Insulin and IGF‐I signaling through the insulin receptor substrate 1

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Abstract
The insulin and insulin‐like growth factor‐I (IGF‐I) receptors are tyrosine kinases. Consequently, an approach to investigating signaling pathways from these receptors is to characterize proteins rapidly phosphorylated on tyrosine in response to insulin and IGF‐I. In many cell types the most prominent phosphotyrosine (Ptyr) protein, in addition to the receptors themselves, is a protein of ˜160 kD, now known as the insulin receptor substrate 1 (IRS‐1). We have purified IRS‐1 from mouse 3T3‐L1 adipocytes, obtained the sequences of tryptic peptides, and cloned its cDNA based on this information. Mouse IRS‐1 is a protein of 1,231 amino acids. It contains 12 tyrosine residues in sequence contexts typical for tyrosine phosphorylation sites. Six of these begin the sequence motif YMXM and two begin the motif YXXM. Recent studies have shown that the enzyme phosphatidylinositol 3‐kinase (PI 3‐kinase) binds tightly to the activated platelet‐derived growth factor (PDGF) and colony‐stimulating factor‐1 (CSF‐1) receptors, through interaction of the src homology 2 (SH2) domains on the 85 kD subunit of PI 3‐kinase with Ptyr in one of these motifs on the receptors. We have found that, upon insulin treatment of 3T3‐L1 adipocytes, a portion of the Ptyr form of IRS‐1 becomes tightly complexed with PI 3‐kinase. Since IRS‐1 binds to fusion proteins containing the SH2 domains of PI 3‐kinase, association most likely occurs through this domain. The association of IRS‐1 with PI 3‐kinase activates the enzyme about fivefold. Thus, one signaling pathway from the insulin and IGF‐I receptors probably proceeds as follows: tyrosine phosphorylation of IRS‐1, tight association of IRS‐1 with PI 3‐kinase with accompanying activation of the kinase, elevation of the PI 3‐phosphates.