Mechanisms of enhanced transmembrane signaling by an insulin receptor lacking a cytoplasmic β-subunit domain

Abstract

We have recently characterized a mutant insulin receptor (HIRΔ978) in which the insulin receptor β-subunit was truncated at amino acid residue 978. Compared with parental Rat1 cells, the cells expressing the truncated receptor exhibited enhanced sensitivity to insulin's biologic actions. All of these effects are now extended to transcriptional events, since we now show enhanced sensitivity to insulin stimulation of c-fos mRNA expression. These effects were insulin-specific, since insulin-like growth factor-1 stimulation of glucose incorporation into glycogen, α-aminoisobutyric acid uptake, and thymidine incorporation into DNA were normal. In addition, the truncated receptor exhibited enhanced sensitivity only in vivo, but not in vitro, since the kinase activity of wheat germ agglutinin-purified receptor preparations was comparable between HIRΔ978 and parental Rat1 insulin receptors. Parental rat endogenous insulin-like growth factor-1 receptors and transfected human insulin receptors form hybrid receptors as well as homologous tetrameric receptors. The normal heterotetrameric receptors possess kinase activity in viva leading to tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and its association with the p85 regulatory subunit of phosphatidyl inositol 3-kinase. Interestingly, preincubation with human-specific anti- insulin receptor antibody abolished the increased insulin sensitivity in glucose incorporation into glycogen in HIRΔ978 cells. Furthermore, microinjection of anti-IRS-1 antibody into HIRΔ978 cells inhibited insulin stimulation of DNA synthesis. In summary: 1) truncated receptors on the cell surface confer enhanced insulin sensitivity in viva; 2) the normal heterotetrameric receptors are functionally active and couple to IRS-1 efficiently; and 3) IRS-1 is an important molecule transmitting insulin's biological signals in HIRΔ978 cells.