MECHANISMS OF ENHANCED TRANSMEMBRANE SIGNALING BY AN INSULIN-RECEPTORLACKING A CYTOPLASMIC BETA-SUBUNIT DOMAIN

We have recently characterized a mutant insulin receptor (HIR Delta 97 8) in which the insulin receptor beta-subunit was truncated at amino a cid residue 978, Compared with parental Rat1 cells, the cells expressi ng the truncated receptor exhibited enhanced sensitivity to insulin's biologic actions, All of these effects are now extended to transcripti onal events, since we now show enhanced sensitivity to insulin stimula tion of c-fos mRNA expression, These effects were insulin-specific, si nce insulin-like growth factor-1 stimulation of glucose incorporation into glycogen, cy-aminoisobutyric acid uptake, and thymidine incorpora tion into DNA were normal, In addition, the truncated receptor exhibit ed enhanced sensitivity only in vivo, but not in vitro, since the kina se activity of wheat germ agglutinin-purified recep tor preparations w as comparable between HIR Delta 978 and parental Rat1 insulin receptor s, Parental rat endogenous insulin like growth factor-1 receptors and transfected human insulin receptors form hybrid receptors as well as h omologous tetrameric receptors, The normal heterotetrameric receptors possess kinase activity in vivo 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 aboli shed the increased insulin sensitivity in glucose incorporation into g lycogen in HIR Delta 978 cells, Furthermore, microinjection of anti-IR S-1 antibody into HIR Delta 978 cells inhibited insulin stimulation of DNA synthesis, In summary: 1) truncated receptors on the cell surface confer enhanced insulin sensitivity in vivo; 2) the normal heterotetr americ receptors are functionally active and couple to IRS-1 efficient ly; and 3) IRS-1 is an important molecule transmitting insulin's biolo gical signals in HIR Delta 978 cells.