulin glulisine; D061267:Insulin Aspart; D017526:Receptor, IGF Type 1