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Insulin and insulin-like growth factor-I receptors similarly stimulate deoxyribonucleic acid synthesis despite
C C Mastick1, H Kato, C T Roberts
1Department of Signal Transduction, Parke-Davis Pharmaceutical Research Division, Warner-Lambert Co., Ann Arbor, Michigan 48105.
Abstract:
Signal transduction pathways stimulated by insulin or insulin-like growth factor-I (IGF-I) were compared in transfected NIH3T3 fibroblast cell lines expressing the human insulin receptor, IGF-I receptor, or a chimeric IGF-I receptor with its carboxy-terminal tail replaced with that of the insulin receptor (approximately 1 x 10(6) receptors/cell). Although receptor autophosphorylation was very similar in the three cell lines overexpressing receptors (EC50 = 1-3 nM), there were differences detected in the protein tyrosine phosphorylation stimulated by insulin and IGF-I in these cells. Although no substrates specific for the insulin receptor were detected, phosphorylation of a 170-kilodalton (kDa; IRS-1) and a 70-kDa protein was 10 times more sensitive to insulin than to IGF-I (EC50 = 1.5-2.5 vs. 14-23 nM). The chimeric receptor stimulated significantly lower levels of phosphorylation of several proteins relative to the wild-type IGF-I receptor. Activation of phosphatidylinositol 3'-kinase paralleled phosphorylation of the 170- and 70-kDa proteins. Despite these differences in protein tyrosine phosphorylation, stimulation of mitogen-activated protein (MAP) kinase and DNA synthesis were very similar in the three cell lines overexpressing receptors. Little difference was detected in Shc phosphorylation or MAP kinase activation through the three receptors, although activation of MAP kinase was more efficiently coupled to the platelet-derived growth factor receptor than to any of the overexpressed receptors. All three receptors stimulated DNA synthesis to levels comparable to 10% serum, with similar sensitivities (EC50 = 1.5-3.5 nM).
Insights
Insulin and IGF-I receptors show similar effects on cell growth but differ in specific protein phosphorylation, revealing distinct signaling pathways. This comparison aids understanding of insulin receptor and IGF-I receptor signaling.
Area of Science:
- Cellular signaling
- Receptor biology
- Molecular endocrinology
Background:
- Insulin and Insulin-like Growth Factor-I (IGF-I) are crucial hormones regulating cell growth and metabolism.
- Their receptors, the insulin receptor (IR) and IGF-I receptor (IGF-IR), share structural similarities but mediate distinct cellular responses.
- Understanding the nuances of their signal transduction pathways is key to deciphering cellular regulation.
Purpose of the Study:
- To compare signal transduction pathways activated by insulin and IGF-I.
- To investigate the roles of specific receptor domains in mediating downstream signaling.
- To elucidate differences in protein tyrosine phosphorylation, kinase activation, and DNA synthesis.
Main Methods:
- Transfection of NIH3T3 fibroblasts with human IR, IGF-IR, or a chimeric receptor.
- Stimulation with insulin and IGF-I to compare receptor autophosphorylation and downstream protein tyrosine phosphorylation.
- Assays for phosphatidylinositol 3'-kinase activation, mitogen-activated protein (MAP) kinase activation, and DNA synthesis.
Main Results:
- Receptor autophosphorylation was similar across cell lines, but protein tyrosine phosphorylation patterns differed.
- Phosphorylation of IRS-1 and a 70-kDa protein was more sensitive to insulin than IGF-I.
- Chimeric receptors showed reduced protein phosphorylation compared to wild-type IGF-IR; MAP kinase activation and DNA synthesis were similar across receptors.
Conclusions:
- Despite similarities in MAP kinase activation and DNA synthesis, insulin and IGF-I receptors exhibit distinct substrate phosphorylation profiles.
- The carboxy-terminal tail of the receptor plays a role in modulating downstream signaling.
- These findings highlight differential signaling capabilities of insulin and IGF-I receptors.